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Title: The Aeroplane Speaks. Fifth Edition

Author: H. Barber

Release date: June 10, 2007 [eBook #21791]

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK THE
AEROPLANE SPEAKS. FIFTH EDITION ***

THE FLIGHT FOLK.

THE AEROPLANE SPEAKS

BY
H. BARBER, A.F.Ae.S.
(CAPTAIN, ROYAL FLYING CORPS)

WITH 36 FULL PAGES OF "TYPES OF AEROPLANES"
AND 87 SKETCHES AND DIAGRAMS

FIFTH EDITION

LONDON
McBRIDE, NAST & CO., LTD.

THE AEROPLANE SPEAKS.

First edition—December, 1916
Second edition—February, 1917
Third edition—April, 1917
Fourth edition—July, 1917
Fifth edition—December, 1917

FIRST REVIEWS:

C. G. G. in the AEROPLANE: "One hopes that the Subaltern
Flying Officer will appreciate the gift which the author has given
him out of his own vast store of experience, for the book contains
the concentrated knowledge of many expensive years in tabloid
form, or perhaps one should say in condensed milk form, seeing
that it is easy to swallow and agreeable to the taste, as well as
wholesome and nourishing. And, besides the young service aviator,
there are thousands of young men, and women also, now employed
in the aircraft industry, who will appreciate far better the value of
the finicky little jobs they are doing if they will read this book and
see how vital is their work to the man who flies."
THE FIELD: "Entirely different from any other text-book on
the subject, not merely in its form, but in its capacity to convey a
knowledge of the principles and practice of flying. Undoubtedly it
is the best book on its subject."
THE UNITED SERVICE GAZETTE: "Should be in the hands
of every person interested in aviation."
THE OUTLOOK: "As amusing as it is instructive."
THE MORNING POST: "Should be read and re-read by the
would be and even the experienced pilot."

PRINTED IN ENGLAND BY
BILLING AND SONS, LIMITED
GUILDFORD

DEDICATED
TO THE
SUBALTERN FLYING OFFICER

MOTIVE
The reasons impelling me to write this book, the maiden effort of my
pen, are, firstly, a strong desire to help the ordinary man to understand the
Aeroplane and the joys and troubles of its Pilot; and, secondly, to produce
something of practical assistance to the Pilot and his invaluable assistant
the Rigger. Having had some eight years' experience in designing, building,
and flying aeroplanes, I have hopes that the practical knowledge I have
gained may offset the disadvantage of a hand more used to managing the
"joy-stick" than the dreadful haltings, the many side-slips, the irregular
speed, and, in short, the altogether disconcerting ways of a pen.
The matter contained in the Prologue appeared in the Field of May 6th,
13th, 20th, and 27th, 1916, and is now reprinted by the kind permission of
the editor, Sir Theodore Cook.
I have much pleasure in also acknowledging the kindness of Mr. C. G.
Grey, editor of the Aeroplane, to whom I am indebted for the valuable
illustrations reproduced at the end of this book.

CONTENTS
PROLOGUE
PAGE
PART I.— THE ELEMENTARY PRINCIPLES AIR THEIR
GRIEVANCES 1
II.— THE PRINCIPLES, HAVING SETTLED THEIR
DIFFERENCES, FINISH THE JOB 15
III.— THE GREAT TEST 27
IV.— CROSS COUNTRY 38

CHAPTER I.— FLIGHT 55
II.— STABILITY AND CONTROL 70
III.— RIGGING 90
IV.— PROPELLERS 115
V.— MAINTENANCE 126
TYPES OF AEROPLANES 130
GLOSSARY 133

THE AEROPLANE SPEAKS

PROLOGUE
PART I
THE ELEMENTARY PRINCIPLES AIR THEIR
GRIEVANCES

The Lecture Hall at the Royal Flying Corps School for Officers was
deserted. The pupils had dispersed, and the Officer Instructor, more fagged
than any pupil, was out on the aerodrome watching the test of a new
machine.
Deserted, did I say? But not so. The lecture that day had been upon the
Elementary Principles of Flight, and they lingered yet. Upon the
Blackboard was an illustration thus:

"I am the side view of a Surface," it said, mimicking the tones of the
lecturer. "Flight is secured by driving me through the air at an angle
inclined to the direction of motion."
"Quite right," said the Angle. "That's me, and I'm the famous Angle of
Incidence."
"And," continued the Surface, "my action is to deflect the air downwards,
and also, by fleeing from the air behind, to create a semi-vacuum or rarefied
area over most of the top of my surface."
"This is where I come in," a thick, gruff voice was heard, and went on:
"I'm the Reaction. You can't have action without me. I'm a very
considerable force, and my direction is at right-angles to you," and he

looked heavily at the Surface. "Like this," said he, picking up the chalk with
his Lift, and drifting to the Blackboard.

The action of the surface upon the air.

"I act in the direction of the arrow R, that is, more or less, for the
direction varies somewhat with the Angle of Incidence and the curvature of
the Surface; and, strange but true, I'm stronger on the top of the Surface
than at the bottom of it. The Wind Tunnel has proved that by exhaustive
research—and don't forget how quickly I can grow! As the speed through
the air increases my strength increases more rapidly than you might think—
approximately, as the Square of the Speed; so you see that if the Speed of
the Surface through the air is, for instance, doubled, then I am a good deal
more than doubled. That's because I am the result of not only the mass of
air displaced, but also the result of the Speed and consequent Force with
which the Surface engages the Air. I am a product of those two factors, and
at the speeds at which Aeroplanes fly to-day, and at the altitudes and
consequent density of air they at present experience, I increase at about the
Square of the Speed.
"Oh, I'm a most complex and interesting personality, I assure you—in
fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr. Hyde.
There's Lift, my vertical part or component, as those who prefer long words
would say; he always acts vertically upwards, and hates Gravity like poison.
He's the useful and admirable part of me. Then there's Drift, my horizontal
component, sometimes, though rather erroneously, called Head Resistance;
he's a villain of the deepest dye, and must be overcome before flight can be
secured."

"And I," said the Propeller, "I screw through the air and produce the
Thrust. I thrust the Aeroplane through the air and overcome the Drift; and
the Lift increases with the Speed, and when it equals the Gravity or Weight,
then—there you are—Flight! And nothing mysterious about it at all."
"I hope you'll excuse me interrupting," said a very beautiful young lady,
"my name is Efficiency, and, while, no doubt, all you have said is quite true,
and that, as my young man the Designer says, 'You can make a tea-tray fly
if you slap on Power enough,' I can assure you that I'm not to be won quite
so easily."
"Well," eagerly replied the Lift and the Thrust, "let's be friends. Do tell us
what we can do to help you to overcome Gravity and Drift with the least
possible Power. That obviously seems the game to play, for more Power
means heavier engines, and that in a way plays into the hands of our enemy,
Gravity, besides necessitating a larger Surface or Angle to lift the Weight,
and that increases the Drift."
"Very well," from Efficiency, "I'll do my best, though I'm so shy, and I've
just had such a bad time at the Factory, and I'm terribly afraid you'll find it
awfully dry."

"Buck up, old dear!" This from several new-comers, who had just
appeared. "We'll help you," and one of them, so lean and long that he took
up the whole height of the lecture room, introduced himself.
"I'm the High Aspect Ratio," he said, "and what we have got to do to help
this young lady is to improve the proportion of Lift to Drift. The more Lift
we can get for a certain area of Surface, the greater the Weight the latter can
carry; and the less the Drift, then the less Thrust and Power required to
overcome it. Now it is a fact that, if the Surface is shaped to have the
greatest possible span, i.e., distance from wing-tip to wing-tip, it then
engages more air and produces both a maximum Reaction and a better
proportion of Lift to Drift.
"That being so, we can then well afford to lose a little Reaction by
reducing the Angle of Incidence to a degree giving a still better proportion
of Lift to Drift than would otherwise be the case; for you must understand
that the Lift-Drift Ratio depends very much upon the size of the Angle of
Incidence, which should be as small as possible within certain limits. So
what I say is, make the surface of Infinite Span with no width or chord, as
they call it. That's all I require, I assure you, to make me quite perfect and
of infinite service to Miss Efficiency."

"That's not practical politics," said the Surface. "The way you talk one
would think you were drawing £400 a year at Westminster, and working up
a reputation as an Aeronautical Expert. I must have some depth and chord
to take my Spars and Ribs, and again, I must have a certain chord to make it
possible for my Camber (that's curvature) to be just right for the Angle of
Incidence. If that's not right the air won't get a nice uniform compression
and downward acceleration from my underside, and the rarefied 'suction'
area over the top of me will not be as even and clean in effect as it might be.
That would spoil the Lift-Drift Ratio more than you can help it. Just thrust
that chalk along, will you? and the Blackboard will show you what I mean."
"Well," said the Aspect Ratio, "have it your own way, though I'm sorry to
see a pretty young lady like Efficiency compromised so early in the game."
"Look here," exclaimed a number of Struts, "we have got a brilliant idea
for improving the Aspect Ratio," and with that they hopped up on to the
Spars. "Now," excitedly, "place another Surface on top of us. Now do you
see? There is double the Surface, and that being so, the proportion of
Weight to Surface area is halved. That's less burden of work for the Surface,
and so the Spars need not be so strong and so deep, which results in not so
thick a Surface. That means the Chord can be proportionately decreased
without adversely affecting the Camber. With the Chord decreased, the
Span becomes relatively greater, and so produces a splendid Aspect Ratio,
and an excellent proportion of Lift to Drift."
"I don't deny that they have rather got me there," said the Drift; "but all
the same, don't forget my increase due to the drift of the Struts and their
bracing wires."
"Yes; I dare say," replied the Surface, "but remember that my Spars are
less deep than before, and consequently I am not so thick now, and shall for

that reason also be able to go through the air with a less proportion of Drift
to Lift."
"Remember me also, please," croaked the Angle of Incidence. "Since the
Surface has now less weight to carry for its area, I may be set at a still lesser
and finer Angle. That means less Drift again. We are certainly getting on
splendidly! Show us how it looks now, Blackboard." And the Blackboard
obligingly showed them as follows:

"Well, what do you think of that?" they all cried to the Drift.
"You think you are very clever," sneered the Drift. "But you are not
helping Efficiency as much as you think. The suction effect on the top of
the lower Surface will give a downward motion to the air above it and the
result will be that the bottom of the top Surface will not secure as good a
Reaction from the air as would otherwise be the case, and that means loss of
Lift; and you can't help matters by increasing the gap between the surfaces
because that means longer Struts and Wires, and that in itself would help
me, not to speak of increasing the Weight. You see it's not quite so easy as
you thought."
At this moment a hiccough was heard, and a rather fast and rakish-
looking chap, named Stagger, spoke up. "How d'ye do, miss," he said
politely to Efficiency, with a side glance out of his wicked old eye. "I'm a
bit of a knut, and without the slightest trouble I can easily minimize the
disadvantage that old reprobate Drift has been frightening you with. I just
stagger the top Surface a bit forward, and no longer is that suction effect
dead under it. At the same time I'm sure the top Surface will kindly extend
its Span for such distance as its Spars will support it without the aid of
Struts. Such extension will be quite useful, as there will be no Surface at all
underneath it to interfere with the Reaction above." And the Stagger leaned
forward and picked up the Chalk, and this is the picture he drew:

Said the Blackboard, "That's not half bad! It really begins to look
something like the real thing, eh?"
"The real thing, is it?" grumbled Drift. "Just consider that contraption in
the light of any one Principle, and I warrant you will not find one of them
applied to perfection. The whole thing is nothing but a Compromise." And
he glared fixedly at poor Efficiency.
"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. What
will the Designer say?"
"Never mind, my dear," said the Lift-Drift Ratio, consolingly. "You are
improving rapidly, and quite useful enough now to think of doing a job of
work."
"Well, that's good news," and Efficiency wiped her eyes with her Fabric
and became almost cheerful. "Suppose we think about finishing it now?
There will have to be an Engine and Propeller, won't there? And a body to
fix them in, and tanks for oil and petrol, and a tail, and," archly, "one of
those dashing young Pilots, what?"
"Well, we are getting within sight of those interesting Factors," said the
Lift-Drift Ratio, "but first of all we had better decide upon the Area of the
Surfaces, their Angle of Incidence and Camber. If we are to ascend as
quickly as possible the Aeroplane must be slow in order to secure the best
possible lift-drift ratio; for the drift of the struts, wires, body, etc., increases
approximately as the square of the speed, but it carries with it no lift as it
does in the case of the Surface. The less speed then, the less such drift, and
the better the Aeroplane's proportion of lift to drift; and, being slow, we
shall require a large Surface in order to secure a large lift relative to the
weight to be carried. We shall also require a large Angle of Incidence
relative to the horizontal, in order to secure a proper inclination of the
Surface to the direction of motion, for you must remember that, while we

shall fly upon an even keel and with the propeller thrust horizontal (which
is its most efficient attitude), our flight path, which is our direction of
motion, will be sloping upwards, and it will therefore be necessary to fix the
Surface to the Aeroplane at a very considerable angle relative to the
horizontal Propeller Thrust in order to secure a proper angle to the upwards
direction of motion. Apart from that, we shall require a larger Angle of
Incidence than in the case of a machine designed purely for speed, and that
means a correspondingly large Camber.
"On the other hand, if we are thinking merely of Speed, then a small
Surface, just enough to lift the weight off the ground, will be best; also a
small Angle to cut the Drift down, and that, of course, means a relatively
small Camber.
"So you see the essentials for Climb or quick ascent and for Speed are
diametrically opposed. Now which is it to be?"
"Nothing but perfection for me," said Efficiency. "What I want is
Maximum Climb and Maximum Speed for the Power the Engine produces."
And each Principle fully agreed with her beautiful sentiments, but work
together they would not.
The Aspect Ratio wanted infinite Span, and hang the Chord.
Maximum Climb.

Maximum Speed.

The Angle of Incidence would have two Angles and two Cambers in one,
which was manifestly absurd; the Surface insisted upon no thickness

whatever, and would not hear of such things as Spars and Ribs; and the
Thrust objected to anything at all likely to produce Drift, and very nearly
wiped the whole thing off the Blackboard.
There was, indeed, the makings of a very pretty quarrel when the Letter
arrived. It was about a mile long, and began to talk at once.
"I'm from the Inventor," he said, and hope rose in the heart of each heated
Principle. "It's really absurdly simple. All the Pilot has to do is to touch a
button, and at his will, vary the area of the Surface, the Angle of Incidence,
and the Camber! And there you are—Maximum Climb or Maximum Speed
as required! How does that suit you?"
"That suits us very well," said the Surface, "but, excuse me asking, how
is it done without apparatus increasing the Drift and the Weight out of all
reason? You won't mind showing us your Calculations, Working Drawings,
Stress Diagrams, etc., will you?"
Said the Letter with dignity, "I come from an Inventor so brilliantly
clever as to be far above the unimportant matters you mention. He is no
common working man, sir! He leaves such things to Mechanics. The point
is, you press a button and——"
"Look here," said a Strut, rather pointedly, "where do you think you are
going, anyway?"
"Well," from the Letter, "as a matter of fact, I'm not addressed yet, but, of
course, there's no doubt I shall reach the very highest quarters and
absolutely revolutionize Flight when I get there."
Said the Chalk, "I'll address you, if that's all you want; now drift along
quickly!" And off went the Letter to The Technical Editor, "Daily Mauler,"
London.
And a League was formed, and there were Directors with Fees, and
several out-of-service Tin Hats, and the Man-who-takes-the-credit, and a
fine fat Guinea-pig, and all the rest of them. And the Inventor paid his
Tailor and had a Hair-Cut, and is now a recognized Press Expert—but he is
still waiting for those Mechanics!

"I'm afraid," said the Slide-rule, who had been busy making those
lightning-like automatic calculations for which he is so famous, "it's quite
impossible to fully satisfy all of you, and it is perfectly plain to me that we
shall have to effect a Compromise and sacrifice some of the Lift for Speed."
Thud! What was that?
Efficiency had fainted dead away! The last blow had been too much for
her. And the Principles gathered mournfully round, but with the aid of the
Propeller Slip1 and a friendly lift from the Surface she was at length revived
and regained a more normal aspect.
Said the Stagger with a raffish air, "My dear young lady, I assure you that
from the experiences of a varied career, I have learned that perfection is
impossible, and I am sure the Designer will be quite satisfied if you become
the Most Efficient Compromise."
"Well, that sounds so common sense," sighed Efficiency, "I suppose it
must be true, and if the Designer is satisfied, that's all I really care about.
Now do let's get on with the job."

So the Chalk drew a nice long slim body to hold the Engine and the
tanks, etc., with room for the Pilot's and Passenger's seats, and placed it
exactly in the middle of the Biplane. And he was careful to make its
position such that the Centre of Gravity was a little in advance of the Centre
of Lift, so that when the Engine was not running and there was
consequently no Thrust, the Aeroplane should be "nose-heavy" just to the
right degree, and so take up a natural glide to Earth—and this was to help
the Pilot and relieve him of work and worry, should he find himself in a fog
or a cloud. And so that this tendency to glide downwards should not be in

evidence when the Engine was running and descent not desired, the Thrust
was placed a little below the Centre of Drift or Resistance. In this way it
would in a measure pull the nose of the Aeroplane up and counter-balance
the "nose-heavy" tendency.
And the Engine was so mounted that when the Propeller-Thrust was
horizontal, which is its most efficient position, the Angle of Incidence and
the Area of the surfaces were just sufficient to give a Lift a little in excess of
the Weight. And the Camber was such that, as far as it was concerned, the
Lift-Drift Ratio should be the best possible for that Angle of Incidence. And
a beautifully simple under-carriage was added, the outstanding features of
which were simplicity, strength, light-weight, and minimum drift. And, last
of all, there was the Elevator, of which you will hear more by-and-by. And
this is what it looked like then:

And Efficiency, smiling, thought that it was not such a bad compromise
after all, and that the Designer might well be satisfied.
"Now," said she, "there's just one or two points I'm a bit hazy about. It
appears that when the Propeller shaft is horizontal and so working in its
most efficient attitude, I shall have a Lift from the Surfaces slightly in
excess of the Weight. That means I shall ascend slightly, at the same time
making nearly maximum speed for the power and thrust. Can't I do better
than that?"
"Yes, indeed," spoke up the Propeller, "though it means that I must
assume a most undignified attitude, for helicopters2 I never approved of. In
order to ascend more quickly the Pilot will deflect the Elevator, which, by
the way, you see hinged to the Tail. By that means he will force the whole
Aeroplane to assume a greater Angle of Incidence. And with greater Angle,
the Lift will increase, though I'm sorry to say the Drift will increase also.
Owing to the greater Drift, the Speed through the air will lessen, and I'm
afraid that won't be helpful to the Lift; but I shall now be pointing upwards,
and besides overcoming the Drift in a forward direction, I shall be doing my

best to haul the Aeroplane skywards. At a certain angle known as the Best
Climbing Angle, we shall have our Maximum Margin of Lift, and I'm
hoping that may be as much as almost a thousand feet altitude a minute."

The angles shown above are only roughly approximate, as they vary with
different types of aeroplanes.

"Then, if the Pilot is green, my chance will come," said the Maximum
Angle of Incidence. "For if the Angle is increased over the Best Climbing
Angle, the Drift will rush up; and the Speed, and with it the Lift, will, when
my Angle is reached, drop to a point when the latter will be no more than
the Weight. The Margin of Lift will have entirely disappeared, and there we
shall be, staggering along at my tremendous angle, and only just
maintaining horizontal flight."
"And then with luck I'll get my chance," said the Drift. "If he is a bit
worse than green, he'll perhaps still further increase the Angle. Then the
Drift, largely increasing, the Speed, and consequently the Lift, will become
still less, i.e., less than the Weight, and then—what price pancakes.3 Eh?"
"Thank you," from Efficiency, "that was all most informing. And now
will you tell me, please, how the greatest Speed may be secured?"
"Certainly, now it's my turn," piped the Minimum Angle of Incidence.
"By means of the Elevator, the Pilot places the Aeroplane at my small
Angle, at which the Lift only just equals the Weight, and, also, at which we
shall make greater speed with no more Drift than before. Then we get our
greatest Speed, just maintaining horizontal flight."
"Yes; though I'm out of the horizontal and thrusting downwards,"
grumbled the Propeller, "and that's not efficient, though I suppose it's the

best we can do until that Inventor fellow finds his Mechanics."
"Thank you so much," said Efficiency. "I think I have now at any rate an
idea of the Elementary Principles of Flight, and I don't know that I care to
delve much deeper, for sums always give me a headache; but isn't there
something about Stability and Control? Don't you think I ought to have a
glimmering of them too?"
"Well, I should smile," said a spruce Spar, who had come all the way
from America. "And that, as the Lecturer says, 'will be the subject of our
next lecture,' so be here again to-morrow, and you will be glad to hear that it
will be distinctly more lively than the subject we have covered to-day."
1 (return)
Propeller Slip: As the propeller screws through the air, the latter to a certain
extent gives back to the thrust of the propeller blades, just as the shingle on the
beach slips back as you ascend it. Such "give-back" is known as "slip," and
anyone behind the propeller will feel the slip as a strong draught of air.
2 (return)
Helicopter: An air-screw revolving upon a vertical axis. If driven with sufficient
power, it will lift vertically, but, having regard to the mechanical difficulties of
such construction, it is a most inefficient way of securing lift compared with the
arrangement of an inclined surface driven by a propeller revolving about a
horizontal axis.
3 (return)
Pancakes: Pilot's slang for stalling an aeroplane and dropping like a pancake.

PART II
THE PRINCIPLES, HAVING SETTLED THEIR
DIFFERENCES, FINISH THE JOB

Another day had passed, and the Flight Folk had again gathered together
and were awaiting the arrival of Efficiency who, as usual, was rather late in
making an appearance.
The crowd was larger than ever, and among the newcomers some of the
most important were the three Stabilities, named Directional, Longitudinal,
and Lateral, with their assistants, the Rudder, Elevator, and Ailerons. There
was Centrifugal Force, too, who would not sit still and created a most
unfavourable impression, and Keel-Surface, the Dihedral Angle, and several
other lesser fry.
"Well," said Centrifugal Force, "I wish this Efficiency I've heard so much
about would get a move on. Sitting still doesn't agree with me at all. Motion
I believe in. There's nothing like motion—the more the better."
"We are entirely opposed to that," objected the three Stabilities, all in a
breath. "Unless it's in a perfectly straight line or a perfect circle. Nothing
but perfectly straight lines or, upon occasion, perfect circles satisfy us, and
we are strongly suspicious of your tendencies."
"Well, we shall see what we shall see," said the Force darkly. "But who in
the name of blue sky is this?"
And in tripped Efficiency, in a beautifully "doped" dress of the latest
fashionable shade of khaki-coloured fabric, a perfectly stream-lined bonnet,
and a bewitching little Morane parasol,4 smiling as usual, and airily
exclaiming, "I'm so sorry I'm late, but you see the Designer's such a funny
man. He objects to skin friction,5 and insisted upon me changing my fabric
for one of a smoother surface, and that delayed me. Dear me, there are a lot
more of us to-day, aren't there? I think I had better meet one at a time." And

turning to Directional Stability, she politely asked him what he preferred to
do.
"My purpose in life, miss," said he, "is to keep the Aeroplane on its
course, and to achieve that there must be, in effect, more Keel-Surface
behind the Vertical Turning Axis than there is in front of it."

Efficiency looking a little puzzled, he added: "Just like a weathercock,
and by Keel-Surface I mean everything you can see when you view the
Aeroplane from the side of it—the sides of the body, struts, wires, etc."
"Oh, now I begin to see light," said she; "but just exactly how does it
work?"
"I'll answer that," said Momentum. "When perhaps by a gust of air the
Aeroplane is blown out of its course and points in another direction, it
doesn't immediately fly off on that new course. I'm so strong I pull it off the
new course to a certain extent, and towards the direction of the old course.
And so it travels, as long as my strength lasts, in a more or less sideways
position."
"Then," said the Keel-Surface, "I get a pressure of air all on one side, and
as there is, in effect, most of me towards the tail, the latter gets pressed
sideways, and the Aeroplane thus tends to assume its first position and
course."

"I see," said Efficiency, and, daintily holding the Chalk, she approached
the Blackboard. "Is this what you mean?"
"Yes, that's right enough," said the Keel-Surface, "and you might
remember, too, that I always make the Aeroplane nose into the gusts rather
than away from them."
"If that was not the case," broke in Lateral Stability, and affecting the
fashionable Flying Corps stammer, "it would be a h-h-h-o-r-rible affair! If
there were too much Keel-Surface in front, then that gust would blow the
Aeroplane round the other way a very considerable distance. And the right-
hand Surface being on the outside of the turn would have more speed, and
consequently more Lift, than the Surface on the other side. That means a
greater proportion of the Lift on that side, and before you could say Warp to
the Ailerons over the Aeroplane would go—probable result a bad side-slip"
(see illustration A, over-leaf).
"And what can the Pilot do to save such a situation as that?" said
Efficiency.
"Well," replied Lateral Stability, "he will try to turn the Aeroplane
sideways and back to an even keel by means of warping the Ailerons or
little wings which are hinged on to the Wing-tips, and about which you will
hear more later on; but if the side-slip is very bad he may not be able to
right the Aeroplane by means of the Ailerons, and then the only thing for
him to do is to use the Rudder and to turn the nose of the Aeroplane down
and head-on to the direction of motion. The Aeroplane will then be meeting
the air in the direction it is designed to do so, and the Surfaces and also the
controls (the Rudder, Ailerons, and Elevator) will be working efficiently;
but its attitude relative to the earth will probably be more or less upside-
down, for the action of turning the Aeroplane's nose down results, as you
will see by the illustration B, in the right wing, which is on the outside of
the circle, travelling through the air with greater speed than the left-hand
wing. More Speed means more Lift, so that results in overturning the
Aeroplane still more; but now it is, at any rate, meeting the air as it is
designed to meet it, and everything is working properly. It is then only
necessary to warp the Elevator, as shown in illustration C, in order to bring
the Aeroplane into a proper attitude relative to the earth."

"Ah!" said the Rudder, looking wise, "it's in a case like that when I
become the Elevator and the Elevator becomes me."
"That's absurd nonsense," said the Blackboard, "due to looseness of
thought and expression."
"Well," replied the Rudder, "when the Aeroplane is in position A and I am
used, then I depress or elevate the nose of the machine; and, if the Elevator
is used, then it turns the Aeroplane to right or left, which is normally my
function. Surely our rôles have changed one with the other, and I'm then the
Elevator and the Elevator is me!"

Said Lateral Stability to the Rudder, "That's altogether the wrong way of
looking at it, though I admit"—and this rather sarcastically—"that the way
you put it sounds rather fine when you are talking of your experiences in
the air to those 'interested in aviation' but knowing little about it; but it
won't go down here! You are a Controlling Surface designed to turn the
Aeroplane about a certain axis of the machine, and the Elevator is a
Controlling Surface designed to turn the Aeroplane about another axis.
Those are your respective jobs, and you can't possibly change them about.
Such talk only leads to confusion, and I hope we shall hear no more of it."
"Thanks," said Efficiency to Lateral Stability. "And now, please, will you
explain your duties?"
"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
First of all, I sometimes arrange with the Rigger to wash-out, that is
decrease, the Angle of Incidence on one side of the Aeroplane, and to effect
the reverse condition, if it is not too much trouble, on the other side."
"But," objected Efficiency, "the Lift varies with the Angle of Incidence,
and surely such a condition will result in one side of the Aeroplane lifting
more than the other side?"
"That's all right," said the Propeller, "it's meant to off-set the tendency of
the Aeroplane to turn over sideways in the opposite direction to which I
revolve."
"That's quite clear, though rather unexpected; but how do you counteract
the effect of the gusts when they try to overturn the Aeroplane sideways?"
said she, turning to Lateral Stability again.

"Well," he replied, rather miserably, "I'm not nearly so perfect as the
Longitudinal and Directional Stabilities. The Dihedral Angle—that is, the
upward inclination of the Surfaces towards their wing-tips—does what it
can for me, but, in my opinion, it's a more or less futile effort. The
Blackboard will show you the argument." And he at once showed them two
Surfaces, each set at a Dihedral Angle like this:

H.E., Horizontal equivalent.

"Please imagine," said the Blackboard, "that the top V is the front view of
a Surface flying towards you. Now if a gust blows it into the position of the
lower V you see that the horizontal equivalent of the Surface on one side
becomes larger, and on the other side it becomes smaller. That results in
more lift on the lower side and less on the higher side, and if the V is large
enough it should produce such a difference in the lift of one side to the
other as to quickly turn the Aeroplane back to its former and normal
position."
"Yes," said the Dihedral Angle, "that's what would happen if they would
only make me large enough; but they won't do it because it would too
greatly decrease the total horizontal equivalent, and therefore the Lift, and
incidentally it would, as Aeroplanes are built to-day, produce an excess of
Keel Surface above the turning axis, and that in itself would spoil the
Lateral Stability. The Keel Surface should be equally divided above and
below the longitudinal turning axis (upon which the Aeroplane rolls
sideways), or the side upon which there is an excess will get blown over by
the gusts. It strikes me that my future isn't very promising, and about my
only chance is when the Junior Draughtsman makes a mistake, as he did the
other day. And just think of it, they call him a Designer now that he's got a

job at the Factory! What did he do? Why, he calculated the weights wrong
and got the Centre of Gravity too high, and they didn't discover it until the
machine was built. Then all they could do was to give me a larger Angle.
That dropped the bottom of the V lower down, and as that's the centre of the
machine, where all the Weight is, of course that put the Centre of Gravity in
its right place. But now there is too much Keel Surface above, and the
whole thing's a Bad Compromise, not at all like Our Efficiency."
And Efficiency, blushing very prettily at the compliment, then asked,
"And how does the Centre of Gravity affect matters?"
"That's easy," said Grandfather Gravity. "I'm so heavy that if I am too low
down I act like a pendulum and cause the Aeroplane to roll about sideways,
and if I am too high I'm like a stick balanced on your finger, and then if I'm
disturbed, over I go and the Aeroplane with me; and, in addition to that,
there are the tricks I play with the Aeroplane when it's banked up,6 i.e.,
tilted sideways for a turn, and Centrifugal Force sets me going the way I'm
not wanted to go. No; I get on best with Lateral Stability when my Centre is
right on the centre of drift, or, at any rate, not much below it." And with that
he settled back into the Lecturer's Chair and went sound asleep again, for he
was so very, very old, in fact the father of all the Principles.
And the Blackboard had been busy, and now showed them a picture of
the Aeroplane as far as they knew it, and you will see that there is a slight
Dihedral Angle, and also, fixed to the tail, a vertical Keel Surface or fin, as
is very often the case in order to ensure the greater effect of such surface
being behind the vertical turning axis.

But Efficiency, growing rather critical with her newly gained knowledge,
cried out: "But where's the horizontal Tail Surface? It doesn't look right like
that!"

"This is when I have the pleasure of meeting you, my dear," said
Longitudinal Stability. "Here's the Tail Surface," he said, "and in order to
help me it must be set in effect at a much less Angle of Incidence than the
Main Surface. To explain we must trouble the Blackboard again," and this
was his effort:

"I have tried to make that as clear as possible," he said. "It may appear a
bit complicated at first, but if you will take the trouble to look at it for a
minute you will find it quite simple. A is the normal and proper direction of
motion of the Aeroplane, but, owing to a gust of air, it takes up the new
nose-down position. Owing to Momentum, however, it does not fly straight
along in that direction, but moves more or less in the direction B, which is
the resultant of the two forces, Momentum and Thrust. And so you will note
that the Angle of Incidence, which is the inclination of the Surfaces to the
Direction of Motion, has decreased, and of course the Lift decreases with it.
You will also see, and this is the point, that the Tail Surface has lost a higher
proportion of its Angle, and consequently its Lift, than has the Main
Surface. Then, such being the case, the Tail must fall and the Aeroplane
assume its normal position again, though probably at a slightly lower
altitude."
"I'm afraid I'm very stupid," said Efficiency, "but please tell me why you
lay stress upon the words 'in effect.'"
"Ah! I was wondering if you would spot that," he replied. "And there is a
very good reason for it. You see, in some Aeroplanes the Tail Surface may
be actually set at the same Angle on the machine as the Main Surface, but
owing to the air being deflected downwards by the front Main Surface it

meets the Tail Surface at a lesser angle, and indeed in some cases at no
angle at all. The Tail is then for its surface getting less Lift than the Main
Surface, although set at the same angle on the machine. It may then be said
to have in effect a less Angle of Incidence. I'll just show you on the
Blackboard."

"And now," said Efficiency, "I have only to meet the Ailerons and the
Rudder, haven't I?"
"Here we are," replied the Ailerons, or little wings. "Please hinge us on to
the back of the Main Surfaces, one of us at each Wing-tip, and join us up to
the Pilot's joystick by means of the control cables. When the Pilot wishes to
tilt the Aeroplane sideways, he will move the stick and depress us upon one
side, thus giving us a larger Angle of Incidence and so creating more Lift on
that side of the Aeroplane; and, by means of a cable connecting us with the
Ailerons on the other side of the Aeroplane, we shall, as we are depressed,
pull them up and give them a reverse or negative Angle of Incidence, and
that side will then get a reverse Lift or downward thrust, and so we are able
to tilt the Aeroplane sideways.
"And we work best when the Angle of Incidence of the Surface in front
of us is very small, for which reason it is sometimes decreased or washed-
out towards the Wing-tips. The reason of that is that by the time the air
reaches us it has been deflected downwards—the greater the Angle of
Incidence the more it is driven downwards—and in order for us to secure a
Reaction from it, we have to take such a large Angle of Incidence that we
produce a poor proportion of Lift to Drift; but the smaller the Angle of the
Surface in front of us the less the air is deflected downwards, and

consequently the less Angle is required of us, and the better our proportion
of Lift to Drift, which, of course, makes us much more effective Controls."

"Wash out" on both sides.

"Yes," said the Lateral and Directional Stabilities in one voice, "that's so,
and the wash-out helps us also, for then the Surfaces towards their Wing-
tips have less Drift or 'Head-Resistance,' and consequently the gusts will
affect them and us less; but such decreased Angle of Incidence means
decreased Lift as well as Drift, and the Designer does not always care to
pay the price."
"Well," said the Ailerons, "if it's not done it will mean more work for the
Rudder, and that won't please the Pilot."
"Whatever do you mean?" asked Efficiency. "What can the Rudder have
to do with you?"
"It's like this," they replied: "when we are deflected downwards we gain a
larger Angle of Incidence and also enter an area of compressed air, and so
produce more Drift than those of us on the other side of the Aeroplane,
which are deflected upwards into an area of rarefied air due to the suction
effect (though that term is not academically correct) on the top of the
Surface. If there is more Drift, i.e., Resistance, on one side of the Aeroplane
than on the other side, then of course it will turn off its course, and if that
difference in Drift is serious, as it will very likely be if there is no wash-out,
then it will mean a good deal of work for the Rudder in keeping the
Aeroplane on its course, besides creating extra Drift in doing so."

"I think, then," said Efficiency, "I should prefer to have that wash-out,7
and my friend the Designer is so clever at producing strength of
construction for light weight, I'm pretty sure he won't mind paying the price
in Lift. And now let me see if I can sketch the completed Aeroplane."

"Well, I hope that's all as it should be," she concluded, "for to-morrow
the Great Test in the air is due."
4 (return)
Morane parasol: A type of Morane monoplane in which the lifting surfaces are
raised above the pilot in order to afford him a good view of the earth.
5 (return)
Skin friction is that part of the drift due to the friction of the air with roughness
upon the surface of the aeroplane.
6 (return)
Banking: When an aeroplane is turned to the left or the right the centrifugal force
of its momentum causes it to skid sideways and outwards away from the centre
of the turn. To minimize such action the pilot banks, i.e., tilts, the aeroplane
sideways in order to oppose the underside of the planes to the air. The aeroplane
will not then skid outwards beyond the slight skid necessary to secure a
sufficient pressure of air to balance the centrifugal force.
7 (return)
An explanation of the way in which the wash-out is combined with a wash-in to
offset propeller torque will be found on p. 82.

PART III
THE GREAT TEST

It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
out of its shed on to the greensward of the Military Aerodrome. There is
every promise of a good flying day, and, although the sun has not yet risen,
it is light enough to discern the motionless layer of fleecy clouds some five
thousand feet high, and far, far above that a few filmy mottled streaks of
vapour. Just the kind of morning beloved of pilots.
A brand new, rakish, up-to-date machine it is, of highly polished,
beautifully finished wood, fabric as tight as a drum, polished metal, and
every part so perfectly "stream-lined" to minimize drift, which is the
resistance of the air to the passage of the machine, that to the veriest tyro
the remark of the Pilot is obviously justified.
"Clean looking 'bus, looks almost alive and impatient to be off. Ought to
have a turn for speed with those lines."
"Yes," replies the Flight-Commander, "it's the latest of its type and looks
a beauty. Give it a good test. A special report is required on this machine."
The A.M.'s8 have now placed the Aeroplane in position facing the gentle
air that is just beginning to make itself evident; the engine Fitter, having
made sure of a sufficiency of oil and petrol in the tanks, is standing by the
Propeller; the Rigger, satisfied with a job well done, is critically "vetting"
the machine by eye; four A.M.'s are at their posts, ready to hold the
Aeroplane from jumping the blocks which have been placed in front of the
wheels; and the Flight-Sergeant is awaiting the Pilot's orders.
As the Pilot approaches the Aeroplane the Rigger springs to attention and
reports, "All correct, sir," but the Fitter does not this morning report the
condition of the Engine, for well he knows that this pilot always personally
looks after the preliminary engine test. The latter, in leathern kit, warm
flying boots and goggled, climbs into his seat, and now, even more than

before, has the Aeroplane an almost living appearance, as if straining to be
off and away. First he moves the Controls to see that everything is clear, for
sometimes when the Aeroplane is on the ground the control lever or "joy-
stick" is lashed fast to prevent the wind from blowing the controlling
surfaces about and possibly damaging them.
The air of this early dawn is distinctly chilly, and the A.M.'s are
beginning to stamp their cold feet upon the dewy grass, but very careful and
circumspect is the Pilot, as he mutters to himself, "Don't worry and flurry,
or you'll die in a hurry."
At last he fumbles for his safety belt, but with a start remembers the Pitot
Air Speed Indicator, and, adjusting it to zero, smiles as he hears the Pitot-
head's gruff voice, "Well, I should think so, twenty miles an hour I was
registering. That's likely to cause a green pilot to stall the Aeroplane.
Pancake, they call it." And the Pilot, who is an old hand and has learned a
lot of things in the air that mere earth-dwellers know nothing about,
distinctly heard the Pitot Tube, whose mouth is open to the air to receive its
pressure, stammer, "Oh Lor! I've got an earwig already—hope to goodness
the Rigger blows me out when I come down—and this morning air simply
fills me with moisture; I'll never keep the Liquid steady in the Gauge. I'm
not sure of my rubber connections either."
"Oh, shut up!" cry all the Wires in unison, "haven't we got our troubles
too? We're in the most horrible state of tension. It's simply murdering our
Factor of Safety, and how we can possibly stand it when we get the Lift
only the Designer knows."
"That's all right," squeak all the little Wire loops, "we're that
accommodating, we're sure to elongate a bit and so relieve your tension."
For the whole Aeroplane is braced together with innumerable wires, many
of which are at their ends bent over in the form of loops in order to connect
with the metal fittings on the spars and elsewhere—a cheap and easy way of
making connection.
"Elongate, you little devils, would you?" fairly shout the Angles of
Incidence, Dihedral and Stagger, amid a chorus of groans from all parts of
the Aeroplane. "What's going to happen to us then? How are we going to
keep our adjustments upon which good flying depends?" "Butt us and screw

us,"9 wail the Wires. "Butt us and screw us, and death to the Loops. That's
what we sang to the Designer, but he only looked sad and scowled at the
Directors."
"And who on earth are they?" asked the Loops, trembling for their
troublesome little lives.
"On earth indeed," sniffed Efficiency, who had not spoken before, having
been rendered rather shy by being badly compromised in the Drawing
Office. "I'd like to get some of them up between Heaven and Earth, I would.
I'd give 'em something to think of besides their Debits and Credits—but all
the same the Designer will get his way in the end. I'm his Best Girl, you
know, and if we could only get rid of the Directors, the little Tin god, and
the Man-who-takes-the-credit, we should be quite happy."
Then she abruptly subsides, feeling that perhaps the less said the better
until she has made a reputation in the Air. The matter of that Compromise
still rankled, and indeed it does seem hardly fit that a bold bad Tin god
should flirt with Efficiency. You see there was a little Tin god, and he said
"Boom, Boom, BOOM! Nonsense! It MUST be done," and things like that
in a very loud voice, and the Designer tore his hair and was furious, but the
Directors, who were thinking of nothing but Orders and Dividends, had the
whip-hand of him, and so there you are, and so poor beautiful Miss
Efficiency was compromised.
All this time the Pilot is carefully buckling his belt and making himself
perfectly easy and comfortable, as all good pilots do. As he straightens
himself up from a careful inspection of the Deviation Curve10 of the
Compass and takes command of the Controls, the Throttle and the Ignition,
the voices grow fainter and fainter until there is nothing but a trembling of
the Lift and Drift wires to indicate to his understanding eye their state of
tension in expectancy of the Great Test.
"Petrol on?" shouts the Fitter to the Pilot.
"Petrol on," replies the Pilot.
"Ignition off?"
"Ignition off."

Round goes the Propeller, the Engine sucking in the Petrol Vapour with
satisfied gulps. And then—
"Contact?" from the Fitter.
"Contact," says the Pilot.
Now one swing of the Propeller by the Fitter, and the Engine is awake
and working. Slowly at first though, and in a weak voice demanding, "Not
too much Throttle, please. I'm very cold and mustn't run fast until my Oil
has thinned and is circulating freely. Three minutes slowly, as you love me,
Pilot."
Faster and faster turn the Engine and Propeller, and the Aeroplane,
trembling in all its parts, strains to jump the blocks and be off. Carefully the
Pilot listens to what the Engine Revolution Indicator says. At last, "Steady
at 1,500 revs. and I'll pick up the rest in the Air." Then does he throttle
down the Engine, carefully putting the lever back to the last notch to make
sure that in such position the throttle is still sufficiently open for the Engine
to continue working, as otherwise it might lead to him "losing" his Engine
in the air when throttling down the power for descent. Then, giving the
official signal, he sees the blocks removed from the wheels, and the Flight-
Sergeant saluting he knows that all is clear to ascend. One more signal, and
all the A.M.'s run clear of the Aeroplane.
Then gently, gently mind you, with none of the "crashing on" bad Pilots
think so fine, he opens the Throttle and, the Propeller Thrust overcoming its
enemy the Drift, the Aeroplane moves forward.
"Ah!" says the Wind-screen, "that's Discipline, that is. Through my little
Triplex window I see most things, and don't I just know that poor discipline
always results in poor work in the air, and don't you forget it."
"Discipline is it?" complains the Under-carriage, as its wheels roll swiftly
over the rather rough ground. "I'm bump getting it, and bump, bump, all I
want, bang, bump, rattle, too!" But, as the Lift increases with the Speed, the
complaints of the Under-carriage are stilled, and then, the friendly Lift
becoming greater than the Weight, the Aeroplane swiftly and easily takes to
the air.

Below is left the Earth with all its bumps and troubles. Up into the clean
clear Air moves with incredible speed and steadiness this triumph of the
Designer, the result of how much mental effort, imagination, trials and
errors, failures and successes, and many a life lost in high endeavour.
Now is the mighty voice of the Engine heard as he turns the Propeller
nine hundred times a minute. Now does the Thrust fight the Drift for all it's
worth, and the Air Speed Indicator gasps with delight "One hundred miles
an hour!"
And now does the burden of work fall upon the Lift and Drift Wires, and
they scream to the Turnbuckles whose business it is to hold them in tension,
"This is the limit! the Limit! The Limit! Release us, if only a quarter turn."
But the Turnbuckles are locked too fast to turn their eyes or utter a word.
Only the Locking Wires thus: "Ha! ha! the Rigger knew his job. He knew
the trick, and there's no release here." For an expert rigger will always use
the locking wire in such a way as to oppose the slightest tendency of the
turnbuckle to unscrew. The other kind of rigger will often use the wire in
such a way as to allow the turnbuckle, to the "eyes" of which the wires are
attached, to unscrew a quarter of a turn or more, with the result that the
correct adjustment of the wires may be lost; and upon their fine adjustment
much depends.
And the Struts and the Spars groan in compression and pray to keep
straight, for once "out of truth" there is, in addition to possible collapse, the
certainty that in bending they will throw many wires out of adjustment.
And the Fabric's quite mixed in its mind, and ejaculates, "Now, who
would have thought I got more Lift from the top of the Surface than its
bottom?" And then truculently to the Distance Pieces, which run from rib to
rib, "Just keep the Ribs from rolling, will you? or you'll see me strip. I'm an
Irishman, I am, and if my coat comes off—— Yes, Irish, I said. I used to
come from Egypt, but I've got naturalized since the War began."
Then the Air Speed Indicator catches the eye of the Pilot. "Good
enough," he says as he gently deflects the Elevator and points the nose of
the Aeroplane upwards in search of the elusive Best Climbing Angle.

"Ha! ha!" shouts the Drift, growing stronger with the increased Angle of
Incidence. "Ha! ha!" he laughs to the Thrust. "Now I've got you. Now who's
Master?" And the Propeller shrieks hysterically, "Oh! look at me. I'm a
helicopter. That's not fair. Where's Efficiency?" And she can only sadly
reply, "Yes, indeed, but you see we're a Compromise."
And the Drift has hopes of reaching the Maximum Angle of Incidence
and vanquishing the Thrust and the Lift. And he grows very bold as he
strangles the Thrust; but the situation is saved by the Propeller, who is now
bravely helicopting skywards, somewhat to the chagrin of Efficiency.
"Much ado about nothing," quotes the Aeroplane learnedly.
"Compromise or not, I'm climbing a thousand feet a minute. Ask the
Altimeter. He'll confirm it." And so indeed it was. The vacuum box of the
Altimeter was steadily expanding under the decreased pressure of the
rarefied air, and by means of its little levers and its wonderful chain no
larger than a hair it was moving the needle round the gauge and indicating
the ascent at the rate of a thousand feet a minute.
And lo! the Aeroplane has almost reached the clouds! But what's this? A
sudden gust, and down sinks one wing and up goes the other. "Oh, my
Horizontal Equivalent!" despairingly call the Planes; "it's eloping with the
Lift, and what in the name of Gravity will happen? Surely there was enough
scandal in the Factory without this, too!" For the lift varies with the
horizontal equivalent of the planes, so that if the aeroplane tilts sideways
beyond a certain angle, the lift becomes less than the weight of the machine,
which must then fall. A fall in such a position is known as a "side-slip."
But the ever-watchful Pilot instantly depresses one aileron, elevating the
other, with just a touch of the rudder to keep on the course, and the Planes
welcome back their precious Lift as the Aeroplane flicks back to its normal
position.
"Bit bumpy here under these clouds," is all the Pilot says as he heads for
a gap between them, and the next minute the Aeroplane shoots up into a
new world of space.
"My eye!" ejaculates the Wind-screen, "talk about a view!" And indeed
mere words will always fail to express the wonder of it. Six thousand feet

up now, and look! The sun is rising quicker than ever mortal on earth
witnessed its ascent. Far below is Mother Earth, wrapt in mists and deep
blue shadows, and far above are those light, filmy, ethereal clouds now
faintly tinged with pink. And all about great mountains of cloud, lazily
floating in space. The sun rises and they take on all colours, blending one
with the other, from dazzling white to crimson and deep violet-blue. Lakes
and rivers here and there in the enormous expanse of country below refract
the level rays of the sun and, like so many immense diamonds, send
dazzling shafts of light far upwards. The tops of the hills now laugh to the
light of the sun, but the valleys are still mysterious dark blue caverns,
crowned with white filmy lace-like streaks of vapour. And withal the
increasing sense with altitude of vast, clean, silent solitudes of space.
Lives there the man who can adequately describe this Wonder? "Never,"
says the Pilot, who has seen it many times, but to whom it is ever new and
more wonderful.
Up, up, up, and still up, unfalteringly speeds the Pilot and his mount.
Sweet the drone of the Engine and steady the Thrust as the Propeller
exultingly battles with the Drift.
And look! What is that bright silver streak all along the horizon? It
puzzled the Pilot when first he saw it, but now he knows it for the Sea, full
fifty miles away!
And on his right is the brightness of the morn and the smiling Earth
unveiling itself to the ardent rays of the Sun; and on his left, so high is he,
there is yet black night, hiding innumerable Cities, Towns, villages, and all
those places where soon teeming multitudes of men shall awake, and by
their unceasing toil and the spirit within them produce marvels of which the
Aeroplane is but the harbinger.
And the Pilot's soul is refreshed, and his vision, now exalted, sees the
Earth a very garden, even as it appears at that height, with discord banished
and a happy time come, when the Designer shall have at last captured
Efficiency, and the Man-who-takes-the-credit is he who has earned it, and
when kisses are the only things that go by favour.

Now the Pilot anxiously scans the Barograph, which is an instrument
much the same as the Altimeter; but in this case the expansion of the
vacuum box causes a pen to trace a line upon a roll of paper. This paper is
made by clockwork to pass over the point of the pen, and so a curved line is
made which accurately registers the speed of the ascent in feet per minute.
No longer is the ascent at the rate of a thousand feet a minute, and the
Propeller complains to the Engine, "I'm losing my Revs. and the Thrust.
Buck up with the Power, for the Lift is decreasing, though the Weight
remains much the same."
Quoth the Engine: "I strangle for Air. A certain proportion, and that of
right density, I must have to one part of Petrol, in order to give me full
power and compression, and here at an altitude of ten thousand feet the Air
is only two-thirds as dense as at sea-level. Oh, where is he who will invent a
contrivance to keep me supplied with air of right density and quality? It
should not be impossible within certain limits."
"We fully agree," said the dying Power and Thrust. "Only maintain Us
and you shall be surprised at the result. For our enemy Drift decreases in
respect of distance with the increase of altitude and rarity of air, and there
is no limit to the speed through space if only our strength remains. And with
oxygen for pilot and passengers and a steeper pitch11 for the Propeller we
may then circle the Earth in a day!"
Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
There may be greater wonders yet. Consider that as the speed increases, so
does the momentum or stored-up force in the mass of the aeroplane become
terrific. And, bearing that in mind, remember that with altitude gravity
decreases. There may yet be literally other worlds to conquer.12
Now at fifteen thousand feet the conditions are chilly and rare, and the
Pilot, with thoughts of breakfast far below, exclaims, "High enough! I had
better get on with the Test." And then, as he depresses the Elevator, the
Aeroplane with relief assumes its normal horizontal position. Then, almost
closing the Throttle, the Thrust dies away. Now, the nose of the Aeroplane
should sink of its own volition, and the craft glide downward at flying
speed, which is in this case a hundred miles an hour. That is what should
happen if the Designer has carefully calculated the weight of every part and

arranged for the centre of gravity to be just the right distance in front of the
centre of lift. Thus is the Aeroplane "nose-heavy" as a glider, and just so to
a degree ensuring a speed of glide equal to its flying speed. And the Air
Speed Indicator is steady at one hundred miles an hour, and "That's all
right!" exclaims the Pilot. "And very useful, too, in a fog or a cloud," he
reflects, for then he can safely leave the angle of the glide to itself, and give
all his attention, and he will need it all, to keeping the Aeroplane horizontal
from wing-tip to wing-tip, and to keeping it straight on its course. The latter
he will manage with the rudder, controlled by his feet, and the Compass
will tell him whether a straight course is kept. The former he will control by
the ailerons, or little wings hinged to the tips of the planes, and the bubble
in the Inclinometer in front of him must be kept in the middle.
A pilot, being only human, may be able to do two things at once, but
three is a tall order, so was this pilot relieved to find the Design not at fault
and his craft a "natural glider." To correct this nose-heavy tendency when
the Engine is running, and descent not required, the centre of Thrust is
arranged to be a little below the centre of Drift or Resistance, and thus acts
as a counter-balance.
But what is this stream of bad language from the Exhaust Pipe,
accompanied by gouts of smoke and vapour? The engine, now revolving at
no more than one-tenth its normal speed, has upset the proportion of petrol
to air, and combustion is taking place intermittently or in the Exhaust Pipe,
where it has no business to be. "Crash, Bang, Rattle——!——!——!" and
worse than that, yells the Exhaust, and the Aeroplane, who is a gentleman
and not a box kite,13 remonstrates with the severity of a Senior Officer. "See
the Medical Officer, you young Hun. Go and see a doctor. Vocal diarrhœa,
that's your complaint, and a very nasty one too. Bad form, bad for
discipline, and a nuisance in the Mess. What's your Regiment? Special
Reserve, you say? Humph! Sounds like Secondhand Bicycle Trade to me!"
Now the pilot decides to change the straight gliding descent to a spiral
one, and, obedient to the Rudder, the Aeroplane turns to the left. But the
Momentum (two tons at 100 miles per hour is no small affair) heavily
resents this change of direction, and tries its level best to prevent it and to
pull the machine sideways and outwards from its spiral course—that is, to
make it "side-skid" outwards. But the Pilot deflects the Ailerons and

"banks" up the planes to the correct angle, and, the Aeroplane skidding
sideways and outwards, the lower surfaces of the planes press up against the
air until the pressure equals the centrifugal force of the Momentum, and the
Aeroplane spirals steadily downwards.
Down, down, down, and the air grows denser, and the Pilot gulps largely,
filling his lungs with the heavier air to counteract the increasing pressure
from without. Down through a gap in the clouds, and the Aerodrome
springs into view, appearing no larger than a saucer, and the Pilot, having
by now got the "feel" of the Controls, proceeds to put the Aeroplane
through its paces. First at its Maximum Angle, staggering along tail-down
and just maintaining horizontal flight; then a dive at far over flying speed,
finishing with a perfect loop; then sharp turns with attendant vertical
"banks," and then a wonderful switchback flight, speeding down at a
hundred and fifty miles an hour with short, exhilarating ascents at the rate
of two thousand feet a minute!
All the parts are now working well together. Such wires as were before in
undue tension have secured relief by slightly elongating their loops, and
each one is now doing its bit, and all are sharing the burden of work
together.
The Struts and the Spars, which felt so awkward at first, have bedded
themselves in their sockets, and are taking the compression stresses
uncomplainingly.
The Control Cables of twisted wire, a bit tight before, have slightly
lengthened by perhaps the eighth of an inch, and, the Controls instantly
responding to the delicate touch of the Pilot, the Aeroplane, at the will of its
Master, darts this way and that way, dives, loops, spirals, and at last, in one
long, magnificent glide, lands gently in front of its shed.
"Well, what result?" calls the Flight-Commander to the Pilot.
"A hundred miles an hour and a thousand feet a minute," he briefly
replies.
"And a very good result too," says the Aeroplane, complacently, as he is
carefully wheeled into his shed.

That is the way Aeroplanes speak to those who love them and understand
them. Lots of Pilots know all about it, and can spin you wonderful yarns,
much better than this one, if you catch them in a confidential mood—on
leave, for instance, and after a good dinner.
8 (return)
A.M.'s: Air Mechanics.
9 (return)
Butt means to thicken at the end. Screw means to machine a thread on the butt-
end of the wire, and in this way the wire can make connection with the desired
place by being screwed into a metal fitting, thus eliminating the disadvantage of
the unsatisfactory loop.
10 (return)
Deviation Curve: A curved line indicating any errors in the compass.
11 (return)
A propeller screws through the air, and the distance it advances during one
revolution, supposing the air to be solid, is known as the pitch. The pitch, which
depends upon the angle of the propeller blades, must be equal to the speed of the
aeroplane, plus the slip, and if, on account of the rarity of the air, the speed of the
aeroplane increases, then the angle and pitch should be correspondingly
increased. Propellers with a pitch capable of being varied by the pilot are the
dream of propeller designers. For explanation of "slip" see Chapter IV. on
propellers.
12 (return)
Getting out of my depth? Invading the realms of fancy? Well, perhaps so, but at
any rate it is possible that extraordinary speed through space may be secured if
means are found to maintain the impulse of the engine and the thrust-drift
efficiency of the propeller at great altitude.
13 (return)
Box-kite. The first crude form of biplane.

PART IV
'CROSS COUNTRY

The Aeroplane had been designed and built, and tested in the air, and now
it stood on the Aerodrome ready for its first 'cross-country flight.
It had run the gauntlet of pseudo-designers, crank inventors, press
"experts," and politicians; of manufacturers keen on cheap work and large
profits; of poor pilots who had funked it, and good pilots who had expected
too much of it. Thousands of pounds had been wasted on it, many had gone
bankrupt over it, and others it had provided with safe fat jobs.
Somehow, and despite every conceivable obstacle, it had managed to
muddle through, and now it was ready for its work. It was not perfect, for
there were fifty different ways in which it might be improved, some of them
shamefully obvious. But it was fairly sound mechanically, had a little
inherent stability, was easily controlled, could climb a thousand feet a
minute, and its speed was a hundred miles an hour. In short, quite a
creditable machine, though of course the right man had not got the credit.
It is rough, unsettled weather with a thirty mile an hour wind on the
ground, and that means fifty more or less aloft. Lots of clouds at different
altitudes to bother the Pilot, and the air none too clear for the observation of
landmarks.
As the Pilot and Observer approach the Aeroplane the former is clearly
not in the best of tempers. "It's rotten luck," he is saying, "a blank shame
that I should have to take this blessed 'bus and join X Reserve Squadron,
stationed a hundred and fifty miles from anywhere; and just as I have licked
my Flight into shape. Now some slack blighter will, I suppose, command it
and get the credit of all my work!"
"Shut up, you grouser," said the Observer. "Do you think you're the only
one with troubles? Haven't I been through it too? Oh! I know all about it!
You're from the Special Reserve and your C.O. doesn't like your style of

beauty, and you won't lick his boots, and you were a bit of a technical knut
in civil life, but now you've jolly well got to know less than those senior to
you. Well! It's a very good experience for most of us. Perhaps conceit won't
be at quite such a premium after this war. And what's the use of grousing?
That never helped anyone. So buck up, old chap. Your day will come yet.
Here's our machine, and I must say it looks a beauty!"
And, as the Pilot approaches the Aeroplane, his face brightens and he
soon forgets his troubles as he critically inspects the craft which is to
transport him and the Observer over the hills and far away. Turning to the
Flight-Sergeant he inquires, "Tanks full of petrol and oil?"
"Yes, sir," he replies, "and everything else all correct. Propeller, engine,
and body covers on board, sir; tool kit checked over and in the locker;
engine and Aeroplane logbooks written up, signed, and under your seat;
engine revs. up to mark, and all the control cables in perfect condition and
tension."
"Very good," said the Pilot; and then turning to the Observer, "Before we
start you had better have a look at the course I have mapped out (see p. 40).
"A is where we stand and we have to reach B, a hundred and fifty miles
due North. I judge that, at the altitude we shall fly, there will be an East
wind, for although it is not quite East on the ground it is probably about
twenty degrees different aloft, the wind usually moving round clockways to
about that extent. I think that it is blowing at the rate of about fifty miles an
hour, and I therefore take a line on the map to C, fifty miles due West of A.
The Aeroplane's speed is a hundred miles an hour, and so I take a line of
one hundred miles from C to D. Our compass course will then be in the
direction A–E, which is always a line parallel to C–D. That is, to be exact, it
will be fourteen degrees off the C–D course, as, in this part of the globe,
there is that much difference between the North and South lines on the map
and the magnetic North to which the compass needle points. If the compass
has an error, as it may have of a few degrees, that, too, must be taken into
account, and the deviation or error curve on the dashboard will indicate it.

A–B, 150 miles,
A–C, 50 miles; direction and miles per hour of wind.
C–D, 100 miles; airspeed of aeroplane.
A–D, Distance covered by aeroplane in one hour.
A–E, Compass course.

"The Aeroplane will then always be pointing in a direction parallel to A–
E, but, owing to the side wind, it will be actually travelling over the course
A–B, though in a rather sideways attitude to that course.
"The distance we shall travel over the A–B course in one hour is A–D.
That is nearly eighty-seven miles, so we ought to accomplish our journey of
a hundred and fifty miles in about one and three-quarter hours.
"I hope that's quite clear to you. It's a very simple way of calculating the
compass course, and I always do it like that."
"Yes, that's plain enough. You have drafted what engineers call 'a
parallelogram of forces'; but suppose you have miscalculated the velocity of
the wind, or that it should change in velocity or direction?"

"Well, that of course will more or less alter matters," replies the Pilot.
"But there are any number of good landmarks such as lakes, rivers, towns,
and railway lines. They will help to keep us on the right course, and the
compass will, at any rate, prevent us from going far astray when between
them."
"Well, we'd better be off, old chap. Hop aboard." This from the Observer
as he climbs into the front seat from which he will command a good view
over the lower plane; and the Pilot takes his place in the rear seat, and, after
making himself perfectly comfortable, fixing his safety belt, and moving
the control levers to make sure that they are working freely, he gives the
signal to the Engine Fitter to turn the propeller and so start the engine.
Round buzzes the Propeller, and the Pilot, giving the official signal, the
Aeroplane is released and rolls swiftly over the ground in the teeth of the
gusty wind.
In less than fifty yards it takes to the air and begins to climb rapidly
upwards, but how different are the conditions to the calm morning of
yesterday! If the air were visible it would be seen to be acting in the most
extraordinary manner; crazily swirling, lifting and dropping, gusts viciously
colliding—a mad phantasmagoria of forces!
Wickedly it seizes and shakes the Aeroplane; then tries to turn it over
sideways; then instantly changes its mind and in a second drops it into a
hole a hundred feet deep; and if it were not for his safety belt the Pilot
might find his seat sinking away from beneath him.
Gusts strike the front of the craft like so many slaps in the face; and
others, with the motion of mountainous waves, sometimes lift it hundreds of
feet in a few seconds, hoping to see it plunge over the summit in a death-
dive—and so it goes on, but the Pilot, perfectly at one with his mount and
instantly alert to its slightest motion, is skilfully and naturally making
perhaps fifty movements a minute of hand and feet; the former lightly
grasping the "joy-stick" which controls the Elevator hinged to the tail, and
also the Ailerons or little wings hinged to the wing-tips; and the latter
moving the Rudder control-bar.

The Pilot's Cock-pit.

A strain on the Pilot? Not a bit of it, for this is his Work which he loves
and excels in; and given a cool head, alert eye, and a sensitive touch for the
controls, what sport can compare with these ever-changing battles of the
air?
The Aeroplane has all this time been climbing in great wide circles, and
is now some three thousand feet above the Aerodrome which from such
height looks absurdly small. The buildings below now seem quite squat; the
hills appear to have sunk away into the ground, and the whole country
below, cut up into diminutive fields, has the appearance of having been
lately tidied and thoroughly spring-cleaned! A doll's country it looks, with
tiny horses and cows ornamenting the fields and little model motor-cars and
carts stuck on the roads, the latter stretching away across country like
ribbons accidentally dropped.
At three thousand feet altitude the Pilot is satisfied that he is now
sufficiently high to secure, in the event of engine failure, a long enough
glide to earth to enable him to choose and reach a good landing-place; and,
being furthermore content with the steady running of the engine, he decides
to climb no more but to follow the course he has mapped out. Consulting
the compass, he places the Aeroplane on the A–E course and, using the
Elevator, he gives his craft its minimum angle of incidence at which it will
just maintain horizontal flight and secure its maximum speed.

Swiftly he speeds away, and few thoughts he has now for the changing
panorama of country, cloud, and colour. Ever present in his mind are the
three great 'cross-country queries. "Am I on my right course? Can I see a
good landing-ground within gliding distance?" And "How is the Engine
running?"
Keenly both he and the Observer compare their maps with the country
below. The roads, khaki-coloured ribbons, are easily seen but are not of
much use, for there are so many of them and they all look alike from such
an altitude.
Now where can that lake be which the map shows so plainly? He feels
that surely he should see it by now, and has an uncomfortable feeling that
he is flying too far West. What pilot is there indeed who has not many times
experienced such unpleasant sensation? Few things in the air can create
greater anxiety. Wisely, however, he sticks to his compass course, and the
next minute he is rewarded by a sight of the lake, though indeed he now
sees that the direction of his travel will not take him over it, as should be the
case if he were flying over the shortest route to his destination. He must
have slightly miscalculated the velocity or direction of the side-wind.
"About ten degrees off," he mutters, and, using the Rudder, corrects his
course accordingly.
Now he feels happier and that he is well on his way. The gusts, too, have
ceased to trouble him as, at this altitude, they are not nearly so bad as they
were near the ground, the broken surface of which does much to produce
them; and sometimes for miles he makes but a movement or two of the
controls.
The clouds just above race by with dizzy and uniform speed; the country
below slowly unrolls, and the steady drone of the Engine is almost hypnotic
in effect. "Sleep, sleep, sleep," it insidiously suggests. "Listen to me and
watch the clouds; there's nothing else to do. Dream, dream, dream of
speeding through space for ever, and ever, and ever; and rest, rest, rest to
the sound of my rhythmical hum. Droning on and on, nothing whatever
matters. All things now are merged into speed through space and a sleepy
monotonous d-d-r-r-o-o-n-n-e———." But the Pilot pulls himself together
with a start and peers far ahead in search of the next landmark. This time it

is a little country town, red-roofed his map tells him, and roughly of
cruciform shape; and, sure enough, there in the right direction are the
broken outlines of a few red roofs peeping out from between the trees.
Another minute and he can see this little town, a fairy place it appears,
nestling down between the hills and its red roofs and picturesque shape, a
glowing and lovely contrast with the dark green of the surrounding moors.
So extraordinarily clean and tidy it looks from such a height, and laid out
in such orderly fashion with perfectly defined squares, parks, avenues, and
public buildings, it indeed appears hardly real, but rather as if it has this
very day materialized from some delightful children's book!
Every city and town you must know has its distinct individuality to the
Pilot's eye. Some are not fairy places at all, but great dark ugly blots upon
the fair countryside, and with tall shafts belching forth murky columns of
smoke to defile clean space. Others, melancholy-looking masses of grey,
slate-roofed houses, are always sad and dispirited; never welcoming the
glad sunshine, but ever calling for leaden skies and a weeping Heaven.
Others again, little coquettes with village green, white palings everywhere,
bright gravel roads, and an irrepressible air of brightness and gaiety.
Then there are the rivers, silvery streaks peacefully winding far, far away
to the distant horizon; they and the lakes the finest landmarks the Pilot can
have. And the forests. How can I describe them? The trees cannot be seen
separately, but merge altogether into enormous irregular dark green masses
sprawling over the country, and sometimes with great ungainly arms half
encircling some town or village; and the wind passing over the foliage at
times gives the forest an almost living appearance, as of some great dragon
of olden times rousing itself from slumber to devour the peaceful villages
its arms encircle.
And the Pilot and Observer fly on and on, seeing these things and many
others which baffle my poor skill to describe—things, dear Reader, that you
shall see, and poets sing of, and great artists paint in the days to come when
the Designer has captured Efficiency. Then, and the time is near, shall you
see this beautiful world as you have never seen it before, the garden it is,
the peace it breathes, and the wonder of it.

The Pilot, flying on, is now anxiously looking for the railway line which
midway on his journey should point the course. Ah! There it is at last, but
suddenly (and the map at fault) it plunges into the earth! Well the writer
remembers when that happened to him on a long 'cross-country flight in the
early days of aviation. Anxiously he wondered "Are tunnels always
straight?" and with what relief, keeping on a straight course, he picked up
the line again some three miles farther on!
Now at last the Pilot sees the sea, just a streak on the north-eastern
horizon, and he knows that his flight is two-thirds over. Indeed, he should
have seen it before, but the air is none too clear, and he is not yet able to
discern the river which soon should cross his path. As he swiftly speeds on
the air becomes denser and denser with what he fears must be the beginning
of a sea-fog, perhaps drifting inland along the course of the river. Now does
he feel real anxiety, for it is the duty of a Pilot to fear fog, his deadliest
enemy. Fog not only hides the landmarks by which he keeps his course, but
makes the control of the Aeroplane a matter of the greatest difficulty. He
may not realize it, but, in keeping his machine on an even keel, he is
unconsciously balancing it against the horizon, and with the horizon gone
he is lost indeed. Not only that, but it also prevents him from choosing his
landing-place, and the chances are that, landing in a fog, he will smash into
a tree, hedge, or building, with disastrous results. The best and boldest pilot
'wares a fog, and so this one, finding the conditions becoming worse and yet
worse, and being forced to descend lower and lower in order to keep the
earth within view, wisely decides to choose a landing-place while there is
yet time to do so.
Throttling down the power of the engine he spirals downwards, keenly
observing the country below. There are plenty of green fields to lure him,
and his great object is to avoid one in which the grass is long, for that would
bring his machine to a stop so suddenly as to turn it over; or one of rough
surface likely to break the under-carriage. Now is perfect eyesight and a
cool head indispensable. He sees and decides upon a field and, knowing his
job, he sticks to that field with no change of mind to confuse him. It is none
too large, and gliding just over the trees and head on to the wind he skilfully
"stalls" his machine; that is, the speed having decreased sufficiently to
avoid such a manœuvre resulting in ascent, he, by means of the Elevator,
gives the Aeroplane as large an angle of incidence as possible, and the

undersides of the planes meeting the air at such a large angle act as an air-
brake, and the Aeroplane, skimming over the ground, lessens its speed and
finally stops just at the farther end of the field.
Then, after driving the Aeroplane up to and under the lee of the hedge, he
stops the engine, and quickly lashing the joy-stick fast in order to prevent
the wind from blowing the controlling surfaces about and possibly
damaging them, he hurriedly alights. Now running to the tail he lifts it up
on to his shoulder, for the wind has become rough indeed and there is
danger of the Aeroplane becoming unmanageable. By this action he
decreases the angle at which the planes are inclined to the wind and so
minimizes the latter's effect upon them. Then to the Observer, "Hurry up,
old fellow, and try to find some rope, wire, or anything with which to picket
the machine. The wind is rising and I shan't be able to hold the 'bus steady
for long. Don't forget the wire-cutters. They're in the tool kit." And the
Observer rushes off in frantic haste, before long triumphantly returning with
a long length of wire from a neighbouring fence. Blocking up the tail with
some debris at hand, they soon succeed, with the aid of the wire, in stoutly
picketing the Aeroplane to the roots of the high hedge in front of it; done
with much care, too, so that the wire shall not fray the fabric or set up
dangerous bending-stresses in the woodwork. Their work is not done yet,
for the Observer remarking, "I don't like the look of this thick weather and
rather fear a heavy rain-storm," the Pilot replies, "Well, it's a fearful bore,
but the first rule of our game is never to take an unnecessary risk, so out
with the engine and body covers."
Working with a will they soon have the engine and the open part of the
body which contains the seats, controls, and instruments snugly housed
with their waterproof covers, and the Aeroplane is ready to weather the
possible storm. Says the Observer, "I'm remarkably peckish, and methinks I
spy the towers of one of England's stately homes showing themselves just
beyond that wood, less than a quarter of a mile away. What ho! for a raid.
What do you say?"
"All right, you cut along and I'll stop here, for the Aeroplane must not be
left alone. Get back as quickly as possible."

And the Observer trots off, leaving the Pilot filling his pipe and anxiously
scrutinizing the weather conditions. Very thick it is now, but the day is yet
young, and he has hopes of the fog lifting sufficiently to enable the flight to
be resumed. A little impatiently he awaits the return of his comrade, but
with never a doubt of the result, for the hospitality of the country house is
proverbial among pilots! What old hand among them is there who cannot
instance many a forced landing made pleasant by such hospitality? Never
too late or too early to help with food, petrol, oil, tools, and assistants.
Many a grateful thought has the writer for such kind help given in the days
before the war (how long ago they seem!), when aeroplanes were still more
imperfect than they are now, and involuntary descents often a part of 'cross-
country flying.
Ah! those early days! How fresh and inspiring they were! As one started
off on one's first 'cross-country flight, on a machine the first of its design,
and with everything yet to learn, and the wonders of the air yet to explore;
then the joy of accomplishment, the dreams of Efficiency, the hard work
and long hours better than leisure; and what a field of endeavour—the
realms of space to conquer! And the battle still goes on with ever-increasing
success. Who is bold enough to say what its limits shall be?
So ruminates this Pilot-Designer, as he puffs at his pipe, until his reverie
is abruptly disturbed by the return of the Observer.
"Wake up, you airman," the latter shouts. "Here's the very thing the
doctor ordered! A basket of first-class grub and something to keep the fog
out, too."
"Well, that's splendid, but don't call me newspaper names or you'll spoil
my appetite!"
Then, with hunger such as only flying can produce, they appreciatively
discuss their lunch, and with many a grateful thought for the donors—and
they talk shop. They can't help it, and even golf is a poor second to flight
talk. Says the Pilot, who must have his grievance, "Just observe where I
managed to stop the machine. Not twenty feet from this hedge! A little more
and we should have been through it and into Kingdom Come! I stalled as
well as one could, but the tail touched the ground and so I could not give
the Aeroplane any larger angle of incidence. Could I have given it a larger

angle, then the planes would have become a much more effective air-brake,
and we should have come to rest in a much shorter distance. It's all the fault
of the tail. There's hardly a type of Aeroplane in existence in which the tail
could not be raised several feet, and that would make all the difference. A
high tail means a large angle of incidence when the machine touches ground
and, with enough angle, I'll guarantee to safely land the fastest machine in a
five-acre field. You can, I am sure, imagine what a difference that would
make where forced landings are concerned!" Then rapidly sketching in his
notebook, he shows the Observer the following illustration:
The Pilot's Aeroplane.

The Change of Design He Would Like.

"That's very pretty," said the Observer, "but how about Mechanical
Difficulties, and Efficiency in respect of Flight? And, anyway, why hasn't
such an obvious thing been done already?"
"As regards the first part of your question I assure you that there's
nothing in it, and I'll prove it to you as follows——"
"Oh! That's all right, old chap. I'll take your word for it," hurriedly replies
the Observer, whose soul isn't tuned to a technical key.
"As regards the latter part of your inquiry," went on the Pilot, a little
nettled at having such a poor listener, "it's very simple. Aeroplanes have
'just growed' like Topsy, and they consequently contain this and many
another relic of early day design when Aeroplanes were more or less thrown
together and anything was good enough that could get off the ground."
"By Jove," interrupts the Observer, "I do believe the fog is lifting. Hadn't
we better get the engine and body covers off, just in case it's really so?"

"I believe you're right. I am sure those hills over there could not be seen a
few minutes ago, and look—there's sunshine over there. We'd better hurry
up."
Ten minutes' hard work and the covers are off, neatly folded and stowed
aboard; the picketing wires are cast adrift, and the Pilot is once more in his
seat. The Aeroplane has been turned to face the other end of the field, and,
the Observer swinging round the propeller, the engine is awake again and
slowly ticking over. Quickly the Observer climbs into his seat in front of the
Pilot, and, the latter slightly opening the throttle, the Aeroplane leisurely
rolls over the ground towards the other end of the field, from which the
ascent will be made.
Arriving there the Pilot turns the Aeroplane in order to face the wind and
thus secure a quick "get-off." Then he opens the throttle fully and the
mighty voice of the Engine roars out "Now see me clear that hedge!" and
the Aeroplane races forward at its minimum angle of incidence. Tail up, and
with ever-increasing speed, it rushes towards the hedge under the lee of
which it has lately been at rest; and then, just as the Observer involuntarily
pulls back an imaginary joy-stick, the Pilot moves the real one and places
the machine at its best climbing angle. Like a living thing it responds, and
instantly leaves the ground, clearing the hedge like a—well, like an
Aeroplane with an excellent margin of lift. Upwards it climbs with even and
powerful lift, and the familiar scenes below again gladden the eyes of the
Pilot. Smaller and more and more squat grow the houses and hills; more
and more doll-like appear the fields which are clearly outlined by the
hedges; and soon the country below is easily identified with the map. Now
they can see the river before them and a bay of the sea which must be
crossed or skirted. The fog still lingers along the course of the river and
between the hills, but is fast rolling away in grey, ghost-like masses. Out to
sea it obscures the horizon, making it difficult to be sure where water ends
and fog begins, and creating a strange, rather weird, effect by which ships at
a certain distance appear to be floating in space.
Now the Aeroplane is almost over the river, and the next instant it
suddenly drops into a "hole in the air." With great suddenness it happens,
and for some two hundred feet it drops nose-down and tilted over sideways;
but the Pilot is prepared and has put his craft on an even keel in less time

than it takes to tell you about it; for well he knows that he must expect such
conditions when passing over a shore or, indeed, any well-defined change in
the composition of the earth's surface. Especially is this so on a hot and
sunny day, for then the warm surface of the earth creates columns of
ascending air, the speed of the ascent depending upon the composition of
the surface. Sandy soil, for instance, such as borders this river produces a
quickly ascending column of air, whereas water and forests have not such a
marked effect. Thus, when our Aeroplane passed over the shore of the river,
it suddenly lost the lift due to the ascending air produced by the warm
sandy soil, and it consequently dropped just as if it had fallen into a hole.
Now the Aeroplane is over the bay and, the sea being calm, the Pilot
looks down, down through the water, and clearly sees the bottom, hundreds
of feet below the surface. Down through the reflection of the blue sky and
clouds, and one might think that is all, but it isn't. Only those who fly know
the beauties of the sea as viewed from above; its dappled pearly tints; its
soft dark blue shadows; the beautiful contrasts of unusual shades of colour
which are always differing and shifting with the changing sunshine and the
ever moving position of the aerial observer. Ah! for some better pen than
mine to describe these things! One with glowing words and a magic rhythm
to express the wonders of the air and the beauty of the garden beneath—the
immensity of the sea—the sense of space and of one's littleness there—the
realization of the Power moving the multitudes below—the exaltation of
spirit altitude produces—the joy of speed. A new world of sensation!
Now the bay is almost crossed and the Aerodrome at B. can be
distinguished....

On the Aerodrome is a little crowd waiting and watching for the arrival
of the Aeroplane, for it is of a new and improved type and its first 'cross-
country performance is of keen interest to these men; men who really know
something about flight.
There is the Squadron Commander who has done some real flying in his
time; several well-seasoned Flight-Commanders; a dozen or more Flight-
Lieutenants; a knowledgeable Flight-Sergeant; a number of Air Mechanics,
and, a little on one side and almost unnoticed, the Designer.

"I hope they are all right," says someone, "and that they haven't had
difficulties with the fog. It rolled up very quickly, you know."
"Never fear," remarks a Flight-Commander. "I know the Pilot well and
he's a good 'un; far too good to carry on into a fog."
"They say the machine is really something out of the ordinary," says
another, "and that, for once, the Designer has been allowed full play; that he
hasn't been forced to unduly standardize ribs, spars, struts, etc., and has
more or less had his own way. I wonder who he is. It seems strange we hear
so little of him."
"Ah! my boy. You do a bit more flying and you'll discover that things are
not always as they appear from a distance!"
"There she is, sir!" cries the Flight-Sergeant. "Just a speck over the
silvery corner of that cloud."
A tiny speck it looks, some six miles distant and three thousand feet high;
but, racing along, it rapidly appears larger and soon its outlines can be
traced and the sunlight be seen playing upon the whirling propeller.
Now the distant drone of the engine can be heard, but not for long, for
suddenly it ceases and, the nose of the Aeroplane sinking, the craft
commences gliding downwards.
"Surely too far away," says a subaltern. "It will be a wonderful machine
if, from that distance and height, it can glide into the Aerodrome." And
more than one express the opinion that it cannot be done; but the Designer
smiles to himself, yet with a little anxiety, for his reputation is at stake, and
Efficiency, the main reward he desires, is perhaps, or perhaps not, at last
within his grasp!
Swiftly the machine glides downwards towards them, and it can now be
seen how surprisingly little it is affected by the rough weather and gusts; so
much so that a little chorus of approval is heard.
"Jolly good gliding angle," says someone; and another, "Beautifully
quick controls, what?" and from yet another, "By Jove! The Pilot must be
sure of the machine. Look, he's stopped the engine entirely."

Then the Aeroplane with noiseless engine glides over the boundary of the
Aerodrome, and, with just a soft soughing sound from the air it cleaves,
lands gently not fifty yards from the onlookers.
"Glad to see you," says the Squadron Commander to the Pilot. "How do
you like the machine?" And the Pilot replies:
"I never want a better one, sir. It almost flies itself!"
And the Designer turns his face homewards and towards his beloved
drawing-office; well satisfied, but still dreaming dreams of the future and ...
looking far ahead who should he see but Efficiency at last coming towards
him! And to him she is all things. In her hair is the morning sunshine; her
eyes hold the blue of the sky, and on her cheeks is the pearly tint of the
clouds as seen from above. The passion of speed, the lure of space, the
sense of power, and the wonder of the future ... all these things she holds for
him.
"Ah!" he cries. "You'll never leave me now, when at last there is no one
between us?"
And Efficiency, smiling and blushing, but practical as ever, says:
"And you will never throw those Compromises in my face?"
"My dear, I love you for them! Haven't they been my life ever since I
began striving for you ten long years ago?"
And so they walk off very happily, arm-in-arm together; and if this hasn't
bored you and you'd like some more of the same sort of thing, I'd just love
to tell you some day of the wonderful things they accomplish together, and
of what they dream the future holds in store.

And that's the end of the Prologue.

CHAPTER I
FLIGHT

Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum. It
therefore obeys Newton's laws14 and resists movement. It is that resistance
or reaction which makes flight possible.
Flight is secured by driving through the air a surface15 inclined upwards
and towards the direction of motion.

S = Side view of surface.
M = Direction of motion.
Chord.—The Chord is, for practical purposes, taken to be a straight line
from the leading edge of the surface to its trailing edge.
N = A line through the surface starting from its trailing edge. The
position of this line, which I call the Neutral Lift Line, is found by means of
wind-tunnel research, and it varies with differences in the camber
(curvature) of surfaces. In order to secure flight, the inclination of the
surface must be such that the neutral lift line makes an angle with and
above the line of motion. If it is coincident with M, there is no lift. If it
makes an angle with M and below it, then there is a pressure tending to
force the surface down.
I = Angle of Incidence. This angle is generally defined as the angle the
chord makes with the direction of motion, but that is a bad definition, as it
leads to misconception. The angle of incidence is best described as the
angle the neutral lift line makes with the direction of motion relative to the
air. You will, however, find that in nearly all rigging specifications the angle
of incidence is taken to mean the angle the chord makes with a line parallel

to the propeller thrust. This is necessary from the point of view of the
practical mechanic who has to rig the aeroplane, for he could not find the
neutral lift line, whereas he can easily find the chord. Again, he would
certainly be in doubt as to "the direction of motion relative to the air,"
whereas he can easily find a line parallel to the propeller thrust. It is a pity,
however, that these practical considerations have resulted in a bad definition
of the angle of incidence becoming prevalent, a consequence of which has
been the widespread fallacy that flight may be secured with a negative
inclination of the surface. Flight may conceivably be secured with a
negative angle of chord, but never with a negative inclination of the surface,
if, as seems reasonable, we regard the surface from the point of view of the
neutral lift line. All this is only applicable to cambered surfaces. In the case
of flat surfaces the neutral lift line coincides with the chord and the
definition I have criticized adversely is then applicable. Flat lifting surfaces
are, however, never used.
The surface acts upon the air in the following manner:

As the bottom of the surface meets the air, it compresses it and
accelerates it downwards. As a result of this definite action there is, of
course, an equal and opposite reaction upwards.
The top surface, in moving forward, tends to leave the air behind it, thus
creating a semi-vacuum or rarefied area over the top of the surface.
Consequently the pressure of air on the top of the surface is decreased, thus
assisting the reaction below to lift the surface upwards.
The reaction increases approximately as the square of the velocity. It is
the result of (1) the mass of air engaged, and (2) the velocity and
consequent force with which the surface engages the air. If the reaction was
produced by only one of those factors it would increase in direct proportion
to the velocity, but, since it is the product of both factors, it increases as V2.

Approximately three-fifths of the reaction is due to the decrease of
density (and consequent decrease of downward pressure) on the top of the
surface; and only some two-fifths is due to the upward reaction secured by
the action of the bottom surface upon the air. A practical point in respect of
this is that, in the event of the fabric covering the surface getting into bad
condition, it is more likely to strip off the top than off the bottom.

The direction of the reaction is, at efficient angles of incidence,
approximately at right-angles to the neutral lift line of the surface, as
illustrated above; and it is, in considering flight, convenient to divide it into
two component parts or values, thus:
1. The vertical component of the reaction, i.e., Lift, which is opposed to
Gravity, i.e., the weight of the aeroplane.
2. The horizontal component, i.e., Drift (sometimes called Resistance), to
which is opposed the thrust of the propeller.
The direction of the reaction is, of course, the resultant of the forces Lift
and Drift. The Lift is the useful part of the reaction, for it lifts the weight of
the aeroplane.
The Drift is the villain of the piece, and must be overcome by the Thrust
in order to secure the necessary velocity to produce the requisite lift for
flight.
Drift.—The drift of the whole aeroplane (we have considered only the
lifting surface heretofore) may be conveniently divided into three parts, as
follows:
Active Drift, which, is the drift produced by the lifting surfaces.

Passive Drift, which is the drift produced by all the rest of the aeroplane
—the struts, wires, fuselage, under-carriage, etc., all of which is known as
"detrimental surface."
Skin Friction, which is the drift produced by the friction of the air with
roughness of surface. The latter is practically negligible having regard to the
smooth surface of the modern aeroplane, and its comparatively slow
velocity compared with, for instance, the velocity of a propeller blade.
Lift-Drift Ratio.—The proportion of lift to drift is known as the lift-drift
ratio, and is of paramount importance, for it expresses the efficiency of the
aeroplane (as distinct from engine and propeller). A knowledge of the
factors governing the lift-drift ratio is, as will be seen later, an absolute
necessity to anyone responsible for the rigging of an aeroplane, and the
maintenance of it in an efficient and safe condition.
Those factors are as follows:
1. Velocity.—The greater the velocity the greater the proportion of
drift to lift, and consequently the less the efficiency. Considering
the lifting surfaces alone, both the lift and the (active) drift,
being component parts of the reaction, increase as the square of
the velocity, and the efficiency remains the same at all speeds.
But, considering the whole aeroplane, we must remember the
passive drift. It also increases as the square of the velocity (with
no attendant lift), and, adding itself to the active drift, results in
increasing the proportion of total drift (active + passive) to lift.
But for the increase in passive drift the efficiency of the
aeroplane would not fall with increasing velocity, and it would
be possible, by doubling the thrust, to approximately double the
speed or lift—a happy state of affairs which can never be, but
which we may, in a measure, approach by doing everything
possible to diminish the passive drift.
Every effort is then made to decrease it by "stream-lining,"
i.e., by giving all "detrimental" parts of the aeroplane a form by
which they will pass through the air with the least possible drift.
Even the wires bracing the aeroplane together are, in many

cases, stream-lined, and with a markedly good effect upon the
lift-drift ratio. In the case of a certain well-known type of
aeroplane the replacing of the ordinary wires by stream-lined
wires added over five miles an hour to the flight speed.

Head-resistance is a term often applied to passive drift, but it
is apt to convey a wrong impression, as the drift is not nearly so
much the result of the head or forward part of struts, wires, etc.,
as it is of the rarefied area behind.
Above is illustrated the flow of air round two objects moving
in the direction of the arrow M.
In the case of A, you will note that the rarefied area DD is of
very considerable extent; whereas in the case of B, the air flows
round it in such a way as to meet very closely to the rear of the
object, thus decreasing DD.
The greater the rarefied area DD, then, the less the density,
and, consequently, the less the pressure of air upon the rear of
the object. The less such pressure, then, the better is head-
resistance D able to get its work in, and the more thrust will be
required to overcome it.
The "fineness" of the stream-line shape, i.e., the proportion of
length to width, is determined by the velocity—the greater the
velocity, the greater the fineness. The best degree of fineness for
any given velocity is found by means of wind-tunnel research.
The practical application of all this is, from a rigging point of
view, the importance of adjusting all stream-line parts to be
dead-on in the line of flight, but more of that later on.

2. Angle of Incidence.—The most efficient angle of incidence
varies with the thrust at the disposal of the designer, the weight
to be carried, and the climb-velocity ratio desired.
The best angles of incidence for these varying factors are
found by means of wind-tunnel research and practical trial and
error. Generally speaking, the greater the velocity the smaller
should be the angle of incidence, in order to preserve a clean,
stream-line shape of rarefied area and freedom from eddies.
Should the angle be too great for the velocity, then the rarefied
area over the top of the surface becomes of irregular shape with
attendant turbulent eddies. Such eddies possess no lift value, and
since it has taken power to produce them, they represent drift
and adversely affect the lift-drift ratio. Also, too great an angle
for the velocity will result in the underside of the surface
tending to compress the air against which it is driven rather than
accelerate it downwards, and that will tend to produce drift
rather than the upwards reaction, or lift.
From a rigging point of view, one must presume that every
standard aeroplane has its lifting surface set at the most efficient
angle, and the practical application of all this is in taking the
greatest possible care to rig the surface at the correct angle and
to maintain it at such angle. Any deviation will adversely affect
the lift-drift ratio, i.e., the efficiency.
3. Camber.—(Refer to the second illustration in this chapter.) The
lifting surfaces are cambered, i.e., curved, in order to decrease
the horizontal component of the reaction, i.e., the drift.
The bottom camber: If the bottom of the surface was flat,
every particle of air meeting it would do so with a shock, and
such shock would produce a very considerable horizontal
reaction or drift. By curving it such shock is diminished, and the
curve should be such as to produce a uniform (not necessarily
constant) acceleration and compression of the air from the
leading edge to the trailing edge. Any unevenness in the
acceleration and compression of the air produces drift.

The top camber: If this was flat it would produce a rarefied
area of irregular shape. I have already explained the bad effect
this has upon the lift-drift ratio. The top surface is then curved to
produce a rarefied area the shape of which shall be as stream-
line and free from attendant eddies as possible.
The camber varies with the angle of incidence, the velocity,
and the thickness of the surface. Generally speaking, the greater
the velocity, the less the camber and angle of incidence. With
infinite velocity the surface would be set at no angle of
incidence (the neutral lift line coincident with the direction of
motion relative to the air), and would be, top and bottom, of
pure stream-line form—i.e., of infinite fineness. This is, of
course, carrying theory to absurdity as the surface would then
cease to exist.
The best cambers for varying velocities, angles of incidence,
and thickness of surface, are found by means of wind-tunnel
research. The practical application of all this is in taking the
greatest care to prevent the surface from becoming distorted and
thus spoiling the camber and consequently the lift-drift ratio.
4. Aspect Ratio.—This is the proportion of span to chord. Thus, if
the span is, for instance, 50 feet and the chord 5 feet, the surface
would be said to have an aspect ratio of 10 to 1.
For a given velocity and a given area of surface, the higher
the aspect ratio, the greater the reaction. It is obvious, I think,
that the greater the span, the greater the mass of undisturbed air
engaged, and, as already explained, the reaction is partly the
result of the mass of air engaged. I say "undisturbed" advisedly,
for otherwise it might be argued that, whatever the shape of the
surface, the same mass of air would be engaged. The word
"undisturbed" makes all the difference, for it must be
remembered that the rear part of the underside of the surface
engages air most of which has been deflected downwards by the
surface in front of it. That being so, the rear part of the surface
has not the same opportunity of forcing; the air downwards

(since it is already flowing downwards) and securing there from
an upwards, reaction as has the surface in front of it. It is
therefore of less value for its area than the front part of the
surface, since it does less work and secures less reaction—i.e.,
lift. Again, the rarefied area over the top of the surface is most
rare towards the front of it, as, owing to eddies, the rear of such
area tends to become denser.

Thus, you see, the front part of the surface is the most
valuable from the point of view of securing an upwards reaction
from the air; and so, by increasing the proportion of front, or
"span," to chord, we increase the amount of reaction for a given
velocity and area of surface. That means a better proportion of
reaction to weight of surface, though the designer must not
forget the drift of struts and wires necessary to brace up a
surface of high aspect ratio.
Not only that, but, provided the chord is not decreased to an
extent making it impossible to secure the best camber owing to
the thickness of the surface, the higher the aspect ratio, the
better the lift-drift ratio. The reason of this is rather obscure. It is
sometimes advanced that it is owing to the "spill" of air from
under the wing-tips. With a high aspect ratio the chord is less
than would otherwise be the case. Less chord results in smaller
wing-tips and consequently less "spill." This, however, appears
to be a rather inadequate reason for the high aspect ratio
producing the high lift-drift ratio. Other reasons are also
advanced, but they are of such a contentious nature I do not
think it well to go into them here. They are of interest to
designers, but this is written for the practical pilot and rigger.
5. Stagger.—This is the advancement of the top surface relative to
the bottom surface, and is not, of course, applicable to a single
surface, i.e., a monoplane. In the case of a biplane having no
stagger, there will be "interference" and consequent loss of
efficiency unless the gap between the top and bottom surfaces is
equal to not less than about 1-1/2 times the chord. If less than
that, the air engaged by the bottom of the top surface will have a
tendency to be drawn into the rarefied area over the top of the

bottom surface, with the result that the surfaces will not secure
as good a reaction as would otherwise be the case.
It is not practicable to have a gap of much more than a
distance equal to the chord, owing to the drift produced by the
great length of struts and wires such a large gap would
necessitate. By staggering the top surface forward, however, it is
removed from the action of the lower surface and engages
undisturbed air, with the result that the efficiency can in this way
be increased by about 5 per cent. Theoretically the top plane
should be staggered forward for a distance equal to about 30 per
cent. of the chord, the exact distance depending upon the
velocity and angle of incidence; but this is not always possible
to arrange in designing an aeroplane, owing to difficulties of
balance, desired position, and view of pilot, observer, etc.

H.E., Horizontal equivalent.
D., Dihedral angle.

6. Horizontal Equivalent.-The vertical component of the reaction,
i.e., lift, varies as the horizontal equivalent (H.E.) of the surface,
but the drift remains the same. Then it follows that if H.E. grows
less, the ratio of lift to drift must do the same.
A, B, and C are front views of three surfaces.
A has its full H.E., and therefore, from the point of view from
which we are at the moment considering efficiency, it has its
best lift-drift ratio.

B and C both possess the same surface as A, but one is
inclined upwards from its centre and the other is straight but
tilted. For these reasons their H.E.'s are, as illustrated, less than
in the case of A, That means less vertical lift, and, the drift
remaining the same (for there is the same amount of surface as
in A to produce it), the lift-drift ratio falls.
The Margin of Power is the power available above that
necessary to maintain horizontal flight.
The Margin of Lift is the height an aeroplane can gain in a
given time and starting from a given altitude. As an example,
thus: 1,000 feet the first minute, and starting from an altitude of
500 feet above sea-level.
The margin of lift decreases with altitude, owing to the
decrease in the density of the air, which adversely affects the
engine. Provided the engine maintained its impulse with
altitude, then, if we ignore the problem of the propeller, which I
will go into later on, the margin of lift would not disappear.
Moreover, greater velocity for a given power would be secured
at a greater altitude, owing to the decreased density of air to be
overcome. After reading that you may like to light your pipe and
indulge in dreams of the wonderful possibilities which may
become realities if some brilliant genius shows us some day how
to secure a constant power with increasing altitude. I am afraid,
however, that will always remain impossible; but it is probable
that some very interesting steps may be taken in that direction.
The Minimum Angle of Incidence is the smallest angle at
which, for a given power, surface (including detrimental
surface), and weight, horizontal flight can be maintained.
The Maximum Angle of Incidence is the greatest angle at
which, for a given power, surface (including detrimental
surface), and weight, horizontal flight can be maintained.
The Optimum Angle of Incidence is the angle at which the
lift-drift ratio is highest. In modern aeroplanes it is that angle of

incidence possessed by the surface when the axis of the
propeller is horizontal.
The Best Climbing Angle is approximately half-way between
the maximum and the optimum angles.
All present-day aeroplanes are a compromise between Climb
and horizontal Velocity. We will compare the essentials for two
aeroplanes, one designed for maximum climb, and the other for
maximum velocity.
Essentials for Maximum Climb:
1. Low velocity, in order to secure the best lift-drift ratio.
2. Having a low velocity, a large surface will be necessary in order
to engage the necessary mass of air to secure the requisite lift.

3. Since (1) such a climbing machine will move along an upward
sloping path, and (2) will climb with its propeller thrust
horizontal, then a large angle relative to the direction of the
thrust will be necessary in order to secure the requisite angle
relative to the direction of motion.
The propeller thrust should be always horizontal, because the most
efficient flying-machine (having regard to climb or velocity) has, so far,
been found to be an arrangement of an inclined surface driven by a
horizontal thrust—the surface lifting the weight, and the thrust overcoming
the drift. This is, in practice, a far more efficient arrangement than the
helicopter, i.e., the air-screw revolving about a vertical axis and producing a
thrust opposed to gravity. If, when climbing, the propeller thrust is at such
an angle as to tend to haul the aeroplane upwards, then it is, in a measure,

acting as a helicopter, and that means inefficiency. The reason of a
helicopter being inefficient in practice is due to the fact that, owing to
mechanical difficulties, it is impossible to construct within a reasonable
weight an air-screw of the requisite dimensions. That being so, it would be
necessary, in order to absorb the power of the engine, to revolve the
comparatively small-surfaced air screw at an immensely greater velocity
than that of the aeroplane's surface. As already explained, the lift-drift ratio
falls with velocity on account of the increase in passive drift. This applies to
a blade of a propeller or air-screw which is nothing but a revolving surface
set at angle of incidence, and which it is impossible to construct without a
good deal of detrimental surface near the central boss.
4. The velocity being low, then it follows that for that reason also
the angle of incidence should be comparatively large.
5. Camber.—Since such an aeroplane would be of low velocity, and
therefore possess a large angle of incidence, a large camber
would be necessary.
Let us now consider the essentials for an aeroplane of maximum velocity
for its power, and possessing merely enough lift to get off the ground, but
no margin of lift.
1. Comparatively high velocity.
2. A comparatively small surface, because, being of greater velocity
than the maximum climber, a greater mass of air will be engaged
for a given surface and time, and therefore a smaller surface will
be sufficient to secure the requisite lift.
3. A small angle relative to the propeller thrust, since the latter
coincides with the direction of motion.
4. A comparatively small angle of incidence by reason of the high
velocity.
5. A comparatively small camber follows as a result of the small
angle of incidence.

ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN
AEROPLANE DESIGNED AS A COMPROMISE BETWEEN
VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF
LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS
HORIZONTAL.

Minimum Angle.
This gives the greatest velocity during horizontal flight at a low altitude.
Greater velocity would be secured if the surface, angle, and camber were
smaller and designed to just maintain horizontal flight with a horizontal
thrust. Also, in such case, the propeller would not be thrusting downwards,
but along a horizontal line which is obviously a more efficient arrangement
if we regard the aeroplane merely from one point of view, i.e., either with
reference to velocity or climb.
Optimum Angle. (Thrust horizontal).
The velocity is less than at the smaller minimum angle, and, as
aeroplanes are designed to-day, the area and angle of incidence of the
surface is such as to secure a slight ascent at a low altitude. The camber of
the surface is designed for this angle of incidence and velocity. The lift-drift
ratio is best at this angle.
Best Climbing Angle.
The velocity is now still less by reason of the increased angle producing
increase of drift. Less velocity at a given angle produces less lift, but the

increased angle more or less offsets the loss of lift due to the decreased
velocity; and, in addition, the thrust is now hauling the aeroplane upwards.
Maximum Angle.
The greater angle has now produced so much drift as to lessen the
velocity to a point where the combined lifts from the surface and from the
thrust are only just able to maintain horizontal flight. Any greater angle will
result in a still lower lift-drift ratio. The lift will then become less than the
weight and the aeroplane will consequently fall. Such a fall is known as
"stalling" or "pancaking."
NOTE.—The golden rule for beginners: Never exceed the Best
Climbing Angle. Always maintain the flying speed of the aeroplane.
SUMMARY.

Essentials for Maximum Climb. Essentials for Maximum Velocity.

1. Low velocity. 1. High velocity.
2. Large surface. 2. Small surface.
3. Large angle relative to 3. Small angle relative to
propeller thrust. propeller thrust.
4. Large angle relative to 4. Small angle relative to
direction of motion. direction of motion.
5. Large camber. 5. Small camber.

It is mechanically impossible to construct an aeroplane of reasonable
weight of which it would be possible to vary the above opposing essentials.
Therefore, all aeroplanes are designed as a compromise between Climb and
Velocity.
As a rule aeroplanes are designed to have at low altitude a slight margin
of lift when the propeller thrust is horizontal. By this means, when the
altitude is reached where the margin of lift disappears (on account of loss of
engine power), and which is, consequently, the altitude where it is just
possible to maintain horizontal flight, the aeroplane is flying with its thrust

horizontal and with maximum efficiency (as distinct from engine and
propeller efficiency).
The margin of lift at low altitude, and when the thrust is horizontal,
should then be such that the higher altitude at which the margin of lift is lost
is that altitude at which most of the aeroplane's horizontal flight work is
done. That ensures maximum velocity when most required.
Unfortunately, where aeroplanes designed for fighting are concerned, the
altitude where most of the work is done is that at which both maximum
velocity and maximum margin of lift for power are required.
Perhaps some day a brilliant inventor will design an aeroplane of
reasonable weight and drift of which it will be possible for the pilot to vary
at will the above-mentioned opposing essentials. Then we shall get
maximum velocity, or maximum margin of lift, for power as required. Until
then the design of the aeroplane must remain a compromise between
Velocity and Climb.
14 (return)
See Newton's laws in the Glossary at the end of the book.
15 (return)
See "Aerofoil" in the Glossary.

CHAPTER II
STABILITY AND CONTROL

Stability is a condition whereby an object disturbed has a natural
tendency to return to its first and normal position. Example: a weight
suspended by a cord.
Instability is a condition whereby an object disturbed has a natural
tendency to move as far as possible away from its first position, with no
tendency to return. Example: a stick balanced vertically upon your finger.
Neutral Instability is a condition whereby an object disturbed has no
tendency to move farther than displaced by the force of the disturbance, and
no tendency to return to its first position.
In order that an aeroplane may be reasonably controllable, it is necessary
for it to possess some degree of stability longitudinally, laterally, and
directionally.
Longitudinal Stability in an aeroplane is its stability about an axis
transverse to the direction of normal horizontal flight, and without which it
would pitch and toss.
Lateral Stability is its stability about its longitudinal axis, and without
which it would roll sideways.
Directional Stability is its stability about its vertical axis, and without
which it would have no tendency to keep its course.
For such directional stability to exist there must be, in effect,16 more
"keel-surface" behind the vertical axis than there is in front of it. By keel-
surface I mean everything to be seen when looking at an aeroplane from the
side of it—the sides of the body, undercarriage, struts, wires, etc. The same
thing applies to a weathercock. You know what would happen if there was
insufficient keel-surface behind the vertical axis upon which it is pivoted. It

would turn off its proper course, which is opposite to the direction of the
wind. It is very much the same in the case of an aeroplane.

The above illustration represents an aeroplane (directionally stable)
flying along the course B. A gust striking it as indicated acts upon the
greater proportion of keel-surface behind the turning axis and throws it into
the new course. It does not, however, travel along the new course, owing to
its momentum in the direction B. It travels, as long as such momentum
lasts, in a direction which is the resultant of the two forces Thrust and
Momentum. But the centre line of the aeroplane is pointing in the direction
of the new course. Therefore its attitude, relative to the direction of motion,
is more or less sideways, and it consequently receives an air pressure in the
direction C. Such pressure, acting upon the keel-surface, presses the tail
back towards its first position in which the aeroplane is upon its course B.
What I have described is continually going on during flight, but in a well-
designed aeroplane such stabilizing movements are, most of the time, so
slight as to be imperceptible to the pilot.
If an aeroplane was not stabilized in this way, it would not only be
continually trying to leave its course, but it would also possess a dangerous
tendency to "nose away" from the direction of the side gusts. In such case
the gust shown in the above illustration would turn the aeroplane round the
opposite way a very considerable distance; and the right wing, being on the
outside of the turn, would travel with greater velocity than the left wing.
Increased velocity means increased lift; and so, the right wing lifting, the
aeroplane would turn over sideways very quickly.
Longitudinal Stability.—Flat surfaces are longitudinally stable owing to
the fact that with decreasing angles of incidence the centre line of pressure

(C.P.) moves forward.
The C.P. is a line taken across the surface, transverse to the direction of
motion, and about which all the air forces may be said to balance, or
through which they may be said to act.

Imagine A to be a flat surface, attitude vertical, travelling through the air
in the direction of motion M. Its C.P. is then obviously along the exact
centre line of the surface as illustrated. In B, C, and D the surfaces are
shown with angles of incidence decreasing to nothing, and you will note
that the C.P. moves forward with the decreasing angle.17
Now, should some gust or eddy tend to make the surface decrease the
angle, i.e., dive, then the C.P. moves forward and pushes the front of the
surface up. Should the surface tend to assume too large an angle, then the
reverse happens—the C.P. moves back and pushes the rear of the surface
up. Flat surfaces are, then, theoretically stable longitudinally. They are not,
however, used, on account of their poor lift-drift ratio.
As already explained, cambered surfaces are used, and these are
longitudinally unstable at those angles of incidence producing a reasonable
lift-drift ratio, i.e., at angles below about 12°.
A is a cambered surface, attitude approximately vertical, moving through
the air in the direction M. Obviously the C.P. coincides with the transverse
centre line of the surface.
With decreasing angles, down to angles of about 30°, the C.P. moves
forward as in the case of flat surfaces (see B); but angles above 30° do not
interest us, since they produce a very low ratio of lift to drift.

Below angles of about 30° (see C) the dipping front part of the surface
assumes a negative angle of incidence resulting in the downward air
pressure D, and the more the angle of incidence is decreased, the greater
such negative angle and its resultant pressure D. Since the C.P. is the
resultant of all the air forces, its position is naturally affected by D, which
causes it to move backwards. Now, should some gust or eddy tend to make
the surface decrease its angle of incidence, i.e., dive, then the C.P. moves
backwards, and, pushing up the rear of the surface, causes it to dive the
more. Should the surface tend to assume too large an angle, then the reverse
happens; the pressure D decreases, with the result that C.P. moves forward
and pushes up the front of the surface, thus increasing the angle still further,
the final result being a "tail-slide."
It is therefore necessary to find a means of stabilizing the naturally
unstable cambered surface. This is usually secured by means of a stabilizing
surface fixed some distance in the rear of the main surface, and it is a
necessary condition that the neutral lift lines of the two surfaces, when
projected to meet each other, make a dihedral angle. In other words, the rear
stabilizing surface must have a lesser angle of incidence than the main
surface—certainly not more than one-third of that of the main surface. This
is known as the longitudinal dihedral.

I may add that the tail-plane is sometimes mounted upon the aeroplane at
the same angle as the main surface, but, in such cases, it attacks air which

has received a downward deflection from the main surface, thus:

The angle at which the tail surface attacks the air (the angle of incidence)
is therefore less than the angle of incidence of the main surface.
I will now, by means of the following illustration, try to explain how the
longitudinal dihedral secures stability:

First, imagine the aeroplane travelling in the direction of motion, which
coincides with the direction of thrust T. The weight is, of course, balanced
about a C.P., the resultant of the C.P. of the main surface and the C.P. of the
stabilizing surface. For the sake of illustration, the stabilizing surface has
been given an angle of incidence, and therefore has a lift and C.P. In
practice the stabilizer is often set at no angle of incidence. In such case the
proposition remains the same, but it is, perhaps, a little easier to illustrate it
as above.
Now, we will suppose that a gust or eddy throws the machine into the
lower position. It no longer travels in the direction of T, since the
momentum in the old direction pulls it off that course. M is now the
resultant of the Thrust and the Momentum, and you will note that this

results in a decrease in the angle our old friend the neutral lift line makes
with M, i.e., a decrease in the angle of incidence and therefore a decrease in
lift.
We will suppose that this decrease is 2°. Such decrease applies to both
main surface and stabilizer, since both are fixed rigidly to the aeroplane.
The main surface, which had 12° angle, has now only 10°, i.e., a loss of
one-sixth.
The stabilizer, which had 4° angle, has now only 2°, i.e., a loss of one-
half.
The latter has therefore lost a greater proportion of its angle of incidence,
and consequently its lift, than has the main surface. It must then fall relative
to the main surface. The tail falling, the aeroplane then assumes its first
position, though at a slightly less altitude.
Should a gust throw the nose of the aeroplane up, then the reverse
happens. Both main surface and stabilizer increase their angles of incidence
in the same amount, but the angle, and therefore the lift, of the stabilizer
increases in greater proportion than does the angle and lift of the main
surface, with the result that it lifts the tail. The aeroplane then assumes its
first position, though at a slightly greater altitude.
Do not fall into the widespread error that the angle of incidence varies as
the angle of the aeroplane to the horizontal. It varies with such angle, but
not as anything approaching it. Remember that the stabilizing effect of the
longitudinal dihedral lasts only as long as there is momentum in the
direction of the first course.
These stabilizing movements are taking place all the time, even though
imperceptible to the pilot.
Aeroplanes have, in the past, been built with a stabilizing surface in front
of the main surface instead of at the rear of it. In such design the main
surface (which is then the tail surface as well as the principal lifting surface)
must be set at a less angle than the forward stabilizing surface, in order to
secure a longitudinal dihedral. The defect of such design lies in the fact that
the main surface must have a certain angle to lift the weight—say 5°. Then,

in order to secure a sufficiency of longitudinal stability, it is necessary to set
the forward stabilizer at about 15°. Such a large angle of incidence results
in a very poor lift-drift ratio (and consequently great loss of efficiency),
except at very low velocities compared with the speed of modern
aeroplanes. At the time such aeroplanes were built velocities were
comparatively low, and this defect was, for that reason, not sufficiently
appreciated. In the end it killed the "canard" or "tail-first" design.
Aeroplanes of the Dunne and similar types possess no stabilizing surface
distinct from the main surface, but they have a longitudinal dihedral which
renders them stable.
The main surface towards the wing-tips is given a decreasing angle of
incidence and corresponding camber. The wing-tips then act as longitudinal
stabilizers.

This design of aeroplane, while very interesting, has not proved very
practicable, owing to the following disadvantages: (1) The plan design is
not, from a mechanical point of view, so sound as that of the ordinary
aeroplane surface, which is, in plan, a parallelogram. It is, then, necessary
to make the strength of construction greater than would otherwise be the
case. That means extra weight. (2) The plan of the surface area is such that
the aspect ratio is not so high as if the surface was arranged with its leading
edges at right angles to the direction of motion. The lower the aspect ratio,
then, the less the lift. This design, then, produces less lift for weight of
surface than would the same surface if arranged as a parallelogram. (3) In
order to secure the longitudinal dihedral, the angle of incidence has to be
very much decreased towards the wing-tips. Then, in order that the lift-drift
ratio may be preserved, there must be a corresponding decrease in the
camber. That calls for surface ribs of varying cambers, and results in an
expensive and lengthy job for the builder. (4) In order to secure directional

stability, the surface is, in the centre, arranged to dip down in the form of a
V, pointing towards the direction of motion. Should the aeroplane turn off
its course, then its momentum in the direction of its first course causes it to
move in a direction the resultant of the thrust and the momentum. It then
moves in a more or less sideways attitude, which results in an air pressure
upon one side of the V, and which tends to turn the aeroplane back to its
first course. This arrangement of the surface results in a bad drift. Vertical
surfaces at the wing-tips may also be set at an angle producing the same
stabilizing effect, but they also increase the drift.
The gyroscopic action of a rotary engine will affect the longitudinal
stability when an aeroplane is turned to right or left. In the case of a Gnome
engine, fitted to a "pusher" aeroplane, such gyroscopic action will tend to
depress the nose of the aeroplane when it is turned to the left, and to elevate
it when it is turned to the right. When fitted to a "tractor" aeroplane, the
engine is reversed so that a reverse condition results. In modern aeroplanes
this tendency is not sufficiently important to bother about, except in the
matter of spiral descents (see section headed "Spinning"). In the old days of
crudely designed and under-powered "pusher" aeroplanes this gyroscopic
action was very marked, and led the majority of pilots to dislike turning an
aeroplane to the right, since, in doing so, there was some danger of
"stalling."
Lateral Stability is far more difficult for the designer to secure than is
longitudinal or directional stability. Some degree of lateral stability may be
secured by means of the "lateral dihedral," i.e., the upward inclination of the
surface towards its wing-tips thus:

Imagine the top V, illustrated opposite, to be the front view of a surface
flying towards you. The horizontal equivalent (H.E.) of the left wing is the
same as that of the right wing. Therefore, the lift of one wing is equal to the
lift of the other, and the weight, being situated always in the centre, is
balanced.
If some movement of the air causes the surface to tilt sideways, as in the
lower illustration, then you will note that the H.E. of the left wing increases,
and the H.E. of the right wing decreases. The left wing then, having the
greatest lift, rises; and the surface assumes its first and normal position.
Unfortunately, however, the righting effect is not proportional to the
difference between the right and left H.E.'s.

R, Direction of reaction of wing indicated.
R R, Resultant direction of reaction of both wings.
M, Horizontal (sideway) component of reaction.
L, Vertical component of reaction (lift).

In the case of A, the resultant direction of the reaction of both wings is
opposed to the direction of gravity or weight. The two forces R R and
gravity are then evenly balanced, and the surface is in a state of equilibrium.
In the case of B, you will note that the R R is not directly opposed to
gravity. This results in the appearance of M, and so the resultant direction of
motion of the aeroplane is no longer directly forward, but is along a line the
resultant of the Thrust and M. In other words, it is, while flying forward, at
the same time moving sideways in the direction M.

In moving sideways, the keel-surface receives, of course, a pressure from
the air equal and opposite to M. Since such surface is greatest in effect
towards the tail, then the latter must be pushed sideways. That causes the
aeroplane to turn; and, the highest wing being on the outside of the turn, it
has a greater velocity than the lower wing. That produces greater lift, and
tends to tilt the aeroplane over still more. Such tilting tendency is, however,
opposed by the difference in the H.E.'s of the two wings.
It then follows that, for the lateral dihedral angle to be effective, such
angle must be large enough to produce, when the aeroplane tilts, a
difference in the H.E.'s of the two wings, which difference must be
sufficient to not only oppose the tilting tendency due to the aeroplane
turning, but sufficient to also force the aeroplane back to its original
position of equilibrium.
It is now, I hope, clear to the reader that the lateral dihedral is not quite so
effective as would appear at first sight. Some designers, indeed, prefer not
to use it, since its effect is not very great, and since it must be paid for in
loss of H.E. and consequently loss of lift, thus decreasing the lift-drift ratio,
i.e., the efficiency. Also, it is sometimes advanced that the lateral dihedral
increases the "spill" of air from the wing-tips and that this adversely affects
the lift-drift ratio.
The disposition of the keel-surface affects the lateral stability. It should
be, in effect, equally divided by the longitudinal turning axis of the
aeroplane. If there is an excess of keel-surface above or below such axis,
then a side gust striking it will tend to turn the aeroplane over sideways.
The position of the centre of gravity affects lateral stability. If too low, it
produces a pendulum effect and causes the aeroplane to roll sideways.
If too high, it acts as a stick balanced vertically would act. If disturbed, it
tends to travel to a position as far as possible from its original position. It
would then tend, when moved, to turn the aeroplane over sideways and into
an upside-down position.
From the point of view of lateral stability, the best position for the centre
of gravity is one a little below the centre of drift. This produces a little
lateral stability without any marked pendulum effect.

Propeller torque affects lateral stability. An aeroplane tends to turn over
sideways in the opposite direction to which the propeller revolves.

This tendency is offset by increasing the angle of incidence (and
consequently the lift) of the side tending to fall; and it is always advisable,
if practical considerations allow it, to also decrease the angle upon the other
side. In that way it is not necessary to depart so far from the normal angle of
incidence at which the lift-drift ratio is highest.
Wash-in is the term applied to the increased angle.
Wash-out is the term applied to the decreased angle.
Both lateral and directional stability may be improved by washing out the
angle of incidence on both sides of the surface, thus:

The decreased angle decreases the drift and therefore the effect of gusts
upon the wing-tips, which is just where they have the most effect upon the
aeroplane, owing to the distance from the turning axis.
The wash-out also renders the ailerons (lateral controlling services) more
effective, as, in order to operate them, it is not then necessary to give them
such a large angle of incidence as would otherwise be required.

The less the angle of incidence of the ailerons, the better their lift-drift
ratio, i.e., their efficiency. You will note that, while the aileron attached to
the surface with washed-out angle is operated to the same extent as the
aileron illustrated above it, its angle of incidence is considerably less. Its
efficiency is therefore greater.
The advantages of the wash-in must, of course, be paid for in some loss
of lift, as the lift decreases with the decreased angle.
In order to secure all the above described advantages, a combination is
sometimes effected, thus:

Banking.—An aeroplane turned off its course to right or left does not at
once proceed along its new course. Its momentum in the direction of its first
course causes it to travel along a line the resultant of such momentum and
the thrust. In other words, it more or less skids sideways and away from the
centre of the turn. Its lifting surfaces do not then meet the air in their correct
attitude, and the lift may fall to such an extent as to become less than the
weight, in which case the aeroplane must fall. This bad effect is minimized
by "banking," i.e., tilting the aeroplane sideways. The bottom of the lifting

surface is in that way opposed to the air through which it is moving in the
direction of the momentum and receives an opposite air pressure. The
rarefied area over the top of the surface is rendered still more rare, and this,
of course, assists the air pressure in opposing the momentum.
The velocity of the "skid," or sideways movement, is then only such as is
necessary to secure an air pressure equal and opposite to the centrifugal
force of the turn.
The sharper the turn, the greater the effect of the centrifugal force, and
therefore the steeper should be the "bank." Experientia docet.
The position of the centre of gravity affects banking. A low C.G. will tend
to swing outward from the centre of the turn, and will cause the aeroplane
to bank—perhaps too much, in which case the pilot must remedy matters by
operating the ailerons.
A high C.G. also tends to swing outward from the centre of the turn. It
will tend to make the aeroplane bank the wrong way, and such effect must
be remedied by means of the ailerons.
The pleasantest machine from a banking point of view is one in which
the C.G. is a little below the centre of drift. It tends to bank the aeroplane
the right way for the turn, and the pilot can, if necessary, perfect the bank by
means of the ailerons.
The disposition of the keel-surface affects banking. It should be, in effect,
evenly divided by the longitudinal axis. An excess of keel-surface above the
longitudinal axis will, when banking, receive an air pressure causing the
aeroplane to bank, perhaps too much. An excess of keel-surface below the
axis has the reverse effect.
Side-Slipping.—This usually occurs as a result of over-banking. It is
always the result of the aeroplane tilting sideways and thus decreasing the
horizontal equivalent, and therefore the lift, of the surface. An excessive
"bank," or sideways tilt, results in the H.E., and therefore the lift, becoming
less than the weight, when, of course, the aeroplane must fall, i.e., side-slip.

When making a very sharp turn it is necessary to bank very steeply
indeed. If, at the same time, the longitudinal axis of the aeroplane remains
approximately horizontal, then there must be a fall, and the direction of
motion will be the resultant of the thrust and the fall as illustrated above in
sketch A. The lifting surfaces and the controlling surfaces are not then
meeting the air in the correct attitude, with the result that, in addition to
falling, the aeroplane will probably become quite unmanageable.
The pilot, however, prevents such a state of affairs from happening by
"nosing-down," i.e., by operating the rudder to turn the nose of the
aeroplane downward and towards the direction of motion as illustrated in
sketch B. This results in the higher wing, which is on the outside of the turn,
travelling with greater velocity, and therefore securing a greater reaction
than the lower wing, thus tending to tilt the aeroplane over still more. The
aeroplane is now almost upside-down, but its attitude relative to the
direction of motion is correct and the controlling surfaces are all of them
working efficiently. The recovery of a normal attitude relative to the Earth
is then made as illustrated in sketch C.
The pilot must then learn to know just the angle of bank at which the
margin of lift is lost, and, if a sharp turn necessitates banking beyond that
angle, he must "nose-down."
In this matter of banking and nosing-down, and, indeed, regarding
stability and control generally, the golden rule for all but very experienced
pilots should be: Keep the aeroplane in such an attitude that the air

pressure is always directly in the pilot's face. The aeroplane is then always
engaging the air as designed to do so, and both lifting and controlling
surfaces are acting efficiently. The only exception to this rule is a vertical
dive, and I think that is obviously not an attitude for any but very
experienced pilots to hanker after.
Spinning.—This is the worst of all predicaments the pilot can find
himself in. Fortunately it rarely happens.
It is due to the combination of (1) a very steep spiral descent of small
radius, and (2) insufficiency of keel-surface behind the vertical axis, or the
jamming of the rudder and/or elevator into a position by which the
aeroplane is forced into an increasingly steep and small spiral.
Owing to the small radius of such a spiral, the mass of the aeroplane may
gain a rotary momentum greater, in effect, than the air pressure of the keel-
surface or controlling surfaces opposed to it; and, when once such a
condition occurs, it is difficult to see what can be done by the pilot to
remedy it. The sensible pilot will not go beyond reasonable limits of
steepness and radius when executing spiral descents.

In this connection every pilot of an aeroplane fitted with a rotary engine
should bear in mind the gyroscopic effect of such engine. In the case of
such an engine fitted to a "pusher" aeroplane, its effect when a left-hand
turn is made is to depress the nose of the machine. If fitted to a "tractor" it is
reversed, so the effect is to depress the nose if a right-hand turn is made.
The sharper the turn, the greater such effect—an effect which may render
the aeroplane unmanageable if the spiral is one of very small radius and the
engine is revolving with sufficient speed to produce a material gyroscopic
effect. Such gyroscopic effect should, however, slightly assist the pilot to
navigate a small spiral if he will remember to (1) make right-hand spirals in
the case of a "pusher," (2) make left-hand spirals in the case of a "tractor."
The effect will then be to keep the nose up and prevent a nose-dive. I say
"slightly" assist because the engine is, of course, throttled down for a spiral
descent, and its lesser revolutions will produce a lesser gyroscopic effect.

On the other hand, it might be argued that if the aeroplane gets into a
"spin," anything tending to depress the nose of the machine is of value,
since it is often claimed that the best way to get out of a spin is to put the
machine into a nose-dive—the great velocity of the dive rendering the
controls more efficient and better enabling the pilot to regain control. It is,
however, a very contentious point, and few are able to express opinions
based on practice, since pilots indulging in nose-dive spins are either not
heard of again or have usually but a hazy recollection of exactly what
happened to them.
Gliding Descent Without Propeller Thrust.—All aeroplanes are, or
should be, designed to assume their correct gliding angle when the power
and thrust is cut off. This relieves the pilot of work, worry, and danger
should he find himself in a fog or cloud. The pilot, although he may not
realize it, maintains the correct attitude of the aeroplane by observing its
position relative to the horizon. Flying into a fog or cloud the horizon is lost
to view, and he must then rely upon his instruments—(1) the compass for
direction; (2) an inclinometer (arched spirit-level) mounted transversely to
the longitudinal axis, for lateral stability; and (3) an inclinometer mounted
parallel to the longitudinal axis, or the airspeed indicator, which will
indicate a nose-down position by increase in air speed, and a tail-down
position by decrease in air speed.
The pilot is then under the necessity of watching three instruments and
manipulating his three controls to keep the instruments indicating
longitudinal, lateral, and directional stability. That is a feat beyond the
capacity of the ordinary man. If, however, by the simple movement of
throttling down the power and thrust, he can be relieved of looking after the
longitudinal stability, he then has only two instruments to watch. That is no
small job in itself, but it is, at any rate, fairly practicable.

Aeroplanes are, then, designed, or should be, so that the centre of gravity
is slightly forward of centre of lift. The aeroplane is then, as a glider, nose-
heavy—and the distance the C.G. is placed in advance of the C.L. should be
such as to ensure a gliding angle producing a velocity the same as the
normal flying speed (for which the strength of construction has been
designed).
In order that this nose-heavy tendency should not exist when the thrust is
working and descent not required, the centre of thrust is placed a little
below the centre of drift or resistance, and thus tends to pull up the nose of
the aeroplane.
The distance the centre of thrust is placed below the centre of drift should
be such as to produce a force equal and opposite to that due to the C.G.
being forward of the C.L. (see illustration above).
Looping and Upside-Down Flying.—If a loop is desired, it is best to
throttle the engine down at point A. The C.G. being forward of the C.P.,
then causes the aeroplane to nose down, and assists the pilot in making a
reasonably small loop along the course C and in securing a quick recovery.
If the engine is not throttled down, then the aeroplane may be expected to
follow the course D, which results in a longer nose dive than in the case of
the course C.

A steady, gentle movement of the elevator is necessary. A jerky
movement may change the direction of motion so suddenly as to produce
dangerous air stresses upon the surfaces, in which case there is a possibility
of collapse.
If an upside-down flight is desired, the engine may, or may not, be
throttled down at point A. If not throttled down, then the elevator must be
operated to secure a course approximately in the direction B. If it is
throttled down, then the course must be one of a steeper angle than B, or
there will be danger of stalling.
16 (return)
"In effect" because, although there may be actually the greatest proportion of
keel-surface in front of the vertical axis, such surface may be much nearer to the
axis than is the keel-surface towards the tail. The latter may then be actually less
than the surface in front, but, being farther from the axis, it has a greater
leverage, and consequently is greater in effect than the surface in front.
17 (return)
The reason the C.P. of an inclined surface is forward of the centre of the surface
is because the front of the surface does most of the work, as explained on p. 62.

CHAPTER III
RIGGING

In order to rig an aeroplane intelligently, and to maintain it in an efficient
and safe condition, it is necessary to possess a knowledge of the stresses it
is called upon to endure, and the strains likely to appear.
Stress is the load or burden a body is called upon to bear. It is usually
expressed by the result found by dividing the load by the number of
superficial square inches contained in the cross-sectional area of the body.

Thus, if, for instance, the object illustrated above contains 4 square
inches of cross-sectional area, and the total load it is called upon to endure
is 10 tons, the stress would be expressed as 2-1/2 tons.
Strain is the deformation produced by stress.
The Factor of Safety is usually expressed by the result found by
dividing the stress at which it is known the body will collapse by the
maximum stress it will be called upon to endure. For instance, if a control
wire be called upon to endure a maximum stress of 2 cwts., and the known
stress at which it will collapse is 10 cwts., the factor of safety is then 5.
Compression.—The simple stress of compression tends to produce a
crushing strain. Example: the interplane and fuselage struts.
Tension.—The simple stress of tension tends to produce the strain of
elongation. Example: all the wires.

Bending.—The compound stress of bending is a combination of
compression and tension.

The above sketch illustrates a straight piece of wood of which the top,
centre, and bottom lines are of equal length. We will now imagine it bent to
form a circle, thus:

The centre line is still the same length as before being bent; but the top
line, being farther from the centre of the circle, is now longer than the
centre line. That can be due only to the strain of elongation produced by the
stress of tension. The wood between the centre line and the top line is then
in tension; and the farther from the centre, the greater the strain, and
consequently the greater the tension.
The bottom line, being nearest to the centre of the circle, is now shorter
than the centre line. That can be due only to the strain of crushing produced
by the stress of compression. The wood between the centre and bottom lines
is then in compression; and the nearer the centre of the circle, the greater
the strain, and consequently the greater the compression.
It then follows that there is neither tension nor compression, i.e., no
stress, at the centre line, and that the wood immediately surrounding it is
under considerably less stress than the wood farther away. This being so,
the wood in the centre may be hollowed out without unduly weakening
struts and spars. In this way 25 to 33 per cent. is saved in the weight of
wood in an aeroplane.

The strength of wood is in its fibres, which should, as far as possible, run
without break from one end of a strut or spar to the other end. A point to
remember is that the outside fibres, being farthest removed from the centre
line, are doing by far the greatest work.
Shear Stress is such that, when material collapses under it, one part
slides over the other. Example: all the locking pins.

Some of the bolts are also in shear or "sideways" stress, owing to lugs
under their heads and from which wires are taken. Such a wire, exerting a
sideways pull upon a bolt, tries to break it in such a way as to make one
piece of the bolt slide over the other piece.
Torsion.—This is a twisting stress compounded of compression, tension,
and shear stresses. Example: the propeller shaft.
Nature of Wood under Stress.—Wood, for its weight, takes the stress of
compression far better than any other stress. For instance: a walking-stick
of less than 1 lb. in weight will, if kept perfectly straight, probably stand up
to a compression stress of a ton or more before crushing; whereas, if the
same stick is put under a bending stress, it will probably collapse to a stress
of not more than about 50 lb. That is a very great difference, and, since
weight is of the greatest importance, the design of an aeroplane is always
such as to, as far as possible, keep the various wooden parts of its
construction in direct compression. Weight being of such vital importance,
and designers all trying to outdo each other in saving weight, it follows that
the factor of safety is rather low in an aeroplane. The parts in direct
compression will, however, take the stresses safely provided the following
conditions are carefully observed.
Conditions to be Observed:
1. All the spars and struts must be perfectly straight.

The above sketch illustrates a section through an interplane
strut. If the strut is to be kept straight, i.e., prevented from
bending, then the stress of compression must be equally
disposed about the centre of strength. If it is not straight, then
there will be more compression on one side of the centre of
strength than on the other side. That is a step towards getting
compression on one side and tension on the other side, in which
case it may be forced to take a bending stress for which it is not
designed. Even if it does not collapse it will, in effect, become
shorter, and thus throw out of adjustment the gap and all the
wires attached to the top and bottom of the strut, with the result
that the flight efficiency of the aeroplane will be spoiled.

The only exception to the above condition is what is known
as the Arch. For instance, in the case of the Maurice Farman, the
spars of the centre-section plane, which have to take the weight
of the nacelle, are arched upwards. If this was not done, it is
possible that rough landings might result in the weight causing
the spars to become slightly distorted downwards. That would
produce a dangerous bending stress, but, as long as the wood is
arched, or, at any rate, kept from bending downwards, it will
remain in direct compression and no danger can result.

2. Struts and spars must be symmetrical. By that I mean that the
cross-sectional dimensions must be correct, as otherwise there
will be bulging places on the outside, with the result that the
stress will not be evenly disposed about the centre of strength,
and a bending stress may be produced.
3. Struts, spars, etc., must be undamaged. Remember that, from
what I have already explained about bending stresses, the
outside fibres of the wood are doing by far the most work. If
these get bruised or scored, then the strut or spar suffers in
strength much more than one might think at first sight; and, if it
ever gets a tendency to bend, it is likely to collapse at that point.
4. The wood must have a good, clear grain with no cross-grain,
knots, or shakes. Such blemishes produce weak places and, if a
tendency to bend appears, then it may collapse at such a point.

5. The struts, spars, etc., must be properly bedded into their sockets
or fittings. To begin with, they must be of good pushing or
gentle tapping fit. They must never be driven in with a heavy
hammer. Then again, a strut must bed well down all over its
cross-sectional area as illustrated above; otherwise the stress of
compression will not be evenly disposed about the centre of
strength, and that may produce a bending stress. The bottom of
the strut or spar should be covered with some sort of paint,
bedded into the socket or fitting, and then withdrawn to see if
the paint has stuck all over the bed.
6. The atmosphere is sometimes much damper than at other times,
and this causes wood to expand and contract appreciably. This

would not matter but for the fact that it does not expand and
contract uniformly, but becomes unsymmetrical, i.e., distorted. I
have already explained the danger of that in condition 2. This
should be minimized by well varnishing the wood to keep the
moisture out of it.
Function of Interplane Struts.—These struts have to keep the lifting
surfaces or "planes" apart, but this is only part of their work. They must
keep the planes apart, so that the latter are in their correct attitude. That is
only so when the spars of the bottom plane are parallel with those of the top
plane. Also, the chord of the top plane must be parallel with the chord of the
bottom plane. If that is not so, then one plane will not have the same angle
of incidence as the other one. At first sight one might think that all that is
necessary is to cut all the struts to be the same length, but that is not the
case.

Sometimes, as illustrated above, the rear spar is not so thick as the main
spar, and it is then necessary to make up for that difference by making the
rear struts correspondingly longer. If that is not done, then the top and
bottom chords will not be parallel, and the top and bottom planes will have
different angles of incidence. Also, the sockets or fittings, or even the spars
upon which they are placed, sometimes vary in thickness owing to faulty
manufacture. This must be offset by altering the length of the struts. The
best way to proceed is to measure the distance between the top and bottom
spars by the side of each strut, and if that distance, or "gap" as it is called, is
not as stated in the aeroplane's specifications, then make it correct by
changing the length of the strut. This applies to both front and rear
interplane struts. When measuring the gap, always be careful to measure
from the centre of the spar, as it may be set at an angle, and the rear of it
may be considerably lower than its front.

Boring Holes in Wood.—It should be a strict rule that no spar be used
which has an unnecessary hole in it. Before boring a hole, its position
should be confirmed by whoever is in charge of the workshop. A bolt-hole
should be of a size to enable the bolt to be pushed in, or, at any rate, not
more than gently tapped in. Bolts should not be hammered in, as that may
split the spar. On the other hand, a bolt should not be slack in its hole, as, in
such a case, it may work sideways and split the spar, not to speak of
throwing out of adjustment the wires leading from the lug or socket under
the bolt-head.
Washers.—Under the bolt-head, and also under the nut, a washer must
be placed—a very large washer compared with the size which would be
used in all-metal construction. This is to disperse the stress over a large
area; otherwise the washer may be pulled into the wood and weaken it,
besides possibly throwing out of adjustment the wires attached to the bolt or
the fitting it is holding to the spar.
Locking.—Now as regards locking the bolts. If split pins are used, be
sure to see that they are used in such a way that the nut cannot possibly
unscrew at all. The split pin should be passed through the bolt as near as
possible to the nut. It should not be passed through both nut and bolt.
If it is locked by burring over the edge of the bolt, do not use a heavy
hammer and try to spread the whole head of the bolt. That might damage
the woodwork inside the fabric-covered surface. Use a small, light hammer,
and gently tap round the edge of the bolt until it is burred over.
Turnbuckles.—A turnbuckle is composed of a central barrel into each
end of which is screwed an eye-bolt. Wires are taken from the eyes of the
eye-bolt, and so, by turning the barrel, they can be adjusted to their proper
tension. Eye-bolts must be a good fit in the barrel; that is to say, not slack
and not very tight. Theoretically it is not necessary to screw the eye-bolt
into the barrel for a distance greater than the diameter of the bolt, but, in
practice, it is better to screw it in for a considerably greater distance than
that if a reasonable degree of safety is to be secured.
Now about turning the barrel to secure the right adjustment. The barrel
looks solid, but, as a matter of fact, it is hollow and much more frail than it
appears. For that reason it should not be turned by seizing it with pliers, as

that may distort it and spoil the bore within it. The best method is to pass a
piece of wire through the hole in its centre, and to use that as a lever. When
the correct adjustment has been secured, the turnbuckle must be locked to
prevent it from unscrewing. It is quite possible to lock it in such a way as to
allow it to unscrew a quarter or a half turn, and that would throw the wires
out of the very fine adjustment necessary. The proper way is to use the
locking wire so that its direction is such as to oppose the tendency of the
barrel to unscrew, thus:

Wires.—The following points should be carefully observed where wire is
concerned:
1. Quality.—It must not be too hard or too soft. An easy practical way of
learning to know the approximate quality of wire is as follows:
Take three pieces, all of the same gauge, and each about a foot in length.
One piece should be too soft, another too hard, and the third piece of the
right quality. Fix them in a vice, about an inch apart and in a vertical
position, and with the light from a window shining upon them. Burnish
them if necessary, and you will see a band of light reflected from each wire.
Now bend the wires over as far as possible and away from the light.
Where the soft wire is concerned, it will squash out at the bend, and this
will be indicated by the band of light, which will broaden at that point. In
the case of the wire which is too hard, the band of light will broaden very
little at the turn, but, if you look carefully, you will see some little
roughness of surface. In the case of the wire of the right quality, the band of
light may broaden a very little at the turn, but there will be no roughness of
surface.
By making this experiment two or three times one can soon learn to
know really bad wire from good, and also learn to know the strength of
hand necessary to bend the right quality.

2. It must not be damaged. That is to say, it must be unkinked, rustless,
and unscored.
3. Now as regards keeping wire in good condition. Where outside wires
are concerned, they should be kept well greased or oiled, especially where
bent over at the ends. Internal bracing wires cannot be reached for the
purpose of regreasing them, as they are inside fabric-covered surfaces. They
should be prevented from rusting by being painted with an anti-rust
mixture. Great care should be taken to see that the wire is perfectly clean
and dry before being painted. A greasy finger-mark is sufficient to stop the
paint from sticking to the wire. In such a case there will be a little space
between the paint and the wire. Air may enter there and cause the wire to
rust.
4. Tension of Wires.—The tension to which the wires are adjusted is of
the greatest importance. All the wires should be of the same tension when
the aeroplane is supported in such a way as to throw no stress upon them. If
some wires are in greater tension than others, the aeroplane will quickly
become distorted and lose its efficiency.
In order to secure the same tension of all wires, the aeroplane, when
being rigged, should be supported by packing underneath the lower surfaces
as well as by packing underneath the fuselage or nacelle. In this way the
anti-lift wires are relieved of the weight, and there is no stress upon any of
the wires.
As a general rule the wires of an aeroplane are tensioned too much. The
tension should be sufficient to keep the framework rigid. Anything more
than that lowers the factor of safety, throws various parts of the framework
into undue compression, pulls the fittings into the wood, and will, in the
end, distort the whole framework of the aeroplane.
Only experience will teach the rigger what tension to employ. Much may
be done by learning the construction of the various types of aeroplanes, the
work the various parts do, and in cultivating a touch for tensioning wires by
constantly handling them.
5. Wires with no Opposition Wires.—In some few cases wires will be
found which have no opposition wires pulling in the opposite direction. For

instance, an auxiliary lift wire may run from the bottom of a strut to a spar
in the top plane at a point between struts. In such a case great care should be
taken not to tighten the wire beyond barely taking up the slack.

Such a wire must be a little slack, or, as illustrated above, it will distort
the framework. That, in the example given, will spoil the camber
(curvature) of the surface, and result in changing both the lift and the drift at
that part of the surface. Such a condition will cause the aeroplane to lose its
directional stability and also to fly one wing down.
I cannot impress this matter of tension upon the reader too strongly. It is
of the utmost importance. When this, and also accuracy in securing the
various adjustments, has been learned, one is on the way to becoming a
good rigger.
6. Wire Loops.—Wire is often bent over at its end in the form of a loop,
in order to connect with a turnbuckle or fitting. These loops, even when
made as perfectly as possible, have a tendency to elongate, thus spoiling the
adjustment of the wires. Great care should be taken to minimize this as
much as possible. The rules to be observed are as follows:

(a) The size of the loop should be as small as possible within
reason. By that I mean it should not be so small as to create the

possibility of the wire breaking.
(b) The shape of the loop should be symmetrical.
(c) It should have well-defined shoulders in order to prevent the
ferrule from slipping up. At the same time, a shoulder should not
have an angular place.
(d) When the loop is finished it should be undamaged, and it should
not be, as is often the case, badly scored.
7. Stranded Wire Cable.—No splice should be served with twine until it
has been inspected by whoever is in charge of the workshop. The serving
may cover bad work.
Should a strand become broken, then the cable should be replaced at once
by another one.
Control cables have a way of wearing out and fraying wherever they pass
round pulleys. Every time an aeroplane comes down from flight the rigger
should carefully examine the cables, especially where they pass round
pulleys. If he finds a strand broken, he should replace the cable.
The ailerons' balance cable on the top of the top plane is often forgotten,
since it is necessary to fetch a high pair of steps in order to examine it.
Don't slack this, or some gusty day the pilot may unexpectedly find himself
minus the aileron control.
Controlling Surfaces.—The greatest care should be exercised in rigging
the aileron, rudder, and elevator properly, for the pilot entirely depends
upon them in managing the aeroplane.

The ailerons and elevator should be rigged so that, when the aeroplane is
in flight, they are in a fair true line with the surface in front and to which
they are hinged.

If the surface to which they are hinged is not a lifting surface, then they
should be rigged to be in a fair true line with it as illustrated above.
If the controlling surface is, as illustrated, hinged to the back of a lifting
surface, then it should be rigged a little below the position it would occupy
if in a fair true line with the surface in front. This is because, in such a case,
it is set at an angle of incidence. This angle will, during flight, cause it to
lift a little above the position in which it has been rigged. It is able to lift
owing to a certain amount of slack in the control wire holding it—and one
cannot adjust the control wire to have no slack, because that would cause it
to bind against the pulleys and make the operation of it too hard for the
pilot. It is therefore necessary to rig it a little below the position it would
occupy if it was rigged in a fair true line with the surface in front.
Remember that this only applies when it is hinged to a lifting surface. The
greater the angle of incidence (and therefore the lift) of the surface in front,
then the more the controlling surface will have to be rigged down.
As a general rule it is safe to rig it down so that its trailing edge is 1/2 to
3/4 inch below the position it would occupy if in a fair line with the surface
in front; or about 1/2 inch down for every 18 inches of chord of the
controlling surface.
When making these adjustments the pilot's control levers should be in
their neutral positions. It is not sufficient to lash them. They should be
rigidly blocked into position with wood packing.
The surfaces must not be distorted in any way. If they are held true by
bracing wires, then such wires must be carefully adjusted. If they are
distorted and there are no bracing wires with which to true them, then some
of the internal framework will probably have to be replaced.
The controlling surfaces should never be adjusted with a view to altering
the stability of the aeroplane. Nothing can be accomplished in that way. The
only result will be to spoil the control of the aeroplane.

Fabric-Covered Surfaces.—First of all make sure that there is no
distortion of spars or ribs, and that they are perfectly sound. Then adjust the
internal bracing wires so that the ribs are parallel to the direction of flight.
The ribs usually cause the fabric to make a ridge where they occur, and, if
such ridge is not parallel to the direction of flight, it will produce excessive
drift. As a rule the ribs are at right angles to both main and rear spars.
The tension of the internal bracing wires should be just sufficient to give
rigidity to the framework. They should not be tensioned above that unless
the wires are, at their ends, bent to form loops. In that case a little extra
tension may be given to offset the probable elongation of the loops.
The turnbuckles must now be generously greased, and served round with
adhesive tape. The wires must be rendered perfectly dry and clean, and then
painted with an anti-rust mixture. The woodwork must be well varnished.
If it is necessary to bore holes in the spars for the purpose of receiving,
for instance, socket bolts, then their places should be marked before being
bored and their positions confirmed by whoever is in charge of the
workshop. All is now ready for the sail-maker to cover the surface with
fabric.
Adjustment of Control Cables.—The adjustment of the control cables
is quite an art, and upon it will depend to a large degree the quick and easy
control of the aeroplane by the pilot.
The method is as follows:
After having rigged the controlling surfaces, and as far as possible
secured the correct adjustment of the control cables, then remove the
packing which has kept the control levers rigid. Then, sitting in the pilot's
seat, move the control levers smartly. Tension the control cables so that
when the levers are smartly moved there is no perceptible snatch or lag. Be
careful not to tension the cables more than necessary to take out the snatch.
If tensioned too much they will (1) bind round the pulleys and result in hard
work for the pilot; (2) throw dangerous stresses upon the controlling
surfaces, which are of rather flimsy construction; and (3) cause the cables to
fray round the pulleys quicker than would otherwise be the case.

Now, after having tensioned the cables sufficiently to take out the snatch,
place the levers in their neutral positions, and move them to and fro about
1/8 inch either side of such positions. If the adjustment is correct, it should
be possible to see the controlling surfaces move. If they do not move, then
the control cables are too slack.
Flying Position.—Before rigging an aeroplane or making any
adjustments it is necessary to place it in what is known as its "flying
position." I may add that it would be better termed its "rigging position."
In the case of an aeroplane fitted with a stationary engine this is secured
by packing up the machine so that the engine foundations are perfectly
horizontal both longitudinally and laterally. This position is found by
placing a straight-edge and a spirit-level across the engine foundations
(both longitudinally and laterally), and great care should be taken to see that
the bubble is exactly in the centre of the level. The slightest error will
assume magnitude towards the extremities of the aeroplane. Great care
should be taken to block up the aeroplane rigidly. In case it gets
accidentally disturbed while the work is going on, it is well to constantly
verify the flying position by running the straight-edge and spirit-level over
the engine foundations. The straight-edge should be carefully tested before
being used, as, being generally made of wood, it will not remain true long.
Place it lightly in a vice, and in such a position that a spirit-level on top
shows the bubble exactly in the centre. Now slowly move the level along
the straight-edge, and the bubble should remain exactly in the centre. If it
does not do so, then the straight-edge is not true and must be corrected. This
should never be omitted.
In the case of aeroplanes fitted with engines of the rotary type, the "flying
position" is some special attitude laid down in the aeroplane's
specifications, and great care should be taken to secure accuracy.
Angle of Incidence.—One method of finding the angle of incidence is as
follows:

First place the aeroplane in its flying position. The corner of the straight-
edge must be placed underneath and against the centre of the rear spar, and
held in a horizontal position parallel to the ribs. This is secured by using a
spirit-level. The set measurement will then be from the top of the straight-
edge to the centre of the bottom surface of the main spar, or it may be from
the top of the straight-edge to the lowest part of the leading edge. Care
should be taken to measure from the centre of the spar and to see that the
bubble is exactly in the centre of the level. Remember that all this will be
useless if the aeroplane has not been placed accurately in its flying position.
This method of finding the angle of incidence must be used under every
part of the lower surface where struts occur. It should not be used between
the struts, because, in such places, the spars may have taken a slight
permanent set up or down; not, perhaps, sufficiently bad to make any
material difference to the flying of the machine, but quite bad enough to
throw out the angle of incidence, which cannot be corrected at such a place.
If the angle is wrong, it should then be corrected as follows:
If it is too great, then the rear spar must be warped up until it is right, and
this is done by slackening all the wires going to the top of the strut, and
then tightening all the wires going to the bottom of the strut.
If the angle is too small, then slacken all the wires going to the bottom of
the strut, and tighten all the wires going to the top of the strut, until the
correct adjustment is secured.
Never attempt to adjust the angle by warping the main spar.
The set measurement, which is of course stated in the aeroplane's
specifications, should be accurate to 1/16 inch.

Lateral Dihedral Angle.—One method of securing this is as follows,
and this method will, at the same time, secure the correct angle of
incidence:

The strings, drawn very tight, must be taken over both the main and rear
spars of the top surface. They must run between points on the spars just
inside the outer struts. The set measurement (which should be accurate to
1/16 inch or less) is then from the strings down to four points on the main
and rear spars of the centre-section surface. These points should be just
inside the four centre-section struts; that is to say, as far as possible away
from the centre of the centre-section. Do not attempt to take the set
measurement near the centre of the centre-section.
The strings should be as tight as possible, and, if it can be arranged, the
best way to accomplish that is as shown in the above illustration, i.e., by
weighting the strings down to the spars by means of weights and tying their
ends to struts. This will give a tight and motionless string.
However carefully the above adjustment is made, there is sure to be some
slight error. This is of no great importance, provided it is divided equally
between the left- and right-hand wings. In order to make sure of this, certain
check measurements should be taken as follows:
Each bay must be diagonally measured, and such measurements must be
the same to within 1/16 inch on each side of the aeroplane. As a rule such
diagonal measurements are taken from the bottom socket of one strut to the

top socket of another strut, but this is bad practice, because of possible
inaccuracies due to faulty manufacture.
The points between which the diagonal measurements are taken should
be at fixed distances from the butts of the spars, such distances being the
same on each side of the aeroplane, thus:

Points A, B, and C, must be the same fixed distances from the butt as are Points
D, E, and F. Distances 1 and 2 must equal distances 3 and 4.

The above applies to both front and rear bays.
It would be better to use the centre line of the aeroplane rather than the
butts of the spars. It is not practicable to do so, however, as the centre line
probably runs through the petrol tanks, etc.
The Dihedral Board.—Another method of securing the dihedral angle,
and also the angle of incidence, is by means of the dihedral board. It is a
light handy thing to use, but leads to many errors, and should not be used
unless necessary. The reasons are as follows:
The dihedral board is probably not true. If it must be used, then it should
be very carefully tested for truth beforehand. Another reason against its use
is that it has to be placed on the spars in a position between the struts, and
that is just where the spars may have a little permanent set up or down, or
some inaccuracy of surface which will, of course, throw out the accuracy of
the adjustment. The method of using it is as follows:

The board is cut to the same angle as that specified for the upward
inclination of the surface towards its wing-tips. It is placed on the spar as

indicated above, and it is provided with two short legs to raise it above the
flanges of the ribs (which cross over the spars), as they may vary in depth.
A spirit-level is then placed on the board, and the wires must be adjusted to
give the surface such an inclination as to result in the bubble being in the
centre of the level. This operation must be performed in respect of each bay
both front and rear. The bays must then be diagonally measured as already
explained.
Yet Another Method of finding the dihedral angle, and at the same time
the angle of incidence, is as follows:
A horizontal line is taken from underneath the butt of each spar, and the
set measurement is either the angle it makes with the spar, or a fixed
measurement from the line to the spar taken at a specified distance from the
butt. This operation must be performed in respect of both main and rear
spars, and all the bays must be measured diagonally afterwards.

Whichever method is used, be sure that after the job is done the spars are
perfectly straight.
Stagger.—The stagger is the distance the top surface is in advance of the
bottom surface when the aeroplane is in flying position. The set
measurement is obtained as follows:

Plumb-lines must be dropped over the leading edge of the top surface
wherever struts occur, and also near the fuselage. The set measurement is

taken from the front of the lower leading edge to the plumb-lines. It makes
a difference whether the measurement is taken along a horizontal line
(which can be found by using a straight-edge and a spirit-level) or along a
projection of the chord. The line along which the measurement should be
taken is laid down in the aeroplane's specifications.
If a mistake is made and the measurement taken along the wrong line, it
may result in a difference of perhaps 1/4 inch or more to the stagger, with
the certain result that the aeroplane will, in flight, be nose-heavy or tail-
heavy.
After the adjustments of the angles of incidence, dihedral, and stagger
have been secured, it is as well to confirm all of them, as, in making the last
adjustment, the first one may have been spoiled.
Over-All Adjustments.—The following over-all check measurements
should now be taken.

The dotted lines on the surface represent the spars within it.

The straight lines AC and BC should be equal to within 1/8 inch. The
point C is the centre of the propeller, or, in the case of a "pusher" aeroplane,
the centre of the nacelle. The points A and B are marked on the main spar,
and must in each case be the same distance from the butt of the spar. The
rigger should not attempt to make A and B merely the sockets of the outer
struts, as they may not have been placed quite accurately by the
manufacturer. The lines AC and BC must be taken from both top and
bottom spars—two measurements on each side of the aeroplane.

The two measurements FD and FE should be equal to within 1/8 inch. F
is the centre of the fuselage or rudder-post. D and E are points marked on
both top and bottom rear spars, and each must be the same fixed distance
from the butt of the spar. Two measurements on each side of the aeroplane.
If these over-all measurements are not correct, then it is probably due to
some of the drift or anti-drift wires being too tight or too slack. It may
possibly be due to the fuselage being out of truth, but of course the rigger
should have made quite sure that the fuselage was true before rigging the
rest of the machine. Again, it may be due to the internal bracing wires
within the lifting surfaces not being accurately adjusted, but of course this
should have been seen to before covering the surfaces with fabric.
Fuselage.—The method of truing the fuselage is laid down in the
aeroplane's specifications. After it has been adjusted according to the
specified directions, it should then be arranged on trestles in such a way as
to make about three-quarters of it towards the tail stick out unsupported. In
this way it will assume a condition as near as possible to flying conditions,
and when it is in this position the set measurements should be confirmed. If
this is not done it may be out of truth, but perhaps appear all right when
supported by trestles at both ends, as, in such case, its weight may keep it
true as long as it is resting upon the trestles.
The Tail-Plane (Empennage).—The exact angle of incidence of the tail-
plane is laid down in the aeroplane's specifications. It is necessary to make
sure that the spars are horizontal when the aeroplane is in flying position
and the tail unsupported as explained above under the heading of Fuselage.
If the spars are tapered, then make sure that their centre lines are horizontal.
Undercarriage.—The undercarriage must be very carefully aligned as
laid down in the specifications.
1. The aeroplane must be placed in its flying position and sufficiently
high to ensure the wheels being off the ground when rigged. When in this
position the axle must be horizontal and the bracing wires adjusted to
secure the various set measurements stated in the specifications.
2. Make sure that the struts bed well down into their sockets.

3. Make sure that the shock absorbers are of equal tension. In the case of
rubber shock absorbers, both the number of turns and the lengths must be
equal.
How to Diagnose Faults in Flight, Stability, and Control.
Directional Stability will be badly affected if there is more drift (i.e.,
resistance) on one side of the aeroplane than there is on the other side. The
aeroplane will tend to turn towards the side having the most drift. This may
be caused as follows:
1. The angle of incidence of the main surface or the tail surface may be
wrong. The greater the angle of incidence, the greater the drift. The less the
angle, the less the drift.
2. If the alignment of the fuselage, fin in front of the rudder, the struts or
stream-line wires, or, in the case of the Maurice Farman, the front
outriggers, are not absolutely correct—that is to say, if they are turned a
little to the left or to the right instead of being in line with the direction of
flight—then they will act as a rudder and cause the aeroplane to turn off its
course.
3. If any part of the surface is distorted, it will cause the aeroplane to turn
off its course. The surface is cambered, i.e., curved, to pass through the air
with the least possible drift. If, owing perhaps to the leading edge, spars, or
trailing edge becoming bent, the curvature is spoiled, that will result in
changing the amount of drift on one side of the aeroplane, which will then
have a tendency to turn off its course.
Lateral Instability (Flying One Wing Down).—The only possible
reason for such a condition is a difference in the lifts of right and left wings.
That may be caused as follows:
1. The angle of incidence may be wrong. If it is too great, it will produce
more lift than on the other side of the aeroplane; and if too small, it will
produce less lift than on the other side—the result being that, in either case,
the aeroplane will try to fly one wing down.
2. Distorted Surfaces.—If some part of the surface is distorted, then its
camber is spoiled, and the lift will not be the same on both sides of the

aeroplane, and that, of course, will cause it to fly one wing down.
Longitudinal Instability may be due to the following reasons:
1. The stagger may be wrong. The top surface may have drifted back a
little owing to some of the wires, probably the incidence wires, having
elongated their loops or having pulled the fittings into the wood. If the top
surface is not staggered forward to the correct degree, then consequently the
whole of its lift is too far back, and it will then have a tendency to lift up the
tail of the machine too much. The aeroplane would then be said to be "nose-
heavy."
A 1/4-inch area in the stagger will make a very considerable difference to
the longitudinal stability.
2. If the angle of incidence of the main surface is not right, it will have a
bad effect, especially in the case of an aeroplane with a lifting tail-plane.
If the angle is too great, it will produce an excess of lift, and that may lift
up the nose of the aeroplane and result in a tendency to fly "tail-down." If
the angle is too small, it will produce a decreased lift, and the aeroplane
may have a tendency to fly "nose-down."
3. The fuselage may have become warped upward or downward, thus
giving the tail-plane an incorrect angle of incidence. If it has too much
angle, it will lift too much, and the aeroplane will be "nose-heavy." If it has
too little angle, then it will not lift enough, and the aeroplane will be "tail-
heavy."
4. (The least likely reason.) The tail-plane may be mounted upon the
fuselage at a wrong angle of incidence, in which case it must be corrected.
If nose-heavy, it should be given a smaller angle of incidence. If tail-heavy,
it should be given a larger angle; but care should be taken not to give it too
great an angle, because the longitudinal stability entirely depends upon the
tail-plane being set at a much smaller angle of incidence than is the main
surface, and if that difference is decreased too much, the aeroplane will
become uncontrollable longitudinally. Sometimes the tail-plane is mounted
on the aeroplane at the same angle as the main surface, but it actually
engages the air at a lesser angle, owing to the air being deflected

downwards by the main surface. There is then, in effect, a longitudinal
dihedral as explained and illustrated in Chapter I.
Climbs Badly.—Such a condition is, apart from engine or propeller
trouble, probably due to (1) distorted surfaces, or (2) too small an angle of
incidence.
Flight Speed Poor.—Such a condition is, apart from engine or propeller
trouble, probably due to (1) distorted surfaces, (2) too great an angle of
incidence, or (3) dirt or mud, and consequently excessive skin-friction.
Inefficient Control is probably due to (1) wrong setting of control
surfaces, (2) distortion of control surfaces, or (3) control cables being badly
tensioned.
Will not "Taxi" Straight.—If the aeroplane is uncontrollable on the
ground, it is probably due to (1) alignment of undercarriage being wrong, or
(2) unequal tension of shock absorbers.

CHAPTER IV
THE PROPELLER, OR "AIR-SCREW"

The sole object of the propeller is to translate the power of the engine
into thrust.
The propeller screws through the air, and its blades, being set at an angle
inclined to the direction of motion, secure a reaction, as in the case of the
aeroplane's lifting surface.
This reaction may be conveniently divided into two component parts or
values, namely, Thrust and Drift (see illustration overleaf).
The Thrust is opposed to the Drift of the aeroplane, and must be equal
and opposite to it at flying speed. If it falls off in power, then the flying
speed must decrease to a velocity, at which the aeroplane drift equals the
decreased thrust. The Drift of the propeller may be conveniently divided
into the following component values:
Active Drift, produced by the useful thrusting part of the propeller.
Passive Drift, produced by all the rest of the propeller, i.e., by its
detrimental surface.
Skin-Friction, produced by the friction of the air with roughness of
surface.
Eddies attending the movement of the air caused by the action of
the propeller.
Cavitation (very marked at excessive speed of revolution). A
tendency of the propeller to produce a cavity or semi-vacuum in
which it revolves, the thrust decreasing with increase of speed
and cavitation.

Thrust-Drift Ratio.—The proportion of thrust to drift is of paramount
importance, for it expresses the efficiency of the propeller. It is affected by
the following factors:
Speed of Revolution.—The greater the speed, the greater the
proportion of drift to thrust. This is due to the increase with
speed of the passive drift, which carries with it no increase in
thrust. For this reason propellers are often geared down to
revolve at a lower speed than that of the engine.
Angle of Incidence.—The same reasons as in the case of the
aeroplane surface.
Aspect Ratio.—Ditto.
Camber.—Ditto.

M, Direction of motion of propeller (rotary).
R, Direction of reaction.
T, Direction of thrust.
AD, Direction of the resistance of the air to the passage of the aeroplane, i.e.,
aeroplane drift.

D, Direction of propeller drift (rotary).
P, Engine power, opposed to propeller drift and transmitted to the propeller
through the propeller shaft.

In addition to the above factors there are, when it comes to actually
designing a propeller, mechanical difficulties to consider. For instance, the
blades must be of a certain strength and consequent thickness. That, in
itself, limits the aspect ratio, for it will necessitate a chord long enough in
proportion to the thickness to make a good camber possible. Again, the
diameter of the propeller must be limited, having regard to the fact that
greater diameters than those used to-day would not only result in excessive
weight of construction, but would also necessitate a very high undercarriage
to keep the propeller off the ground, and such undercarriage would not only
produce excessive drift, but would also tend to make the aeroplane stand on
its nose when alighting. The latter difficulty cannot be overcome by
mounting the propeller higher, as the centre of its thrust must be
approximately coincident with the centre of aeroplane drift.
Maintenance of Efficiency.
The following conditions must be observed:
1. Pitch Angle.—The angle, at any given point on the propeller, at which
the blade is set is known as the pitch angle, and it must be correct to half a
degree if reasonable efficiency is to be maintained.
This angle secures the "pitch," which is the distance the propeller
advances during one revolution, supposing the air to be solid. The air, as a
matter of fact, gives back to the thrust of the blades just as the pebbles slip
back as one ascends a shingle beach. Such "give-back" is known as Slip. If
a propeller has a pitch of, say, 10 feet, but actually advances, say, only 8
feet owing to slip, then it will be said to possess 20 per cent. slip.
Thus, the pitch must equal the flying speed of the aeroplane plus the slip
of the propeller. For example, let us find the pitch of a propeller, given the
following conditions:

Flying speed ... 70 miles per hour.
Propeller revolutions ... 1,200 per minute.
Slip ... 15 per cent.

First find the distance in feet the aeroplane will travel forward in one
minute. That is—
369,600 feet (70 miles)
= 6,160 feet per minute.
60 " (minutes)
Now divide the feet per minute by the propeller revolutions per minute,
add 15 per cent. for the slip, and the result will be the propeller pitch:
6,160
+ 15 per cent. = 5.903 feet.
1,200
In order to secure a constant pitch from root to tip of blade, the pitch
angle decreases towards the tip. This is necessary, since the end of the blade
travels faster than its root, and yet must advance forward at the same speed
as the rest of the propeller. For example, two men ascending a hill. One
prefers to walk fast and the other slowly, but they wish to arrive at the top
of the hill simultaneously. Then the fast walker must travel a farther
distance than the slow one, and his angle of path (pitch angle) must then be
smaller than the angle of path taken by the slow walker. Their pitch angles
are different, but their pitch (in this case altitude reached in a given time) is
the same.

In order to test the pitch angle, the propeller must be mounted upon a
shaft at right angles to a beam the face of which must be perfectly level,
thus:

First select a point on the blade at some distance (say about 2 feet) from
the centre of the propeller. At that point find, by means of a protractor, the
angle a projection of the chord makes with the face of the beam. That angle
is the pitch angle of the blade at that point.
Now lay out the angle on paper, thus:

The line above and parallel to the circumference line must be placed in a
position making the distance between the two lines equal to the specified
pitch, which is, or should be, marked upon the boss of the propeller.
Now find the circumference of the propeller where the pitch angle is
being tested. For example, if that place is 2 feet radius from the centre, then
the circumference will be 2 feet x 2 = 4 feet diameter, which, if multiplied
by 3.1416 = 15.56 feet circumference.
Now mark off the circumference distance, which is represented above by
A–B, and reduce it in scale for convenience.
The distance a vertical line makes between B and the chord line is the
pitch at the point where the angle is being tested, and it should coincide
with the specified pitch.
You will note, from the above illustration, that the actual pitch line
should meet the junction of the chord line and top line.
The propeller should be tested at several points, about a foot apart, on
each blade; and the diagram, provided the propeller is not faulty, will then

look like this:

A, B, C, and D, Actual pitch at points tested.
I, Pitch angle at point tested nearest to centre of propeller.
E, Circumference at I.
J, Pitch angle at point tested nearest to I.
F, Circumference at J.
K, Pitch angle at next point tested.
G, Circumference at K.
L, Pitch angle tested at point nearest tip of blade.
H, Circumference at L.

At each point tested the actual pitch coincides with the specified pitch: a
satisfactory condition.
A faulty propeller will produce a diagram something like this:

At every point tested the pitch angle is wrong, for nowhere does the
actual pitch coincide with the specified pitch. Angles A, C, and D, are too
large, and B is too small. The angle should be correct to half a degree if
reasonable efficiency is to be maintained.
A fault in the pitch angle may be due to (1) faulty manufacture, (2)
distortion, or (3) the shaft hole through the boss being out of position.
2. Straightness.—To test for straightness the propeller must be mounted
upon a shaft. Now bring the tip of one blade round to graze some fixed
object. Mark the point it grazes. Now bring the other tip round, and it
should come within 1/8 inch of the mark. If it does not do so, it is due to (1)

faulty manufacture, (2) distortion, or (3) to the hole through the boss being
out of position.
3. Length.—The blades should be of equal length to 1/16 inch.
4. Balance.—The usual method of testing a propeller for balance is as
follows: Mount it upon a shaft, which must be on ball-bearings. Place the
propeller in a horizontal position, and it should remain in that position. If a
weight of a trifle over an ounce placed in a bolt-hole on one side of the boss
fails to disturb the balance, then the propeller is usually regarded as unfit
for use.

The above method is rather futile, as it does not test for the balance of
centrifugal force, which comes into play as soon as the propeller revolves.
It can be tested as follows:

The propeller must be in a horizontal position, and then weighed at fixed
points, such as A, B, C, D, E, and F, and the weights noted. The points A, B,
and C must, of course, be at the same fixed distances from the centre of the
propeller as the points D, E, and F. Now reverse the propeller and weigh at
each point again. Note the results. The first series of weights should
correspond to the second series, thus:
Weight A should equal weight F.
Weight B should equal weight E.
Weight C should equal weight D.

There is no standard practice as to the degree of error permissible, but if
there are any appreciable differences the propeller is unfit for use.
5. Surface Area.—The surface area of the blades should be equal. Test
with calipers thus:

The distance A–B should equal K–L.
The distance C–D should equal I–J.
The distance E–F should equal G–H.
The points between which the distances are taken must, of course, be at
the same distance from the centre in the case of each blade.
There is no standard practice as to the degree of error permissible. If,
however, there is an error of over 1/8 inch, the propeller is really unfit for
use.
6. Camber.—The camber (curvature) of the blades should be (1) equal,
(2) decrease evenly towards the tips of the blades, and (3) the greatest depth
of the curve should, at any point of the blade, be approximately at the same
percentage of the chord from the leading edge as at other points.
It is difficult to test the top camber without a set of templates,18 but a
fairly accurate idea of the concave camber can be secured by slowly passing
a straight-edge along the blade, thus:

The camber can now be easily seen, and as the straight-edge is passed
along the blade, the observer should look for any irregularities of the

curvature, which should gradually and evenly decrease towards the tip of
the blade.
7. The Joints.—The usual method for testing the glued joints is by
revolving the propeller at greater speed than it will be called upon to make
during flight, and then carefully examining the joints to see if they have
opened. It is not likely, however, that the reader will have the opportunity of
making this test. He should, however, examine all the joints very carefully,
trying by hand to see if they are quite sound. Suspect a propeller of which
the joints appear to hold any thickness of glue. Sometimes the joints in the
boss open a little, but this is not dangerous unless they extend to the blades,
as the bolts will hold the laminations together.
8. Condition of Surface.—The surface should be very smooth, especially
towards the tips of the blades. Some propeller tips have a speed of over
30,000 feet a minute, and any roughness will produce a bad drift or
resistance and lower the efficiency.
9. Mounting.—Great care should be taken to see that the propeller is
mounted quite straight on its shaft. Test in the same way as for straightness.
If it is not straight, it is possibly due to some of the propeller bolts being too
slack or to others having been pulled up too tightly.
Flutter.—Propeller "flutter," or vibration, may be due to faulty pitch
angle, balance, camber, surface area, or to bad mounting. It causes a
condition sometimes mistaken for engine trouble, and one which may easily
lead to the collapse of the propeller.
Care of Propellers.—The care of propellers is of the greatest
importance, as they become distorted very easily.
1. Do not store them in a very damp or a very dry place.
2. Do not store them where the sun will shine upon them.
3. Never leave them long in a horizontal position or leaning up
against a wall.
4. They should be hung on horizontal pegs, and the position of the
propellers should be vertical.

If the points I have impressed upon you in these notes are not attended to,
you may be sure of the following results:
1. Lack of efficiency, resulting in less aeroplane speed and climb
than would otherwise be the case.
2. Propeller "flutter" and possible collapse.
3. A bad stress upon the propeller shaft and its bearings.
Tractor.—A propeller mounted in front of the main surface.
Pusher.—A propeller mounted behind the main surface.
Four-Bladed Propellers.—Four-bladed propellers are suitable only
when the pitch is comparatively large. For a given pitch, and having regard
to "interference," they are not so efficient as two-bladed propellers.
Spiral Courses of Two-Blade Tips.
Spiral Courses of Four-Blade Tips.

Pitch the same in each case.

The smaller the pitch, the less the "gap," i.e., the distance, measured in
the direction of the thrust, between the spiral courses of the blades (see
illustration on preceding page).
If the gap is too small, then the following blade will engage air which the
preceding blade has put into motion, with the result that the following blade
will not secure as good a reaction as would otherwise be the case. It is very
much the same as in the case of the aeroplane gap.
For a given pitch, the gap of a four-bladed propeller is only half that of a
two-bladed one. Therefore the four-bladed propeller is only suitable for

large pitch, as such pitch produces spirals with a large gap, thus offsetting
the decrease in gap caused by the numerous blades.
The greater the speed of rotation, the less the pitch for a given aeroplane
speed. Then, in order to secure a large pitch and consequently a good gap,
the four-bladed propeller is usually geared to rotate at a lower speed than
would be the case if directly attached to the engine crank-shaft.

18 (return)
I have heard of temporary ones being made quickly by bending strips of lead
over the convex side of the blade, but I should think it very difficult to secure a
sufficient degree of accuracy in that way.

CHAPTER V
MAINTENANCE

Cleanliness.—The fabric must be kept clean and free from oil, as that
will rot it. To take out dirt or oily patches, try acetone. If that will not
remedy matters, then try petrol, but use it sparingly, as otherwise it will take
off an unnecessary amount of dope. If that will not remove the dirt, then hot
water and soap will do so, but, in that case, be sure to use soap having no
alkali in it, as otherwise it may injure the fabric. Use the water sparingly, or
it may get inside the planes and rust the internal bracing wires, or cause
some of the wooden framework to swell.
The wheels of the undercarriage have a way of throwing up mud on to
the lower surface. This should, if possible, be taken off while wet. It should
never be scraped off when dry, as that may injure the fabric. If dry, then it
should be moistened before being removed.
Measures should be taken to prevent dirt from collecting upon any part of
the aeroplane, as, otherwise, excessive skin-friction will be produced with
resultant loss of flight speed. The wires, being greasy, collect dirt very
easily.
Control Cables.—After every flight the rigger should pass his hand over
the control cables and carefully examine them near pulleys. Removal of
grease may be necessary to make a close inspection possible. If only one
strand is broken the wire should be replaced. Do not forget the aileron
balance wire on the top surface.
Once a day try the tension of the control cables by smartly moving the
control levers about as explained elsewhere.
Wires.—All the wires should be kept well greased or oiled, and in the
correct tension. When examining the wires, it is necessary to place the
aeroplane on level ground, as otherwise it may be twisted, thus throwing

some wires into undue tension and slackening others. The best way, if there
is time, is to pack the machine up into its "flying position."
If you see a slack wire, do not jump to the conclusion that it must be
tensioned. Perhaps its opposition wire is too tight, in which case slacken it,
and possibly you will find that will tighten the slack wire.
Carefully examine all wires and their connections near the propeller, and
be sure that they are snaked round with safety wire, so that the latter may
keep them out of the way of the propeller if they come adrift.
The wires inside the fuselage should be cleaned and regreased about once
a fortnight.
Struts and Sockets.—These should be carefully examined to see if any
splitting has occurred.
Distortion.—Carefully examine all surfaces, including the controlling
surfaces, to see whether any distortion has occurred. If distortion can be
corrected by the adjustment of wires, well and good; but if not, then some
of the internal framework probably requires replacement.
Adjustments.—Verify the angles of incidence, dihedral, and stagger, and
the rigging position of the controlling surfaces, as often as possible.
Undercarriage.—Constantly examine the alignment and fittings of the
undercarriage, and the condition of tyres and shock absorbers. The latter,
when made of rubber, wear quickest underneath. Inspect axles and skids to
see if there are any signs of them becoming bent. The wheels should be
taken off occasionally and greased.
Locking Arrangements.—Constantly inspect the locking arrangements
of turnbuckles, bolts, etc. Pay particular attention to the control cable
connections, and to all moving parts in respect of the controls.
Lubrication.—Keep all moving parts, such as pulleys, control levers, and
hinges of controlling surfaces, well greased.
Special Inspection.—Apart from constantly examining the aeroplane
with reference to the above points I have made, I think that, in the case of
an aeroplane in constant use, it is an excellent thing to make a special

inspection of every part, say, once a week. This will take from two to three
hours according to the type of aeroplane. In order to carry it out
methodically, the rigger should have a list of every part down to the
smallest split-pin. He can then check the parts as he examines them, and
nothing will be passed over. This, I know from experience, greatly increases
the confidence of the pilot, and tends to produce good work in the air.
Windy Weather.—The aeroplane, when on the ground, should face the
wind; and it is advisable to lash the control lever fast, so that the controlling
surfaces may not be blown about and possibly damaged.
"Vetting" by Eye.—This should be practised at every opportunity, and, if
persevered in, it is possible to become quite expert in diagnosing by eye
faults in flight efficiency, stability, and control.
The aeroplane should be standing upon level ground, or, better than that,
packed up into its "flying position."
Now stand in front of it and line up the leading edge with the main spar,
rear spar, and trailing edge. Their shadows can usually be seen through the
fabric. Allowance must, of course, be made for wash-in and wash-out;
otherwise, the parts I have specified should be parallel with each other.
Now line up the centre part of the main-plane with the tail-plane. The
latter should be symmetrical with it. Next, sight each interplane front strut
with its rear strut. They should be parallel.
Then, standing on one side of the aeroplane, sight all the front struts. The
one nearest to you should cover all the others. This applies to the rear struts
also.
Look for distortion of leading edges, main and rear spars, trailing edges,
tail-plane, and controlling surfaces.
This sort of thing, if practised constantly, will not only develop an expert
eye for diagnosis of faults, but will also greatly assist in impressing upon
the memory the characteristics and possible troubles of the various types of
aeroplanes.

Mishandling on the Ground.—This is the cause of a lot of unnecessary
damage. The golden rule to observe is, Produce no Bending Stresses.
Nearly all the wood in an aeroplane is designed to take merely the stress
of direct compression, and it cannot be bent safely. Therefore, in packing an
aeroplane up from the ground, or in pulling or pushing it about, be careful
to stress it in such a way as to produce, as far as possible, only direct
compression stresses. For instance, if it is necessary to support the lifting
surface, then the packing should be arranged to come directly under the
struts so that they may take the stress in the form of compression for which
they are designed. Such supports should be covered with soft packing in
order to prevent the fabric from becoming damaged.
When pulling an aeroplane along, if possible, pull from the top of the
undercarriage struts. If necessary to pull from elsewhere, then do so by
grasping the interplane struts as low down as possible. Never pull by means
of wires.
Never lay fabric-covered parts upon a concrete floor. Any slight
movement will cause the fabric to scrape over the floor with resultant
damage.
Struts, spars, etc., should never be left about the floor, as in such position
they are likely to become scored. I have already explained the importance
of protecting the outside fibres of the wood. Remember also that wood
becomes distorted easily. This particularly applies to interplane struts. If
there are no proper racks to stand them in, then the best plan is to lean them
up against the wall in as near a vertical position as possible.
Time.—Learn to know the time necessary to complete any of the various
rigging jobs. This is really important. Ignorance of this will lead to bitter
disappointments in civil life; and, where Service flying is concerned, it will,
to say the least of it, earn unpopularity with senior officers, and fail to
develop respect and good work where men are concerned.
The Aeroplane Shed.—This should be kept as clean and orderly as
possible. A clean, smart shed produces briskness, energy, and pride of work.
A dirty, disorderly shed nearly always produces slackness and poor quality
of work, lost tools, and mislaid material.

GLOSSARY
The numbers at the right-hand side of the page indicate the parts
numbered in the preceding diagrams.
Aeronautics—The science of aerial navigation.
Aerofoil—A rigid structure, of large superficial area relative to its
thickness, designed to obtain, when driven through the air at an
angle inclined to the direction of motion, a reaction from the air
approximately at right angles to its surface. Always cambered when
intended to secure a reaction in one direction only. As the term
"aerofoil" is hardly ever used in practical aeronautics, I have,
throughout this book, used the term SURFACE, which, while
academically incorrect, since it does not indicate thickness, is the
term usually used to describe the cambered lifting surfaces, i.e., the
"planes" or "wings," and the stabilizers and the controlling aerofoils.
Aerodrome—The name usually applied to a ground used for the practice of
aviation. It really means "flying machine," but is never used in that
sense nowadays.
Aeroplane—A power-driven aerofoil fitted with stabilizing and controlling
surfaces.
Acceleration—The rate of change of velocity.
Angle of Incidence—The angle at which the "neutral lift line" of a surface
attacks the air.
Angle of Incidence, Rigger's—The angle the chord of a surface makes
with a line parallel to the axis of the propeller.
Angle of Incidence, Maximum—The greatest angle of incidence at which,
for a given power, surface (including detrimental surface), and
weight, horizontal flight can be maintained.

Angle of Incidence, Minimum—The smallest angle of incidence at which,
for a given power, surface (including detrimental surface), and
weight, horizontal flight can be maintained.
Angle of Incidence, Best Climbing—That angle of incidence at which an
aeroplane ascends quickest. An angle approximately halfway
between the maximum and optimum angles.
Angle of Incidence, Optimum—The angle of incidence at which the lift-
drift ratio is the highest.
Angle, Gliding—The angle between the horizontal and the path along
which an aeroplane, at normal flying speed, but not under engine
power, descends in still air.
Angle, Dihedral—The angle between two planes.
Angle, Lateral Dihedral—The lifting surface of an aeroplane is said to be
at a lateral dihedral angle when it is inclined upward towards its
wing-tips.
Angle, Longitudinal Dihedral—The main surface and tail surface are said
to be at a longitudinal dihedral angle when the projections of their
neutral lift lines meet and produce an angle above them.
Angle, Rigger's Longitudinal Dihedral—Ditto, but substituting "chords"
for "neutral lift lines."
Angle, Pitch—The angle at any given point of a propeller, at which the
blade is inclined to the direction of motion when the propeller is
revolving but the aeroplane stationary.
Altimeter—An instrument used for measuring height.
Air-Speed Indicator—An instrument used for measuring air pressures or
velocities. It consequently indicates whether the surface is securing
the requisite reaction for flight. Usually calibrated in miles per hour,
in which case it indicates the correct number of miles per hour at
only one altitude. This is owing to the density of the air decreasing
with increase of altitude and necessitating a greater speed through
space to secure the same air pressure as would be secured by less

speed at a lower altitude. It would be more correct to calibrate it in
units of air pressure. [1]
Air Pocket—A local movement or condition of the air causing an aeroplane
to drop or lose its correct attitude.
Aspect-Ratio—The proportion of span to chord of a surface.
Air-Screw (Propeller)—A surface so shaped that its rotation about an axis
produces a force (thrust) in the direction of its axis. [2]
Aileron—A controlling surface, usually situated at the wing-tip, the
operation of which turns an aeroplane about its longitudinal axis;
causes an aeroplane to tilt sideways. [3]
Aviation—The art of driving an aeroplane.
Aviator—The driver of an aeroplane.
Barograph—A recording barometer, the charts of which can be calibrated
for showing air density or height.
Barometer—An instrument used for indicating the density of air.
Bank, to—To turn an aeroplane about its longitudinal axis (to tilt sideways)
when turning to left or right.
Biplane—An aeroplane of which the main lifting surface consists of a
surface or pair of wings mounted above another surface or pair of
wings.
Bay—The space enclosed by two struts and whatever they are fixed to.
Boom—A term usually applied to the long spars joining the tail of a
"pusher" aeroplane to its main lifting surface. [4]
Bracing—A system of struts and tie wires to transfer a force from one point
to another.
Canard—Literally "duck." The name which was given to a type of
aeroplane of which the longitudinal stabilizing surface (empennage)
was mounted in front of the main lifting surface. Sometimes termed

"tail-first" aeroplanes, but such term is erroneous, as in such a
design the main lifting surface acts as, and is, the empennage.
Cabre—To fly or glide at an excessive angle of incidence; tail down.
Camber—Curvature.
Chord—Usually taken to be a straight line between the trailing and leading
edges of a surface.
Cell—The whole of the lower surface, that part of the upper surface directly
over it, together with the struts and wires holding them together.
Centre (Line) of Pressure—A line running from wing-tip to wing-tip, and
through which all the air forces acting upon the surface may be said
to act, or about which they may be said to balance.
Centre (Line) of Pressure, Resultant—A line transverse to the
longitudinal axis, and the position of which is the resultant of the
centres of pressure of two or more surfaces.
Centre of Gravity—The centre of weight.
Cabane—A combination of two pylons, situated over the fuselage, and
from which the anti-lift wires are suspended. [5]
Cloche—Literally "bell." Is applied to the bell-shaped construction which
forms the lower part of the pilot's control lever in a Bleriot
monoplane, and to which the control cables are attached.
Centrifugal Force—Every body which moves in a curved path is urged
outwards from the centre of the curve by a force termed
"centrifugal."
Control Lever—A lever by means of which the controlling surfaces are
operated. It usually operates the ailerons and elevator. The "joy-
stick." [6]
Cavitation, Propeller—The tendency to produce a cavity in the air.
Distance Piece—A long, thin piece of wood (sometimes tape) passing
through and attached to all the ribs in order to prevent them from

rolling over sideways. [7]
Displacement—Change of position.
Drift (of an aeroplane as distinct from the propeller)—The horizontal
component of the reaction produced by the action of driving through
the air a surface inclined upwards and towards its direction of
motion plus the horizontal component of the reaction produced by
the "detrimental" surface plus resistance due to "skin-friction."
Sometimes termed "head-resistance."
Drift, Active—Drift produced by the lifting surface.
Drift, Passive—Drift produced by the detrimental surface.
Drift (of a propeller)—Analogous to the drift of an aeroplane. It is
convenient to include "eddies" and "cavitation" within this term.
Drift, to—To be carried by a current of air; to make leeway.
Dive, to—To descend so steeply as to produce a speed greater than the
normal flying speed.
Dope, to—To paint a fabric with a special fluid for the purpose of
tightening and protecting it.
Density—Mass of unit volume; for instance, pounds per cubic foot.
Efficiency—
Output

Input.
Efficiency (of an aeroplane as distinct from engine and propeller)—
Lift and Velocity

Thrust (= aeroplane drift).
Efficiency, Engine—

Brake horse-power

Indicated horse-power.
Efficiency, Propeller—
Thrust horse-power

Horse-power received from engine
(= propeller drift).
Note.—The above terms can, of course, be expressed in foot-
pounds. It is then only necessary to divide the upper term by the
lower one to find the measure of efficiency.
Elevator—A controlling surface, usually hinged to the rear of the tail-
plane, the operation of which turns an aeroplane about an axis which
is transverse to the direction of normal horizontal flight. [8]
Empennage—See "Tail-plane."
Energy—Stored work. For instance, a given weight of coal or petroleum
stores a given quantity of energy which may be expressed in foot-
pounds.
Extension—That part of the upper surface extending beyond the span of
the lower surface. [9]
Edge, Leading—The front edge of a surface relative to its normal direction
of motion. [10]
Edge, Trailing—The rear edge of a surface relative to its normal direction
of motion. [11]
Factor of Safety—Usually taken to mean the result found by dividing the
stress at which a body will collapse by the maximum stress it will be
called upon to bear.
Fineness (of stream-line)—The proportion of length to maximum width.

Flying Position—A special position in which an aeroplane must be placed
when rigging it or making adjustments. It varies with different types
of aeroplanes. Would be more correctly described as "rigging
position."
Fuselage—That part of an aeroplane containing the pilot, and to which is
fixed the tail-plane. [12]
Fin—Additional keel-surface, usually mounted at the rear of an aeroplane.
[13]
Flange (of a rib)—That horizontal part of a rib which prevents it from
bending sideways. [14]
Flight—The sustenance of a body heavier than air by means of its action
upon the air.
Foot-pound—A measure of work representing the weight of 1 lb. raised 1
foot.
Fairing—Usually made of thin sheet aluminium, wood, or a light
construction of wood and fabric; and bent round detrimental surface
in order to give it a "fair" or "stream-like" shape. [15]
Gravity—Is the force of the Earth's attraction upon a body. It decreases
with increase of distance from the Earth. See "Weight."
Gravity, Specific—
Density of substance

Density of water.
Thus, if the density of water is 10 lb. per unit volume, the same unit
volume of petrol, if weighing 7 lb., would be said to have a specific
gravity of 7/10, i.e., 0.7.
Gap (of an aeroplane)—The distance between the upper and lower surfaces
of a biplane. In a triplane or multiplane, the distance between any
two of its surfaces. [16]

Gap, Propeller—The distance, measured in the direction of the thrust,
between the spiral courses of the blades.
Girder—A structure designed to resist bending, and to combine lightness
and strength.
Gyroscope—A heavy circular wheel revolving at high speed, the effect of
which is a tendency to maintain its plane of rotation against
disturbing forces.
Hangar—An aeroplane shed.
Head-resistance—Drift. The resistance of the air to the passage of a body.
Helicopter—An air-screw revolving about a vertical axis, the direction of
its thrust being opposed to gravity.
Horizontal Equivalent—The plan view of a body whatever its attitude
may be.
Impulse—A force causing a body to gain or lose momentum.
Inclinometer—A curved form of spirit-level used for indicating the attitude
of a body relative to the horizontal.
Instability—An inherent tendency of a body, which, if the body is
disturbed, causes it to move into a position as far as possible away
from its first position.
Instability, Neutral—An inherent tendency of a body to remain in the
position given it by the force of a disturbance, with no tendency to
move farther or to return to its first position.
Inertia—The inherent resistance to displacement of a body as distinct from
resistance the result of an external force.
Joy-Stick—See "Control Lever."
Keel-Surface—Everything to be seen when viewing an aeroplane from the
side of it.

King-Post—A bracing strut; in an aeroplane, usually passing through a
surface and attached to the main spar, and from the end or ends of
which wires are taken to spar, surface, or other part of the
construction in order to prevent distortion. When used in connection
with a controlling surface, it usually performs the additional
function of a lever, control cables connecting its ends with the pilot's
control lever. [17]
Lift—The vertical component of the reaction produced by the action of
driving through the air a surface inclined upwards and towards its
direction of motion.
Lift, Margin of—The height an aeroplane can gain in a given time and
starting from a given altitude.
Lift-Drift Ratio—The proportion of lift to drift.
Loading—The weight carried by an aerofoil. Usually expressed in pounds
per square foot of superficial area.
Longeron—The term usually applied to any long spar running length-ways
of a fuselage. [18]
Mass—The mass of a body is a measure of the quantity of material in it.
Momentum—The product of the mass and velocity of a body is known as
"momentum."
Monoplane—An aeroplane of which the main lifting surface consists of
one surface or one pair of wings.
Multiplane—An aeroplane of which the main lifting surface consists of
numerous surfaces or pairs of wings mounted one above the other.
Montant—Fuselage strut.
Nacelle—That part of an aeroplane containing the engine and/or pilot and
passenger, and to which the tail-plane is not fixed. [19]
Neutral Lift Line—A line taken through a surface in a forward direction
relative to its direction of motion, and starting from its trailing edge.

If the attitude of the surface is such as to make the said line
coincident with the direction of motion, it results in no lift, the
reaction then consisting solely of drift. The position of the neutral
lift line, i.e., the angle it makes with the chord, varies with
differences of camber, and it is found by means of wind-tunnel
research.
Newton's Laws of Motion—1. If a body be at rest, it will remain at rest; or,
if in motion, it will move uniformly in a straight line until acted
upon by some force.
2. The rate of change of the quantity of motion (momentum) is
proportional to the force which causes it, and takes place in the
direction of the straight line in which the force acts. If a body be
acted upon by several forces, it will obey each as though the others
did not exist, and this whether the body be at rest or in motion.
3. To every action there is opposed an equal and opposite
reaction.
Ornithopter (or Orthopter)—A flapping wing design of aircraft intended
to imitate the flight of a bird.
Outrigger—This term is usually applied to the framework connecting the
main surface with an elevator placed in advance of it. Sometimes
applied to the "tail-boom" framework connecting the tail-plane with
the main lifting surface. [20]
Pancake, to—To "stall."
Plane—This term is often applied to a lifting surface. Such application is
not quite correct, since "plane" indicates a flat surface, and the
lifting surfaces are always cambered.
Propeller—See "Air-Screw."
Propeller, Tractor—An air-screw mounted in front of the main lifting
surface.
Propeller, Pusher—An air-screw mounted behind the main lifting surface.

Pusher—An aeroplane of which the propeller is mounted behind the main
lifting surface.
Pylon—Any V-shaped construction from the point of which wires are
taken.
Power—Rate of working. [21]
Power, Horse—One horse-power represents a force sufficient to raise
33,000 lb. 1 foot in a minute.
Power, Indicated Horse—The I.H.P. of an engine is a measure of the rate
at which work is done by the pressure upon the piston or pistons, as
distinct from the rate at which the engine does work. The latter is
usually termed "brake horse-power," since it may be measured by an
absorption brake.
Power, Margin of—The available quantity of power above that necessary
to maintain horizontal flight at the optimum angle.
Pitot Tube—A form of air-speed indicator consisting of a tube with open
end facing the wind, which, combined with a static pressure or
suction tube, is used in conjunction with a gauge for measuring air
pressures or velocities. (No. 1 in diagram.)
Pitch, Propeller—The distance a propeller advances during one revolution
supposing the air to be solid.
Pitch, to—To plunge nose-down.
Reaction—A force, equal and opposite to the force of the action producing
it.
Rudder—A controlling surface, usually hinged to the tail, the operation of
which turns an aeroplane about an axis which is vertical in normal
horizontal flight; causes an aeroplane to turn to left or right of the
pilot. [22]
Roll, to—To turn about the longitudinal axis.

Rib, Ordinary—A light curved wooden part mounted in a fore and aft
direction within a surface. The ordinary ribs give the surface its
camber, carry the fabric, and transfer the lift from the fabric to the
spars. [23]
Rib, Compression—Acts as an ordinary rib, besides bearing the stress of
compression produced by the tension of the internal bracing wires.
[24]
Rib, False—A subsidiary rib, usually used to improve the camber of the
front part of the surface. [25]
Right and Left Hand—Always used relative to the position of the pilot.
When observing an aeroplane from the front of it, the right hand
side of it is then on the left hand of the observer.
Remou—A local movement or condition of the air which may cause
displacement of an aeroplane.
Rudder-Bar—A control lever moved by the pilot's feet, and operating the
rudder. [26]
Surface—See "Aerofoil."
Surface, Detrimental—All exterior parts of an aeroplane including the
propeller, but excluding the (aeroplane) lifting and (propeller)
thrusting surfaces.
Surface, Controlling—A surface the operation of which turns an aeroplane
about one of its axes.
Skin-Friction—The friction of the air with roughness of surface. A form of
drift.
Span—The distance from wing-tip to wing-tip.
Stagger—The distance the upper surface is forward of the lower surface
when the axis of the propeller is horizontal.
Stability—The inherent tendency of a body, when disturbed, to return to its
normal position.

Stability, Directional—The stability about an axis which is vertical during
normal horizontal flight, and without which an aeroplane has no
natural tendency to remain upon its course.
Stability, Longitudinal—The stability of an aeroplane about an axis
transverse to the direction of normal horizontal flight, and without
which it has no tendency to oppose pitching and tossing.
Stability, Lateral—The stability of an aeroplane about its longitudinal axis,
and without which it has no tendency to oppose sideways rolling.
Stabilizer—A surface, such as fin or tail-plane, designed to give an
aeroplane inherent stability.
Stall, to—To give or allow an aeroplane an angle of incidence greater than
the "maximum" angle, the result being a fall in the lift-drift ratio, the
lift consequently becoming less than the weight of the aeroplane,
which must then fall, i.e., "stall" or "pancake."
Stress—Burden or load.
Strain—Deformation produced by stress.
Side-Slip, to—To fall as a result of an excessive "bank" or "roll."
Skid, to—To be carried sideways by centrifugal force when turning to left
or right.
Skid, Undercarriage—A spar, mounted in a fore and aft direction, and to
which the wheels of the undercarriage are sometimes attached.
Should a wheel give way the skid is then supposed to act like the
runner of a sleigh and to support the aeroplane. [28]
Skid, Tail—A piece of wood or other material, orientable, and fitted with
shock absorbers, situated under the tail of an aeroplane in order to
support it upon the ground and to absorb the shock of alighting.
[28a]
Section—Any separate part of the top surface, that part of the bottom
surface immediately underneath it, with their struts and wires.

Spar—Any long piece of wood or other material.
Spar, Main—A spar within a surface and to which all the ribs are attached,
such spar being the one situated nearest to the centre of pressure. It
transfers more than half the lift from the ribs to the bracing. [29]
Spar, Rear—A spar within a surface, and to which all the ribs are attached,
such spar being situated at the rear of the centre of pressure and at a
greater distance from it than is the main spar. It transfers less than
half of the lift from the ribs to the bracing. [30]
Strut—Any wooden member intended to take merely the stress of direct
compression.
Strut, Interplane—A strut holding the top and bottom surfaces apart.
[31]
Strut, Fuselage—A strut holding the fuselage longerons apart. It should be
stated whether top, bottom, or side. If side, then it should be stated
whether right or left hand. Montant. [32]
Strut, Extension—A strut supporting an "extension" when not in flight. It
may also prevent the extension from collapsing upwards during
flight. [33]
Strut, undercarriage— [33a]
Strut, Dope—A strut within a surface, so placed as to prevent the tension
of the doped fabric from distorting the framework. [34]
Serving—To bind round with wire, cord, or similar material. Usually used
in connection with wood joints and wire cable splices.
Slip, Propeller—The pitch less the distance the propeller advances during
one revolution.
Stream-Line—A form or shape of detrimental surface designed to produce
minimum drift.
Toss, to—To plunge tail-down.

Torque, Propeller—The tendency of a propeller to turn an aeroplane about
its longitudinal axis in a direction opposite to that in which the
propeller revolves.
Tail-Slide—A fall whereby the tail of an aeroplane leads.
Tractor—An aeroplane of which the propeller is mounted in front of the
main lifting surface.
Triplane—An aeroplane of which the main lifting surface consists of three
surfaces or pairs of wings mounted one above the other.
Tail-Plane—A horizontal stabilizing surface mounted at some distance
behind the main lifting surface. Empennage. [36]
Turnbuckle—A form of wire-tightener, consisting of a barrel into each end
of which is screwed an eyebolt. Wires are attached to the eyebolts
and the required degree of tension is secured by means of rotating
the barrel.
Thrust, Propeller—See "Air-Screw."
Undercarriage—That part of an aeroplane beneath the fuselage or nacelle,
and intended to support the aeroplane when at rest, and to absorb the
shock of alighting.
Velocity—Rate of displacement; speed.
Volplane—A gliding descent.
Weight—Is a measure of the force of the Earth's attraction (gravity) upon a
body. The standard unit of weight in this country is 1 lb., and is the
force of the Earth's attraction on a piece of platinum called the
standard pound, deposited with the Board of Trade in London. At
the centre of the Earth a body will be attracted with equal force in
every direction. It will therefore have no weight, though its mass is
unchanged. Gravity, of which weight is a measure, decreases with
increase of altitude.
Web (of a rib)—That vertical part of a rib which prevents it from bending
upwards. [37a]

Warp, to—To distort a surface in order to vary its angle of incidence. To
vary the angle of incidence of a controlling surface.
Wash—The disturbance of air produced by the flight of an aeroplane.
Wash-in—An increasing angle of incidence of a surface towards its wing-
tip. [38]
Wash-out—A decreasing angle of incidence of a surface towards its wing-
tip. [39]
Wing-tip—The right or left-hand extremity of a surface. [40]
Wire—A wire is, in Aeronautics, always known by the name of its
function.
Wire, Lift or Flying—A wire opposed to the direction of lift, and used to
prevent a surface from collapsing upward during flight. [41]
Wire, Anti-lift or Landing—A wire opposed to the direction of gravity,
and used to sustain a surface when it is at rest. [42]
Wire, Drift—A wire opposed to the direction of drift, and used to prevent a
surface from collapsing backwards during flight.
Wire, Anti-drift—A wire opposed to the tension of a drift wire, and used to
prevent such tension from distorting the framework. [44]
Wire, Incidence—A wire running from the top of an interplane strut to the
bottom of the interplane strut in front of or behind it. It maintains
the "stagger" and assists in maintaining the angle of incidence.
Sometimes termed "stagger wire." [45]
Wire, Bracing—Any wire holding together the framework of any part of
an aeroplane. It is not, however, usually applied to the wires
described above unless the function performed includes a function
additional to those described above. Thus, a lift wire, while strictly
speaking a bracing wire, is not usually described as one unless it
performs the additional function of bracing some well-defined part
such as the undercarriage. It will then be said to be an
"undercarriage bracing lift wire." It might, perhaps, be acting as a

drift wire also, in which case it will then be described as an
"undercarriage bracing lift-drift wire." It should always be stated
whether a bracing wire is (1) top, (2) bottom, (3) cross, or (4) side.
If a "side bracing wire," then it should be stated whether right- or
left-hand.
Wire, Internal Bracing—A bracing wire (usually drift or anti-drift) within
a surface.
Wire, Top Bracing—A bracing wire, approximately horizontal and situated
between the top longerons of fuselage, between top tail booms, or at
the top of similar construction. [46]
Wire, Bottom Bracing—Ditto, substituting "bottom" for "top." [47]
Wire, Side Bracing—A bracing wire crossing diagonally a side bay of
fuselage, tail boom bay, undercarriage side bay or centre-section
side bay. This term is not usually used with reference to incidence
wires, although they cross diagonally the side bays of the cell. It
should be stated whether right- or left-hand. [48]
Wire, Cross Bracing—A bracing wire, the position of which is diagonal
from right to left when viewing it from the front of an aeroplane.
[49]
Wire, Control Bracing—A wire preventing distortion of a controlling
surface. [50]
Wire, Control—A wire connecting a controlling surface with the pilot's
control lever, wheel, or rudder-bar. [51]
Wire, Aileron Gap—A wire connecting top and bottom ailerons. [52]
Wire, Aileron Balance—A wire connecting the right- and left-hand top
ailerons. Sometimes termed the "aileron compensating wire." [53]
Wire, Snaking—A wire, usually of soft metal, wound spirally or tied round
another wire, and attached at each end to the framework. Used to
prevent the wire round which it is "snaked" from becoming, in the
event of its displacement, entangled with the propeller.

Wire, Locking—A wire used to prevent a turnbuckle barrel or other fitting
from losing its adjustment.
Wing—Strictly speaking, a wing is one of the surfaces of an ornithopter.
The term is, however, often applied to the lifting surface of an
aeroplane when such surface is divided into two parts, one being the
left-hand "wing," and the other the right-hand "wing."
Wind-Tunnel—A large tube used for experimenting with surfaces and
models, and through which a current of air is made to flow by
artificial means.
Work—Force × displacement.
Wind-Screen—A small transparent screen mounted in front of the pilot to
protect his face from the air pressure.

Types of Aeroplanes.

Plate I.

The first machine to fly—of which there is anything like authentic record—
was the Ader "Avion," after which the more notable advances were made as
shown above.

Plate II.

The Henri Farman was the first widely used aeroplane. Above are shown
the chief steps in its development.

Plate III.

THE AVRO.—The aeroplane designed and built by Mr. A. V. Roe was the
first successful heavier-than-air flying machine built by a British subject.
Mr. Roe's progress may be followed in the picture, from his early "canard"
biplane, through various triplanes, with 35 J.A.P. and 35 h.p. Green engines,
to his successful tractor biplane with the same 35 h.p. Green, thence

through the "totally enclosed" biplane 1912, with 60 h.p. Green, to the
biplane 1913-14, with 80 h.p. Gnome.

Plate IV.

THE SOPWITH LAND-GOING BIPLANES.—The earliest was a pair of
Wright planes with a fuselage added. Next was the famous tractor with 80
h.p. Gnome. Then the "tabloid" of 1913, which set a completely new
fashion in aeroplane design. From this developed the Gordon-Bennett racer
shown over date 1914. The gun-carrier was produced about the same time,
and the later tractor biplane in a development of the famous 80 h.p. but with
100 h.p. monosoupape Gnome.

Plate V.

THE MAURICE FARMAN.—First, 1909, the 50-60 h.p. Renault and coil-
spring chassis. 1910, the same chassis with beginning of the characteristic
bent-up skids. 1911 appeared the huge French Military Trials 3-seater; also
the round-ended planes and tails and "Henry" type wheels. This developed,
1912, into the square-ended planes and upper tail, and long double-acting
ailerons of the British Military Trials. The 1913 type had two rectangular
tail-planes and better seating arrangements, known affectionately as the
"mechanical cow"; the same year came the first "shorthorn," with two tail-
planes and a low nacelle. This finally developed into the carefully
streamlined "shorthorn" with the raised nacelle and a single tail-plane.

Plate VI.

THE SHORT "PUSHERS."—In 1909 came the semi-Wright biplane, with
35 h.p. Green, on which Mr. Moore-Brabazon won the "Daily Mail's"
£1000 prize for the first mile flight on a circuit on a British aeroplane. Then
the first box-kite flown by Mr. Grace at Wolverhampton. Later the famous
"extension" type on which the first Naval officers learned to fly. Then the
"38" type with elevator on the nacelle, on which dozens of R.N.A.S. pilots
were taught.

Plate VII.

SHORT TRACTORS, 1911–1912.—They were all co-existent, but the first
was the "tractor-pusher" (bottom of picture). Then came the "twin-tractor

plus propeller" (at top). A development was the "triple-tractor" (on the
right), with two 50 h.p. Gnomes, one immediately behind the other under
the cowl, one driving the two chains, the other coupled direct. Later came
the single-engined 80 h.p. tractor (on the left), the original of the famous
Short seaplanes.

Plate VIII.

THE VICKERS MACHINES: First the Vickers-R.E.P. of 1911, which
developed into the full-bodied No. V. with R.E.P. engine, then the Military
Trials "sociable" with Viale engine, and so to the big No. VII with a 100
h.p. Gnome. Contemporary with the No. V and No. VI were a number of
school box-kites of ordinary Farman type, which developed into the curious
"pumpkin" sociable, and the early "gun 'bus" of 1913. Thence arrived the
gun-carrier with 100 h.p. monosoupape Gnome.

Plate IX.

THE BRISTOL AEROPLANES.—First, 1910, Farman type box-kites
familiar to all early pupils. Then the miniature Maurice-Farman type
biplane of the "Circuit of Britain." Contemporaneous was the "floating tail"
monoplane designed by Pierre Prier, and after it a similar machine with
fixed tail. Then came the handsome but unfortunate monoplane designed by
M. Coanda for the Military Trials, 1912.

Plate X.

THE BRISTOL TRACTORS.—Late 1912 came the round fuselaged
tractor, with Gnome engine, designed by Mr. Gordon England for Turkey.

1912-13 came the biplane built onto the Military Trials monoplane type
fuselage, also with a Gnome, designed by M. Coanda for Roumania. Then
the Renault-engined Coanda tractor 1913, followed by 80 h.p. Gnome-
engined scout, designed by Messrs. Barnwell and Busteed, which with
Gnomes, le Rhones and Clergets, has been one of the great successes.
Almost contemporary was the two-seater Bristol.

Plate XI.

THE MARTINSYDES.—1909, first experimental monoplane built with
small 4-cylinder engine. J.A.P.-engined machine, 1910, followed by the
Gnome-engined machine, 1911. 1912, first big monoplane with Antoinette
engine was built, followed by powerful Austro-Daimler monoplane, 1913.
Then came the little Gnome-engined scout biplanes, 1914, some with, some
without, skids.

Plate XII.

THE CURTISS BIPLANES.—In 1909 came the "June-bug," the united
product of Glen Curtiss, Dr. Graham Bell, and J. A. D. McCurdy. Then the
box-kite type, 1909, on which Mr. Curtiss won the Gordon-Bennett Race at
Reims. Next the "rear-elevator" pusher, 1912, followed by first tractor,
1913, with an outside flywheel. All purely Curtiss machines to that date had
independent ailerons intended to get away from Wright patents. Following
these came tractors with engines varying from 70 to 160 h.p., fitted with
varying types of chassis. All these have ordinary ailerons.

Plate XIII.

THE BLERIOT (1).—The first engine-driven machine was a "canard"
monoplane. Then came the curious tractor monoplanes 1908–1909, in order

shown. Famous "Type XI" was prototype of all Bleriot successes. "Type
XII" was never a great success, though the ancestor of the popular "parasol"
type. The big passenger carrier was a descendant of this type.

Plate XIV.

THE BLERIOT (2):—1910, "Type XI," on which Mr. Grahame-White won
Gordon-Bennett Race, with a 14-cylinder 100 h.p. Gnome. 1911 came the
improved "Type XI," with large and effective elevator flaps. On this type,
with a 50 h.p. Gnome, Lieut. de Conneau (M. Beaumont) won Paris-Rome
Race and "Circuit of Britain." Same year saw experimental "Limousine"
flown by M. Legagneux, and fast but dangerous "clipped-wing" Gordon-
Bennett racer with the fish-tail, flown by Mr. Hamel. About the same time
came the fish-tailed side-by-side two-seater, flown by Mr. Hamel at Hendon
and by M. Perreyon in 1912 Military Trials. 1911, M. Bleriot produced the
100 h.p. three-seater which killed M. Desparmets in French Military Trials.
1912-13, M. Bleriot produced a quite promising experimental biplane, and a
"monocoque" monoplane in which the passenger faced rearward.

Plate XV.

THE BLERIOT (3)—1912 tandem two-seater proved one of the best
machines of its day. 1913 "canard" lived up to its name. A "pusher"
monoplane was built in which the propeller revolved on the top tail boom.
This machine came to an untimely end, with the famous pilot, M. Perreyon.
1912 "tandem" was developed in 1914 into the type shown in centre; almost
simultaneously "parasol" tandem appeared. 1914, M. Bleriot built a
monoplane embodying a most valuable idea never fully developed. The
engine tanks and pilot were all inside an armoured casing. Behind them the
fuselage was a "monocoque" of three-ply wood bolted onto the armour. And
behind this all the tail surfaces were bolted on as a separate unit.

Plate XVI.

THE CAUDRON.—1910, came the machine with ailerons and a 28 h.p.
Anzani. 1911 this was altered to warp control and a "star" Anzani was
fitted. From this came the 35 h.p. type of 1912, one of the most successful
of school machines. Small fast monoplane, 1912, was never further
developed. 1913 appeared the familiar biplanes with 80 h.p. Gnomes, and
5-seater with 100 h.p. Anzani for French "Circuit of Anjou." 1914 produced
the "scout" biplane which won at Vienna. 1915 appeared the twin-engined
type, the first successful "battle-plane."

Plate XVII.

THE DEPERDUSSIN.—In 1911 the little monoplane with a Gyp. engine.
Then the Gnome-engined machine of the "Circuit of Europe." In 1912 came
the Navy's machine with 70 h.p. Gnome, and Prevost's Gordon-Bennett
"Bullet," 135 miles in the hour. The last was the British-built "Thunder-
Bug," familiar at Hendon.

Plate XVIII.

THE BREGUET.—First to fly was the complicated but business-like
machine of 1909. Then came the record passenger carrier, 1910 (which
lifted 8 passengers). 1911 the French Military Trials machine with geared-
down 100 h.p. Gnome appeared. 1912 produced the machine with 130 h.p.
Salmson engine on which the late Mr. Moorhouse flew the Channel with
Mrs. Moorhouse and Mr. Ledeboer as passengers; also the machine with
130 h.p. horizontal Salmson, known as the "Whitebait." The last before the
war was the rigid wing machine with 200 h.p. Salmson.

Plate XIX.

THE CODY.—First the Military Experiment of 1908, with an Antoinette
engine, then improved type 1909 with a Green engine. Next the
"Cathedral," 1910, with a Green engine, which won Michelin Prize. In 1911
"Daily Mail" Circuit machine, also with a Green, won the Michelin. This
was modified into 1912 type which won Military Competition and £5,000
in prizes, with an Austro-Daimler engine, and later the Michelin Circuit
Prize, again with a Green. 1912 the only Cody Monoplane was built. 1913 a
modified biplane on which the great pioneer was killed.

Plate XX.

THE NIEUPORT.—The first Nieuport of 1909 was curiously like a
monoplane version of a Caudron. In 1910 came the little two-cylinder
machine with fixed tail-plane and universally jointed tail. In 1911 the
French Trials machine was built with 100 h.p. 14 cylinder Gnome, and is

typical of this make. Also the little two-cylinder record breaker. A
modification of 1913 was the height record machine of the late M.
Legagneux.

Plate XXI.

THE R.E.P. MONOPLANES.—First came the curious and highly
interesting experiments of 1907, 1908, 1909, and 1910. 1910–1911, the
World's Distance Record breaker was produced; after it, the "European
Circuit," all with R.E.P. engines. In 1913-14 came the French military type
with Gnome engine and finally the "parasol," 1915.

Plate XXII.

THE MORANE: First the European Circuit and Paris-Madrid type. Then
the 1912 types, with taper wing and modern type wing. The 1913 types, the
"clipped wing," flown by the late Mr. Hamel, one of the standard tandem
types now in use. About the same time came the "parasol." 1914-15 came a
little biplane like a Nieuport, and the "destroyer" type with a round section
body, flown by Vedrines.

Plate XXIII.

THE VOISIN.—1908, the first properly controlled flight on a European
aeroplane was made on a Voisin of the type shown with fixed engine. Then
followed the record breaker of 1909 with a Gnome engine. In 1909 also the
only Voisin tractor was produced. 1910 the Paris-Bordeaux type was built;
1911 the amphibious "canard" and the "military" type with extensions, and
the type without an elevator. 1913 came the type with only two tail-booms
and a geared-down engine, which developed into the big "gun" machine
with a Salmson engine.

Plate XXIV.

THE HANRIOT AND PONNIER MONOPLANES.—In 1909 came the
first Hanriot with 50 h.p. 6-cylinder Buchet engine, and in 1910 the famous
"Henrietta" type with E.N.Vs. and stationary Clergets. 1911 came the
Clerget two-seater entered in French Military Trials, and 1912 the 100 h.p.
Hanriot-Pagny monoplane which took part in British Military Trials. Sister
machines of the same year were the single seater with 50 h.p. Gnome and
the 100 h.p. Gnome racer with stripped chassis. In 1913 the Ponnier-Pagny
racing monoplane with 160 h.p. Le Rhone competed in the Gordon-Bennett
race, doing about 130 miles in the hour. The 60 h.p. Ponnier biplane was the
first successful French scout tractor biplane.

Plate XXV.

THE WRIGHT BIPLANE.—The first power flights were made, 1903, on a
converted glider fitted with 16 h.p. motor. The prone position of the pilot

will be noted. By 1907 the machine had become reasonably practical with
40 h.p. motor. On this the first real flying in the world was done. In 1910
the miniature racing Wright was produced; also the type with a rear elevator
in addition to one in front. Soon afterwards the front elevator disappeared,
and the machine became the standard American exhibition and school
machine for four years. In 1915 a machine with enclosed fuselage was
produced.

Plate XXVI.

THE BLACKBURN MONOPLANES.—In 1909 was built the curious
four-wheeled parasol-type machine with 35 h.p. Green engine and chain
transmission, on which flying was done at Saltburn. In 1911 the Isaacson-
engined machine was built, together with a 50 h.p. Gnome single-seater on
which Mr. Hucks started in the Circuit of Britain race. In 1912 another 50
h.p. single-seater was built on which a good deal of school work was done.
A more advanced machine appeared in 1913 and a two-seater with 80 h.p.
Gnome did a great deal of cross-country work in 1913-14.

Plate XXVII.

In 1908 the first Antoinette monoplane was produced by MM. Gastambide
and Mengin. Then followed a machine with central skids, a single wheel,
and wing skids. In 1909 came the machine with four-wheeled chassis and
ailerons and later an improved edition which reverted to the central skid
idea. On this M. Latham made his first cross-channel attempt. The next
machine shed the wing skids and widened its wheelbase. During 1910-11
the ailerons vanished, warp control was adopted and the king-post system
of wing-bracing was used. In 1911 the curious machine with streamlined
"pantalette" chassis, totally enclosed body and internal wing-bracing, was
produced for French Military Trials. In 1912 the three-wheeled machine
was used to a certain extent in the French Army. Then the type disappeared.

Plate XXVIII.

In 1908 and 1909 detached experimental machines in various countries
attained a certain success. The late Capt. Ferber made a primitive tractor
biplane 1908. The Odier-Vendome biplane was a curious bat-winged pusher
biplane built 1909. The tailless Etrich monoplane, built in Austria, 1908,
was an adaptation of the Zanonia leaf. M. Santos-Dumont made primitive
parasol type monoplanes known as "Demoiselles," in which bamboo was
largely used. 1909 type is seen above. A curious steel monoplane was built
by the late John Moisant, 1909. The twin-pusher biplane, built by the
Barnwell Bros. in Scotland, made one or two straight flights in 1909. The
Clement-Bayard Co. in France constructed in 1909 a biplane which did
fairly well. Hans Grade, the first German to fly, made his early efforts on a
"Demoiselle" type machine, 1908.

Plate XXIX.

In 1910 a number of novel machines were produced. The Avis with Anzani
engine was flown by the Hon. Alan Boyle. Note the cruciform universally
jointed tail. The Goupy with 50 h.p. Gnome was an early French tractor,
notable for its hinging wing-tips. The Farman was a curious "knock-up"
job, chiefly composed of standard box-kite fittings. The Sommer with 50
h.p. Gnome was a development of the box-kite with a shock-breaking
chassis. The Savary, also French, was one of the first twin tractors to fly.
The model illustrated had an E.N.V. engine. Note position of the rudders on
the wing tips. The Austrian Etrich was the first successful machine of the
Taube class ever built.

Plate XXX.

INTERESTING MACHINES, 1910.—The Werner monoplane with E.N.V.
engine, combined shaft and chain drive, was a variant of the de Pischoff.
The Macfie biplane was a conventional biplane with 50 h.p. Gnome and
useful originalities. The Valkyrie monoplane, another British machine, was
a "canard" monoplane with propeller behind the pilot and in front of main
plane. The Weiss monoplane was a good British effort at inherent stability.
The Tellier monoplane was a modified Bleriot with Antoinette proportions.
The Howard Wright biplane was a pusher with large lifting monoplane tail.
The Dunne biplane was another British attempt at inherent stability. The
Jezzi biplane was an amateur built twin-propeller.

Plate XXXI.

SOME INTERESTING MACHINES, 1911.—The Compton-Paterson
biplane was very similar to the early Curtiss pusher; it had a 50 h.p. Gnome.
The Sloan bicurve was a French attempt at inherent stability with 50 h.p.
Gnome and tractor screw. The Paulhan biplane was an attempt at a machine
for military purposes to fold up readily for transport. The Sanders was a
British biplane intended for rough service. The Barnwell monoplane was
the first Scottish machine to fly; it had a horizontally opposed Scottish
engine. The Harlan monoplane was an early German effort; note position of
petrol tank.

Plate XXXII.

The Clement-Bayard monoplane, 1911, was convertible into a tractor
biplane. The standard engine was a 50 h.p. Gnome. The machine was
interesting, but never did much. The Zodiac was one of the earliest to
employ staggered wings. With 50 h.p. Gnome engine it was badly
underpowered, so never did itself justice. The Jezzi tractor biplane, 1911,
was a development of an earlier model built entirely by Mr. Jezzi, an
amateur constructor. With a low-powered J.A.P. engine it developed an
amazing turn of speed, and it may be regarded as a forerunner of the scout
type and the properly streamlined aeroplane. The Paulhan-Tatin monoplane,
1911, was a brilliant attempt at high speed for low power; it presented
certain advantages as a scout. A 50 h.p. Gnome, fitted behind the pilot's seat
in the streamlined fuselage, was cooled through louvres. The propeller at
the end of the tail was connected with the engine by a flexible coupling.
This machine was, in its day, the fastest for its power in the world, doing 80

miles per hour. Viking 1 was a twin tractor biplane driven by a 50 h.p.
Gnome engine through chains. It was built by the author at Hendon in 1912.

Plate XXXIII.

Much ingenuity was exerted by the French designers in 1911 to produce
machines for the Military Trials. Among them was the 100 h.p. Gnome-
Borel monoplane with a four-wheeled chassis, and the Astra triplane with a
75 h.p. Renault engine. This last had a surface of about 500 square feet and
presented considerable possibilities. Its principal feature was its enormous
wheels with large size tyres as an attempt to solve difficulties of the severe
landing tests. The Clement-Bayard biplane was a further development of
the Clement-Bayard monoplane; the type represented could be converted
into a monoplane at will. The Lohner Arrow biplane with the Daimler
engine was an early German tractor biplane built with a view to inherent
stability, and proved very successful. The Pivot monoplane was of
somewhat unconventional French construction, chiefly notable for the
special spring chassis and pivoted ailerons at the main planes; this pivoting
had nothing to do with the name of the machine, which was designed by M.
Pivot.

Plate XXXIV.

The Flanders monoplane, 1912, with 70 h.p. Renault engine, was one of the
last fitted with king-post system of wing bracing. The Flanders biplane
entered for British Military Trials. Notable features: the highly staggered
planes, extremely low chassis and deep fuselage. Also, the upper plane was
bigger in every dimension than the lower; about the first instance of this
practice. The Bristol biplane, with 100 h.p. Gnome engine, was also entered
for the Trials, but ultimately withdrawn. The Mars monoplane, later known
as the "D.F.W.," was a successful machine of Taube type with 120 h.p.
Austro-Daimler engine. The building of the engine into a cowl, complete
with radiator in front, followed car practice very closely. The tail of the
monoplane had a flexible trailing edge; its angle of incidence could be
varied from the pilot's seat, so that perfect longitudinal balance was attained
at all loadings and speeds. The Handley-Page monoplane, with 70 h.p.
Gnome engine, was an early successful British attempt at inherent stability.

Plate XXXV.

The Sommer monoplane, with 50 h.p. Gnome, was a 1911-12 machine; it
did a good deal of cross-country flying. The Vendome monoplane of 1912,
also with 50 h.p. Gnome engine, was notable chiefly for its large wheels
and jointed fuselage, which enabled the machine to be taken down for
transport. The Savary biplane took part in the French Military Trials, 1911.
It had a four-cylinder Labor aviation motor. Notable features are twin chain-
driven propellers, rudders between the main planes, the broad wheel-base
and the position of the pilot. The Paulhan triplane, which also figured in the
French Military Trials, was a development of the Paulhan folding biplane. It
had a 70 h.p. Renault engine. For practical purposes it was a failure. The
R.E.P. biplane, with 60 h.p. R.E.P. engine, was a development of the famous
R.E.P. monoplanes. Its spring chassis, with sliding joints, marked an
advance. Like the monoplanes, it was built largely of steel.

Plate XXXVI.

In 1912 came the first really successful Handley-Page monoplane, with 50
h.p. Gnome engine. The Short monoplane, was built generally on Bleriot
lines. Its chassis was an original feature. The Coventry Ordnance biplane
was a two-seater tractor built for the British Military Trials. It had a 100 h.p.
14-cylinder Gnome engine, with propeller geared down through a chain
drive. The machine was an interesting experiment, but not an unqualified
success. The Moreau "Aerostable," fitted with a 50 h.p. Gnome, was a
French attempt to obtain automatic stability, but it only operated
longitudinally. The pilot's nacelle was pivoted under the main planes, wires
were attached to the control members so that the movements of the nacelle
in its efforts to keep a level keel brought them into operation. The Mersey
monoplane, an entrant for the British Military Trials, was designed to
present a clear field of view and fire. The 45 h.p. Isaacson engine was
connected by a shaft to a propeller mounted behind the nacelle on the top
tail boom. It was a promising experiment, but came to grief. The Radley-
Moorhouse monoplane was a sporting type machine on Bleriot lines, with
50 h.p. Gnome engine. It was notable for its streamlined body and disc
wheels.

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