CHAPTER I. FLIGHT

Produced by Charles Keller

THE AEROPLANE SPEAKS

By H. Barber

(Captain, Royal Flying Corps)

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

PART

I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES

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

III. THE GREAT TEST

IV. CROSS COUNTRY

CHAPTER

I. FLIGHT

II. STABILITY AND CONTROL

III. RIGGING

IV. PROPELLERS

V. MAINTENANCE

TYPES OF AEROPLANES

GLOSSARY

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 the illustration you see in the frontispiece.

“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.

“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 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 of

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 L400 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.

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 Slip[1] 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

counterbalance 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 helicopters[2] 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.”

“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.”

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”

“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 roles 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 its vertical axis, and the

Elevator is a Controlling Surface designed to turn the Aeroplane about

its lateral 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:

“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 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.”

“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.”

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 “streamlined” 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.'s[8] 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

Pilot Air Speed Indicator, and, adjusting it to zero, smiles as he hears

the Pilot-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 Pilot 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--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.

“Oh 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

Curve[10] 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 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, clowned 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

pitch[11] 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 diarrhoea, 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 lowest 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.

PART IV. 'CROSS COUNTRY

The Aeroplane had been designed and built, and tested in the air, and

now 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 to 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 jolly 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, “Tank 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.

“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.

“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.

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 the 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 the 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 town it appears,

nestling down between the hills with 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 which 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 manoeuvre

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. High tails

mean 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:

“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,” said someone, “and that they haven't had

difficulties with the fog. It rolled up very quickly, you know.”

“Never fear,” remarked 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,” said

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 whom 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 walked 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 laws[14] and resists movement. It is that

resistance or reaction which makes flight possible.

Flight is secured by driving through the air a surface[15] 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. 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 criticised 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 V(2S).

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 approximately at right-angles to the

chord 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 roughnesses 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 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.

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 streamline 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

thicknesses 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 greater the

aspect ratio, the greater the reaction. It is obvious, I think, that the

greater the span, the greater the mass of air engaged, and, as already

explained, the reaction is partly the result of the mass of air engaged.

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 greater 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 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.

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 requisit 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.

SUMMARY.

Essentials for Maximum Essentials for Maximum

Climb. Velocity

1. Low velocity. High velocity.

2. Large surface. Small surface.

3. Large angle relative to Small angle relative to

propeller thrust. propeller thrust.

4. Large angle relative to Small angle relative to direction

direction of motion. of motion.

5. Large camber. Small camber.

It is mechanically impossible to construct an aeroplane of reasonable

weight of which it would be possible to very 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.

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.

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.