The Aeroplane Speaks Part 6

"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 ma.s.ses. 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 s.p.a.ce.

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 pa.s.sing over a sh.o.r.e 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 pa.s.sed over the sh.o.r.e 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 s.p.a.ce and of one's littleness there--the realization of the Power moving the mult.i.tudes below--the exaltation of spirit alt.i.tude produces--the joy of speed. A new world of sensation!

Now the bay is almost crossed and the Aerodrome at B. can be distinguished....

On the Aerodrome is a little crowd waiting and watching for the arrival of the Aeroplane, for it is of a new and improved type and its first 'cross-country performance is of keen interest to these men; men who really know something about flight.

There is the Squadron Commander who has done some real flying in his time; several well-seasoned Flight-Commanders; a dozen or more Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air Mechanics, and, a little on one side and almost unnoticed, the Designer.

"I hope they are all right," says someone, "and that they haven't had difficulties with the fog. It rolled up very quickly, you know."

"Never fear," remarks a Flight-Commander. "I know the Pilot well and he's a good 'un; far too good to carry on into a fog."

"They say the machine is really something out of the ordinary," says another, "and that, for once, the Designer has been allowed full play; that he hasn't been forced to unduly standardize ribs, spars, struts, etc., and has more or less had his own way. I wonder who he is. It seems strange we hear so little of him."

"Ah! my boy. You do a bit more flying and you'll discover that things are not always as they appear from a distance!"

"There she is, sir!" cries the Flight-Sergeant. "Just a speck over the silvery corner of that cloud."

A tiny speck it looks, some six miles distant and three thousand feet high; but, racing along, it rapidly appears larger and soon its outlines can be traced and the sunlight be seen playing upon the whirling propeller.

Now the distant drone of the engine can be heard, but not for long, for suddenly it ceases and, the nose of the Aeroplane sinking, the craft commences gliding downwards.

"Surely too far away," says a subaltern. "It will be a wonderful machine if, from that distance and height, it can glide into the Aerodrome."

And more than one express the opinion that it cannot be done; but the Designer smiles to himself, yet with a little anxiety, for his reputation is at stake, and Efficiency, the main reward he desires, is perhaps, or perhaps not, at last within his grasp!

Swiftly the machine glides downwards towards them, and it can now be seen how surprisingly little it is affected by the rough weather and gusts; so much so that a little chorus of approval is heard.

"Jolly good gliding angle," says someone; and another, "Beautifully quick controls, what?" and from yet another, "By Jove! The Pilot must be sure of the machine. Look, he's stopped the engine entirely."

Then the Aeroplane with noiseless engine glides over the boundary of the Aerodrome, and, with just a soft soughing sound from the air it cleaves, lands gently not fifty yards from the onlookers.

"Glad to see you," says the Squadron Commander to the Pilot. "How do you like the machine?" And the Pilot replies:

"I never want a better one, sir. It almost flies itself!"

And the Designer turns his face homewards and towards his beloved drawing-office; well satisfied, but still dreaming dreams of the future and ... looking far ahead who should he see but Efficiency at last coming towards him! And to him she is all things. In her hair is the morning sunshine; her eyes hold the blue of the sky, and on her cheeks is the pearly tint of the clouds as seen from above. The pa.s.sion of speed, the lure of s.p.a.ce, the sense of power, and the wonder of the future ... all these things she holds for him.

"Ah!" he cries. "You'll never leave me now, when at last there is no one between us?"

And Efficiency, smiling and blushing, but practical as ever, says:

"And you will never throw those Compromises in my face?"

"My dear, I love you for them! Haven't they been my life ever since I began striving for you ten long years ago?"

And so they walk off very happily, arm-in-arm together; and if this hasn't bored you and you'd like some more of the same sort of thing, I'd just love to tell you some day of the wonderful things they accomplish together, and of what they dream the future holds in store.

[Ill.u.s.tration]

_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.

[Ill.u.s.tration]

S = Side view of surface.

M = Direction of motion.

CHORD.--The Chord is, for practical purposes, taken to be a straight line from the leading edge of the surface to its trailing edge.

N = A line through the surface starting from its trailing edge. The position of this line, which I call the _Neutral Lift Line_, is found by means of wind-tunnel research, and it varies with differences in the camber (curvature) of surfaces. In order to secure flight, the inclination of the surface must be such that the neutral lift line makes an angle with and _above_ the line of motion. If it is coincident with M, there is no lift. If it makes an angle with M and _below_ it, then there is a pressure tending to force the surface down.

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

The surface acts upon the air in the following manner:

[Ill.u.s.tration]

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 a.s.sisting 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 ma.s.s 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^2.