Twilight falls, and we turn our steps towards home, but even as we do so comes upon the wind a faint but ever-increasing murmur, a deep and steady hum growing to a roar, that resembles the noise of a powerful motor-car, and yet is unmistakably different. There is a cry of ' An aeroplane !' and the strollers on the common turn their faces eagerly upwards as there swoops across the sky, proudly and steadily, with infinite dignity of bearing and grace of swift unswerving flight, a large biplane flying , high ; appearing first as a couple of parallel lines against the blue, and then as it nears, revealing its tapering body and full spread of white wings—a thrilling and beautiful sight that familiarity never robs of its charm.
In one short hour we have seen three examples, in progressive sequence, of the flying machine. Kites have been flown from time immemorial, elastic driven models were introduced by Penaud forty years ago, aeroplanes are the invention of to-day, but the fundamental principle is the same in all three: the light supporting surface set at an angle against the force of the wind. Only whereas in the kite it is the wind of heaven that supports the plane, and the pull of the string that keeps it at the right angle, in the power-driven machine it is construction and human skill that maintain the angle against the self-made wind of the swift onward motion.
The requisite angle, known as the angle of incidence, is a matter of prime importance. Naturally the more you tilt your plane upwards the more resistance it will offer to the air, and the more power you will need to drive it along. This air resistance is exerted horizontally, and the aeroplane constructor has a sort of slang term for it: he calls it 'drift,' in contradistinction to ' lift,' the force that would raise the plane vertically upwards. As we have seen, a plane set at an angle against the wind has a tendency to 'lift' as well as 'drift,' and as a compromise between the two goes slantwise up into the sky. The more ' lift' it has and the less ' drift' the steeper will be the angle at which it rises. There is both lift and drift in the flight of every aeroplane ; but the aeroplane designer aims at obtaining the least possible amount of drift to the greatest amount of lift in his machine, and he does this, in the first place, by setting his planes at a very small angle to the horizon.
But he does more than this: he makes his planes of a particular shape. It was Sir George Cayley, as we have seen, more than a hundred years ago, who first pointed out, from the study of the wings of a bird, that more lifting power could be obtained from an inclined plane by making it not flat but arched, with its front edge curved downwards, or as we now say, 'cambered,' Later investigation has proved this point to be of the very vastest importance; in fact it is certain that but for the discovery of the properties of the ' cambered plane ' we should never have risen into the air at all. The history of the discovery of flight supplies many useful parables on the necessity for the theoretical man and the practical man to work together in the paths of progress. The mathematician, with pencil and paper, demonstrates irrefutably, from the laws of motion, that flight is impossible because more power is required to raise the planes than could ever be profitably employed. (According to Newton's laws alone a swallow would need the strength of a man to move at the speed it actually attains.) Meantime a Lilienthal or an Orville Wright builds himself a glider, practises with it down a hill, and presently proves that flight is already within man's grasp, because by simply bending his planes he gets a lifting power out of them at which the theorist could not even guess.
Exactly why a cambered plane lifts better than a flat one is only just beginning to be understood, and is still matter of experiment and discussion. It is due entirely to the behaviour of the air. We have all of us noticed the infinitely complex eddies and currents set up in water by passing objects—the blade of an oar or the hull of a boat. We have all watched the broad path of churning waves that marks the trail of a steamer across the sea. We ignore, because we cannot see them, the currents and eddies that passing objects are for ever causing in the air, vastly more complicated because air is so vastly more unstable. But if these water eddies are sufficiently important to have to be carefully reckoned with, as we know is the case, in designing the hull of a battleship, we can guess what great effect the air eddies must exercise upon a light aeroplane in swift motion aloft, and how all-important it is to understand their action.
In making diagrams and pictures which are to represent the flow of currents of air or water, it is customary to indicate them by drawing a number of long parallel lines in the direction in which the current is flowing. In similar fashion we may consider, for convenience sake, that water or air is composed of an infinite number of minute threads of fluid arranged regularly side by side ; and for these hypothetical threads there is a recognized term —the 'stream-lines,' It is the breaking up of these stream-lines which causes eddies and resistance when an object is moved through them. To reduce the resistance to a moving body as far as possible it is necessary to make the body of such a shape that the stream-lines—either of air or water—flow round it smoothly and unbrokenly, and the correct form of this much-to-be-desired 'stream-line shape' has been the subject of endless experiment. One result has been to prove that Nature, as usual, has been ahead of us, for in the end, after all his experiments, the designer finds he cannot do better than shape his torpedo like a fish, while every year sees the form of the body of a monoplane more nearly resembling that of a bird. A stream-line shape travels blunt end foremost, and tapers smoothly off towards the tail. This is speaking broadly. Latest researches have revealed the subject as infinitely more complex even than first supposed, and it would appear as if correct stream-line shape varies with the speed at which the object travels. There is still much to be learned in this direction, and the subject becomes yet more difficult and complex when we consider the motion of the stream-lines of air about a cambered plane driven rapidly forward, and the effect of the eddies thus set up.