Why does an aeroplane fly? The question is worthy of close examination. There is one common enemy to aeroplanes—the force of gravity. Were it not for the existence of this force, which, as Newton put it, “is unseen and unheard and yet dominates the universe,” the problem of the aeroplane would have been solved years ago.

Most readers have handled the toy kite, and since the principles governing the flight of a kite are precisely the same as those which apply to the aeroplane, the latter will be the more readily understood if the principles are explained through this medium. Full-size aeroplanes to which certain models approximate are shown in Fig. 1.

If a kite is launched in a wind it speedily attains a certain height or altitude, at which it remains so long as the wind does not drop. The wind is overcoming gravity, which constantly endeavours to bring the kite to earth, and hence, since the kite remains in the air, the forces acting on the kite are said to be in equilibrium—that is, balanced. The forces are shown diagrammatically in Fig. 1A, and include gravity, which is practically constant and remains unaltered under all conditions, the air pressure which, when sufficiently intense, lifts the kite against the action of gravity, and the pull of the string. The air pressure is really a combination of two forces—lift and drift. The drift or resistance tends to move the kite in the direction of the wind, and lift to raise the kite in opposition to gravity. Since, therefore, drift is an undesirable factor, the resistance of the machine must be made as low as possible, as it absorbs power, as will clearly be seen. If the velocity of the wind drops, the kite drops also, increasing its angle with the horizon, thereby causing it to capture and force down more air until equilibrium is again restored. If the string of a kite breaks, the balance of the forces is destroyed, drift and gravity taking command and so bringing the kite to earth.

If it takes a wind of fifteen miles an hour to lift a kite, similarly it would lift to exactly the same elevation if the holder of the kite-string commenced to run at a rate of fifteen miles per hour in calm air.

Now, an aeroplane is merely a kite with a mechanical arrangement (the engine and propeller) which supplies the motion necessary to fly it, and eliminates the necessity for a wind. This statement can easily be followed. In the aforementioned parallel it was seen that it was immaterial whether the kite-flyer was standing still with the wind moving at fifteen miles per hour, or whether he was moving at the rate of fifteen miles per hour in still air. The result in each case is the same—the kite flies.

It has been stated that if the kite-string fractured the kite would fall to the ground. If, however, it were possible at the moment of rupture to attach a weightless engine and air-screw to the kite capable of exerting a forward push equal to the drift, the kite would still remain in the air.

Again, if the wind were suddenly to stop, and the engine and air-screw were capable of moving the kite forward at the same rate at which the wind was blowing, the kite would fly, and in all important respects would constitute an aeroplane.

The kite, it will be assumed, requires a minimum speed of fifteen miles per hour in order to sustain itself. If the wind be blowing at fifteen miles an hour the operator can remain stationary. If it blows at ten miles an hour he must run at five miles an hour against the wind. If it blows at five miles an hour he must run at ten miles an hour against the wind, or twenty miles per hour with the wind to maintain the kite.

Hence an aeroplane really has two speeds—its speed relative to the earth and its air speed. The former is the rate of which it would travel a given distance, and the latter is the sum of the speed relative to the earth and the velocity of the wind.

It can readily be seen that an aeroplane travelling at ten miles an hour relative to the earth against a fifteen-mile-an-hour wind has really an air speed of twenty-five miles an hour. When the aeroplane, however, is travelling with the wind, the air speed is the speed relative to the earth minus the velocity of the wind.

It is also convenient to draw a parallel between the ship and the aeroplane. The weight of a ship must equal the weight of water it displaces in order to float. Similarly an aeroplane, by its motion through the air, must deflect a volume of air equal at least to its own weight. The aeroplane then would just lift itself from the ground; and the more air it deflects the higher does it ascend.

Now, if a 1-lb. weight be laid on a table, the table presses against the weight with a force of 1 lb. If the hand is pressed against the wall, the wall presses back with an equal pressure. If a person fires a revolver, the force of explosion tends to force the revolver and the person in the opposite direction to the travel of the bullet. These are merely illustrations of the law that action and reaction are equal and opposite. It is in reality due to this law that the aeroplane can resist gravity.

Fig. 2 represents an end view of a kite—or, for that matter, of an aeroplane. The arrows indicate the direction of motion of the wind. Upon contact with the kite the air has a downward action, and the consequent reaction lifts the kite. Hence the motion of an aeroplane through the air causes a pressure on the latter, and the resultant is what is termed lift.

So far, then, the reason why an aeroplane lifts has been dealt with. Further considerations have to be dealt with after the machine has left the ground. In technical language these could be summarised into a single sentence—that is, the centres of pressure and gravity must be made to coincide, and the machine must also be stable in both lateral and longitudinal directions.

An ordinary paper glider, cut from a stiff sheet of cartridge paper, will serve admirably to demonstrate this statement, which at first sight will convey as much to the reader as Choctaw or other remote language.

Cut the paper to the dimensions given in Fig. 3 and make sure that it is flat, by pressing between the leaves of a book. Then project it horizontally into the air. It does not attain gliding motion. It performs a series of evolutions, too quickly for the eye to perceive; but what happens is this. After launching, the front edge turns up and the sheet glides back. Now the back edge turns up and the glider dives forward. Again the front edge turns up, the glider slides back, the back edge turns up, it glides forward, and so on until the glider reaches the ground. Now fix a couple of small brass paper-fasteners in the front edge (the correct number of fasteners can however only be found by experiment, but two will usually be sufficient for the size of glider indicated), and launch the glider again. It will be noticed that it glides steadily at a small angle to the ground.

The explanation of this phenomenon is simple. When it was launched in the first place, the centre of gravity of the plane lay along a line running through the geometrical centre, parallel with the front edge, and the glider merely rocked or oscillated about this axis. The centre of pressure of the surface would be approximately in the position shown in the illustration. When the correct number of paper-fasteners, however, are fixed, the centre of gravity is moved forward to a position coincident with the centre of pressure, the result being that the glider came to earth in steadiness and poise. But, even though it is now balanced, it will still show a tendency to rock sidewise or laterally, and if the wings are bowed up to the dihedral angle shown in Fig. 4, the rock will be eliminated, and the machine is said to be laterally stable. Either of the dihedral angles may be used, although B is much to be preferred.

What of stability in a longitudinal direction? Just as important this, but not quite so easy to obtain.

Fig. 5 is a side elevation of two surfaces fixed to a spar, and shows how stability is obtained longitudinally. The surfaces of the elevator or tail, according to whether the machine is “canard” or tractor (canard being the term for propeller-behind or “pusher” machines), is placed at a positive angle with the horizon. The correct angle can, of course, only be found by experiment.

Now, from the foregoing certain laws can be deduced. Firstly, in order to be stable longitudinally, the centre of pressure must be kept as near to the centre of gravity as possible, and secondly, the main surface of the aeroplane must be inclined to preserve lateral stability. With full-size aeroplanes there are, however, several exceptions to this rule, as the faster a machine travels the more stable does it become, and hence the dihedral angle is really unnecessary.

It may be well at this point to describe the action of a plane. Strictly speaking, the terms “plane” and “aeroplane” are misnomers, since no full-size machine has surfaces which even approximate to planes.

The reason why a perfectly flat plane is never used on full-size aeroplanes will be followed from Fig. 6, which shows the flow of air over an inclined plane, the term “plane” being used here in its technical sense.

It will be noticed that a region of “dead air” or partial vacuum is caused, which seriously affects the lift of the plane. Fig. 7 shows the flow of air over a cambered aerofoil (or to use the popular colloquialism “plane”). Less disturbance occurs in this instance, the air following very approximately the contour of the surface. It has been proved by test in the Wind Tunnel at the National Physical Laboratory at Teddington that an efficiently-designed aerofoil section has a lift two-thirds greater than a true plane. For a similar reason all struts or aeroplanes are “streamlined,” as shown by Fig. 8. The air flow, it will be seen, is less disturbed than by a square strut (Fig. 9).

Fig. 10 shows the air flow round a square-ended and taper-ended plane respectively. It will be noticed that the air has a tendency to leak over the end of the square plane, which is obviated by the tapered wing.

It may be thought that such details as these are unimportant; but when it is remembered that an aeroplane, correctly streamlined, will fly for one-half the power required to fly a machine not so designed, the enormous saving in power will be manifest.

The actual thrust required to lift a model aeroplane is roughly equal to a quarter of its total weight. Thus a model weighing 6 oz. will require 1½-oz. thrust.