The Boys' Book of Model Aeroplanes / How to Build and Fly Them: With the Story of the Evolution of the Flying Machine

by Francis A. Collins

The action of the air currents had first to be carefully studied before flight became possible. Although the air is invisible we now know exactly how the air currents act upon the wings or planes. When a plane surface, such as the wing of an aëroplane, moves horizontally through the air, the air is caught for a moment underneath it and is pressed down slightly and a moment later slips out again from under the other edges at the sides and back. It is this air under pressure which yields a slight support.

It has been proven by many experiments that this supporting power varies with the shape of the plane or surface driven horizontally through the air. A long narrow surface driven sideways gains much more support from the air than the same area in the form of a square or any other shape. In other words, a square surface ten feet square containing 100 square feet will not travel as far as a surface twenty feet long and five feet wide.

The explanation is very simple. As the square surface moves along, the air is momentarily compressed under the front edge, but instantly slips off at the back and sides. As the broad surface of the rectangular plane cuts the air, however, few of the air currents can escape at the sides while the most of them are crowded together and held in place until they slip off at the back. The supporting power of the plane is therefore in direct proportion to the length of the front or, as it is called, the entering edge of the plane.

Here we find one of the secrets of the flight of birds. The spread between the tips of their outstretched wings is much greater than the width of the wings themselves. It also explains why the Wright model, for instance, should be so oddly shaped and should move sideways like a crab. If you study the models of the successful monoplanes with this in mind they have a new meaning. The law of the proportion of the entering edge is very important in designing an aëroplane.

It is so important for the air to be confined as long as possible beneath the gliding plane that many devices have been tried to hold it. Some planes are built with a slight edge running around the sides and back, on the under surface, to hem in the air. Some of the biplanes are built with closed sides, the cellular form they are called, to keep the air from slipping away. The box kite is constructed with this in view. The builder of model aëroplanes will find, however, that the slight edge formed by turning the cloth over the frame of the plane is sufficient to hold the air.

The flight of a kite, by the way, appears a very simple matter once this law is understood. The air currents strike the kite at an angle and are deflected or carrom off at exactly the same angle. A line drawn through the middle of this angle, exactly bisecting it, will give you the direction of the force exerted by the wind. Meanwhile the kite string holds the plane rigidly in position. As the kite darts from side to side it is merely obeying this law and adjusting itself so that its surface will stand at right angles to this thrust of the wind. An aëroplane is simply a kite which makes its own wind or air currents.

The kite is, of course, balanced against the wind currents and kept more or less stable by its cord, but an aëroplane must balance itself. The secret of insuring stability was discovered only after years of experience with gliders in actual flights.

The stability of the aëroplane depends upon the proper adjustment of the pressure of the air on the machine. There is, of course, a center of pressure, just as there is a center of gravity in every aëroplane of whatever form or size. It may be laid down as a general rule that a plane traveling horizontally in a quiet atmosphere is kept horizontal and stable by making the centers of pressure and gravity coincide.

The air currents, as we pointed out, are never entirely at rest but are constantly tilting the plane about. Hold a sheet of stiff paper horizontally and let it fall. It will flutter to the ground or perhaps be twirled away, indicating the presence of a number of unexpected air currents. The aëroplane which would remain stable in a perfectly quiet atmosphere must overcome all these twists and turns. The problem of stability has not yet of course been solved. Having reached this stage in the evolution of the aëroplane the aviator next began to experiment by bending his wings or planes and throwing out lateral or stability planes to help him keep his balance.

It was now found that a very little tilting of the planes upward or downward would serve to right the machine when it leaned over. The secret, like so many others, was gained by watching the flights of birds. You have perhaps seen a great albatross or sea gull soar without the slightest effort and apparently without motion. Look more closely and you will see that the tips of the broad wings move slightly from time to time, while the main body of the wings remains rigid, which is the great secret of stability in flight.