CHAPTER VIII DIRECTIONAL CONTROL
The unerring flight of birds is, of course, the model for the builders of heavier-than-air machines. Much of the birds' skill in directing their motive power remains a mystery to us, but we are learning to analyze and, in a measure, imitate them. The builder of model aeroplanes again must not alone imitate the methods of the birds; he must make their system of maintaining stability automatic. A study of a variety of successful models shows that there is great difference of opinion as to the best plan for stabilizing the aeroplane.
Directional stability is gained by the use of horizontal elevators or tails for controlling vertical movement, by vertical rudders or fins for steering to right or left, and by flexible wing tips to guard against tipping.
In designing any system of rudders, or ailerons, for gaining stability, one should always have in mind the general principles upon which such surfaces act. The movement of the horizontal planes or ailerons has an important effect upon the direction of the flight, because they change the angle of incidence.
In other words, they alter the angle of the plane with the line along which the aeroplane is flying. If you bend the rear edge of the plane, or aileron, downward, the angle of incidence is increased. What happens is this. As the plane is lowered, the air is compressed beneath it, which tends to lift the plane, throwing up the front edge and changing the course of the flight.
This method of securing stability, which was invented and patented by the Wright Brothers, has been widely imitated. In their later machines, the Wrights have even abandoned the front elevating surfaces and depend upon the movement of the main plane and a small elevating plane placed just back of the rear rudder for their directional control.
They have thus done away with the friction encountered by the front planes, which has resulted in giving the machine greatly increased speed. Now in the model aeroplane, it is, of course, impossible to flex the planes up or down during flight. Some adjustment must be hit upon which will give the machine automatic stability. The principle of the action of the stabilizer remains, of course, exactly the same.
In designing rudders for controlling horizontal flight, it should be borne in mind that their stabilizing power varies largely in proportion to their distance from the center of gravity. In most models the further they are removed, either front or back, the greater is their leverage, and the smaller need be their surface. By placing the rudder on an outrigger carried far out, a very small plane will suffice.
A serviceable model showing excellent workmanship built by Cecil Peoli