FIG 3. Diagram showing the makeup of a biplane (Wright).
The most prominent machines of the biplane group are the Voisin, Wright, Curtiss and Farman aeroplanes. The old practice of placing the elevator in the front of a biplane is gradually being abandoned and it is safe to say that by the time this book has been printed all these machines will be of the "headless" variety with the elevator in the rear.
The vertical fins shown between the planes of the elevator in the old type of biplane, counterbalance the effect of gusts of wind striking the vertical rudder from the sides and also act as a pivot for turning to the right and left. Together with the steering rudder, they constitute a sort of keel which keeps the machine straight to its course.
In order for an aeroplane to fly in the accepted sense of the word, it must possess supporting surfaces, an elevator or tail and a propeller driven by a motor. These are essentially the sustaining, propelling and steering members of the machine.
FIG. 4. Two methods of controlling the lateral stability of an aeroplane.
The machine must, however, also possess "lateral stability," that is, the wings of the apparatus must not incline from the right to left or vice versa during the flight. The machine must be so constructed at it rights itself by its own effort or is under the immediate control of the aviator.
This is accomplished by "warping the wings," that is, the extreme tips of the planes can be moved up and down so as to present a greater or lesser angle and corresponding increase or decrease the lifting capacity of those portions.
The same result is also reached by means of small subsidiary moving planes attached to the rear of the main supporting surfaces called "aileron." When one aileron is lowered, the other is raised. The action of the air on the ailerons is to depress the one which is raised and to raise the one which is lowered as shown by the arrows in the illustration.