The use of wing tips of any form is intended to control both the horizontal and vertical movement. The general theory or equilibrium, of course, applies in both cases. The most perfectly adjusted model is subject to many forces which tend to tip it to one side or the other. A gust of wind,—and the air is never perfectly quiet,—will tip one end of the plane up or down. [Illustration: Various Steering Devices. "a" and "b," simple aileron forms. "A" novel fin on Vinet plane. "B" L-shaped aileron. "C" vertical rudder (Bleriot type). "D" "Blinkers," an effective rudder. "E" stability planes not unlike the runners of a sleigh.] In the early models, this tendency was met by fixing the plane at a dihedral angle. An examination of last year’s models will show how common was this design. The dihedral angle lowers the center of gravity. Now, after one side of the model is raised and the plane rights itself, the center of gravity swings through a considerable arc, like a pendulum, before it can come to rest, so that the center must swing back and forth several times. This tendency to tipping is fatal to a steady flight. It was first observed by the Wright Brothers while studying the early Langeley type of machine. The Wrights abandoned the dihedral angle entirely, as all the world knows, and replaced it by the horizontal plane with a straight entering edge. The keel will in a measure overcome this side motion. Much of the advantage of the dihedral angle may be borrowed, however, by turning up the extreme ends of the plane, without materially lowering the center of gravity. In several of the successful models of the year, these tips have been made equal to about one-fifth the width of the plane, and are raised as high as forty-five degrees. The theory is that, when the oscillation commences, these surfaces damp out the swinging tendency, and bring the model to an even keel. Sometimes the tips are rounded off, although in some cases they are made triangular and brought to a point. As a rule, they are added to the rear plane, although one notable exception is the case of the Lester Robinson model, which carries these tips on both planes. The tips, or ailerons, at the ends of the planes maybe made of the same material as the planes themselves. In case you are using wire frames, it is, of course, a very simple matter to bend up the tips to any desired angle. When the frames of the planes are made of reed, as is generally the case, the tips should be made separately. Bend your reeds to the desired shape and cover them with the same material used for the planes. It is quite as important that they be covered smoothly as in the case of the planes. They may then be attached to the ends of the planes by wiring rigidly in position. It will be found convenient to adjust them so that they may be bent up or down to suit conditions. The same plan should be followed in building and attaching the ailerons to the rear of the main stability planes.