Lateral Stability
This particular difficulty has considerably delayed the development of the aeroplane. It may, however, be overcome by very simple methods—simple, at least as far as their mechanical features are concerned. If the outer limb of the plane is tilted upward, it is because the wind pressure is greater there. The wind pressure is greater because the velocity is greater. We have only to increase the wind pressure at the inner limb, in order to restore equilibrium. This cannot be done by adjusting the velocity, because the velocity is fixed by the curvature of path required: but the total wind pressure depends upon the sail area as well as the velocity; so that by increasing the surface at the inner limb we may equalize the value of L, the lifting force, at the two ends of the plane. This increase of surface must be a temporary affair, to be discontinued when moving along a straight course.
The Aileron
Let us stand in the rear of an aeroplane, the main wing of which is represented by ab. Let the small fan-shaped wings c and d be attached near the ends, and let the control wires, e, f, passing to the operator at g, be employed to close and unclasp the fans. If these fans are given a forward inclination at the top, as indicated in the end view, they will when spread out exert an extra lifting force. A fan will be placed at each end. They will be ordinarily folded up: but when rounding a curve the aviator will open the fan on the inner or more slowly moving limb of the main plane. This represents one of the first forms of the aileron or wing-tip for lateral control.
The more common present form of aileron is that shown in the lower sketch, at s and t. The method of control is the same.
Wing Tipping
The cellular Voisin biplanes illustrate an attempt at self-sufficing control, without the interposition of the aviator. Between the upper and lower sails of the machine there were fore and aft vertical partitions. The idea was that when the machine started to revolve, the velocity of rotation would produce a pressure against these partitions which would obstruct the tipping. But rotation may take place slowly, so as to produce an insufficient pressure for control, and yet be amply sufficient to wreck the apparatus. The use of extra vertical rudder planes, hinged on a horizontal longitudinal axis, is open to the same objection.