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.
[Illustration: 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.
[Illustration: 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.