The _gyroplane_ or _helicopter_ was the type of flying machine
regarded by Lord Kelvin as alone likely to survive. It lifts itself
by screw propellers acting vertically. This form was suggested in
1852. When only a single screw was used, the whole machine rotated
about its vertical axis. It was attempted to offset this by the use
of vertical fin-planes: but these led to instability in the presence
of irregular air currents. One early form had two oppositely-pitched
screws driven by a complete steam engine and boiler plant. One of the
Cornu helicopters had adjustable inclined planes under the two large
vertically propelling screws. The air which slipped past the screws
imposed a pressure on the inclined planes which was utilized to produce
horizontal movement in any desired direction--if the wind was not too
adverse. A gasoline engine was carried in a sort of well between the
screws.
[Illustration: BRÉGUET GYROPLANE DURING CONSTRUCTION
(Helicopter type)]
The helicopter may be regarded as the limiting type of aeroplane, the
sail area being reduced nearly to zero; the wings becoming mere fins,
the smaller the better. It therefore requires maximum motor power and
is particularly dependent upon the development of an excessively light
motor. It is launched and descends under perfect control, without
regard to horizontal velocity. It has very little exposed surface and
is therefore both easy to steer and independent of wind conditions. By
properly arranging the screws it can be amply balanced: but it must
have a particularly stout and strong frame.
The development of this machine hinges largely on the propeller. It is
not only necessary to develop _power_ (which means force multiplied by
velocity) but actual propulsive vertical _force_: and this must exceed
or at least equal the whole weight of the machine. From ten to forty
pounds of lifting force per horse-power have been actually attained:
and with motors weighing less than five pounds there is evidently some
margin. The propellers are of special design, usually with very large
blades. Four are commonly used: one, so to speak, at each "corner" of
the machine. The helicopter is absolutely dependent upon its motors.
It cannot descend safely if the power fails. If it is to do anything
but ascend and descend it must have additional propulsive machinery for
producing horizontal movement.