landing, since, if it were effective, it would obviously be able
to rise from and alight on any ground capable of containing its
own bulk; a further advantage claimed is that the helicopter
would be able to remain stationary in the air, maintaining
itself in any position by the vertical lift of its propeller.

These potential assets do not take into consideration the fact
that efficiency is required not only in rising, landing, and
remaining stationary in the air, but also in actual flight. It
must be evident that if a certain amount of the motive force is
used in maintaining the machine off the ground, that amount of
force is missing from the total of horizontal driving power.
Again, it is often assumed by advocates of this form of flight
that the rapidity of climb of the helicopter would be far
greater than that of the driven plane; this view overlooks the
fact that the maintenance of aerodynamic support would claim the
greater part of the engine-power; the rate of ascent would be
governed by the amount of power that could be developed surplus
to that required for maintenance.

This is best explained by actual figures: assuming that a
propeller 15 ft. in diameter is used, almost 50 horse-power
would be required to get an upward lift of 1,000 pounds; this
amount of horse-power would be continually absorbed in
maintaining the machine in the air at any given level; for
actual lift from one level to another at a speed of eleven feet
per second a further 20 horse-power would be required, which
means that 70 horse-power must be constantly provided for; this
absorption of power in the mere maintenance of aero-dynamic
support is a permanent drawback.