of flight by
this method is modified by a very definite disadvantage of which lovers
of the helicopter seem to take little account. It is always claimed for
a machine of this type that it possesses great advantages both in rising
and in 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.

The attraction of the helicopter lies, probably, in the ease with which
flight is demonstrated by means of models constructed on this principle,
but one truism with regard to the principles of flight is that the
problems change remarkably, and often unexpectedly, with the size of
the machine constructed for experiment. Berriman, in a brief but very
interesting manual entitled Principles of Flight, assumed that 'there is
a significant dimension of which the effective area is an expression
of the second power, while the weight became an expressi