vertically, but in order to move horizontally he should be
supplied with a tail in the shape of another pterophore. When
he wished to stop for a little time, valves fixed firmly across
the end of the space between the blades would automatically
close the openings through which the air flows, and change the
pterophore into an unbroken surface which would resist the flow
of air and retard the fall of the machine to a considerable
degree.'

The doctrine thus set forth might appear plausible, but it is
based on the common misconception that all the force which might
be put into the helicopter or 'pterophore' would be utilised for
lifting or propelling the vehicle through the air, just as a
propeller uses all its power to drive a ship through water.
But, in applying such a propelling force to the air, most of the
force is utilised in maintaining aerodynamic support--as a
matter of fact, more force is needed to maintain this support
than the muscle of man could possibly furnish to a lifting
screw, and even if the helicopter were applied to a full-sized,
engine-driven air vehicle, the rate of ascent would depend on
the amount of surplus power that could be carried. For example,
an upward lift of 1,000 pounds from a propeller 15 feet in
diameter would demand an expenditure of 50 horse-power under the
best possible conditions, and in order to lift this load
vertically through such atmospheric pressure as exists at
sea-level or thereabouts, an additional 20 horsepower would be
required to attain a rate of 11 feet per second--50 horse-power
must be continually provided for the mere support of the load,
and the additional 20 horse-power must be continually provided
in order to lift it. Although, in model form, there is nothing