of 50 miles an hour. This is an
equivalent of about 134 pounds per horsepower. For an average modern
flying machine, with a total load, machine and passengers, of 1,200
pounds, and equipped with a 50-horsepower engine, 50 miles an hour
is the maximum. Here we have the equivalent of exactly 24 pounds per
horsepower. Why this great difference?

No less an authority than Mr. Octave Chanute answers the question in a
plain, easily understood manner. He says:

"In the case of an automobile the ground furnishes a stable support;
in the case of a flying machine the engine must furnish the support and
also velocity by which the apparatus is sustained in the air."

Pressure of the Wind.

Air pressure is a big factor in the matter of aeroplane horsepower.
Allowing that a dead calm exists, a body moving in the atmosphere
creates more or less resistance. The faster it moves, the greater is
this resistance. Moving at the rate of 60 miles an hour the resistance,
or wind pressure, is approximately 50 pounds to the square foot of
surface presented. If the moving object is advancing at a right angle
to the wind the following table will give the horsepower effect of the
resistance per square foot of surface at various speeds.

Horse Power
Miles per Hour per sq. foot
10 0.013
15 0 044
20 0.105
25 0.205
30 0.354
40 0.84
50 1.64
60 2.83
80 6.72
100 13.12

While the pressure per square foot at 60 miles an hour, is only 1.64
horsepower, at 100 miles, less than double the speed, it has increased
to 13.12 horsepower, or exactly eight times as much. In other words the
pressure of the wind increases with the square of the velocity. Wind at
10 miles an hour has four times more pressure than wind at 5 miles an
hour.

How to Determine Upon Power.

This element of air resistance must be taken into consideration in
determining the engine horsepower required. When the machine is under
headway sufficient to raise it from the ground (about 20 miles an hour),
each square foot of surface resistance, will require nearly nine-tenths
of a horsepower to overcome the wind pressure, and propel the machine
through the air. As shown in the table the ratio of power required
increases rapidly as