The Theory and Practice of Model Aeroplaning

by V. E. Johnson

∴ ⁵⁶/₇₄₆ = ¹/₁₃ h.p. approx.

at the observed number of revolutions per minute.

§ 19. N.B.—The h.p. required to drive a propeller varies as the cube of the revolutions.

Proof.—Double the speed of the screw, then it strikes the air twice as hard; it also strikes twice as much air, and the motor has to go twice as fast to do it.

§ 20. To compare one model with another the formula

^{Weight × velocity (in ft. per sec.)}/_horse-power

is sometimes useful.

§ 21. A Horse-power is 33,000 lb. raised one foot in one minute, or 550 lb. one foot in one second.

A clockwork spring raised 1 lb. through 4½ ft. in 3 seconds. What is its h.p.?

1 lb. through 4½ ft. in 3 seconds
is 1 lb. " 90 ft. " 1 minute.