prop rpm question

"Bob Fry" wrote in message
...
At 1000 rpm or so, my airplane will taxi and get up to, what, 15-20
kts? But at double the rpm it will fly at 80-90 kts, though it would
take a long time to take off. Surely double the rpm produces more
than double the propellor thrust...or does it? Anyway, it seems very
nonlinear, that is, double the rpm and I get much more than double the
performance. Why is that?

The laws of physics (certainly those relating to mechanics - velocity,
acceleration, thrust, drag and all that) are rarely linear. On the ground
your aircraft is probably not in an optimum attitude for drag reduction, so
it's probably not fair to compare it with an aircraft in the sky. And will
your aircraft fly straight and level at 1000rpm? If so, how fast?

The main thing dictating how your aircraft performs is drag. "Normal" drag
increases with the square of the speed you fly at - so if you double the
speed, you roughly quadruple the drag (hence everything has a terminal
velocity when falling to earth - as you get faster, the drag increases
faster than your speed increases, and you stop accelerating once drag equals
the acceleration caused by gravity). Remember also that at low speeds you
have induced drag, which is high at low speeds but vanishes as you get
faster.

The moral of the story, though, is that if you think something ought to
behave in a linear manner, you're probably mistaken.

Cheers,

D.