"Bob Fry" wrote in message
...
Uh, thanks for trying...I guess:
snip
Anyway I'll restate the question, plus post to r.a.'s garbage heap,
r.a.piloting.

At 1000 rpm the prop produces some amount of thrust (lift), call it
T[1000]. This thrust is only enough to move the plane in a moderate
taxi.

At double that rpm, 2000 rpm, the prop produces another amount of
thrust, call it T[2000]. Now I'm not positive, but it sure seems that

T[2000] T[1000]

Why, if rpm only doubles, does thrust (seem to) much more than double?

The short way around is to look at your engine's power chart and evaluate
how many excess HP it is developing at 1000 rpm vs 2,000 rpm.

If we assume the plane in question is a C-152, the engine is making very
little power at 1,000 rpm. I'd guess 15 hp, of which at least 5 hp is spent
in friction inside the engine, leaving 10 hp for thrust. At 2,000 rpm, the
engine is probably making 60 hp, of which 10 is spent on internal friction.
Therefore, you have 50 hp for thrust.

Another way to look at it is that your prop has an advance rate. Let's say
it the advance rate is 4 feet per revolution. At 1,000 rpm, and no drag on
the airplane (rolling or aerodynamic), the airplane would have a terminal
velocity of 4,000 fpm, or about 48 mph. Of course, there is rolling and
aerodynamic drag, and there is prop drag too, so the engine can only drag
the plane along at, say, 30 mph, assuming a flat smooth runway.

At 2,000 rpm, with no drag, the terminal velocity would be 8,000 fpm, or
about 85 mph. Of course, there is still aerodynamic and prop drag, but there
is no rolling resistance, so you get more bang for your RPM buck. Of course,
it helps that your engine is delivering 60 hp, as opposed to 15 hp when it
turns 1,000 rpm.

Yet another way to look at it is that when your prop spins at 2x the speed,
it requires 4x the power to turn it... KE=1/2MV^2..

And yes, all of this stuff is ideal world, no prop efficiency losses, etc...

KB

KB