prop rpm question

Bob Fry wrote:

I wish to leave the engine out of the discussion, but let's
continue...


"KB" == Kyle Boatright writes:


KB If we assume the plane in question is a C-152,

Close enough, it's an Aircoupe with a C90.

But let's look just at the prop. Why does a prop produce so much more
thrust, much more than double, when it's turned at only twice the
rate?

KB Another way to look at it is that your prop has an advance
KB rate. Let's say it the advance rate is 4 feet per
KB revolution.

Yep, 48" pitch.

KB At 1,000 rpm, and no drag on the airplane (rolling
KB or aerodynamic), the airplane would have a terminal velocity
KB of 4,000 fpm, or about 48 mph. Of course, there is rolling and
KB aerodynamic drag, and there is prop drag too, so the engine
KB can only drag the plane along at, say, 30 mph, assuming a flat
KB smooth runway.

KB At 2,000 rpm, with no drag, the terminal velocity would be
KB 8,000 fpm, or about 85 mph.

Hmmmm...so prop thrust is indeed only twice at double the
rpm?...ideally speaking of course.

The idealized (no viscosity etc.) math seems to say that it is linear,
but intuitive feel says not.

Is aerodynamic drag of an airfoil linear with speed? Is airfoil lift
linear with speed? Since a prop is just an airfoil going in a circle,
why would you expect it to be linear?

Matt