prop rpm question

Do you really think including factual data is likely to resolve the
question?

And do you know Dr Dan?

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

Bob Fry wrote:

Uh, thanks for trying...I guess:

DC "Bob Fry" wrote in message
DC ...
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?

DC The laws of physics (certainly those relating to mechanics -
DC velocity, acceleration, thrust, drag and all that) are rarely
DC linear.

Eh? F=ma and many others are.

DC On the ground your aircraft is probably not in an
DC optimum attitude for drag reduction, so it's probably not fair
DC to compare it with an aircraft in the sky. And will your
DC aircraft fly straight and level at 1000rpm? If so, how fast?

Kee-rist. Drag, and attitude (angle of attack, really) have little to
do with the explanation I was looking for. No, of course it won't fly
straight and level at 1000 rpm. That's nearly full idle landing rpm.

DC The main thing dictating how your aircraft performs is
DC drag. "Normal" drag increases with the square of the speed you
DC fly at

I think you mean parasitic drag.

DC - so if you double the speed, you roughly quadruple the
DC drag (hence everything has a terminal velocity when falling to
DC earth - as you get faster, the drag increases faster than your
DC speed increases, and you stop accelerating once drag equals
DC the acceleration caused by gravity). Remember also that at low
DC speeds you have induced drag, which is high at low speeds but
DC vanishes as you get faster.

Induced drag--drag caused by the wing producing lift at a vector not
perpendicular to flight--never vanishes unless lift vanishes.

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?

Because prop thrust increases as the square of the RPM and thus you get
four times more thrust at twice the RPM. There are other factors that
come into play as well such as advance ratio, but I assume you can Google...

Matt