prop rpm question

This won't be a complete ( or maybe even correct answer) but ...

The prop is an airfoil. Here *lift* will be *thrust*. The thrust force
generated will be

T = C A d/2 V^2. V is the speed of the airfoil (prop). So the force is
nonlinear, it goes as the square.
For the rest of the parameters: C is coeff of lift (depends on angle of
attack), A is airfoil area, d= air density.

The prop will have induced and parasitic drag. The relative wind angle of
attack on the prop foil will depend on the aircraft speed. At low speed it
is at high angle of attack. The induced drag is large and the engine can't
get up to full rpm. As the aircraft speed increases, the AOA decreases, the
induced drag decreases and the can rev up to higher rpm (as constnt
throttle). There will be an AOA where the drag is minimum, This is where the
prop is most efficient. It is a narrow range, because, as you go faster,
parasitic drag on the prop kicks in. Constant speed props are used to adjust
the pitch to remian efficient over a wider raneg of airspeeds.

Kershner states that the maximum thrust force occurs when the plane is
standing still (at a fixed throttle setting, I guess), and decreases as you
go faster. I do not understand this. Is it beacese AOA is largest? I am
trying to see how this relates to power. Power would be force*distance/time
or force*velocity. Maybe the thrust decreases slowly with airspeed, but the
power still goes up as you go faster.

This is just a hand waving argument. Please, anyone who knows more, feel
free to correct this picture.

Dave






"Bob Fry" wrote in message
...
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.