In article , "coriolis" wrote:

I find the aerodynamics of fixed pitch props relatively easy to understand -
the angle of attack varies with both forward speed and RPM.

However, something puzzles me about constant speed prop thrust. I realise
that the blades twist to give a different pitch, and therefore when you
increase MP to maintain the same RPM the blades will be at a higher helix
angle. However, won't the angle of attack (calculated by the relative wind)
by the same? Isn't that the idea? If the prop is at the same angle of
attack, why is the thrust higher? Once again, I know the pitch is higher,
but isn't the angle of attack the important thing in generating lift (which
is what thrust is) rather than simply the pitch angle? I've seen texts that
say you get a bigger bite, but that doesn't seem to explain why that matters
since lift is predicated on angle of attack.

Anyone have a good explanation?

TIA

On a fixed-pitch prop AOA reduces as airspeed increases. RPM has to
increase in order to maintain a constant AOA until it reaches redline.

Essentially, the variable-pitch propeller increases angle-of-attack
(AOA) as speed or RPM decreases and MP remains the same.

Those propellers are also a set of compromises, since their twist is set
up to give maximum efficiency at a given airspeed/RPM. Deviations from
those conditions reduce efficiency.

You can make a simple program to map airspeed, RPM, AOA at various
stations along the prop span.

--
Remove _'s from email address to talk to me.