It's certainly true that for a constant prop pitch and fixed IAS:
- a high engine RPM gives a positive prop AoA and therefore positive thrust
- some lower RPM gives zero prop AoA and therefore zero thrust, and
- an even lower RPM gives negative prop AoA and therefore negative thrust
.....but that explanation holds whether or not the engine is running. The bit that I can't get my brain around is how to prove (I agree that it's intuitively obvious) that the overall drag at some (low) RPM is lower when the prop is being driven by the engine than when it is windmilling at the same RPM.
[This message has been edited by hugh flung_dung (edited 16 July 1999).]