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.
If the RPM is the same, then the drag is the same, engine running or not - the thing is that if the engine is running, the RPM will be greater than if the prop is windmilling.
Rolling Circle:
Prop spinning fast: thrust
Prop spinning slower: zero thrust/drag
Prop windmilling (slower again): drag
Starting to see the pattern? The slower the prop is rotating, the
more drag it produces (assuming fixed pitch, of course). You assert that if it stops, the drag
reduces :I don't think so!