In article ,
Mike Ash wrote:

In article ,
Ron Garret wrote:

There's also the conservation of energy argument. If the engine is
turning, the energy to overcome friction and compression has to come
from somewhere.

I don't think this works. In the non-spinning case, you're dissipating
all that energy into the air, and there's no real limit as to how much
that could be. Now, it would seem that the conservation-of-energy
argument gets you the right answer, but IMO not for the right reasons.

I'll grant you it's not a slam-dunk argument, but it sure seems
plausible that it takes a lot more energy to turn a dead engine than it
does to move a stopped prop through the air at ~100 knots.

rg