Surely a dead engine with a fully open throttle can windmill easier than one with a closed throttle?
Therefore if one experienced engine failure and needed to maximise glide range to make land or clear an obstacle would setting full throttle reduce the resistance of a windmilling prop and improve glide range?
Of course make sure alternator is turned off and battery used purely for ELT and mayday radio calls. Maybe NAV too if IMC.
Update: I should have mentioned this is for a fixed prop with no feathering capability eg C150/152/172. Obviously if you have a prop lever or autofeather that would provide the best outcome.
I’ve never heard of anything like this. I suspect that the amount of resistance involved in windmilling the prop doesn’t affect the aerodynamic drag created by a windmilling prop in any meaningful way.
What you really want to do is STOP the prop from windmilling. That’s why multiengine aircraft have the capability to feather the prop. From a single engine aircraft standpoint, you’re better off with a seized engine than a windmilling one.
In a piston twin both mixtures full rich both props full forward both throttles full forward - Identify ( DEAD FOOT Dead Engine )suspected failed engine- Verify by slowly reducing suspected engine throttle if engine is producing partial power leave running if not mixture off and feather turn 5 degrees toward good engine the rudder pedal on the good engine side will get a workout.