Where's the mystery?
A stopped prop will cause less drag than a windmilling one. An idleing engine turning the prop will cause less drag than a windmilling or stopped prop. Which ever you are faced with, you simply judge the glide angle it is giving you and fly the approach accordingly. Aircraft engines will windmill unless the speed is bled off sufficient for the tuning force of the air hitting the prop to be overcome by forces acting within the engine to resist motion. Newer engines that are still tight, or ones with higher compression ect will stop at higher airspeed than older or low compression ones. Also prop pitch/diameter will also have an effect. Certainly, if I turn off the ignition off, on my VW it will stop dead! Mind you, my max airspeed is only 70knots
I don't fly glide approaches every time, if I did in my T31 I'd be moaned at every time I went to an airfield other than my home strip. However, I prefer to be within gliding distance of the field if possible and get very peeved at the bomber circuits adopted by some (noise abatement permitting).
SS