You can tell, because it's putting enegry into the engine, about 30% of the idle power need.
If you want to try it, make sure you have a starter sytem, 'cause sometime you can hardly dive fast enough to restart. As Larry says, " Don't ask me how I know". Ron amd Martha Freiberger mail to ronandmar...@earthlink.net During my 40 hour test flying on the Mustang I got bored to tears and decided to test the windmilling or not theory. I was surprised to find that on that plane I had a better glide ratio with the prop windmilling than with it stopped. With my particular prop and engine combination the prop would windmill until I slowed to about 90. It would not start spinning again until I speed up to about 140 so I had a pretty good range where I could choose a windmilling or stopped prop. I actually did something like 5-10% better glide with it still windmilling. Keep in mind that this was one particular airplane, prop, and engine combination and I don't doubt that many, if not most, other combinations will glide further with the prop stopped. With a lower pitch prop and a higher compression engine you also might not be able to keep it windmilling at anywhere near your best glide speed. It all boils down to if you want to know try it yourself in your plane. Now that I have cut two inches off the prop and had it pitched 4" lower I will try again and see if I still get the same results. Oh, and don't try this experiment if you don't have a mixture control. Shut your engine with the mag switch and then turn it back on with the prop windmilling and you are going to get the backfire from hell.
