I was flying my Midget Mustang today (39 hours down, 1 to go!) and I wanted to see how much drag a windmilling prop really causes. I started from 9,000' and did some idle power glide testing and some engine stopped glide testing both with the prop windmilling and stopped. With the engine at idle it took me 70 seconds to descend 1,000'. With the engine stopped and the prop windmilling it took 65 seconds. With the prop stopped I expected a better glide, but it took 60 seconds. I guess the idea of a stopped prop giving less drag than a windmilling prop does not hold up for every plane and prop combination.
Incidently, on this plane it does not make that much of a difference anyway because the best glide speed is about 80 MPH and at that speed the prop will not windmill. It stops windmilling at about 90. To get it windmilling again after it stops I had to accelerate t about 140. So if the fan stops blowing at low altitude my best choice is to keep the speed at about 90 and the prop windmilling. That gives mejust about the same glide as 80 with the prop stopped and it gives the engine a chance to restart without having to use the starter or a serious dive. I recommend doing this testing on your own planes so you know what works best. If you are not confident enough in your engine or your landing ability to shut the engine at a very high altitude right over the airport then you need to fly more and get more confidence before the engine stops when you least expect it. Just remember that if you are at a high altitude with the mixture pulled half way back and you pull it all the way out to shut the engine that you should not go and push it all the way back rich to start it again. Also, shut the engine by going to idel and pulling the mixture, not turning off the ignition. If you do that and then turn the ignition back on you are going to get one hell of a backfire. Brian Kraut Engineering Alternatives, Inc. www.engalt.com
