I found a Thesis by Peter Sprunger, Goshen College, on windmilling propeller drag. His data shows that it depends a lot on the pitch of the prop. Of course a feathered prop will have the least drag, but I cannot feather my fixed pitch prop and because the pitch is very little I'm convinced the best thing to do is stop the prop and prevent windmilling in an engine out condition.
A normal internal combustion engine on an ultralight has enough compression to stop the prop from windmilling however my gas turbine has zero compression and whats more - it's "two spool" turbine which means it has a free turbine - which spins free - which means windmill city big time. It's like draging a 6 foot diameter parachute behind! It wrecked my 7 to 1 glide ratio.
What to do?
In addition to that, I now have an amphibious aircraft.
How can I stop in the water without stopping the engine?
--- duh, how about I stop the prop?
I found a fairly low cost Hayes GX-2 Mechanical Disc Break for about $45 and adapted it to the propeller shaft.
I didn't think it would work and it took a lot of effort to find out. I had to modify my hub to accept the disc and make a very strong bracket to hold the caliper.
Last year I had to install wheel brakes because the turbine would not idle low enough to taxi on the ground let alone on the water. Wheel brakes don't work in the water. So this year I'll brake the prop and solve two problems, 1) water taxi and 2) the windmilling problem.