I'll throw this into the discussion. There are a bazzillion free flight rubber designs that use plastic store bought props. They have a built in free wheeling device. In competition P-30 rubber, it is usually desired that a precisely place wheel collar on the prop shaft take the load on the bearing while the prop is being driven by the rubber, and as it unwinds, there is plenty of room for the prop to move back from the hook or latch, and easily free wheel. If you have ever seem a free flight model with the prop hung up and not turning it is almost like a dethermalizer! It's another one of those things that you can stay up all night and drink a lot of beer while arguing about it! I got to see one of Windy's flight with his Typhoon (?) that had the Z-Tron system in it at the NATS one year, and he landed that with some power on, and killed it when wheels contacted the ground. I think this actually hurt him as his landing took several laps and wasn't as is described in the rule book for the landing maneuver. All things considered, I like my I/C engines and having the prop come to a nice, horizontal position for landing.
Type at you later,
Dan McEntee