I believe that turbo prop model engine has a constant speed prop but I think it sells for $10 grand. I think you would need a geared engine so you can have some prop efficiency because a prop tuning 10000 rpm would be hard to control accurately, and the governor would be pretty big to work so there would be a lot of weight. The prop would be pretty hefty too, to be strong enough not to destruct. There has been controlled props in pattern but they were never strong to be safe, but today you could mimic it with the heli throttle collective mix in your radio. Back in the 80's the Russian control line scale team had home made featherable props; dont know how they worked.