RE: HP VT21 worth using?
Hello; I have one of those VT21's, I last saw it fly in a Dynaflite Piece of Cake. I couldn't get it to idle below 3300 rpm's, but it would run happily, and very quietly, but you would have to fly the tank out because it wouldn't shut off. I tried it on my speedy bee and it wouldn't get it off our grass runway. I would say run it, it is a well designed well made engine with some clever ideas with the rotary valve.
Diesel Dan, I have one of those open rocker Enyas' too, runs great, very quiet. I haven't mounted it into a plane yet.