Well the Drone engines were setup up originally for control line stunt using s 11x10 prop. Drone used to sell the 11x10 prop for the engines too.
With the fuel, I was using in one of my Drone engines, a 12x8 felt a little bit under compressed and a 13x6 seemed to be about right. So I was planning on trying some other props but didn't get around to it. I was thinking that a 12x9 prop would probably be correct and maybe a 13x7 prop too a 14x6 might do the trick as well. Since the engine doesn't rev up high, one would have to use a 11x10 prop or equivalent to get the air speed up for control line flying.