Forgot to ask ref your comment below. From the point of view of just freeing the engine and getting it to start, which is the best size prop from below?

"A 10x4 is a perfect match for peak revs-or if you had CL in mind a 9x6.....in the unlikely event you felt compelled to fly it in vintage team race-then an 8x8 might be appropriate. For vintage R/C perhaps a 10x5-but if you want to chug around gently it will have no trouble hanging on to an 11x4, 11x5 or even larger diameter for something like a lightly loaded slow flying vintage model. Its a big heavy motor, and produces just over 1/4 HP. "