for the cv-a engines I would go online and find some c/l fast combat props..buy a variety of six or so and try them stock and then modify as needed, its likely that you will find that a 5 inch ish prop is whats right....the old rossi 15 engines used cut down and thinned 6x5 revup props..they aimed for 30k rpm on the ground..if it wouldn't turn 30k rpm it was to much prop....