A 13 X6 APC prop flys my heavy bi wing Arrowmaster with extended upper wing for IMAA legal events. All flight characteristics are normal. It appears to fly a bit slowly, and has short vertical climbs, but the airplane is totally controllable. This plane would have a bit more of the Circus plane zip with the OS 75 AX, and experience tells me to go long and light on pitch. This Always makes OS happy. This characteristic emanates from the way engineers at OS spec out their engines. I would use a 14 X 6 prop and let the engine fully breath after take off as she unloads and RPM grow. This engine is stronger than you think and CAN handle 14 size props. We all know that looking for a vertical climb the higher pitch props lose RPM's too fast.and the short prop with low pitch will take away the speed element.
Enjoy the OS 75AX, I'm planning to own mine soon.
samiamaustin