Bruce,


Most current sport .61 engines are under-propped with an 11x7 prop.

The late 70s/early 80s piped F3A engines, like those used by Dick Hanson on his Tiporare and by other pattern competitors in those days, which would spin an 11x7-11x7.75 prop, around 14,300-15,300 RPM, in ground static condition, would unload for more RPM in flight, reaching their real HP peak RPM of 16-17,000.

Most current sport .61 engines, as they come, with their original silencers, peak around 13,000 RPM, despite anything their manufacturers claim they can do...


So the right size prop is a 12x6, or a 12x7 (or one of equal load) that will have the engine spinning around 11,500 RPM on the ground.

The 11x7 size will not cause the engine to overheat, but if extra cooling is desired, a higher pitch prop of equal load will be much better than chopping the 11x7. A 10x9 is the best choice, if over-heating is feared with an 11x7.