Hi Mark
It can be pretty much any combination of pitch and rpm, a 20x15 with the right setup can spin into the mid 6000's. I think around 90 mph and change is about as high a pitch speed as you can get with the current 20-22" electric setups.
ORIGINAL: mmattockx
Chad - If I assume the 12" pitch goes with the 6500rpm end of the range and the 15" with the 5800rpm end, that gives a pitch speed range of 78-87mph. My example of a 10" pitch at 7500rpm or so should be OK then, considering the unloading that will take place in the air with the gas engine. Good to know it sounds like a decent starting point.
Mark