ORIGINAL: jimmyjames213
if your using a 11x6 or 7 on a 40-47 engine your have way to much prop for the engine. While the engine may ''turn'' it it will not like you and won't give you as much power as you could be having. A lot of people fail to understand that our engines (like all engines) have a torque curve where they make the most power. This is around 12500-14000 on the ground. For most .46 engines a 11x5 will put you in the 13000-13500 range and 10x7 in the 14000 range.
Prop selection is a pilots choice thing, it's up to the pilot to test out different props to find the size that will fly the plane the way they like. As long as the engine is working within it's recommended rpm range there is no correct size. The 11X6 APC is one of the most often used props on .46 two stroke engines. I found the 11X6 worked just the way I wanted on my .46 OS LA engines on my Up-Roar planes for fun fly compition. For racing my pilot liked the APC 10X8 on his webra .55, I thought he was nuts but he liked it and it worked for him. There is really no blanket answer to props, as long as the engine is within it's working RPM range it's pilots choice. All my engines run bigger props with smaller pitch in there lower RPM range, I like it like that, others may want a faster plane and it wouldn't work for them. Works for me.