Being poverty-stricken, I was limited to a ocuple engines and manipulated prop sizes to get a plane to handle the way I needed it to. My Goldberg Cub used an Enya .40SS (bushing) engine and 11x5 prop. My brother's Cub had a K&B .61 with a 12x4, I think - maybe a 13x4. It's an issue getting 'em down out of the sky if you have an engine that doesn't have a really slow idle speed.......