Larry,

A quick a dirty rule of thumb I use is to figure out the middle of the road prop for sport flying, for example a 10X6 for a 40 engine. This provides an optimized loading on the engine. So if you want more thrust, you increase diameter. However, doing this at the same pitch (i.e. 11X6) will increase the engine loading over optimal, but if you increase diameter and decrease pitch (11X5) then the loading doesn't go up quite as much. Therefore, you get more thrust, but slower flight. Same thing going the other way, trade a 10X6 for a 9X7.

This is not scientifically accurate and also varies by prop type, etc. However, it carries the message in an easy to understand way. Each engine has an optimum rpm where it produces max power. By juggling pitch and diameter, you can attempt to stay as close to that point as possible.

Also, try a few different prop types also. A 10X6 master airscrew is a much different animal than a 10X6 APC or wooden prop for that matter.

Duke