ORIGINAL: jaka

Hi!
Even in pylonracing we don't calculate anything ...we go from what we have (long winding road of cut and try)and use our knowledge to modify the prop so it works as good as possible.
Don't belive for a minute that you can calculate anything if you want outmost performance.

Use simple knowledge that says large prop disc : good climbing performance at slow speed.
Small prop disc and higher pitch: Higher air speed and not so good climbing performance.
These are the basic prop rules!

For your airplane that weights around 2,6-3kg and is powered by a .82 four stroke I would say 13x7-13x8 APC. Or a 14x4W -14x5N APC. Why APC ...?? because these are the best props at these prop sizes. There are some other good prop brands too like Bolly, RAM and Graupner Cam prop. All of these are 2-blade props. 2-blade props are always better than 3-blade props at these small sizes.

Also remember that all props, plastic as well as wood , could be modified to perform better. Less blade surface means higher rpm and sometimes better climbing performance and speed.Less diameter could mean better speed (but not always).
Remember to keep the rpm of any four stroke lower than 10000rpm for best power and reliablility.