Sorry to keep bothering everyone with my questions, but I need to learn about correct prop selection, without buying a whole heap and experimenting to much and not knowing what I am doing. It seems for 3D you want a bigger diameter prop with less pitch? Why is this?

And for speed you want less diameter and more pitch, which I can kind of understand for resistance and RPM reasons, or am I completely off track?

Any advise would be greatly appreciated, specifically pertaining to 55AX and ST G-51

Thankyou