Originally Posted by Daniel Z
As a fact, 40 old sized planes were flown at more speed on 10´props, today we use 13 and 14´for 3d planes (if we think about nitro), and 8´on pylon racers
Right. So when you slap a 17" prop on a 40 sized plane the gyroscopic effect starts to make the plane harder to depart from it's heading into 3D. The control surfaces on most 40 sized planes aren't large enough to overcome the effect of the 17" prop, but can manage a 14" prop. What would you do, put a 20" prop on a 40 sized plane? It works against you.
And I will add that I have 40 sized electric 3D planes and have tried the larger and the smaller props.