RE: prop diametre ?
Next example:
This time it is explained why a .60 powered speed plane can be considerably faster than a .90 powered speed plane.
As we can see the below pictured plane has no engine/pipe cowl. The fusealage section is quite fat as well as the airfoil thickness’ of wing, stab and fin. This plane comes with the strong and reliable OS 91 VR-DF ducted fan engine plus BVM tuned pipe (about 4.7 HP of ouput). The fastest runs of this combo do reach about 195 mph using a 9x10 prop made of carbon fiber (only carbon provides the needed strength). If using more pitch (and less diameter to unload the engine better) top speeds will be slower than 195 mph because of the inability of this airframe to transform the increased propeller slip stream into higher top speed.