RE: prop diametre ?
Well, the 10x7 prop size will only provide better top speed (because of higher revs thus higher slip stream) IF the airframe does allow that.
So it is crucial to minimise the drag, both induced and parasitic.
If using a “perfectly“ shaped airframe such like some of the speed contenders at the SpeedCup in Germany this weekend, very high pitched props (oversquared) can be used successfully.
Even props like the mentioned 5x10 will work fine then (only for a typical speed flight pattern, not for pylon racing). With such oversquared props the only critical situation can be the start procedure, due to a quite low static thrust. Once in the air the (stage two) pipe resonance starts (during the first dive at the latest).
It does require some routine to find the “sweet spot“ where the engine will work at the optimum (=rev and torque peak). Meaning that airframe and all components should harmonise with each other:
- airframe design (drag components)
- engine timings and compression setup
- tuned pipe length
- prop size (the prop diameter has to be adjusted to the target engine rpm to avoid transsonic prop tip speeds, the pitch on the other hand does determine the potential speed plane target top speed)