prop size

[Lsaircraft]

I am wondering why the sonex only swings a 54 inch prop there is more than enough room to swing a way bigger one, and with the 3300 you have the power. I know the plane is slippery but i would still think the more air you grab the faster you go. just curious thanks

[Bryan Cotton]

Lsaircraft,
Prop diameter is chosen by RPM. The goal is to keep the tips subsonic. If you go bigger diameter, the excess power gets converted to noise rather than thrust. With a high RPM high power engine, you absorb the power by increasing pitch or chord.

Also, big props are good for static thrust at low speeds. Small props are good for thrust and efficiency at high speeds. There was a axiom in the helicopter industry that holds true here: what is good for low speed is bad for high speed, and visa versa. If you have sufficient climb performance with a small prop you will help your high end - a good tradeoff.