Was the 15x8 a zinger??? Ha! On my 120's I used 14x8's! in the old days of the original 120's. Heck even 3 or 4 props is a lot of money. Those carbon props tend to unload well. The Xoar does too so be sure to throttle back in a dive. -Tom
Bob Boucher's (From Astro Flight) Electric Motor Handbook is a sophisticated manual that has excellent discussions on prop efficiency, thrust vs normalized speed etc. Not easy reading! It was published long ago but might still be available. All the info directly applies to engines. It has been valuable to me. -Tom