Until a year ago I was using an older airplane with the YS .61 in it for competition. I started with 12x12 APC props on it turning around 9200 RPM on 10% nitro. Only problem with the 12x12 was that it never seemed to want to wind down for landing, so I went down to the 12x10. It turned slightly faster, gave better vertical performance, and slowed nicely for landing.