Tom, YS should spin a 15x8 (I like the APC). OS should use 14x6 as others have stated. The YS has the power of a 1.20 of other brands. I have the Skybolt ARF with an OS 91, spin 14x6 MAS, and it has plenty of power and balanced perfectly. The YS is a little heavier. If you build the Skybolt kit (not the ARF) it will be heavier than the ARF, and might make better use of the YS power.