The OS 46AX engines have tons of power in their size range but the 61 is only mediocre power wise. A 13-6 prop on the OS 61 really loads it down. I would go for an APC 12-7 instead. You would get a quick take off with a 13-6 but not much speed and it takes a few seconds to wind up when you go from say half to full throttle. [8D]