I am on my second AX55. Call me crazy, but I used a 14 x 4 Zinger on a UCan 46 and it would pull out of a hover consistently on YS20-20 and had terrific overall performance. It never died in the air. Never. I used it for about 150 flights. The transmitter read 30+ hours, some of which was for programming the transmitter. It gradually lost power because of a worn piston, sleeve, and bearings, and Hobby Services rebuilt it for me for $85. Meanwhile, while I was waiting for the return repair, I bought another one new and put it on the same plane. This time I went with a 13 X 4, and it flies great with still good verticals. I actually think the 14 x 4 was asking a lot, but the engine would spin-start with three fingers almost every time when it was warm. Great idle, smooth power. I might just go back to the 14 X 4. There's an APC hanging around here somewhere. Rock on OS.