I have had VERY good luck with the os 91 fs in the ucd3d ...However if you haven't bought the engine yet then go with the YS110fz I did a swap and found that the plane will hover @ 1/3rd throttle instead of 3/4 or more with the OS...The YS is the perfect engine for the ucd3d-60... I'm swinging a 16x6 APC prop, the OS liked the 14x6 top flite and the 14x4 APC....Good luck ...JW