I fly a Ultra Sport 60/OS 91 Surpass with a 13x9 APC prop. It turn 9400 rpm on 11.5% Cool Power. The motor is ever reliable, it has about 30 to 40 flights on it, yet I can't say I like flying it as well as my buddies Ultra Sport 60/ST 75. He also flies mine and he said it needs a 14 inch prop however the ground clearance is close when the tail goes up prior to take-off. This is my 1st 4s and I just can't catch the 4s fever. I tell every one I love the plane. I'm really not into engine sounds however I do like the 4s's smaller muffler and I really liked the the looks of this plane with a side mounted 4s engine. I'm really working at developing a likin for this ole plane.