i plugged this formula in and figured without factoring slippage and whatnot a 31cc ryobi on GGL spinning roughly 8200 rpm spinning a 16x8 prop it would only go about 50 mph!? this formula must be a bit out of whack b/c i know a guy that has a big 'ol cub with a similar setting as this, and his plane is alot faster than that! no radar to prove it, but that plane would fly circles around my trainer which is a lt 25 with a webra 40 on it.