You are getting into a very low pitch to diameter ratio with that prop. Good for 3D anyway. As it unloads in the air its efficiency will become poor. I have never had a prop of that size. A much more appropriate prop would be a 22x12. This prop would also give good performance at part throttle settings. These engines are not to be run at full throttle continuously. I fly my 91 inch corsair at 1/2 throttle most of the time!