stang, I think a 15x4W would probably be the best 3-D prop for that .91. I ran a 14" on my Twist for a few flights and I think I remember the prop just clearing with the plane level. You may have to bend the gear down some or go with aftermarket gear. I think there have been a few Twist pilots running .91 four strokes. You might scroll down to where it says "search this thread" and try typing in ".91 four stroke" or something like that. I'm looking forward to your flight report with that big .91 in the Twist.