A fellow in our club has a Butterfly and has a .25 something or other on it. He made the wings almost straight-- took out most of the polyhedral, but kept the basic dihedral, and put a small set of wheels on it, flies it like a trainer. This past summer he flew it so high we lost sight of it unless the setting sun hit it just right, couldn't hear the engine at all. He'd finally close the throttle and glide back, total flight times in excess of 20 minutes. Nice plane.