I would recommend a Saito(or OS) size .82. The Saito will be the bower to weight to ratio and the .82 will launch a 7 lb plane vertical, like a rocket. A Saito .72 will do it as well but not fly towards th moon in a matter of seconds. The Saito .82 weighs only 16 oz and will swing a 15x6 to 12,000 RPM's!