As a general rule most manufacturers recommend an engine typically as from "A"-"C" with A being small and C being larger. The Dynaflite Butterfly says .10 to .15 I believe. If you don't live near sea level that may be true especially if you build light. I fly at 5000' ASL and believe me the Butterfly won't get out of its own way let alone off the ground very well with a .15. I've built two already so I don't need to hear from all the people that say mine would lift five pounds with a .15. To qualify any such claims please tell us the altitude of your flying site. For the guy who believes the kit seller look around and be sure to ask the right questions. I did build a Slow Poke when they first came out because I thought it was cute. So much weight was needed to balance it that even an OS .20FP wouldn't make much of an airplane out of it. Changed to an OS.25FX and it was at least flyable.