My first Butterfly had the wing fold in flight. The weakest part seems to be the outboard polyhedral joint. Built the next one with a considerable upgrade in the strength department at that point. The Center section with the 1/4" steel bar was plenty strong. Put an OS.25fx in it because our field is 5,000' ASL. It's still a powered glider no matter what else they call it. Also an excellent trainer I might add.