Well. I put a ST .51. However, the CG says that it should balance "about" where the cabane strut attaches to the lower wing. I had to add several ounces to the tail, and move the battery back behind the compartment. However, when I ran her up for her first flight, she would nose over as soon as she got to takeoff speed. Lost two props before I packed it in. I guess I need a much smaller engine, probably a .25 to .35 size. Any ideas?