Two X is a good ratio of cc to weight.

Generally speaking I believe it is good for the cc to be about twice the all up weight. A 15-pound airplane will usually have a 25 to 31 cc engine. A 25-pound airplane will just barely fly with a 25 cc engine and is an accident desperately looking for a place to happen. Don’t ask how I know. The same airplane broken and fixed many times flies well with a 57 cc engine at 35 pounds.

Bill