Thanks jeffEE and WacoNut. I have a 1/4 SpaceWalker that flies very scale-like on a 22 cc engine. Scaling that up to 40 cc on the 1/3 bird seemed reasonable to me too.

I have noted that the Sig 1/3 scale plane is really a bit smaller than 1/3 scale. The real plane is 28 ft. span - 1/3 should be 112 inches. at 104 inches, the actual scale is about 31%. I am guessing this was done to make use of standard sawed lengths of wood - 48 inches?