Do a static balance (on the bench) and then read this http://www.hebertcompetitiondesigns....ngulation.aspx

To answer your question, F3A models are balanced upside down due to the vertical CG. Don't confuse this with CG along the length of the model. A high wing trainer would be balanced upright because most of the weight (vertical CG) is below the wings. Trying to balance a trainer (on the bench) inverted would be next to impossible.

Regards,
Jason.