Glowfuel,

With out going into a ton of aerodynamics.....I think it is the flat bottomed wing, not really a good design for inverted flight. It should loop and roll just fine, but not really very good for sustained inverted. The airfoil design makes it want to lift more to the top of the wing....hence the more up elevator when inverted.

I'm sure the experts will follow with the aerodynamics of a symetrical airfoil......[X(]

Hope this helped........and it is a nice looking airplane.....!

EDIT: Spelling...should have spent that singing lesson money on a dictionary......