Give an upside down wing enough angle of attack and or speed and it will create enough lift for level flight. The more the wing was designed for efficient upright flight, the more inefficient it'll be when inverted. Something with a symmetrical wing will be somewhat inefficient both upright and inverted, but chuck a big enough engine on the front and you can drag it through the sky making it do what you want. It's always a compromise. Just remember when inverted that some design features that are designed to reduce adverse yaw when upright will contribute to it when inverted. A Pitts needs a lot more rudder to balance an inverted turn than it does to balance an upright turn.
Last edited by AerocatS2A; 10th Sep 2012 at 12:30.