Before reading my comments, bring up this page in another browser window and refer to it as you read. The pictures on the right and figure 3.1 toward the bottom will help to make things clearer. You can read the text on the page if you like, but I am only using the pictures here. http://www.allstar.fiu.edu/aero/Wing31.htm

Before I begin, while I have basic understanding of aerodynamics, I am not an aircraft designer. I am just a guy who likes to get into the technicals. I am told I am pretty good at simplifying those technicals so I can explain them to others at a practical level. I guess that comes from 23 years in sales in the computer business. Let's see how I do.

Simplistically, there are three types of wings. There are variations and degrees, but basically three:

Wings with flat bottoms, wings with bulged bottoms or bottoms that curve down, and wings with concave or curved up bottoms. As I understand it, the last one is called undercambered because the bottom of the wing actually curves up following or approaching the camber, or shape, of the top of the wing.

This kind of wing produces very high lift at low speeds. I expect that is why a lot of our parkflyers have this kind of wing. The aerobird, Sky Scooter, Slow Stick, Tiger Moth, are all examples. This is an intentional design feature to provide a plane that behaves in a certain way. This is one of the reasons the Aerobird glides so well at slow speeds and why you can climb so fast with a firebird without an elevator, just by using the throttle. If it had a flat bottom wing, it would still climb, but very slowly. Neat design!

However this design fights inverted flight. When you roll it over that high lift factor is pushing the wing down so the plane does not want to fly upside down. I would guess that you would need a huge elevator to fight that downward lift in inverted flight, and most likely the plane would just lose altitude in a level fashion.

Add to that the dihedral, the fact that the end of the wing is higher at the tips than the root, where it attaches to the fuselage. This is a characteristic of most entry level planes because it makes them fairly stable and makes it easier to turn with rudder control alone. Polyhedral wings, wings that go up, then up again at a sharper angle are even more self stabilizing so a poly wing is REALLY hard to fly inverted.

So, put undercamber together with dihedral and you get a plane that REALLY REALLY wants to right itself. The combination makes it very hard to remain inverted for very long. You are fighting two intentional design features of the plane that want you to be right side up.

This is perfect behavior for a beginner plane. This is a major reason why the Aerobird, and frankly the entire bird family is so easy to learn to fly. The are very stable and want to right themselves.

I hope that was not too confusing.