Very simply stated an airfoil when passing through the air creates lift. How this occurs is the wing, if it is semi or symetrical is curved at the top and bottomand it takes longer for the wind to pass over this curved surface. As a result based on a principle called bournoulis (forgive my spelling)principle, (same as a venturi cut in half) this causes a low pressure moment on the part of the wing that is pointed up. (An airplane flying level will always have a high angle of attack eitherinverted or right side up.As a result the wing wants to lift towards the low pressure area to fill the void, this is called lift.

A flat bottom wing is more stable and will want to right itself due to the nature of the wing being flat on the bottom and curved on the top.

A semi or full symetrical wing is more aerobatic in nature and will tend to stay where you point it.

Di-hedral is another aspect that helps flat bottom wings be more stable and self righting, the more dihedral the more stable the airplane, on a flat bottom wing.

hope this helps

Glenn Williams