If an airplane wing provides lift (an airfoil), how does a plane fly upside down?
The same way that your hand gets pulled upwards if you stick it out the car window and tilt it. The lift of an airfoil is determined by two things - the shape of the wing, and it's angle of attack. Angle of attack is the angle between the flat surface of the wing, and the oncoming air stream. So when I roll the airplane upside down... if I push the stick forward, that pushes the nose of the plane towards the sky, and increases my angle of attack, even though I'm upside down. So, the shape of the wing doesn't change, and it pulls me down, but the angle of attack I control, and I can make it push me up. So when the lift from angle of attack in the up direction, exceeds the lift from the shape of the wing, in the down direction, you can balance the force of gravity acting on the plane. You can see the influence of angle of attack, because even if your hand isn't shaped like a wing, you can see how the angle and the force of the air push it in whichever direction you choose.
Frank DiBonaventuro, B.S., Physics, The Citadel, Air Force officer
'As long as men are free to ask what they must; free to say what they think; free to think what they will; freedom can never be lost and science can never regress. '