In a factory...
A wing when looked upon from the wing tip is curved at the top and straight along the bottom. So, now there is more surface area on the top in comparison to the bottom. As air rushes along on both...
They don't sometimes.
To provide the lift that is required to glide through the air without plummeting to the ground.
It provides lift to keep the plane airborne.
The air that travels over the wing is travelling at a higher velocity and is at a lower pressure than the air travelling under the wing.