The Principle behind Airfoil

Let's take a look!

An airplane can stay in the air because of the lift acting on the main wing.
The wing has a rounded front end and a sharp rear end. This kind of shape is called a “streamline.” This shape offers less resistance.
However, this shape alone will not make the airplane fly. The back is tilted down a little (it has an angle of attack). Furthermore, the entire shape is curved (cambered) as well.
Air on the bottom of the wing is bent downward, thereby receiving a downward force from the wing. As a reaction, the wing receives an upward force from the air.
Air on top of the wing follows the wing contour and is bent downward, thereby receiving a downward force from the wing. As a reaction, the wing receives an upward force from the air.
Therefore, both the top and bottom bend air downward and receive upward forces as a reaction. This force is called “lift” and this is the force that makes airplanes fly.
The above explanation is based on momentum theory. An explanation can also be made using the streamline curvature theorem.

[Note]	This phenomenon is often explained as follows: "The distance at the top is longer than the distance at the bottom. Since the air has to travel those distances with the same amount of time, the flow is faster at the top side and slower at the bottom side. From Bernoulli's theorem, the top side will have negative pressure while the bottom side will have positive pressure, thereby causing "lift". This explanation is incorrect. The statement "travel those distances with the same amount of time" is incorrect. Please be careful about this as this incorrect explanation is used in several scientific books.
[Keywords]	lift, momentum theory
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 168-169 and 156-161 “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 218-219 and 84-87

Last Update：9.7.2013