Two Explanations on How Lift is Created


Newtonís Explanation

Isaac Newton said that for every action there is an equal, and opposite reaction (Newton's Third Law). I thought a good example of this is watching two skaters on ice. If one pushes on the other, they both move -- one due to the action force and the other due to the reaction force. In the late 1600s, Isaac Newton theorized that air molecules behave like individual particles, and that the air hitting the bottom surface of a wing behaves like shotgun pellets bouncing off a metal plate. Each individual particle bounces off the bottom surface of the wing and is deflected downward. As the particles strike the bottom surface of the wing, they transfer some of their momentum to the wing. As a result of each molecule striking the wing, it will slightly move up each time it is struck.

Bernoulliís Principle

Bernoulliís Principle is probably one of the most recognized principles behind lift. This principle uses the fact that the top surface of a wing is more curved than the bottom surface. Air particles that approach the leading edge of the wing must travel either over or under the wing. For an example, say that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing. Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster. Bernoulli's equation states that as the speed of a fluid flow increases, its pressure decreases. Bernoulliís Principle derives that this faster moving air develops a lower pressure on the top surface, while the slower moving air maintains a higher pressure on the bottom surface. This pressure difference essentially pushes the wing upward, thus causing an airplane to climb.

Hopefully I made these two explanations clear enough to understand, and you now know more about how lift is created.








Willits, Pat, ed. Private Pilot Manual. Englewood, CO: Jeppesen Sanderson, Inc., 1998.