The Newtonian Explanation

What is it?
Isaac Newton stated that for every action there is an equal, and opposite, reaction (Newton's Third Law). You can see a good example of this by watching two skaters at an ice rink. If one pushes on the other, 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 impart some of their momentum to the wing, thus incrementally nudging the wing upward with every molecular impact.

Note: Actually, Newton's theories on fluids were developed for naval warfare, in order to help decrease the resistance that ships encounter in the water -- the goal was to build a faster boat, not a better airplane. Still, the theories are applicable, since water and air are both fluids.

Why is it not entirely correct?
The Newtonian explanation provides a pretty intuitive picture of how the wing turns the air flowing past it, with a couple of exceptions:

1. The top surface of the wing is left completely out of the picture. The top surface of a wing contributes greatly to turning the fluid flow. When only the bottom surface of the wing is considered, the resulting lift calculations are very inaccurate.

2. Almost a hundred years after Newton's theory of ship hulls, a man named Leonhard Euler noticed that fluid moving toward an object will actually deflect before it even hits the surface, so it doesn't get a chance to bounce off the surface at all. It seemed that air did not behave like individual shotgun pellets after all. Instead, air molecules interact and influence each other in a way that is difficult to predict using simplified methods. This influence also extends far beyond the air immediately surrounding the wing.

Why is it not entirely wrong?
While a pure Newtonian explanation does not produce accurate estimates of lift values in normal flight conditions (for example, a passenger jet's flight), it predicts lift for certain flight regimes very well. For hypersonic flight conditions (speeds exceeding five times the speed of sound), the Newtonian theory holds true. At high speeds and very low air densities, air molecules behave much more like the pellets that Newton spoke of. The space shuttle operates under these conditions during its re-entry phase.

Unlike the Longer Path explanation, the Newtonian approach predicts that the air is deflected downward as it passes the wing. While this may not be due to molecules bouncing off the bottom of the wing, the air is certainly deflected downward, resulting in a phenomenon called downwash. (See NASA: Glenn Research Center for more on downwash.)