Computer drawing of an airfoil with description of the incorrect
 Skipping Stone Theory.

There are many theories of how lift is generated. Unfortunately, many of the theories found in encyclopedias, on web sites, and even in some textbooks are incorrect, causing unnecessary confusion for students.

The theory described on this slide is often seen on web sites and in popular literature. The theory is based on the idea that lift is the reaction force to air molecules striking the bottom of the airfoil as it moves through the air. Because this is similar to the way in which a flat rock thrown at a shallow angle skips across a body of water, it is called the "Skipping Stone" theory of lift. (It is sometimes called a Newtonian theory of lift, since it involves Newton's third law, but to avoid confusion with the correct Newtonian theory of flow turning, we shall call it the "Skipping Stone" theory.)

Before considering what is wrong with this theory, let's investigate the actual flow around an airfoil by doing an experiment using a Java simulator which is solving the correct flow equations.

This interactive Java applet shows flow going past a symmetric airfoil. The flow is shown by a series of moving particles. You can change the angle of attack of the airfoil by using a slider, and the angle of attack generates the lift through flow turning. There is also a translating probe with a gage on the simulator which lets you investigate the flow.

This is a secondary Java applet which uses a text box to describe some experiments for the student to perform using the previous applet.

Let's use the information we've just learned to evaluate the "Skipping Stone" Theory.

BUT..... this theory is not totally inaccurate. In certain flight regimes, where the velocity is very high and the density is very low, few molecules can strike the upper airfoil surface and the Newtonian theory gives the correct predictions. These are the conditions which occur on the Space Shuttle during the early phases of its re-entry into the Earth's atmosphere at altitudes above about 50 miles and at velocities above 10,000 mph (hypersonic conditions). For these flight conditions, the theory gives a correct prediction. However, for most normal flight conditions, like those on an airliner (35,000 feet, 500 mph), this theory does not give the right answer.

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byTom Benson
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