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Skipping Stone Theory Interactive

Image of an incorrect theory of lift
Incorrect Theory#2

Incorrect Theories of Lift

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.

Reaction Force of an Airfoil

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. Below the simulator is a text box with instructions. Be sure that the slider on the right of the text box is pulled to the top to begin the experiments.

Skipping Stone Theory

Let’s use the information we’ve just learned to evaluate the “Skipping Stone” Theory.

Flight Conditions

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 very accurate 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 good 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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