Computer drawing of an airfoil with description of the incorrect
 Equal Transit 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 one of the most widely circulated, incorrect explanations. The theory can be labeled the "Longer Path" theory, or the "Equal Transit Time" theory. The theory states that airfoils are shaped with the upper surface longer than the bottom. The air molecules (the little colored balls on the figure) have farther to travel over the top of the airfoil than along the bottom in order to meet up at the trailing edge. Therefore, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, then, from Bernoulli's equation, the pressure is lower. The difference in pressure across the airfoil produces the lift.

Before considering what is wrong with this theory, let's investigate the actual flow around an airfoil by doing a couple of experiments 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 various parts of the "Equal Transit" Theory.

You can further investigate all the factors affecting lift by using the FoilSim II Java Applet. You can also download your own copy of FoilSim to play with for free.

 


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byTom Benson
Please send suggestions/corrections to: benson@grc.nasa.gov