Computer drawing of an airfoil with description of the incorrect
  Venturi 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 a model that the airfoil upper surface is shaped to act as a nozzle which accelerates the flow. This nozzle configuration is known as a Venturi nozzle and it can be analyzed classically. By considering the conservation of mass, the mass flowing past any point on the airfoil (nozzle) is a constant. This leads to the relation that the mass flow rate (the density times the velocity times the area) of the Venturi is a constant. For a constant density, at any point where the area is decreased, the velocity will be increased. Therefore, over the top of the airfoil the velocity will be increased due to the flow constriction. Now, coupling this with Bernoulli's equation, at any point where the area is decreased the pressure will be decreased. The low pressure over the upper surface of the airfoil then 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 "Venturi" Theory.

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byTom Benson
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