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   Daniel Bernoulli, an eighteenth-century Swiss scientist, discovered that as the velocity of a fluid increases, its pressure decreases. How and why does this work, and what does it have to do with aircraft in flight?

   Bernoulli's principle can be seen most easily through the use of a venturi tube (see Animation or Figure 3-2). The venturi will be discussed again in the unit on propulsion systems, since a venturi is an extremely important part of a carburetor. A venturi tube is simply a tube which is narrower in the middle than it is at the ends. When the fluid passing through the tube reaches the narrow part, it speeds up. According to Bernoulli's principle, it then should exert less pressure. Let's see how this works.

       As the fluid passes over the central part of the tube, shown in Animation or Figure 3-2, more energy is used up as the molecules accelerate. This leaves less energy to exert pressure, and the pressure thus decreases. One way to describe this decrease in pressure is to call it a differential pressure. This simply means that the pressure at one point is different from the pressure at another point. For this reason, the principle is sometimes called Bernoulli's Law of Pressure Differential.

To see the animation 3-2 press here.

       Bernoulli's principle applies to any fluid, and since air is a fluid, it applies to air. The camber of an airfoil causes an increase in the velocity of the air passing over the airfoil.

   This results in a decrease in the pressure in the stream of air moving over the airfoil. This decrease in pressure on the top of the airfoil causes lift.

Many believe that this explanation is incorrect because flat wings (such as seen on balsa wood airplanes, paper planes and others) also have managed to create lift.   Please read How planes fly:  the physical description of flight as well to get a fuller understanding of the creation of lift.

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Updated: March 12, 2004