Aeronautics - Principles of Flight (BERNOULLI'S PRINCIPLE)


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BERNOULLI'S PRINCIPLE

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 below). 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 the Figure to the right, 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.

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 top of the airfoil. On the lower side of the airfoil, the pressure is higher than on the top of the airfoil. The difference in pressure acting on the airfoil produces a force pushing the airfoil up and backwards in relation to the direction of the airflow. The upward part of the force acting on the airfoil is what causes lift.

If you don't see the animation, click here to download Macromedia Shockwave Player.

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From
Civil Air Patrol
Educational Materials

Updated: February 23, 1999