Bernoullis Principle of Lift

The force (called lift) that enables an airplane to fly is produced by the application of a physical principle discovered in 1738 by Swiss scientist Daniel Bernoulli (1700-1782). Called “Bernoulli’s Law of Differential Pressure,” or simply Bernoulli’s Principle, it states that as the speed of a fluid increases, its pressure decreases. Bernoulli did his experiments using liquids. Later, scientists proved this principle applies to any fluid, including gases. The reason pressure decreases as the speed of a fluid increases is because of the law of conservation of energy. There is a finite amount of energy in the fluid (whether liquid or gas). When the fluid speeds up, it leaves less to exert pressure, which therefore declines.

There are many examples of Bernoulli’s principle in everyday life. For instance, suppose you get caught in a rainstorm and open an umbrella. If it is windy, you feel an upward pull on the umbrella. Air blowing under the umbrella passes directly from one side to the other. The air passing over the top of the umbrella has to travel farther and so it speeds up. According to Bernoulli’s principle, this means the air pressure on the top of the umbrella decreases. Since the air on the bottom is at a higher pressure, it pushes up harder than the air above is pressing down, producing a net upward force. You feel the upward force as a tugit seems as if the wind is trying to snatch the umbrella out of your hand.

It was not until near the end of the 1800s that it was realized this might be useful for designing a flying machine. But in 1893 Edward Huffaker suggested this idea to Samuel Langley, director of the Smithsonian Institution. The Wright brothers later drew on this idea to solve a crucial puzzle in their efforts to build a heavier-than-air flying machine.

The top of an airplane wing is curved, while the lower surface is almost flat (much like the umbrella mentioned above). When the airplane is moving, air flows over and under the wing. The air flowing over the curved top of the wing must travel farther and speeds up, causing the air pressure on the top of the wing to drop. This creates a pressure differential between the upper and lower surfaces of the wings.

This is what produces most of the lift that enables a plane to fly (about 75-80%). The remainder is produced by the “angle of attack” of the wings. Aircraft are designed with the forward edge of the wings tilted up at a slight angle (about 5-10 degrees) so more air strikes the underside of the wings as it moves, adding to the air pressure on the wings’ underside and increasing the lift.

Although all airplane wings have a curved top and flat underside, there are many different types of wings. For example, when an engineer designs an aircraft, one goal is to maximize the lift the wings produce. Another is to minimize the air resistance or “drag.” The less drag, the faster the plane will fly; besides, if there is too much drag, the plane won’t be able to get off the ground!

Airplanes that fly relatively slowly have thick wings. The air has further to travel over the top of the wing, increasing the pressure differential and creating more lift. However, that thick wing also increases the drag. If you want to build a really fast plane, like a supersonic fighter jet, that won’t work. To fly that fast, a thin wing is needed. This design doesn’t produce as much lift at a given speed, but supersonic aircraft make up for this because the plane flies so much faster that the high rate of airflow creates enough lift.

Whatever the exact design, all airplanes depend on this application of Bernoulli’s Principle to produce the lift they need to leave the ground and fly.