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# Aerodynamics: What Causes Lift?

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Source: NOVA: "Nazi Prison Escape"

This resource can be found on the NOVA: “Nazi Prison Escape" Web site.

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Bernoulli's principle, an important principle of fluid dynamics, is often used to explain what causes lift, the upward force that keeps an airplane or glider in flight. This media-enhanced essay from the NOVA Web site presents an additional explanation of lift, based on Newton's third law of motion, which holds that in order to produce lift a wing must push air down.

Background Essay

To get an airplane airborne or climbing in flight, its wings must produce more lift than the total weight of the aircraft. One popular explanation for how this happens is based on Bernoulli's principle, which describes the relationship between the velocity and pressure exerted by a fluid in motion. It states that as the velocity of a fluid increases, the pressure exerted by that fluid decreases, and vice versa. For most planes, the wings, viewed in profile, are curved on top and flat on the bottom. As air passes over the wing, it has further to travel and moves faster than the air passing beneath the wing. According to Bernoulli, this creates a difference in pressure that results in a net upward force.

But that explanation may be incomplete because it doesn't state, for one thing, why air moving above and beneath a wing must meet at the wing's trailing edge. An alternative, perhaps even complementary, explanation calls on Newton's third law of motion: for every action, there is an equal and opposite reaction. The focus here is on the tilt of the wing, called its angle of attack, and its influence on airflow. It holds that as a wing is tilted upward, it generates more lift. This is because more air molecules strike the bottom surface of the wing and get deflected downward. This in turn transfers upward momentum to the wing. Thus, if the angle of attack is increased, the plane rises; if it is decreased, the plane descends.

Airfoil is a term for the cross-section of an airplane wing. While thick airfoils provide lots of lift, they also produce lots of drag, a force that tends to slow the motion of a plane through the air. For this reason, planes with thick airfoils are not well suited for high-speed or long-duration flight. Thin airfoils, by contrast, minimize drag and are both fast and fuel-efficient. Fighter jet wings are almost symmetrical: the curve of the upper surface is almost identical to the curve on the bottom surface. This results in less lift compared with other wing profiles. To compensate, the plane has to move through the air at high speed to stay aloft.

Discussion Questions

• Why does the illustration at the beginning of the essay provide an incomplete explanation of what causes lift? What is missing?
• How does Newton' third law of motion help explain what causes lift?
• Why is the shape of an airplane's wing important?

• Standards

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