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Factors Affecting Lift

What Factors Affect Lift?

The size and shape of the wing, the angle at which it meets the oncoming air, the speed at which it moves through the air, even the density of the air, all affect the amount of lift a wing creates. Let’s begin with the shape of a wing intended for subsonic flight.

Why Does a Wing Have a Rounded Front?

Air divides smoothly around a wing’s rounded leading edge, and flows neatly off its tapered trailing edge. You might think a sharp leading edge would be better. However, air cannot turn a sharp corner, so tilting a sharp wing even slightly would disrupt the smooth airflow over the wing. This would cause a loss of lift and increase drag. A rounded leading edge divides the airflow smoothly, even as the wing is tilted up or down.

Why Does a Wing Have a Sharp Rear Edge?

If the trailing edge were rounded, the higher-pressure air flowing along the lower side would try to follow the rounded surface and spill upward into the lower-pressure air above the wing. A sharp trailing edge prevents this upward spill, because air cannot make a sharp turn. Instead, the air flowing off the top and bottom surfaces rejoins smoothly.

How Does Tilting a Wing Affect the Air Flowing Over It?

"Tilting" a wing up or down changes the wing's angle of attack to the oncoming airstream and affects a wing's ability to produce lift.  Tilting the wing upward (or increasing the angle of attack) increases lift—to a point—but decreases airspeed. If you tilt it too much, the airflow pulls away from the upper surface, and the smooth flow turns turbulent. The wing suddenly loses lift, a condition known as a stall. You can reestablish a smooth airflow by tilting the wing back to a more level position.  Tilting the wing downward (or decreasing the angle of attack) decreases lift, but increases airspeed.

Credit: Bill Tinkler

What Are Flaps For?

Flaps change a wing’s curvature, increasing lift. Airplanes use flaps to maintain lift at lower speeds, particularly during takeoff and landing. This allows an airplane to make a slower landing approach and a shorter landing. Flaps also increase drag, which helps slow the airplane and allows a steeper landing approach.

The Museum’s Jet Aviation gallery contains a working model of a Lockheed L-1011 jetliner wing. It demonstrates how the wing’s complex system of ailerons, flaps, and other devices works during a landing.

Related Images

Flying Upside Down

Flaps at Takeoff

Flaps at Landing

Ask an Explainer

Q:

How much lift can wings produce?

A:

An airliner wing may produce a pound of lift per square inch in level flight. That doesn't seem like much, but over the entire surface of the wings these pounds-per-square-inch add up. The wings of a Boeing 747 have a surface area of about 510 square meters (5,500 square feet) and can produce as much as 390 tons (850,000 pounds) of lift.

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Did You Know?

A hydrofoil generates lift in the same way that an airfoil does. A hydrofoil generates lift to raise its hull out of the water to reduce drag.

Pop Quiz

Air travels faster over the top surface of a wing, because it has to "catch up" with the air traveling along the shorter distance below the wing.

A) True
B) False

When an airplane flies, it pushes air out of the way. That air must go somewhere so it "squeezes" between the wings and the surrounding air.  The wings are shaped and tilted so that the air moving over the top has less room than the air moving below the wings.  The air moving over the top must speed up more than the air below the wing, because of conservation of mass.  As it moves faster, the air on top of the wing also loses pressure and push.  The slower moving air below the wing maintains more of its pressure, which pushes the wing, and the plane, up.