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.
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.