What Do Wing Flaps Do?

Modern jet transport aircraft have wings designed to operate efficiently within a wide range of air speeds. Transport aircraft must be able to fly at speeds of from 100 miles per hour to close to the speed of sound. A look at the wing's secondary control surfaces -- flaps and slats -- will explain how the highly adaptable wing accommodates this range of speed.

Flaps are located on the aft or "trailing edge" of the wing and increase both lift and drag. Because of the increased drag, flaps are only partially extended on takeoff, and fully extended on landing. The added drag created by full flaps on landing allows the aircraft to approach at a steep angle without excessive air speed. Think of a car as it rolls downhill. If the hill is too steep, the car's speed becomes excessive. By using the drag of the brakes, the speed stays under control.

The greater the wing's camber, or curvature, the better the wing will perform at slow air speeds. Flaps extend downward and rearward, thereby increasing both camber and wing area. The flaps extend in several segments, creating gaps, known as slots, in the wing. The slots allow high energy air from below the wing to vent over the top of the wing. This high energy air "tricks the wing into thinking" it is traveling faster than it actually is, enhancing the low speed performance. On the leading edge of the wing are the slats. As the slats extend forward another slot is opened on the leading edge of the wing. The combination of the slats and flaps creates an airfoil of significant camber for low speed operations.

The wing is said to be "dirty" when the flaps and slats are extended, referring to the high amount of drag the wing experiences. In the dirty configuration the wing is capable of low speed flight; the high drag would, however, prohibit high speed flight. After takeoff the wing is "cleaned up." After climbing through 400 feet the air speed is allowed to increase, reducing the need for the flaps and slats. After the completion of the flap and slat retraction, the aircraft can accelerate to cruise speed. During the approach for landing, the flaps and slats are again extended. The flaps are set to an initial setting, usually one degree. The leading edge slats are later deployed. When initiating the final descent for landing, the flaps are fully extended. Now that the wing is again in its low speed configuration, the aircraft can be slowed to its final approach speed.

Aircraft designers know that no wing is efficient at all speeds. By using flaps, slats and slots, a wing can take on beneficial low speed characteristics for takeoff and landing, and characteristics beneficial for high speed flight while at cruise. If it were not for flaps and slats, the takeoff and landing speeds would have to be much greater and runways would be prohibitively long.

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