Definition: Hinged sections on the trailing edges of wings.
Significance: Both ailerons and flaps can be deflected to change local wing camber and to increase or decrease local lift. Ailerons are used to control the airplane in roll, while flaps allow flight at lower speeds for landing and takeoff.
Ailerons
Early experimenters with gliders turned their vehicles by shifting their bodies so their weight was to the left or right of their wing’s lifting center. This action made the glider roll or bank to help it turn. Wilbur and Orville Wright improved on this effect by twisting or warping their wood and fabric wing with ropes and pulleys so that one wingtip was at a higher angle of attack than the other and the difference in lift on the two wingtips helped it roll. This design gave their airplane much greater maneuverability than early European designs which tried to turn using only a rudder. This wing-warping control system was the essential element in the Wright patent on the first successful airplane.
Glenn H. Curtiss, another American aviation pioneer, patented a different way to control an airplane in roll, using ailerons, originally small, separate wings that were placed between the upper and lower wings of his biplane near the wingtips. The pilot could change the angle between these small wings and the flow to increase the lift on one wing and decrease that of the other. The Wrights claimed that this was a violation of their patent, and the case spent many years in the courts until the U.S. government stepped into resolve the dispute.
Today’s ailerons are built into the trailing edge of wings near the wingtips, and they work by changing the wing’s camber, or curvature, instead of its angle of attack. The ailerons deflect either up or down opposite to each other to increase the lift near one wingtip while lowering lift on the other wingtip. This makes the wing roll, with one wing moving up and the other down. Usually the aileron deflecting up produces more drag than the one moving down, which helps the airplane turn. In most turns, the aileron movements are coordinated with the movement of the rudder to create a turn which is balanced so that the airplane passengers feel only a downward force and no sideward force. A coordinated turn not only feels better but also is more aerodynamically efficient.
If the pilot wants to roll the airplane without turning, the rudder must also be used to oppose the turning motion caused by aileron drag; this is called a cross-control maneuver. A similar cross-control use of rudder and ailerons can make the airplane rotate to the left or right in a sideslip motion without rolling.
Flaps
Flaps often resemble ailerons except that they are placed on the wing near the fuselage rather than near the wing-tips. Flaps normally are only deflected downward since they are used to increase temporarily the wing’s lift on landing and sometimes on takeoff. This maneuver allows flight at lower speeds and landing and takeoff in shorter distances.
Early aircraft did not need flaps because they flew at low speeds and could land in much shorter distances than today’s planes; however, as airplanes became more streamlined and could cruise at higher speeds and altitudes, they needed higher speeds for takeoff and landing. Designers added flaps to give additional lift and drag and to reduce landing speeds. The famous DC-3 airliner was one of the first commercial planes to use flaps to combine good cruise performance with reasonable landing and takeoff distances.
There are many types of flaps, from simple plates that deflect down from the bottom of the wing to very sophisticated combinations of little wings that extend down and behind a wing. The split flap was used on the DC-3 and many World War II airplanes. Fowler flaps are more common today, but many smaller aircraft use simple hinges on the rear part of their wings to deflect a plain flap. The Fowler flap increases the wing camber while increasing the wing area. The space that opens up between the deployed Fowler flap and the wing allows an airflow that helps control the pressures over the flap and delay wing stall.
Many airliners designed in the mid-to-late twentieth century used complex flap systems that worked like the Fowler flap but had two or more flap elements that opened out below and behind the wing. These flap systems were very carefully designed temporarily to give very high lift at low speeds on sleek, modern wings that were shaped for flight near the speed of sound. They allowed airplanes that cruise at 500 to 600 miles per hour to land at low speeds and come to a stop on relatively short runways.
Today’s commercial transport designs do not need these complex flap systems and tend to use simpler Fowler flaps, which are lighter and easier to build and maintain. This shift is partly because of improvements in wing and airfoil design and partly because most major airports now have longer runways.
Flaperons and Slats
Occasionally, an airplane design needs extra flap area to get lower landing speeds and the ailerons are also used as flaps. This kind of aileron is called a flaperon, and it requires a more complex hookup to the aircraft controls than a standard aileron and flap system.
Some aircraft have flaps on the front of their wings that can also be deflected downward to increase the wing camber. These leading edge flaps, or slats, help control the flow over the wing at high angles of attack and allow the wing to go to a higher angle of attack before it stalls.
