Flap (aircraft) - Types of Flaps

Types of Flaps

• Plain flap: the rear portion of airfoil rotates downwards on a simple hinge mounted at the front of the flap. Used in this form as early as 1917 (during World War I) on the widely produced Breguet 14 and possibly earlier on experimental types. Due to the greater efficiency of other flap types, the plain flap is normally only used where simplicity is required. A modern variation on the plain flap exploits the ability of composites to be designed to be rigid in one direction, while flexible in another. When such a material forms the skin of the wing, its camber can be altered by the geometry of the internal supporting structure, allowing such a surface to be used either as a flap or as an aileron. While most currently use a complex system of motors and actuators, the simplest such installation uses ribs that resemble bent carrots - when the bend is nearly horizontal, there is no deflection, but when the carrot is rotated so the bend is downward, the camber of the airfoil is changed in the same manner as on a plain flap.

• Split flap: the rear portion of the lower surface of the airfoil hinges downwards from the leading edge of the flap, while the upper surface stays immobile. Like the plain flap, this can cause large changes in longitudinal trim, pitching the nose either down or up, and tends to produce more drag than lift. At full deflection, a split flaps acts much like a spoiler, producing lots of drag and little or no lift. It was invented by Orville Wright and James M. H. Jacobs in 1920 but only became common in the 1930s but was quickly superseded. The Douglas DC-3 & C-47 used a split flap.

• Slotted flap: a gap between the flap and the wing forces high pressure air from below the wing over the flap helping the airflow remain attached to the flap, increasing lift compared to a split flap. Additionally, lift across the entire chord of the primary airfoil is greatly increased as the velocity of air leaving its trailing edge is raised, from the typical non-flap 80% of freestream, to that of the higher-speed, lower-pressure air flowing around the leading edge of the slotted flap. Any flap that allows air to pass between the wing and the flap is considered a slotted flap. The slotted flap was a result of research at Handley-Page, a variant of the slot and dates from the 1920s but wasn't widely used until much later. Some flaps use multiple slots to further boost the effect.

• Fowler flap: split flap that slides backwards flat, before hinging downwards, thereby increasing first chord, then camber. The flap may form part of the uppersurface of the wing, like a plain flap, or it may not, like a split flap but it must slide rearward before lowering. It may provide some slot effect but this is not a defining feature of the type. Invented by Harlan D. Fowler in 1924, and tested by Fred Weick at NACA in 1932. They were first used on the Martin 146 prototype in 1935, and in production on the 1937 Lockheed Electra, and is still in widespread use on modern aircraft, often with multiple slots. As mentioned under the plain flaps, variable geometry wings are making a comeback, and an F-111 was modified with such a system that acted as fowler flaps by NASA for trials on the AFTI/F-111 Mission Adaptive Wing.

• Junkers Flap: a slotted plain flap where the flap is fixed below the trailing edge of the wing, rotating about its forward edge. When not in use it has more drag than other types but is more effective at creating additional lift than a plain or split flap, while retaining their mechanical simplicity. Invented by O. Mader at Junkers in the late 1920s, it was widely used on the Junkers Ju 52, though it can be found on many modern ultralights.
• Gouge flap: a type of split flap that slides backwards along curved tracks that force the trailing edge downwards, increasing chord and camber without affecting trim or requiring any additional mechanisms. It was invented by Arthur Gouge for Short Brothers in 1936 and used on the Short Empire and Short Sunderland flying boats which used the very thick Shorts A.D.5 airfoil. Short Brothers may have been the only company to use this type.

• Fairey-Youngman flap: drops down (becoming a Junkers Flap) before sliding aft and then rotating up or down. Fairey was one of the few exponents of this design, which was used on the Fairey Firefly and Fairey Barracuda. When in the extended position, it could be angled up (to a negative angle of incidence) so that the aircraft could be dived vertically without needing excessive trim changes.

• Zap Flap or commonly but incorrectly Zapp Flap: Invented by Edward F. Zaparka while he was with Berliner/Joyce and tested on a General Aircraft Corporation Aristocrat in 1932 and on other types periodically thereafter, but it saw little use on production aircraft other than on the Northrop P-61 Black Widow. The leading edge of the flap is mounted on a track, while a point at mid chord on the flap is connected via an arm to a pivot just above the track. When the flap's leading edge moves aft along the track, the triangle formed by the track, the shaft and the surface of the flap (fixed at the pivot) gets narrower and deeper, forcing the flap down.

• Krueger flap: hinged flap which folds out from under the wing's leading edge while not forming a part of the leading edge of the wing when retracted. This increases the camber and thickness of the wing, which in turn increases lift and drag. This is not the same as a leading edge droop flap, as that is formed from the entire leading edge. Invented by Werner Krüger in 1943 and evaluated in Goettingen, Krueger flaps are found on many modern swept wing airliners.

• Gurney flap: A small fixed perpendicular tab of between 1 and 2% of the wing chord, mounted on the high pressure side of the trailing edge of an airfoil. It was named for racing car driver Dan Gurney who rediscovered it in 1971, but has since used on some helicopters such as the Sikorsky S-76B to correct control problems without having to resort to a major redesign. It boosts the efficiency of even basic theoretical airfoils (made up of a triangle and a circle overlapped) to the equivalent of a conventional airfoil. The principle was discovered in the 1930s but was never proceeded with and was then forgotten.

• Leading edge droop: entire leading edge of the wing rotating downwards, effectively increasing camber but slightly reducing chord. Most commonly found on fighters with very thin wings unsuited to other leading edge high lift devices.

• Blown flaps: also known as Boundary Layer Control Systems, are systems that blow engine air over the upper surface of any of the previously mentioned types of flap to improve lift characteristics. Two types exist - the original type blew air out of channels or holes in the surface of the flap, while newer systems simply blow engine exhaust over the top of the flap. These require ample reserves of power and are maintenance intensive thus limiting their use but they provide lots of lift at low airspeeds. Although invented by the British, the first production aircraft with blown flaps was the Lockheed F-104 Starfighter. The later type was trialled on the Boeing YC-14 in 1976.

• Controls that look like flaps but are not:
  • Handley Page leading edge slats/slots may be confused for flaps but are mounted on the top of the wings' leading edge and while they may be either fixed or retractable, when deployed they provide a slot or gap under the slat to force air against the top of the wing which is absent on a Krueger flap. Other types of flaps may be equipped with one or more slots to increase their effectiveness, a typical setup on many modern airliners. These are known as slotted flaps as described above.

• • Spoilers may also be confused for flaps but are intended solely to create drag and not lift. A spoiler is much larger than a Gurney flap, and can be retracted.

• • Ailerons are similar to flaps (and work the same way) but are intended to provide lateral control, rather than to change the lifting characteristics of both wings together, and so operate differentially - when an aileron on one wing increases the lift, the opposite aileron does not, and will often work to decrease lift. Some aircraft use flaperons, which combine both the functionality of flaps and ailerons in a single control, working together to increase lift, but to slightly different degrees so the aircraft will roll toward the side generating the least lift. Flaperons were used by the Fairey Aviation Company as early as 1916 but didn't become common until after World War II.

• A fully extended double slotted fowler flaps before landing

• Triple-slotted trailing-edge flaps and leading edge Krueger flaps fully extended on a Boeing 747 for landing.

• Junkers flaps, doubling as ailerons clearly visible on a Kitfox Lite ]]