View source for Flap (aeronautics)

{{Short description|Anti-stalling high-lift device on aircraft}}
{{More citations needed|date=February 2013}}
{{Use dmy dates|date=February 2020}}
[[File:wing.slat.600pix.jpg|thumb|Trailing edge flaps extended on the right on a typical airliner (an [[Airbus A310|Airbus A310-300]]). [[Leading edge slats]] are also extended, on the left.]]

A '''flap''' is a [[high-lift device]] used to reduce the [[stall (flight)|stalling speed]] of an [[aircraft]] [[wing]] at a given weight. Flaps are usually mounted on the wing [[trailing edge]]s of a [[fixed-wing aircraft]]. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in [[Drag (physics)|drag]] so they are retracted when not needed.

The flaps installed on most aircraft are partial-span flaps; spanwise from near the wing root to the inboard end of the [[aileron]]s. When partial-span flaps are extended they alter the spanwise [[lift distribution]] on the wing by causing the inboard half of the wing to supply an increased proportion of the lift, and the outboard half to supply a reduced proportion of the lift. Reducing the proportion of the lift supplied by the outboard half of the wing is accompanied by a reduction in the [[angle of attack]] on the outboard half. This is beneficial because it increases the margin above the [[Stall (fluid dynamics)|stall]] of the outboard half, maintaining aileron effectiveness and reducing the likelihood of asymmetric stall, and [[Spin (aerodynamics)|spinning]]. The ideal lift distribution across a wing is elliptical, and extending partial-span flaps causes a significant departure from the elliptical. This increases [[lift-induced drag]] which can be beneficial during approach and landing because it allows the aircraft to descend at a steeper angle.

Extending the wing flaps increases the [[camber (aerodynamics)|camber]] or curvature of the wing, raising the maximum [[lift coefficient]] or the upper limit to the lift a wing can generate. This allows the aircraft to generate the required lift at a lower speed, reducing the minimum speed (known as stall speed) at which the aircraft will safely maintain flight. For most aircraft configurations, a useful side effect of flap deployment is a decrease in aircraft pitch angle which lowers the nose thereby improving the pilot's view of the runway over the nose of the aircraft during landing.

There are many different designs of flaps, with the specific choice depending on the size, speed and complexity of the aircraft on which they are to be used, as well as the era in which the aircraft was designed. Plain flaps, [[slotted flap]]s, and [[Fowler flap]]s are the most common. [[Krueger flap]]s are positioned on the leading edge of the wings and are used on many jet airliners. The Fowler, Fairey-Youngman and Gouge types of flap increase the wing area in addition to changing the camber. The larger lifting surface reduces [[wing loading]], hence further reducing the stalling speed.

Some flaps are fitted elsewhere. Leading-edge flaps form the wing leading edge and when deployed they rotate down to increase the wing camber. The [[de Havilland DH.88 Comet]] racer had flaps running beneath the fuselage and forward of the wing trailing edge. Many of the [[Waco Custom Cabin series]] biplanes have the flaps at mid-[[Chord (aeronautics)|chord]] on the underside of the top wing.

==Principles of operation==
The general airplane lift equation demonstrates these relationships:<ref name=perkins-hage>Perkins, Courtland; Hage, Robert (1949). ''Airplane performance, stability and control'', Chapter 2, John Wiley and Sons. {{ISBN|0-471-68046-X}}.</ref>
:<math>L = \tfrac12 \rho V^2 S C_L</math>

where:
* ''L'' is the amount of ''Lift'' produced,
* ''<math>\rho</math>'' is the air density,
* ''V'' is the [[true airspeed]] of the airplane or the ''Velocity'' of the airplane, relative to the air
* '''S''' is the area of the wing
* '''<math>C_L</math>''' is the ''[[lift coefficient]]'', which is determined by the shape of the airfoil used and the angle at which the wing meets the air (or angle of attack).

Here, it can be seen that increasing the area (S) and lift coefficient (<math>C_L</math>) allow a similar amount of lift to be generated at a lower airspeed (V). Thus, flaps are extensively in use for short takeoffs and landings ([[STOL]]).
[[File:easyjet a319 wing g-ezav arp.jpg|thumb|The three orange pods are [[Aircraft fairing|fairings]] streamlining the flap track mechanisms. The flaps (two on each side, on the [[Airbus A319]]) lie directly above these.]]

Extending the flaps also increases the [[drag coefficient]] of the aircraft. Therefore, for any given weight and airspeed, flaps increase the [[Drag (physics)|drag]] force. Flaps increase the [[drag coefficient]] of an aircraft due to higher [[induced drag]] caused by the distorted spanwise lift distribution on the wing with flaps extended. Some flaps increase the wing area and, for any given speed, this also increases the [[parasitic drag]] component of total drag.<ref name=perkins-hage />

=== Flaps during takeoff ===
Depending on the aircraft type, flaps may be partially extended for [[takeoff]].<ref name=perkins-hage /> When used during takeoff, flaps trade runway distance for climb rate: using flaps reduces ground roll but also reduces the climb rate. The amount of flap used on takeoff is specific to each type of aircraft, and the manufacturer will suggest limits and may indicate the reduction in climb rate to be expected. The ''[[Cessna 172S]] Pilot Operating Handbook'' recommends 10° of flaps on takeoff, when the ground is soft or it is a short runway, otherwise 0 degrees is used.<ref name="cessna-172">Cessna Aircraft Company. ''Cessna Model 172S Nav III''. Revision 3-12, 2006, pp. 4–19 to 4–47.</ref>

=== Flaps during landing ===
[[File:Airplane Flaps.jpg|thumb|Flaps during ground roll after landing, with spoilers up, increasing drag.]]
[[File:T-6 G Musee du Bourget P1020147.JPG|thumb|North American T-6 trainer, showing its split flaps]]
Flaps may be fully extended for [[landing]] to give the aircraft a lower stall speed so the approach to landing can be flown more slowly, which also allows the aircraft to land in a shorter distance. The higher lift and drag associated with fully extended flaps allows a steeper and slower approach to the landing site, but imposes handling difficulties in aircraft with very low [[wing loading]] (i.e. having little weight and a large wing area). Winds across the line of flight, known as ''crosswinds'', cause the windward side of the aircraft to generate more lift and drag, causing the aircraft to [[Aircraft principal axes|roll, yaw and pitch]] off its intended flight path, and as a result many light aircraft land with reduced flap settings in crosswinds. Furthermore, once the aircraft is on the ground, the flaps may decrease the effectiveness of the brakes since the wing is still generating lift and preventing the entire weight of the aircraft from resting on the tires, thus increasing stopping distance, particularly in wet or icy conditions. Usually, the pilot will raise the flaps as soon as possible to prevent this from occurring.<ref name="cessna-172" />

=== Maneuvering flaps ===
Some [[Glider aircraft|gliders]] not only use flaps when landing, but also in flight to optimize the camber of the wing for the chosen speed. While [[thermal]]ling, flaps may be partially extended to reduce the stall speed so that the glider can be flown more slowly and thereby reduce the rate of sink, which lets the glider use the rising air of the thermal more efficiently, and to turn in a smaller circle to make best use of the core of the [[thermal]].{{Citation needed|date=February 2013}}  At higher speeds a negative flap setting is used to reduce the nose-down [[pitching moment]]. This reduces the balancing load required on the [[Stabilizer (aircraft)|horizontal stabilizer]], which in turn reduces the trim drag associated with keeping the glider in longitudinal trim.{{Citation needed|date=February 2013}} Negative flap may also be used during the initial stage of an aerotow launch and at the end of the landing run in order to maintain better control by the [[aileron]]s.{{Citation needed|date=February 2013}}

Like gliders, some [[Fighter aircraft|fighters]] such as the [[Nakajima Ki-43]] also use special flaps to improve maneuverability during air combat, allowing the fighter to create more lift at a given speed, allowing for much tighter turns.<ref>Windrow 1965, p. 4.</ref> The flaps used for this must be designed specifically to handle the greater stresses and most flaps have a [[V speeds#VFE|maximum speed]] at which they can be deployed. [[Control line]] model aircraft built for [[Control line#Precision aerobatics|precision aerobatics]] competition usually have a type of maneuvering flap system that moves them in an opposing direction to the elevators, to assist in tightening the radius of a maneuver.

=== Flap tracks ===
Manufactured most often from PH steels and titanium, flap tracks control the flaps located on the trailing edge of an aircraft's wings. Extending flaps often run on guide tracks. Where these run outside the wing structure they may be faired in to streamline them and protect them from damage.<ref>{{cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960052267.pdf|title=High-Lift Systems on Commercial Subsonic Airliners|author=Rudolph, Peter K. C.|date=September 1996|page=39|publisher=NASA|access-date=7 July 2017|archive-date=21 December 2019|archive-url=https://web.archive.org/web/20191221145349/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960052267.pdf|url-status=live}}</ref> Some [[Aircraft fairing|flap track fairings]] are designed to act as [[anti-shock body|anti-shock bodies]], which reduce drag caused by local sonic shock waves where the airflow becomes [[transonic]] at high speeds.

===Thrust gates===
Thrust gates, or gaps, in the trailing edge flaps may be required to minimise interference between the engine flow and deployed flaps. In the absence of an inboard aileron, which provides a gap in many flap installations, a modified flap section may be needed. The thrust gate on the [[Boeing 757]] was provided by a single-slotted flap in between the inboard and outboard double-slotted flaps.<ref>{{cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960052267.pdf|title=High-Lift Systems on Commercial Subsonic Airliners|author=Rudolph, Peter K. C.|date=September 1996|pages=40, 54|publisher=NASA|access-date=7 July 2017|archive-date=21 December 2019|archive-url=https://web.archive.org/web/20191221145349/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960052267.pdf|url-status=live}}</ref> The [[Airbus A320|A320]], [[Airbus A330|A330]], [[Airbus A340|A340]] and [[Airbus A380|A380]] have no inboard aileron. No thrust gate is required in the continuous, single-slotted flap. Interference in the go-around case while the flaps are still fully deployed can cause increased drag which must not compromise the climb gradient.<ref>{{cite CiteSeerX|last=Reckzeh|first=Daniel|title=Aerodynamic Design of Airbus High-lift Wings in a Multidisciplinary Environment|page=7|citeseerx=10.1.1.602.7484|year=2004}}</ref>

==Types of flap==
[[File:Airfoil lift improvement devices (flaps).png|thumb|upright=1.65|Flaps and high lift devices. Gurney flap exaggerated for clarity. Blown flap skipped as it is modified from any other type. Pale lines indicate line of movement, and green indicates flap setting used during dive.]]

=== Plain flap ===
The rear portion of airfoil rotates downwards on a simple hinge mounted at the front of the flap.<ref>Gunston 2004, p. 452.</ref> The [[Royal Aircraft Factory]] and [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] in the [[United Kingdom]] tested flaps in 1913 and 1914, but these were never installed in an actual aircraft.<ref name=Fairey>Taylor 1974, pp. 8–9.</ref> In 1916, the [[Fairey Aviation Company]] made a number of improvements to a [[Sopwith Baby]] they were rebuilding, including their Patent Camber Changing Gear, making the [[Fairey Hamble Baby]] as they renamed it, the first aircraft to fly with flaps.<ref name=Fairey /> These were full span plain flaps which incorporated ailerons, making it also the first instance of flaperons.<ref name=Fairey /> Fairey were not alone however, as [[Breguet Aviation|Breguet]] soon incorporated automatic flaps into the lower wing of their [[Breguet 14]] reconnaissance/bomber in 1917.<ref>{{cite book|last=Toelle|first=Alan|title=Windsock Datafile Special, Breguet 14 | publisher = Albatros Productions|location=Hertfordshire, Great Britain|year=2003|isbn=978-1-902207-61-2}}</ref> Owing 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.{{Citation needed|date=February 2013}}-->

=== 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.<ref>Gunston 2004, p. 584.</ref> This can cause large changes in longitudinal trim, pitching the nose either down or up. At full deflection, a split flaps acts much like a spoiler, adding significantly to drag coefficient.{{Fact|date=February 2024|reason=See talk page for discussion}} It also adds a little to lift coefficient. It was invented by [[Orville Wright]] and James M. H. Jacobs in 1920, but only became common in the 1930s and was then quickly superseded.<ref>{{cite interview |last=Jacobs |first=James Wilbur |interviewer=Susan Bennet |title=Interview with James Wilbur Jacobs |url=http://ecommons.udayton.edu/cgi/viewcontent.cgi?article=1010&context=archives_wrightkett_oh |publisher=University of Dayton |date=4 March 1967 |work=eCommons |access-date=20 July 2020 |archive-date=18 March 2020 |archive-url=https://web.archive.org/web/20200318185137/https://ecommons.udayton.edu/cgi/viewcontent.cgi?article=1010&context=archives_wrightkett_oh |url-status=live }}</ref>{{Failed verification|date=July 2020}} The [[Douglas DC-1]] (progenitor to the DC-3 and C-47) was one of the first of many aircraft types to use split flaps.

=== 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.<ref>Gunston 2004, p. 569.</ref> 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.<ref name=smith>{{cite journal |last1=Smith |first1=Apollo M. O. |author-link1=Apollo M. O. Smith |title=High-Lift Aerodynamics |journal=Journal of Aircraft |issn=0021-8669 |volume=12 |issue=6 |pages=518–523 |year=1975 |url=http://www.arvelgentry.com/amo/High-Lift_Aerodynamics.pdf |access-date=12 July 2011 |doi=10.2514/3.59830 |archive-url=https://web.archive.org/web/20110707172637/http://www.arvelgentry.com/amo/High-Lift_Aerodynamics.pdf |archive-date=7 July 2011}}</ref> 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 that dates from the 1920s, but was not widely used until much later. Some flaps use multiple slots to further boost the effect.

=== Fowler flap ===
A split flap that slides backwards, before hinging downward, thereby increasing first chord, then camber.<ref>Gunston 2004, p. 249–250.</ref> The flap may form part of the upper surface of the wing, like a plain flap, or it may not, like a split flap, but it must slide rearward before lowering. As a defining feature – distinguishing it from the Gouge Flap – it always provides a slot effect. 

The flap was invented by [[Harlan D. Fowler]] in 1924, and tested by [[Fred Weick]] at [[National Advisory Committee for Aeronautics|NACA]] in 1932. First used on the [[Martin 146]] prototype in 1935, it entered production on the 1937 [[Lockheed Model 14 Super Electra|Lockheed Super Electra]],<ref>{{cite book|title= Wind and Beyond: A Documentary Journey Into the History of Aerodynamics|author=National Aeronautics and Space Administration}}</ref> and remains in widespread use on modern aircraft, often with multiple slots.<ref name="NASA">{{cite web |title=The Wind and Beyond: A Documentary Journey into the History of Aerodynamics in America. Volume 1; The Ascent of the Airplane |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040031410.pdf |website=ntrs.nasa.gov |date=January 2003 |publisher=NASA |access-date=17 July 2020 |archive-date=17 July 2020 |archive-url=https://web.archive.org/web/20200717172017/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040031410.pdf |url-status=live |last1=Hansen |first1=James R. |last2=Taylor |first2=D. Bryan |last3=Kinney |first3=Jeremy |last4=Lee |first4=J. Lawrence }}</ref>{{anchor|Junkers flap}}

===Junkers flap===
A slotted plain flap fixed below the trailing edge of the wing, and rotating about its forward edge.<ref>Gunston 2004, p. 331.</ref> 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 Otto Mader at Junkers in the late 1920s, they were most often seen on the [[Junkers Ju 52]] and the [[Junkers Ju 87|Junkers Ju 87 ''Stuka'']], though the same basic design can also be found on many modern ultralights, like the [[Denney Kitfox]].  This type of flap is sometimes referred to as an external-airfoil flap.<ref>{{cite web|url=https://ntrs.nasa.gov/citations/19930081377|title=Full-scale wind-tunnel and flight tests of a Fairchild 22 airplane equipped with external-airfoil flaps |author=Reed, Warren D. |author2=Clay, William C.|date=30 June 1937|publisher=NACA|access-date=10 August 2020|archive-date=21 October 2020|archive-url=https://web.archive.org/web/20201021051045/https://ntrs.nasa.gov/citations/19930081377|url-status=live}}</ref>

===Gouge flap===
{{Main|Gouge flap}}
A type of split flap that slides backward along curved tracks that force the trailing edge downward, increasing chord and camber without affecting trim or requiring any additional mechanisms.<ref>Gunston 2004, p. 270.</ref> It was invented by [[Arthur Gouge]] for [[Short Brothers]] in 1936 and used on the [[Short Empire]] and [[Short Sunderland|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 Aviation#History|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.{{citation needed|date=September 2020}}

=== Zap flap ===
The Zap flap was invented by Edward F. Zaparka while he was with Berliner/Joyce and tested on a [[General Airplanes 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.<ref>{{cite magazine| editor= C.M. Poulsen| date= 27 July 1933| title= "The Aircraft Engineer - flight engineering section" Supplement to Flight| magazine= Flight Magazine| pages= 754a–d| url= http://www.flightglobal.com/pdfarchive/view/1933/1933%20-%200200.html| archive-date= 27 June 2013| archive-url= https://web.archive.org/web/20130627140403/http://www.flightglobal.com/pdfarchive/view/1933/1933%20-%200200.html| url-status= dead}}</ref>

===Krueger flap===
{{Main|Krueger flap}}
A 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.<ref>{{cite web|url=http://www.hq.nasa.gov/pao/History/SP-468/ch10-5.htm|title=Chapter 10: Technology of the Jet Airplane|website=www.hq.nasa.gov|access-date=11 December 2006|archive-date=15 January 2017|archive-url=https://web.archive.org/web/20170115222921/http://www.hq.nasa.gov/pao/History/SP-468/ch10-5.htm|url-status=live}}</ref><ref>{{cite web|url=http://www.aoe.vt.edu/~jschetz/fluidnature/unit05/unit5e.html|title=Virginia Tech – Aerospace & Ocean Engineering|archive-url=https://web.archive.org/web/20070307081041/http://www.aoe.vt.edu/~jschetz/fluidnature/unit05/unit5e.html|archive-date=7 March 2007}}</ref> This is not the same as a leading edge droop flap, as that is formed from the entire leading edge.<ref>Gunston 2004, p. 335.</ref> Invented by Werner Krüger in 1943 and evaluated in Goettingen, Krueger flaps are found on many modern swept wing airliners.

===Gurney flap===
{{Main|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, and has since been used on some helicopters such as the [[Sikorsky S-76|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 rarely used and was then forgotten. Late marks of the [[Supermarine Spitfire (Griffon-powered variants)|Supermarine Spitfire]] used a bead on the trailing edge of the elevators, which functioned in a similar manner.

=== Leading edge flap ===
The entire leading edge of the wing rotates downward, effectively increasing camber and also slightly reducing chord.<ref>Clancy 1975, pp. 110–112.</ref><ref>Gunston 2004, p. 191.</ref> Most commonly found on fighters with very thin wings unsuited to other leading edge high lift devices. Slats are one of such devices, Slats are extendable high lift devices on the leading edge of the wings of some fixed wing aircraft. Their purpose is to increase lift during low speed operations such as take-off, initial climb, approach and landing.

===Blown flap===
{{Main|Blown flap}}
A type of Boundary Layer Control System, blown flaps pass engine-generated air or exhaust over the flaps to increase lift beyond that attainable with mechanical flaps. Types include the original (internally blown flap) which blows compressed air from the engine over the top of the flap, the externally blown flap, which blows engine exhaust over the upper and lower surfaces of the flap, and upper surface blowing which blows engine exhaust over the top of the wing and flap. While testing was done in Britain and Germany before the [[Second World War]],<ref>{{cite web|url=http://naca.central.cranfield.ac.uk/reports/arc/cp/0209.pdf|title=An Analysis of Aerodynamic Data on Blowing Over Trailing Edge Flaps for Increasing Lift|author=Williams, J.|date=September 1954|page=1|publisher=NACA|access-date=11 January 2016|archive-date=1 October 2015|archive-url=https://web.archive.org/web/20151001192734/http://naca.central.cranfield.ac.uk/reports/arc/cp/0209.pdf|url-status=live}}</ref> and flight trials started, the first production aircraft with blown flaps was not until the 1957 [[Lockheed T2V SeaStar]].<ref>American Military Training Aircraft' E.R. Johnson and Lloyd S. Jones, McFarland & Co. Inc. Publishers, Jefferson, North Carolina</ref> Upper Surface Blowing was used on the [[Boeing YC-14]] in 1976.

===Flexible flap===
{{Main|Adaptive compliant wing}}
Also known as the '''FlexFoil'''. A modern interpretation of wing warping, internal mechanical actuators bend a lattice that changes the airfoil shape. It may have a flexible gap seal at the transition between fixed and flexible airfoils.<ref>{{cite web|title=Shape-shifting flap takes flight|url=http://www.aopa.org/News-and-Video/All-News/2014/November/17/Wing-warping|access-date=19 November 2014|date=2014-11-17|archive-date=29 November 2014|archive-url=https://web.archive.org/web/20141129063706/http://www.aopa.org/News-and-Video/All-News/2014/November/17/Wing-warping|url-status=live}}</ref>

===Flaperon===
{{Main|Flaperon}}
A type of aircraft [[flight control surfaces|control surface]] that combines the functions of both [[flap (aircraft)|flaps]] and [[aileron]]s.

=== Continuous trailing-edge flap ===
As of 2014, [[U.S. Army Research Laboratory]] (ARL) researchers at NASA's Langley Research Center developed an active-flap design for helicopter rotor blades. The Continuous Trailing-Edge Flap (CTEF) uses components to change blade camber during flight, eliminating mechanical hinges in order to improve system reliability. Prototypes were constructed for wind-tunnel testing.<ref>{{Cite book|title=Technical Committees Present the Year in Review|publisher=Aerospace America|year=2014|page=15}}</ref>

A team from ARL completed a live-fire test of a rotor blade with individual blade control technology in January 2016. The live fire experiments explored the ballistic vulnerability of blade control technologies. Researchers fired three shots representative of typical ground fire on a 7-foot-span, 10-inch-chord rotor blade section with a 4-foot-long CTEF at ARL's Airbase Experimental Facility.<ref>{{Cite web|url=https://www.arl.army.mil/www/default.cfm?article=2758|title=Army researchers explore future rotorcraft technologies {{!}} U.S. Army Research Laboratory|website=www.arl.army.mil|language=en|access-date=2018-07-10|archive-date=10 July 2018|archive-url=https://web.archive.org/web/20180710195820/https://www.arl.army.mil/www/default.cfm?article=2758|url-status=live}}</ref>

===Related devices===
* '''[[Leading edge slats]] and [[Leading edge slot|slot]]s''' 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. They offer excellent lift and enhance controllability at low speeds. Leading edge slats allow the wing to fly at a higher angle of attack which decrease takeoff and landing distances.<ref>{{Cite web|url=https://www.flightglobal.com/pdfarchive/view/1921/1921%20-%200845.html|title=fig {{!}} slot opffh {{!}} pbar slot {{!}} 1921 {{!}} 0845 {{!}} Flight Archive|website=www.flightglobal.com|access-date=2019-04-18|archive-date=15 May 2019|archive-url=https://web.archive.org/web/20190515150512/https://www.flightglobal.com/pdfarchive/view/1921/1921%20-%200845.html|url-status=live}}</ref> 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. Frederick Handley Page experimented with fore and aft slot designs in the 20s and 30s.
* '''[[Spoiler (aeronautics)|Spoilers]]''' are intended to create drag and reduce lift by "spoiling" the airflow over the wing. A spoiler is much larger than a Gurney flap, and can be retracted. Spoilers are usually installed mid chord on the upper surface of the wing, but may also be installed on the lower surface of the wing as well.
* '''[[Air brake (aircraft)|Air brakes]]''' are used to increase drag, allowing the aircraft to decelerate rapidly. When installed on the wings they differ from flaps and spoilers in that they are not intended to modify the lift and are built strongly enough to be deployed at much higher speeds.
* '''[[Aileron]]s''' 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. When ailerons are designed to lower in conjunction with flaps, they are usually called [[flaperon]]s, while those that spoil lift (typically placed on the upper surface before the trailing edge) they are called [[spoileron]]s.
{{Clear}}

<gallery>
File:ILA 2008 PD 750.JPG|Plain flap at full deflection.
File:Avro Lancaster flap Flickr 4841178432.jpg|Split flap on a World War II bomber
File:A fully extended flap.jpg|Double slotted Fowler flaps extended for landing
File:Undercarriage.b747.arp.jpg|Krueger flaps and triple-slotted trailing-edge flaps of a [[Boeing 747]] extended for landing
File:Kitfox Lite.jpg|Junkers flaps, doubling as [[ailerons]].
</gallery>

==See also==
{{Commons category|Trailing-edge flaps}}
*[[Air brake (aeronautics)]]
*[[Aircraft flight control system]]
*[[Aileron]]
*[[Body flaps]], a type of high-drag set of aerosurfaces designed for very high angle-of-attack descent of rocket-powered vehicles, particularly used during [[atmospheric entry]] of space vehicles.  Body flaps are being designed<!-- currently just beginning the flight test program in 4Q2020 --> to bleed off as much kinetic and potential energy as possible during a near-vertical descent through the atmosphere.<ref name=spacex20191020>{{cite AV media |url=https://www.youtube.com/watch?time_continue=94&v=bysu8XN5OfY |title=SpaceX - Mars Society Convention 2019 |date=20 October 2019 |people=Paul Wooster |time=47:30-49:00 |access-date=25 October 2019 |medium=video |via=YouTube |quote=''Vehicle is designed to be able to land at the Earth, Moon or Mars. Depending on which ... the ratio of the energy dissipated aerodynamically vs. propulsively is quite different. In the case of the Moon, it's entirely propulsive. ... Earth: over 99.9% of the energy is removed aerodynamically ... Mars: over 99% of the energy is being removed aerodynamically at Mars''.}}</ref><ref name=musk20200805>{{cite tweet |user=ElonMusk |number=1290826885375696899 |date=5 August 2020 |title=We will do several short hops to smooth out launch process, then go high altitude with body flaps |archive-date=6 August 2020 |archive-url=https://web.archive.org/web/20200806161427/https://twitter.com/elonmusk/status/1290826885375696899 |url-status=live}}</ref><ref name=sx20201207>{{cite web |url=https://www.spacex.com/vehicles/starship/index.html |title=UPCOMING TEST: Starship high-altitude flight test |website=spacex.com |date=7 December 2020 |access-date=8 December 2020 |archive-date=27 November 2020 <!-- needs to be 7 Dec as SpaceX updated the page late on 7 Dec -->|archive-url=https://web.archive.org/web/20201127095415/https://www.spacex.com/vehicles/starship/index.html |url-status=live }}</ref><!-- leaving this in "See also" rather than the article body since, as a flap used on a space vehicle, this technically seems just a bit out of scope the rest of the article discussing aircraft flaps -->
*[[Circulation control wing]]
*[[High-lift device]]
*[[Leading-edge slat]]

==References==
{{Reflist|colwidth=35em}}

===Bibliography===
* {{cite book | last=Clancy | first=L.J. | chapter =6 | title=Aerodynamics | location=London | publisher=Pitman Publishing Limited | year=1975 | isbn=978-0-273-01120-0}}
* Gunston, Bill, The Cambridge Aerospace Dictionary Cambridge, Cambridge University Press 2004, {{ISBN|978-0-521-84140-5}}/{{ISBN|0-521-84140-2}}
* Windrow, Martin C. and René J. Francillon. ''The Nakajima Ki-43 Hayabusa''. Leatherhead, Surrey, UK: Profile Publications, 1965.

{{Aircraft components}}
{{Authority control}}

{{DEFAULTSORT:Flap (Aircraft)}}
[[Category:Aircraft controls]]
[[Category:Aircraft wing components]]