Flap (aircraft) - AbsoluteAstronomy.com

Flaps are normally hinged surfaces mounted on the trailing edges of the wing

Wing

A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...

s of a fixed-wing aircraft

Fixed-wing aircraft

A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as improving climb rate. It does this by increasing the amount of lift generated, by lowering the stall speed

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

and increasing the drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

.

Extending flaps primarily increases the camber

Camber (aerodynamics)

Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...

or curvature of the wing airfoil raising the maximum lift coefficient

Lift coefficient

The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...

- or the lift a specific wing section may generate. This allows the aircraft to generate as much lift but at a lower speed, reducing the stalling speed

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

of the aircraft, or the minimum speed at which the aircraft will maintain flight. Extending flaps increases drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

which can be beneficial during approach and landing because it slows the aircraft. On some aircraft a useful side effect of flap deployment is a decrease in aircraft pitch angle which improves the pilot's view of the runway over the nose of the aircraft during landing, however the flaps may also cause pitchup, depending on the type of flap and the location of the wing.

There are many different types of flaps used, with the specific choice depending on the size, speed and complexity of the aircraft they are to be used on, as well as the era in which the aircraft was designed. Plain flaps, slotted flaps, and Fowler flaps are the most common. Flaps used on the leading edge of the wings of many jet airliners are called Krueger flaps

Krueger flaps

Krueger flaps are lift enhancement devices that may be fitted to the leading edge of an aircraft wing. Unlike slats or drooped leading edges, the main wing upper surface and its nose is not changed...

.
The Fowler, Fairey-Youngman and Gouge types of flap increase the planform area of the wing in addition to changing the camber. The larger lifting surface reduces wing loading

Wing loading

In aerodynamics, wing loading is the loaded weight of the aircraft divided by the area of the wing. The faster an aircraft flies, the more lift is produced by each unit area of wing, so a smaller wing can carry the same weight in level flight, operating at a higher wing loading. Correspondingly,...

and allows the aircraft to generate more lift at a given speed and reduces stalling speed but although the effect is similar to increasing the lift coefficient, raising the planform area of the wing does not necessarily change the lift coefficient which is dependent on the camber, not the chord

Chord

Chord may refer to:* Chord , an aggregate of musical pitches sounded simultaneously.** Chord an aggregate of musical pitches played simultaneously on a guitar* Chord , a line segment joining two points on a curve...

Physics explanation

The general airplane lift equation demonstrates these relationships:

where:

L is the amount of Lift produced,
is the air density,
V is the indicated airspeed
Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....

of the airplane or the Velocity of the airplane, relative to the air
S is the planform area or Surface area of the wing and
is the lift coefficient
Lift coefficient
The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...

which is determined by the camber of the airfoil used, the chord of the wing 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 (

) allow a similar amount of lift to be generated at a lower airspeed (V).
Extending the flaps also increases the drag coefficient

Drag coefficient

In fluid dynamics, the drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment such as air or water. It is used in the drag equation, where a lower drag coefficient indicates the object will have less aerodynamic or...

of the aircraft. Therefore, for any given weight and airspeed, flaps increase the drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

force. Flaps increase the drag coefficient

Drag coefficient

of an aircraft because of higher induced drag caused by the distorted spanwise lift distribution on the wing with flaps extended. Some flaps increase the planform area of the wing and, for any given speed, this also increases the parasitic drag

Parasitic drag

Parasitic drag is drag caused by moving a solid object through a fluid medium . Parasitic drag is made up of many components, the most prominent being form drag...

component of total drag.

Flaps during takeoff

Depending on the aircraft type, flaps may be partially extended for takeoff

Takeoff

Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.For horizontal takeoff aircraft this usually involves starting with a transition from moving along the ground on a runway. For balloons, helicopters and some specialized fixed-wing aircraft , no...

. When used during takeoff, flaps trade runway distance for climb rate - using flaps reduces ground roll and 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 172

Cessna 172

The Cessna 172 Skyhawk is a four-seat, single-engine, high-wing fixed-wing aircraft. First flown in 1955 and still in production, more Cessna 172s have been built than any other aircraft.-Design and development:...

Pilot Operating Handbook recommends not using any flaps on takeoff unless the ground is rough or soft.

Flaps during landing

Flaps may be fully extended for landing

Landing

thumb|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...

to give the aircraft a lower stalling 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

Wing loading

(the ratio between the wing area and the weight of the aircraft). 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 roll, yaw and pitch off its intended flight path, and as a result many light aircraft have limits on how strong the crosswind can be while using flaps. 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.

Maneuvering flaps

Some gliders

Glider aircraft

Glider aircraft are heavier-than-air craft that are supported in flight by the dynamic reaction of the air against their lifting surfaces, and whose free flight does not depend on an engine. Mostly these types of aircraft are intended for routine operation without engines, though engine failure can...

not only use flaps when landing but also in flight to optimize the camber of the wing for the chosen speed. When thermal

Thermal

A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...

ling, flaps may be partially extended to reduce the stalling speed so that the glider can be flown more slowly and thereby turn in a smaller circle to make best use of the core of the thermal

Thermal

.
At higher speeds a negative flap setting is used to reduce the nose-down pitching moment

Pitching moment

In aerodynamics, the pitching moment on an airfoil is the moment produced by the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center of the airfoil...

. This reduces the balancing load required on the horizontal stabilizer

Stabilizer (aircraft)

In aviation, a stabilizer provides stability when the aircraft is flying straight, and the airfoil of the horizontal stabilizer balances the forces acting on the aircraft....

, which in turn reduces the trim drag associated with keeping the glider in longitudinal trim. 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

Aileron

Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...

s.

Like gliders, some fighters

Fighter aircraft

A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

also use special flaps to improve maneuverability during air combat, allowing the fighter to bleed off speed quickly, allowing for much tighter turns. The flaps used for this must be designed specifically to handle the greater stresses as most flaps have a maximum speed at which they can be deployed.

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
Breguet 14
-See also:-References:*Tomasz J. Kowalski, Samolot Breguet 14, TBiU no.197, Warsaw 2002, ISBN 83-11-09461-6...

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 suface 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
Douglas DC-3
The Douglas DC-3 is an American fixed-wing propeller-driven aircraft whose speed and range revolutionized air transport in the 1930s and 1940s. Its lasting impact on the airline industry and World War II makes it one of the most significant transport aircraft ever made...

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
Fred Weick
Fred Ernest Weick was one of the United States' earliest aviation pioneers, working as an airmail pilot, research engineer, and aircraft designer....

at NACA
National Advisory Committee for Aeronautics
The National Advisory Committee for Aeronautics was a U.S. federal agency founded on March 3, 1915 to undertake, promote, and institutionalize aeronautical research. On October 1, 1958 the agency was dissolved, and its assets and personnel transferred to the newly created National Aeronautics and...

in 1932. They were first used on the Martin 146 prototype in 1935, and in production on the 1937 Lockheed Electra
Lockheed Model 14 Super Electra
|-See also:-References:NotesBibliography* Francillon, René J. Lockheed Aircraft since 1913. Annapolis: Naval Institute Press, 1987. ISBN 0-85177-835-6.-External links:*...

, 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
Junkers
Junkers Flugzeug- und Motorenwerke AG , more commonly Junkers, was a major German aircraft manufacturer. It produced some of the world's most innovative and best-known airplanes over the course of its fifty-plus year history in Dessau, Germany. It was founded there in 1895 by Hugo Junkers,...

: 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 additonal 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
Junkers Ju 52
The Junkers Ju 52 was a German transport aircraft manufactured from 1932 to 1945. It saw both civilian and military service during the 1930s and 1940s. In a civilian role, it flew with over 12 air carriers including Swissair and Deutsche Luft Hansa as an airliner and freight hauler...

, though it can be found on many modern ultralights.

Gouge flap
Gouge flap
The Gouge flap, invented by Arthur Gouge of Short Brothers in 1936, allowed the pilot to increase both the wing area and the chord of an aircraft's wing. This provided the benefit of a shorter take-off distance for a given load, a shorter distance to achieve a given height and a lower take-off speed...

: 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
Short Brothers
Short Brothers plc is a British aerospace company, usually referred to simply as Shorts, that is now based in Belfast, Northern Ireland. Founded in 1908, Shorts was the first company in the world to make production aircraft and was a manufacturer of flying boats during the 1920s, 1930s, and 1940s...

in 1936 and used on the Short Empire
Short Empire
The Short Empire was a passenger and mail carrying flying boat, of the 1930s and 1940s, that flew between Britain and British colonies in Africa, Asia and Australia...

and Short Sunderland
Short Sunderland
The Short S.25 Sunderland was a British flying boat patrol bomber developed for the Royal Air Force by Short Brothers. It took its service name from the town and port of Sunderland in northeast England....

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
Fairey Firefly
The Fairey Firefly was a British Second World War-era carrier-borne fighter aircraft and anti-submarine aircraft of the Fleet Air Arm ....

and Fairey Barracuda
Fairey Barracuda
The Fairey Barracuda was a British carrier-borne torpedo- and dive bomber used during the Second World War, the first of its type used by the Royal Navy's Fleet Air Arm to be fabricated entirely from metal. It was introduced as a replacement for the Fairey Swordfish and Fairey Albacore biplanes...

. 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
General Aircraft Corporation
The General Aircraft Company was an American aircraft design and manufacturing company formed in the 1940s and ceased involvement with aircraft in 1976.-History:...

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.

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.

Gurney flap
Gurney flap
The Gurney Flap is a small flat tab projecting from the trailing edge of a wing. Typically it is set at a right angle to the pressure side surface of the airfoil,and projects 1% to 2% of the wing chord....

: 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
Dan Gurney
Daniel Sexton Gurney is an American racing driver, race car constructor, and team owner.The son of a Metropolitan Opera star, he was born in Port Jefferson, New York, but moved to California as a teenager...

who rediscovered it in 1971, but has since used on some helicopters such as the Sikorsky S-76B
Sikorsky S-76
The Sikorsky S-76 is an American medium-size commercial utility helicopter. The S-76 is powered by two turboshaft engines, which drive both the main and tail rotors, each with four blades...

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.

Blown flap
Blown flap
Blown flaps are a powered aerodynamic high-lift device invented by the British and used on the wings of certain aircraft to improve low-speed lift during takeoff and landing. The process is sometimes called a boundary layer control system . They were a popular design feature in the 1960s, but fell...

s: 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
Boeing YC-14
The Boeing YC-14 was a twin-engine short take-off and landing tactical transport. It was Boeing's entrant into the United States Air Force's Advanced Medium STOL Transport competition, which aimed to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport...

in 1976.

Controls that look like flaps but are not:
- Handley Page leading edge slats/slots
  Leading edge slats
  Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower...
  
  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
  Spoiler (aeronautics)
  In aeronautics, a spoiler is a device intended to reduce lift in an aircraft. Spoilers are plates on the top surface of a wing which can be extended upward into the airflow and spoil it. By doing so, the spoiler creates a carefully controlled stall over the portion of the wing behind it, greatly...
  
  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.

- Aileron
  Aileron
  Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
  
  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. 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
  World War II
  World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
  
  .

Physics explanation

Flaps during takeoff

Flaps during landing

Maneuvering flaps

Types of flaps

See also