Editing Flap (aeronautics) (section)

==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 drag and lower stalling speed associated with fully extended flaps allow 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, reducing the stalling speed and 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>