page_title: Flap (aeronautics)
section_title: Principles of operation
hierarchical_section_title: Flap (aeronautics) / Principles of operation
caption_attribution_description: The escape rope bracket (yellow) on an Easyjet Airbus A319-100 (G-EZAV) during a flight to Bristol, England from Palma Airport, Majorca, Spain.
context_page_description: A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges 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 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 ailerons. When partial-span flaps are extended they alter the spanwise lift loading 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 of the outboard half, maintaining aileron effectiveness and reducing the likelihood of asymmetric stall, and spinning.
  Extending the wing flaps increases the camber or curvature of the wing, raising the maximum lift coefficient or the upper limit to the lift a wing can generate.
context_section_description: The general airplane lift equation demonstrates these relationships:
  where:
  L is the amount of Lift produced,
   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
   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 () allow a similar amount of lift to be generated at a lower airspeed (V).
  Extending the flaps also increases the drag coefficient of the aircraft. Therefore, for any given weight and airspeed, flaps increase the 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.
  Thus, flaps are extensively in use for short takeoffs and landings (STOL).
caption: The three orange pods are fairings streamlining the flap track mechanisms. The flaps (two on each side, on the Airbus A319) lie directly above these.
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