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Flaps are a high lift device consisting of a hinged panel or panels mounted on the trailing edge of the wing. When extended, they increase the camber and, in most cases, the chord and surface area of the wing resulting in an increase of both lift and drag and a reduction of the stall speed. These factors result in an improvement in takeoff and landing performance.
There are many different flap designs and configurations in use. Large aircraft sometimes incorporate more than one type, utilising different flap designs on the inboard and outboard sections of the wing. The following are descriptions of some of the more common flap designs:
- Plain Flap - The rear portion of the wing aerofoil rotates downwards on a simple hinge arrangement mounted at the front of the flap.
- Split Flap - The rear portion of the lower surface of the wing aerofoil hinges downwards from the leading edge of the flap, while the upper surface remains immobile.
- Slotted Flap - Similar to a Plain Flap but incorporates a gap between the flap and the wing to force high pressure air from below the wing over the upper surface of the flap. This helps reduce boundary layer separation and allows the airflow over the flap to remain laminar.
- Fowler Flap - A split flap that slides rearwards level for a distance prior to hinging downwards. It thereby first increases chord (and wing surface area) and then increases camber. This produces a flap which can optimise both takeoff (partial extension for optimal lift) and landing (full extension for optimal lift and drag) performance. This type of flap or one of its variations is found on most large aircraft.
- Double Slotted Fowler Flap - This design improves the performance of the Fowler flap by incorporating the boundary layer energising features of the slotted flap.