Slats are small aerodynamic surfaces on the leading edge of an airplane wing which, when deployed, allow the wing to operate at a higher angle of attack. Lift is a product of angle of attack and speed, so by deploying slats an aircraft can fly slower or take off and land in a shorter distance. They are usually used while landing or performing manoeuvres which take the aircraft close to the stall, but are usually retracted in normal flight to minimise drag.

The position of the leading edge slats on an airliner (Airbus A310). In this picture, the slats are extended.

Types include:

• Automatic - the slat lies flush with the wing leading edge until reduced aerodynamic forces allow it to extend by way of springs when needed. This type is typically used on light aircraft.
• Fixed - the slat is permanently extended. This is rarely used, except on specialist low-speed aircraft.
• Powered - the slat extension can be controlled by the pilot. This is commonly used on airliners.

The chord of the slat is typically only a few percent of the wing chord. They may extend over the outer third of the wing or may cover the entire leading edge. Slats work by increasing the camber of the wing, and also by opening a small gap (the slot) between the slat and the wing leading edge, allowing a small amount of high-pressure air from the lower surface to reach the upper surface, where it helps postpone the stall.

The slat has a counterpart found in the wings of some birds, the alula – a feather or group of feathers which the bird can extend under control of its "thumb".

History

Slats were first developed by Handley-Page in 1919; licensing the design became one of their major sources of income in the 1920s. The original designs were in the form of a fixed slot in the front of the wing, a design that was found on a number of STOL aircraft. During World War II German aircraft commonly fitted a more advanced version that pushed back flush against the wing by air pressure to reduce drag, popping out on springs when the airflow decreased during slower flight. In the post-war era slats have generally been hydraulically or electrically operated, allowing for more complex and efficient designs.

Today slats are generally one of several high-lift devices used on airliners, complex flap systems running along the trailing edge of the wing as well.

See also:

• Aviation
• Flight
• Flight controls