Aslat is anaerodynamic surface on the leading edge of thewing of afixed-wing aircraft. When retracted, the slat lies flush with the rest of the wing. A slat is deployed by sliding forward, opening a slot between the wing and the slat. Air from below the slat flows through the slot and replaces the boundary layer that has travelled at high speed around the leading edge of the slat, losing a significant amount of itskinetic energy due toskin friction drag. When deployed, slats allow the wings to operate at a higherangle of attack beforestalling. With slats deployed an aircraft can fly at slower speeds, allowing it to take off and land in shorter distances. They are used during takeoff and landing and while performing low-speed maneuvers which may take the aircraft close to a stall. Slats are retracted in normal flight to minimizedrag.
Slats arehigh-lift devices typically used on aircraft intended to operate within a wide range of speeds. Trailing-edgeflap systems running along the trailing edge of the wing are common on all aircraft.
Types include:
Thechord of the slat is typically only a few percent of the wing chord. The slats may extend over the outer third of the wing, or they may cover the entireleading edge. Many early aerodynamicists, includingLudwig Prandtl, believed that slats work by inducing a high energy stream to the flow of the mainairfoil, thus re-energizing itsboundary layer and delaying stall.[1] In reality, the slat does not give the air in the slot a high velocity (it actually reduces its velocity) and also it cannot be called high-energy air since all the air outside the actual boundary layers has the sametotal heat. The actual effects of the slat are:[2][3]
The slat has a counterpart found in the wings of some birds, thealula, a feather or group of feathers which the bird can extend under control of its "thumb".
Slats were first developed byGustav Lachmann in 1918. The stall-related crash in August 1917 of aRumpler C aeroplane prompted Lachmann to develop the idea, and a small wooden model was built in 1917 inCologne. In Germany in 1918 Lachmann presented a patent for leading-edge slats.[4] However, the German patent office at first rejected it, as the office did not believe the possibility of postponing the stall by dividing the wing.
Independently of Lachmann,Handley Page Ltd in Great Britain also developed the slotted wing as a way to postpone the stall by delaying separation of the flow from the upper surface of the wing at high angles of attack, and applied for a patent in 1919; to avoid a patent challenge, they reached an ownership agreement with Lachmann. That year, anAirco DH.9 was fitted with slats and test flown.[5] Later, anAirco DH.9A was modified as a monoplane with a large wing fitted with full-span leading edge slats and trailing-edge ailerons (i.e. what would later be called trailing-edge flaps) that could be deployed in conjunction with the leading-edge slats to test improved low-speed performance. This was later known as theHandley Page H.P.20[6] Several years later, having subsequently taken employment at the Handley-Page aircraft company, Lachmann was responsible for a number of aircraft designs, including theHandley Page Hampden.
Licensing the design became one of the company's major sources of income in the 1920s. The original designs were in the form of a fixed slot near the leading edge of the wing, a design that was used on a number ofSTOL aircraft.
During World War II, German aircraft commonly fitted a more advanced version of the slat that reduceddrag by being pushed back flush against the leading edge of the wing byair pressure, popping out when the angle of attack increased to a critical angle. Notable slats of that time belonged to the GermanFieseler Fi 156Storch. These were similar in design to retractable slats, but were fixed and non-retractable. This design feature allowed the aircraft to takeoff into a light wind in less than 45 m (150 ft), and land in 18 m (60 ft). Aircraft designed by theMesserschmitt company employed automatic, spring-loaded leading-edge slats as a general rule, except for theAlexander Lippisch-designedMesserschmitt Me 163BKomet rocket fighter, which instead used fixed slots built integrally with, and just behind, the wing panel's outer leading edges.
Post-World War II, slats have also been used on larger aircraft and generally operated byhydraulics orelectricity. The A-4 Skyhawk slats were spring loaded and deployed by the air load below certain speeds.
Several technology research and development efforts exist to integrate the functions of flight control systems such asailerons,elevators,elevons,flaps, andflaperons into wings to perform the aerodynamic purpose with the advantages of less: mass, cost, drag,inertia (for faster, stronger control response), complexity (mechanically simpler, fewer moving parts or surfaces, less maintenance), andradar cross-section forstealth. These may be used in manyunmanned aerial vehicles (UAVs) and 6th generationfighter aircraft.
One promising approach that could rival slats are flexible wings. In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow. TheX-53 Active Aeroelastic Wing is aNASA effort. Theadaptive compliant wing is a military and commercial effort.[7][8][9]
