Leading-edge slat

Lore & Background

Slats were first developed by Gustav Lachmann in 1918, prompted by the stall-related crash of a Rumpler C aeroplane in August 1917. A small wooden model was built in 1917 in Cologne, Germany. Lachmann presented a patent in 1918, but the German patent office initially rejected it, doubting the possibility of postponing a stall by dividing the wing. Independently, Handley Page Ltd in Great Britain also developed the slotted wing and applied for a patent in 1919; to avoid a patent challenge, they reached an ownership agreement with Lachmann. That year, an Airco DH.9 was fitted with slats and test flown. Later, an Airco DH.9A was modified as a monoplane with full-span leading edge slats and trailing-edge ailerons to test improved low-speed performance, later known as the Handley Page H.P.20. Licensing the design became a major source of income for Handley Page in the 1920s.

Types include automatic (spring-loaded, deployed by reduced aerodynamic force), fixed (permanently extended, used on specialist low-speed aircraft or for simplicity), and powered (pilot-controlled, common on airliners). During World War II, German aircraft commonly used a more advanced version that reduced drag by being pushed back flush by air pressure, popping out at critical angles of attack. The Fieseler Fi 156 Storch had fixed, non-retractable slats, allowing takeoff in less than 45 m and landing in 18 m. Messerschmitt aircraft typically used automatic, spring-loaded slats, except the Me 163B Komet, which used fixed slots. Post-World War II, slats have been used on larger aircraft, generally operated by hydraulics or electricity; the A-4 Skyhawk slats were spring-loaded and deployed by air load below certain speeds.

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

Reader's Guide

The leading-edge slat is a high-lift device typically used on aircraft intended to operate within a wide range of speeds. Its significance lies in enabling slower flight and shorter takeoff and landing distances by allowing the wing to operate at a higher angle of attack before stalling. The article describes several aerodynamic effects of the slat: the slat effect reduces velocities at the leading edge of the main airfoil, reducing pressure peaks; the circulation effect improves the aerodynamic performance of the slat; the dumping effect increases discharge velocity at the slat's trailing edge, alleviating separation or increasing lift; off the surface pressure recovery decelerates the slat wake efficiently; and the fresh boundary layer effect allows each new element to start with a thin boundary layer that can withstand stronger adverse gradients. The article notes that early aerodynamicists, including Ludwig Prandtl, believed slats work by inducing a high-energy stream to re-energize the boundary layer, but in reality the slat does not give the air in the slot a high velocity and cannot be called high-energy air. The legacy of slats includes their use on many aircraft from the 1920s onward, with licensing becoming a major income source for Handley Page. Research efforts exist to integrate flight control functions with wings for reduced mass, cost, drag, inertia, complexity, and radar cross section, with flexible wings being a promising approach that could rival slats.

Did You Know?

• The German patent office initially rejected Lachmann's patent because it did not believe postponing a stall by dividing the wing was possible.
• The Fieseler Fi 156 Storch, with fixed slats, could take off in less than 45 m and land in 18 m.
• The slat has a counterpart in birds called the alula, a feather or group of feathers controlled by the bird's thumb.