Leading-edge slat

Last updated

A slat is an aerodynamic surface on the leading edge of the wing of a fixed-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 its kinetic energy due to skin friction drag. When deployed, slats allow the wings to operate at a higher angle of attack before stalling. 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 minimize drag.

Contents

Slats are high-lift devices typically used on aircraft intended to operate within a wide range of speeds. Trailing-edge flap systems running along the trailing edge of the wing are common on all aircraft.

The position of the leading-edge slats on an airliner (Airbus A310-300). In this picture, the slats are drooped. Note also the extended trailing-edge flaps. Wing.slat.600pix.jpg
The position of the leading-edge slats on an airliner (Airbus A310-300). In this picture, the slats are drooped. Note also the extended trailing-edge flaps.
Slats on the leading edge of an Airbus A318 of Air France Airfrance.a318-100.f-gugj.arp.jpg
Slats on the leading edge of an Airbus A318 of Air France
Automatic slats of a Messerschmitt Bf 109 Bundesarchiv Bild 146-1980-005-05, Flugel einer Messerschmitt Me 109.jpg
Automatic slats of a Messerschmitt Bf 109
The wing of a landing Airbus A319-100. The slats at the leading edge and the flaps at the trailing edge are extended. Bmi a319-100 g-dbca closeup arp.jpg
The wing of a landing Airbus A319-100. The slats at the leading edge and the flaps at the trailing edge are extended.
The Fieseler Fi 156 Storch had permanently extended slots on its leading edges (fixed slats). Argus As 10 C & Storch.jpg
The Fieseler Fi 156 Storch had permanently extended slots on its leading edges (fixed slats).

Types

Types include:

Automatic
The spring-loaded slat lies flush with the wing leading edge, held in place by the force of the air acting on them. As the aircraft slows down, the aerodynamic force is reduced and the springs extend the slats. Sometimes referred to as Handley-Page slats.
Fixed
The slat is permanently extended. This is sometimes used on specialist low-speed aircraft (these are referred to as slots) or when simplicity takes precedence over speed.
Powered
The slat extension can be controlled by the pilot. This is commonly used on airliners.

Operation

The chord 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 entire leading edge. Many early aerodynamicists, including Ludwig Prandtl, believed that slats work by inducing a high energy stream to the flow of the main airfoil, thus re-energizing its boundary 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 same total heat. The actual effects of the slat are: [2] [3]

The slat effect
The velocities at the leading edge of the downstream element (main airfoil) are reduced due to the circulation of the upstream element (slat) thus reducing the pressure peaks of the downstream element.
The circulation effect
The circulation of the downstream element increases the circulation of the upstream element thus improving its aerodynamic performance.
The dumping effect
The discharge velocity at the trailing edge of the slat is increased due to the circulation of the main airfoil thus alleviating separation problems or increasing lift.
Off the surface pressure recovery
The deceleration of the slat wake occurs in an efficient manner, out of contact with a wall.
Fresh boundary layer effect
Each new element starts with a fresh boundary layer at its leading edge. Thin boundary layers can withstand stronger adverse gradients than thick ones. [3]

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

A319 slats during and after landing Voilure A319.jpg
A319 slats during and after landing

Slats were first developed by Gustav Lachmann in 1918. The stall-related crash in August 1917 of a Rumpler C aeroplane prompted Lachmann to develop the idea, and a small wooden model was built in 1917 in Cologne. 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, an Airco DH.9 was fitted with slats and test flown. [5] Later, an Airco 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 the Handley 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 the Handley 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 of STOL aircraft.

During World War II, German aircraft commonly fitted a more advanced version of the slat that reduced drag by being pushed back flush against the leading edge of the wing by air pressure, popping out when the angle of attack increased to a critical angle. Notable slats of that time belonged to the German Fieseler Fi 156 Storch. 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 the Messerschmitt company employed automatic, spring-loaded leading-edge slats as a general rule, except for the Alexander Lippisch-designed Messerschmitt Me 163B Komet 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 by hydraulics or electricity.

Research

Several technology research and development efforts exist to integrate the functions of flight control systems such as ailerons, elevators, elevons, flaps, and flaperons 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), and radar cross-section for stealth. These may be used in many unmanned aerial vehicles (UAVs) and 6th generation fighter 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. The X-53 Active Aeroelastic Wing is a NASA effort. The adaptive compliant wing is a military and commercial effort. [7] [8] [9]

See also

Related Research Articles

<span class="mw-page-title-main">Lift (force)</span> Force perpendicular to flow of surrounding fluid

When a fluid flows around an object, the fluid exerts a force on the object. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it is defined to act perpendicular to the flow and therefore can act in any direction.

<span class="mw-page-title-main">Wing</span> Appendage used for flight

A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total drag on the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift.

<span class="mw-page-title-main">Stall (fluid dynamics)</span> Abrupt reduction in lift due to flow separation

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number.

<span class="mw-page-title-main">Leading-edge extension</span> Anti-stall control surface on aircraft

A leading-edge extension (LEX) is a small extension to an aircraft wing surface, forward of the leading edge. The primary reason for adding an extension is to improve the airflow at high angles of attack and low airspeeds, to improve handling and delay the stall. A dog tooth can also improve airflow and reduce drag at higher speeds.

<span class="mw-page-title-main">Vortex generator</span> Aerodynamic device

A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface or a rotor blade of a wind turbine. VGs may also be attached to some part of an aerodynamic vehicle such as an aircraft fuselage or a car. When the airfoil or the body is in motion relative to the air, the VG creates a vortex, which, by removing some part of the slow-moving boundary layer in contact with the airfoil surface, delays local flow separation and aerodynamic stalling, thereby improving the effectiveness of wings and control surfaces, such as flaps, elevators, ailerons, and rudders.

<span class="mw-page-title-main">Airfoil</span> Cross-sectional shape of a wing, blade of a propeller, rotor, or turbine, or sail

An airfoil or aerofoil is a streamlined body that is capable of generating significantly more lift than drag. Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils.

<span class="mw-page-title-main">Blown flap</span> High-lift device on some aircraft wings

Blown flaps, blown wing or jet flaps are powered aerodynamic high-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the rear edge of the wing, directing the flow downward to increase the lift coefficient. There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure air bled off of a jet engine's compressor and then redirected to follow the line of trailing-edge flaps.

<span class="mw-page-title-main">High-lift device</span> Wing surface area adjuster, typically for shortening take-off and landing

In aircraft design and aerospace engineering, a high-lift device is a component or mechanism on an aircraft's wing that increases the amount of lift produced by the wing. The device may be a fixed component, or a movable mechanism which is deployed when required. Common movable high-lift devices include wing flaps and slats. Fixed devices include leading-edge slots, leading edge root extensions, and boundary layer control systems.

<span class="mw-page-title-main">Flap (aeronautics)</span> Anti-stalling high-lift device on aircraft

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.

<span class="mw-page-title-main">Leading-edge cuff</span> Fixed aerodynamic wing device

A leading-edge cuff is a fixed aerodynamic wing device employed on fixed-wing aircraft to improve the stall and spin characteristics. Cuffs may be either factory-designed or an after-market add-on modification.

<span class="mw-page-title-main">Leading-edge slot</span> Anti-stall control surface on aircraft

A leading-edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of attack and thus reduce the stall speed.

<span class="mw-page-title-main">Supercritical airfoil</span> Airfoil designed primarily to delay the onset of wave drag in the transonic speed range

A supercritical aerofoil is an airfoil designed primarily to delay the onset of wave drag in the transonic speed range.

Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers.

<span class="mw-page-title-main">Circulation control wing</span> Aircraft high-lift device

A circulation control wing (CCW) is a form of high-lift device for use on the main wing of an aircraft to increase the maximum lift coefficient and reduce the stalling speed. CCW technology has been in the research and development phase for over sixty years. Blown flaps were an early example of CCW.

<span class="mw-page-title-main">Wing configuration</span> Describes the general shape and layout of an aircraft wing

The wing configuration of a fixed-wing aircraft is its arrangement of lifting and related surfaces.

<span class="mw-page-title-main">Krueger flap</span> Aerodynamic device

Krueger flaps, or Krüger flaps, are lift enhancement devices that may be fitted to the leading edge of an aircraft wing. Unlike slats or droop flaps, the main wing upper surface and its nose is not changed. Instead, a portion of the lower wing is rotated out in front of the main wing leading edge. The Boeing 707 and Boeing 747 used Krueger flaps on the wing leading edge. Several modern aircraft use Krueger flaps between the fuselage and closest engine, but use slats outboard of the closest engine. The Boeing 727 also used a mix of inboard Krueger flaps and outboard slats, although it had no engine between them.

<span class="mw-page-title-main">Handley Page H.P.20</span> Type of aircraft

The Handley Page H.P.20 was an experimental monoplane modification of a de Havilland DH.9A, built to study controllable slots and slotted ailerons as high lift devices. It was the first aircraft to fly with controllable slots.

A supersonic airfoil is a cross-section geometry designed to generate lift efficiently at supersonic speeds. The need for such a design arises when an aircraft is required to operate consistently in the supersonic flight regime.

<span class="mw-page-title-main">Villiers XXIV</span> French night fighter prototype

The Villiers XXIV or Villiers 24 CAN2 was a French army night fighter most notable as the first French military aircraft to be fitted with leading edge slats.

The Bodiansky 20, a French four-seat touring aircraft flown in the early 1930s, was one of the first French aircraft to adopt Handley Page slots to delay the stall and lower landing speed.

References

  1. Theory of wing sections, Abbott and Doenhoff, Dover Publications
  2. High-Lift Aerodynamics, A.M.O. Smith, Journal of Aircraft, 1975
  3. 1 2 High-Lift Aerodynamics, by A. M. O. Smith, McDonnell Douglas Corporation, Long Beach, June 1975 Archived 2011-07-07 at the Wayback Machine
  4. Gustav Lachmann - National Advisory Committee for Aeronautics (November 1921). "Experiments with slotted wings" (PDF). Archived from the original (PDF) on 2012-11-29. Retrieved 2018-10-14.
  5. Handley Page, F. (December 22, 1921), "Developments In Aircraft Design By The Use Of Slotted Wings", Flight, vol. XIII, no. 678, p. 844, archived from the original on 2012-11-03 via Flightglobal Archive
  6. F. Handley Page "Developments In Aircraft Design By The Use Of Slotted Wings" Archived 2012-11-03 at the Wayback Machine Flight, December 22nd 1921, photo page 845 of converted D.H.4 for testing of slotted wings
  7. Scott, William B. (27 November 2006), "Morphing Wings", Aviation Week & Space Technology, archived from the original on 26 April 2011
  8. "FlexSys Inc.: Aerospace". Archived from the original on 16 June 2011. Retrieved 26 April 2011.
  9. Kota, Sridhar; Osborn, Russell; Ervin, Gregory; Maric, Dragan; Flick, Peter; Paul, Donald. "Mission Adaptive Compliant Wing – Design, Fabrication and Flight Test" (PDF). Ann Arbor, MI; Dayton, OH, USA: FlexSys Inc., Air Force Research Laboratory. Archived from the original (PDF) on 22 March 2012. Retrieved 26 April 2011.