A leading-edge slot is a fixed

aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

feature of the wing of some aircraft to reduce the

stall speed In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. This occurs when the ...

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 In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...

and thus reduce the stall speed.

Purpose and development

At an

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...

above about 15° many

airfoil An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of an object whose motion through a gas is capable of generating significant lift, such as a wing, a sail, or the blades of propeller, rotor, or turbine. ...

s enter the stall. Modification of such an airfoil with a fixed leading-edge slot can increase the stalling angle to between 22° and 25°.Clancy, L.J., ''Aerodynamics'', Section 6.9 Slots were first developed by

Handley Page Handley Page Limited was a British aerospace manufacturer. Founded by Frederick Handley Page (later Sir Frederick) in 1909, it was the United Kingdom's first publicly traded aircraft manufacturing company. It went into voluntary liquidation a ...

in 1919 and the first aircraft to fly with them was the experimental H.P.17, a modified

Airco DH.9A The Airco DH.9A was a British single-engined light bomber designed and first used shortly before the end of the First World War. It was a development of the unsuccessful Airco DH.9 bomber, featuring a strengthened structure and, crucially, repl ...

. Their invention is credited jointly to Sir

Frederick Handley Page Sir Frederick Handley Page, CBE, FRAeS (15 November 1885 – 21 April 1962) was an English industrialist who was a pioneer in the aircraft industry and became known as the father of the heavy bomber. His company Handley Page Limited was ...

and

Gustav Lachmann Gustav Victor Lachmann (3 February 1896 – 30 May 1966) was a German aeronautical engineer who spent most of his professional life working for the British aircraft company Handley Page. He was, with Frederick Handley Page, the co-inventor of the ...

. The first aircraft fitted with controllable slots was the Handley Page H.P.20. Licensing the design became one of Handley Page's major sources of income in the 1920s. Similar, but retractable, leading-edge devices are called

slats Super Low Altitude Test Satellite (SLATS) or Tsubame was a JAXA satellite intended to demonstrate operations in very low Earth orbit (VLEO, below 200 km), using ion engines to counteract aerodynamic drag from the Earth's atmosphere which i ...

.Aviation Publishers Co. Limited, ''From the Ground Up'', page 26 (27th revised edition) When the slat opens, it creates a slot between the slat and the remainder of the wing; retracted, the drag is reduced. A fixed leading-edge slot can increase the maximum

lift coefficient In fluid dynamics, the lift coefficient () is a dimensionless quantity that relates the lift generated by a lifting body to the fluid density around the body, the fluid velocity and an associated reference area. A lifting body is a foil or a com ...

of an airfoil section by 40%. In conjunction with a slat, the increase in maximum lift coefficient can be 50% or even 60%. Unlike trailing edge flaps, leading-edge slots do not increase the lift coefficient at zero angle of attack since they do not alter the

camber Camber may refer to a variety of curvatures and angles: * Camber angle, the angle made by the wheels of a vehicle * Camber beam, an upward curvature of a joist to compensate for load deflection due in buildings * Camber thrust in bike technology * ...

Operation

A leading-edge slot is a fixed (non-closing) gap behind the wing's

leading edge The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...

. Air from below the wing can accelerate through the slot towards the low pressure region above the wing, and exit from the slot moving parallel to the upper wing surface. This high-speed flow then mixes with the

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...

attached to the upper surface and delays

boundary layer separation In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous fo ...

from the upper surface. Slots naturally exact a penalty on the aircraft in which they are used. This is because they contribute to drag compared to an unslotted wing.Abbott and Von Doenhoff, ''Theory of Wing Sections'', Section 8.6 The extra drag at low speed is acceptable because of the beneficial reduction in stall speed and improvement in handling characteristics, but at higher speeds the extra drag contributed by slots is a significant disadvantage because it reduces cruising speed and increases

fuel consumption A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...

per unit distance flown. One way to reduce the cruise drag of slots is to make them able to be closed. This arrangement is known as

leading-edge slat Slats are aerodynamic surfaces on the leading edge of the wing of a fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, ...

s. Aerodynamically, slats work in the same way as fixed slots but slats can be retracted at higher speeds when they are not needed. Slats, in turn, are heavier and more complex than slots. At low angles of attack the airflow through the slot is insignificant, although it contributes to drag. At progressively higher angles of attack, the flow of air through the slot becomes increasingly significant, accelerating from the higher pressure region below the wing to the lower pressure region on top of the wing. At high angles of attack the fastest airspeed relative to the airfoil is very close to the leading edge, on the upper surface. In this region of high local airspeed, skin friction (

viscous force The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

) is very high and the boundary layer arriving at the slot on the upper wing has lost much of its total pressure (or total

mechanical energy In Outline of physical science, physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, t ...

) due to this friction. In contrast, the air passing through the slot has not experienced this high local airspeed or high skin friction, and its total pressure remains close to the free-stream value. The mixing of the upper surface boundary layer with air arriving through the slot re-energises the boundary layer which then remains attached to the upper surface of the wing to a higher angle of attack than if the slot were not there. The leading-edge slot was therefore one of the earliest forms of

boundary layer control Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers. It may be desirable to reduce flow separation on fast vehicles to reduce the size of the wake (streamlining), which may reduce drag. Boundary l ...

Application of leading-edge slots

Leading-edge slots are generally of two types: those that are full-span and those that are partial-span. Full-span slots are generally found on Short Take-off and Landing

STOL A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh conditio ...

aircraft like the

Fieseler Storch The Fieseler Fi 156 ''Storch'' (, "stork") was a German liaison aircraft built by Fieseler before and during World War II. Production continued in other countries into the 1950s for the private market. It was notable for its excellent short fiel ...

Dornier Do 27 The Dornier Do 27 is a German single-engine STOL utility aircraft that was designed and manufactured by Dornier GmbH (later DASA Dornier and Fairchild-Dornier). It was notable for being the first mass-produced aircraft in Germany following the ...

, PZL-104M Wilga 2000, and Zenair CH 701 STOL. Their primary purpose is to allow the aircraft to fly at a higher angle of attack before reaching the stalling angle.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 471. Aviation Supplies & Academics, 1997. In aircraft other than specialist STOL aircraft, full-span slots have serious drawbacks because, to take advantage of the high angle of attack at the stall, they usually necessitate long undercarriage legs that either cause high drag or are longer than can be accommodated easily inside the airframe.Kermode, A.C., ''Mechanics of Flight'', Figures 6.6 and 6.7 Partial-span slots are usually found only on the outboard portion of the wing where they ensure airflow over that portion of the wing will remain unstalled at higher angles of attack than the inboard portions of the wing. This ensures the wing root stalls first and contributes to docile stall behaviour and maintaining aileron control throughout the stall. Using slots in this manner produces a similar result to employing washout on a wing, but through a different means. Examples of aircraft with partial-span, fixed slots are the

Stinson 108 The Stinson 108 was a popular general aviation aircraft produced by the Stinson division of the American airplane company Consolidated Vultee, from immediately after World War II to 1950. It was developed from the prewar Model 10A Voyager. St ...

Bristol Beaufort The Bristol Beaufort (manufacturer designation Type 152) is a British twin-engined torpedo bomber designed by the Bristol Aeroplane Company, and developed from experience gained designing and building the earlier Blenheim light bomber. At l ...

Lockheed Hudson The Lockheed Hudson is a light bomber and coastal reconnaissance aircraft built by the American Lockheed Aircraft Corporation. It was initially put into service by the Royal Air Force shortly before the outbreak of the Second World War and prim ...

, and Dornier Do 28D-2 Skyservant.

References

*Ira H. Abbott, and Albert E. Von Doenhoff (1959), ''Theory of Wing Sections'', Dover Publications Inc., New York SBN 486-60586-8 *Aviation Publishers Co Limited: ''From the Ground Up, Twenty-Seventh Revised Edition''. Aviation Publishers Co Limited, 1996. *Clancy, L.J. (1975), ''Aerodynamics'', Chapter 6 ''High Lift Devices'', Pitman Publishing Limited, London *Kermode, A.C. (1972), ''Mechanics of Flight'', Chapter 3 ''Aerofoils – Subsonic Speeds'' (8th edition) Pitman Publishing Limited, London

External links

History of patent and Handley Page
an article by

published in a 1921 issue of ''

Flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...

'' {{Aircraft components Aircraft wing design Handley Page

Purpose and development

Operation

Application of leading-edge slots

See also

References

External links