Leading edge slats
Encyclopedia
Slats are aerodynamic surfaces on the leading edge of the wing
s of 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, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are usually used while landing or performing maneuvers which take the aircraft close to the stall
, but are usually retracted in normal flight to minimize drag
.
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. In reality, the slat does not give the air in the slot 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 head. The actual effects of the slat are:
The slat has a counterpart found in the wings of some bird
s, the alula
, a feather or group of feathers which the bird can extend under control of its "thumb".
in 1918. A crash in August 1917, with a Rumpler C aeroplane on account of stalling caused the idea to be put in a concrete form, and a small wooden model was built in 1917 in Cologne
. In 1918, Lachmann presented a patent for leading edge slats in Germany. However, the German patent office at first rejected it as the office did not believe in the possibility of increasing lift by dividing the wing.
Independently of Lachmann, Handley-Page Ltd in Great Britain also developed the slotted wing as a way to postpone stall by reducing the turbulence over 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 a De Havilland D.H.9 was fitted with slats and flown. Later a D.H.4 was modified as a monoplane with a large wing fitted with full span leading edge and back ailerons (ie what would later be called flaps) that could be deployed in conjunction with the leading edge slats to test improved low speed performance. 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 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 when the airflow decreased during slower flight. Notable slats of that time belonged to the German Fieseler Fi 156
Storch. These were similar in design to retractable slats, but were fixed non-retractable slots. The slotted wing allowed this aircraft to take off 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 leading-edge slats as a general rule.
Post World War II, slats have generally been operated by hydraulics
or electricity
.
Slats are one of several high-lift device
s used on airliner
s, such as flap
systems running along the trailing edge of the wing.
s, elevators
, elevon
s, flap
s, and flaperon
s 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 vehicle
s (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.
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
s of fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
which, when deployed, allow the wing to operate at a higher angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are usually used while landing or performing maneuvers which take the aircraft close to the stall
Stall (flight)
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...
, but are usually retracted in normal flight to minimize drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
.
Types
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.
- Fixed - the slat is permanently extended. This is sometimes used on specialist low-speed aircraft (these are referred to as slotsLeading edge slotA leading edge slot is an 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 span-wise gap in each wing, allowing air to flow from below the wing to its upper surface...
) 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 chordChord (aircraft)
In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...
of the slat is typically only a few percent of the wing chord
Chord (aircraft)
In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...
. 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
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
thus re-energizing its boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
and delaying stall. In reality, the slat does not give the air in the slot 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 head. The actual effects of the slat are:
- The slat effect: The velocities at the leading edge of the downstream element (main airfoilAirfoilAn airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
) 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 out with a fresh boundary layerBoundary layerIn physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
at its leading edgeLeading edgeThe leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one....
. Thin boundary layers can withstand stronger adverse gradientGradientIn vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
s than thick ones.
The slat has a counterpart found in the wings of some bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, the alula
Alula
The alula, or bastard wing, is a small projection on the anterior edge of the wing of modern birds. The alula is the freely moving first digit, a bird's "thumb," and is typically covered with three to five small feathers, with the exact number depending on the species...
, a feather or group of feathers which the bird can extend under control of its "thumb".
History
Slats were first developed by Gustav LachmannGustav Lachmann
Gustav Victor Lachmann was a German aeronautical engineer who spent most of his professional life working for the British company of Handley Page....
in 1918. A crash in August 1917, with a Rumpler C aeroplane on account of stalling caused the idea to be put in a concrete form, and a small wooden model was built in 1917 in Cologne
Cologne
Cologne is Germany's fourth-largest city , and is the largest city both in the Germany Federal State of North Rhine-Westphalia and within the Rhine-Ruhr Metropolitan Area, one of the major European metropolitan areas with more than ten million inhabitants.Cologne is located on both sides of the...
. In 1918, Lachmann presented a patent for leading edge slats in Germany. However, the German patent office at first rejected it as the office did not believe in the possibility of increasing lift by dividing the wing.
Independently of Lachmann, Handley-Page Ltd in Great Britain also developed the slotted wing as a way to postpone stall by reducing the turbulence over 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 a De Havilland D.H.9 was fitted with slats and flown. Later a D.H.4 was modified as a monoplane with a large wing fitted with full span leading edge and back ailerons (ie what would later be called flaps) that could be deployed in conjunction with the leading edge slats to test improved low speed performance. 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
Handley Page Hampden
The Handley Page HP.52 Hampden was a British twin-engine medium bomber of the Royal Air Force serving in the Second World War. With the Whitley and Wellington, the Hampden bore the brunt of the early bombing war over Europe, taking part in the first night raid on Berlin and the first 1,000-plane...
.
Licensing the design became one of the company's major sources of income
Income
Income is the consumption and savings opportunity gained by an entity within a specified time frame, which is generally expressed in monetary terms. However, for households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings...
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
STOL
STOL is an acronym for short take-off and landing, a term used to describe aircraft with very short runway requirements.-Definitions:There is no one accepted definition of STOL and many different definitions have been used by different authorities and nations at various times and for a myriad of...
aircraft.
During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
aircraft commonly fitted a more advanced version that pushed back flush against the wing by air pressure to reduce drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
, popping out when the airflow decreased during slower flight. Notable slats of that time belonged to the German Fieseler Fi 156
Fieseler Fi 156
The Fieseler Fi 156 Storch was a small German liaison aircraft built by Fieseler before and during World War II, and production continued in other countries into the 1950s for the private market...
Storch. These were similar in design to retractable slats, but were fixed non-retractable slots. The slotted wing allowed this aircraft to take off into a light wind in less than 45 m (150 ft), and land in 18 m (60 ft). Aircraft designed by the Messerschmitt
Messerschmitt
Messerschmitt AG was a famous German aircraft manufacturing corporation named for its chief designer, Willy Messerschmitt, and known primarily for its World War II fighter aircraft, notably the Bf 109 and Me 262...
company employed leading-edge slats as a general rule.
Post World War II, slats have generally been operated by hydraulics
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
or electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
.
Slats are one of several high-lift device
High-lift device
In aircraft design, high-lift devices are moving surfaces or stationary components intended to increase lift during certain flight conditions. They include common devices such as flaps and slats, as well as less common features such as leading edge extensions and blown flaps.-Purpose:Aircraft...
s used on airliner
Airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...
s, such as flap
Flap (aircraft)
Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...
systems running along the trailing edge of the wing.
Research
Several technology research and development efforts exist to integrate the functions of flight control systems such as aileronAileron
Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
s, elevators
Elevator (aircraft)
Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down...
, elevon
Elevon
Elevons are aircraft control surfaces that combine the functions of the elevator and the aileron , hence the name. They are frequently used on tailless aircraft such as flying wings. An elevon that is not part of the main wing, but instead is a separate tail surface, is a stabilator...
s, flap
Flap (aircraft)
Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...
s, and flaperon
Flaperon
A flaperon is a type of aircraft control surface that combines aspects of both flaps and ailerons. In addition to controlling the roll or bank of an aircraft as do conventional ailerons, both flaperons can be lowered together to function similarly to a dedicated set of flaps...
s into wings to perform the aerodynamic purpose with the advantages of less: mass, cost, drag, inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
(for faster, stronger control response), complexity (mechanically simpler, fewer moving parts or surfaces, less maintenance), and radar cross section
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
for stealth
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...
. These may be used in many unmanned aerial vehicle
Unmanned aerial vehicle
An unmanned aerial vehicle , also known as a unmanned aircraft system , remotely piloted aircraft or unmanned aircraft, is a machine which functions either by the remote control of a navigator or pilot or autonomously, that is, as a self-directing entity...
s (UAVs) and 6th generation fighter aircraft
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
. 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
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
effort. The Adaptive Compliant Wing
Adaptive Compliant Wing
The Adaptive Compliant Wing is a wing design developed by FlexSys, Inc., which is flexible so that aspects of its shape can be changed in flight....
is a military and commercial effort.
See also
- Flap (aircraft)Flap (aircraft)Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...
- Flight control surfaces
- Leading edge slotLeading edge slotA leading edge slot is an 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 span-wise gap in each wing, allowing air to flow from below the wing to its upper surface...
- Krueger flapsKrueger flapsKrueger flaps are lift enhancement devices that may be fitted to the leading edge of an aircraft wing. Unlike slats or drooped leading edges, the main wing upper surface and its nose is not changed...