Leading edge slot

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A leading edge slot on a STOL aircraft. Leading edge slot.jpg
A leading edge slot on a STOL 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. [1]

Aerodynamics branch of dynamics concerned with studying the motion of air

Aerodynamics, from Greek ἀήρ aer (air) + δυναμική (dynamics), is the study of motion of air, particularly as interaction with a solid object, such as an airplane wing. It is a sub-field of fluid dynamics and gas dynamics, and many aspects of aerodynamics theory are common to these fields. The term aerodynamics is often used synonymously with gas dynamics, the difference being that "gas dynamics" applies to the study of the motion of all gases, and is not limited to air. The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as aerodynamic drag were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving heavier-than-air flight, which was first demonstrated by Otto Lilienthal in 1891. Since then, the use of aerodynamics through mathematical analysis, empirical approximations, wind tunnel experimentation, and computer simulations has formed a rational basis for the development of heavier-than-air flight and a number of other technologies. Recent work in aerodynamics has focused on issues related to compressible flow, turbulence, and boundary layers and has become increasingly computational in nature.

Angle of attack angle between the chord of the wing and the undisturbed airflow

In fluid dynamics, angle of attack is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air.

Contents

Purpose and development

Full-span leading edge slot in the wing of the PZL-104M Wilga 2000 PZL-104M Wilga 2000 of Polish Border Guard (reg. SN-41YG), static display, Radom AirShow 2005, Poland.jpg
Full-span leading edge slot in the wing of the PZL-104M Wilga 2000

At an angle of attack above about 15° many airfoils enter the stall. Modification of such an airfoil with a fixed leading edge slot can increase the stalling angle to between 22° and 25°. [2]

Airfoil

An airfoil or aerofoil is the cross-sectional shape of a wing, blade, or sail.

Slots were first developed by Handley Page in 1919 and the first aircraft to fly with them was the experimental H.P.17, a modified Airco DH.9A. 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. [3]

Handley Page defunct aircraft manufacturer

Handley Page Limited was founded by Frederick Handley Page in 1909 as the United Kingdom's first publicly traded aircraft manufacturing company. It went into voluntary liquidation and ceased to exist in 1970. The company, based at Radlett Aerodrome in Hertfordshire, was noted for its pioneering role in aviation history and for producing heavy bombers and large airliners.

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, replacing the under-powered and unreliable inline 6-cylinder Siddeley Puma engine of the DH.9 with the American V-12 Liberty engine.

Handley Page H.P.20 experimental monoplane

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.

Similar, but retractable, leading edge devices are called slats. [4] 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 of an airfoil section by 40%. In conjunction with a slat, the increase in maximum lift coefficient can be 50% or even 60%. [2] [5]

The lift coefficient (CL) is a dimensionless coefficient 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 complete foil-bearing body such as a fixed-wing aircraft. CL is a function of the angle of the body to the flow, its Reynolds number and its Mach number. The lift coefficient cl refers to the dynamic lift characteristics of a two-dimensional foil section, with the reference area replaced by the foil chord.

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. [6]

In aeronautics and aeronautical engineering, camber is the asymmetry between the two acting surfaces of an aerofoil, with the top surface of a wing commonly being more convex. An aerofoil that is not cambered is called a symmetric aerofoil. The benefits of cambering were discovered and first utilized by Sir George Cayley in the early 19th century.

Operation

Partial-span leading edge slot in the wing of a Stinson 108-3 Stinson108-3photo03.jpg
Partial-span leading edge slot in the wing of a Stinson 108-3
Fieseler Storch with full-span slots. Fieseler Fi156.jpg
Fieseler Storch with full-span slots.
A Zenith STOL CH 701 showing its full-span slots. ZenairCH701photo01.JPG
A Zenith STOL CH 701 showing its full-span slots.

A leading edge slot is a fixed (non-closing) gap behind the wing’s leading edge. 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 attached to the upper surface and delays boundary layer separation 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. [7] 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 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 slats. 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. [4] [7]

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) 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) 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. [2] The leading edge slot was therefore one of the earliest forms of boundary layer control. [2]

Application of leading edge slots

Leading edge slots are generally of two types: those that are full-span and those that are partial-span. [4]

Full-span slots are generally found on Short Take-off and Landing STOL aircraft like the Fieseler Storch, Dornier Do 27, 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. [8]

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. [9]

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. [2] [4] 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, Bristol Beaufort, Lockheed Hudson, and Dornier Do 28D-2 Skyservant.

See also

Related Research Articles

Lift (force) force; aerodynamics term

A fluid flowing past the surface of a body exerts a force on it. 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 can act in any direction at right angles to the flow.

Wing Surface used for flight, for example by insects, birds, bats and airplanes

A wing is a type of fin that produces lift, while moving through air or some other fluid. As such, wings have streamlined cross-sections that are subject to aerodynamic forces and act as an 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.

Stall (fluid dynamics) 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 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Leading-edge extension

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.

Vortex generator

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.

Blown flap

Blown flaps, 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.

High-lift device arcraft component to increase lift

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.

Flap (aeronautics) aircraft wing device used to increase lift by extending the trailing edge of the wing

Flaps are a kind of high-lift device used to increase the lift of an aircraft wing at a given airspeed. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used for extra lift on takeoff. Flaps also cause an increase in drag in mid-flight, so they are retracted when not needed.

Leading-edge cuff

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.

Supercritical airfoil

A supercritical airfoil is an airfoil designed primarily to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered ("downward-curved") aft section, and larger leading-edge radius compared with NACA 6-series laminar airfoil shapes. Standard wing shapes are designed to create lower pressure over the top of the wing. The camber of the wing determines how much the air accelerates around the wing. As the speed of the aircraft approaches the speed of sound, the air accelerating around the wing reaches Mach 1 and shockwaves begin to form. The formation of these shockwaves causes wave drag. Supercritical airfoils are designed to minimize this effect by flattening the upper surface of the wing.

Wing fence

Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Often seen on swept-wing aircraft, wing fences are flat plates fixed to the upper surfaces parallel to the airflow, typically wrapping around the leading edge. By obstructing span-wise airflow along the wing, they prevent the entire wing from stalling at once, as opposed to wingtip devices, which increase aerodynamic efficiency by seeking to recover wing vortex energy.

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

Wing configuration 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.

Leading-edge slat aerodynamic surface on the leading edge of the wings of fixed-wing aircraft

Slats are aerodynamic surfaces on the leading edge of the wings 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. They decrease stall speed.

Supersonic airfoils

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.

Vortilon

Vortilons are fixed aerodynamic devices on aircraft wings used to improve handling at low speeds.

Villiers XXIV

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.

References

  1. Kermode, A.C., Mechanics of Flight, Chapter 3
  2. 1 2 3 4 5 Clancy, L.J., Aerodynamics, Section 6.9
  3. Centennial of Flight Archived 2012-10-10 at the Wayback Machine . Retrieved 2008-02-19
  4. 1 2 3 4 Aviation Publishers Co. Limited, From the Ground Up, page 26 (27th revised edition) ISBN   0-9690054-9-0
  5. Kermode, A.C., Mechanics of Flight, Figure 3.36
  6. Kermode, A.C., Mechanics of Flight, Figure 3.37
  7. 1 2 Abbott and Von Doenhoff, Theory of Wing Sections, Section 8.6
  8. Crane, Dale: Dictionary of Aeronautical Terms, third edition, page 471. Aviation Supplies & Academics, 1997. ISBN   1-56027-287-2
  9. Kermode, A.C., Mechanics of Flight, Figures 6.6 and 6.7