High-lift device

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High lift devices on an Air New Zealand Boeing 747-400 (ZK-SUH) on arrival to London Heathrow, England. The triple-slotted trailing edge flaps are well displayed and the Krueger flaps on the leading edge also are visible. Air New Zealand Boeing 747-400 (ZK-SUH) arrives London Heathrow 17Oct2010 arp.jpg
High lift devices on an Air New Zealand Boeing 747-400 (ZK-SUH) on arrival to London Heathrow, England. The triple-slotted trailing edge flaps are well displayed and the Krueger flaps on the leading edge also are visible.

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.



The size and lifting capacity of a fixed wing is chosen as a compromise between differing requirements. For example, a larger wing will provide more lift and reduce the distance and speeds required for takeoff and landing, but will increase drag, which reduces performance during the cruising portion of flight. Modern passenger jet wing designs are optimized for speed and efficiency during the cruise portion of flight, since this is where the aircraft spends the vast majority of its flight time. High-lift devices compensate for this design trade-off by adding lift at takeoff and landing, reducing the distance and speed required to safely land the aircraft, and allowing the use of a more efficient wing in flight. The high-lift devices on the Boeing 747-400, for example, increase the wing area by 21% and increase the lift generated by 90%. [1]

Types of device


The most common high-lift device is the flap, a movable portion of the wing that can be lowered to produce extra lift. When a flap is lowered this re-shapes the wing section to give it more camber. Flaps are usually located on the trailing edge of a wing, while leading edge flaps are used occasionally. There are many kinds of trailing-edge flap.

Simple hinged flaps came into common use in the 1930s, along with the arrival of the modern fast monoplane which had higher landing and takeoff speeds than the old biplanes.

In the split flap, the lower surface hinges downwards while the upper surface remains either fixed to the wing or moves independently.

Travelling flaps also extend backwards, to increase the wing chord when deployed, increasing the wing area to help produce yet more lift. These began to appear just before World War II due to the efforts of many different individuals and organizations in the 1920s and 30s.

Slotted flaps comprise several separate small airfoils which separate apart, hinge and even slide past each other when deployed. Such complex flap arrangements are found on many modern aircraft. [2] Large modern airliners make use of triple-slotted flaps to produce the massive lift required during takeoff.

Slats and slots

Another common high-lift device is the slat, a small aerofoil shaped device attached just in front of the wing leading edge. The slat re-directs the airflow at the front of the wing, allowing it to flow more smoothly over the upper surface when at a high angle of attack. This allows the wing to be operated effectively at the higher angles required to produce more lift. A slot is the gap between the slat and the wing. [3] The slat may be fixed in position, with a slot permanently in place behind it, or it may be retractable so that the slot is closed when not required. If it is fixed, then it may appear as a normal part of the leading edge of a wing, with the slot buried in the wing surface immediately behind it.

A slat or slot may be either full-span, or may be placed on only part of the wing (usually outboard), depending on how the lift characteristics need to be modified for good low speed control. Slots and slats are sometimes used just for the section in front of the ailerons, ensuring that when the rest of the wing stalls, the ailerons remain usable.

The first slats were developed by Gustav Lachmann in 1918 and simultaneously by Handley-Page who received a patent in 1919. By the 1930s automatic slats had been developed, which opened or closed as needed according to the flight conditions. Typically they were operated by airflow pressure against the slat to close it, and small springs to open it at slower speeds when the dynamic pressure reduced, for example when the speed fell or the airflow reached a predetermined angle-of-attack on the wing.

Modern systems, like modern flaps, can be more complex and are typically deployed hydraulically or with servos. [4] [5] [6]

Boundary layer control and blown flaps

Powered high-lift systems generally use airflow from the engine to shape the flow of air over the wing, replacing or modifying the action of the flaps. Blown flaps take "bleed air" from the jet engine's compressor or engine exhaust and blow it over the rear upper surface of the wing and flap, re-energising the boundary layer and allowing the airflow to remain attached at higher angles of attack. A more advanced version of the blown flap is the circulation control wing, a mechanism that ejects air backwards over a specially designed airfoil to create lift through the Coandă effect. The Blackburn Buccaneer had a sophisticated boundary layer control (BLC) system which involved compressor air blown onto the wings and tailplane to reduce the stalling speed and facilitate operations from smaller aircraft carriers.

Another approach is to use the airflow from the engines directly, by placing a flap so that it deploys into the path of the exhaust. Such flaps require greater strength due to the power of modern engines and also greater heat resistance to the hot exhaust, but the effect on lift can be significant. Examples include the C-17 Globemaster III.

Leading edge root extensions

More common on modern fighter aircraft but also seen on some civil types, is the leading-edge root extension (LERX), sometimes called just a leading edge extension (LEX). A LERX typically consist of a small triangular fillet attached to the wing leading edge root and to the fuselage. In normal flight the LERX generates little lift. At higher angles of attack, however, it generates a vortex that is positioned to lie on the upper surface of the main wing. The swirling action of the vortex increases the speed of airflow over the wing, so reducing the pressure and providing greater lift. LERX systems are notable for the potentially large angles in which they are effective.

Co-Flow Jet

A Co-Flow Jet (CFJ) wing has an upper surface with an injection slot after the leading edge and a suction slot before the trailing edge, to augment lift, increase the stall margin and reduce drag. CFJ is promoted by the mechanical and aerospace engineering department of the University of Miami. For a hybrid-electric regional aircraft based on the ATR 72 with the same wing area, size and weight, CFJ improves its cruise lift coefficient for a higher wing loading, allowing more fuel and batteries for longer range. [7]

See also

Related Research Articles

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<span class="mw-page-title-main">Tailplane</span> Small lifting surface of a fixed-wing aircraft

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

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<span class="mw-page-title-main">Takeoff</span> Phase of flight in which a vehicle leaves the land or water surface

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<span class="mw-page-title-main">Angle of attack</span> Angle between the chord of a wing and the undisturbed airflow

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<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.

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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">Flight control surfaces</span> Surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

<span class="mw-page-title-main">Blown flap</span>

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.

<span class="mw-page-title-main">Spoiler (aeronautics)</span> Device for reducing lift and increasing drag on aircraft wings

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<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.

Aircraft flight mechanics are relevant to fixed wing and rotary wing (helicopters) aircraft. An aeroplane, is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight".

<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">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. 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">Leading-edge slat</span> Device increasing the lift of the wing at low speed (take-off and landing)

A slat is an aerodynamic surface on the leading edge of the wing of a fixed-wing aircraft. Slats, when deployed, allow the wings 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 used during takeoff and landing or while performing low speed maneuvers which may take the aircraft close to a stall. Slats are retracted in normal flight to minimize drag.

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

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. Several modern aircraft use this design between the fuselage and closest engine, where the wing is thickest. Outboard of the engine, slats are used on the leading edge. The Boeing 727 also used a mix of inboard Krueger flaps and outboard slats, although it had no engine between them. Most early jet airliners, such as the Boeing 707 and Boeing 747, used Krueger flaps only.

<span class="mw-page-title-main">Strake (aeronautics)</span> Flight control surface

In aviation, a strake is an aerodynamic surface generally mounted on the fuselage of an aircraft to improve the flight characteristics either by controlling the airflow or by a simple stabilising effect.

<span class="mw-page-title-main">Leading-edge droop flap</span> Aerodynamic device

The leading-edge droop flap is a device on the leading edge of aircraft wings designed to improve airflow at high angles of attack. The droop flap is similar to the leading-edge slat and the Krueger flap, but with the difference that the entire leading edge section rotates downwards, whereas the slat and Krueger flap are panels which move away from the wing leading edge when it is deployed.

The 1958 Lanier Paraplane Commuter 110 or 110 Paraplane Commuter PL-8 was one of the last designs stemming from Edward H. Lanier's 1930s patents, and aircraft incorporating apertures in the upper surfaces, which claimed to give benefits in safety, lift and STOL ability.



  1. Colin Cutler (November 19, 2014). "16 Little Known Facts About The Boeing 747". www.boldmethod.com. Retrieved March 22, 2016.
  2. Taylor 1990, p. 337.
  3. Kermode, A.C. Mechanics of flight, 8th Edn., Pitman, 1972
  4. Taylor 1990, p. 346
  5. Taylor 1990, p. 399.
  6. Abzug, Malcomb (2005). Airplane Stability and Control: A History of the Technologies that Made Aviation Possible. 231: Cambridge University Press. p. 416. ISBN   9780521021289.{{cite book}}: CS1 maint: location (link)
  7. Graham Warwick (Jan 21, 2019). "The Week In Technology, Jan. 21-26, 2019". Aviation Week & Space Technology.


  • Taylor, John W.R. The Lore of Flight, London: Universal Books Ltd., 1990. ISBN   0-9509620-1-5.