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]
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.
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]
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.
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.
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]
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.
A tailplane, also known as a horizontal stabilizer, is a small lifting surface located on the tail (empennage) behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed-wing aircraft such as helicopters and gyroplanes. Not all fixed-wing aircraft have tailplanes. Canards, tailless and flying wing aircraft have no separate tailplane, while in V-tail aircraft the vertical stabilizer, rudder, and the tail-plane and elevator are combined to form two diagonal surfaces in a V layout.
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.
Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft traveling vertically, this is known as liftoff.
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.
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.
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.
Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.
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.
In aeronautics, a spoiler is a device which intentionally reduces the lift component of an airfoil in a controlled way. Most often, spoilers are plates on the top surface of a wing that can be extended upward into the airflow to spoil the streamline flow. By so doing, the spoiler creates a controlled stall over the portion of the wing behind it, greatly reducing the lift of that wing section. Spoilers differ from airbrakes in that airbrakes are designed to increase drag without disrupting the lift distribution across the wing span, while spoilers disrupt the lift distribution as well as increasing drag.
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".
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.
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.
The wing configuration of a fixed-wing aircraft is its arrangement of lifting and related surfaces.
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.
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.
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.
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.
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