High-lift device
Encyclopedia
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 flap
s and slat
s, as well as less common features such as leading edge extension
s and blown flap
s.
. Flaps are usually located on the trailing edge of a wing, while leading edge flaps are occasionally used as well. Some flap designs also increase the wing chord
when deployed, increasing the wing area to help produce more lift; such complex flap arrangements are found on many modern aircraft. The first "travelling flaps" that moved rearward were starting to be used just before World War II
due to the efforts of many different individuals and organizations in the 1920's and 30's, and have been followed by increasingly complex systems made up of several parts with gaps in between, known as slotted flaps. Large modern airliners make use of triple-slotted flaps to produce the massive lift required during takeoff.
. 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. The slat may be fixed in position, or it may be retractable. If it is fixed, then it may appear as a normal part of the leading edge of a wing which has slot. The slat or slot may be either full span, or may occur on only part of the wing (usually outboard), depending on how the lift characteristics need to be modified for good low speed control. Often it is desirable for part of the wing where there are no controls to stall first, allowing aileron control well into the stall.
The first slats were developed by Gustav Lachmann
in 1918 and simultaeously by Handley-Page who received a patent in 1919, and by the 1930s had developed into a system that operated automatically when the airflow over the wing reduced pressure on the leading edge, small springs would then push the slat out. Modern systems, like modern flaps, are more complex and are typically deployed hydraulically or with servos.
(LEX). A LERX typically consist of a small triangular fillet between the wing leading edge root and 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, and are commonly found on modern fighter aircraft
.
s use "bleed air
" from the jet engine
's compressor or engine exhaust which is blown 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 tangentially ejects air over a specially designed airfoil
to create lift through the Coanda effect
.
A more common system uses the airflow from the engines directly, by placing a flap in the path of the exhaust. The flap requires greater strength due to the power of modern engines, and most designs deliberately "split" the flap so the portions directly behind the engines do not move into the airflow. However, if the flaps can be made strong enough, the effects can be large. The C-17 Globemaster III
utilizes this concept as its high lift device.
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
design, high-lift devices are moving surfaces or stationary components intended to increase lift during certain flight conditions. They include common devices 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...
s and slat
Leading edge slats
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...
s, as well as less common features such as leading edge extension
Leading edge extension
A leading edge extension is a small extension to an aircraft wing surface, forward of the leading edge. Different kinds of extensions have been used for different reasons.-Leading edge slats:...
s and blown flap
Blown flap
Blown flaps are a powered aerodynamic high-lift device invented by the British and used on the wings of certain aircraft to improve low-speed lift during takeoff and landing. The process is sometimes called a boundary layer control system . They were a popular design feature in the 1960s, but fell...
s.
Purpose
Aircraft designs include compromises intended to maximize performance for a particular role. One of the most fundamental of these is the size of the wing; a larger wing will provide more lift and reduce takeoff and landing distance, but will increase drag during cruising flight and thereby lead to lower than optimum fuel economy. High-lift devices are used to smooth out the differences between the two goals, allowing the use of an efficient cruising wing, and adding lift for takeoff and landing.Flaps
The most common high-lift device is the flap, a movable portion of the wing that can be lowered into the airflow to produce extra lift. Their purpose is to re-shape the wing section into one that has more camberCamber (aerodynamics)
Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...
. Flaps are usually located on the trailing edge of a wing, while leading edge flaps are occasionally used as well. Some flap designs also increase 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...
when deployed, increasing the wing area to help produce more lift; such complex flap arrangements are found on many modern aircraft. The first "travelling flaps" that moved rearward were starting to be used just before 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...
due to the efforts of many different individuals and organizations in the 1920's and 30's, and have been followed by increasingly complex systems made up of several parts with gaps in between, known as slotted flaps. 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 while at a high angle of attackAngle 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...
. 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. The slat may be fixed in position, or it may be retractable. If it is fixed, then it may appear as a normal part of the leading edge of a wing which has slot. The slat or slot may be either full span, or may occur on only part of the wing (usually outboard), depending on how the lift characteristics need to be modified for good low speed control. Often it is desirable for part of the wing where there are no controls to stall first, allowing aileron control well into the stall.
The first slats were developed by Gustav Lachmann
Gustav 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 and simultaeously by Handley-Page who received a patent in 1919, and by the 1930s had developed into a system that operated automatically when the airflow over the wing reduced pressure on the leading edge, small springs would then push the slat out. Modern systems, like modern flaps, are more complex and are typically deployed hydraulically or with servos.
Leading edge root extensions
Although not as common, another high-lift device is the leading edge root extension (LERX) or leading edge extensionLeading edge extension
A leading edge extension is a small extension to an aircraft wing surface, forward of the leading edge. Different kinds of extensions have been used for different reasons.-Leading edge slats:...
(LEX). A LERX typically consist of a small triangular fillet between the wing leading edge root and fuselage. In normal flight the LERX generates little lift. At higher angles of attack, however, it generates a vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called 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, and are commonly found on modern 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...
.
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 flapBlown flap
Blown flaps are a powered aerodynamic high-lift device invented by the British and used on the wings of certain aircraft to improve low-speed lift during takeoff and landing. The process is sometimes called a boundary layer control system . They were a popular design feature in the 1960s, but fell...
s use "bleed air
Bleed air
Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft...
" from the jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
's compressor or engine exhaust which is blown over the rear upper surface of the wing and flap, re-energising the 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 allowing the airflow to remain attached at higher angles of attack. A more advanced version of the blown flap is the circulation control wing
Circulation control wing
A circulation control wing is a form of high-lift device for use on the main wing of an aircraft to increase the lift coefficient. CCW technology has been in the research and development phase for over sixty years, and the early models were called blown flaps.The CCW works by increasing the...
; a mechanism that tangentially ejects air over a specially designed airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
to create lift through the Coanda effect
Coanda effect
The Coandă effect is the tendency of a fluid jet to be attracted to a nearby surface. The principle was named after Romanian aerodynamics pioneer Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development....
.
A more common system uses the airflow from the engines directly, by placing a flap in the path of the exhaust. The flap requires greater strength due to the power of modern engines, and most designs deliberately "split" the flap so the portions directly behind the engines do not move into the airflow. However, if the flaps can be made strong enough, the effects can be large. The C-17 Globemaster III
C-17 Globemaster III
The Boeing C-17 Globemaster III is a large military transport aircraft. Developed for the United States Air Force from the 1980s to the early 1990s by McDonnell Douglas, the C-17 is used for rapid strategic airlift of troops and cargo to main operating bases or forward operating bases throughout...
utilizes this concept as its high lift device.