|High lift devices on a Qantas Boeing 747-400 at takeoff. Krueger flaps are extended from the wing leading edge and flaps from the wing trailing edge.|
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.
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.
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 camber. 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 1920s and 30s, 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 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. 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 simultaneously by Handley-Page who received a patent in 1919, and by the 1930s had developed into a system that operated by airflow pressure against the slat to close and small springs to open at slower speeds or automatically when the airflow reached a predetermined angle-of-attack on the wing, aerodynamic forces 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 extension (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.
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 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.
- Taylor, John W.R. The Lore of Flight, London: Universal Books Ltd., 1990. ISBN 0-9509620-1-5.