High-lift device
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In aircraft design, high-lift devices are a variety of mechanisms intended to add lift during certain portions of flight. They include common devices such as flaps and slats, as well as less common devices such as leading edge extensions and blown flaps. Generally they are divided into two classes by engineers, powered and unpowered.
All aircraft designs include a number of 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 make takeoffs and landings shorter and easier, but increase drag during cruising and thereby lead to lower "gas mileage". 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 rear wing that can be bent down into the airflow to produce extra lift. The effect of the flap is not as obvious as it may seem, the real goal is to re-shape the wing as a whole into one that has more camber as well as being longer. In general wings with more camber and chord will produce more lift for any given amount of drag. It is the second goal, making the wing longer, than results in the complex flap arrangements found on many modern aircraft. The first "travelling flaps" that moved rearward were starting to be used just before World War II due to efforts at Arado, and have been followed by increasingly complex systems made up of several parts, known as slots. Large modern airliners make use of triple-slotted flaps to produce the massive lift needed to lift them on takeoff.
Another common high-lift device is the slat, what appears to be a flap at the front of the wing. In fact the action of the slat is very different than the flap, as it does not directly produce extra lift. Instead the slat re-directs the airflow at the front of the wing, allowing it to flow more smoothly over the surface while at a high angle of attack. This allows the wing to be operated effectively at higher angles, which produce more lift. The original slats were patented by Handley-Page 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. More modern systems, like modern flaps, are more complex and typically operated hydraulically.
Although not as common, another high-lift device is the leading edge extension, or LEX. LEX systems typically consist of a thin delta wing mounted in front of the main wing, which normally generates little lift. At higher angles of attack, however, the LEX generates a vortex that is positioned to lie on the upper surface of the main wing. This reduces the pressure over the wing, leading to greater lift. LEX systems are notable for their potentially huge angles in which they operate, and are commonly found on modern fighter aircraft.
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 which is blown over the rear of the wing and flap, adding airflow and allowing the air to remain attached at higher angles of attack. In effect the airflow acts as a sort of a slat for the flaps.
A more common system uses the airflow from the engines directly, by placing a flap directly in the path of the exhaust. This isn't a trivial exercise due to the power of modern engines, and most aircraft deliberately "split" the flap so the portion behind the engines do not move into the airflow. However, if the flaps can be made strong enough, the effects can be enormous. Oddly the effect is most pronounced if the engines are mounted above the wing due to the Coanda effect, which led to a number of aircraft such as the Boeing YC-14 and Antonov An-72 with high-mounted engines. The C-17 Globemaster III uses a more conventional low-mounted engine design based on the same general idea.
