Leading edge extension
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Image:Hornet.f18.750pix.jpg Image:FA18 LEX.jpg
Leading edge extensions or LEX (also referred to as leading edge root extensions or LERX) are fillets added to the front of a modern fighter aircraft's wings in order to provide usable airflow at high angles of attack. They are typically roughly triangular in shape, running from the leading edge of the wing root to a point near the cockpit along the fuselage. They tend to be fairly small in span, extending out less than a metre.
In cruising flight the effect of the LEX is minimal. However: when the angle of attack increases, as in a dog fight, the LEX starts to generate a high-speed vortex that remains attached to the top of the wing. Due to the effects described by Bernoulli's principle the wing therefore has a low pressure zone on top, and continues to generate lift past the normal stall point. The F/A-18 Hornet has especially large examples of LEX, as does the Sukhoi Su-27. Early prototypes of the Su-27 crashed due to poorly designed LEX, causing it to freeze at angles of attack above 5 degrees. This has since been overcome. In fact, the LEX help in making advanced maneuvres such as the Pugachev's Cobra, the Cobra Turn and the Frolov Chakra possible.
Airplanes using LEX
A few examples of aircraft with leading edge extensions are listed below. Russia/USSR
- Sukhoi Su-17
- Mikoyan MiG-31
- All Flankers and derivatives, including the Sukhoi Su-34
USA
- F-5 Freedom Fighter
- F-15 Eagle and F-15E Strike Eagle
- F-16 Fighting Falcon and it's unsuccessful competitor the YF-17
- F/A-18 Hornet
Similar concepts
A concept similar to LEX can be found on many aircraft, including airliners. These vortex generators take the form of small fins sticking up from the surface of the wing just behind the leading edge. In this case the purpose is to generate small vortices that ensure airflow over the control surfaces at high angles of attack, typically during takeoff and landing.
Another way to prevent stalling at high angles of attack is to use canards. They can be used in conjunction with leading edge extensions, as is shown in the Sukhoi Su-33.
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