Leading edge extensions, or LEX, are filets added to the front of a wing on modern fighter aircraft in order to provide usable airflow over the wing at high angles of attack. They are typically roughly triangular in shape, running from the leading edge of the wing 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 as the angle of attack increases, 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 far beyond the stall point.
Smaller versions of LEX can be found on many aircraft, including airliners. These 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 smaller vortexes that keep airflow over the flight controls at the rear of the wing at high angles of attack, notably during takeoff and landing. In this case the devices are known as vortex generators.
