Wing fence
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Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Not to be confused with wingtip fences, wing fences are flat metal plates fixed to the upper surfaces (and often wrapping around the leading edge) parallel to the airflow. They are often seen on swept-wing aircraft. They obstruct span-wise airflow along the wing, and prevent the entire wing from stalling at once. Wing fences are often used in addition to or instead of slats.
As a swept-wing aircraft slows toward the stall speed of the wing, the angle of the leading edge forces some of the airflow sidewise, toward the wing tip. This process is progressive, airflow near the middle of the wing is affected not only by the leading edge angle, but also the spanwise airflow from the wing root. At the wing tip the airflow can end up being almost all spanwise, as opposed to front-to-back over the wing, meaning that the effective airspeed drops well below the stall. Because the geometry of swept wings typically places the wingtips of an aircraft's aft of its center of gravity, lift generated at the wingtips tends to create a nose-down pitching moment. When the wingtips stall, both the lift and the associated nose-down pitching moment rapidly diminish. The loss of the nose-down pitching moment leaves the previously balanced aircraft with a net nose-up pitching moment. This forces the nose of the aircraft up, increasing the angle of attack and leading to stall over a greater portion of the wing. The result is a rapid and powerful pitch-up followed by a complete stall, a difficult situation for a pilot to recover from. The "Sabre dance" -which caused many F-86 Sabre's to crash- is a notable example of this behavior.
Wing fences delay, or eliminate, this effect by preventing the spanwise flow from moving too far along the wing and gaining speed. When meeting the fence, the air is directed back over the wing surface. Similar solutions included a notch in the leading edge, as seen on the Avro Arrow, or the use of slats, as on the later versions of the F-86. Slats can act as fences directly, in the form of their actuators, but also reduce the problem by improving the angle of attack response of the wing and moving the stall point to a lower speed.
Wolfgang Liebe is generally credited with inventing wing fences, receiving a patent for it in 1938 while working on the Messerschmitt Me.109B. After World War II, Soviet military aircraft designers became known for their habit of using wing fences, using them on aircraft as varied as MiG-15 'Fagots' and Tu-22M 'Backfires'. It was joked in the West that the purpose of wing fences on Soviet military aircraft was to "prevent air from defecting over the tips of the wings."
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