Wing fence

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A Polish Sukhoi Su-20, with large wing fences on inner wings.

Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Wing fences are flat metal plates fixed to the upper surfaces(and often wrapping around the leading edge) parallel to the airflow. They work by obstructing the crossflow along the wing and preventing the entire wing from stalling at once. They are commonly seen on swept-wing aircraft, as they remedy the stall characteristics of swept wings. Wing fences are often used in addition to or instead of slats.

Wolfgang Liebe is generally credited with inventing wing fences, receiving a patent for it in 1938 while working on the Messerschmitt Bf 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 Mikoyan MiG-15s and Tupolev Tu-22Ms. 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."