Vortilon is a contraction of VORTex and pYLON words (generating vortices as a leading-edge engine pylon does).
Vortilons consist of one or more flat plates attached to the underside of the wing near its leading edge, aligned with the flight direction. When the speed is reduced and the aircraft approaches stall, the local flow at the leading edge is diverted outwards; this spanwise component of velocity around the vortilon creates a vortex streamed around the top surface, which energises the boundary layer. A more turbulent boundary layer, in turn, delays the local flow separation.
Vortilons are often used to improve low-speed aileron performance, thereby increasing resistance to spin. They can be used as an alternative to wing fences, which also restrict airflow along the span of the wing. Vortilons only stream vortices at high angles of attack and produce less drag at higher speeds than wing fences. Pylons used to mount jet engines under the wing produce a similar effect.
The occurrence of span-wise flow at high angles of attack, such as observed on swept wings, is an essential requirement for vortilons to become effective. According to Burt Rutan, vortilons installed on straight wings would not have any effect.
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