Wing fences and Vortilons

https://en.wikipedia.org/wiki/Wing_fence

Wing fences, also known as boundary layer fences and potential fences are fixed
aerodynamic devices attached to aircraft wings. Often seen on swept-wing
aircraft, wing fences are flat plates fixed to the upper surfaces parallel to
the wing chord and in line with the free stream airflow, typically wrapping
around the leading edge. By obstructing span-wise airflow along the wing, they
prevent the entire wing from stalling at once, as opposed to wingtip devices,
which increase aerodynamic efficiency by seeking to recover wing vortex energy.

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 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-100 Super Sabres to crash) is a notable example of this
behavior.

Wing fences delay, or eliminate, these effects 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 earlier 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, who is generally credited with inventing wing fences, filed 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-15, Mikoyan MiG-17,
Mikoyan MiG-19, Mikoyan MiG-21, Mikoyan MiG-25, Mikoyan MiG-31, Tupolev Tu-128,
Tupolev Tu-95, Tupolev Tu-160 in swept-wing mode, and Tupolev Tu-22Ms.

Vortilons

https://en.wikipedia.org/wiki/Vortilon

Vortilons are fixed aerodynamic devices on aircraft wings used to improve
handling at low speeds.

The vortilon was invented by aerodynamicists working at Douglas Aircraft who had
previously developed the engine pylons for the Douglas DC8. The original pylons
which wrapped around the leading edge of the wing had to be cut back to reduce
excessive cruise drag. Wind tunnel testing of the next Douglas commercial
aircraft, the Douglas DC9 which had no under-wing engines, showed a cutback
engine pylon would be beneficial to wing lift and upwash at the tail at the low
speed stall. The pylon was reduced in size and became the vortilon (
VORTex-generating-pYLON).

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.

Vortilons were first introduced with the McDonnell Douglas DC-9 to achieve a
strong nose down pitching moment just beyond the normal stall and their
influence ceased to have any effect beyond 30 degrees angle of attack. They have
been used on subsequent aircraft, including:

Rutan VariEze, replacing leading edge cuffs (1984)
Rutan Long-EZ
BAe 125 1000 series
Hawker 800XP
Embraer ERJ 145 family
Learjet 45
Aériane Swift



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