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The question being answered is: What are stall fences?
Created by: Matt Daskilewicz
Stall fences are small chordwise plates that protrude from the upper wing surface and leading edge. Their purpose is to disrupt spanwise flow, shielding the outboard wing section from a developing inboard stall. By keeping the outboard section from stalling, aileron effectiveness is maintained during a stall. This allows the pilot to keep the aircraft level and exit the stall safely.
The image to the right shows an aircraft with stall fences installed on its wing.
How Stall Fences Work Edit
To understand how stall fences work, we must first understand what a stall is. A stall occurs when flow over the wing separates because it has insufficient kinetic energy to stay attached. This may be caused by flying too slowly, resulting in an angle of attack that is too large.(Ref 2 p.162) Low energy flow is found in the boundary layer, which thickens along a surface in the direction of the flow, resulting in the lowest energy at the trailing edge of the surface.
For aircraft with swept wings, the local flow along the wing is not in the direction of flight. There is also a spanwise flow along the wing from the center toward the tips. This causes the boundary layer to thicken towards the tips, making them more susceptible to stalling than the inboard portion of the wing. This is dangerous because the ailerons, used to control roll, are located near the wing tips (where they will have the greatest moment arm). If the tips stall the ailerons will lose their effectiveness, making the aircraft uncontrollable.
Stall fences work by creating a boundary condition that prevents spanwise flow. Because the flow cannot go through the fence, it will stagnate when it reaches the fence. Just like the stagnation point on the leading edge of the airfoil, this will essentially cause a new (thin) boundary layer to start on the other side of the fence. Thus instead of the boundary layer getting thicker and thicker along the span, it restarts when it reaches the stall fence, and stays thin along the entire span.
Stall Fence Geometry Edit
The image below shows four types of stall fences. These are not established 'names,' but rather are just descriptions of the different types of geometry. Note that all types protrude from the leading edge of the wing. The true fences extend along the upper surface of the wing, since this is the low pressure (suction) surface where flow separation occurs.
Stall strips are sharp spanwise protrusions places on the inboard section of the leading edge. They cause the inboard section to stall at a lower angle of attack than the outboard section, ensuring the the tips do not stall first. These are often used in conjunction with true fences.
Vortilons are short fences that extend forward below the wing. At high angles of attack the tip will extend above the wing, producing vortices that energize the flow to delay stall, and buffet the tail to warn the pilot of an imminent stall.
Effectiveness of Stall Fences Edit
The graph to the right shows the effects of three types of stall fences on CL and CD for a full-scale low-wing general aviation aircraft. The CL curves are the upper set, and the CD curves are the lower set. The long fence increases CLmax and stalling angle of attack, but it causes a steep stall characteristic. The short fence (skewed in) does not increase CLmax, but makes the stall behavior more gradual. The short fence (skewed out) actually worsens stall performance. Note that below 10 degrees angle of attack the stall fences have no effect on either lift or drag.
Aircraft that Use Stall FencesEdit
Stall fences are not very common on modern aircraft, but may be found on highly swept wings and STOL aircraft. (In other words, aircraft that have significant spanwise flow or large lift coefficients.)
In most modern aircraft, stall fences have been replaced with vortex generators, which prevent stalls by creating turbulent vortices that energize the flow. The picture below shows an aircraft with prominent vortex generators (they are usually not so numerous).
Aircraft that use stall fences include:
(1) Newsom, W. Satran, D. Johnson, J. Effects of Wing-Leading-Edge Modifications on a Full-Scale, Low-Wing General Aviation Airplane. NASA TP-2011, 1982
(2) Shevell, R. Fundamentals of Flight. Prentice Hall, 1989.