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Klaus's Info Aerodynamic Flow Fence

last update:
January 01, 2007

Flow Fences
Flow fences shown here on Shirl Dickey's vintage Vari Eze.

Aerodynamic Flow Fence Technicalities

Span wise flow characteristics of swept wings are well understood and often treated with flow correcting devices. Most commonly they are installed on top of the wing and rarely behind or below.

Airliners do not have them since they have fully articulating trailing edges.

The following text only applies to the Vari Eze wing directly and other swept wings indirectly.

While span wise flow still exists on trailing edges of unswept wings, it is so small that restricting flow fences would probably not have a significant benefit.

On swept wings flow fences should not be installed on aileron control surfaces since this loads them up to the point that roll authority is all but lost.

In an effort to reduce approach and landing speeds of Light Speed Engineering's Vari Eze, span wise flow fences were installed on the trailing edge and flight tested.

The overall change in low speed performance was remarkable. It was immediately noticed that take- off distance is reduced 10-15% climb rate is improved 20% and most noticeably approaches can be flown at least 10-15% slower resulting in a significantly shorter landing distance, nearly 30% less. There was no measurable decrease in top speed. The configuration of the airplane tested had the standard 3 Vortilons on each wing leading edge with three roughly equally spaced flow fences between the outboard end of the aileron and the winglet. The shape of each fence does not appear to be critical but it is believed that it is important that they exist on top of, behind and below the wing surface to completely isolate the section outboard of the fence from any span wise flow that is developed inboard of it. Span wise flow on swept wings at high angle of attack around the trailing edge redevelop in a short span thus several fences are more beneficial.

Technically speaking the slope of the CL/alpha curve of the main wing is increased especially in the +5 to +14 deg AoA region. Max AoA at equal flight conditions was reduced by 2.2 deg. from 16.2 down to 14 deg. Vmin is not noticeably reduced since it is elevator limited. Due to the higher main wing C/L generated at max AoA the elevator is more loaded even though max alpha is reduced. The increased pitching moment loads the elevator to stall at a lower deck angle.

The only negative observation is a reduced roll response at the lower speeds, probably due to lower dynamic pressure.

Special thanks to all the customers of Light Speed Engineering who have made this and other research possible.

Klaus Savier
Light Speed Engineering, LLC




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