Experimenter

NOV 2014

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

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24 Vol.3 No.11 / November 2014 VORTICES, VGS, AND FENCES…OH, MY! WING FENCES: WHAT ARE THEY? Sometimes called "boundary layer fences" or "fl ow fences," wing fences are nonmoving plates attached perpendicular to a wing. Unlike slanted VGs, fences are aligned with the free- stream airfl ow. They typically extend no farther than one-third of the wing chord and often—but not always—wrap around the leading edge of the wing. They stick out above the boundary layer, farther than a VG would. Fences are not a new invention; the patent was fi led in 1938. Figure 14 shows an example of an old-fashioned wing fence. This fence is unusual in that it is not fl at—the curved ridge makes the fence stronger than a fl at plate would be. Although most wing fences are on the leading edge of the wing, the photo below shows one mounted on the trail- ing edge. Fences at the wingtip are distinct from winglets; tip fences extend aft of the trailing edge of the wing. Figure 16 contrasts winglets from fences. The red winglet on the Boeing 737-800 does not extend beyond the trailing edge of the wing, like the yellow fence on the Airbus A319 does. HOW DO FENCES WORK? The exact workings of wing fences are somewhat complex and still being analyzed. However, here is a simplifi ed way of think- ing about them: Fences are boundary layer control devices used to help the air move to the back of the wing rather than toward the wingtip. Generally, high-pressure air from below the wing wants to fl ow toward low pressure. Normally that would be toward the back of the wing. However, for some confi gurations, most notably swept wings, the air wants to fl ow "spanwise," or down the wing toward the tip. The air along the wing has more or less tendency to do that. At the wing root, the air has a slight desire to fl ow spanwise, but it really wants to do that at the wingtip. This spanwise fl ow can cause the fl ow to separate on the outer wing, which leads to a variety of ills, most notably really bad stall performance. To understand this poor stall performance, imagine a swept- wing airplane whose spanwise fl ow has caused the tips to stall. The tips are behind the rest of the wing, so losing that lift at the back of the wing means that the plane would pitch up. A pitch-up during a stall doesn't correct itself when the pilot backs of from the nose-up command. The plane continues to pitch up, worsen- Figure 14 – A wing fence. Figure 15 – A fence attached to the trailing edge of a wing. Figure 16 – A wingtip winglet. Figure 17 – A wing fence shot from the wing Photography courtesy of Lynne Wainfan and Boeing

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