DEC 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.

Issue link: http://experimenter.epubxp.com/i/434207

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Page 28 of 44

EAA Experimenter 29 edge of the wing. Trailing-edge fences have many advan- tages over similar devices. They have less drag than vortilons because they affect less of the wing; they have less drag than regular fences for the same reason; they have less wetted area than regular fences; and they are mostly inside the boundary layer. Figure 9 (and Figure 7) show Klaus' 4-inch-tall trailing-edge fences. Fences can be installed at various locations on a wing, but Klaus' are placed on the wing near each end of the ailerons to increase aileron ef ectiveness and to reduce spanwise fl ow, which is highest during aileron defl ection. "On swept wings, fences should not be installed on aileron control surfaces since this loads them up to the point that roll authority is all but lost," he cautioned. The trailing-edge fences' ef ect on low-speed performance was remarkable. Klaus' testing on the VariEze showed "it was immediately noticed that takeof distance is reduced 10 to 15 percent; climb rate is improved 20 percent; and most notice- ably, approaches can be fl own at least 10 to 15 percent slower, resulting in a signifi cantly shorter landing distance—nearly 30 percent less. There was a measurable increase in top speed above 10,000 feet." Recall from my article "Vortilons, VGs, and Fences, Oh My" that it is sometimes helpful to trip the boundary layer from laminar to turbulent slightly forward of where it would naturally transition. Klaus used his fl ow visualization methods to fi nd the transition zone and the ideal place to trip the fl ow to turbulent. Figure 10 shows the zigzag tape that Klaus some- times uses to trip the boundary layer on his wing. When Klaus oiled up his airplane, he was surprised to fi nd that the transition to turbulent fl ow was not where convention- al wisdom predicted. "Surprise!" he reported. "The transition was at 62 percent of the chord, 5 percent aft of the predicted location of the GAW-2 airfoil. It was way farther back than ex- pected." If he hadn't understood the situation by visualizing his fl ow, he would have put the zigzag tape too far forward, giving up those precious few inches of laminar fl ow. So why did he fi nd that his boundary layer transitioned later than experts predicted? Klaus believes that the turbulence in free air is much lower than in any wind tunnel. Readers are invited to send in thoughts on this potentially important theory. Boundary layer trippers aren't just useful on wings. Figure 11 shows Klaus' landing gear, with zigzag tape on the strut and wheelpant. Figure 9: A trailing edge fence. Figure 10: Klaus occasionally uses this zig-zag tape to trip the boundary layer on his wing. Figure 11. More zig-zag tape on the wheelpant trips the boundary layer there as well.

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