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/418587
22 Vol.3 No.11 / November 2014 VORTICES, VGS, AND FENCES…OH, MY! of the way of the other wing. Another tradeoff faced was effectiveness versus simplic- ity. Yes, the round VGs are taller than one would expect, and again draggier. But these round VGs are off-the-shelf parts— no fabrication is needed. As you can see, there are many factors to consider—and trade-offs—when designing aerody- namic devices. As an interesting side note, VGs are not new and are not always designed by engineers. The figure below shows a duck (or a "canard" in French), with VGs popped up for landing. HOW DO VGS WORK? Notice from Figure 7 that the red VGs aren't aimed into the apparent wind; they are angled. As the air hits the VGs, vortices are formed, which energizes the airflow. This helps to keep the boundary layer attached when it might separate otherwise. Figure 12 illustrates the airfl ow with and without VGs, for dif erent angles of attack. For this top wing without VGs shown in Figure 12, the boundary layer is laminar until it begins to separate at some angle of attack (middle top picture). At a higher angle of attack, the fl ow becomes almost completely separated—the fl ow swirls around in a big way—and the wing stalls. As the lower left-hand picture of Figure 12 shows, VGs create a small vortex all the time. At higher angles of attack, the flow stays attached because the vortices energize the boundary layer. Moving to the rightmost drawing, you can see that the boundary layer remains attached even at higher angles of attack, whereas the boundary layer without the VGs separates. Similarly, for VGs placed in front of control surfaces, fl ow can stay attached longer. Figure 13 shows how vortices could be created to help to improve fl ap ef ectiveness. WHY ARE VGS GOOD? VGs can be helpful in many ways. As described above, the fi rst benefi t is to improve the stall characteristics of the airplane— fl ow stays attached at higher angles of attack than without the VGs. They can lower the stall speed of the aircraft. Another way VGs can help is to improve control surface ef ectiveness. Since fl ow separation is more likely near the trailing edge of the wing, stabilizer, and fi n, then ailerons, fl aps, elevators, and rudders have lousy air fl owing over them if the fl ow is separated. A control surface in separated fl ow means that the pilot may not be able to control the aircraft. To improve control surface ef ectiveness, VGs can be placed just ahead of the control surface. Figure 8 shows the VG positioned ahead of the rudder in order to maintain its ef ectiveness. VG DRAWBACKS VGs introduce a few problems when added to experimental aircraft. The fi rst is that they increase drag during normal fl ight conditions. Anything sticking up into the air will increase the skin-friction drag of the airplane. Not only are they perpen- dicular surfaces directly in the boundary layer airfl ow, but the fl ange that attaches the VG to the airplane is draggy also. By Figure 10 – How draggy are cylinders? Figure 11 – Natural vortex generators. Figure 12 – Airfl ow with and without VGs. Figure 13 – VGs can also improve fl ap effectiveness. Photography courtesy of WingSailor.com; Illustration courtesy of Micro AeroDynamics