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
18 Vol.3 No.11 / November 2014 VORTICES, VGS, AND FENCES…OH, MY! IF YOU'RE AROUND experimental airplanes very often, you prob- ably have seen little things sticking up from the airplanes' wings, fuselages, and tails. You may have fi gured correctly that they are meant to redirect airfl ow, but what do they do and why are they good? This article covers three of the most popular of those protu- berances—vortex generators (VGs), fences, and vortilons. Each of them looks a little dif erent, and each of them af ects the air in dif erent ways, with their own pros and cons. WHY ARE THEY THERE? Some people consider these devices to be "fi xes" for surprises that are discovered during fl ight testing. Others believe that they allow airplane producers to improvise ways to save on tooling (or the certifi cation process) between dif erent versions of similar airplanes. Aerospace engineers know that many times these are indeed the reasons to attach aerodynamic devices to an airplane, but sometimes there is an additional reason. An airplane designer has to compromise on various performance goals; they can't design the perfect airplane, because improving performance in one area makes it worse in another. Vortilons, vortex generators, and fences are air "tweakers" in an aerody- namicist's toolbox—tools that let the designer af ect the air in order to deal with surprises and compromises as the designer improvises a solution. These devices are not only available to designers but also to owners, if they are legally allowed to modify their airplane to get performance dif erent from the stock confi guration. Howev- er, you must not change the aerodynamic confi guration of your airplanes without knowing what you're doing. This article does not give suf cient information for you to modify your airplane. Instead, it is meant to present an overview of vortilons, vortex generators, and fences. After reading this, you will at least know some terminology so you can go talk to an expert without em- barrassing yourself. Before we describe what these devices are and how they work, it is important to know some basic aerodynamic con- cepts. There is no math here, just pretty pictures. WHAT IS A VORTEX? Vortices are easy to see in water: When you watch the swirling motion of water going down the drain, you're seeing a vortex. If you look up at planes during the right atmospheric conditions, you can sometimes catch the swirling white-tip vortices form- ing a spiral behind the wing. Sometimes the clouds help us see an airplane's tip vortices, as in Figure 1 below. In this case, the air starts swirling because the high-pressure air on the bottom of the wing wants to go toward the low-pres- sure air on the top of the wing. The air can go around the wing most easily at the wingtip, so a tip vortex is particularly strong. Fundamentally, a vortex is a swirling mass of fl uid. Yup, aerodynamicists think of air as a fl uid. That's why many aerody- namicists use computational fl uid dynamics (CFD) tools. Vortices, VGs, and Fences…Oh, My! What they are, what they do BY LYNNE WAINFAN, PH.D., E A A LIFE TIME 50408 Figure 1 – Wingtip vortices enhanced by clouds. Photography courtesy of fl yingindian.wordpress.com