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
26 Vol.3 No.11 / November 2014 VORTICES, VGS, AND FENCES…OH, MY! HOW DO VORTILONS WORK? From the two photographs in Figure 18, you can see that the fi rst vortilon is more rounded than the ones in the bottom pic- ture and sticks out in front of the leading edge more. Vortilon shapes, placement, and amount of protrusion past the leading edge are not standardized. In fact, there is not much informa- tion available about vortilon design. However, we know a bit about how they work. Recall that at higher angles of attack, the stagnation point (where the air smacks perpendicularly into the wing) moves to a point below the leading edge of the wing. This is shown by the grey stream- line under the leading edge in Figure 19. For some angles of attack, this high-pressure air on the bottom of the wing actually fl ows "up" around the leading edge to the top of the wing before proceeding "down" to the trailing edge. For swept wings at high angles of attack, the air wants to travel toward the tip. This spanwise fl ow hits the vortilon, cre- ating a vortex that travels up, around the leading edge, and over the top surface of the wing. Just like a VG or a fence, this vortex helps to delay fl ow separation over the top surface of the wing and can help control surface ef ectiveness. The vortilon acts as a mini fence, reducing the spanwise fl ow. This reduces the tendency in swept-wing airplanes to tip stall. Another vortilon ef ect was discovered when Douglas found that at high angles of attack its vortilon produced an "upwash fi eld" inboard of the vortilon that af ected the airfl ow over the plane's horizontal tail. The ef ect was to produce a nose-down pitching moment at the stall. WHY ARE VORTILONS GOOD? There's a lot to love about vortilons. Like VGs and fences, they can delay fl ow separation, which delays stall and reduces drag. They also can improve control surface ef ectiveness. They can help to create a nose-down pitching moment near stall. Unlike VGs, they do not produce a vortex except at high angles of at- tack where you may need a vortex. In addition to this drag im- provement, they are lined up with the direction of fl ight so they do not have the frontal area drag of VGs. Vortilons have less surface area than a fence, so they have less skin-friction drag. THE DRAWBACK OF VORTILONS Vortilons are only ef ective when there is the problem of span- wise fl ow, for instance, on swept-wing airplanes. VGs would be more ef ective for straight-winged airplanes. Vortilons are not a cure-all for poor stall characteristics. Very little has been writ- ten about vortilon design: their shape, amount of protrusion in front of the wing, where they should be placed, how many of them are desired, etc. This makes experimenting with them dif- fi cult, time-consuming, and dangerous if you don't know what you're doing. SOME FINAL THOUGHTS As with any aerodynamic changes, one must proceed with caution. There are several ways you could get in trouble slapping on your own device and going flying. The first is that the device may adversely affect some characteristic of your airplane. Another is that you may find that the device works as planned, for instance, delaying stall; but in high, hot, or humid conditions, your engine does not have a simi- lar device. Then there's the problem of the device falling off. Not only would you lose the benefits of the vortilon, VG, or fence, but you might lose it on one side and not the other. Also, a device that comes off in flight might fly back and hit something on the back of the airplane. There are other fail- ure modes, but you get the picture: Don't experiment with these at home unless you know very well what you are doing, or unless you have confidence that the devices others have designed have been carefully configured and fully tested. All these aerodynamic devices have been proved useful for some planes in certain situations. VGs are the most com- mon of the three devices, and a number of companies sell VG kits that have been flight-tested on specific aircraft types, including many experimental planes. Some of the VG kits come with a contact-paper-like template that makes installa- tion easy and accurate. You stick the template down onto the wing, and glue the VGs onto the wing through the holes in the template. This approach takes much of the guesswork out of the process of installing VGs. Fences and vortilons are some- times specified by the designer for a particular type of experimental airplane, but they have not been designed or tested for the vast majority of experimental airplanes. Just because a device is useful on one aircraft type, it doesn't mean that it will work on a different type. As we gain more experience with fences and vortilons, we may see something similar to VGs in the future. Companies may invest their time and money designing and testing these devices to make them reliable and easy for homebuilders to use. Lynne Wainfan has been a private pilot for more than 30 years. Originally an aerospace engineer, then a manager at Boeing Space, Lynne now consults and teaches at California State University, Long Beach. Readers may remember the Facetmobile experimental airplane, which was built by Barnaby and Lynne Wainfan and Rick Dean. Figure 19 Illustration courtesy of Institute of Space and Astronautical Science