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
EAA Experimenter 25 ing the stall. At some angle of attack, the horizontal tail stalls, and the pilot cannot push the nose down to recover from the stall. A fence can reduce this spanwise fl ow, redirecting the air so it moves toward the trailing edge instead of toward the tip. Another way the fence works is somewhat like a VG. When the spanwise fl ow tries to move toward the wingtip, some of it spills over the fence and creates a vortex. This energizes the boundary layer outboard of the fence. Both of these modes—the redirection of the air to the trailing edge and the vortex—can delay separation/ stall and improve control surface ef ectiveness. There are other ways a fence can work. Some are used on the back of the wing between the fl aps and ailerons to improve aileron ef ectiveness. This placement is shown in Figure 14. Al- ternatively, Airbus advertisements say that their wingtip fences reduce wingtip vortices. Wingtip vortices energize the fl ow behind the wing, where they don't do any good. Airbus fences don't produce useful vortices; they reduce drag. The detailed design approach for fences is not well docu- mented—this is still an art that some companies do not share because they see it as a competitive advantage. However, there are some design principles that are generally true: Fences are taller than VGs because they must stick out above the boundary layer. It is believed that increasing the fence length to beyond 50 percent of the chord is not useful. Similarly, fences rarely extend past the leading or trailing edge of the wing more than one-third of the chord length. WHY ARE FENCES GOOD? Like VGs, fences can delay fl ow separation and lower stall speed; they can improve control surface ef ectiveness at high angles of attack; and they can reduce the airplane's tendency to tip stall, improving stall recovery performance. Fences may re- duce drag by minimizing wingtip vortices. An additional benefi t is that if fences are placed correctly, they can shield the pilot from the glare of the wingtip lights! THE DRAWBACKS OF FENCES Like VGs, fences are devices that scrub against the moving air and therefore they create skin-friction drag. On the plus side, fences are lined up with the airfl ow whereas VGs are slanted into the wind. On the minus side, fences stick up out of the boundary layer where the air is moving fast, producing more skin-friction drag than a VG. Fence shape seems somewhat straightforward, but where to place the fences and how long they should be are still somewhat proprietary information. Like all aerodynamic devices, they must be carefully designed, placed, and tested. Fences should not be installed on ailerons as this can excessively load the aileron and drastically reduce the aile- ron's effectiveness. Because fences are designed to control spanwise fl ow, they are used almost exclusively on swept-wing aircraft. There is some spanwise fl ow on straight wings. But it is so small that a fence's ef ectiveness would be small, and it probably wouldn't be worth the drawbacks. However, fences are placed on a straight-winged aircraft between the fl aps and ailerons could improve aileron performance when the fl aps are deployed. These considerations lead us to the last of our aerody- namic devices. VORTILONS: WHAT ARE THEY? Like wing fences, vortilons are nonmoving plates attached perpendicular to a wing, lined up with the free-stream airfl ow. Vortilons are smaller than fences and extend from the bottom of the wing to beyond the wing's leading edge. Figure 18 shows two examples of vortilons. There is evidence that vortilons were invented for the DC-9 airliner. When the DC-9 designers moved the engines (and pylons) from the DC-8's under-wing placement, they saw less lift in the landing confi guration. Douglas engineers didn't know why this happened, but when they put the old engine pylons from the DC-8 onto the DC-9, the lift returned. Rumor has it that the engineers gradually trimmed away the pylons until the lift degraded again, and then they added some back. What was left of the pylon made up the fi rst vortilon. The word "vortilon" comes from the fact that they are vortex-generating pylons. Figure 18 – Two types of vortilons.