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 21 It is helpful to stop and summarize the difference be- tween two related concepts: flow transition happens when the airflow changes from laminar to turbulent. This is dis- tinct from the concept of flow separation, where the air can no longer follow the shape of the wing. Separation is bad: Drag increases, lift reduces, wings stall, and control surfaces behind the wake can become less effective. Turbulent flow— unless the flight attendant has just handed you your coffee— is not necessarily bad, as we have seen with the golf ball and will see with planes. Okay, now that you've had your introductory aerodynam- ics lesson, let's get back to aerodynamic devices. VORTEX GENERATORS…WHAT ARE THEY? Vortex generators are typically small plates that stick up per- pendicular to the wing surface. Their height is usually about 80 percent of the boundary layer thickness. Often they are placed right in front of the transition zone shown in Figure 7, where the air goes from laminar to turbulent. The figure below shows an example of vortex generators. Vortex generators (VGs) can be found in other places beyond the wing transition zone. For instance, they can be placed farther aft on the wing, just before the control surface's hinge. VGs also can be placed on surfaces other than wings. They can be seen on the horizontal stabilizer to improve control effectiveness of the airplane's elevator. They can be seen on engine nacelles, turbine blades, and even cars. VGs are sometimes placed on airplane fins to improve flow over rudders, as shown in figure 8. Not all VGs are small plates—other shapes can generate vortices as well. Figure 9 shows a weight-shift trike with round VGs: small cylinders with a conical top. As you may know, cylinders are extremely draggy. How draggy are cylinders? Figure 10 shows two shapes—an airfoil and a cylindrical wire—with the same drag. The first shape is a big, old airfoil, and the bottom is a tiny round object. If something the size of a wire has the same drag as that airfoil, then surely these round VGs would be very draggy—what was the designer thinking? All designers face trade-offs. In this case, the first trade- off that led to the round VGs was effectiveness versus user- friendliness. Yes, the round VGs have more drag than con- ventional VGs, but these are mounted on a fabric airplane. Conventional VGs would slice through the fabric when the wing folded; these are made of rubber and they just bend out Figure 7 – Vortex generators are often placed right in front of the transition zone where the air goes from laminar to turbulent. Figure 8 – Vortex generators are also used on other surfaces, such as the elevator. Figure 9 – Vortex generators on this trike wing have a conical top. Photography courtesy of Lynne Wainfan