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
20 Vol.3 No.11 / November 2014 VORTICES, VGS, AND FENCES…OH, MY! fl ow around the wing. Farther away, they are less af ected by the airfoil. On the left side of the illustration, we can see the streamline that comes up from below to hit the wing perpen- dicularly. If a bug were to be fl ying in that streamline, it would hit the wing directly and not slide of . For a wing at zero angle of attack, this "stagnation point" is typically right on the lead- ing edge. As the angle of attack increases, the stagnation point moves to a point below the wing. In Figure 4, this is the point below where the green fl ow forms. Also note that the air on the top of the wing near the leading edge is still following the shape of the wing. Do you see how it starts out laminar (the green fl ow) and then transitions to turbulent (the blue fl ow)? At this point, the fl ow is not separated—you can see from the stream- lines that it pretty much follows the shape of the wing. This is an important point that we'll return to later. But then about halfway to the trailing edge, the fl ow moves up from the wing (the big orange blob). Continuing aft, you can see that the air behind this has trouble following the shape of the top of the wing. It follows the swirling air instead. Because the air is no longer following the shape of the wing, the fl ow is considered separated. This separation produces a much broader wake than would be there if the air were still attached. This large wake after separation increases drag. Separated fl ow produces dra- matically lower lift. It also can af ect the airfl ow behind it—if an aileron were in separated fl ow, you can imagine that it wouldn't be as ef ective as it would be if the fl ow were attached. Some- times fl ow separation can af ect fl ight surfaces way behind it as well—a horizontal tail or elevator fl ying in that wake might lose ef ectiveness too. Another important observation from the fi gure above pertains to the boundary layer. Of course, we would expect the laminar portion of the boundary layer to be attached to the wing. You can see this in the green area near the leading edge. The streamline above it is roughly parallel to the wing. But here's the surprise—the turbulent boundary layer in Figure 4 (the blue zone right behind the green zone) also has attached fl ow. You can squint and see that the boundary layer is thicker when the fl ow transitions to turbulent, but it still follows the shape of the wing. This is somewhat counterintuitive, turbulent air following the shape of the wing. Even more amazing is that turbulent fl ow can stay attached longer than laminar fl ow—far- ther down the wing—or a golf ball. Figure 5 shows the wind tunnel results of a sphere in regular fl ow (left) versus a sphere whose boundary layer has been tripped to turbulent by a small wire attached at about 25 percent "chord." See how the fl ow on the left picture starts out laminar (where the boundary layer is so thin that it is barely visible) and then transitions to turbulent at around 40 percent chord. This produces a big wake, and therefore, high drag. The fi gure on the right shows how the air starts out laminar, and then transitions to turbulent early, at the wire—if you look closely, you can see the very small swirls in the turbulent boundary layer at around 60 percent chord. But notice that the more turbulent fl ow confi guration's wake is much smaller than the ball on the left. Tripping the boundary layer to turbulent earlier—more forward on the shape—actually lowered the drag. This is the main reason that golf balls have dimples; they trip the boundary layer to turbulent earlier than it would transition normally, like the wire in the fi gure. This produces a smaller wake and reduces the golf ball's drag. But let's return to our colorful wing. As the angle of attack increases, the separation gets worse until the fl ow fully sepa- rates and the wake becomes quite large. This wing is stalled. Figure 6 shows this condition. Figure 4 – As angle of attack increases, air has a tougher time of adhering to the wing. Figure 5 —The wind tunnel results of a sphere in regular fl ow (left) versus a sphere whose boundary layer has been tripped to turbulent by a small wire attached at about 25 percent "chord." Figure 6 — As the angle of attack increases, the separation gets worse until the fl ow fully separates and the wake becomes quite large. This wing is stalled. Illustration courtesy of Institute of Space and Astronautical Science