Experimenter

NOV 2014

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

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EAA Experimenter 19 WHAT IS A BOUNDARY L AYER? If you visualize yourself fl ying and looking out at your right wing, you can imagine that the air touching the wing moves slower—relative to the airplane—than the air 10 feet above the wing. Why is this? Air is a little viscous (sticky) and is slowed down by being dragged along the wing. At the wing, the air molecules that stick to the wing are at zero velocity (relative to the airplane). If you move up from the wing, the air molecules right above the ones stuck to the wing stick to the air molecules below; they are not moving at zero velocity, but they are moving slowly. As we move farther upwards, the air moves faster and faster until it reaches "free-stream" velocity. The boundary layer is the thin layer of air between a surface and the air that is going 9 percent of the free-stream veloc- ity. Boundary layers are not very thick—generally less than a half inch during cruise. Boundary layers exist on anything that touches the air—fuselages, fi ns, stabilizers, etc. From here on, we will only consider boundary layers on the wing. There are two types of boundary layers. Laminar, or layered, boundary layers occur when the air motion can be thought of as following the shape of the wing. The reason it is called "layered" is because as you go farther and farther up from the wing, the airspeed changes at a uniform rate. The other type is a turbulent boundary layer. Here the air is unsteady—changing with time—and swirling. Laminar fl ow has less skin-friction drag than turbulent fl ow, but laminar fl ow is very dif cult to maintain as the air moves across a surface. Any imperfections, such as fuel caps, lights, antennas, rivets, seams, ripples, hinges, and even bugs can trip the fl ow from laminar to turbulent. On a conventional wing, the boundary layer starts out as laminar at the leading edge and then transitions to turbulent as the friction of the air against the wing slows the air. This transi- tion occurs over a segment of air called the "transition zone." Figure 2 below illustrates the laminar fl ow, the transition zone, and the turbulent fl ow as the air moves from the leading edge toward the trailing edge of the wing. Where does this transition point occur on a wing? It de- pends on a number of factors: airfoil shape, angle of attack, sur- face ripples, surface roughness or cleanliness, and other factors that were described by my aerodynamics professor using Greek letters and graphs. Aerospace engineers love to see fl ow transitions. We search almost obsessively, in rivers, faucets, traf c, and even pictures of Jupiter. We are on duty even while watching movies. In Figure 3, Humphrey Bogart holds his signature cigarette. Near his cigarette, the smoke shows that the fl ow is laminar—it is in a neat column. As it rises, however, the fl ow transitions to tur- bulent. This leads to an important warning: Excessive interest in aerodynamics can cause you to watch for fl ow visualization opportunities rather than sexy movie stars. WHAT HAPPENS TO THE WING NEAR STALL? Let's imagine that your airplane is fl ying straight and level. As you raise the nose, you increase the angle of attack—the angle between the oncoming air and a reference line on the wing. For small angles of attack, the fl ow follows the shape of the wing. As we increase the angle of attack, the air has a tougher time following the entire top of the wing—it separates. Figure 4 shows a CFD drawing of this ef ect. There are a few things to note in the drawing in Figure 4. The light gray lines represent streamlines—the path that the air takes as it moves past the airfoil. At the wing, the streamlines Figure 2 – Laminar fl ow. Figure 3 – Humphrey Bogart's cigarette exhibits laminar fl ow. Photography courtesy of Recreational Flying.com and LAIRgacy.com

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