DEC 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: https://experimenter.epubxp.com/i/434207

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Page 26 of 44

EAA Experimenter 27 Internet and in paint shops. To this, he adds motor oil which is viscous—a handy trait that helps it not fall of the airplane. Then Klaus reduces this mixture with diesel fuel or kerosene, but this combination has too much surface tension. To counteract that problem, he adds a lot of dish soap…One begins to understand ex- actly how obsessive engineers are about visualizing airfl ow. (I have found that dish soap makes carbon-black cleanup a breeze.) Klaus sometimes uses tufts as well. Tufts are small pieces of string that are attached by tape to a surface. In fl ight, the tufts line up with the air that fl ows past them. Tufts that stay lined up with the direction of fl ight represent an aerodynamicist's dream. If they aim elsewhere or bounce around, they show that there is opportu- nity for airfl ow improvement. Figure 4 is a picture of Klaus' tufts (while the plane is on the ground) that he used to fi ne-tune the canard tip shape. There are several reasons that oil fl ow visualization is more useful than tufts. First, the tufts sometimes fl y up above the boundary layer into free-stream air. Second, the tufts can trip the fl ow from laminar to turbulent, which could af ect the air- plane's fl ying qualities and mess up your measurements. Third, tufts only show the air motion when the aircraft is in fl ight, unlike the oil which stays in place after landing. Finally, tufts are only helpful if you can actually see them during fl ight. In- stalling cameras to see the tuft movement in fl ight is often not feasible. Figure 5 shows the same shaped wing on the ground after a fl ight with the oil visualization method. The other method that Klaus uses for fl ow visualization is impingement. Things in the air can strike the fl ight surfaces and leave their mark. Most aerodynamicists know about the bug method—if bugs hit the airplane, most of them will slide right of . The ones that hit the plane perpendicularly—at the stagnation point—tend to stick. This bug splat method of fl ow visualization approach is not limited to wings. Figure 6 shows a duct where the air is supposed to fl ow smoothly from left to right. The bug splats tell the story: The air is hitting the duct perpendicularly and aerodynamic improvements can be made. An unexpected impingement opportunity came when Klaus and his co-pilot Jenny Tackabury fl ew through smoke from a California wildfi re. The particles in the smoke rubbed on the freshly waxed airplane. Where there was laminar fl ow on the forward part of the fl ight surfaces, the particles slid of cleanly. But the turbulent boundary layer embedded dust particles into the wax. This left the laminar region polished and made the turbulent area dull. As you may remember from my article on aerodynamic devices last month, this transition from laminar to turbulent fl ow is important to locate. So Klaus' fl ow visualization methods let him accomplish step one of the process: Understand the situation. From here, Klaus identifi ed several ways that the air was not fl owing ef ciently. The following shows some of his improvements which have been designed and tested. Klaus worked on the cowling fi rst, lowering its profi le in order to reduce drag ("it pushes less air out of the way"), and to blank less of the pusher prop. Figure 6a shows Klaus' sleek cowling. Figure 4: Tufting testing on the canard tip. Figure 5: Using oil visualization to determine airfl ow. Figure 6: The bugs inside the cooling duct show that the airflow into the duct isn't smooth. Figure 6a: The Determinator's sleek cowling. Photography courtesy of Light Speed Engineering

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