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/434207
26 Vol.3 No.12 / December 2014 KL AUS SAVIER'S DETERMINATOR from the original cylinder's exhaust. This means that the second cylinder's exhaust gas doesn't completely leave the cylinder. A tuned exhaust, on the other hand, reduces the exhaust pressure right before the port closes, using resonances like those occurring in the intake system. This lets spent gas out of the cylinder and fresh mixture into it, improving engine ef ciency. "I talked to all sorts of people who were knowledgeable about exhaust systems," Klaus said. "One let me use his shop. I spent six weekends and $2,000 worth of material. After all these modifi ca- tions, the best I could do was 10 pounds more weight and 6 knots less speed. This was one of my bigger mistakes." Klaus urges cau- tion when talking to experts, as it's easy to get bad advice. Because of that experience and because there is so little room for a longer exhaust on a pusher airplane, the Long-EZ's exhaust is still on his list of future improvements. Next, Klaus decided on a timed-sequential, fuel-injection system. Traditional injectors fi re all or two injectors as a group, regardless of whether the cylinder is ready or not. A timed-se- quential system opens the injector during a specifi ed period in the intake cycle. Because of cowling constraints, Klaus had to buy a smaller injector so he could put it in a better location. A dif erent kind of problem came when Klaus put a header tank behind the cockpit to replace the traditional external sump blisters. He installed a transfer pump to fi ll the header from the left main tank. A standpipe prevents accidental overfi lling of the header tank. In fl ight, when the air hit the common vent, the dynamic pressure actually pushed fuel into the header tank, keeping it full. The solution? Reduce the area of the vent opening. Klaus made a new vent by wrapping carbon around the cap of a felt marker. Figure 3 shows Klaus' header tank—with visual fuel gauge—and carbon vent tube. What about the propeller—was there anything to be gained from modifi cations there? Klaus, through his company Light Speed Engineering, has designed, built, and tested more than 50 propellers for dif erent aircraft. He knows how to optimize their shape. "It's really hard to get speed out of a propeller," he said. "It's tough to increase the propeller ef ciency of a good propeller. All you can do is increase the maximum rpm where the engine makes more hp." Propeller modifi cations let you set your optimal performance at lower rpm, saving wear and tear on the engine and reducing fuel costs. But again, he has made discoveries that go against the conventional wisdom. For the Determinator, he uses a carbon, fi xed-pitch propeller that he designed and hand carved to an amazing 100-plus inches of pitch. It turns at 2,600 rpm. Making real propeller improvements aerodynamically is also structurally challenging. Klaus cautions that metal props don't work on pusher airplanes. "The blades get excited by the wake of the wing and cowling, and the aluminum, with its characteristi- cally poor fatigue life, will fail sooner or later," he warned. "All of the wood and most of the composite props are fairly thick in order to have adequate structure. Aerodynamic improvements come mainly from using a much thinner airfoil. But these thin blades are dif cult to shape and require a vacuum-bagged laminate of very high strength. Thin airfoils are also very sensitive to angle of at- tack. This means that if the pitch distribution does not correspond to the infl ow angles, the blade will stall." Klaus said that a racing propeller is also on his list of future changes. Klaus has an unusual take on prop mounting. "In the 1930s, we stopped using wood propellers due to the increase in horse- power and the better performance of metal props," he said. "We went from eight [mounting] bolts used for wood props to six bolts, which are plenty for a metal hub." Wood is less stif than metal, so the bolts see more bending load. On the Long-EZ project, he actually broke bolts at two dif erent occasions before having a set of custom bolts made. "Very expensive," he said. For his 250-hp engine, he hopes that the industry standard for wood props lowers the bolt ending load, returning to eight bolts instead of six, and a crush plate that is splined to the shaft. AERODYNAMIC IMPROVEMENTS Klaus has made dozens of changes to the way air fl ows over his airplane. Recall that the fi rst of our four-step process is to under- stand the situation. How does Klaus understand the air? Readers of last month's Experimenter will recall from my ar- ticle that we aeronautical engineers are obsessive about visualizing airfl ow. We look for it in smoke trails, cof ee cream, Saturn's swirls, and movie stars' cigarette smoke. Klaus is widely recognized as the world's fl ow visualization guru. He uses whatever method he can fi nd, but the main techniques are oil fl ow and impingement. In the oil fl ow method, special dark-colored oil is put on the airplane before fl ight. After landing, the oil tracks show where the air was—or was not. Klaus has a fi nely tuned mixture for his oil: He starts with carbon black, the fi ne black powder that is added to paint base to make it black. Carbon black is available on the Figure 3: The header tank with a visual gauge. Photography by Lynne Wainfan