October 2012

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/84816

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Page 29 of 47

Under the Cowl that's usually the cylinder heads. Continental, in fact, showed some experimental O-200 cylinders a few years ago that had no fi ns at all. They run cool enough, but it turns out that the fi ns also serve as mechanical reinforcement to keep the cylinders round. (The newest O-200-D cylin- ders have much smaller fi ns that taper ever smaller toward the case, saving about a pound per cylinder.) Better yet is to have equal pressure to all the cylinders, and one excellent solution is to build a single box that attaches to the top of the engine and encloses all the heads, supplying air more or less equally to all areas. This "plenum" design can get scientifi c, but some general principles always apply: 1. The plenum should not leak. Lost air is wasted drag. 2. The plenum should not chafe against the cowl or any other part of the airframe. 3. Baffl ing inside the plenum, if used, should concentrate airfl ow to the heads. 4. The plenum should not enclose exhaust components. Hot air won't cool your engine! 5. The plenum should have as much volume as is practically possible to allow uniform pres- sure inside the cowl and to allow maximum expansion of the outside air. T e Junkers Tri-Motor's No. 3 engine shows a typical approach to forcing air through a radial engine. 6. Inlets to the plenum should be as long as possible (it some- times helps to use a prop shaft extension), and the inlets should expand as they approach the plenum, at no more than a 15-degree included angle. Inlets need to be smooth on the inside as well as outside! If you can do all that, you're halfway there. The other half involves letting the air back out, while causing the smallest amount of drag. Obviously, the outlet should be located in a low-pressure area. Although some cowls vent out the top, most use an opening on the bottom. And many of these openings are much larger than they need to be. First, it's not suffi cient to rely on the pressure (inlet) side to do the work of pumping cooling air through the fi ns. The outlet should provide some vacuum (lower pressure). By making the outlet side (below the tray) a larger volume than above (or in the plenum), a natural vacuum can often be achieved. The opening itself can be made to help: Many fi nd that a small "lip" bent into the lower trailing edge of the cowl will help create a pressure drop more useful than the drag it creates, like a spoiler on the rear deck of a car. "Small" is the byword! A potentially more potent pressure- dropper can be made by exploiting the exhaust outlets. Many Reno racers help accelerate the air out of the cowl by "persuading" it with speedy exhaust gases. (The photos do a better job of explaining this.) In next month's article, look for a few special-case solutions (e.g., pusher/ seaplanes) and some traditional and not-so-traditional expedient measures (e.g., cowl fl aps, spray bars, and elec- tric fans). Baffl ing, remember, is simple. Like Mozart's music. Bill King's Curtiss Pusher replica uses rudimentary "eyebrow" air catchers to put more air on his Lycoming's six cylinders. 30 NO. 2/OCTOBER 2012 Tim Kern is a private pilot who lives near Indianapolis, Indiana. He has written for more than 40 different aviation magazines and also provides writing and marketing services to the aviation industry. He was key builder on two aircraft and has earned the title of Certifi ed Aviation Manager from the NBAA.

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