Experimenter

JAN 2013

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/101874

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close to the edge, and who has different prop choices available to compensate for, say, temperature and humidity? Yeah, some racers do. Are you a racer?) Ground clearance, particularly in a tricycle-gear airplane, can also be a factor. The minimum clearance from the hub center to the ground must be measured with the nose suspension at full compression and front tire flat—plus don't forget a little more clearance to clear a bump, hangar door track, or fuel filler! If you live at a high-altitude airport with a short runway and you carry a lot of luggage, "flat" pitch (a smaller pitch dimension allows more revolutions for less forward motion, effectively simulating a "low" gear ratio in your car) is the only way to go. If all you do is run laps at the Reno Air Racers, where you have a mile-long runway and top speed is paramount, you'll probably run the highest pitch your engine will pull. Even in the certificated world with its FAA-limited choices, many aircraft owners have "cruise," "standard," "climb," and sometimes even "seaplane" props to consider. (Note: the pilot's operating handbook usually lists performance for only the standard prop, so when you buy any used airplane, even if it's certificated, it's important to know which prop you're getting!) A Note on Using Two-Stroke Props on Four-Stroke Engines Even among props of nominally identical pitch and diameter, the blade design matters. Consider that a two-stroke engine makes its power in a relatively narrow, high-rpm band. In order to make good power, it needs to rev up. A four-stroke aircraft engine has a wide torque curve; it produces useful power over a wider range of rpm. Prop designers know this and design two-stroke propellers to be efficient only in their top rpm range. (If they made props that pulled hard at lower revs, the engine would never be able to climb up into its sweet spot!) A typical mistake is to put such a two-stroke design on a four-stroke engine. Although (other factors optimized) it will perform okay at Care and Feeding of Propellers All Props • If you use self-locking nuts to mount your propeller, use them once only. • Do not use any lube on the bolt threads, unless the prop manufacturer specifically calls for it; lubes can drastically change the tension in the bolt, at any given torque setting! • Check your prop mounting bolts for any kind of wear, damage, or corrosion; at least during every annual inspection. (Check your hub while you're there!) • Always follow the prop manufacturer's instructions and specifications. All manufacturers are happy to offer advice on the care and maintenance of their products. • Use only approved finishes, paints, finishing procedures, etc. Wood Props Be sure your prop plate is of sufficient thickness that it does not distort, and that it is of sufficient diameter to cover the prop's hub. A general rule of thumb for thickness is a minimum of 1/4 inch (steel) or 3/8 inch (aluminum). • Check the torque on the propeller bolts regularly. But "check" does not mean "tighten them some more." • Protect the propeller from the elements. Keep the airplane indoors, or at least use a good propeller cover whenever you're not flying. • Even with leading-edge protection, avoid flying in rain. Consider von Richthofen's account: "The rain fell in streams. Sometimes it hailed. Afterwards, the propeller had the most extraordinary aspect. The blades looked like saws." Metal Props Never reuse the prop bolt washers. These have a specific amount of "squish" designed into them, and the value will change after their first use. Any little nick needs to be (ad)dressed. A buildup of weld and a filing down to duplicate the original shape are not endorsed by any prop manufacturer. This is done only for "museum restoration" projects, where the prop will never be flown again. (No matter how good it looks, don't use a prop you don't know!) If the propeller is painted, any exposed metal should be touched up with paint to prevent corrosion. Adjustable-Pitch Props Getting the pitch the same on every blade is critical. Unless the prop has a self-indexing design (e.g., Sensenich), be ultracareful when setting it up. Then after you've torqued and checked everything, check it one more time. Composites can deteriorate in UV light. Keep your blades covered when you're not flying. EAA Experimenter 39

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