Why a wooden prop?[The following article first appeared in the Aero News Network email newsletter, written by Tim Kern, ANN News Editor. Personally, I prefer aluminum for easier starting and greater efficiency, though I do admit that the wood look good. -- Dan Ford]
We thought we'd start our interview with Steve Boser, Sensenich Wood Propeller's Chief Engineer, with a waist-high fast ball. He swung at it, and connected: "One big reason is economy. Wood is the least-expensive suitable material, with good strength-to-weight; it's light weight; they're pretty easy to install -- six bolts at the right torque range -- it's pretty easy." In two words: "They're simple."
There's more, too. Wood is great stuff. "Compared with other materials, wood, with its inherent damping, tends to run smoother. It's also the material you want, if you have a tip strike; it acts as a 'fuse' when you get a tip strike -- the strike makes a bunch of toothpicks; but your engine probably won't be wrecked -- it's cheaper to replace a prop than a crankshaft."
...and it's pretty. "They also look authentic on classic airplanes, and particularly show airplanes."
Wood Also Has Its Particular NeedsThere are two areas where wood is more-sensitive than other propeller materials, Steve noted: mounting-bolt torque and erosion. "You have to maintain proper mounting-bolt torque," Mr. Boser explained. "Wood will shrink and swell with changes in moisture content, just like a wooden door; so we recommend checking bolt torque every 3 months or 50 hours, to maintain proper pre-load." Remember: 'Checking the torque' does not necessarily mean, 'tightening it some more.'
There's also the abrasion resistance factor. "If you fly through rain, for instance, you'll erode the wood." Different types of flying call for different approaches to rain resistance: "On little ultralights, for example, there's little rain-flying; a urethane tape on the leading edge may suffice. In experimentals, a urethane resin is popular." What's the difference? "We carve out a section of wood, and fill it with a urethane resin. It will handle light rain, and it's very economical. After that," he said, things get serious: "you may use a metal leading edge; it was the primary protection for wood props for a long time. It's expensive, though, and it doesn't breathe -- so inspections are more-critical."
Of course, if you plan on flying in the rain a lot, or in severe showers, the Chief Engineer said, there is the old standby. "A metal prop is better in rain, of course." What about composite props? They're good for a lot of things, "...but a composite prop, without a metal leading edge or urethane on the leading edge, will erode pretty fast, too," he said. Other than for rain flying, why consider a metal prop?
"Metal props can be made thinner than a wood prop, so they can be a little more-efficient," Mr. Boser reflected; and he added, "They may give you a little more speed; they don't need to re-torqued; they're more-durable for abrasion." ...and the downside? "The downside is: they're heavy -- aluminum is typically 4X heavier than [the same volume of a typical] wood; even taking account the thinness, they're still 2-3x heavier." There are other things, too: "They're more expensive." This is due to more than just the cost of materials. Because of some of metals' inherent properties (fatigue, for instance), "There's a lot of testing involved. Metal has essentially no inherent damping, so metal props will fatigue with use." Some arrangements are worse than others: "The worst things for them are the engine's compression ratio (worse in a diesel), and the arrangement of the cylinders."
Most aircraft piston engines are opposed and flat, and many have four cylinders. In a flat four engine, the crankshaft actually stops and starts momentarily, twice each revolution, as the pistons go to the tops and bottoms of their bores. The resulting vibration is really effective at exacerbating metal fatigue. Even worse, Steve explained, "The metal prop is like a tuning fork -- at some frequencies, some locations on the blade will maintain a harmonic. That's why there's extensive testing of a metal prop before it's released."
All that testing isn't just expensive. It also means that many innocent-looking modifications can easily ruin the prop; and testing can also reveal certain operating limitations: "Sometimes you'll see placards -- rpm range, no cutting down, etc; and they're very specific to the application of the engine and airplane," Steve told us. "Therefore, for a custom application, a wood prop will be much quicker-designed and cheaper."
What about plastic?"Composite props are in the middle," we were told. "They're usually available as ground-adjustable, or even as variable-pitch. They are generally more-durable than wood, not quite as durable as metal. They're more-resistant to harmonics than metal, but they can be sensitive to it -- certain engines can cause trouble. Because they're ground-adjustable, they're pretty versatile." Steve offered a caveat for newbies or do-it-yourselfers: "If you design a blade for a particular application, the blade may be pretty good, at a significant variation in pitch. Stay in one design 'family,' though," Steve warned. A prop design that works great on a geared engine, for instance, may disintegrate in a direct-drive application.
Maintain that wood prop"The main means of getting your engine torque to the prop is friction" between the prop's mounting face and the prop hub face, Steve reminded us. "If you over-torque the prop bolts, you'll crush the wood; then it may shrink or swell faster. If you under-torque, the prop will literally jiggle back and forth; you'll get broken bolts, a broken hub; or even a scorched hub, or possibly a fire. Once it starts to heat up, it will shrink the wood that much faster, making even a looser fit. There have been cases when that has happened, and the pilot will see the smoke." Sometimes the evidence isn't apparent until the prop is removed for some reason, and there's a coating of charcoal on the mounting face. Good-bye, prop.
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