by Tim Kern (Aero News Network)
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."
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."
"UV radiation and sunlight will accelerate the deterioration of the finish," Boser said. It should be obvious, but he warned, "Leave it out all year long, and you will kill the finish -- it will fade and flake, and lose that moisture-protection. Use even an inexpensive prop cover -- something that will shield the prop from the sun, but not hold moisture."
Steve advises hundreds of homebuilders, airframe and engine manufacturers, and restorers every year, and he can walk you through the process of finding the best way to turn horsepower into thrust: "Start with engine's power rating, the speed range of the airplane, the blade design. More twist: more speed. Diameter -- that's a big consideration. You want to run the largest prop, for as long as you can." There are limits, though. Even if you have all the ground clearance in the world, you might not have enough horsepower: "Tips can't go supersonic, and the blade can't be too thin." If you have a lot of flexibility, "Look for a Mach tip number of about 837 fps. Target 850 as a top number, except for special applications. Metal props, not as-effected by erosion, can go to about 900."
Huh? Steve made it simpler: "RPM x diameter in inches / 256,000 = Mach number at tip, due to rotation." If the diameter in inches times maximum rpm is under 256,000, you're theoretically cool, in other words. 'Safety' and 'fudge factors' bring the number down to around 220,000 or so, for mere mortals.
He continued, "Merely adding blades isn't the answer. You'll get some improvement in low-speed thrust; but it will hurt fuel consumption in cruise. It depends on the blade -- are you already max'd out? Will it add to the potential of the package? You run into diminishing returns. If you scale the blade appropriately, you won't lose that much efficiency going from two to three blades; but it's better, in most of our size applications, to just design a good 2-blade prop." [That's a lot of props. Sensenich Wood Propellers uses 600~700 basic prop designs, from 5 through 800 hp -- everything from a wind machine, down to a target drone --ed.]
Steve doesn't mind the trend toward geared engines. "Reduction drives help vibration issues quite a bit. More cylinders help; low compression ratios help." With direct-drive engines, "Stiffer crankshafts -- I've been told that the Lycoming 360, for instance -- the heavier-crank model (290hp) is better for vibrations. That's a specific application, though -- I don't know for sure if it applies to aircraft use."
Just for fun, here are a few things to think about, when you
find that you're a propeller engineer:
1) diameter -- the longer the reach, the more air it can grab (but
don't go supersonic)
2) chord -- when you have to have a short prop, the wider, the
better (within reason)
3) number of blades -- more blades will do more; but there are
greatly-diminishing returns as you add blades
4) pitch -- limited by stall (hey -- it's just like a wing!)
5) airfoil (camber) -- there are some typical airfoils; but
there's a limit to AOA
6) dropping tip speed below about 700 fps will really force
compromise -- the prop will work in a much-narrower range. A
constant-speed prop will help in climb; at the expense of speed --
its versatility is useful
Steve reminded us, it doesn't get simpler as you add engines: "With twins, you gear everything to (single-engine) climb performance; with a single, you look to cruise."
If your baby already has a wood prop, have another look -- check the torque, repair little finish imperfections, give it some wax.
If you're undecided about which prop to put on the love of your life, read this article again; then call Steve.
Question? Comment? Newsletter? Send me an email. Blue skies! -- Dan Ford
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