The Only War We've Got

The case for a wooden prop

[The following article first appeared in the Aero News Network email newsletter, written by the ANN News Editor. Personally, I prefer aluminum for easier starting when I had to prop the Cub myself, and for its greater efficiency, though I do admit that the wood looks good. -- Daniel Ford]

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."

Wood Also Has Its Particular Needs

There 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.

Keeping the bolts at the right torque is #1

Then, he advised, "The second thing to do is to keep the finish in good shape. We use either spar varnish or a polyurethane. There is nothing that will totally seal a wood prop; the coatings slow that down, a lot." Take care of your wood: "Anything you can do to keep the finish in good shape will prolong your prop's life. Re-wax every six months or as needed; if the finish is worn or missing in a small spot, lightly sand the area, and you can put new spar varnish on it. If you have large areas missing finish, you'll need to refinish the whole prop, and recheck balance." Store in a cool, dark, dry place.

"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."

The High Country

A diamond is forever. So's a wood prop?

"A wood prop has no TBO -- it's based on condition," Steve explained. "'It's literally as good as it looks,' provided you know what you're looking for. Look for cracks, finish problems. A metal prop with 99% of its fatigue life used up may look fine, but it's not; with wood, the fatigue limit and ultimate strength are quite close. Wood props are essentially free from fatigue [damage]." How to Pick a Prop:

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.

Is 'more,' really 'more?'

"Multi-blade props tend to run smoother; they can give better climb performance." Steve went back to a basic engineering premise: once again, you don't get something for nothing. "Added blades add weight, cost, and complexity. With wood, it's just so much easier to make a four-blade, that we don't make a three-. A ground-adjustable hub -- that makes it easy. We make 2,3,4, and 5-blade hubs for our composite airboat props. Airboats -- they just can't get a big-enough prop -- the prop would be too big for the road. We build big, 'paddle' blades, or we run 3-blade or 4-blade; sometimes, you'll see 2- or 5-blade configurations on airboats."

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.]

Look it up

"Sensenich has a huge library of props that they've built," Steve said. "We've got a really broad experience, in history -- 70 years plus. There's a very good chance that we'll have a prop off the shelf -- from 80 hp through about 225, in light aircraft. Lower horsepower -- most of that work is for unmanned aircraft; UAVs tend to be pretty slow -- a lot of power, in a small prop -- that's because of vehicle constraints, like landing gear length considerations. Sometimes, they're pretty fast -- we did a target drone, 140 hp and 300+mph -- but that's unusual. When you get above 225 hp, 220mph in a manned aircraft, you'll do well to look at constant-speed props. In the past five or so years, we've really done a lot of work in the 80-120 hp range -- 912S, Jabiru, VW 2180s." What about constant-speed props? "Constant-speed? you'll like the versatility, and the smoothness of the multiple-blade designs that populate that market." Gears...

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."

What's the worst vehicle for a prop?

Florida-based Sensenich knows all too well: "Airboats are the toughest applications -- we get to test our props 'on the ground' first -- materials, processes, and so on." It's not just that the prop diameters have to be held relatively small, and that they often deliver big horsepower; it's all the stuff that 'happens' to them: "Tools, coolers, magnetos, beer cans, wildlife, foliage -- it all goes through those props. It's wild to see some things sticking out of props -- they'll bring props in on Monday mornings, asking if we can remove something strange from them. It's a harsh environment -- and probably the lowest maintenance. Rescue, law enforcement -- there's over 20,000 licensed airboats in Florida alone. Car engines have taken a lot of business from the older, airplane engine conversions; but now they're all multi-blade, due to speed reduction of the PSRUs. A car engine will run longer blades, generally -- they pretty much need reduction drives." (Engineers' Checklist)

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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