Why a wooden prop? (part 2)
Keeping the bolts at the right torque is #1Then, 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."
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.