A half decade ago when diesel engines gained some traction as aircraft powerplants, the skeptics got quite a little whisper campaign going by noting that diesel engines could swing only wooden props. Their torque pulses and resonances were just too harsh for the fatigue limits of metal props. The naysayers forgot to mention—or perhaps didn’t know—that wood props cost half what metal ones do, last just as long and, on most engines, run smoother than metal and composite props do.
More than a century after the Wrights carved their paddles out of select spruce and while composites continue to make strides, wood as a prop material is, if not enjoying a resurgence, at least holding its own in the propeller market. Surprisingly, if it weren’t for carefully crafted birch props, more than a few edge-of-tech UAVs would be beached. In fact, the rise of robotic flight has pushed propeller design forward across the board, technology that’s trickled down into the civil market from drones to airline turboprops.
For many Experimental aircraft, prop material choice spans the spectrum: wood, metal or composite. But for some airplanes, wood is the only practical choice, for economic and, in the case of our recently re-engined J-3C Cub, aesthetic reasons. No self-respecting Cub driver would be caught dead explaining away a composite prop on a classic yellow bear.
When our Cub recently tanked its A-65 Continental, we upgraded to an A-75 and that required a replacement prop in order to generate the required rpm. That seemed like the perfect opportunity to visit nearby Sensenich Propeller for a look at how wooden props are made. As is so often the case with legacy aviation technology, wooden prop manufacture is an amalgam of traditional woodworking technique and state-of-the-art digital production methods.
The Prop Market
Sensenich holds onto a piece of the propeller market for Experimentals, although it’s unclear how big that piece is. Sensenich general manager Don Rowell told us he’d like it to be bigger, but the company finds plenty of competition against other wooden prop makers plying the Experimental market, not to mention composite manufacturers, who enjoy favorable economics since they’re free to develop props without the constraints and expense of FAA certification.
Nonetheless, Sensenich is happy to prop any engine up to about 225 horsepower and actually builds props for legacy radial engines larger than that.
What’s the attraction of wood? What it always was. Wood is relatively cheap, its properties are well understood and although labor costs are higher than for metal or composite, as a material, it’s predictable and readily workable.
Rowell describes the economics as three tiered. “Metal props are basically high materials costs and lower labor costs. You have a forging and that’s a significant investment in forging dies,” he explains. “For a wood prop, the materials costs are relatively low. But there’s a very high labor cost. You do have to start from bare wood, CNC it to a profile and hand carve it,” he says.
Composites are exactly between the two; lower labor costs than wood, but higher than metal. For composites, materials cost is not a key driver.
There’s a bottom line for everything and wood comes out ahead here. For certified aircraft with low output engines, wood is a no brainer. It costs less than half as much as an equivalent metal prop. For Experimentals, the price delta may be a little less because E/AB props don’t carry the cert overhead load. Composite props—many of which are actually wood cores clad in composite—dwell between wood and metal in price.
Wood excels in two other characteristics. “A wood prop naturally damps harmonics. It eats them up and dissipates them. A metal prop is like a tuning fork, it takes a harmonic and really enhances it and you can feel that back in the airframe. A composite prop, again, is in the middle. It doesn’t damp as well as wood, but it doesn’t enhance harmonics like a metal prop does,” Rowell explains.
Sensenich conducts vibration analysis on all its props, even the Experimental-only models which, technically, don’t require it. Rowell says in his 35 years at Sensenich, he’s only seen two instances in which a wooden prop developed sympathetic vibration that required redesign.
“You really don’t see resonances in wood. It’s very, very rare. Occasionally you can see flutter, but generally you don’t even see that,” he adds. Pilots who have flown the same engine/airframe combination with both wood and metal are quite likely to notice the difference. Metal props, no matter how well designed and balanced, tend to have harsh spots at certain rpms. “There’s a fallacy out there that you have to get rid of the harmonics, but you can’t. They’re there in every propeller. If the frequency is higher than we would like, we’ll try to move that frequency where you just go through it; it’s not somewhere where you operate,” Rowell says. But wood simply soaks up the vibes.
It soaks up something else, too: prop strike energy. “You’re going to have occasional prop strikes and a wood prop acts like a fuse. It breaks very easily so you won’t have sudden engine stoppage and damage to internal components the way you will with metal,” Rowell explains. Composite props don’t do quite as well because the composite cladding adds stiffness. But that’s a tradeoff, too. A stiffer composite prop can accommodate a couple of inches of pitch over a comparable wooden prop, thanks to the additional stiffness.
Building in Wood
Sensenich builds between 3000 and 4500 props a year, an output made more impressive by the fact that the factory employs only 30 people in a relatively compact 12,500 square feet not too far from the Plant City Airport in Florida. If it weren’t for all those props stacked around in various stages of completion, Sensenich would look like a moderate-sized cabinet shop, albeit one with a few digital upgrades.
What’s most noticeable is the large number of small, unrecognizable props—production carts full of them. Decorative props for the pedal airplane market, perhaps? Nope, UAV props by the dozen. As with every other airplane-related manufacturing entity, Sensenich has been touched by the burgeoning UAV market, which, at least for small in-theatre tactical drones, tends to go through props like candy through a kid. Sensenich has also developed processes to build props using internal pressure molding technology and carbon fiber. Very trick. This method finds application in the Light Sport and Experiment segments, but probably won’t migrate to certified aircraft. The certification costs are just too high for the return, according to Sensenich.
Sensenich does use this method for airboat props, which constitute a large portion of its business and the reason that part of the factory was moved from Pennsylvania to Florida in 1990.
Composite work aside, building a wooden prop is basic laminate woodworking, shaping and carving. As would be expected, Sensenich has a vast library of prop designs, which exist both as drawings and tables of specs kept in binders on the shop floor. The specs describe overall dimensions for each prop, blade width and, critically, chordwise blade angle at a series of defined stations along the blade span.
Each prop begins with a glue-up of three to as many as 12 plies for the largest props, ranging in thickness up to about a half inch, but usually less. Sensenich uses yellow birch—betula alleghaniensis—for both its workability, but also its exceptional shear strength; few commercial timbers beat birch’s shear numbers. Hard maple has also been used, and around the shop, you’ll see the occasional walnut decorative prop winding through the production process. For the laminating process, Sensenich uses old school resorcinol, the original urea formaldehyde glue developed for the lamination work in the de Havilland Mosquito during WW-II. During my days as a commercial cabinetmaker, I had used resorcinol for some outdoor millwork, but I was surprised to see it was still in use. But Rowell says for making props, the glue has unbeatable characteristics, even though it’s getting harder to obtain. When mixed, resorcinol is the color of dried blood and in the visible glue lines of a prop, it appears as dark red, almost black. It’s considered waterproof and doesn’t degrade much with age.
Machines First, Hands Last
Every wooden prop begins with the raw lumber, with the prop’s rough shape marked out on birch boards with templates that have been in the company for years. The marked-out planks are roughly sawn to shape, then glued up into rough-laminated propeller blanks. Sensenich has custom presses to compress the lams while the resorcinol cures.
Back in the day, the cured blank was largely hand carved with various woodworking edge tools, but Sensenich soon developed a hand-operated duplicating router that relied on a template to get the rough shape right. Rowell compares it to a large-scale key cutter.
But all of that has been replaced with CNC technology that first automatically machines the hub and mounting bolts to serve as a datum for future operations and then by a CNC router that zips the blank to within 0.050 inch of its final profile within a matter of 30 or 40 minutes. The blanks are clamped into the router three at a time and once the button is pushed, the technician goes on to other work while the carving carries on robotically.
Although the props emerging from this process appear finished, they’re not quite there yet. The machine routing just isn’t capable of the precise profiles called for in Sensenich’s drawings, so the props move on to hand carving stations where the finish work is done. It’s all hand-eye work with razor-sharp spokeshaves and a series of templates and protractors that match the precise blade angle called for at up to nine stations along each blade’s length. For handwork, the tolerances are fairly demanding.
“At the inboard stations, he’s got 0.090 above drawing and 0.060 under. Outboard, where the blade is thinner, there’s no tolerance under, but 0.030 over,” Rowell explains. Birch isn’t a nasty roey wood like ash or beech, but will chip out under an edge tool, so the craftsman has to know both his tool and the wood. A pneumatic orbital sander at every station tamps down the chip-outs and brings the prop to its final profile.
After carving to the final profile, the prop is ready for finishing or composite cladding, which happens in a separate cool room inside the shop.
Rowell said 75% of what the Florida shop manufactures has composite covering of some kind. They use a wet layup process with fiberglass, carbon fiber or Kevlar. Although a painted composite prop has the look of a thick layer of cladding, the covering is actually quite thin; only two plies of 0.089-inch thick 9-ounce fiberglass for typical aircraft props.
Rowell says Kevlar has proven highly successful for airboat props because it’s so resistant to FOD passing through the prop—hats, sunglasses, beer cans and the occasional starter or alternator.
“I’ve seen airboat props come in here for repair that would have exploded if they didn’t have the Kevlar,” he says. This material may find its way onto aircraft props eventually, but so far, it hasn’t.
Before leaving the factory, every Sensenich wooden prop gets some final touches. You’ve probably noticed that many wooden props have painted tips, but in Sensenich products, that’s actually aircraft-grade cotton glued and painted, then covered with varnish. Sensenich developed this technique years ago as a low-tech means of protecting the tips against erosion and FOD damage.
The prop’s leading edges also get protection in the form of either an inlaid urethane leading edge or an applied brass edge that’s screwed and riveted in place. To create a smooth, finished look, the fasteners are countersunk and then filled with solder.
Most wooden props are finished clear with marine spar varnish, and there’s an option to have the back of the prop painted black to reduce glare. Sensenich has tried other finishes on props, Rowell says, but as with the resorcinol glue, old school still works best so they’ve stuck with spar varnish. On boat brightwork, the spar varnish often gets renewed every year, but on props, it lasts for years. That’s probably because most wooden props are—or should be—hangared, and thus don’t see much UV light, which breaks down the varnish surface film.
To help protect against that, Sensenich recommends regular coating with ordinary automotive paste wax.
And by the way, Sensenich says it’s okay to fly wooden props in the rain. The metal leading edge will protect against edge erosion, and the finish is robust enough to resist rain damage. They do, however, recommend throttling back to reduce even minimal erosion.
How long can a wooden prop last? There’s no specific overhaul period for wooden props as there are for metal designs; and, technically, wood can’t be overhauled since there are no specs for post-overhaul performance checks, as there are with engines. Wooden props can, however, be stripped and reconditioned and also, if necessary, rebalanced and even re-tracked. Where a metal prop’s tracking can be adjusted by bending the blades, that’s not an option on wooden props, so the hub surface is minutely machined to adjust tracking angle. Warping of the blades—a relative rarity—is a certain killer of a wooden prop. That’s why when one is removed, it should be stored sitting flat in a temperature-controlled environment, and on an idle airplane, the prop should be rotated to the horizontal. That keeps moisture from migrating into the end grain exposed inside the hub bore.
If it sounds like a properly cared for wooden prop can last almost forever, that may not be too much of an exaggeration. Sensenich sees its share of 50-year-old props arriving for refinishing that perform as well as they did the day they first left the factory. Wouldn’t it be nice if you could say that about everything in aviation?
This article oriiginally appeared in the April 2014 issue of Kitplanesmagazine.