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• History of Flight

Good Wood

Wooden propellers are like Louisville Sluggers: The distance.

• By Tom Harpole
• Air & Space Magazine, July 01, 2003

ONE SUMMER DAY IN 1928, HARRY AND MARTIN SENSENICH TOOK THEIR PROPELLER-DRIVEN farm wagon on its maiden flight, so to speak, hurtling down the narrow dirt roads of Lancaster County, Pennsylvania. It was a maelstrom of an outing, leaving in its wake stampeding livestock and a series of transgressions against the rules of the road. The next day, authorities banned the brothers from operating their go-devil on the Pennsylvania byways.

Having experienced the exhilaration of being propelled by pushing air, the Sensenichs sought another course. By that winter they had transplanted their engine and propeller to an ice sled, which they tethered by a hundred-foot hawser to a stout stake set in the frozen Susquehanna River. They enjoyed speeding around the circle until eventually the rope frayed, the sled catapulted into the brushy bank, and snow settled upon the remains of the boys’ conveyance. Their newly purchased propeller was splintered into kindling. The Sensenich brothers (pronounced “Sen-sen-ick”), impecunious but ingenious, borrowed a drawknife and a spoke shave—tools used in making wagon wheels—and, encouraged by local aviators, began experimenting with the graceful compound curve of a propeller.

Seventy-five years later the company that bears their name has produced more than 450,000 wooden propellers—as well as thousands of metal props. The Sensenich Brothers Propeller Company meets almost the entire demand for Federal Aviation Administration-certified fixed-pitch wooden propellers in the United States. “We still carve about 4,000 props per year,” says general manager Don Rowell.

Rowell’s career with Sensenich spans a third of the company’s history. He started when he was in high school, sweeping sawdust at the company’s original location in Lancaster County. Rowell has performed every job in the prop-making process. He spent 10 years hand-carving more than 10,000 rough-cut hardwood blanks into perfectly balanced, laminated-wood props. In 1994, Rowell, by then general manager of Sensenich, moved the wood prop shop to Plant City, Florida, to get closer to the market for a big chunk of its production: swamp boats, also called airboats. He provides 20 craftspeople with some of the best-paying jobs in the area, having trained them in skills that aren’t taught at the local vo-tech.

On the majority of aircraft, metal and composite props have replaced wood, but, wooden propellers still own 10 percent of the aviation market. The demand derives from attributes including performance (“It’s much easier to design the optimum wood propeller for custom aircraft,” says Rowell; “Wood propellers inherently have less vibration”), price, and, in the case of vintage aircraft, authenticity—right down to the 1940s-vintage Sensenich decals that the company applies to the finished product.

“Wood makes sense,” says Steve Boser, the design engineer at Sensenich. “Metal props are much more sensitive to engine vibrations. All props flex in flight, due to harmonics, the high-frequency oscillations excited by engine vibrations. Wood props damp out engine-induced vibrations by several magnitudes better than metal. But countless flexion cycles don’t affect wood significantly, while metal props accumulate invisible flaws from vibrations and flexing.

“A wood prop is as good as it looks. We’ve had 30-year-old wood props come in that only needed refurbishing, cosmetics. And we’ve had wood props come back in two years that were unairworthy. It all depends on proper maintenance.”

During World War II, when the Sensenich company employed 400 people and cranked out more than 5,000 propellers a week, wood was the only material the company used. Sensenich didn’t begin producing metal propellers until the late 1940s. Metal props initially had a performance advantage over wood—because metal is so strong, metal props can be made thinner than wood and are therefore more efficient. But the benefits were obviated in the early 1950s by the design of a new airfoil for wooden propellers. Wood props traditionally had a flat backside, which worked well, but the thickness that was required to keep them from flexing cost some efficiency, measured by the percentage of shaft horsepower converted to thrust horsepower. Sensenich engineer Henry Rose designed a wooden-blade airfoil that was curved on both sides—now called the Rose “E”—which brought wood prop performance within a few percentage points of the performance of metal props.

In performance, a few points make a huge difference. Formula One props, for instance, at 90 percent efficiency, propel air racers at 275 to 300 mph. “We get the physics from customers who tell us the engine power, prop diameter, and rpms [revolutions per minute], and we make a prop that has maximum efficiency at full power cruising at 7,000 feet,” Boser says. “But it’s still like choosing one gear ratio for a car.”

Efficiency in fixed-pitch props is always a compromise: They either take off and climb well, or optimize the engine’s horsepower at cruise, but they can’t do both. Sensenich makes a high-speed target-drone prop that is rated 90 percent efficient for cruising at 300 mph, but the propeller’s pitch is so inefficient at takeoff that a catapult is required to get the drone airborne.

Propeller pitch is determined by the ratio of forward speed to the propeller’s rotational speed. Theoretically, a 41-pitch prop would move forward 41 inches along an imaginary line during the time it takes for the propeller to make a single revolution. Outfit identical Piper Cubs, one with a 46-pitch prop and the other with a 50: The 46-pitch, at 2,300 rpm, cruises at 85 mph. The 50, under the same conditions, cruises at 100 mph.

In addition to airboat props, a portion of the Sensenich production goes to uncertified, experimental, and amateur-built aircraft, and for powered paragliders and hang gliders. Steve Boser is a powered paraglider pilot, and he tests every new design the factory manufactures for the paraglider market, heading to a nearby cow pasture and flying in the sultry evenings.

John Monnet, an experimental-aircraft designer in Oshkosh, Wisconsin, who has sold more than 2,000 kit planes and 500 Sonex aircraft, uses wood props exclusively. “We’ve designed aircraft that use engines that run from 2,750 to 6,000 rpms,” he says. “You can’t safely cover that range with metal props. Wood is durable, experiences far less torsional vibrations, and we can experiment with different tuning of the prop. It costs a few hundred dollars for Sensenich people to change a computer program and carve a new pitch and diameter. It costs thousands for recasting, grinding, and polishing a forged-metal prop.”

General aviation aircraft take 40 percent of the company’s wood props, airboats take 20 percent, and display props—for decoration only—10 percent. The biggest market niche the company supplies is unmanned aerial vehicles, the small recon and attack aircraft, which account for 30 percent. The niche is growing, due in part to the fact that ship-launched UAVs are designed to return to the ship and crash-land into a Kevlar net, breaking, it is hoped, only the propeller. “You’ve got a million-dollar plane, a $20,000 net, and a 300-dollar prop,” Rowell says, adding with a grin: “Props are cheap.” The Sensenich price range runs from the low-end UAV props to $3,000 for a 98-inch Stearman.

Business has been steady, and customers seem to accept that the process of making a wooden propeller may require a wait. “The last time we got caught up with orders was after September 11th, when the aviation world slowed down,” Rowell says.

The craftsmen at Sensenich begin their workday at 6:00 a.m., switching on fans of all descriptions—including a couple of wooden four-footers built at the shop—to counter the torpid Florida air that wafts in from the loading dock.

At each station where the props are taken through another stage of development, there’s a fan and a boom box. The ambient noise in the gymnasium-size building never lets up, a gnashing din of whines, grinding, chipping, and hammering that competes with rock, salsa, rap, bluegrass, and oldies blaring at the various workstations.

Props follow a spiral path around the factory floor, starting at the center of the building, where rough-sawed yellow birch planks, all one inch thick and random widths and lengths, are stacked on carts that crowd an area the size of a parking space.

Here at the center, inspector Charlie Brown sorts through the birch boards, which are harvested from New England tree farms. Before Brown begins making a stack, he has a specific propeller in mind. He balances each board on the edge of his hand, culling nearly half of them as he stacks up as many as 16 laminations, swapping ends so that heavy sides alternate. (A stack of five 0.75-inch-thick laminations is stronger than a solid 3.5-inch-thick piece of wood.)

The shop, near the Plant City airport, resembles an industrial museum, with tools that haven’t changed in 75 years. Not even the glue has been improved in decades. Resorcinol has been the only adhesive used by Sensenich since “somewhere back in World War II,” Rowell says. “We’ve never had a glue failure. If we tried to use some new glue we’d have to go through recertification. Way too expensive to replace something that has worked flawlessly for 50 years.”

The purplish glue coats nearly every surface in a far corner of the factory where Jose Mendez and Wayne Allen are making up a blank for a 98-inch Stearman prop. The set-up time of the resorcinol, at 90 degrees, allows them less than an hour to apply the glue to each surface of the eight laminations and get the blank clamped. “It’s no big deal,” Mendez says. “We’ll get it in 25 minutes.” Twenty-two minutes later, the two have applied 112 C-clamps to the laminations. There is nowhere on the prop that they can insert a fist between clamps. Resorcinol drips down the terraced edges of the big blank in one-inch intervals. After the initial machine carving, the glue lines along the laminations of yellow birch will define the prop’s curve from root to tip and provide guidelines for the master carvers to follow.

After letting the glue set for up to 24 hours, the workers rough-shape the blanks in a computer-controlled carving machine, or on the duplicating router, a 70-year-old machine designed by Martin Sensenich that rough-cuts laminated wood blanks into propellers. Like a three-dimensional key-cutting machine, the big router, modeled after a gunstock-duplicating machine, tracks the shape of a pattern prop and transfers the profile to the blank. When the blanks are rough-cut they are within 3/16 of an inch of their final shape. A worker carries the rough-cut blanks to a drill press, where a large hole is milled out of the hub, then a drill press is used to bore the bolt holes. The hub hole doubles as the fastening point for vises that are designed to allow the carvers to set the props at any angle they wish.

By 2:00 p.m., at the master carvers’ station, it is 94 degrees in the shop, and Chris Thorpe and Ben Smith are bent over, scrutinizing a tiny area on a prop. They beckon Justin Bryant, a burly apprentice carver who moonlights as a mud wrestler. Bryant bends over and puts his face about a foot away from the blade, and three close-cropped heads hover within inches of one another as they stroke the spot with their palms and fingers. “Things you don’t feel with your fingers alone you’ll notice if you use your whole hand,” Thorpe instructs Bryant. Their shirts flutter in the stiff breeze from the fan.

Thorpe turns to his workbench and picks up masking tape and a razor blade. Smith turns the fan off and Thorpe goes to work with the blade, chipping off a glue line that is an inch long and perhaps half the width of a toothpick. Smith makes a little masking tape dam and Thorpe dribbles some epoxy into the minuscule gouge in the prop.

Thorpe bends over his props in an unbroken curve from hams to head. Later he will wield an air-driven drum sander that is revolving at 1,800 rpm and sand down the epoxy patch to a .03-inch tolerance—the thickness of a piece of paper. The carvers also use the drum sanders to sand away the leading edge so that they can make another dam to fill with urethane for erosion protection on props that don’t get protective sheet metal applied to their leading edges.

By early afternoon the carvers are ankle deep in shavings from countless strokes they pull with Stanley #64 spoke shaves. “This here is the heart of the prop carving business,” Thorpe says, holding out a six-inch-long spoke shave with a two-inch blade centered between spoon-like handles. The way he holds it tells me he is not offering it for closer inspection. “Got to take care of what makes the truck payment,” he says, cradling the tool in his palms.

When the drum sander’s whine gives way to the soft scritching of overlapping spoke shave strokes at the carvers’ station, Smith and Thorpe pass the time in blurted exchanges on topics as diverse as the taste and texture of wild frog legs versus farm-raised frogs and the heartbreaking error of feeding piglets a pickup load of rotten cantaloupes. “They couldn’t digest the seeds and swelled up and croaked, but not like frogs,” Thorpe notes. They exchange recipes for soft-tailed turtles. Most of the day, however, they are busy with a tricky bit of wood grain a few inches from their eyes, and they are absorbed into the swift strokes of their spoke shaves, and with sandwiching metal templates on the front and back of the props until they meet at a perfectly carved spot along its length.

Thorpe and Smith, arguably the two best prop carvers around, don’t race each other or the clock. “The people who pay this kind of money for a propeller deserve something as nice as humanly possible,” Smith says.

What’s humanly possible, especially without the aid of computers and duplicating routers, is what Holmes Beach, Florida freelance prop carver Ed Sterba has been doing for 20 years. He is one of perhaps a dozen independent carvers in the country who advertise their non-FAA-certified propellers in the classifieds of aviation magazines.

Sterba, a lean, tanned pilot and sailor and single parent of three teenagers, also projects the contentment and pride that seems inveterate in woodworkers. Working out of a shop smaller than the Sensenich rough lumber stack, he makes three propellers per week. He keeps a library of hundreds of index cards on which he has recorded mathematical descriptions of the angles, thicknesses, curves, and pitches of hundreds of props at four to eight stations along their two blades. His power tools are a band saw, hand-held planer, drum sander, and a drill press that he built from mismatched parts and wood scraps. Working without templates, he relies on his eyes and two decades of standing over laminated-maple blanks clamped to a workbench the size of an ironing board. He balances a maple plank using a point like an old-fashioned tire balancer. Then he drills a hub into it and sketches lines along its length to guide him when he bandsaws the laminated planks into rough props. As he listens to the predictable cadences of NPR, he planes and sands away at the rough form until it looks like the propeller he wants.

Sterba regards the first Wright brothers’ propeller, the one they carved in their Ohio bicycle shop, as a stunning achievement of calculus and craftsmanship, as amazing as the airplane itself. “Their propeller was easily as great a discovery as anything they had to learn,” he says.

His tiny shop is located in a row of commercial spaces that are rented by artisans: antique restorers, a couple of jewelry makers, a potter. He regards his work as a critical trade compared to the craftsmanship of, say, furniture or musical instrument making. “Lives are at stake,” he says. “This is closer to boat building. But real artists can do things that I can’t.”

Art is amorphous by nature, and Sterba “repitches,” or slightly adjusts the blades’ angles, at no charge if they aren’t as efficient or perfectly tuned to the aircraft’s performance as possible. “That’s the nature of experimental aircraft,” he shrugs. It’s also the nature of pilots who seek the ideal fixed-pitch prop: one that takes off and climbs taking advantage of max horsepower, but doesn’t let the engine overspeed at cruise.

Like the Sensenich company’s, Sterba’s material costs are limited to wood, glue, leading-edge materials, and coatings. Sterba buys one-inch-thick maple boards from a local lumber dealer and uses random widths from two to eight inches wide to lay up laminated blanks. Like any prop maker, he strives for balance at every step of the process.

Back at the Sensenich factory, Don Rowell notes: “Everyone who handles these props can affect, for good or bad, the balance.” After a prop leaves the master carvers, placing a paper clip on the tip, as it rests on the horizontal balance beam, would cause it to rotate out of level.

The master carvers move perfectly balanced props on to have the leading edge tipping applied: stainless steel and brass work that conforms to the twisting curve of the leading edge so closely and finely as to be barely palpable. Jesse Sims, who fabricates and fastens the sheet metal tipping, adds as much as 12 ounces of brass or stainless steel sheet metal plus the weight of as many as a hundred screws and rivets. He makes the sheets into trough-shaped pieces that wrap the edge for an inch and a half, then he clamps the metal onto the prop, adjusting a bungee cord so that it doesn’t cover the holes he had earlier drilled for the screws and rivets that fasten the metal to the wood. Using a torch and solder, he drips molten lead into the bugled depressions in the edge, which he will then grind and polish until he achieves a fine sheen.

“The paint booth boys can change the balance with an extra coat of paint here and there, but these props have to be within a couple grams of perfect when I’m done,” Sims says.

One afternoon at quitting time, I watched Sims punch his time card and walk by a rack of finished propellers on his way out. He paused to run his index finger down the polished brass edge of a Stearman prop with more than 100 rivets and screws fastening the bright brass to the leading edge. The tips of that prop, at 2,100 rpm, will travel at Mach .80—roughly 612 mph—without perceptible vibration for thousands of hours.

“These props are sculpture,” I said.

Sims, nonplused, replied, “This is a production outfit.” He plucked his cap off and scratched his sweaty gray thatch. “We fight tooth and nail to get these props perfect. We have keen eyes and hands here. Nothing gets shipped with an imperfection.”