Good Wood

Wooden propellers are like Louisville Sluggers: The distance

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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.

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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.

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