Wooden propellers are like Louisville Sluggers: The distance.
- By Tom Harpole
- Air & Space magazine, July 2003
(Page 5 of 5)
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.”