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.