Wooden propellers are like Louisville Sluggers: The distance.
- By Tom Harpole
- Air & Space magazine, July 2003
(Page 2 of 5)
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.