The Electric Airplane
Quiet, smooth, dependable—shouldn’t we be flying these by now?
- By Peter Garrison
- Air & Space magazine, August 2009
Sonex Aircraft, LLC
(Page 2 of 4)
Magnetic forces—attraction and repulsion—cause the rotor (an electromagnet) of an electric motor to spin. Some types use two metal tabs, or brushes, with opposite charges; during each revolution, the rotor comes into contact with first one brush, then the other, each time switching its polarity. To perform the same function, a brushless electric motor relies on a solid-state switching device called a controller. Rapid switching of high-voltage currents, however, turns out to be difficult. The currents have momentum, just like moving water, and a random surge can quickly vaporize even quite massive transistors. Another problem is more mundane: The motors are hard to start.
“The controller is really where it’s at,” Buck says. “It should be cookbook, but it’s not that easy. None of us recognized the complexity. There are only a few people who know how to do it, and they aren’t talking.”
The battery pack consists of a stack of thin lithium-polymer cells that resemble foil-wrapped legal pads. “We always thought the batteries would come to us,” Buck says—meaning that they sized the airplane and motor for batteries that didn’t yet exist. “There are batteries out there that have five times the energy density of those we can buy today, but they’re only in the lab.” And the Sonex team wanted the electric airplane to be comparable in price to the aircraft now being built from the Waiex kits. “We’ve always believed in an airplane that would be available at a price the average pilot could afford,” says Buck, “so that the whole airplane, including the engine, would cost about the same as a new car, around $26,000.”
Batteries are, in the final analysis, the key to the whole project. Controllers are tricky but feasible; motors are delicate and expensive, but technically straightforward. It’s really on batteries—developing ones that are powerful, durable, and not prone to burst into flame if mistreated—that the future of electric airplanes hangs.
A gasoline powerplant, with its fuel, accounts for about a quarter of an airplane’s takeoff weight. An electric powerplant is somewhat heavier to begin with; it adds 75 pounds to the weight of the Waiex because the batteries alone weigh 200 pounds. The big disadvantage is that the energy available from all those batteries is equivalent to only a couple of gallons of gasoline. Observes Buck: “We pilots would consider that ‘unusable’ ”—the technical term for dregs in the bottom of the fuel tank that may not be available in all flight attitudes.
Buck aims at an airplane of conventional dimensions—with a little more wingspan than most, but able to be tucked comfortably into an ordinary hangar—and having climbing and cruising capability comparable to that of a gasoline-powered airplane in every respect except, perhaps, duration of flight. In other words, he and Monnett want to prove that an electric airplane can look and fly just like a gas-powered one.
Greg Cole sees things a little differently.
Cole, 46, is a freelance aeronautical engineer. His Oregon company, Windward Performance, produces a carbon-fiber sailplane called the SparrowHawk, which, at 155 pounds empty, weighs less than many of the pilots who fly it. Cole is a bit of a visionary. He is concerned not just about the price of gasoline, but also about aviation as a whole—the possibility that the cost and the complexity and stress of flying modern airplanes might drive people away from flying. He is not just an engineer; he is a reformer. Cole, like Monnett, is preparing to manufacture an electric two-seater. The wingspan of his design is a glider-like 51 feet—a rather cumbersome size for taxiing, parking, and hangarage at many general-aviation airports. The longer an airplane’s wingspan, though, the less power it needs to lift a given weight. Cole’s motor, similar in design to Buck’s but smaller, is rated at just 40 hp. If he can keep his airplane’s empty weight below 500 pounds or so—the SparrowHawk demonstrates his ability to engineer very light, yet strong structures—he will be able to climb at 660 feet a minute and cruise at 70 mph on the electrical equivalent of one gallon of gas per hour. “We need to get into lower-power airplanes,” he says. “We need to do smaller.” He brushes aside objections that his design will not mesh easily with existing infrastructure. Electric—smooth, quiet, non-polluting, and with motors that will never fail or wear out—is “a completely viable way to revolutionize aviation.”