JULY 23RD, 2011
BY STEVEN ASHLEY
Several years ago, when the auto industry faced government pressure to minimize its environmental footprint by reducing fuel consumption and tailpipe emissions, manufacturers turned first to hybrid-electric and then battery-electric powertrains that now move Chevy Volts and Nissan Leafs.
Today, confronted with substantially the same environmental mandates, the aviation industry has begun gearing up to use those same green power plants to propel aircraft.
The electric car is so yesterday; electric airplanes are coming.
EADS (European Aeronautic Defense and Space Company), the parent firm of Airbus, for example, has been flying a battery electric-powered ultralight aircraft for the last year, and at the recent Paris Air Show it introduced a series-hybrid motor glider as well as an ambitious future concept for an all-electric, 50-seat passenger plane powered by superconducting drive motors. Last fall, Boeing released details of a NASA-funded effort to use a hybrid battery-electric/gas turbine propulsion system to power a future 737-class commercial transport. A few months earlier, at the 2010 Oshkosh event, both Cessna and Sikorsky announced plans to fly some time this year electric-powered demonstrators—respectively, a light plane and a light helicopter.
Boeing’s SUGAR Volt (Subsonic Ultra Green Aircraft Research) twin-engine future concept airliner. The 737-size transport would be powered by hybrid propulsion system that would combine gas turbine and battery/electric motor technology. Courtesy Boeing
A slow transition to electric
But just how practical are electric aircraft? After all, powering a car down the road is different than powering an airplane through the air, and the electric automobile is still struggling to find its niche.
Aviation experts say that ultra-green aviation technology will take a good deal of time. The necessary technical advances are still speculative. On the other hand, they add, aerospace engineers can see the path to eventual success, if not the actual stepping stones along the way.
“Next-generation aircraft will feature more and more electronics,” said Dale Carlson, Executive for Advanced Engine Systems at GE Aviation, but “the last thing to convert to electric power will be the power plant. This is because the batteries that would be required to supply the amount of electricity for large commercial aircraft, weigh a lot.” (General Electric is the sponsor of this magazine) The power capacity of battery technology, he continued, would have to grow by “at least a factor of four before we are near where we need to be to accomplish this.” Larger electric airliners would need hundreds of thousands of horsepower.
Last year EADS Innovation Works modified a French Cri-Cri single-seat ultralight aircraft by replacing its two piston engines with four electric motors that are each driven by a lithium polymer battery. Courtesy EADS
Johannes Stuhlberger, head of the Global Innovation Network, Power and Flight Propulsion at EADS, agreed with this assessment. “The development of electric aircraft not only depends on the speed at which battery technology improves, but also how fast electrical equipment—the motors—get better.” Electric motors would need efficiencies of around 95 percent, he continued. “We will need tremendous improvements in the power-to-mass ratio of the entire propulsion system, while still keeping it affordable.”
Still, many researchers can already envision how the switch to electric will take place. Electric propulsion will be a “game-changer and transform aeronautics in the next 20 to 30 years,” predicted Mark Moore, an aerospace engineer and conceptual design expert at NASA Langley Research Center. The first breakthroughs will occur with small aircraft, according to Moore, perhaps personal air vehicles that could replace the car on some trips. He envisions electric-powered unmanned aerial vehicles first transitioning from military to civilian use, followed in time by much more environmentally responsible commercial transport planes.
The changeover to fully electric propulsion will be assisted by the adoption of hybrid-electric drive trains, Stuhlberger noted. “Hybrids, which can implemented more easily, provide a way to compensate for the high masses of batteries and so afford a way to improve the power-to-mass ratio” in the interim. Carlson speculated that at some point “we’ll probably see a hybrid engine that combines fuel cells with turbines—where the turbine technology would be there for takeoff and the fuel cells utilized during cruise.”
Who knows what’s really possible? Just last week, Scaled Composites, the innovative engineering firm founded by famed engineer, Burt Rutan, tested a two-seat, hybrid-powered aircraft that might prove to be a practical solution to the long-held dream of a flying car. Dubbed the Model 367 BiPod, the novel vehicle features removable wings, stabilizer and tail tips to enable rapid conversion into a road vehicle.
Top image: The VoltAir all-electric airliner concept, which was developed by EADS Innovation Works, feature next-generation lithium-air batteries which feed electricity to two ultra-efficient superconducting electric motor
Steven Ashley is a contributing editor at Scientific American magazine, where he writes and edits articles on general science and technology topics. Ashley’s work has been published in Popular Science, MIT’s Technology Review and Physics Today, among others.