A look forward to electric aircraft

  • 16-Sep-2011 04:03 EDT

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.

Electric aircraft, a relatively new concept in the aviation world, are taking to the air in greater numbers of late. In fact, you could say the electric aircraft are suddenly flying high.

One of the stars at the recent EAA Oshkosh Airventure show, for example, was the Taurus G4 aircraft, a four-seat test-bed that is powered by a 150-kw electric motor, the most powerful yet seen in an all-electric airplane. The one-of-a-kind, twin-glider body airframe was designed and built in five months by Slovenian electric aircraft manufacturer, Pipistrel, and a Penn State team. The battery-electric G4 will soon vie for the upcoming NASA/CAFE Green Flight Challenge, a green aviation fly-off.

Meanwhile in mid-August an all-electric helicopter flew for the first time when French electrical/aerospace engineer Pascal Chretien piloted his single-seat e-chopper for 2 minutes and 10 seconds.

In true fact, the momentum in the aviation industry toward electric and hybrid aeropropulsion technology has been building for some time.

EADS and Siemens, for instance, has been flying a battery electric-powered ultralight aircraft for the last year. Moreover, EADS 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 at the recent Paris Air Show.

Last fall, Boeing released details of aNASA-funded effort to use an advanced hybrid battery-electric/gas turbine propulsion system to power a future 737-class commercial transport.

Even at last year’s Oshkosh event, Cessna and Sikorsky announced plans to fly some time in 2011 electric-powered demonstrators—respectively, a light plane and a light helicopter.

And all along pioneering firms such as Pipistrel and Electric Aircraft Corp. have already been flying small electric planes.

This nascent trend stems from the same kind of government pressures that automakers faced not so long ago. Regulations mandated that the car industry must minimize both fuel use and tailpipe emissions. To help shrink their environmental footprints, the auto manufacturers developed and introduced hybrid-electric and battery-electric power trains. Today a small, but growing number of Toyota Priuses, Chevrolet Volts,and Nissan Leafs (and, soon, BMW i3s and i8s) cruise the world’s highways and byways.

Now, facing substantially the same environmental restrictions, the aviation industry has begun gearing up to power future aircraft using those same hybrid and electric power plants.

Notwithstanding the early electric flight demonstrations, aerospace engineers and managers do not expect to see larger electric aircraft any time soon. Providing power to propel a car down the road differs from markedly from sending an airplane through the air. Aircraft propulsion systems need considerably more power while meeting significantly tougher weight constraints and near "fail-safe" safety standards.

It is notable then that even after significant investment the electric automobile still struggles to expand its commercial market, hindered as it is by high costs and heavy, sub-optimal batteries. Likewise, the aviation industry will take decades to implement ultra-green aviation technology, industry observers believe. At this point, they said, researchers can only speculate about the technical advances that would be necessary for success.

“Next-generation aircraft will feature more and more electronics, but the last thing to convert to electric power will be the power plant,” said Dale Carlson, Executive for Advanced Engine Systems at GE Aviation. “This is because the batteries that would be required to supply the amount of electricity for large commercial aircraft weigh a lot.”

Battery capacity, he noted, would need 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, of course, need hundreds of thousands of horsepower to develop enough thrust to fly.

“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,” said Johannes Stuhlberger, Head of the Global Innovation Network, Power and Flight Propulsion at EADS. Electric motors would need efficiencies of around 95%, he noted.

“We will need tremendous improvements in the power-to-mass ratio of the entire propulsion system, while still keeping it affordable,” said Stuhlberger. “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 meanwhile mused that “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.”

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Thermal imaging data obtained from a FLIR high-performance camera shows that the expected turbine output temperature is approximately 285°C when the helicopter is in forward flight. However, during hover operations a steady state temperature of about 343°C will be reached.

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