Thursday, November 1, 2012
Designer of Electric Helo Reflects on 2011 Flight Tests
After having been virtually absent from the media for more than one year, Pascal Chretien, the designer and pilot of the world’s first electrically powered helicopter, accepted an interview with Rotor & Wing. He reflected on the flight tests that took place in the summer of 2011 near Aix-en-Provence, France. There, with help from Solution F, a small company specializing in motorsports, he managed to do what no helicopter manufacturer has done so far. Chretien sees his feat as just one step towards a more ambitious goal—persuading the industry that the time has come for hybrid helicopters, where a combination of an internal combustion engine and electric motors would revolutionize safety.
|To avoid “wasting” relatively scarce power, the designer of the world’s first electric helicopter chose a coaxial main rotor configuration instead of a conventional single main rotor and accompanying tail rotor.|
Chretien and Solution F’s managing director, Eric Chantriaux, agreed to work together in 2010. Design work actually started in August, Chretien recalled. It turned out Sikorsky introduced its electric Firefly almost simultaneously at EAA Air Venture Oshkosh, late in July that year. Chretien was under pressure, as he wanted to fly first. The Firefly, a modified S-300C, eventually never took off.
Hard work—seven days a week, 10 to 12 hours a day, Chretien said—led to the first ground tests in March 2011. However, the following month, a motor incident delayed the program. The delay had some positive outcome. It gave Chretien time to rethink several safety issues. For example, “I had started the ground tests without a helmet or a safety belt,” he said. Also, he installed additional controls on the cyclic stick to disengage the drive in case of emergency. Besides, the aircraft was fitted with a new set of blades using asymmetrical profiles. They raised out-of-ground-effect lift from 570 to 680-lbs force.
Safety was not always Chretien’s top priority. To limit the risk of battery-induced fire in case of crash, common design practice would suggest installing distributed fuses along the chain of cells. But fuse weight was “unacceptably high,” at 3.3 lbs per fuse. So Chretien decided to “trade safety for weight.”
|Chretien’s helicopter design features four rotor blades.|
Power never was a problem. Two Agni DC motors supplied a total 32 kilowatts (43 hp). This was enough to hover, at a takeoff weight of 545 lbs. For energy storage, Chretien chose Kokam lithium-ion polymer pouch cells—128 lbs of them, located under the pilot seat. He used a honeycomb frame and an Aerogel-based firewall. A battery management system, supplied by Lithium Balance, provided battery management and charger control. It was a key element to success, he said.
Why did Chretien choose a dual, coaxial main rotor? “I wanted to avoid wasting power,” he answered. A tail rotor uses an estimated 8-10 percent of the total power available. The designer was essentially pursuing lift.
Chretien, who has engineering degrees in electronics and aerospace, in addition to its commercial helicopter pilot license, sees further than his single-occupant demonstrator. He and Solution F have filed patents relating to a “serial hybrid” helicopter concept. The principle is built around an engine that produces electric power via a generator. The generator feeds batteries, which enable a distributed stack of electric direct drives to turn the blades.
“Gears are medieval technology; you can’t have a redundant main gearbox on a helicopter,” Chretien insisted. On the contrary, “you can design a stack of redundant of motors,” he said. This brings a true redundancy up to the rotor mast. Hence a major enhancement in safety, if one thinks of the mind-boggling problems gearboxes have caused to helicopter manufacturers.
Moreover, an electric system would be much more durable—“about 12,000 to 15,000 hours of service life, compared to 2,000 to 4,000 hours with a main gearbox,” Chretien asserted. Price would be a factor, too. On a five-seat turbine single, he estimated the price of the main gearbox at EUR250,000 ($325,000) versus EUR145,000 ($188,000) for the entire distributed electric motor unit.
In the future, superconduction might help. Chretien mentioned demonstrated superconductive temperature as high as 95 Kelvin (minus 289 F). “Even considering the weight of the cryocooler, such drive systems would be much lighter, with a power density way above 15 kilowatts per kilogram,” he told Rotor & Wing. By contrast, common aircraft-grade gearboxes with their oil circuit and cooling accessories are around 6.7 kW/kg.
A major obstacle remains, though. Major helicopter manufacturers draw a large part of their revenues from maintenance, repair and overhaul. This market would be much smaller, should rotor power get rid of gearboxes. Helicopter makers may soon have to decide on a paradigm shift. — By Thierry Dubois, on Twitter: @aerodub