Pascal Chretien’s prototype aircraft hovered merely three feet high for a total of 2 minutes and 10 seconds on August 12 in France—but he made aviation history last month as the pilot of the first untethered electric helicopter.
The project was privately funded and executed by Solution F, a racing car and engine supplier based in France.
With battery technology advancing steadily, it was only a matter of time until somebody tried to produce an EH (electric helicopter) or an HEH (hybrid electric helicopter). Indeed, Connecticut-based Sikorsky Aircraft Corporation had been working on its own project to launch the first manned electric helicopter and had unveiled its pre-flight Firefly prototype at the Experimental Aircraft Association AirVenture exhibition in July 2010.
Just one month later, in August 2010, Solution F contacted Chretien and challenged him to develop its electric helicopter prototype.
But Sikorsky’s Firefly is a much more conventional helicopter than Chretien's creation: It uses a standard, enclosed body shape, a single top rotor, and a tail rotor —all of which add considerable weight, requiring additional heavy battery packs. A tail rotor, for example, drains somewhere between 8 percent and 10 percent of total hover power.
Any extra weight could hinder a successful flight, because helicopters literally require more “get up and go” than other aircraft. Electric airplanes, for example, require lots of power to leave the ground, but can cruise and land on lesser amounts. In contrast, helicopters rely on strong power delivery during all three phases of flight—takeoff, hovering, and landing.
Thus, although Chretien’s prototype helicopter reached a very modest altitude, the liftoff and landing were dangerous. Before strapping in, Chretien, an experienced aerospace engineer, said, “In case of crash, I stand good chances to end up in kebab form.”
In comparison to Sikorsky’s still Earth-bound design, Chretien built an ultra-light aircraft that could achieve a decent flight time on battery power. His creation is oddball in appearance, bearing more resemblance to a large insect than to any air-worthy apparatus. Chretien developed a coaxial design with two counter-rotating rotors on top —and a feather-weight tail fin, to stop the aircraft from rotating out of control.
In place of the typical cyclic control, which uses a variable blade tilting system to control which way the helicopter tilts and advances, Chretien chose a weight-shifting system —a big set of handlebars that literally tilts the main weight of the aircraft underneath the rotors —as his steering assembly. But this increased Chretien's risk factor by a significant margin, according to Gizmag, because it meant the controls would be reversed compared to a normal helicopter.
As Chretien put it, “Weight savings were substantial – [9 pounds to 11 pounds] compared to a conventional arrangement... But habits being second nature, after 15 years of flying conventional machines, the risks of crash were quite high.” Chretien used a pendular weight shift training machine to get some practice during the development of the aircraft and try to ensure that he didn't react the wrong way when he first took to the air.
The frame of the helicopter was built from welded 7020 aluminum tubing. Composite materials would have been lighter, but time was of the essence, and crash-worthy aluminum frames can be built very quickly and cost-effectively.
Chretien had been given a target of 10 to 12 minutes' flight time by Solution F, so the most critical part of the aircraft design was working out how to store enough power for this kind of flight, as well as how to use that power most efficiently. Early on, he sought the help of Denmark-based Lithium Balance , which supplied the battery management system and consulted on power storage issues.
The rechargeable battery cells are Lithium ion polymer pouch cells, with an energy density of 160 Watt-hours per kilogram. Although reasonably lightweight, these cells presented probably the biggest danger to Chretien in the test flight phase. He commented, “The infamous thermal instability of lithium/cobalt chemistry does not leave room for error.... It is important to take it slowly, if I don't want to wreck tens of thousands of Euros worth of hardware… as LiPo batteries are notoriously infamous for bursting to flames once distorted. The chemical reaction is violently exothermic. This machine looks like a toy, and flies like a toy, but there is a raging tiger under the seat, waiting to bite at the first mistake.”
Now that he has done the groundwork and testing, Chretien is working on making the prototype more flight-worthy. The electric drive train needs more development, for example, and Chretien believes that “looking at the excellent power reserve we have today, it appears that we could have used a conventional cyclic [control] stick.”
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Cheryl Kaften is an accomplished communicator who has written for consumer and corporate audiences. She has worked extensively for MasterCard (News - Alert) Worldwide, Philip Morris USA (Altria), and KPMG, and has consulted for Estee Lauder and the Philadelphia Inquirer Newspapers. To read more of her articles, please visit her columnist page.Edited by
Jennifer Russell
