Pascal Chrétien, the designer and pilot of an electric rotorcraft that flew in 2011, is forming a company that aims to offer hybrid power for aircraft, notably helicopters. The patented technology is called Tetraero, and according to its promoters its main benefit would be in safety.
“You can’t design a redundant main gearbox–if one pinion breaks, the entire thing fails,” Chrétien told AIN. As recent ditching events in the North Sea have proved, the main gearbox remains the Achilles heel of helicopters, even expensive models such as the 19-seat Eurocopter EC225 and Sikorsky S-92. Statistically, he sees transmission failures as the primary cause of helicopter crashes after human error.
With that in mind, Chrétien is pitching a “series hybrid” architecture in which a turbine engine drives a distributed generator, which in turn feeds a battery pack that sends current to a distributed stack of electric direct drives to spin the rotors. “If you lose part of the motor stack, degradation is progressive,” he explained.
If the conventional turbine engine fails, the batteries still have enough energy to provide seven minutes of flight. If the engine failure happens at altitude, the pilot could perform an autorotation down to a few hundred feet agl and then use the battery power to land normally.
New high-performance magnets make this sort of architecture possible. Also, in just the past two years battery power density has increased from 73 to 181 Watt-hours per pound (meaning one pound of battery can supply 181 Watts for one hour). Likewise, motors have improved from 0.90 to 2.7 kW per pound.
Chrétien has calculated the cost of “hybridizing” an existing production helicopter. He claims a “hybridized” variant of the $1.4 million MD530F would cost only $1.33 million, thanks in part to elimination of the expensive main gearbox.
There is a downside, however, in that the hybrid configuration is 8 percent less fuel-efficient than a conventional turbine and transmission. “You have two power conversions–one between the engine and the batteries and another between the batteries and the motors,” Chrétien conceded. But the turbine engine can run constantly at its thermodynamically most efficient speed, offsetting this disadvantage to some extent.
A hybrid MD530F would be 150 pounds heavier, Chrétien told AIN, but theextra weight would not affect available payload because the hybrid architecture produces more takeoff power. “That’s the paradox of going hybrid,” he said.
Total hourly operating costs would be close, Chrétien asserts, since the longer component lives of the hybrid system would reduce maintenance.
Chrétien bases his calculations on the use of off-the-shelf semiconductors such as silicon-carbide power transistors. Current technology for these devices limits the maximum power of a hybrid to about 550 shp, but future advances in superconduction could allow higher output.
A more immediate challenge is protecting onboard avionics against electromagnetic interference. “This is often overlooked,” Chrétien said, explaining that on the electric single-seater he flew last year, he created his own mix for a protective paint.
Future fuel cells are expected to bring roughly 50 percent greater energy efficiency, he said, compared with the 25 percent that current turbine engines allow. “We are at the bottom of a promising innovation curve,” Chrétien concluded, and he sees conventional turbine powertrains as the wrong route toward progress since they provide only marginal efficiency gains from costly engineering efforts.