One day Sikorsky (Booth No. 2737) may be here at a Heli-Expo with an electrically powered helicopter. The U.S. manufacturer is getting closer to the first flight of the Firefly, a modified S-300C light single where the piston engine has been replaced with an electric motor and batteries. The proof-of-concept rotorcraft is expected to have limited endurance but is seen as a stepping stone toward a commercially viable electric helicopter.
The company had hoped to fly the Firefly by the end of 2010, but did not meet that target. “While there are no technical issues, there are a lot of moving pieces, as with any experimental aircraft program,” program manager Jonathan Hartman told AIN. Therefore, he said, the first flight will take place “as soon as we are ready.”
USHybrid, a long-time supplier for ground applications, is providing the motor. The power--190 shp--is the same as in the original S-300C. GAIA of Germany is supplying lithium-ion batteries.
“One challenge is adapting these cutting-edge technologies to an aviation application,” Hartman said. Sikorsky’s Firefly team wants to make sure both the motor and the batteries are properly tested for aircraft application. “If an aircraft loses a charge, it is much more serious than a ground vehicle losing a charge,” he pointed out.
The permanent-magnet motor is one of the first that USHybrid is designing with air cooling. “We did not want the weight and complexity of a liquid-cooling system,” Hartman explained. Batteries have a forced air cooling system, while the motor is cooled by ram air. A small cooling fan for the electronic components is embedded in the motor.
Gaia has performed some custom chemistry on the batteries for the power profile needed by Sikorsky. “These batteries do not have the highest power density available on the market but are suitable for the Firefly,” Hartman said. Delivering only 15 minutes of endurance, the batteries have an installed weight of 1,150 pounds.
The aircraft is thus several hundred pounds heavier than the original S-300C. The empty weight is close to the 2,050-pound mtow. Nevertheless, Sikorsky expects the Firefly will have the same flight characteristics (including speed, with maximum cruise remaining at 86 knots) as the S-300C. “We want it to look, feel and fly like an S-300C, except there is an electric motor attached,” Hartman said. He said there is no change from the motor mount forward.
Another major challenge has been human factors. The aircraft is much quieter, which deprives the pilot of the normal noise cues. “We have included an interactive health monitoring display in the cockpit,” Hartman said. In addition, design engineers have also introduced load-limiting control laws because an electric motor has its full torque available from zero rpm.
Asked about the benefits, Hartman said an electric helicopter design is inherently more efficient–300 percent better in the case of the Firefly/S-300C comparison. If designed from a clean sheet of paper, an electric helicopter is less complex. It also has fewer parts. This favorably impacts direct operating costs (DOC). Also, electricity is much cheaper than fuel, so the bottom line is about 30 percent lower DOC. Moreover, the reduced vibration allows a lighter structure.
Electric energy storage is the main challenge but is progressing very fast. Seeing an electric helicopter on commercial offer is a matter of years, not decades, Hartman estimated.