Officially announced at AirVenture 2010, Sikorsky's Project Firefly sets out to demonstrate the feasibility and showcase the benefits of electrically powered helicopters. The technology demonstrator is based on an S-300C light helicopter, with a 190-horsepower electric motor replacing the standard piston-engine and lithium ion battery packs added to either side of the cabin. The result is a significantly more efficient system that – although it's expected to fall short of typical helicopter performance when the first flight takes place later this year – is just the tip of the iceberg for the new era of manned electric rotorcraft.

Along with the electric motor and battery packs, the demonstrator includes a new digital motor controller and next generation cockpit display. The drive train, rotors and flight controls from the S-300C have been retained and very little has been changed on the airframe, as the electric motor has been designed to pick up the same hard points as the piston engine it replaces.

The 190 hp (142 kW) motor built by Californian firm U.S. Hybrid from California is derived from a unit developed for ground based vehicles but, as Sikorsky's Tim Lauder explained to Gizmag, it's been specifically tailored for use in rotorcraft. These changes include an additional 40 horsepower, constant operation at 3200 RPM and forced air cooling as opposed to water cooling.

The battery pack from German manufacturer GAIA consists of 300 cells – 150 in each bay – with an energy density of 0.13 kW per kilogram.

Inside, real-time aircraft information including temperature for left and right battery packs, flight time remaining and voltage is shown on an interactive LCD monitor.

Performance and efficiency

The comparative figures Sikorsky has released for the testbed helicopter show huge improvements in efficiency. The main rotor output, for example, is 76.3 percent efficient in the electric version versus 25.5 percent for a Sikorsky 76D running two PW210S turboshaft engines on JetA fuel. The other notable comparison is heat loss, where the electrical system loses just 15 percent compared to 73.4 percent on the 76D according.

“Through the electrical conversion, propulsion efficiency of the aircraft has been increased roughly 300 percent from baseline," said Mark Miller, Vice President, Sikorsky Research & Engineering. "Electric propulsion also inherently simplifies the complexity of the propulsion system by reducing the quantity of moving parts, increasing reliability while reducing direct operating costs.”

While both the cruising speed of 79 knots and the gross weight of 2150 lbs are comparable to the S-300C helicopter, the big issue, as with electric cars, is range. The demonstrator has an expected flight time of 15 minutes (compare this to the 3.7 hrs the S-300C gets from its 32 gallons of fuel) but as battery technologies get better, this will improve exponentially.

Lauder sees lithium air batteries as one of the most promising of these developing technologies. "Lithium air batteries would be lighter (because they have no anode) and provide nine times the energy density of the current batteries, enabling three hour flights and rivaling combustion engines."

The problem at this stage is that these batteries are non-rechargeable, but research and development of rechargeable versions is ongoing.

It's safe to say that the future for electric helicopters is bright. As battery technology matures the value of the systems showcased on Sikorsky's testbed can only increase. Although the company has chosen to use an existing airframe to showcase the technology at this early stage of development, it is already looking towards ground-up designs that will run two electric motors – one in the main rotor and another in the tail rotor. This approach, like electric car designs that place motors at or in each wheel, will further reduce weight and improve efficiency by bypassing the need for any conventional drivetrain.

    Sikorsky electric helicopter specifications

  • Max Gross weight - 2,150 lb
  • Power 190 hp (142 kW) Permanent Magnet electric motor
  • Lithium Ion Battery - 45 Ah, 3.6 V, 3 cells in Parallel = 135 Ah, 100 set in series = 360 V
  • Forced Air Cooling
  • Endurance - 15 min
  • Max Velocity - 79 kt
  • System Efficiency - 79 % (Cruise), 91 % (Max Power efficiency)
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