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Tuesday, November 23, 2010

Silent flight: Sikorsky's electric helicopter

A team at Sikorsky Innovation, the technology development arm of Sikorsky Aircraft, is attempting to develop the world’s first all-electric, near-silent helicopter in a programme known as ’Project Firefly’. The team’s plan is to validate the benefits of the electric rotorcraft in flight while addressing some of the challenges of making an environmentally friendly helicopter both commercially and technically viable.

’We’ve had electric cars, electric buses and electric street sweepers for a while now, but we really want to understand the unique difficulties associated with integrating electric propulsion into rotorcraft,’ said Jonathan Hartman, a programme manager for Project Firefly. ’In an ideal world, we would want an electric helicopter to compete on a one-to-one basis with your internal combustion aircraft today.’

Despite advances in electric powertrains for automotive applications, progress in aviation has been slow. The industry faces a very different set of challenges due to its high power requirements and Hartman believes that technology development has to begin now

’You can either wait until your technology is fully commercially ready and then just integrate it into your product, or you can get out there ahead of time and start learning about all these issues that will come up during construction. That’s what we’re doing.’

The Firefly team has retrofitted a 50-year-old military S-300C design with an electric motor and digital controller, together weighing in at 180lb. Much of the S-300C’s original design has remained the same to keep costs down, with the main difference being the addition of an interactive LCD monitor in the cockpit. Two battery packs, each made up of 150 individual 45amp-hour (Ah) lithium-ion cells, are the backbone of the energy-storage system. They sit outside the aircraft much like an agricultural spraying kit, and run at 370V to provide enough power for a continuous 15-minute flight.

’That’s obviously not a lot of time,’ said Hartman. ’There is one technology that needs to mature to make this a commercial reality and that is energy-storage distribution. There are projects out there that we are watching. If some of the technologies that are on the bench now can come out and provide the energy-surge capacity they are claiming, we could well see helicopter endurances in the half hour to the hour range instead of just 15 minutes, which is exciting.’

Sikorsky claims it has increased the propulsion efficiency of the helicopter by around 300 per cent from baseline. The use of an electric motor reduces the complexity of the helicopter, cutting down on the number of components and reducing maintenance costs. There is also a reduction in vibrations and associated acoustics with electric propulsion, which could one day lead to a new series of low-observable, optionally manned aircraft for both military and civil applications.

The benefits, however, sit against a long list of challenges. The motor must be air cooled rather than liquid cooled, and has to remain at operating temperature while sitting on the ground. Sikorsky presented this problem to US Hybrid, which came up with an adaptation of a motor it uses for surface-vehicle programmes, such as the electric-powered Humvee for the US Marine Corps. The group provided 40 per cent more power and added a number of control laws to allow the motor to run at a constant high rpm.

While a conventional motor requires time to engage and build up torque, an electric motor can do this straight away so further control laws were added to prevent pilots from taking off and landing too quickly. Feedback from pilots also showed that the lack of noise could affect handling. ’Our toughest challenge was on the human factor side,’ said Hartman.’We have this electric helicopter that produces substantially less noise than a traditional helicopter. All of the pilot cueing is basically gone and that presents some unique difficulties that a pilot never had to worry about before.’

To address this, integrated sensors were added that feed real-time aircraft health information to the pilot through an interactive LCD monitor. Monitoring of health data is done automatically through the flight-control computer and any potentially hazardous conditions are avoided with automatic system re-routing or electric shutdown. As well as providing information to the pilot, the system tracks critical stages of flight in real time allowing the team to rapidly prototype new ideas as it goes through the development programme.

The issue that poses the greatest challenge, however, is weight. ’The helicopter itself weighs empty several hundred pounds more than it did in its legacy configuration,’ said Hartman. ’What we did in the project was take the maximum weight of the aircraft, subtract out the motor, the weight of the cockpit, control and the pilot and fill the rest of it up with batteries. They are not the most weight-efficient batteries that are on the market today, but we went for reliability over weight and selected safe components that we knew would work… If a bus loses charge that’s one thing, but if a helicopter suddenly loses charge then you have a problem.’

Hartman is confident that with improvements in battery technology, weight will be reduced and reliability improved significantly allowing electric helicopters to compete with conventional aircraft. Hartman added that in the shorter term electric helicopters could have some interesting applications using endurances of between half-an-hour and an hour. ’My director has a fantastic vision. He would love to see a helicopter fly on a two-seat tour of the Grand Canyon. You and the pilot, no shaking, you don’t even have to wear headphones - you just enjoy the view. You can imagine something similar for applications in urban transportation.’

The Firefly project is undergoing component bench testing and is scheduled to move to ground testing and first flight early next year. It seems inevitable that at some point full-scale electric rotorcraft will be seen in our skies, but just when that will happen is anyone’s guess. ’I would need a crystal ball to tell you,’ said Hartman. ’But if the trajectory continues as it is, the next five to 10 years are going to be very interesting.’

The specs

Airframe: converted S-300C
System efficiency: 89 per cent during cruise, 91 per cent maximum power efficiency
Power: 190hp permanent magnet electric motor
Endurance: 15min
Lithium-ion battery: 150, 45Ah lithium-ion cells
Maximum velocity: 79knots

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