Airbus: Hybrid-Power Helicopters Not Practical Yet
Jul 12, 2016
Graham Warwick | Aviation Week & Space Technology
Hybrid-electric propulsion could improve the safety of helicopters, but to become practical it demands even more dramatic improvements in energy storage than required for fixed-wing applications because of the need for light weight.
That is the conclusion of studies conducted by
Airbus Helicopters, which has already test-flown a single-turbine AS350/H125 with a backup battery-powered electric motor to assist with autorotation after engine failure.
The 2011 test put AHC ahead of other rotorcraft manufacturers also exploring hybrid propulsion. But the studies, presented at AHS International’s Forum 72 convention in May, concluded that an emergency electric power source to increase safety was not ready for use because of the additional cost and weight.
HOW HYBRID POWER COULD BE USED—AND WHAT AIRBUS HELICOPTERS THINKS
Help turbine meet 30-sec. one-engine inoperative rating—Feasible now
Provide autorotation assistance if turbine fails—Payload penalty too high
Provide emergency power if engine fails in a twin—Cost too high
Enable all-electric variable-rpm tail rotor—Too heavy
Full hybrid and full electric propulsion—Batteries nowhere near ready
Airbus studied architectures ranging from mild hybridization to back up the turbine powerplant to a full electric-powered helicopter, but concluded: “Very significant progress is still needed . . . especially for the storage device, the battery being the best fit but still far away from the weight/power ratio required for the helicopter, which is the most demanding aircraft in terms of lightness.”
Airbus is not the first to study electric propulsion. In 2010,
Sikorsky modified an S-300C light helicopter as the all-electric Firefly, replacing the piston engine with a 140-kW (190-hp) electric motor and mounting lithium-ion battery packs in external panniers. But the aircraft was never flown.
Under Project Zero,
AgustaWestland, now Leonardo’s helicopter division, in 2012 flew an unmanned, all-electric, vertical-takeoff-and-landing (VTOL) aircraft with tilting ducted rotors. Germany’s e-Volo is flight testing a VTOL two-seat model with a fixed overhead array of 18 battery-powered propellers.
As tested on this A350, electric autorotation assistance kicked in after engine failure to slow rotor droop, then again to help the flare for landing. Credit: Airbus Helicopters
Bell Helicopter has begun studying hybrid propulsion, while
Boeing in February received a U.S. patent (9,248,908) for a hybrid electric helicopter in which a diesel engine-driven generator and batteries provide power to motors on the main and tail rotor to provide the safety of a twin-engine design.
Architectures studied by Airbus Helicopters include a microhybrid with up to 50 kW of electric power to provide transient assistance to the turbine engine. This would provide electric power to the turbine’s gas generator to boost output at the power turbine to meet the 30-sec. one-engine-inoperative rating. Such an application is possible with available technology, the study concludes.
A mild hybrid architecture, meanwhile, would provide up to 300 kW of electric power, either to the main gearbox for emergency power in the event of engine failure or the tail rotor to make it fully electric, says Christian Mercier, chief engineer, research and technology, for innovative power systems at Airbus Helicopters.
Providing 15-30 sec. of power after engine failure in a single-turbine light helicopter, an automatic electric backup system would reduce reaction time and rotor-speed droop, preventing too high a descent rate and improving controllability and safety in an autorotation landing.
“Available motors and power electronics, and one-shot-specific batteries, are close to enabling a complete [backup] system weighing less than 50 kg [110 lb.],” he says. But the development and recurring costs would not provide customer value “because of the loss in payload near to one passenger in the absence of a regulatory constraint that would impose it on all manufacturers.”
In a twin-turbine helicopter, an electric backup system could increase one-engine-inoperative takeoff performance where it is limited by the emergency power rating of the surviving engine, and the study says that Airbus tested such a system on a midsize helicopter demonstrator in 2015.
“The outcome can be a payload benefit equivalent to up to 2-3 passengers in takeoff performance on confined or ground helipad configurations,” the study says, but it concludes the recurring and high-development cost for such systems prevent their near-term application.
Airbus is also experimenting with idling or shutting down one engine in the cruise on twin-turbine helicopters, to reduce fuel consumption, and flight tested the concept in 2015 on its Bluecopter technology demonstrator, a modified EC145 light twin.
A mild hybrid could help in so-called super-idle or single-engine operations by providing the extra power required to sustain level flight on one turbine without a significant reduction in cruise speed. When the batteries are drained, the second engine would restart to sustain cruise and recharge them.
But this architecture would reduce fuel burn by only a few percent, increase system complexity, add about 250 kg in batteries, motor and electronics, and “is not promising yet,” Mercier concludes.
Several manufacturers are looking at potentially powering the anti-torque tail rotor electrically, to decouple it from main-rotor rpm and optimize the design for increased performance and reduced noise.
But Airbus’s study concludes that a redundant, variable-rpm tail-rotor drive system, at about 70 kg, incurs an unacceptable weight penalty roughly equivalent to one passenger. Motor weight will have to reduce by a factor of five before an electric tail-rotor can be feasible, the study finds.
The team also studied a full hybrid architecture for an H125 light helicopter, where the single turbine engine is reduced in size and operated to minimize its fuel burn, and batteries are used to meet peak power demand, such as on takeoff.
A modified EC145, the Bluecopter tested a range of technologies including an “eco mode” in which one engine is shut down during cruise. Credit: Airbus Helicopters
In this serial hybrid example, the downsized turbine drives a 500 kW-class generator that supplies electric power, along with that from batteries, to a 450-kW motor driving the main rotor at 350 rpm and a 70-kW motor driving the tail rotor at 4,200 rpm.
The fuel-burn reduction from running a smaller turbine at its optimum specific fuel consumption could be almost 10%, but losses in the electric system could approach 20%, the study says. And the electric system would weigh more than 300 kg.
Battery power density will have to increase by a factor of at least seven and, even then, downsizing the turbine will not be sufficient to offset the empty-weight penalty. “This architecture has no future with the electrical component characteristics currently forecast,” the study concludes.
While small electric multirotor unmanned aircraft that fly for short durations are now common, AHC determined all-electric propulsion for full-size rotorcraft is impractical because the power and energy required to fly a 3,000-kg helicopter for 2 hr. “are way too much with current battery technology.”
Because of volume constraints, current technology would provide only 10 min. of cruise flight and the batteries and motors would weigh roughly 1,000 kg. Battery energy density would have to improve by a factor of 14 for an all-electric helicopter to become practical, the study concludes.
The study also highlights that the electric motors, power electronics and batteries would need cooling, adding weight, unless used for only short durations, such as the emergency backup system. Battery system volume is also an issue in helicopters, where it could eat into baggage and passenger volumes.
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