E-Fan cross-Channel flight could herald a green future for hybrid-electric passenger aircraft
The Airbus E-Thrust concept aircraft
Thu 23 Jul 2015 – Following in the steps of Louis Blériot’s pioneering cross-Channel flight 106 years ago, the recent flight of Airbus Group’s all-electric E-Fan technology demonstrator aircraft in the opposite direction – between Lydd Airport on England’s south coast and Calais Airport – could herald a new era in green aviation travel. The E-Fan is a key element in the European aircraft manufacturer’s electric aircraft roadmap towards achieving emission-free and almost noiseless flight and is targeting advanced technological breakthroughs that could one day bring hybrid-electric propulsion to passenger aircraft. While Airbus is now working towards commercialisation of two-seater and four-seater versions of the E-Fan, it is also collaborating in a long-term project with Rolls-Royce on a completely new regional aircraft design incorporating a radically more efficient electrical distributed propulsion system that would result in significantly lower fuel consumption, fewer emissions and less noise.
“E-Fan is a crucial step on Airbus Group’s journey towards all-electric aviation,” commented Detlef Müller-Wiesner, Head of E-Aircraft Programmes, on the cross-Channel flight. “Our initial steps today potentially are leading to giant leaps forward in the future.”
With two seats and a weight of just 600kg, the E-Fan 1.0 is equipped with two electric motors that provide a combined power of 60 kilowatts, each driving a ducted, variable pitch fan. Airbus is investing €20 million ($22m) in E-Fan 2.0, a two-seater aimed at pilot training with a first flight planned for 2017. When it enters service, which is scheduled for 2018, it will be the world’s first all-electric plane certified to international civil airworthiness standards, claims Airbus.
This will be followed in 2019 by the E-Fan 4.0 four-seat airplane for full pilot licensing and the general aviation market. It will include an internal combustion engine that serves as a ‘range extender’ by recharging the batteries during longer flights. Construction on a final assembly line to industrialise both versions is due to begin next year in Pau, France, with production initially targeted at around 10 aircraft annually.
Airbus estimates the operating costs of both versions to be around one-third of traditional piston-engine light aircraft. A ground-based charging unit will be able to bring the aircraft batteries to their full flight endurance in 1.5 hours.
E-Fan is just one activity in Airbus Group’s short, medium and long term development of electric planes and e-aircraft technology, which is being led by the E-Aircraft System House based near Munich. It is responsible for the design and verification of electric and hybrid propulsion system architectures, harmonising research activities across the company and setting Group-wide project targets.
As part of plans for a regional aircraft with hybrid-electric propulsion, Airbus is looking to build a 1MW hybrid ground demonstrator in 2016, to be followed at a later date by a 2-6MW version. Airbus EVP Engineering Charles Champion foresees a hybrid-electric regional aircraft within the 2030 timeframe that would have a 25% lower fuel burn on a typical 300 nautical mile mission.
The E-Thrust regional airliner concept is being developed by research and technology arm Airbus Group Innovations and Rolls-Royce, with the UK’s Cranfield University as a partner. With a 2050 timeframe, the Distributed Electrical Aerospace Propulsion (DEAP) project, which started in 2012, is exploring innovative technologies that will enable improved fuel economy and reduced exhaust gas and noise emissions.
Compared to engines on existing commercial airliners, a distributed propulsion system will require a much higher level of integration with the airframe design than that of today’s aircraft. The concept involves a number of electrically-powered fans – six are being considered as a starting point – distributed in clusters along the wing span, with one advanced gas power unit providing the electrical power for the fans and for re-charging of the energy storage system. Initial results show that a single large gas power unit has advantages over two or more smaller units, providing better overall noise reduction and allowing filtering of particles in the long exhaust duct at the back of the engine. As well as allowing for a more aerodynamic overall design, having a number of smaller fans integrated in the airframe instead of large wing-mounted turbofans is also expected to reduce the total propulsion system noise.
To optimise propulsive efficiency requires a fundamental increase in the bypass ratio beyond values of 12:1 achieved by today’s most efficient turbofans to over 20:1, which would lead to significant reductions in fuel consumption and emissions.
During the aircraft’s take-off and climb, power would come from the gas power unit and the energy storage system, the latter being sized to ensure safety should the gas power unit fail during this phase. In the cruise phase, the gas power unit would provide the cruise power and the power to recharge the energy storage system. In the initial descent phase, no power is provided to the fans and the gas power unit switched off so the aircraft is effectively gliding, with the energy storage system providing the power for the aircraft’s on-board systems. The fans would then windmill in the second phase of the descent and produce electrical power to top-up the energy storage system. For the landing phase, the gas power unit is re-started to provide power at a low level for the propulsion system. This is a safety feature to cover a hypothetical loss of power from the energy storage system.
For the distributed propulsion concept to work, step changes are required in enabling technologies such as energy storage and superconductivity.
Airbus expects new generations of energy storage systems to exceed energy densities of 1,000 watt hours per kg within the next two decades, more than doubling today’s best performance. Although still under development and not yet commercially available, lithium-air batteries provide the best solution for E-Thrust’s energy storage requirements, as they have a higher energy density than lithium-ion batteries, which are fitted to the E-Fan.
For the power levels in the megawatt range that are required in an electrical distributed propulsion network, a new high-voltage superconducting electrical system has to be designed and validated to meet stringent efficiency requirements when transferring electrical power from the gas power unit and energy storage to the fans. Superconductivity is a quantum mechanical phenomenon of exactly zero electrical resistance, which occurs in certain materials when they are cooled below a critical temperature – normally minus 245 degrees C – and allows the electrical system components to be much smaller, lighter and more efficient than conventional technology. The necessary cooling can be achieved either by supplying cryogenic fluids or by using a cryocooler, a technology used today in space applications and in MRI scanners.
Airbus sees the paradigm shift DEAP project as a vital element in achieving the EU’s ‘Flightpath 2050’ environmental targets of reducing aircraft CO2 emissions by 75%, along with NOx reductions of 90% and 65% lower noise levels by 2050, compared to standards in the year 2000.