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Electric aircraft propulsion offers quiet operation but big challenges

Electrification is having profound impacts on the automotive industry. Electric aircraft may have a similar impact on the aerospace sector.

Attracted by the low weight and cost of electric motors compared with gas turbine jet engines, and particularly by their near-silent operation and potential for zero emissions, the civil aviation sector is showing increasing interest in using electricity to power future flight, at least over the relatively short distances that dominate city-to-city aviation.

While jets are unassailable as the prime movers for intercontinental long-haul airliners — nobody’s suggesting that electric aircraft could transport passengers from Europe to Asia, across the Atlantic or even between the East and West coasts of the US — the short-haul routes between both European and US cities, currently plied mostly by twin-engined single-aisle aircraft, are the targets for a new generation of radically different aircraft whose development is being fostered by the aerospace majors. These machines could take the sector into areas of science and technology very different from those it is used to.

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Airbus aspires to develop electric short-haul airlines

Of the two biggest players in civil aviation, Airbus is taking the more active role in this initiative by developing its own aircraft, while Boeing has invested in a start-up company that is conducting the process. Airbus’s plans have undergone considerable change, however.

The last time The Engineer covered this subject in detail was in an interview with the then-chief technology officer of Airbus, Jean Botti, in late 2015. Electric aircraft were a major focus of Botti’s tenure as CTO but he left the company in April 2016. His successor, while still attaching considerable importance to the project, has proved to have different ideas about how it should be conducted.

Paul Eremenko joined Airbus in 2015 to launch and run its Silicon Valley-based innovation offshoot, A3; he took over the CTO’s position in June 2016. An aeronautics and astronautics graduate, he has worked at DARPA — where he headed the office responsible for drones, satellites, robotics and X-planes — and at Motorola and Google, where he initiated development of modular, customisable smartphones. In a lecture at Purdue University in 2014, he stated that his career had been motivated partly by a desire to build a starship, and while at DARPA he was involved in a study to define an organisation that might meet such a goal.

On moving to Airbus, one of Eremenko’s first actions was to revamp the E-Fan electric aircraft project. The first E-Fan, a two-seater with two 30kW electric motors, flew in 2014, crossing the English Channel as a symbolic feat. Airbus’s original plan was to develop this light aircraft further: a production version was slated for 2017 and marketed as a pilot trainer; and a four-seat version with a hybrid powertrain was due to fly in 2019, in which a kerosene-fuelled engine would act as a range extender, operating a generator to charge the batteries and boosting endurance to around three-and-a-half hours.

A hybrid configuration – E-Fan Plus, with a 68hp (50kW) two-stroke combustion engine from German manufacturer Solo Aircraft Systems, capable of 30 minutes’ battery endurance and up to two hours 15 minutes in hybrid mode – was developed and flown in 2016, but subsequent plans for the electric aircraft project changed.

“To put it simply, we decided that this plan was just not ambitious enough,” Glenn Llewellyn, Airbus general manager for electrification, told The Engineer.

Electric aircraft development has continued under Siemens, which in 2016 formed a partnership with Airbus after co-sponsoring – but not providing equipment for – E-Fan. Siemens supplied the 50kg, 260kW motor for an Extra 330LE aerobatic aircraft that first took to the air in July 2016. An order of magnitude more powerful than the E-Fan, this aircraft spurred Airbus to abandon its roadmap of gradual power increases from the kW to the MW range, as Botti had envisaged, and instead look to progress directly to a 2MW demonstrator, as a stepping stone to a hybrid-drive aircraft that could match current single-aisle, short-haul aircraft in terms of range and passenger capacity, and operate from conventional airports. Such an aircraft would likely be another order of magnitude more powerful than the 2MW demonstrator, Llewellyn said.

“We might be able to do that in a single step; we might not,” he told The Engineer. “We expect to encounter all sorts of challenges in developing the 2MW aircraft; we just don’t know what all of those might be yet. It’s those ‘unknown unknowns’ that make it difficult to predict how we’d take the step after that.”

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Artist’s impression of City Airbus aircraft

The 2MW option has its own goals as a basis for urban aerial mobility vehicles. This sector – currently mostly speculative, of course – was one that Airbus had barely targeted before Eremenko’s appointment but, at the Geneva motor show in March this year, the company unveiled the result of a collaboration with Italian automotive development house Italdesign: a modular concept vehicle called Pop.Up. This consisted of a two-seater passenger capsule that could be linked to an electric ground module (essentially a four-wheeled chassis) or an octocopter air module.

Airbus’s ambitions in this sector are likely to favour a larger-capacity vehicle, Llewellyn indicated, and not one that could operate in either road or air mode but one that would be able to take off and land from ‘vertiports’ within cities. “Electric propulsion is ideally suited to urban environments because it’s so quiet, which is important when you want to fly near occupied buildings and at low altitude. And, when you’re in pure electric mode, operating off batteries, you’re producing no emissions and are not affecting air quality,” he said.

However, Llewellyn would not rule out such vehicles also having a hybrid capability. “It gives us greater flexibility and, of course, with a motor and generator complementing the electric system you can operate in a different mode. A gas turbine acting as a range extender wouldn’t have to handle all the varying loads you encounter during flight – the electric propulsion system would deal with those – so you can optimise its operation so that it runs at maximum efficiency.”

Such hybridisation could be with a hydrogen fuel cell or a gas turbine, or even a diesel engine, coupled to a generator, Llewellyn added. “Even with existing technologies, a hybrid could have significant range.”

Another factor in favour of hybrids crops up regularly with electric aircraft: much of the technology required to enable the type of aircraft that industry would like to fly is simply not yet advanced enough.

“The power-to-weight ratios for battery technology are still a long way short of what is required,” Llewellyn said. “Current batteries would be far too heavy as the only source of energy to feasibly fly a large passenger aircraft. This is where hybrid solutions, even at low levels, can be of interest. That said, battery research perhaps has the most investment of any technology in the world right now, so we need to prepare ourselves for a future in which this is possible.”

Pop.Up is not Airbus’s only urban aerial mobility concept. At a presentation to media earlier this year, Eremenko described four other electric technology demonstrator projects: CityAirbus, a multi-propeller vehicle to carry up to four passengers, intended to be autonomous once regulations are in place for such aircraft, which is being developed by Airbus Helicopters and targeted for first flight by 2018; Skyways, a collaboration with the University of Singapore for delivery of small packages by drones, to be tested initially around the university; Vahana, a single-passenger self-piloted VTOL vehicle being developed by A3; and a less tangible A3 project, Voom, an on-demand shared helicopter booking app for megacity dwellers (with applications beyond only electric aircraft).

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Artist’s impression of Airbus Vahana

One thing that hasn’t changed from Botti’s vision is the design philosophy Airbus wants to apply to its larger electric aircraft. Rather than simply ‘bolting on’ engines to the wings or fuselage, it would integrate electric motors into the airframe at multiple points within aerodynamic housings.

“One advantage that electric motors have over gas turbine engines is that it’s relatively cheap, in terms of both cost and weight, to add more of them,” Llewellyn said. “But using integration we can explore things like boundary layer ingestion, which could be another way to recover energy.”

Boundary layer ingestion uses the air immediately around the aircraft to feed the fans that accelerate air to generate thrust; it reduces drag and, in theory, means that the engines do not have to work so hard, both of which would reduce fuel consumption (or, in the case of electric motors, their demand for electricity). However, it is not a simple matter because boundary layer airflow is distorted, so fans need to be designed to take account of this.

Aerospace technology is often a matter of trade-offs – for example, material strength

is always balanced against lightness – and electrification is no exception. Heat management is often a sticking point, Llewellyn said, with more powerful motors’ components running hotter and therefore requiring more equipment to handle the heat load, so power output must be balanced against the amount of equipment that can be carried. This may also limit the use of another technology whose potential is attractive for electric aviation: superconductivity.

This is another area where technology has not developed to the point of usefulness. “Even the highest-temperature superconductors need to be cooled to about 70° above absolute zero,” Llewellyn said, “so, although you’d gain from the very efficient, low-loss transmission of energy in a superconducting system, you’d have to balance that against the extra mass of all that cooling equipment, plus you’d have to handle the heat extracted from the system.”

Development of higher-temperature superconductors could be a sector of fundamental research where the aerospace industry might want to inject funds, he speculated, although these might be limited to projects directly relevant to aviation.

Discover more about how firms and organisations are addressing the potential of electric aircraft propulsion