TRANSPORT ELECTRIC AIRCRAFT 77
E-Fan displays at
AIR POWER
the Farnborough
International
Airshow
THE RISE OF
ELECTRIC
AIRCRAFT
Europe appears to be leading the charge for
electric aircraft propulsion, but a number of
technological hurdles remain.
By Mark Williamson
ELECTRICALLY-PROPELLED cars are experimental planes covered in solar cells, Airbus Group’s E-Fan, which first flew in
becoming more common on our streets, like Pathfinder or Helios developed under prototype form in March 2014, aims to change
but what are the prospects for electric Nasa’s Environmental Research Aircraft that impression by kick-starting a revolution
aircraft? Can you imagine boarding a technology programmes of the late 20th in aircraft propulsion.
Boeing 737 for your summer holiday century. A more recent version is the Swiss
without the smell of aviation fuel and ‘Solar Impulse’, currently being prepared E-Fan
the roar of jet engines? If the engineers for a world circumnavigation flight in An aircraft the size of E-Fan – less than 7m
of Airbus Group and its partners have 2015. It has a wingspan of 70m with 17,000 in length with a 9.5m wingspan – is easy to
their way, this is exactly what you will solar cells – and carries a crew of one. overlook amidst the organised chaos of the
be able to do in another 20 years or so. Whichever way you spin it, these vehicles Farnborough International Airshow, but
AIRBUS
Today, any mention of electric are a long way from being what most people even an aircraft novice would recognise it
aircraft tends to stimulate thoughts of would regard as practical aircraft. as a ‘proper aircraft’. It has a streamlined >
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78 TRANSPORT ELECTRIC AIRCRAFT
< canopy covering two in-line seats, and
two ducted-fan engines, one on each side
of the fuselage between the wings and a
high tail. Indeed, a casual observer might
assume the existence of fuel tanks inside
the wings with a system of pipes and pumps
to supply the engines. Instead, however, the
inboard wing sections house a set of 250V
lithium-ion polymer batteries which power
the E-Fan’s 30kW engines. The point is, it’s
a conventional-looking but electrically-
powered aircraft without a solar cell in sight. On-board camera
view from E-Fan
But what about range, that perennial as it displays at
question when it comes to electric the Farnborough
International Airshow
vehicles? The project team expects the
prototype to deliver an ‘endurance’ figure
of about 40 minutes and, as of July, the kerosene-powered generator that will charge market. “The E-Thrust concept is a first
50-hour test programme had reached the batteries in flight – adding an extra one design proposal for how a future hybrid
37 minutes. Assuming the 40-minute goal and a half hours to its range. Moreover, with regional aircraft could look,” he adds, in
is realised, the E-Fan’s 160km/h cruise a cruising speed of around 260km/h (100 reference to a futuristic-looking design
speed and 220km/h maximum might be more than the 2.0), this would make London visualisation known as eConcept.
expected to produce a maximum range to Edinburgh, Paris or Cologne a practical As the electrical propulsion element,
of around 100km (with a fair wind!). In possibility. In addition, according to Airbus, E-Thrust itself is part of a Distributed
other words, it could fly from London to the innovation of electric propulsion means Electrical Aerospace Propulsion (DEAP)
Brighton or from Paris to Rouen. So, even that there is “no reduction of performance at programme, which is co-funded by the UK’s
though the E-Fan is simply – in Airbus altitude and in hot weather, no propeller Technology Strategy Board. Airbus Group,
terminology – a ‘technology demonstrator torque effects and no vibration, providing a along with partners Rolls-Royce and
for an electrically-powered general aviation very smooth flight”. Cranfield University, has been engaged in
training aircraft’, it could actually transport Clearly, batteries are a crucial component DEAP since 2012, researching key
a couple of people over a useful distance. in an all-electric aircraft and the battery technologies to improve fuel economy and
The idea for the project originated at the system will undergo continual development. reduce exhaust-gas emissions using a
2011 Paris Airshow as a follow-on to the New, higher energy-density batteries will distributed propulsion system architecture.
Cri-Cri, the world’s first fully electric replace the off-the-shelf items used in the What this means in practice is designing a
aerobatic plane. Using the Cri-Cri as a flying prototype, along with the introduction of a system that distributes the electrical power
testbed, engineers gained experience with ‘quick-change system’ as an alternative to generated by a single advanced aero engine
battery integration and energy management, recharging. to a number of electrically-powered ducted
while researchers concentrated on aspects The E-Fan variants will be produced by a fans (effectively big-brother versions of the
such as energy recovery and variable new Airbus subsidiary, VoltAir, in a new E-Fan engines).
propeller pitch. factory near Bordeaux’s Merignac airport. A model on display at the Farnborough
Since then, the E-Fan programme appears According to Müller-Wiesner, the first flight Airshow featured a centrally-mounted
to have been fast-tracked from its official of the 2.0 is expected by the end of 2017, with generator-engine at the rear of the vehicle
go-ahead in October 2012 to its public the launch of the 4.0 two years after that. feeding two groups of three electric fans
(ground-based) presentation in June 2013 and Within ten years, Airbus expects to be mounted in pods above the wings. According
its first test-flight in March 2014. Its display making, and selling, 80 aircraft a year. to Nicolas Fouquet, research team leader in
flight at this year’s Farnborough Airshow, This is far from the end of the story, the Power & Energy Management group of
while impressive enough, was somewhat however, simply the commercial leading edge Airbus UK, the engineering elegance of the
understated because of its diminutive size of a much larger technology programme that system lies in the separation of power
and lack of engine roar. Indeed, one Airbus could change the way we all fly. generating and propulsive elements: it allows
executive playfully likened the aircraft to a one to “separate the optimisation of the
“flying hairdryer”. E-Thrust thermal efficiency of the generating unit
Interestingly, although E-Fan is currently One of the interesting aspects of the from the propulsive efficiency of the fans”,
only a prototype, the company intends to Airbus R&D plan is that, while actually he explained. The hybrid concept allows the
industrialise and market the aircraft in two producing saleable electric aircraft in the generator to be optimised, and thus
versions: a two-seater (side-by-side as short term, it incorporates a vision that downsized, for the cruise portion of the
opposed to in-line) and a four-seater. supports the European Commission’s flight, he adds, because “the additional power
According to Detlef Müller-Wiesner, long-term environmental protection goals required for take-off can be provided by
Airbus senior vice president and head of as set out in its roadmap report ‘Flightpath on-board batteries charged on the ground”.
E-Aircraft Programme, flying clubs are a key 2050 – Europe’s Vision for Aviation’. As In fact, the system appears beneficial for
sales target for the E-Fan 2.0, which is Müller-Wiesner confirms, “these goals, all flight phases in terms of fuel efficiency,
designed as a two-seat training aircraft “at a which we are committed to fulfil, include a 75 noise reduction and safety. Using a smaller
price comparable with similar-sized per cent reduction of aircraft CO2 emissions, generator-engine combined with electrical
piston-engine aircraft” but with operating a 90 per cent reduction of nitrogen oxides storage for take-off and climb reduces noise
costs of about a third. Moreover, the and a 65 per cent reduction in noise levels – and pollution in the vicinity of the airport
signature quietness of the electric engines all compared to standards of the year 2000”. while, in the cruise phase, the generator
would score in terms of noise pollution and The current stage of the company’s provides power to propel the aircraft and
other local environmental issues. With an long-term R&D programme is called recharge the batteries. In the initial descent
expected one-hour endurance, and a E-Thrust and aims to develop an ‘electrically phase, the aircraft becomes a glider with
15-minute reserve, Airbus expects the distributed propulsion system concept’ for a on-board power provided by the batteries
two-seater to be used for anything from regional class of aircraft (approximately 90 and, later on, the electric fans act as
glider-towing to aerobatics. seats and two hours flight-time). According windmill-generators to top up the battery
The E-Fan 4.0, meanwhile, is base-lined for to Müller-Wiesner, this aircraft class covers system (akin to regenerative braking in a
an endurance of two hours, but will be more than 90 per cent of passenger miles car). Finally, for landing, the generator-
equipped with a ‘range extender’ – a flown, so it represents a huge potential engine is restarted as a safety back-up and
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79
the system operates in dual-source mode as it Finally, the concept of boundary layer
does on take-off. All taxiing could be ‘As with the E-Fan, or any other ingestion allows the use of airflow that, with
conducted entirely from electrical storage, electric vehicle, the challenge conventional under-wing engines, is
making airport aprons much quieter than of energy storage is a given. effectively wasted. Aerodynamics defines the
they are today.
That said, Airbus expects new boundary layer as the part of the airflow
close to the wing; in the E-Thrust concept,
Technology systems to “more than double the six electrically-driven fans are mounted
Fouquet explained that this innovation today’s best performance”.’ on top of the wing where they intercept, or
in aircraft design will involve “the ingest, this boundary-layer flow. The engine
successful implementation of three itself is called a wake re-energising fan,
key enabling technologies: energy propulsion system and the need to reduce the because it captures part of the wake
storage, superconductivity and resistance of conductors. In practice, this normally generated by an aircraft in flight
boundary layer ingestion”. means cooling the wires to very low (that would otherwise result in drag) and
As with the E-Fan, or any other electric temperatures, either using cryogenic fluids uses it to improve propulsive efficiency.
vehicle come to that, the engineering or a cryocooler (a mechanical heat exchanger
challenge of energy storage is a given. That based on the Stirling engine). Although Competition?
said, Airbus expects new systems to “more superconductors are used today in MRI It’s not yet clear how Airbus’s chief
than double today’s best performance” and scanners and Stirling coolers have been commercial aircraft competitor, Boeing,
believes that lithium-air batteries currently designed for spacecraft, these are not off-the- will respond to these developments.
under development represent the most shelf items readily applicable to aircraft Boeing built and flew a light aircraft
promising solution for E-Thrust. Lithium-air power distribution systems, which is why powered by a fuel cell in 2008 and has been
batteries have a higher energy density than this is one of the key ‘enabling technologies’ involved in research with Nasa on hybrid-
lithium-ion units – more than 1000Wh/kg is for E-Thrust. electric aircraft, which in 2012 produced its
predicted – because of a lighter cathode and One could not help but be impressed by a Sugar Volt concept. The Sugar (Subsonic
the ability to take oxygen freely from the demonstration item in the Airbus pavilion at Ultra Green Aircraft Research) team’s report
environment. As with any such programme, Farnborough: a levitating, briefcase-sized concluded that hybrid-electric engine
there is a degree of crystal-ball gazing object incorporating a superconducting technology “is a clear winner, because it can
involved, but there is time to develop the magnet floating a centimetre or so above a potentially improve performance relative to
technology and Airbus believes that the desk. According to Fouquet, it required a all of the Nasa goals”, but that’s about as far
required energy density can be achieved daily cryogenic top-up to maintain its as it went, pending improvements in battery
“within the 25-year timeframe” of the apparently effortless hover. Arguably more technology. Instead, Boeing appears to be
Distributed Propulsion programme. important was the display of cables it concentrating its R&D efforts on other
The second enabling technology is, if carried, which compared a large cross- emission-reduction concepts applied, for
anything, even more challenging: the section copper conductor with its flimsy instance, to its 787 Dreamliner.
development of a superconducting equivalent in superconducting wire. The For the moment, the momentum seems to
distribution network. The requirement for weight saving alone would probably account be with Airbus and its partners. That said,
this stems from the high voltage and for the difference between carrying 100 there’s still a long way to go on the road for
megawatt power range of the E-Thrust passengers or just the pilot! electric aircraft development. *
Airbus eConcept
airliner
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