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