New companies are emerging in the Silicon Valley region and are already positioning themselves to address the potential market for urban air transport, hoping to help shape its future. In a White Paper published in October 2016, the transport network company Uber identified a number of problem areas in relation to approvals, technology, operations and infrastructure that could be direct obstacles to the introduction of urban air taxis. Electric flight could provide solutions to some of these problems. Electrical propulsion could save energy and reduce noise and emissions, thus making it suitable for inner-city transport.
The first electrically-powered aircraft in general aviation
In recent years, air displays have showcased the first fully-electric fixed-wing aircraft for general aviation. Gliders such as the ASG 32 have been flying with battery-powered sustainer engines in the power range of 25 kilowatts since 2015 and are now available as series-production aircraft. The Alpha Electro has also been flying with a 60-kilowatt electric motor since 2015. Slovenian manufacturer Pipistrel, which manufactures ultralight training aircraft, sees itself as a pioneer of electric flight. In 2011, its all-electric technology demonstrator, the Taurus G4 with a power of 145 kilowatts, won the NASA Green Flight Challenge; the e-Genius, developed by the University of Stuttgart, came in second.
The two Green Flight Challenge contestants approached the problem of extending the range of their aircraft in different ways. While the Stuttgart-designed e-Genius was equipped with a Wankel engine in 2016, Pipistrel partnered with the fuel cell research group at the Stuttgart site of the German Aerospace Center (DLR). This group had developed, built and flown the first fully hydrogen fuel cell-powered electric glider, the Antares DLR-H2, in 2009. In 2016, the team presented the first flying hybrid propulsion system, comprising fuel cells and batteries, with the Taurus G4. The four-seater Hy4 prototype first took off in September 2016, just two weeks after the maiden hybrid flight of the e-Genius.
However, the trend towards electric aircraft is also bringing players from other sectors into the scene. Siemens has been testing what is currently the most powerful electric motor in aviation – at 260 kilowatts – in its Extra 330LE aerobatic aircraft since 2016. In 2011, the electrical engineering company teamed up with EADS and Diamond to develop the world’s first hybrid-electric aircraft, the DA36 E-Star. Since then, the motor’s power-to-weight ratio has increased fivefold. Siemens wants to achieve its self-imposed target of increasing this ratio by a factor of 10 in order to gain a foothold in the aviation industry.
Electric propulsion systems for passenger aircraft
With the E-Thrust (Airbus) and SUGAR Volt (Boeing) concept studies, the major aircraft manufacturers are already looking towards electrically powered aircraft in the Airbus A320 / Boeing 737 class. This long-term objective is set to shape aircraft development over the next 20 years. Both companies are committed to hybrid propulsion and therefore need to provide the necessary means for a gradual shift from smaller to larger systems. European projects are currently leading the way in this area.
In 2016, for example, the two European heavyweights Siemens and Airbus joined forces to create the E-Aircraft System House in Ottobrunn near Munich, with 200 employees. Hundreds of millions of euros will be invested in developing a hybrid-electric two-megawatt powertrain there for a future E-Fan X. In addition, both Airbus and Boeing are increasing investing in start-ups that are looking to revolutionise aviation with new ideas and rapid development.
Such large sums of money and an agile approach are exactly what is needed. This is because some fundamental questions about electric propulsion remain largely unresolved and require further research, development and, perhaps most importantly, experimentation.
Challenges for electric flight
Energy storage is an important consideration in the implementation of all- or hybrid-electric propulsion systems. Highly-efficient gliders can extend their cruising flight phase to over an hour using batteries alone. For less efficient configurations such as multicopters or motorised aircraft, the limit remains stubbornly stuck at 20 minutes of flight time – sufficient for only very short distances in all-electric flight. State-of-the-art batteries are currently much too heavy to be used as primary energy storage for heavier aircraft and longer ranges. For example, more than 50 tonnes of batteries would have to be installed in an all-electric A320 for a 20-minute flight. That would be approximately two-thirds of the maximum take-off weight, which rules it out as an option for commercial aviation.
For this reason, research is being conducted into hybrid solutions, as is the case in the car industry. Hybrid-electric concepts combine the flexibility of batteries with the energy density of chemical energy sources. The basic idea is to cover the continuous power required for cruising flight using high-energy fuels (for example, kerosene) while covering power peaks for take-off and manoeuvres with buffer batteries. As with electric cars, this increases the range. There are currently three different approaches to implementing this kind of propulsion system. In a serial-hybrid architecture, a gas turbine feeds its entire output into the electrical system. The resulting power is used to drive one or more propellers with electric motors. In a partially-hybridised aircraft, the electrical power is taken directly from an existing conventional engine. In a parallel-hybrid architecture, the electric motor drives the propeller in parallel with the conventional engine.
The architecture of a specific hybrid-electric powertrain is able to combine these three basic components as required, in any degree of complexity. Various types of energy converters are also possible for transforming fuels into electrical power. These could be, for example, gas turbines and piston engines with generators, or even fuel cells. Hydrogen fuel cells in particular promise extremely low emissions consisting only of water vapour; this would make aviation as low-emission as possible. Strictly speaking, as in the case of charging the batteries electrically, the manufacturing process and the resulting emissions also have to be taken into account. However, these concepts also allow energy to be saved using conventional power sources. Less fuel is required if the combustion process always runs at its optimal operating point and does not have to cope with variable loads.
Advantages and disadvantages of hybrid propulsion
One important – if not the most important – advantage of electrical aircraft propulsion is the new possibilities it affords for overall aircraft design. As electric motors are lighter and more economical, they can be installed in places that would be completely unsuitable for conventional propulsion systems. This makes it possible to implement new configurations with distributed, highly integrated or even boundary layer ingesting units. NASA’s X-57 demonstrator shows how this might look. This technology testing vehicle is to be powered by 14 electric engines arranged along the wings. The advantages of such configurations range from more efficient airflow around the wing to reduced drag and new control options. For instance, propellers at the tips of the wings could reduce the strength of wake vortices and assist the rudder. The versatile dynamics of electric motors also provide an additional level of freedom. For example, a multicopter can function without any adjustable parts and can be controlled just by varying the speeds of the motors, making the overall system far more straightforward than the complicated transmission systems used in current helicopters.
A hybrid-electric propulsion concept also results in significantly reduced noise emissions. Noisy components such as gas turbines can be installed in better-insulated areas of the aircraft and the turbulent exhaust gas stream can also be minimised and better shielded. In addition, propeller noise can be reduced by exploiting the high torque of an electric motor to drive the propeller at much lower rotational speeds.
One major disadvantage of a hybrid-electric system is its complexity and weight, which are both much higher than a conventional propulsion system. With its extra components, the new architecture also competes with mature gas turbine technology, and it must be operated under optimum conditions in order to achieve the desired efficiency advantages. One important aspect is the waste heat from the electrical components. When electrical energy is generated and transferred, some of it is always lost as heat. The lighter and longer the cable, the greater this power loss becomes. If even one percent of the power is lost as heat, this can result in an energy loss of 400 kilowatts for an A320-class aircraft. This heat must be dissipated in a useful way, and ideally used for a process such as air conditioning. These new propulsion systems also impact other on-board systems. The technical challenges are compounded by uncertainty surrounding certification of the systems. This gives rise to worries, but also opportunities, as the authorities are already thinking about making future certification procedures much more flexible than has hitherto been the case.
Looking to the future
Overall, the view is that the introduction of hybrid-electric propulsion systems could result in energy savings of approximately 20 percent. However, the environmental benefits and increase in efficiency due to electric propulsion only become apparent in the details of the design and are heavily dependent on the specific application. For a fair assessment of the complete lifecycle, it is necessary to consider the entire chain – from the production of fuel to the complete air transport system – in addition to the propulsion system. Some basic technological issues have yet to be resolved.
Addressing these challenges requires a coordinated approach across all technical disciplines and the entire product range. The technological landscape in Germany means that it is perfectly positioned to address this challenge. The country is home to both large industrial companies that are willing to invest in hybrid-electric aviation products and universities that are at the cutting edge of research across all subject areas. Special synergies are also set to emerge in relation to other future-oriented areas such as electromobility on land and at sea, and renewable energy – fields in which Germany and Europe as a whole are leading the way. It will be the task of politicians and interest groups to identify these cross-connections and stimulate exchange.
In particular, fast and flexible testing capabilities for simulations, and on the ground and in the air will be needed if the big step from the small electric aircraft of today to electric commercial aircraft is to be made. Independent large-scale research institutions such as DLR will undoubtedly play an integral role in this process. In any event, major developments are expected in the near future. Exciting times indeed!