The end of the jet age could be in sight. Innovative new electric aircraft are starting to find their way off the drawing board and onto runways, funded by startups, government agencies and the world’s biggest jet makers. They promise flights that are cleaner, quieter and safer than today’s jets, and with a fraction of their carbon footprint.
Earlier this month, Nasa announced that it would be building a high-speed research aircraft called Maxwell that would use electric motors to drive 14 propellers. The four-seater aircraft should be able to fly at speeds of up to 175mph (about as fast as many small aircraft), using a fifth of the energy of a normal private plane.
“Eventually, Nasa would love to replace airplanes like the Boeing 737 or Airbus A320 that represent a large fraction of civil aviation,” says Jack Langelaan, a professor of aerospace engineering at Penn State University. “A huge proportion of today’s emissions and fuel burn comes from these kind of planes.”
Commercial aviation already accounts for 2% of all man-made CO2 emissions, a figure that could rise to 22% (pdf) by 2050. Electric aircraft could slow this trend, or even roll it back.
For a start, electric motors are about twice as efficient as internal combustion or jet engines. “All you’re doing is running wires to motors,” says Carl Schaefer of Aurora Flight Sciences, an aerospace company developing an electric drone called Lightning Strike for the US military. “So instead of one large propeller, you have the freedom to place small motors anywhere on the aircraft.”
Maxwell will have 12 propellers on the wing, and two more mounted on its wingtips – each with its own motor. This configuration enables it to boost fuel efficiency even further – and adds safety. “Having one engine fail during takeoff when you’re using 14 is much less severe than having one fail when you only have three,” says Sean Clarke, one of Maxwell’s designers.
Aviation startups have also been quick to realise that electric motors can enable new vertical takeoff and landing (VTOL) aircraft to replace cars rather than planes. “Vertical takeoff is popular because if we’re going to use these vehicles for commuting, no one wants to land at an airport five miles away from work,” says Langelaan.
German startup Lilium’s two-seater needs a landing zone no larger than 15 metres square, while Joby’s S2, under development in California, tilts 12 rotors to take off vertically, then converts into a conventional plane. Joby claims the 200mph plane will use five times less energy, door to door, than a petrol-driven car.
The Volocopter, another German prototype, has 18 helicopter-style rotors, while Chinese firm EHang’s 184 aircraft resembles an oversized single-person drone. The ultimate aim of such aircraft is on-demand aviation, an Uber for aircraft that provides automated pilot-less aerial transport around, and between, congested cites.
EHang has already got permission to start testing its autonomous 184 aircraft in Nevada, although not with passengers on board.
“There are a lot of things we need to get sorted out to make this work,” says Langelaan. “Collision avoidance and traffic management are important. It’s a big sky but once a lot of these little airplanes start flying around, the sky will start looking an awful lot smaller.”
There are other technical challenges for electric aircraft to overcome too, not least the state of battery technology. Range anxiety takes on a whole new meaning when you’re worried about reaching the next runway. “Lithium ion batteries are not at the point where you have the power density to build an aircraft with long range,” says Schaefer. “You’re limited to about an hour’s flight.”
Aurora gets around this problem by using a hybrid system, where three one-megawatt generators, powered by jet fuel, drive the Lightning Strike’s electric motors.
A greener solution is to generate power as you need it, using photovoltaic cells. Solar Impulse 2 recently became the first aircraft powered solely by renewable energy to cross the Atlantic, as part of a round-the-world tour. But the Solar Impulse can carry only one person, has a top speed lower than most cars and costs millions of dollars to develop.
“Solar right now only works well for light airplanes with large wings,” says Langelaan. “As solar cell efficiencies get better, you’re going to see solar-powered airplanes with more traditional dimensions.”
The good news is that all the technologies behind electric planes, from batteries to solar cells to the motors themselves, are getting better, lighter and cheaper all the time. The large jet makers are paying attention. Boeing and Airbus have experimented with electric aircraft, and even Tesla boss Elon Musk has hinted that he is considering building an all-electric plane.
Just like today’s Tesla, Prius and Leaf cars, hybrid and electric planes will have much lower running costs than traditional propeller or jet aircraft. The maker of the solar-powered Sun Flyer electric plane says it uses just $1 of electricity for each hour of flying time, compared to $40/hour for a fossil-fuel powered light aircraft.
Clarke and Schaefer both say that battery technology is about a decade away from where it needs to be for commercial aviation on the scale of passenger jets to make sense.
“We’re all waiting for battery technology,” says Clarke. “But demonstrating the complex systems now, like we are doing on the Maxwell, will allow the market to pick up those technologies when they are fully mature.”