CityAirbus eVTOL urban air mobility program presses ahead
By Oliver Johnson | February 15, 2018
Estimated reading time 12 minutes, 6 seconds.
Airbus Helicopters has revealed new details of its CityAirbus electric vertical takeoff and landing (eVTOL) program, with a first flight of its demonstrator aircraft still firmly in its sights for the end of this year.
Airbus hopes the aircraft is a vision of the future of “urban air mobility,” and its striking multirotor design and stylized aerodynamic appearance certainly make it look like something taken from the set of a science fiction movie.
But the manufacturer believes it’s the most practical attempt yet to develop a viable product for the next evolution of aviation transportation.
The aircraft’s propulsion is provided by four pairs of propellers: the lower propellers sit towards the bottom of four large ducts on the fuselage’s corners, while the counter-rotating upper rotors sit exposed slightly above the ducts.
“This kind of multicopter architecture allows you to completely redesign the aircraft from scratch,” said Marius Bebesel, head of urban air mobility at Airbus Helicopters, during a recent presentation for aviation media at the company’s facility in Donauwörth, Germany — where the project will be based until it has completed its first flight.
Despite representing something of a technological leap forward, simplicity is at the core of the aircraft’s design. Four 140-kw lithium-ion batteries power the eight Siemens SP200D electric motors, which directly drive the eight fixed-pitch propellers. Instead of pitch control, the aircraft will adjust the revolutions per minute (RPM) of the propellers to move the aircraft forwards.
“The only real mechanical part on this aircraft is the propeller shaft,” said Bebesel. “Of course, this simplicity has a big impact on operational cost, and this simplicity has an impact on maintenance costs.”
The aircraft is also relatively compact at just eight meters wide by eight meters long — a necessary consideration given its ultimate goal to operate in built-up urban environments.
“We want to keep this vehicle very efficient and compact at the same time,” said Bebesel, highlighting the “three-dimensional ducts” as one of the key elements of CityAirbus’s design. “This three-dimensional duct is making this concept really viable, because the ducted propeller allows you to increase the lift capability, while keeping the vehicle very compact.”
This is important to reduce drag during cruise flight, he said, during which the ducts function as “a bit of wing.”
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And it’s because of this that the aircraft will be able to meet one of the team’s key performance targets — being able to achieve cruise speed in forward flight without having to pitch its nose down to any large degree. Bebesel claimed it will even be able to fly at a more neutral attitude than a traditional helicopter.
At the start of the development process, Airbus performed a market study to establish popular requirements for an urban air transport vehicle — and it is these targets for size, speed and range that CityAirbus is being developed to meet.
“The [market study] has proven that you don’t need to fly too fast for this mission, so for us it’s 120 km/h [75 miles per hour],” said Bebesel. “But, as we’re improving the vehicle, I think we we’ll go beyond [that speed].”
The team is aiming for a maximum takeoff weight (MTOW) of 2.2 tonnes (4,850 pounds) for the demonstrator; the airframe represents just 10 to 15 percent of that total, but the four batteries alone weigh about 500 kilograms (1,100 pounds). Bebesel emphasized that the team is constantly improving the aircraft’s weight and that there is plenty of room for optimization due to an oversized motor for the demonstrator, but still, the aircraft will clearly have plenty of room to carry the weight of the final fittings of a production model, as well as the four passengers it is designed to carry.
CityAirbus is currently targeting a range of about 60 miles (96 kilometers), which Bebesel said “will be enough to cover all the megacities of the world.”
The cabin will be accessed through a door in the fuselage underneath the rear of the fore ducts. The clearance to the bottom of the ducts from the ground is just 1.95 meters (6 feet and four inches), so the propellers will need to be completely stopped before passengers embark or disembark. However, this is unlikely to be too much of an issue with the aircraft’s ability to bring the propellers to a complete stop in “a few seconds” if required, said Bebesel.
The inside of the cabin will feel spacious “like in a British taxi” he said. Behind the cabin, the batteries will hang on a rail in the rear of the aircraft’s fuselage, allowing them to be moved fore or aft to adjust the aircraft’s center of gravity according to its payload.
The batteries present one of the main challenges facing the program team. “We need to get the batteries at a point that they have enough energy on board, they are lightweight, you can allow a big recharge, and then lifetime will be an issue as well,” said Bebesel.
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For the first few years of the aircraft’s life as a production model, it’s likely to need to have its batteries replaced once or twice per year, and those batteries would likely require a recharge time of 30 to 60 minutes between flights. But as battery technology improves, Airbus hopes to lengthen their lifetime and reduce charging time to around 15 minutes.
Airbus is using a mix of metal and composite for the demonstrator’s airframe, with metal providing the team greater flexibility to quickly and cheaply adjust the design as it changes during the development.
Any production model would also use a mix of materials, said Bebesel, with composite used “where it is really needed” for its stiffness and weight.
The design is being kept as simple as possible to allow for high volume production if the project proves a success.
Single failure tolerance is built into CityAirbus’s design, with Airbus claiming the aircraft will be capable of losing one propeller, motor, or battery, and still be able to safely land. Bebesel said the team has “a concept” for how the aircraft will cope with a potential bird strike.
“It’s not a cage, but we will work with the design of the duct in front to prevent a direct bird impact,” he said.
The program reached a major milestone last December with the completion and powering-on of the “iron bird” ground test facility, which has the capability to operate CityAirbus’s propulsion system chain from flight controls to the dynamic loads of the propellers.
Once the electric, mechanical and thermal dynamics of the aircraft’s propulsion system have been matured and verified on the test bench, it will be embedded on the demonstrator.
The first flight — scheduled for December 2018 — will be piloted, albeit remotely, through the use of a joystick in a ground control system. However, as Airbus is designing and developing the aircraft to be self-piloting, the flight tests will plan to use manual control as little as possible.
“We will start automatically, we will land automatically, [and] we will fly around a given route and waypoints,” said Bebesel.
The eventual certification of the aircraft will be another challenge, largely because it will again be breaking new ground. Bebesel said his team has “a concept of how to certify” CityAirbus, but that the certification of the batteries and the aircraft’s complex electrical architecture would present the largest challenge.
While using CityAirbus as a personal air vehicle “could be an option,” Bebesel said the ultimate plan for the type is as a mass form of transit between established hubs within cities — similar to existing car-share programs — which could be ordered through the use of an app on a phone.
It’s here that the program is most ambitious — given that it requires large-scale infrastructure development and buy-in from cities and the general public.
“The number of hubs we need are equivalent to the number of metro stations/underground stations [in a city],” said Bebesel. For cities such as London, New York, and Paris, this would equate to over 200 hubs.
Such a proliferation would mean fleets of aircraft in the skies — and the prospect of the noise this would bring. Because of this, Airbus knows it is crucial to keep CityAirbus as quiet as possible.
Airbus is aiming for around 70 A-weighted decibels — less than the noise of cars on a highway at a distance of 100 meters, and much quieter than a helicopter.
“It’s just the beginning, so we are experimenting a lot,” said Bebesel. “There are a lot of [options] in terms of noise, you can really design to get the acoustic [element] very silent.”
The speed of the propellers is one such element. Compared to a helicopter’s rotor tip speed of 210 meters per second, the propellers in CityAirbus have a rotor tip speed of around 120 meters per second.
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An important project
Around 50 engineers are working on the CityAirbus project, and while Airbus hasn’t revealed the exact level of investment it has made in the program, it’s clearly a significant amount — showing just how important it is for the company’s future.
“We really think this will be a very important, a very interesting segment of the vertical lift industry,” said Dr. Wolfgang Schoder, CEO of Airbus Helicopters Deutschland. “CityAirbus is exactly the center of our competence, the center of our market.”
The program is one of several initiatives Airbus is undertaking to stake a claim of the future urban air mobility market. It also has the Skyways project in Singapore for autonomous drone parcel delivery, and its A^3 Silicon Valley outpost is developing the self-piloted Vahana eVTOL aircraft, as well as the Altiscope project to shape future regulations and air traffic control requirements to safely integrate eVTOL aircraft into urban skies.
Finally, Voom — the on-demand helicopter booking platform that launched in São Paulo, Brazil, in April 2017 — recently joined Airbus Helicopters after having been incubated at A^3.
“[Voom] is showing pretty clearly there is a need for urban air transportation in cities,” said Bebesel. “Even with a conventional helicopter — which is a bit more noisy and higher in costs than a CityAirbus will be — even with this concept and the right approach you can deliver an opportunity for cities.”
Visitors to HAI Heli-Expo 2018 will be able to take a closer look at CityAirbus with a 1:5 mockup of the aircraft at Airbus Helicopters’ booth.