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Airbus Helicopters expects to complete the first flight of its CityAirbus urban mobility demonstrator sometime in March, as the vehicle continues its testing program at the manufacturer’s facility in Donauwörth, Germany.
The company had originally targeted the end of 2018 for the flight, but the project required “a bit more fine tuning than we expected,” Marius Bebesel, head of the CityAirbus program, told reporters during a media briefing.
“I think on the vehicle we were ready to go, it was more the [aircraft’s] ground control station and the monitoring [systems that caused the delay],” he said.
The 2.2-tonne aircraft is fully electric, operating at 800 volts with four batteries. This is a contrasting approach to that taken by fellow eVTOL pioneer Bell, which has chosen a hybrid system for its Nexus vehicle.
“What we need is a vehicle to be able to cope with the requirements in a big city,” said Bebesel. “So what we have done with the CityAirbus is really to develop a dedicated electrical vehicle for operation in [an] urban environment.”
CityAirbus is designed to carry four passengers, and the need to be able to do so was a key requirement of the project, said Bebesel. “We wanted to have a certain transport capacity because you need it to really have the right business case,” he said. “The approach we follow here is to be as efficient [and] as compact as possible, so this is why the size of the CityAirbus is relatively compact.”
The dominant feature of the vehicle is its four enormous carbon fiber ducts, two each side at the top of a more conventional-looking, but still highly stylized, cabin. Each duct contains a “pod” consisting of a pair of coaxial counter-rotating propellers, spanning 9.2 feet (2.8 meters) in diameter. For the time being, the propellers are wooden, but carbon fiber propellers have been created and will be tested, potentially offering more efficiency and lighter weight.
“For a start, wooden propellers are quite nice; producing them is easy, and very quick,” said Bebesel. “And wood is, I would say, in terms of failure and dynamic behavior, a very nice material. It’s a bit more forgiving than carbon fiber.”
The lower propellers sit in the inside of the ducts with just three millimeters of clearance between the blade tip and the inner edge of the duct. The upper propeller is positioned above the duct, which is an optimized design for cruise flight, said Bebesel. If the duct was created tall enough to shroud both propellers, it would create too much drag in cruise.
“One of our findings was we are very efficient with a narrow duct on the lower propeller, we achieve tremendous efficiency in hover, but we are still able, due to the narrow duct, to fly at good performance for cruise,” he said.
The ducts weigh just 44 pounds (20 kilograms) each, but despite their light weight, have proved very stiff during testing, with no issues in the clearance between blade tip and the edge of the ducts, said Bebesel.
The duct design generates an impressive amount of extra lift, he said — more than 880 pounds (400 kilograms) in the hover as compared to that produced by the propeller pods without the ducts. The quadcopter-style design also serves to keep the aircraft compact, and provides an important safety purpose, said Bebesel, allowing it to cope with any kind of single failure.
Layers of redundancy are built into the aircraft, with four batteries and distribution boxes, and eight motors.
“More or less we can cope with any kind of [single] failure: on the battery, on the distribution boxes, on the motor controls, [or] on the motor itself,” said Bebesel. “We can cover also the loss of two propellers.”
The aircraft’s skids are one of the only “borrowed” pieces on the demonstrator, and have been taken from the H135. “In the beginning we agreed the landing skid is one of the most tricky parts, so we decided to go with [skids from] a 135 aircraft and to build the aircraft from this landing skid,” said Bebesel.
He said the team has a lot to learn about this kind of vehicle, and said the ground test phase had already produced “a lot of very interesting effects” — though he declined to specify what those were.
“There is a lot of technology on board we want to test and we want to learn, so it’s an incredible learning platform,” he said.
The noise produced by the four sets of coaxial propellers is “a particular sound… [that’s] not easy to describe,” said Bebesel, adding that the lower tip speed of the propellers as compared to a helicopter rotor blades (around 120 to 140 meters per second, compared to over 200) would produce a more “pleasant” noise.
Currently, the team is tuning the aircraft’s flight control system in the ground test program, gradually increasing the rpm of the propellers. To date, they have reached 600 rpm, but hope to reach 750 rpm during testing this week. The aircraft will require around 1,000 rpm for takeoff, said Bebesel, with 950 rpm needed for “nominal thrust.”
The CityAirbus team plans to have “at least two more weeks” of ground runs, and once the first flight has been completed, they will move the program to a military test area in Mannheim to expand the aircraft’s flight envelope. First hovering at a height, then moving step by step to cruise flight.
The aircraft will be unmanned throughout the flight test campaign, which Bebesel said was a more straightforward approach to developing a product that is ultimately designed to be autonomous.
“To be quick and to have proof of concept of such a vehicle, which can be later a product, we decided not to go with a pilot, because it’s easier to go in the beginning with an unmanned vehicle [than switch from piloted to unmanned],” he said.
The demonstrator will instead be piloted remotely from a ground control station, although the plan is to limit the manual controls as much as possible. Flights will be programmed into the aircraft’s systems beforehand, with the aircraft tasked with flying through various waypoints in the flight area. The “pilot” will be able to interact with the aircraft, but the hope is that this will be limited.
Flight tests are due to be completed by the end of this year, when the team hopes to “have all the information we need to get on this kind of vehicle,” said Bebesel. “The aim is to really have a proof of concept that we are able to perform a mission we have defined for ourselves.”
CityAirbus is one of two eVTOL concepts currently being tested by Airbus. The other, known as Vahana, is a self-piloting aircraft powered by eight propellers, and is being developed by Airbus’s A³ Silicon Valley outpost. Vahana has been performing flight tests for over a year, and Bebesel said the two programs are in “close contact” exchanging lessons learned.
These have included how to approach to expansion of the aircraft’s envelope.
“I think what is tricky is at a certain point in time get in the air,” said Bebesel. “You can be too cautious, you can be too bold, so you have to find the right approach. It’s not that easy. We are Airbus, so we tend to be a bit more cautious. So we learn a bit from the [Vahana] guys, how can we be quick and safe.”