When Bell introduced its full-size Nexus concept for an electric vertical takeoff and landing craft at the Consumer Electronics Show (CES) in early 2019, it gave the world a glimpse at where Bell technologists see a possible future for vertical-lift urban transportation.
The original Nexus concept was unveiled as a fuselage mockup at CES 2018, and the 2019 version came in a full-scale, six-rotor configuration with seats for four passengers and one pilot. Plans called for a hybrid-electric propulsion system using a turbine engine to generate electricity to power six tilting ducted fans. Safran was picked for the hybrid propulsion and drive systems, Moog for flight-control actuation, and Garmin for avionics and vehicle-management computer.
“We’re taking the next step this year,” said Bell CEO Mitch Snyder at CES 2020. That's where Bell demonstrated its Smart City concept, with Nexus models flying between scaled-down buildings, carrying out the commands of virtual passengers as expressed by show attendees interacting with tablet computers to plan their trips.
The Nexus itself has evolved and now is a four-rotor design, thus called the Nexus 4EX for four fans, electric power, and experimental. The plan is to field a fully electric-powered version but offer customers the opportunity to buy a hybrid-electric-powered version if they desire. “It’s propulsion-agnostic,” Snyder said. “We can make hybrid work. It depends on customer needs.” Planned range for the electric Nexus is 60 miles, and that would be longer for the hybrid version.
The Nexus 4EX is also designed to be fully autonomous, saving the space needed for a pilot, although the earliest model will be piloted. “Once we’ve proven [the technology] and gained the trust of the public, they will feel more comfortable flying autonomously,” Snyder said.
There are about 70 people out of the 150-strong Bell innovation team working on the Nexus program. “Nexus is taking the front line of development,” said Scott Drennan, v-p of innovation.
Fly-by-wire (FBW) flight controls are a key feature in the Nexus. “Bell’s roots are in fly-by-wire,” he said, with the Bell-Boeing V-22 the company’s first production aircraft with FBW and the not-yet-certified Bell 525 also featuring FBW. “We look at that as our basis to enable autonomy,” he said. “Our foundations in fly-by-wire are secure.”
Bell hasn’t identified its vendor for the Nexus electric-drive system but has finished the downselect process, according to Drennan.
With long experience in aircraft manufacturing, Bell is not willing to compromise safety in the design of a passenger- or even cargo-carrying vehicle like its Autonomous Pod Transport.
Bell is targeting 10-to-the-power-of-nine reliability (the number of failures in one billion operating hours), the current standard that applies to aircraft flight control systems, according to Drennan. “We can have 10-to-the-nine that can be affordable,” he said. “Our vehicles will be at that level.”
He acknowledged that there is some discussion within the eVTOL industry that the reliability standards could be lowered. “We’re hearing lots of folks say 10-to-the-power-of-seven might be sufficient,” he said. “We’re at a fundamental disagreement, and we’re confident this should be done and we can do it. In a helicopter, you can’t lose a rotor or a mast. We engineer reliability into those components to meet or exceed 10-to-the-ninth, and we will do the same on the Nexus rotor and duct system.”
That said, he added, “There is a deeper conversation about what other pieces of the system add to safety: the vehicle, flight operations, and maintenance.” For traditional rotorcraft, he said, “the preponderance of incidents are related to flight operations and maintenance.” Attention will need to be paid to these factors. “We have to bring those to the same level.”
In the Nexus, this translates into making the rotor system as reliable as a helicopter’s.
For electrical system emergencies, redundant battery packs will provide power to bring the Nexus to a safe landing. While the Nexus won’t autorotate like a helicopter, its structure and the ducts provide some lift, and it could land in an airplane-like mode if some minimum amount of power were available. Ultimately, Drennan said, “Power failure is prevented by a combination of redundancy and reliable systems. But [power failure] won’t happen, because we’re designing in appropriate redundancy and reliability.”
“With any design that Bell has done,” concluded Snyder, “the first requirement is safety. With [Nexus], we want to make it safe, quiet, clean, and green, and we want to make it affordable and accessible to everyone.”