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Bell has revealed an evolved design for its Nexus air taxi, with a new “4EX” version using fewer ducted fans and a longer wing than its predecessor, as well as being fully electrically powered.
The 4EX in the aircraft’s name represent the major design changes — “4” ducted fans, “E” for electric, and “X” for experimental. The original Nexus design, now referred to by Bell as the “6HX,” utilized six ducted fans and was envisioned as a hybrid-powered aircraft.
Unveiled in the form of a full-scale mockup at CES 2020, which begins Jan. 6 in Las Vegas, Nevada, the 7,000-pound 4EX can carry four to five passengers up to 60 miles (95 kilometers) at a cruise speed of about 150 mph (240 km/h). While intended to be an electric aircraft, it has been designed to be propulsion agnostic, and a hybrid version would extend that range to beyond 150 miles. In terms of footprint, the 4EX will fit in the same 40-foot-by-40-foot box as the 6HX.
The evolution in design of the Nexus was dictated by market requirements, Bell CEO Mitch Snyder told media ahead of the aircraft’s unveiling. Bell had originally targeted range with its 6HX design, but it became clear that was one of two distinct and separating requirements for an urban mobility vehicle; the other being an aircraft that will serve cities as an intra-urban mobility provider.
“One vehicle was compromising the attributes of each, and so we went at it in a different direction,” said Snyder. “We looked at . . . what was coming from the market and what they wanted. And we were also able to progress our technology further and understand it and say, ‘You know what? I think we can build an all-electrical [aircraft].'”
The main driver in reducing the number of ducts was efficiency, said Scott Drennan, VP of innovation at Bell, with fewer ducts creating less drag. “And when you want to go to an all-electric vehicle, you need to get that efficiency in there,” he said.
The ducts have remained roughly the same size in terms of diameter — about eight feet — but are a little shallower on the 4EX than they were on the 6HX.
“There’s still work to be done around performance optimization in the duct, but we were confident enough to start showing a different chord length because of some of the data that we acquired [in testing],” said Drennan.
The move to all-electric also means a switch in propulsion partner, with the replacement for Safran (who had been working with Bell on the hybrid 6HX) to be announced shortly.
Building a safe aircraft is the main focus for the team, said Snyder. “With this form of transportation, we want to make it safe, we want to make it quiet, [and] we want to make it clean and green,” said Snyder. “And the most important thing here, besides the safety, is we want to make it affordable, [and] accessible to everyone.”
The exact level of safety Bell is targeting is one in a billion (10-9) for a catastrophic failure — which is the level of safety EASA recently identified in its certification standards for small VTOL aircraft.
“You can have a vehicle that’s 10-9 and still be affordable,” said Drennan. “Our vehicles will be at that level. They should be because of our history, [and] they should be because of the use of these — we’re talking about 2,000 hours per year, hundreds, maybe thousands of vehicles across these different areas — and when you start doing math, you want that number to be where it is.”
Despite losing two ducts and one of two forms of propulsion, the 4EX is just as safe as its predecessor, he added, with the reliability of the ducts and rotors preventing them from failing at the same time.
“Redundancy is only a means to reliability; reliability is reliability, that’s what you design into your vehicles,” he said. “A four-ducted Nexus is as safe as a six-ducted [Nexus] and that’s because we understand where to put critical parts, which will be in the ducts themselves. So the rotor system will be critical, and then we know where to put redundancy, which is downstream — flight control computers, electrical buses, battery systems, [and] redundant structural load paths.”
As with its predecessor, the 4EX is designed for autonomous operation from launch, but Bell is anticipating the need for a pilot or “mission manager” on board the aircraft as urban air mobility first establishes itself, while public acceptance of autonomous flight is solidified.
“Once we have proven that [autonomy] works and gained the trust of the public, then I think we’ll feel more comfortable flying them autonomously,” said Snyder.
The issue of autonomy in flight has been thrust in the spotlight with the highly publicized issues relating to the Boeing 737 Max. But while Bell has been following the developments with that aircraft, Michael Thacker, executive vice president of technology and innovation at Bell, said the company’s approach with the Nexus — with full autonomy the goal — required a different approach from the offset.
“If you start with the idea that, ‘I’m going to have a pilot,’ and you add autonomy to a pilot, you end up with a different architecture than if you go into it saying, ‘This is going to be fully autonomous,’ and then back into, ‘What is a pilot’s role in that?’ for a period of time until you can demonstrate the capability of the full autonomy to the public and to the regulators,” he said.
A Flexible Design
While the 4EX is the Nexus design that is moving forward towards a demonstration vehicle, that doesn’t necessarily mean the 6HX will never be seen again.
“We want to make sure that our customer knows that if they want to do just intra-urban with an all-electric vehicle, so 60 miles or so, here is Nexus for that,” said Drennan. “If you need that range to extend or you need more hover capability because you have a unique mission, we can offer hybrid for the range extension or maybe a configuration that looks more like the 6HX if you needed other capabilities.”
In terms of a timeline for a Nexus demonstrator vehicle, Drennan said Bell was looking to have a certified system ready to serve the public in the middle to late 2020s, with a demonstrator to “come in appropriately” between now and then.
“There will be three, maybe four prototypes in an objective program of record, so we’re talking about a time period that doesn’t just contain the tech demonstrator but also the prototypes that help the certification get done,” he added.
Snyder said the speed of development was dictated by more than simply Bell’s capability to design a vehicle. “To some degree what’s pacing us is we’re developing the technologies, [and] we’re working with regulatory authorities as well as the cities and infrastructure to understand this system and we’re progressing it together,” he said. “I can tell you we are designing vehicles, we are maturing the technologies, and we’re working with the regulatory authorities and we’re driving that together.”
The regulatory piece of the puzzle “is actually pretty well advanced,” said Thacker, with the Federal Aviation Administration (FAA) engaged and having learned a lot from the challenges it faced in developing a regulatory system for small unmanned aircraft systems. “The bigger challenges there is really getting the entire FAA system — so aircraft certification, operations, and traffic management — all to have a common vision,” he said. “It has to be a system-wide solution that will work for the entire process.”
There are currently about 70 people working on the Nexus program. “It’s a really nice size team that can move out really quickly on everything,” said Drennan.
The upcoming year will see them focusing on three new key performance indicators for the program, with safety, accessibility and sustainability at the core of its conversations for urban air mobility.
Accessibility refers to not only the right location for vertiports within a city, but making urban air mobility affordable for the masses, and ensuring people with limited mobility, for example, or those travelling with young children or a pet feel able to use the system.
In terms of sustainability, an environmentally sustainable system is clearly key, but noise is a major factor — communities are likely to reject vehicles that are too loud.
Bell completed wind tunnel tests with a full scale eight-foot Nexus duct at Canada’s National Research Center in Ottawa, Ontario, last year, and Drennan said the company has been pleased with the results. “At 400 feet . . . just in taking that raw data, untreated and taking that combined up to four [ducts], it’s somewhere up to seven [decibels] below a traditional helicopter,” he said. “You’ve heard the targets out there being somewhere 15 [decibels] below [a helicopter], so we’ve got some distance to go, but we haven’t done everything we need to do on [the duct] to get it where we want it [in terms of noise reduction].”
Bell will also continue to work on a systems integration lab (SIL) for the Nexus, following in the path of development established by Bell’s fly-by-wire 525 Relentless, V-280 tiltrotor, and 360 Invictus. The SIL combines the aircraft’s avionics, electrical and flight controls systems, tying together its hardware and systems to effectively create a non-flying “aircraft zero.”