Bell eVTOL Goes All-Electric

When it took the wraps off its electric vertical-takeoff-and-landing (eVTOL) air taxi concept in January 2019, Bell revealed a different approach to urban air mobility (UAM).

Unveiled at the CES consumer electronics show in Las Vegas, the Nexus had six tilting ducted fans and hybrid-electric propulsion providing sufficient range for intercity flights as well as intraurban trips.

• Nexus 4EX focused on short-range intraurban flights
• Bell taking mobility-as-a-service approach to UAM

In a year, technology and the market have moved on, and Bell has returned to Las Vegas for CES 2020 with a revised design for the Nexus—with four ducted fans and all-electric propulsion.

And beyond unveiling a new full-scale mockup of the five-seat eVTOL, the company is detailing its plans for mobility as a service (MaaS) and its vision of a future smart city ecosystem.

Evolution of the air taxi design into the Nexus 4EX (for four fans, all-electric and experimental) reflects Bell’s belief, solidified over the past 12 months, that the short-range intraurban market will emerge first and that to succeed the service must be affordable and accessible to everyone.

The new design has two tilting ducts on the forward fuselage and two at the tips of an aft wing—a configuration closely resembling that of the Bell X-22 experimental aircraft first flown in 1966. The change from six to four ducts improves cruise efficiency to maximize range using batteries only.

The Nexus 4EX will have a range of about 60 mi. compared with 150 mi. for the original design, now called the Nexus 6HX (six fans, hybrid, experimental). “We started with six ducts and hybrid electric because we wanted range,” says Mitch Snyder, Bell president and CEO.

Originally, the Nexus was designed to meet two distinct requirements, intraurban and intercity, but the need for more range was compromising the vehicle, he says. The redesigned eVTOL is “propulsion agnostic,” Snyder says. “It will fly all-electric, but we can make hybrid work as well.”

“If customers need more range they can go hybrid. If they need more hover they can go with the 6HX,” says Scott Drennan, vice president of innovation.

The main design changes were driven by the need to increase cruise efficiency to enable all-electric flight. The ducts produce lift in forward flight and, in addition to there being two fewer of them, they are less deep so as to reduce drag in the cruise.  

“The more hover flight you have in a mission, the more and deeper ducts you want,” says Drennan. “For UAM, the focus is on cruise efficiency because you only have about a minute of hover.”

Although there are now fewer ducted fans, there is no reduction in safety, Bell maintains. “Redundancy is only a means to reliability, and reliability is what we design into the vehicle,” says Drennan. “We understand where to put critical parts—in the ducts—and where to put redundancy—upstream in the batteries, flight control computers and load paths.”

For Bell, accessibility equals affordability. “The mix of reliable parts and redundant systems creates the cost equation, for acquisition and operation, and we need to drive that down” Drennan says. The company is aiming to meet commercial-airline safety levels—a 10^-9 per flight-hour probability of catastrophic failure—and still be affordable.

Redundant battery packs and component reliability will prevent a total power failure he says, but the Nexus will be capable of landing in transition mode, with the ducts partially tilted, or fully tilted in airplane mode on less power than in rotorborne flight.

Safety is one of three key performance indicators Bell has set for UAM. The others are accessibility and sustainability. In addition to affordability, and convenience in terms of vertiport location, accessibility includes the passenger experience, as many customers will be new to urban air transport.

Bell defines sustainability as “quiet, clean and relevant.” By relevance, the company means being able to offer the same customer experience in different markets—such as longer regional flights—by using different technologies in terms of payload and range capabilities.

Unveiling of the Nexus 4EX moves Bell a step closer to building a demonstrator. The company is not putting a timetable on flying this aircraft, but is targeting the mid-to-late 2020s for production vehicles and commercial services, Drennan says.

Bell has ground-tested a full-scale duct and rotor, produced an 8-ft.-dia. circular composite duct spar and tested a one-eighth-scale model in the wind tunnel. The company is working on a system integration laboratory (SIL)—“Aircraft 0”—to test avionics, electrics and flight controls.

Flight control computer developer Thales also has a SIL in Canada, and battery supplier EPS is doing some of the integration work, says Drennan. Safran, which would have provided the hybrid-electric propulsion system, is no longer involved. A supplier for the all-electric propulsion system will be announced soon.

Also at CES, Bell is outlining its thinking on urban air mobility as a service. This encompasses not only the vehicles—air taxis and unmanned aircraft for logistics and data-gathering—but also operations, digital and physical infrastructure, and maintenance, repair and overhaul (MRO).

“I removed the bounds from the team and told them to look at every aspect where Bell can be a player,” says Snyder. “Can we operate them more safely? Instead of just providing training for operations and MRO, as we do today, can we do it and raise the bars for safety?”

In Bell’s approach to MaaS, the service is powered by AerOS, an aerospace operating system the company is developing as a digital backbone for UAM, managing fleet information, monitoring aircraft health and controlling the throughput of people, goods and data.

AeroOS will comprise modular microservices in the cloud, such as passenger booking, flight scheduling, route optimization, airspace management and fleet visibility. While Uber will provide these services for its Elevate aerial ridesharing networks, “there will be other customers that do not have them,” says Matthew Holvey, operations manager for innovation. “It needs to be a service Bell provides.”

To manage high-tempo operations safely, AerOS is to be an autonomous system with human oversight. For example, the system will track which aircraft are seeing deeper battery discharges on longer flights. These reduce cycle life, and AeroOS will adjust scheduling to level out battery utilization across the fleet.

Bell has begun development of AerOS with master scheduling. This models expected traffic demand for each day and prepositions aircraft. “We are using artificial intelligence to predict where people are,” says Holvey. As the schedule is executed, and deviations inevitably occur, the master schedule is rerun and the traffic-demand forecast updated to provide a better predictive model.

For Bell, starting development of AerOS now, years before UAM services are expected to begin, is a way to enable the market by offering prospective MaaS providers—whether commercial operators or cities themselves—with an option for the digital infrastructure. “If we don’t think about this now, we won’t be ready,” says Drennan.