A year after teasing the fledgling electric vertical-takeoff-and-landing (eVTOL) industry with a mockup of a futuristic air taxi cabin, rotary-wing giant Bell has unveiled the first major configuration details of its Nexus on-demand urban air mobility (UAM) concept.
Revealed at the Consumer Technology Association’s CES 2019 show in Las Vegas, Bell’s Nexus is distinguished by six tilting ducted fans and sized to carry four passengers and a pilot. Powered by a hybrid-electric propulsion system incorporating batteries and an unspecified Safranturbine engine, the Nexus is designed to “safely and efficiently redefine air travel,” says Bell’s executive vice president for technology and innovation, Michael Thacker.
As one of five companies teamed with Uber to develop urban air taxi demonstration vehicles for trials in Dallas and Los Angeles as early as 2020, Bell is also unveiling the Nexus as part of a broader strategy aimed at applications beyond air taxis, including logistics and military missions. While the idea of air taxis is not new, Thacker says: “What is new is the emergence of technologies that enable quiet, safe, efficient, affordable urban air mobility operations at scale using small, heavily automated electric and hybrid-electric vertical lift aircraft.”
The Nexus is a preliminary product of four integrated frameworks used by Bell to help define a UAM model. “Using operational, regulatory, manufacturing and technology frameworks, we are enabling innovative technology, charting a path toward regulatory support, and ultimately informing aircraft design and operating requirements,” says Thacker.
“While we are not sharing specifics on active projects today, we believe that viable commercial operations are possible in the mid-2020s,” he says. “Tomorrow’s challenges facing our population centers are not going away and will not be solved by conventional means. There is a lot of work to be done to create a viable UAM network, but we believe the future is real and possible and coming soon to a city near you.”

The Nexus is a “pretty serious aircraft,” says Bell Vice President of Innovation Scott Drennan. “[The design is] what we believe is a certifiable vehicle that makes this market real in the future. Hybrid propulsion reflects our belief in a broader capability set. The ducts are great for noise and enable a new set of folks to approach this aircraft comfortably. We want this to reach everyday folks who have to commute to work, visit families and get from Point A to Point B.”
Sized to fit within a 37 X 37-ft. light helicopter landing site, the Nexus is targeted at a range of 150 mi. and a cruise speed of around 130 kt. Bell is aiming for a takeoff gross weight of 6,000 lb. and maximum payload capacity of 800-1,000 lb.
The cabin’s automobile-styled interior is configured with side-by-side seating for four, plus a forward single seat for a pilot. “We think that is where the market is. We think a pilot will be there initially, to help people accept the technology, but we think the pilot will eventually go away and be replaced by an autonomous system,” says Drennan. “In this ecosystem, you will find people who are bullish and some who are bearish. We are in—we are believers, and we are going to make this become real in the marketplace.”
Forward and aft fans are mounted close to the fuselage; the midfan set is mounted on the tips of the stub wing. Credit: Bell Concept

Bell’s confidence stems in part from its extensive heritage in ducted-fan and tiltrotor designs going back to the XV-3 of the 1950s and, more particularly, the X-22 short-takeoff-and-vertical-landing X-plane of the 1960s. Adopting a configuration very similar to that of the much larger X-22, the Nexus features high-mounted ducted fans arranged in pairs. 
The forward and aft fans are located on pivoting struts close to the fuselage, and the midset is attached at the tips of a centrally mounted stub wing. The single-rotor fans, each consisting of four blades and measuring 8 ft. in diameter, are housed on hubs supported within the duct by guide vanes. In helicopter mode, the ducts are tilted horizontally to generate vertical thrust, and in aircraft mode they tilt vertically to provide forward thrust. The ducts are passive lift systems and generate the vast majority of lift in airplane mode, along with the wings in the center,” Drennan says. The duct shape generates lift regardless of what is spinning inside of it in airplane mode.”
A Safran turbine engine is housed on the upper aft fuselage and, along with batteries, provides energy for the vehicle’s hybrid-electric propulsion system. The engine exhaust is ducted aft between the twin vertical stabilizers of the aircraft’s Pi tail.
A hybrid-electric propulsion system was adopted for greater mission flexibility, says Kyle Heironimous, propulsion lead engineer for Nexus. “That does not mean we’ve closed the door on future technologies,” he notes, “so we are always keeping our eyes on the status of batteries, fuel cells and future energy storage, and a lot of technologies that we are developing for hybrid systems including electrical machines, motors, generators and power electronics.”
Beginning at CES Bell is surveying public reaction to advanced single and two-inceptor pilot control designs for the Nexus as alternatives to conventional helicopter controls. Credit: Bell

As well as providing the turbine engine, Safran will be responsible for development of the series hybrid-electric propulsion system and drive system. The company, which began ground runs of a 100-kW hybrid-electric distributed propulsion system in mid-2018 at its Pau-Pyrenees test site in France, also recently unveiled the first of a newly developed family of electric motors up to 500 kW, and is providing a modified version of its Ardiden 3 turboshaft to provide electrical power on the Boeing-backed Zunum 12-seater ZA10 hybrid-powered aircraft.
In the Nexus the turbine-driven generator will produce DC electricity, which will be transferred to motors in each duct via a redundant power distribution network. “In addition to the turbine or turbo-gen as we call it, we have a high-power, high-energy battery energy storage system that provides a redundant and dissimilar source of power to the propulsion system,” Heironimous  says.
The architecture enables the battery to assist the generator’s electrical output “when the aircraft needs it during transition or maneuvers,” he says. “Also, if in the unlikely event that the engine shuts down in flight, the batteries have enough power to land the aircraft safely or potentially continue its mission and land safely. That means we don’t have to carry extra parachutes and can always land safely under our own power.”

Power from the generator and battery combine into the distribution system that feeds the six fans, each of which contains a direct-drive electric motor. “That is important because it is attached directly to the rotor system,” says Heironimous. “There is no intermediate gearbox or lube system. We are keeping it as simple as possible, and that keeps it reliable, cost-effective and safer.”
Logan, Utah-based Electric Power Systems (EPS), which led development of the lithium-ion battery pack for NASA’s X-57 Maxwell electric propulsion demonstrator, will provide the energy storage system, including batteries, power electronics, thermal management and battery management. “Batteries are always a trade between high power with low energy or high energy with low power,” says Heironimous. “But a VTOL mission needs both, which means it is a heavy battery. Only within the past couple of years have we reached the point where batteries are reaching the energy and power densities necessary to enable electric and hybrid eVTOL.”
The advanced EPS battery system makes the hybrid approach feasible and enabled Bell to optimize the size of the generator and turbine. “Using the battery to shave off the peak power demands of the mission allows us to size a smaller turbine, which is more efficient and lighter,” says Heironimous.
The Nexus will have a Thales-developed fly-by-wire flight control system (FCS) that will be “different to anything Bell has done before in many ways,” says flight controls lead engineer Jeff Epp. The flight control system will be distributed, with centralized flight control computers connecting to remote electronics and motor controllers. For the first time on a Bell aircraft, primary flight control surfaces will be moved by all-electric actuators rather than conventional hydraulic systems. The electromechanical actuators and motor controllers will be provided by Moog.

“For the first time, we have integrated the FCS with the propulsion system,” says Epp. Flight control computers will control the motors, rotors and ducts. “The flight control computers control those motors to not only provide thrust but also to use those rotors to dynamically control the aircraft,” he adds. “That allows us to do roll, pitch and yaw control as well as use them for hover mode and forward flight.” In helicopter mode roll, pitch and yaw control is provided by varying rotational speed on the fans and directing thrust using movable vanes in the fan ducts. In airplane mode, flight control is provided by rudders on the canted vertical tails and ailerons on the midfan duct struts.
The flight control system will also work closely with the autonomous vehicle management computer (VMC), which will be developed by Garmin. The VMC is “the brain of the aircraft,” says Nexus avionics lead Frankie Mazzei. For power management, the VMC will take information from the battery and relay it to the engine, while for navigation it will be able to take ground station commands and relay those to the FCS. “Today, that is all managed by the pilot,” says Mazzei. “The goal of the VMC at Bell is to be the ultimate pilot assistant, enhance situational awareness, reduce workload and increase safety. The end goal is to be fully autonomous.” 
Bell is reluctant to detail the development schedule, but Drennan says the program is currently “somewhere between preliminary and critical design reviews.” Entry into service is targeted at the mid-2020s, with demonstrators and prototypes “phased in between now and then,” he says, adding that the first vehicles will be fully autonomous to “unlock speed in the schedule” and enable the early potential for an optionally piloted aircraft. The route to certification is undecided but “could come out of Part 23,” Drennan says.