Bell Nexus Partners Connect
By Kenneth I. Swartz
Vertiflite May/June 2019
Bell and its partners talk about its Nexus eVTOL and the team’s approach to creating a revolutionary new type of aircraft.
As its name suggests, the Bell Nexus hybrid-electric VTOL is all about making connections.
From a development perspective, Bell has leveraged the capabilities of five leading aerospace companies to develop the Nexus in a collaborative, imaginative and exciting way. At Heli-Expo, executives from Bell and four of its partners shared a stage to talk about their collaboration on a panel that was moderated by Elan Head from Vertical magazine.
When Bell set out to find partners to help develop the Nexus, “I was looking for capable believers [that were] capable of generating an aviation-grade, safe, high-quality productionized vehicle [and] had to be fearless with us, believe in the future of vertical lift,” said Scott Drennan, vice president of innovation at Bell.
The five key Nexus partners include Safran, Thales, Garmin, Moog and Electric Power Systems (EPS) employing teams based in the US, Canada and France.
In this collaboration, Safran will provide the hybrid-electric propulsion systems; Thales will lead the flight controls system; Garmin will integrate the avionics and the vehicle management computer (VMC) systems; Moog will lead the design and development of the flight control actuation systems; and EPS will provide the energy storage systems.
Safran began ground testing a 100 MW hybrid-electric propulsion system driving four propellers in July 2018 at a test facility near Pau-Pyrenees Airport in France. Engineers are now developing an affordable 600 kW hybrid system for the Nexus. The Hybrid-Electric Propulsion System (HEPS) is the result of a close collaboration between several subsidiaries of Safran, including Safran Helicopter Engines (formerly Turbomeca), Safran Electrical & Power (ex-Labinal), and Safran Power Units (previously Microturbo).
Thales is providing the Nexus fly-by-wire (FBW) flight control systems. The company’s FBW expertise spans more 35 years and 10,000 aircraft. Thales avionics can also be found on the several helicopters, including the Airbus Tiger HAD, Boeing Chinook, Leonardo AW109 and AW159 Wildcat, NH90, and Sikorsky S-76D and UH-60V.
Garmin will lead the design, development and production of the avionics hardware and software needed for the Nexus’ piloted and autonomous vehicle management, including primary flight information, navigation/communication, flight guidance and flight management systems. Garmin introduced the G1000H integrated flight deck on the 407GX, 407GXP and 407GXi and the fully integrated G5000H cockpit on the Bell 525 Relentless.
Moog is providing the all-electric FBW actuators and electronics used to direct and control the Nexus’s ducts and the flight control surfaces. Moog is also a supplier of flight control computers, software and flight control actuation for the Bell 525 and V-280 Valor.
Electric Power Systems (EPS) is leading the design, development, testing, production and support of the energy storage system for the Nexus’ hybrid propulsion system. This includes technology to improve life, reduce cost, and meet stringent US Federal Aviation Administration (FAA) safety requirements around cell containment in thermal runaway. EPS is also designing and building the energy storage system for the NASA X-57 Maxwell research aircraft.
Bell started seriously looked at hybrid-electric propulsion as an option when its market studies identified that both the military logistics and civil urban air mobility (UAM) markets could utilize an aircraft with a five-seat cabin capacity, 130 nm (240 km) range, and a 130 kt (240 km/h) cruise speed, said Drennan. Bell believed a hybrid system would “let our customers stretch out range” and the hybrid system can be certified sooner than a fully electric system while waiting for batteries to mature.
Safran had been studying hybrid-electric propulsion systems for a few years and signed on early as a Nexus partner. The hybrid-electric system was designed for safety, said Jean-Baptiste Jarin, vice president of Hybrid Propulsion System Programs at Safran Helicopter Engines, using “six rotors with two engines per rotor and multiple segregation.”
Thanks to the contributions of Safran and EPS, “we really have three propulsion systems on board,” said Drennan. The first is an integrated propulsion system that operates through an entire mission. The second is a hybrid-electric system that can operate separate from the battery. And the third is a battery-powered electric system that can operate independently of the turbo-generator, explained Drennan.
“Their integration is the core to our safety approach,” said Drennan. “We have two systems on board. One that can fly us through our entire mission in the form of the hybrid-electric but then [the redundant battery system] that can get us to ground under full power, full control should there be a contingency event.”
“That can’t be underestimated in terms of this team’s philosophy towards safety because that choice doesn’t come for free. It comes from a place of value and safety for our customers but it costs some extra weight, some extra work together.”
EPS’ contribution will also include an active battery management system to ensure that when cells charge and discharge they remain in sync.
Bell is used to working with components at lower technology readiness levels (TRL). But the challenge is more about integrating and controlling a lot of new technology in a different way than the individual TRLs of the various subsystems, said Drennan. “But we’ve created a team that can handle all of them and make a safe product.”
All the Nexus partners are working together as an integrated team to achieve the high systems integration required on the actual aircraft.
“One thing that’s unique with this development that you typically don’t see on other aircraft developments is transparency,” said Nathan Millecam, President & CEO, EPS. “If there is a challenge, we know about it. And we all work on it and we solve it together.”
All the partners are “sitting together and talking about what is the intended function [of a system] and then collaboratively deciding where does that function need to reside,” added Didier Papadopoulos, vice president of Aviation Programs Systems & Business Development at Garmin. “What is the most effective architecture for that to reside so we don’t have three, four or five boxes. Instead, we can maybe have two boxes, so save on weight, power, cost, and all those things.”
Bell has a lot of experience developing innovative VTOL aircraft, but the Nexus presents new challenges. Drennan said there will be a great deal of system-level testing before the first flight of the demonstration aircraft. This includes aircraft “zero,” which will exist in a laboratory environment where all the different components “will be able to speak to each other” even if they physically located at partner facilities.
Safran’s hybrid-electric system is integrated with EPS’ battery management system to power a distributed propulsion system. When it comes to energy storage, Millecam said EPS’s approach is to start where systems are today on aircraft, such as the F-35 or 787 that have already deployed lithium-ion batteries.
Aircraft stability will be provided by a Thales FBW flight control system that will be integrated with Garmin’s flight management systems and Moog’s control actuators.
Like most eVTOL aircraft in development today, Bell’s Nexus initial flight demonstrator vehicle will be completely autonomous. Bell’s vision for the certified vehicle to have a pilot onboard at least initially is based on regulatory concerns, as well as customer and community acceptance.
“One of the things I like about going autonomy first is it helps to squeeze your schedule down a bit, and you can get into the development flight test stage a little faster,” said Drennan. “The data we get off a flight test is quite critical for the future state.”
This approach will also allow Bell to study the man-machine interface for the aircraft, which is something the company does all the time with its future flight control simulators supporting the development of the FBW Bell 525 and V-280 Valor.
Bell has not revealed exactly where the Nexus demo aircraft will be test flown. This may be a function of available engineering resources, as well as the most favorable regulatory environment.
Bell is also designing the Nexus so it will be capable of autonomous passenger operations at some point in the future. “The challenge will be to do all the things a pilot normally does even better in an all-autonomous vehicle,” said Drennan.
Bell believes that the progression to fully autonomous operations begins with augmenting the capabilities of the pilot at an early stage, then gradually increasing the capability so the pilot can just be a safety or mission officer. Then, let the pilot step off the aircraft once a full autonomous system is available and validated, said Drennan.
Garmin believes that the introduction of autonomous systems will eventually change the definition of a pilot over time. For example, using automatic datalinks to communicate with air traffic control will reduce pilot workload and change their role.
Drennan said that in his meetings with the FAA, Transport Canada and the European Union Aviation Safety Agency (EASA), he’s noticed that they are already well informed because the Nexus partners have been talking to regulators early and often. They plan to continue as the integrated Nexus aircraft moves towards type certification.
Safran began discussing certification criteria with the FAA and EASA about a year-and-a-half ago, said Safran’s Jarin. “We do not see any hurdles in terms of specific technologies on board. What’s new is the integration level that we are mentioning today. For us the challenge now is to make sure we have appropriate safety level which is recognized by the authorities and to define and validate the parameters [for] certification.”
“Thales brings to the table a certified product for the demo aircraft,” said Mylène Tassy, Director of Sales and Marketing at Thales. “The risk is low in terms of certification challenges for the demo … but the challenge will be to ensure that the systems are integrated completely and meet the requirements. And we need to make sure we are ready for the production vehicle with the same strong building blocks we use in the demo.”
Millecam added that EPS, “got the regulatory authorities involved very early on and often and included them in a lot of the dialogue, and the FAA has actually been very supportive with emerging technologies.”
With the potential of thousands of VTOL aircraft flying over urban areas, Millecam believes that the safety bar has to be set very high from day one.
Bell has not described the certification path it will follow for the Nexus, probably because the certification rules for eVTOL aircraft are still being defined. In general, the FAA seems to be favoring using FAR Part 23 airplane airworthiness standards as a basis of certification rather than Part 27 used for rotorcraft.
One promising development is that certification authorities in the US have been moving from low-level, prescriptive-focused certification methods to operational- and performance-based ways of certifying systems, said Papadopoulos.
“We have already seen the shift in the FAR 23 world … and I think that we are going to build on top of that to tackle the unmanned systems.”
Bell believes that Nexus pilots will come from traditional sources, including the military and pilots who have funded their own flight training.
Papadopoulos believes that the smart avionics and smart flight management system Garmin is introducing on the Nexus will simplify training and add a safety margin even for the well-trained pilots.
Garmin also believes that on the piloted aircraft there might be an opportunity to introduce biometric sensors into future cockpits in the form of a wearable device or some other system that can monitor a pilot’s workload and “support them through human factors engineering.”
Low Operating Costs
Drennan said that the hybrid Nexus will yield a significant reduction in operating costs compared to a helicopter and there will be an equally significant cost benefit with the transition to a pure-electric VTOL aircraft: “It’s formulated based on a couple easy to understand ideas … think of a helicopter with the big rotors, complex hub, complex mechanical transmission and complex engine installation and a duty cycle that can be quite varied for a turbine… If you look at our [Nexus] aircraft, the gas turbine is generating electricity so its duty cycle will become quite constant and that will improve the maintenance intervals and the cost.”
“Then you look at using a high-voltage power system instead of mechanical transmission that has much lower maintenance costs associated with it, and then the hubs are electric motors with integrated prop rotors that are less complex components than [rotor] blades and therefore require less maintenance,” he added.
In addition, the modularity of the Nexus means that Bell can easily apply a line replacement unit (LRU) system that could see the battery pack, electric motor and other components removed for maintenance. That’s a big change from the on-aircraft maintenance of most helicopters, which keeps an aircraft on the ground where it can’t make money.
The Nexus is designed to operate in urban areas where traditional helicopter operations would be restricted by noise. Rig testing in France and wind tunnel testing in Ottawa has already provided Bell with a positive appreciation of the sound the Nexus will make in flight.
“It sounds like this: ‘shhh’,” said Drennan. “I’m not saying ‘shhh’ as in quiet, I’m saying that there is no longer that thump-thump-thump [of a helicopter]. It actually sounds like a fan or the sound of cars passing [on a highway].”
“We are very excited about its ability to blend in to the ambient urban noise. And we are super-psyched about the loudness level itself.” When it comes to the noise footprint, Drennan said, “three things have changed: the loudness, the tonality and the direction of the tonality. And that has been unlocked by the distributed nature of the proprotor system and the ducts that move.”
One advantage of ducts is that they provide a directionality and can be rotated in flight based on what is below the aircraft on the ground landing, taking off and cruise flight.
“We can use that directionality to our advantage … to be a better neighbor … better citizen … a better part of the community,” said Drennan.
Bell is leveraging the engineering capabilities of a strong set of international partners to disrupt the vertical flight industry in a way that it could never achieve working on its own, Drennan added.