Designing a new aircraft is, relatively speaking, a simple task. If you know the fine detail of the market it is aimed at – who will fly it and why – and the regulatory environment in which it will operate, then you can develop something that addresses those concerns.
And if you are experienced in the field, then you will know how to translate those ideas into a manufacturable, marketable product. Of course, not every programme unfurls without a hitch – and some go more awry than others – but there is at least a known quantity to the process.
But what does a manufacturer do – experienced or otherwise – when attempting to design and build a product that is meant to serve an entirely new market, to operate in an environment that does not yet exist, for which rules are not yet drawn up, using unproven technologies, and which, it is hoped, will be built in such prodigious quantities that the experience drawn from other, entirely alien industries will be of more relevance than anything from aerospace?
Such are the potential rewards from the nascent urban air mobility (UAM) market though, that countless firms are attempting to unravel that tangle of challenges. They include Silicon Valley start-ups, offshoots of the automotive world, and in Airbus and Boeing, two of aerospace's biggest names.
Also present in that list is Bell, the helicopter stalwart. Although having ditched that word from its name, the company is attempting to reshape itself as a more agile, more dynamic, more innovative player in the vertical-lift world.
Its bid to address the market for urban air taxis is the Nexus, a design that bears almost zero resemblance to anything the airframer has previously produced.
Destined to take flight early next decade, the concept features six tilting ducted fans, driven by a hybrid-electric powertrain. Nexus, which is intended to have a 2,720kg (6,000lb) maximum take-off weight, will be able to accommodate four paying passengers and one pilot – although with advances in autonomous flight technology, Bell is already eyeing the 25% efficiency gain that eliminating the crew would deliver to operators.
Nexus, at least according to the company's marketing material, will be "creating a new era of flight" and "helping to solve real-world challenges now and in the future".
But to achieve that vision, Bell has identified the importance of thinking beyond the air vehicle itself, points out Scott Drennan, the company's vice-president of innovation.
To guide the design of the aircraft, "it's important first to define operational requirements it must meet, as well as the transport network it will operate within", he says.
Bell has defined four key pillars – "integrated frameworks", it calls them – that have driven the vehicle's configuration: operational, regulatory, manufacturing, and technological, with the latter very much subordinate to, and informed by, the others.
In other words, Bell has no plans to use technology for technology's sake; instead it must be adapted to serve those other considerations.
While having just four frameworks seems like a nice manageable number, it is worth noting that each is made of up hundreds of different strands.
For instance, on the operational side – which in practice is defining "the functional requirements for on-demand UAM network", as Drennan puts it – that means thinking about how it would be operated commercially, the infrastructural challenges, air traffic management, safety and acoustic requirements and critically, delivering "a solution that is affordable to most people".
Regulatory considerations cover some of the same ground, but are being worked on with NASA and the US Federal Aviation Administration, and other rule makers, to help define an integrated approach across the vehicle, as well as the operational and air traffic management requirements. In addition, the certification authorities have been regularly consulted during the design process, including encouraging their attendance at review meetings "so that we can all see what the challenges are together".
Ultimately the regulatory framework is driven by the need to "clear paths to compliance and [gain] permission to operate for urban air mobility concepts", says Drennan.
And as for manufacturing, Bell needs to develop "dependable and repeatable fabrication and assembly processes", he says. With quality and safety as baseline requirements, notes Drennan, a great deal of focus within the manufacturing stream is instead on "cost, weight and environmental impact".
Manufacturing on the scale envisaged is a puzzle for any traditional airframer like Bell, which is used to turning out at most hundreds of a particular aircraft in a year. If UAM takes off as forecast, then Bell and others will be required to build thousands of vehicles annually.
"At that different scale we need to start thinking less like a [traditional] manufacturer, more like an automobile manufacturer. It's not quite all the way there – but the methodologies in automotive look to be a lot more helpful," says Drennan.
That means, he says, fewer traditional, high-temperature composites that would require a "whole city of autoclaves and presses and ovens" to meet the output requirements. Instead, these would be used selectively – "where we need to have those materials for weight or certification purposes" – with other structures formed via out-of-autoclave processes, which "will start to unlock the rates we are talking about".
Technology, therefore, is not the end itself, but a means to an end; it needs to support one of the other three frameworks.
Although Bell's approach is more or less mirrored by its "more traditional friends", Drennan is not sure that applies universally.
"It does look like the new folks are thinking more about technology and less about certification and manufacturing. There are some cases where it's an 'unknown unknown', but they are certainly getting exposed to some of it now as they develop their flight-test vehicles and are asking questions about how they might do this in the future," says Drennan.
"Manufacturing is not just about capex, but the long-term manufacturing and quality processes."
As he points out, it is not sufficient to focus solely on gaining an aircraft's type certificate: to make any programme a success, a manufacturer also requires a production certificate and continued airworthiness approval. "Of course we have all three of those things, but it's something that others should be thinking about as well," he says.
For all that Bell is looking to the future and to tap into experience from outside the industry, Team Nexus – the programme's core suppliers – are traditional aerospace companies through and through.
Power will be delivered by a Safran-built hybrid-electric system – a gas turbine engine coupled to a generator; excess power will be stored in a battery pack from EP Systems; Moog will supply the all-electric actuators for the control surfaces; Thales will develop the flight-control computer and avionics are to be delivered by Garmin.
Traditional as they may be, the applications are in many cases an entirely new direction for the firms involved.
Take Safran, for example: it has extensive expertise making small small gas turbines, both in its helicopter engines business – the former Turbomeca – and the Safran Power Units operation.
Although it has not previously produced a hybrid-electric architecture, even prior to the announcement of the selection by Bell, Safran built and ran a ground-test version of such a system.
That, says Kyle Heironimus, Bell's propulsion lead for Nexus, is a "testament" to its partner's belief in the technology.
"The fact that Safran already has a cycle of learning under their belt with this type of system clearly means that, number one, they are the perfect partner for Bell, but two, they believe this [system] is going to be the future in VTOL markets."
As with anyone involved in the programme, there has to be a belief that "we can make this happen and this will come true", he says.
The EP Systems-supplied batteries are also vital to the project; as Heironimus puts it, they are an "enabler" for the rest of the propulsion system.
"We could create a system with no storage – but to get the safety and redundancy we would have to have multiple turbines and generators – maybe even more than two in some instances."
That would quickly become both complex and heavy – two things to be avoided at all costs. Instead, the use of the battery allows the size of the turbine and generator to be optimised for the mission, cutting weight, cost and complexity.
But as Heironimus notes, the configuration is not yet a "home run" for VTOL applications, as "until very recently" energy storage and battery technology, notably weight, were ill suited to the mission.
On top of that, VTOL aircraft require lots of both power – for take-off and to hover – and energy for the cruise phase. Traditionally, batteries were able to provide one or the other, not both.
"Only very recently are we getting into a place where batteries are reaching both the power and energy densities necessary to enable this mission," says Heironimus.
Cells, currently using a lithium-ion chemistry, have been carefully selected for the mission that "met all of our requirements at a minimum weight" and then integrated into a pack with a battery management system for safety.
While Heironimus will not be drawn on the specifics of the system, citing supplier confidentiality, the design is such "that even if there were a failure of any single cell… that would not cause any critical failure of the aircraft itself". The design attempts to both prevent a thermal runaway of a cell, and if one does occur, ensure that it does not then affect the safety of the aircraft.
"It's a pretty holistic approach that you have to take to assure aircraft safety, not just the pack itself," he says.
The hybrid architecture should enable an additional degree of safety, argues Drennan, allowing the Nexus to perform a "controlled descent to the ground under power" should the engine fail. In addition, the simplicity of the powertrain, which eliminates gearboxes and transmission shafts in favour of direct-drive electric motors, should bring additional benefits through better reliability and reduced weight and maintenance costs.
One intriguing aspect of Bell's proposal is the acknowledgement that pilots – or at least highly trained aviators – may not be required for the Nexus's operation.
"We may find out that as we go into service, people want to look left and see somebody with them for the initial flights, depending on who you are or how comfortable you feel with autonomy," says Drennan.
"But there might be a lot of folks, or even certain markets, that adopt it more quickly from just an autonomous state off the bat."
However, economic considerations are likely to drive the advance of autonomy: Drennan points out that if an operator can increase payload by 25% through removing a pilot, then it "becomes significant to take them out of there and stop them being a cost on the system".
In the short term, Bell is researching what control interfaces and aircraft responses will be required for Nexus by current and future pilots.
That includes part of its exhibit at the 5-7 March HAI Heli-Expo event, which will present visitors to the stand with a number of potential control options.
The study is with the aim, as company test pilot Jim Gibson puts it, of eventually designing a flight-control system that "allows individuals with limited training to safely and efficiently operate urban air vehicles".
A lower standard of entry for prospective pilots – or "safety officers" as Drennan refers to them – will be key to ensure there are, initially at least, sufficient crew for the thousands of UAM envisaged by Bell and others.
And Gibson believes that aviation regulators are aligned with that view. "I think they understand that something has to change," he says.
"They understand the economics of what it takes to become a pilot today. That's not going to work in a situation where you need 2,000, 3,000 or 4,000 more pilots."
There may be hundreds of other designs competing in the UAM space, but Drennan believes this will only help spur Bell on.
"I try to be really optimistic about people who work on difficult problems like this. I always admire people getting involved. That helps me stay honest about our own position – you can't be too conceited or arrogant," he says.
"But I do think there's going to be a cooling down. You will see it as people get closer to those [operational, regulatory and manufacturing] frameworks. Once they understand the capital investment and process rigour required for manufacturing, we will start to see some folks combine together with people like ourselves or go on to different skills and missions."
Those approaching the problem of UAM from a non-aviation standpoint can sometimes be confounded by the industry's regulations, he says.
There are, Drennan says, "some hugely accomplished software folks" already working in autonomous ground transport applications who are often interested in working with Bell. "Then we sit them down and explain to them [aviation-specific regulations]. They see them as a barrier."
Some also think that simply getting the vehicle flying is "the end-game" itself. "But you have to build in redundancy and you have to build structures that can last for multiple flights."
But for all that, Drennan is still keen to stress that Bell remains open to non-aviation experience: "Our approach doesn't mean we are right on everything," he says.