As a finger-numbing wind whips across Kemble airfield it keens through the empty nacelles of the 747-400 parked behind us.
While the Jumbo Jet was once considered the very cutting edge of aviation, those days are long past for the Queen of the Skies, its production line shuttered and a dwindling number in passenger service.
Instead, or so their proponents argue, the future belongs to the small white aircraft sat nearby, its eight electric-powered propellers whirling in the weak December sun.
That aircraft is the second prototype of the VX4, an electric vertical take-off and landing (eVTOL) vehicle developed by Bristol, UK-headquartered Vertical Aerospace.
A four-passenger, crewed aircraft, the VX4 is designed to perform air taxi missions of up to 100 miles, tapping into a market for urban or advanced air mobility from 2028.
At the time of FlightGlobal’s visit in late December, Vertical had not long graduated to the second phase of flight testing – piloted thrust-borne hovering. That was followed in early January by the start of Phase 2B – piloted thrust-borne manoeuvres capped at 20kt (37km/h).
Another two steps are in Vertical’s flight-test plan: Phase 3 will involve wing-borne flight following a standard rolling take-off at about 90kt, while Phase 4 is the acid test of transition – moving from thrust-borne to wing-borne flight at 40-75kt.
That is all in the future, however: today’s test is a tethered ground run, which will see the VX4’s propulsion system put through its paces with chief test pilot Simon Davies in the cockpit.
Davies, a Royal Navy veteran with experience on multiple fixed- and rotary-wing types, says Vertical’s decision to perform flights of the prototype with a pilot on board puts the company in a “fairly exclusive club”.
Vertical claims it is only the second developer, after Joby Aviation, to have flown a full-scale, piloted, vectored-thrust eVTOL, and while there are multiple qualifiers to that claim which exclude other potential contenders, the field is nonetheless a small one.
An earlier prototype of the VX4 (G-EVTL) was written off during a remotely piloted testing mishap in August 2023 when a propeller separation caused cascading failures across multiple systems, resulting in a loss of control and the aircraft falling from a low height to the tarmac at a rate of almost 20ft/sec, according to the UK Air Accidents Investigation Branch.
Depending on who is talking, the incident is described as a hard landing or a crash, but either way it was a setback for Vertical, putting a halt to flight testing for almost a year while it assembled the second prototype and sought clearance from the UK regulator to resume flights.
The delay has clearly been problematic – investment has been harder to attract without a prototype in the air and certification has inevitably slipped.
Nonetheless, the new flight-test asset (G-EVTA) is a significant improvement over its predecessor. It features multiple enhancements over the previous iteration, using components sourced from the supply chain that will eventually equip the production version of the VX4; familiar names like Leonardo, GKN Aerospace and Honeywell all have a presence on the aircraft.
It also uses in-house-developed battery packs and a second-generation propeller design – removing the point of failure that triggered the accident. Vertical believes there will be a significant benefit to controlling the IP for both systems.
As the VX4 strains against its tethers as the propellers spin up to their take-off speed of around 700rpm, two things are apparent.
Firstly, it is quiet. While there will be arguments around this when the VX4 is in an operational configuration and airborne, the noise from the light aircraft flying above us is much more apparent than that produced by the prototype.
Second, the VX4 – and its pilot – really want to fly.
“Operating this aircraft… it’s starting to feel like it’s ready; it’s starting to feel like a real aeroplane,” says Davies in the terribly matter-of-fact way common to all test pilots. “We are really reaping the benefit of having a pilot on board as well.”
At the time of FlightGlobal’s visit, Davies had taken the untethered VX4 into air on a handful of occasions, but nonetheless had already gained a strong feel for how it handles: although there are visual similarities to a helicopter, in hover “it flies completely differently”, he says.
As a pilot, “you are telling it where to go, telling it the flightpath”, and the Honeywell-supplied digital flight-control system largely does the rest. In that respect, it is, he says, “very like the [Lockheed Martin] F-35B”, adding: “It is incredibly powerful technology.”
Davies says the guiding principle behind the flight controls has been to deliver an aircraft with “reduced complexity” which is “much, much easier to fly”. (FlightGlobal’s ham-fisted attempts on Vertical’s Pilot in the Loop Simulator, during which no damage was caused to the virtual aircraft or scenery, are a testament to the simplicity of operation.)
In the cockpit, the pilot is confronted by twin sticks and no other controls (although the prototype features pedals for differential braking, these will not make it into the production version). The left-hand stick controls the speed – forward for faster, back for slower – while pull back on the right-hand stick and the aircraft climbs, push forward and it descends. That same stick handles lateral manoeuvring, while it also has a twist axis for yaw control.
Crucially, the sticks command the same operations whether the aircraft is hover or cruise flight.
Transitions between different states are automated. For example, when readying for a departure, the flight-control system automatically tilts the rotors from taxi mode to take-off position.
The same is true when transitioning from thrust-borne to wing-borne flight and back again as the aircraft passes through the threshold at around 60kt. And once the airspeed is sufficient for the wing to generate all of the lift with a safe margin from wing stall, the flight-control computer automatically stops the aft propellers.
“It is an order of magnitude less demanding than piloting a helicopter or fixed-wing aircraft,” Davies adds.
The VX4 has multiple control surfaces: the tilt, pitch and speed of the front four rotors can be changed, as can be the speed of the four fixed rear units, albeit within a narrower range; there are also flaps, flaperons and ailerons on the wing, plus four ruddervators on the V-tail.
Such an extensive array of effectors means that “it is far too much for a human to manage at one time”, hence the VX4’s “unified control law” – a principle applied to the F-35B - says head of flight test Peter Scheidler, a rotorcraft and powered-lift veteran, who previously worked on the Leonardo Helicopters’ AW609 tiltrotor.
Of course, the latest VX4 prototype and its sister ship – currently in final assembly at GKN Aerospace’s Global Technology Centre in Bristol – are not fully representative of the aircraft that will eventually enter service.
Obvious differences are the lack of passenger cabin and the presence of an ejection seat for the pilot, while less evident is the more robust – read heavier – landing gear required to cope with the rigours of wingborne take-offs and landings at around 90kt. While needed for the test campaign, such a capability is not, yet, in the production specification, says Scheidler.
Vertical also has not yet fully optimised the VX4’s battery packs for volume, explains Ross Crawford, operations and manufacturing director.
Crawford, prior to a recent promotion, headed the company’s battery facility, also known as the Vertical Energy Centre (VEC).
Housed in an anonymous industrial estate on the outskirts of Bristol the VEC is where Vertical designs, develops and tests its battery systems and the production methods needed to cope with high-volume manufacturing.
Opened in 2023, the VEC was the result of a decision by Vertical to bring this vital piece of the supply chain in house. “The batteries are such a critical part of the aircraft performance… and the quality standards are so rigorous, it is really hard to rely on third parties to do that, they don’t care about weight and quality as much as we do,” says Crawford. “This way we can control the process.”
Cells from supplier Molicel are delivered to the VEC, where they are tested and assembled into battery packs capable of delivering 1.4MW at peak power that are then integrated with the battery management system.
The battery packs flying aboard the latest prototype have a power-to-weight ratio that is 20% better than those on its predecessor, the company points out.
Designed as an R&D facility, the multi-million-pound investment in the VEC – notably in equipment and personnel – allows Vertical to build prototype products and test them quickly. “If we didn’t have the capability in house we’d have to wait six weeks for a test slot externally,” Crawford points out.
In-house testing capabilities include an extensive climatic chamber, able to replicate mission profiles at operating temperatures, a vibration test cell, and a destructive testing facility: cheerfully nicknamed the “abuse chamber”, this is a specially outfitted container where vital but more brutal testing is performed.
Similarly, automated production systems – adhesive application and curing, plus cleaning and welding – can be fine-tuned ahead of commercial roll-out. “All that stuff gets baked in now because we have to focus on high scale and volumes,” says Crawford.
However, certain parts of the process will always remain manual, where either automation is too expensive or for tasks where “sometimes people are just better”.
What the VEC, is not, however, is a production site. Although Vertical will build the battery packs for the prototypes and conforming aircraft there, “we’ll need a larger facility as we’ll need to scale up”.
At service entry, Vertical plans to be using batteries with an energy density of slightly less than 200Wh/kg at pack level but is working with Molicel on the “future product roadmap”. The VX4’s design allows improved battery packs to be swapped in to enhance performance as a future upgrade, boosting range or payload.
While Vertical has not specified that new batteries are in the works, its Flightpath 2030 strategy – published in November last year – promises undisclosed upgrades will be available by the end of the decade.
Release of the Flightpath document marked the start of a busy end to the year for Vertical as it also addressed the persistent issue of its financing.
While Vertical has been relatively parsimonious compared with its big US rivals – its modest headquarters building tucked behind Bristol’s main rail station is more Silicon Alley than Silicon Valley – even a capital-light model still requires money coming through the door to fund continued development work.
Obtaining future investment was hampered by a 2021 agreement with US investment fund Mudrick Capital, which in return for a then $200 million cash injection received floating convertible notes in the business.
The uncertainty inherent in those notes – notably around their pricing – had served to deter other investors. But an agreement finally ratified on 23 December appears to have resolved the issue.
Mudrick has converted half of its outstanding $260 million debt to equity – giving it a 70% stake in Vertical – and the deal fixes the conversion price of the remaining notes.
That agreement has already yielded progress: a funding round that closed on 24 January brought in gross proceeds of $90 million – including $25 from Mudrick – a figure increased from an original target of $75 million due to investor demand . Additionally, the warrants that form part of the offering could generate another $100 million if redeemed for cash.
But even with the latest investment, Vertical will need to return to the market for more funding to push the VX4 through certification and service entry, and to build out necessary production infrastructure. By most estimates, the recent funding round extends the company’s cash runway into 2027.
If investor enthusiasm appears one tangible outcome of the Mudrick deal, another is the return as chair of Domhnal Slattery, the former chief executive of lessor Avolon.
Slattery is no stranger to the Vertical role, having previously held the post from January 2022 to August 2023, but on this occasion Mudrick proposed his return, according to Vertical’s stock market filings, signalling a shift in the balance of power. And if further evidence was needed, company founder Stephen Fitzpatrick on 22 January announced that he would step down from Vertical’s board once the funding round had closed – a departure confirmed at the end of the month.
Although the corporate-level moves do not solve all of Vertical’s challenges, whether technical, financial or regulatory, they do at least allow the team at Kemble to focus on their mission: seeing the VX4 soar.
