How Vertical Aerospace is progressing toward first flight of the VA-X4 eVTOL

Avatar for Elan HeadBy Elan Head | September 1, 2021

Estimated reading time 22 minutes, 3 seconds.

Last year, we visited with Tim Williams, chief engineer for Vertical Aerospace, shortly after the company revealed its commercial eVTOL design. The aircraft — which was then called the VA-1X and is now known as the VA-X4 — is a five-seat, fully electric air taxi designed to fly over 100 miles (160 kilometers) at speeds of up to 200 mph (320 km/h).

A lot has happened since that conversation. Notably, in June, Vertical Aerospace disclosed plans to go public through a merger with the special purpose acquisition company (SPAC) Broadstone Acquisition Corp. At the same time, it announced pre-orders for up to 1,000 of its aircraft from American Airlines, Virgin Atlantic, and the aircraft lessor Avolon.

Like several of its competitors, Vertical has the ambitious goal of launching commercial operations in 2024 — which, in the context of aircraft certification programs, is just around the corner. So we decided to check back with Williams for the latest on the VA-X4 development program. (This interview has been edited for length and clarity.)

Vertical Aerospace VA-X4 eVTOL
Vertical Aerospace is preparing to assemble its VA-X4 demonstrator aircraft. “I keep telling everybody that I’m getting the world’s biggest Airfix kit,” said Tim Williams, referring to the popular brand of model aircraft kits. Vertical Aerospace Image

Elan Head / It’s great to reconnect and I’m excited to hear about your progress. To start, maybe you can just give me an update: What’s the status of the first prototype?

Tim Williams: Last time we spoke, we were just coming out at the end of a series of sprint activities trying to define the scope of the demonstrator aircraft. Since then we’ve gone through a number of design reviews. We’ve gone through our preliminary design review, we’ve gone through our critical design review. We’ve finalized the design of the demonstrator. And we’re currently in a situation where most of our parts are out for manufacture and we’re expecting the delivery of a huge number of components. Some of our big structural components are still being worked, but we’re now gearing up to start receiving all those parts and starting to assemble our aircraft kit anytime now, with a view to flying it before the end of the year. How closely will this demonstrator resemble the production aircraft?

Vertical Aerospace chief engineer Tim Williams

Tim Williams: There are a number of compromises that we’ve included in the demonstrator. The biggest one is that, for ease of access and for safety reasons — we’ve not flown an aircraft like this before — we’ve actually incorporated the batteries in the main body of the fuselage of the aircraft, so where passengers would normally sit. Clearly we need to work out where we package those batteries in the final certification aircraft. We have a view for where they should be going, and that was part of the design studies that we did last year.

Those design studies were really focused on the final aircraft, and then we decided to change to a technology demonstrator program, so that we learn and get some data and then when that’s done, we flip back onto the certification program. From the outside, if you blur your eyes [the demonstrator] will look almost identical to the cert aircraft, but there are a number of compromises in some of the functions inside the aircraft. Have you made any changes in terms of what you expect the final production design to be?

Tim Williams: Not yet, although we have got a lot of data that will feed into the final production design. So, for example, we’ve spent a lot of time in the wind tunnel. We have a wind tunnel near us that we’ve been using, so we’ve been to the wind tunnel twice — we went in to understand our wing aerodynamics and interactions between rotors and wings. We’ve just recently tested a 14% scale model of the aircraft. So we’ve now got a lot of data that helps us understand the lift and the drag of the aircraft.

Vertical Aerospace wind tunnel test
Tim Williams said that the results of Vertical Aerospace’s recent wind tunnel tests were “pretty much in line with expectations.” The company will use insights from the tests to fine-tune the aircraft’s drag profile. Vertical Aerospace Image What are some of the other questions that you’re looking to answer with a demonstrator in terms of nailing down that production design?

Tim Williams: We’re looking to understand the overall performance of the aircraft, looking to understand how our control system is going to work. It’s been widely publicized that we’re working in conjunction with Honeywell for our flight control system. We’re hoping that that’s going to give us a technical edge, so we need to understand and validate that control system, its behavior.

We’ve done a lot of work to try and understand other aspects like noise. We’ve been running some rigs outside; we have what we call a single rotor rig. It’s a big structure we’ve been testing at our local airfield, getting some noise measurements of the rotor blades from that rig — those are pretty much in line with expectations. Some of the data that we need to get off of this demonstrator aircraft is what’s the noise like at an aircraft level and are there any interaction effects between the various different rotors that we haven’t been able to test so far. So that’s something that’ll come out of the flight test program.

We’re looking to get a good understanding of operability, stability, handling characteristics. So yeah, there’s a lot that we need to get out of this program, and the next 12 months is going to be really exciting to get us some of that data for the future programs. I noticed in some of Vertical’s SEC [Securities and Exchange Commission] filings that you’re expecting the noise in a hover to be around 70 decibels, which is a bit higher than some of your competitors. How did you arrive at that estimate, and are you going to be working to reduce the noise level and do further work on the acoustics?

Tim Williams: Yes, we are. And again, that’s a piece of testing that’s just about to start. We’re taking a rig down to the Institute of Sound and Vibration at Southampton University; we’re going into their anechoic chamber to try and understand the noise characteristics of our current rotor blade. We have a program running to explore a next generation of rotor blades which should reduce that noise even further, but we don’t have the data around that yet. What we’ve advertised is what we feel confident of doing on the first iteration. You mentioned Honeywell, and Vertical has announced a number of different partnerships over the past year. Can you describe how you’re working with some of those suppliers on the design and build of the demonstrator and eventually other prototype aircraft?

Tim Williams: Our approach is that we will partner with the people that we consider to be among the best in the industry. Honeywell with their flight control system, for example, have far greater experience of those aspects of the aircraft than us. So we work really closely with them, we have a really good working relationship, we’ve been working with Honeywell for a number of years now. We freely share data, and we’ve been working really closely to try and develop the control laws.

We have a number of different suppliers and partners that we’re working with. The other big announcement over the course of last year was that we will be looking to work more closely with Rolls-Royce, for their electrical motors and power distribution system. And again we’ve developed a really good relationship with them. The demonstrator aircraft won’t have Rolls-Royce motors on there, but we’re already talking with them about requirements and how do we integrate their system onto the aircraft. How do you plan to structure the flight testing for this first demonstrator? Are you still planning on starting with a pilot in the aircraft, are you planning on doing on unpiloted flights, and how do you expect that program to progress?

Tim Williams: We need to get some really urgent data in the first six months of next year. We’re planning to start our initial flight using remotely piloted capability, which I think is almost industry standard for eVTOL manufacturers. We have designed the aircraft to be capable of taking a pilot; it’s been designed to essentially the [European Union Aviation Safety Agency/EASA] SC-VTOL requirements. So, we are exploring, we are planning, and we would very much like to put a pilot into the aircraft once we’ve got some initial understanding of how it behaves and we can convince ourselves that it’s safe to do so. We don’t see any issues there, but it’s something that we need to build up to slowly.

We’re still working out some of the details of the overall flight program. I think it’s fair to say the initial flights will be tethered rotor-borne flights, and then we will progressively expand the rotor-borne envelope. The sequence in which we go from rotor-borne to conventional and the transition in between is yet to be decided.

Vertical Aerospace is exploring medevac and cargo applications for its eVTOL aircraft, in addition to passenger transport. Vertical Aerospace Image When do you expect to move on to subsequent prototypes that are more type representative?

Tim Williams: So as I say we’re just getting most of our parts into the factory now to start building the [demonstrator] aircraft. There’s still going to be a lot of engineering effort to put that aircraft together, but I think the load will probably drop off beginning of quarter four this year. We’re already doing the sizing activity and some of the scope and requirements for the cert aircraft. I think we’ll be starting to develop that cert aircraft “in anger” towards the end of this year, and through next year.

Our hope is that we will have an aircraft that’s more representative of the final cert aircraft by the end of next year. Whether that’s a second prototype that needs subsequent iteration or whether that’s the first sort of final version is still being worked through. Last time we spoke, you were planning on pursuing initial certification with the CAA [U.K. Civil Aviation Authority] and validating that through EASA. Is that still the plan?

Tim Williams: Yes, that’s still the case. The conversation [with the CAA] is going very well, they’re taking a really serious approach. I think it’s fair to say that their approach is going to be broadly similar to the requirements of SC-VTOL and EASA.

I think I mentioned last year we had a really good relationship with EASA in the past. We’d invited them into the organization quite frequently; they witnessed a number of different tests. We’ve had to build our relationship with the CAA. I think it’s becoming very strong, we are getting good engagement with them. We’re looking to get our design organization approval by the end of the year. Circling back to the batteries: The batteries that are in this demonstrator, are those roughly the batteries that are going to be in your production aircraft as well, or are you using a different technology?

Tim Williams: I think the batteries that will be in the certified aircraft will be an evolution or two away from where we are now. I don’t want to give too much away on the batteries, but there will be some changes, I believe. We are currently in discussions with a number of different people around the world in terms of cell supply and what that might mean for battery designs. Related to batteries, crash safety is obviously a big factor. How are you incorporating that into the design of the aircraft?

Tim Williams: We have done a whole load of safety tests on the battery packs. As I mentioned, the relationship we had with EASA previously was really good, and we did a 15-meter drop test on a battery pack and actually invited EASA to come and witness it. We’ve also done naked flame testing on battery packs to demonstrate that there’s no rapid thermal runaway and those were very successful.

We’ve built on that basis over the last year. We’ve done a whole series of vibration tests, we’ve done a whole lot of additional structural tests on the battery. We’ve really thought hard about the final location of batteries and how we keep them out of any hazardous areas. So it’s all going into our thinking about the overall program and we feel comfortable that we’ve got a really sensible approach to our battery pack design for the demonstrator and subsequently for the cert program.

Virgin Atlantic VA-X4 eVTOL

Virgin Atlantic is exploring a joint venture with Vertical Aerospace to bring short-haul electric vehicle connectivity to its U.K. airport hubs. Vertical Aerospace Image Is there anything else that you’ve learned about the aircraft design in the past year that you think is especially interesting or worth highlighting?

Tim Williams: It’s not all been plain sailing. We’ve had a few tests where we’ve had some problems. But I think we’ve got a really good approach to addressing issues and we’ve built the company with some really strong foundations. For example, we failed a vibration test on the battery packs, so we quickly looked at the data, we looked at the evidence, we went and did a really good understanding and got to root cause, and very rapidly were able to introduce a modification to address that particular concern.

I guess one of the big lessons I’ve learned is don’t leave your focus on weight until too late in the program. I think it’s fair to say that for the demonstrator we have struggled with weight. But again, that knowledge you get really sets you up with some great learning, and a great approach to how you might address things slightly differently in the longer-term cert program.

I think what I really like about Vertical is our approach is learning by doing and not being afraid to try something, and if it goes wrong, well, that’s a positive point on the chart somewhere so we can try something different. So we’ve got that sort of entrepreneurial spirit in the engineering team and people aren’t afraid to try things, which is really good news, and it helps us move forward quite quickly.

Notice a spelling mistake or typo?

Click on the button below to send an email to our team and we will get to it as soon as possible.

Report an error or typo

Have a story idea you would like to suggest?

Click on the button below to send an email to our team and we will get to it as soon as possible.

Suggest a story

Join the Conversation

  1. Avatar for Elan Head
  2. Avatar for Elan Head
  3. Avatar for Elan Head


  1. My mind goes back to the ‘flying bedstead’ and its development into the world beating Harrier aircraft. I wish you all the luck in the world in your efforts to introduce ‘green’ travel. God bless you.

Leave a comment

Your email address will not be published.