Arnaud Didey, founder of Neoptera, with a flying subscale model of the tilt-body eOpter aircraft at the Vertical Flight Society eVTOL Symposium. Image: SD

Interview: Arnaud Didey of Neoptera


The eOpter by Neoptera is an odd-looking aircraft design which may well turn out to be a reference configuration in the race to build and certify Electric Vertical Takeoff and Landing (eVTOL) air taxis for Urban Air Mobility (UAM). Compared to aircraft in development by Bell (Nexus), Kittyhawk (Cora), Boeing (PAV), and Airbus (A3 Vahana) for example, the eOpter has a fundamental difference: the body tilts, and not the wings or rotors. Neoptera founder Arnaud Didey says a “tail-sitter” is an inherently safer design, since if the tilt mechanism fails, the airworthiness of the aircraft will not be compromised. We sat down at the Vertical Flight Society’s eVTOL Symposium in late January in Phoenix, Arizona.

The Neoptera eOpter transitions in flight.
The eOpter ready for boarding at a heliport.
The eOpter in flight.

Q: I’m here at the 6th Annual Electric VTOL Symposium of the Vertical Flight Society with Arnaud Didey of Neoptera. Arnaud, thanks for sitting down with us.

Arnaud Didey: Thank you, it’s a pleasure.

Q: Now, why don’t you introduce yourself, Neoptera is a startup, but it seems it’s not your first one. You have been working in aerospace for nearly two decades it seems. And you have also worked extensively with EADS and Airbus and you also have many patents to your name. Tell me about yourself.

Arnaud Didey: I have indeed been working for nearly 20 years now in the aerospace industry. I started working on spacecraft. I was at the time a Systems Architect on Electric Propulsion Systems. It was used for North-South Station Keeping (NSSK). In particular, Intelsat 10 and Inmarsat 4 geostationary telecommunication satellites. So, obviously it is very specific. Spacecraft Electric Propulsion Systems is not what you might have in mind, it is a Plasma Propulsion System (PPS), that relies on accelerating an ionised gas, but nonetheless. I have always worked as a consultant, under fixed-duration contracts and when the spacecraft were launched I had to return to the UK and I was offered a contract to work with Airbus. I moved from spacecraft to aircraft and I have now spent nearly 15 years working on electric actuation systems and more electric aircraft architectures with Airbus. I have never been involved in any of Airbus eVTOL projects however, only commercial aircraft.

That 15-year period was interrupted when I co-founded Reach Robotics, a company based in Bristol where I was in charge of engineering and helped design and develop the world’s first gaming robot called MekaMon, which is now available online and in Apple Stores. It is a rather complex four legged robot with 12 degrees of freedom but I quickly found the product somewhat limited in scope and decided to return to my passion, which is aircraft design. In 2017, I started turning my attention to electric Vertical Take-Off and Landing (eVTOL) aircraft. At first I was intrigued to know if it really was possible to fly viable missions all-electric. I started looking at sizing airframe, motors and batteries and in doing so, I ended up having to look at what was being developed, greatly helped by the Vertical Flight Society dedicated website I wasn’t particularly impressed by the complexity of some of the concepts, having spent quite some time working the design of safety critical systems and their certification implications.

With over 25 patents to my name, many on fault-tolerant actuators, I was against using safety-critical actuation systems and against using excessive levels of redundancy. I rapidly came to the conclusion that the simplest thing to do was a tail-sitting aircraft because transition only relies on conventional control surfaces and differential thrust from the propulsion units. And so I very quickly came up with this unique tandem wing configuration capable of landing vertically and a tilting body designed to ensure that passengers would remain level during flight so as to avoid the drawbacks of past tail-sitters, in which passengers would take-off and land on their backs, with very limited visibility during the vertical flight phases. It became very quickly obvious that this simple configuration had a lot of other benefits starting with the fact that, because of the wing configuration, it was fairly safe to embark and disembark away from propeller hazard, with the upper wing being well above head level. Looking back, this just seemed a logical thing to do and it is only then that we started to realize the full potential of this configuration.

Q: So tell me, the eOpter seems to be a very developed concept already. You’ve already done several models that fly and you were saying yesterday there are some real advantages to this approach.

Arnaud Didey: First of all, we have to remain humble and realize that, although we understand aircraft design, and we have done a lot of subscale testing and full-scale design, modelling and simulations, we still have to go through the motions of building a full-scale aircraft, flying it and demonstrating the merits of this unexplored configuration. If you look at what even the big OEMs [Original Equipment Manufacturer] are doing in this field, they are all tentatively hovering and none of them have transitioned properly, in public at least. So there is a long way to go, but our subscale prototyping, of which the first flew in the fall of 2017, and our analysis of larger scale versions suggests that this is potentially one of tomorrow’s dominant designs. And indeed we have built over the past year and a half a number of prototypes of various scales, the largest having a span of over 1.4m (and 7kg) and it is very encouraging.

But again, however representative these prototypes have been, and we are very careful to make sure that they are as representative as they can be, it can never be fully representative, because not everything scales linearly, obviously. Dimensions will scale linearly, but surfaces scale to the square, volumes/masses to the cube, and crucially, for inertias it is even more critical. So you have to bear that in mind as you move towards a full scale aircraft. We have built and flown eight prototypes so far and each time we try different things. We can test aerodynamic configurations, test different weight distribution and geometries, test different control laws etc. The geometry has been fixed for quite some time now. We iterated two or three times about a year ago on geometry and we now understand how to best configure this aircraft so that it is balanced in vertical flight phase and sufficiently stable in horizontal flight.

Q: So I think you were calling this a tilt-body design because the fuselage, the nacelle stays comfortable for the passengers because it stays level, isn’t that right?

Arnaud Didey: Yes. So it’s very important to understand that there are still people who will look at this and think, “Oh, it’s a tilt-wing”. This is not the case, only the fuselage pivots, and the aircraft does not land in its fuselage but instead lands on its wings. In the event of a failure of the mechanism, the aircraft can still land safely, without endangering its occupants. So if we had elected to tilt wings or to tilt rotors, we would be in a very different situation. We would still be able to design a safe aircraft — all certified VTOL will be safe — but we would require highly fault-tolerant actuators and we would have to deal with potentially severe failure conditions and complex degraded modes. So that is the main benefit of this configuration.

Q: All right. I had seen in your presentation earlier that you’ve incorporated into the design the ability for it to be packaged in a half-length container for travel.

Arnaud Didey: Yes. The aircraft is modular in design for ease of industrialisation and because we wanted to be able to easily service and maintain the aircraft in the field with very little downtime for the operator. And so the aircraft is designed in such a way that the wings can be detached relatively quickly. Obviously it is something that needs to be done by a trained technician, but it is designed to be quite a swift and quick operation. So as a result, it also allows us to transport the aircraft very easily in a small truck or a 20 foot container. And transportation is not only useful for the emergency services and disaster relief operations that we discussed today, it is also useful for shipping the aircraft to your customers!

Q: OK. I read that you have started a new structure in Toulouse [in the south of France], what can you tell me about that?

Arnaud Didey: So in Toulouse, we are working very closely with the Aerospace Valley, they have been extremely supportive and we are also working very closely with ISAE-SUPAERO. So the French side of the company is based on the campus. Olivier Lesbre, the president of ISAE-SUPAERO, was kind enough to allow us to domicile the company in their dedicated space for start-ups. And so until we outgrow this office, we have a presence there and we have access to many facilities such as the “Fablab”, a prototyping lab, and of course we also are surrounded by a wealth of knowledge from world class researchers and professors who can advise on a number of topics including aircraft design, aerodynamics and flight control for example. Not to forget the complex topic of transition flight. So naturally, we set up a small structure there in Toulouse so that we can work at the moment with bright students.

Q: Right, because you already have a structure in Bristol, don’t you?

Arnaud Didey: Yes, in the UK, where we have 2 companies, a consultancy company that I created 20 years ago and that is self-funding a newly created company dedicated to developing this aircraft. So both the UK and French companies are funded by this consultancy company that will eventually disappear when we get external investment and are able to fund our activities and growth that way.

Q: OK, great. And I think you mentioned during your presentation that you’re really seeking that funding to be able to build a full-size prototype.

Arnaud Didey: That’s correct. Today, with the limited resources we have, we can only do subscale prototyping and design work. Really, what needs to happen now is to build and test a full-scale aircraft, our TRL 5 prototype, which will be a two seat manned aircraft. And for that, there is no way we can do it without external investment. So indeed we seek funding to move to the next step, but that will also allow us to expand the team and allow the part-time contributors to become full-time and allow us to recruit further, to scale up both aircraft and team.

Q: Let’s go back for a minute, to the presentation you did just an hour ago. The NASA Transformative Vertical Flight Working Group 4 on Public Services. Tell me a little bit about that group, you’re a contributor now.

Arnaud Didey: Yes. So, towards the end of last year, 2018, we were approached by Johnny Doo, who is the NASA Lead for this working group, to take part as he saw the potential of our aircraft for search and rescue operations, thanks to its reduced footprint. But I think — and I’m reading between the lines here — crucially, that Johnny also realized that we were serious about what we’re doing. And most importantly, realistic about what these aircraft can do, because it’s all very well to sell dreams, at some point you have to deliver, and ultimately you are only kidding yourself. I think this is quite important because if you are not realistic, you can easily develop or try to develop a concept that can never be turned into a viable product. So understanding the limitations of these [VTOL] aircraft, what really matters to achieve the performance is crucial to starting on the right foot because this is not something which you can just later on down the line change and start again. This is a big task, this is very ambitious and pivoting — which is a terminology often used for startups — later down the line isn’t really going to be an option on such large scale projects.

Q: There was a slide I was very interested in, where you were talking about electric or hybrid propulsion. Can you just tell me a little about that?

Arnaud Didey: Yes. So it seems to be a hot topic at the moment. There are advantages to both. It is undeniable that range is limited with a purely electric aircraft. So if your use case requires a longer range, you probably have no choice but to have a hybrid-electric aircraft. But for what we target, which is for example medical evacuation, the air taxi as it exists today, a good example is a return trip Nice-Monaco, these journeys are very short, less than 50 miles (80 km) typically. If you look at the companies that offer private helicopter transportation — tourism, or going from an airport to your final destination if you can afford it — these flights are very short. They are very short because, for a start, in 20 minutes you go quite far in a straight line at speeds above 120 mph (200 km/h). And they are very short because it is expensive. So the use case for these already are compatible with the relatively short range you get from an all-electric aircraft. So a startup like us needs to be realistic, hybrid is a project in its own right, you know, over and beyond the fact that you also have to make an aircraft that can transition from vertical to horizontal flight and back. So we think that hybrid or electric is kind of a red herring. It’s fine if you have the means of a big OEM, if you can afford to look at both. At the same time, for us, there are huge challenges in showing to the world that this is a very effective and simple way of transitioning from vertical to horizontal and that is what we need to focus on to begin with. So this first demonstrator, our TRL [Technology Readiness Level] 5, is going to be all-electric; for TRLs 6/7 we can start looking at hybrid solutions and we have already provisioned for it in the design. We know where we are going to put our power generator, where we would store our fuel, what architecture is required to meet safety requirements and cope with failure modes, etc. None of which is made simpler with a hybrid power unit! So that comes later, it requires more manpower, more money, and it doesn’t solve the very problem that has prevented most past VTOLs from being successful, which is transitioning and flying safely and reliably.

Q: I think you said that you want to place the batteries in the wings for safety reasons.

Arnaud Didey: Well, yes, historically fuel is usually in aircraft wings. So obviously that is the logical place to be store our energy and locate our batteries. This has the benefit obviously of moving away from passengers the safety hazards associated with modern high energy batteries mainly the risks associated with high voltage and fire hazard. So yes, I think most VTOL companies will want to put their batteries in wings, assuming they have wings of course! We do have a lot of batteries as you can imagine, and so it would be a shame not to use that empty space [laughter].

Q: All right, Arnaud, thank you so much for meeting with me today.

Arnaud Didey: My pleasure. Thank you for taking this time and for your questions.