Clear Prop #8 | Forum 79 Paper Spotlight #1
FEATURE: Are eVTOL Aircraft Inherently More Susceptible to the Vortex Ring State than Conventional Helicopters?
This is the first in our Vertical Flight Society Forum 79 Feature Series where we dive deep into exciting papers which will be presented in West Palm Beach, Florida, May 16-18. The VFS Annual Forum, organized by the leading vertical flight non-profit organization around the world, will feature 250+ papers that are the latest & greatest in rotorcraft, drone, eVTOL, and AAM research. There are also two short courses — one on eVTOL Technology and another on Artificial Intelligence (AI) — on Monday, May 15. Highly recommended to attend.
In this edition, I sit down with Richard Brown, the CTO of Sophrodyne Aerospace. Richard published a thought-provoking paper last September on examining the Vortex Ring State (VRS) as it may apply to eVTOLs. In our conversation, we explore the “big picture” of what this means for the industry going forward and dive into some of the technical insights that are hidden in between the lines of the paper.
Below, you can find the key takeaways of the research followed by our conversation.
FEATURE: Are eVTOL Aircraft Inherently More Susceptible to the Vortex Ring State than Conventional Helicopters?
VRS - colloquially known among helicopter pilots as “settling with power” - is a dangerous flight regime where at the right combination of descent rate and forward speed, the rotors lose some of their ability to generate thrust. This is due to the rotor becoming enveloped in a highly unsteady, vortical flow. Counterintuitively, if the pilot attempts to arrest the descent rate of the vehicle by raising the collective, they can end up increasing the descent rate even further. If the pilot is not trained in effective means to escape VRS, the outcome can be catastrophic. Since eVTOLs are envisioned to have higher disc loading than helicopters, comprise multiple rotors that interact aerodynamically with each other, and operate in tight urban configurations with extreme gusts & wind shear, VRS may be critical to the safety of urban air operations.
Key takeaways:
Compared to helicopters where VRS can often be understood in terms of the behaviour of the main rotor in isolation, the paradigm of distributed electric propulsion will require eVTOL designers to examine VRS as a characteristic of the vehicle as a whole. For example, looking at thrust-vectored vehicles (think Joby, Archer, Vertical Aerospace), the rearward tilt of some of the rotors during transition from airplane to helicopter mode while arresting forward speed may be sufficient to precipitate the onset of VRS.
eVTOLs are subject to VRS over a much larger region of their flight envelope compared to similarly sized helicopters such as the Bell 206B. In essence, the increased disc loading of these vehicles compared to helicopters with similar weight means that helicopter operational experience might be a very poor guide to understanding eVTOL susceptibility to VRS.
The closely-spaced, multiple-rotor configurations seen in many eVTOL designs are susceptible to complex wake interactions. Side-by-side rotors and rotors in tandem are particularly susceptible to a type of interaction that can expand the range of descent rates over which the aircraft experiences VRS-like symptoms.
Asymmetric VRS can occur during rolling maneuvers where the rotors on one side of the aircraft might experience a loss of thrust while those on the other side do not, creating a challenge for the design of effective flight control systems.
High disc loading can be a double-edged sword when it comes to gusts, especially in urban environments. High disc loading values can reduce the sensitivity of the eVTOL to turbulence while at the same time increasing the likelihood that gusts may precipitate the onset of VRS. This means that eVTOLs may provide a smooth passenger experience during gusts but at the same time run the risk of abrupt onset of VRS during their approach to land, especially in turbulent conditions.
The BFD: It’s quite common to see flight testing programs during certification expand in time & money due to certain aerodynamic interactions that were unforeseen during preliminary design. Unanticipated problems with VRS may cause similar issues for eVTOL aircraft. Conventional aerodynamic tools struggle to realistically model VRS. Thus, specialist methods will be needed at the design stage to understand the sensitivity of the aircraft to the onset of VRS. Adoption of the appropriate tools will not only save valuable time & capital for OEMs but will make urban air taxi operations safer in the long run.
Pamir: Richard, thanks for being here. Could you tell us a little bit about your background and your motivation to tackle VRS for eVTOL?
Richard: My pleasure. I’ve been in aviation for 30 years now and have dedicated my career to aerodynamics - with a strong emphasis on using mathematical modelling to improve physical insight. I’ve held various academic positions over the years, including at Imperial College London and The University of Glasgow. Six years ago, I decided to leave academia and form my own research consultancy, with the conviction that research can be more effectively applied by working directly with the industry.
A key point in my career was the V-22 Osprey crash back in 2000 that led to the death of 19 US Marines in Arizona while practicing a nighttime tactical descent in the desert. It soon became apparent that the accident was caused by the onset of VRS given the high rate of descent of the aircraft. This accident motivated a group of us to understand VRS properly from first principles. It seemed pretty clear at the time that the basic physical mechanisms leading to the onset of VRS were not particularly well understood - even though a lot of material had been written on the subject. Pilot’s handbooks still describe the phenomenon as “descending into your own wake”. It’s not quite as simple as that. VRS happens when the inherent instability of the rotor wake catches up with the rotor, rather than the wake itself. The pictures that you see in textbooks that suggest rotors producing these perfect helical vortices in their wake are very misleading - the dynamics turn out to be far more complex and interesting.
What this means practically is that VRS can be encountered at much lower descent rates than you might expect if you believe in the more simplistic explanation for its onset. Indeed, our new models agree much better with observations than the older ones do. Having kept a close eye on the eVTOL industry, there seemed to be a real need for someone to come forward and warn about the potential dangers of VRS posed to these novel aircraft with their multiple rotors.
Pamir: How did you integrate eVTOLs into the VRS model? What are some of the assumptions you’ve made about these vehicles?
Richard: The model in the paper does not consider specific eVTOL configurations or designs. When we are mapping various helicopters and generic eVTOLs onto Perry’s VRS diagram - which essentially shows where you are likely to encounter VRS based on your forward speed and descent rate - eVTOLs are characterized purely by their disc loading. In other words, there are no preconceptions made as to whether the eVTOL is a tilt-wing or a multicopter, nor are any assumptions made about the dimensions of the lifting surfaces, blade twist, and such. Keeping the analysis at this level of generality essentially gives the OEMs a recipe, which they can then translate into their own terms.
Going deeper for a specific eVTOL design, we would expect to find that the shape and size of the VRS regime would be roughly similar to that predicted by the generic model, but that the specifics of the vehicle would introduce more texture and structure into the characterization. The reason for this is simply that the aerodynamic interactions between the rotors themselves, and also between the rotors, wings, and the fuselage, as well as the details of the rotor blade design all affect the behaviour of the wake system as the aircraft approaches the onset of VRS. Our hope is eventually to produce a set of heuristics that will allow designers to predict beforehand what features of their configuration might influence its behaviour near the onset of VRS. However, that level of understanding is still a way off in the future. For the moment, we’re reliant on detailed numerical simulations and flight testing to pick up any signs of trouble.
Pamir: You emphasize asymmetric VRS in your paper. What are some of the dangers associated with this flight regime?
Richard: It’s important to understand that when the rotors are distributed around the airframe, they may not all be seeing the same aerodynamic conditions at the same time. An example I give in the paper has the aircraft turning abruptly onto a new heading while descending to land. If the turn takes place at a particularly inopportune point along the aircraft’s trajectory, then it can be imagined that the combination of roll and yaw can create conditions on one side of the aircraft that will take a rotor mounted outboard on the wing into VRS. The rotors on the other side of the aircraft will remain unaffected. In this situation, the danger is that one side of the aircraft loses thrust, generating a powerful rolling moment that the control system will struggle to counteract. Indeed, designing a flight control system that can cope with such a situation will be an extremely challenging task given the highly nonlinear aerodynamic response of the rotors.
Pamir: How are eVTOL OEMs tackling this problem today?
Richard: After publishing our research, a number of eVTOL companies have approached us to understand how to integrate our analysis into their aircraft development processes. It has been very heartening to me to have been able to contribute to an increased awareness within the industry of the potential dangers posed by this particular aerodynamic phenomenon. So, without talking in specific detail, we are actively working with several eVTOL manufacturers to support them in tackling potential issues around the onset of VRS in their designs.
A few firms in the industry have been a little more stubborn, having managed to convince themselves for various reasons that VRS could not possibly be a problem for their specific eVTOL design. Their reasoning is often simply that they have not encountered this issue in their simulations or flight tests.
One or two OEMs also seem to be pretty confident that the envelope protection systems they’ve built will be enough to prevent their aircraft from ever encountering the VRS. The problem that I see here is that the aerodynamic tools that are usually used for aircraft design are often not able to give a realistic appreciation of the dangers posed by VRS. Also, VRS will not immediately be encountered during the course of the normal, prudent approach to envelope expansion that takes place during flight tests - you need to go looking for it. The danger, of course, is that the flight test team ends up being unprepared for an inadvertent entry into a rapid descent, or an encounter with a stronger-than-usual gust that suddenly precipitates the onset of the VRS. The lesson learned 23 years ago in Marana was a hard one, and we should do our best not to forget it.
Pamir: What would be your critical message to eVTOL companies from here?
Richard: I see three main issues that OEMs should consider taking to heart:
The potential onset of VRS under certain flight conditions is an issue that all OEMs definitely need to consider. Designers need to verify that their aircraft do not contain hidden failure modes with regard to VRS. It’s really up to the OEM to do the groundwork in ensuring their aircraft is safe against this phenomenon under all operational and environmental conditions that their design will encounter. Edge cases are important - helicopter experience tells us that entry into the VRS is often the end result of something else having gone wrong. If potential issues are caught during simulation or flight testing, then a lot of time & capital, and potentially lives, might be saved.
The appropriate CFD tools need to be used to model the unsteady, vortical flows that multiple interacting rotors produce across all regimes of flight. There is a tendency in the industry to use pre-packaged, general-purpose simulation tools that, although relatively easy to use, do not have the power to resolve the aerodynamics of these complex configurations to the requisite level of detail. In particular, modelling the flow instabilities that lead to the onset of VRS is particularly challenging from a numerical point of view. This needs simulation capabilities that are carefully constructed to properly represent the pertinent physics.
The potential that eVTOL aircraft might encounter VRS feeds directly into some of the regulatory decisions that will govern type certification and vertiport design. Some recent proposals for how eVTOLs should negotiate the urban environment will require the aircraft to fly an approach trajectory that is very much steeper and slower than your average helicopter operator would be happy with. A real step forward would be for OEMs, vertiport designers, and regulators to come together to ensure that the trajectories envisioned for vertiport operations are within the bounds of what is safe with respect to VRS and other issues such as maximum tolerable outwash damage. Effectively, we need to be realistic as to what these aircraft can achieve performance-wise whilst still being subject to the usual laws of physics!
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