Clear Prop #10 | Forum 79 Paper Spotlight #3
FEATURE: A Summary of Test Results from a NASA Lift + Cruise eVTOL Crash Test
This is the last in our Vertical Flight Society Forum 79 Feature Series where we dive deep into exciting papers which were 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, featured 250+ papers that are the latest & greatest in rotorcraft, drone, eVTOL, and AAM research. If you missed it this year, please access all recorded content and papers here. The next one (Forum 80) will take place on May 7-9, 2024 in Montréal, Canada.
Highly recommended to attend, best technical event of the year - hands down!
In today’s feature, I interview Justin Littell who is a Research Engineer at NASA Langley specializing in aircraft crashworthiness with a major focus on rotorcraft. Recently, as a technical lead in the Revolutionary Vertical Lift Technology (RVLT) Impact Dynamics Project, he has been leading a team of engineers conducting research into various aspects of eVTOL crashworthiness. With his co-author Jacob Putnam, Justin presented an exciting paper that tested a severe but survivable crash with a full-scale lift + cruise reference vehicle designed by NASA.
It is (unfortunately) the inevitable fact that while these vehicles will be certified by the FAA/EASA diligently and thoroughly, we should be expecting eVTOL accidents once they are commercially flying. Therefore, it is essential to design the aircraft and its subsystems with a high survivability rating for the pilot(s) and the occupants.
In the following, I will be walking you through the key takeaways of the paper. Then, we will dive deep in between the lines with Justin, exploring what this research means for the eVTOL space going forward.
FEATURE: A Summary of Test Results from a NASA Lift + Cruise eVTOL Crash Test
eVTOLs not only differ from conventional rotorcraft in terms of aircraft design (i.e. distributed propulsion units around the fuselage, typically a high wing configuration, and a higher useful-load-to-MTOW ratio due to the presence of batteries), but the fact that a large amount of composite materials are used. There is not enough accumulation of research into how composite eVTOLs perform under certain crash configurations. Therefore, a lift + cruise vehicle was fitted with multiple crash test dummies and different energy absorbing systems. It was then dropped from a height of 35 ft under a swinging condition (meaning a combined horizontal and vertical load). 100s of different channels of sensors collected data on the loads experienced by the fuselage, seats, and the dummies, supplemented by a camera feed analyzing the destruction by precision.
Key takeaways:
Energy absorbing subfloors and seats were effective at protecting test dummies, where lumbar loads were below FAA’s established certification limits. On the other hand, crash dummies that did not have energy absorbing systems were over the limits.
The collapse of the roof due to the presence of the overhead mass (think the high wings and the propulsion system) was not predicted in simulations. This was significant as it caused neck loads over established limits.
The energy absorbing subfloors deformed to the extent necessary only for the heaviest occupants (meaning they performed as intended). These structures should be optimized per occupant weight to ensure they provide the protection as intended since their performance is correlated with the size of the passenger.
The researchers used a variety of load thresholds on the test dummies in addition to FAA’s requirements around the 50th percentile occupant by weight. The Neck Injury Criteria from the National Highway Traffic Safety Administration (NHTSA) and further lumbar limits for the 5th and 95th percentile occupant imported from the Full Spectrum Crashworthiness (FSC) Criteria for Rotorcraft, established by the US Army, were used.
Asymmetric damage was observed between the port and starboard sides, which may have been caused by the 2-degree yaw upon impact.
The BFD: Using simulation software to make predictions about novel systems can in certain instances lead to inaccurate results. Thus, NASA’s aim with these series of physical testing is three-fold: 1) increase understanding of eVTOL crashworthiness, 2) optimize their simulation models for future predictions, and 3) support industry in developing consensus standards on crashworthiness. New regulatory standards for composite and/or eVTOL aircraft may be developed as the knowledge base in this space grows. This research also sheds a spotlight on various operational aspects that are not being discussed in the industry today. For example, if energy absorbers need to be optimized based on the occupant’s weight, children and overweight individuals may need to be seated at specific seats assigned to them by the operator prior to the flight. Therefore, in addition to a weight & balance consideration, there may be a “crashworthiness safety consideration” to seating arrangements.
Pamir: It is a pleasure to have you here Justin. For eVTOLs, why can’t we use our existing understanding from rotorcraft and fixed-wing crashworthiness? What’s special about eVTOLs here?
Justin: It’s my pleasure, thank you for reaching out. As a matter of fact, it’s not the novel configurations specific to eVTOLs that are the most important factors in crashworthiness performance here. Rather, it’s the fact that composite structures are novel. And we do not know about them from a crashworthiness perspective as much as we know for metallic structures. Metals buckle, and they have elastic and plastic deformation curves when subject to stress. Composites, on the other hand, do not exhibit a plastic range. Therefore, you do not see permanent deformation with these materials. They simply “pop” without much warning in advance. This characteristic makes it much harder to predict when and how they are likely to fail. For example, this is what happened with the roof collapsing in this particular test due to composite brittle failure. We did not anticipate this coming. Hence, you have to design the material and how it fits within the larger structure of the aircraft differently.
While the material dominates crashworthiness performance, the aircraft design as a whole also matters. In this test, we didn’t really define the exact configuration but took the wings and rotors as point ballast masses. Therefore, this was a more generic aircraft than a lift + cruise configuration - essentially an agnostic 6-seater, composite, high-wing vehicle.
Pamir: You mentioned that there needs to be more research done for us to understand how eVTOLs perform under crashes. What are some other aspects of crashworthiness that we need to test and have a good understanding of?
Justin: First and foremost, batteries. The question is, “how does the impact outcome change when you have a large mass of batteries integrated into the aircraft structure?” Besides deformation and mechanical damage to the occupants, fire hazards inherently associated with batteries need to be understood for crashworthiness.
Secondly, we need to make sure that the overhead mass is accounted for in various impact conditions. As our research shows, the roof crashed on the test dummies causing neck injuries above the limits given by the NHTSA. We need to specifically uncover how different eVTOL designs affect this failure mode (e.g. high wing, low wing, no wing, etc.) and what needs to be done for each condition in terms of structural design.
There are certain other off-nominal events that we need to consider for crashworthiness as well. These include the effect of bird strikes on composite materials and the potential for multiple rotors spaced out along the aircraft to fly away during impact.
Pamir: Is there simulation software that approximates reality well for such crashes and their effect on passengers?
Justin: Simulation is a big part when we are exploring different impact conditions for various designs of aircraft. We use software such as the 3DS Abaqus and the Ansys LS-DYNA, which are commercially available finite element analysis and crash test simulation code. However, it is hard to tell how predictive and accurate they are when compared to real crashes. Therefore, after testing real crashes, we go back to our simulation capabilities and compare & contrast the shortcomings of our models. This is exactly the expertise of my co-author Jacob, who led a paper for this specific test. Jacob updated the assumptions and parameters that form the bases of our simulations and the newly fine-tuned model will be tested on a second test that we’ll be doing on an identical test article. However, we will likely vary the vehicle configuration a bit, adjust the impact condition, use different energy absorbing subfloors, and optimize other aspects based on our learnings from the first crash test.
Pamir: What is the future state-of-the-art for improving crashworthiness safety performance?
Justin: Currently, there is no revolutionary technology out there that will change the playing field on this. What we need to do is optimize the composite materials we have and have them fail predictably under various impact conditions. Hybrid materials is one area we are looking at. They are essentially an emergent type of nanotech that combine two composite materials such as carbon and aramid or carbon and glass fiber. We are using them to study the effect on failure and ultimately crashworthiness of the combined hybrid materials.
Hybrid composites can be engineered to have a great strength-to-weight ratio and energy absorption capabilities while at the same time designed to be lightweight. One thing to note here is that these materials are not entirely new. However, we need to figure out to use them in novel use cases and test them under specific crashworthiness scenarios.
Pamir: What would be your main critical message to the eVTOL OEMs from here?
Justin: Think about crashworthiness when you’re designing your vehicles. Early design decisions including the 3D weight distribution around the fuselage and the location of batteries plus the energy absorption systems for the seats and cabin subfloors may reduce the likelihood of severe injury and save lives. When I look at where the industry is today, we are - for the most part - still at the design phase when we can add crashworthy features relatively easily. Harder to do when you already have a production vehicle you are aiming to certify and you need to reiterate mid-way.
On the regulatory & certification side, the big question right now is that we do not know the actual flight test campaign for impacts. The impact configurations are not known yet so we are hoping that this research and its continuation supports the FAA and other regulatory bodies on their certification standards.
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