Aircraft crashworthiness will be one fundamental element of assuring the overall safety of urban air mobility operations, and must be designed into eVTOLs from an early stage.
Those were two key takeaways from an eVTOL crashworthiness panel held Jan. 22 at the Vertical Flight Society’s (VFS’s) Transformative Vertical Flight 2020 conference in San Jose, California. Speakers from NASA, the Federal Aviation Administration (FAA), Uber, Bell, and Jaunt Air Mobility discussed the role that crash safety will play in urban air mobility, and some of the technical and regulatory challenges associated with protecting the occupants of future eVTOL aircraft.
“The success of eVTOL is going to very much be linked to how well we protect our public image,” pointed out Michael Smith, a long-serving engineer at Bell who is now part of the company’s Innovation team. By mitigating the consequences of otherwise catastrophic accidents, he said, a focus on crashworthiness not only “protects the occupant . . . it protects our industry.”
Ryan Naru, vehicle standards lead for Uber Elevate, described crashworthiness as part of Uber’s “layered” approach to eVTOL safety, which takes into account operational factors as well as aircraft design.
“Crashworthiness is one piece of mitigating the total fatal accidents and reducing fatal accident rates,” he said. “You’re going to have to accept the fact that accidents will occur, [so] what can we do to mitigate those accidents away from fatalities and towards a more survivable incident?”
With more than 250 models currently listed in VFS’s World eVTOL Aircraft Directory, many eVTOL developers may be more focused on simply getting their aircraft aloft than preparing for worst-case scenarios. But the panelists emphasized that early design choices will have significant downstream effects on crash safety.
For example, batteries located in the sub-floor could potentially negate some occupant protection systems, said Justin Littell, an engineer with NASA Langley Research Center’s Structural Dynamics Branch. “If you have a Kevlar honeycomb in your sub-floor and now you put a rigid battery in there, the rigid battery is going to take up the load and any kind of energy-absorbing additions you have are not going to work,” he noted.
Emphasizing the role that crashworthy structures play in protecting aircraft occupants, FAA chief scientific and technical advisor for crash dynamics Joseph Pellettiere urged eVTOL designers to give primacy to structural design, rather than counting on the safety benefits of add-on features.
“Yes, you can come up with other techniques like a deployable absorber, an airbag, a parachute, or something, but all of that is going to add complexity, weight, other aerodynamic issues,” he said. “The more space that you can show for crushable space and attenuating energy is going to allow you to size those systems differently, it’s always going to be reliable, and it’s going to be a much easier way to show that you have some protection for the occupant.”
The importance of crushable space notwithstanding, Jaunt Air Mobility chief technology officer Martin Peryea suggested that external airbags are an add-on feature that “could buy its way onto” some aircraft designs. While Jaunt’s Reduced rotor Operating Speed Aircraft (ROSA) concept has a large overhead rotor and, like helicopters, is capable of autorotation, airbags could be an especially promising solution for other eVTOL designs that can’t perform the same type of emergency landing.
“It is a challenge with these types of vehicles in terms of performance, but the fact that they don’t have large rotor systems to do autorotations or big wing systems to do a run-on landing, we have to visit some of these technologies — and external airbags is probably the answer for a lot of these aircraft,” he said.
Such external technologies are not new. Littell was part of the NASA team that tested a so-called “deployable energy absorber” on an MD 500 helicopter in December 2009. The composite honeycomb concept — which has a flexible hinge design that allows the entire structure to lie flat until deployed — successfully protected the airframe from much structural damage, and the four crash test dummies within from significant injuries.
“I’m a very big proponent for external airbags. They work well on every surface,” said Smith, although he granted that they do add about 1.5% of an aircraft’s gross weight. Like some other panelists, he was less enthusiastic about ballistic parachutes, which don’t necessarily work reliably at the low speeds and altitudes associated with VTOL operations.
While further research and development could conceivably improve the outlook for ballistic parachutes in urban air mobility, Naru said, “a significant amount of research is going to need to be done if we expect the next generation of eVTOL vehicles to include a parachute as an above-and-beyond step to mitigate these potential fatal accidents.”
Meanwhile, Uber sees next-generation energy-absorbing seats as a particularly high-priority research area for eVTOL crashworthiness, according to Naru.
“We’re looking at what would potentially be a mass-market product,” he said. “That means that there will be children flying with us; there will be people who are above the traditional 150 pounds of the anthropomorphic test dummy, and both categories of individuals will not be well protected by a stroking seat. So we need to think about . . . what is the next generation for a seat that can be adapted specifically to the occupant that is in it?”
Several panelists also called attention to the post-crash fire risks associated with the lithium batteries in many eVTOL designs.
Said Peryea, “Obviously we have to design systems that will survive a crash and not catch fire. You do have a lot of stored-up energy in the battery system, and the impact loads could ignite that battery system. And you’ve got to give the occupants enough time to leave the aircraft. . . . This is going to be a challenge.”