4 ms·

eVTOL have to be designed to the same safety standards as other aircraft; and in fact, they will be held to slightly higher design safety standards due to their

by lambda 4y ago

eVTOL have to be designed to the same safety standards as other aircraft; and in fact, they will be held to slightly higher design safety standards due to their novelty, required to meet safety standards that otherwise only apply to larger transport category aircraft. You are right, eVTOL cannot autorotate, but electric motors can be far more reliable than internal combustion engines, with many fewer failure modes. Additionally, they are much lighter, and so you can have much greater redundancy. There is considerable redundancy in the lift motors on these aircraft, so one or in many cases more than one lift motor can fail while still providing safe and controllable flight. The exact level of redundancy needed is verified via probabilistic risk analysis, considering failure probabilities of different components, and as mentioned the target failure rate will be lower for these than for equivalent sized conventional aircraft due to their novelty. While there's still a lot of design and development work to do, I'd be very surprised if these didn't end up much more reliable and less prone to failure than helicopters and single engine airplanes. Note: I work for one of these startups, so I may be a bit biased, but I also have insight into the design process and safety and redundancy of systems are absolutely a top concern.

lambda 4y ago
Oh, I should also say, for a helicopter autorotation is only particularly helpful if you have enough forward airspeed or height to initiate it. Helicopters come with an HV diagram indicating safe flight envelope, such that autoration could be initiated in case of an engine failure, or you are low enough for a crash to be survivable if you do not have enough height or airspeed for autoration. https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velocity_diagram https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velo... In an eVTOL, if you have enough height or velocity, you should be able to glide on the wing. So you'll be wanting to keep yourself in the appropriate region of the HV curve, and your emergency engine out procedure will be much more like a conventional fixed wing aircraft, rather than a helicopter.
theYipster 4y ago
We shall see… I can make a strong counter argument for your first paragraph. The EVTOL community has long lobbied for, and for some time, has gotten away with a whole new set of performance based standards not previously used to certify other aircraft of similar size and weight. This was, of course, until several weeks ago when the FAA made the abrupt rule change going from Part 23 to 21.17b for these vehicles. Much has been said about this in the industry, but I understand that questioning the performance based standards is at the core. We’ll see where the dust lands, but it’s clear that some in the EVTOL community are not interested in applying airliner safety standards to these vehicles. Questions: is your firm following ARP 4754A and 4761 to the letter? What systems are being mandated to DAL A (requiring a 10 to the minus 9 failure rate?) Are these the usual components found in other jets, or new components that typically haven’t required the same level of safety criticality? These are a whole new type of aircraft—there are so many unknowns that it’s impossible to compare safety standards to other aircraft, even if the intent is to meet something like Part 25.1309. Case in point: power distribution systems in airliners are rarely DAL A, but I’d recon they’re pretty damn safety critical in an EVTOL. No power distribution = no spinning propellers = no thrust, and all on a “powered lift vehicle…” cue the classic line about the V22, “a plane that can’t glide AND a heli that can’t autorotate—all in one!”
- lambda 4y ago
  You're right, some other entrants in the eVTOL community seem to have strategies that rely on some kind of regulatory magic. The company I'm working for, Beta, is taking a very different strategic approach, with much less reliance on "then some regulatory magic happens;" we are focusing on cargo first, and a passenger variant later. We are not particularly concerned with the change to 21.17b, we're willing to work with the FAA on that rather than fighting it. And because our initial target is cargo, such as transporting organs between hospitals with existing helipad infrastructure or cargo from airport to distribution center, there are a lot fewer regulatory hurdles about where we operate than the air taxi market. We are following ARP4754A and 4761, yes. We are still in the process of doing our safety assessments at the aircraft and systems levels and finalizing our type certified design, so I can't say which systems are going to be required to be DAL-A, but I expect that the pilot inputs, flight controller, battery systems, high voltage distribution system, low voltage distribution system, lift motors, and control surface servos will be designed to that level. Our strategy is to build ourselves the critical components for an electric aircraft, so we are doing the battery, high voltage distribution, and electric motors, while we are buying the other components (pilot inputs, flight control computer, control surface servos, primary flight displays, etc) from existing suppliers who already have experience in this space. In the meantime, we are also learning a lot from flight test of our experimental proof-of-concept airframes, which we can feed into this process. We've already built one experimental eVTOL under a previous design, and then two proof of concept airframes with a design much closer to the type certified design that we're now working on; so while we go through the process of breaking down the requirements per 4754A and doing safety assessments per 4761 and working on our final type certified design, we're also learning a lot about operational concerns and refining our design on our experimental proof of concept airframes. As for the V22 line, well, our aircraft can't autorotate, but it's not bad as a glider. That was a primary design criterion; it should mostly be a glider that happens to have electric power. We recently did a cross-country trip halfway across the country on our PoC aircraft in CTOL configuration (https://evtol.news/news/beta-alia-250-prototype-flies-cross-country-to-arkansas https://evtol.news/news/beta-alia-250-prototype-flies-cross-...), with a number of stops on the way, of course, our range is limited by physics, and the majority of that trip was done such that there was an airport within the glide cone of our aircraft; we did allow for a few legs in which that wasn't always the case, and we would need to ditch in the event of power failure, but for most of the trip there was an airport within our glide cone if we had a power off event.

by lambda 4y ago

lambda 4y ago

Oh, I should also say, for a helicopter autorotation is only particularly helpful if you have enough forward airspeed or height to initiate it. Helicopters come with an HV diagram indicating safe flight envelope, such that autoration could be initiated in case of an engine failure, or you are low enough for a crash to be survivable if you do not have enough height or airspeed for autoration. https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velocity_diagram https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velo... In an eVTOL, if you have enough height or velocity, you should be able to glide on the wing. So you'll be wanting to keep yourself in the appropriate region of the HV curve, and your emergency engine out procedure will be much more like a conventional fixed wing aircraft, rather than a helicopter.

theYipster 4y ago

We shall see… I can make a strong counter argument for your first paragraph. The EVTOL community has long lobbied for, and for some time, has gotten away with a whole new set of performance based standards not previously used to certify other aircraft of similar size and weight. This was, of course, until several weeks ago when the FAA made the abrupt rule change going from Part 23 to 21.17b for these vehicles. Much has been said about this in the industry, but I understand that questioning the performance based standards is at the core. We’ll see where the dust lands, but it’s clear that some in the EVTOL community are not interested in applying airliner safety standards to these vehicles. Questions: is your firm following ARP 4754A and 4761 to the letter? What systems are being mandated to DAL A (requiring a 10 to the minus 9 failure rate?) Are these the usual components found in other jets, or new components that typically haven’t required the same level of safety criticality? These are a whole new type of aircraft—there are so many unknowns that it’s impossible to compare safety standards to other aircraft, even if the intent is to meet something like Part 25.1309. Case in point: power distribution systems in airliners are rarely DAL A, but I’d recon they’re pretty damn safety critical in an EVTOL. No power distribution = no spinning propellers = no thrust, and all on a “powered lift vehicle…” cue the classic line about the V22, “a plane that can’t glide AND a heli that can’t autorotate—all in one!”

lambda 4y ago
You're right, some other entrants in the eVTOL community seem to have strategies that rely on some kind of regulatory magic. The company I'm working for, Beta, is taking a very different strategic approach, with much less reliance on "then some regulatory magic happens;" we are focusing on cargo first, and a passenger variant later. We are not particularly concerned with the change to 21.17b, we're willing to work with the FAA on that rather than fighting it. And because our initial target is cargo, such as transporting organs between hospitals with existing helipad infrastructure or cargo from airport to distribution center, there are a lot fewer regulatory hurdles about where we operate than the air taxi market. We are following ARP4754A and 4761, yes. We are still in the process of doing our safety assessments at the aircraft and systems levels and finalizing our type certified design, so I can't say which systems are going to be required to be DAL-A, but I expect that the pilot inputs, flight controller, battery systems, high voltage distribution system, low voltage distribution system, lift motors, and control surface servos will be designed to that level. Our strategy is to build ourselves the critical components for an electric aircraft, so we are doing the battery, high voltage distribution, and electric motors, while we are buying the other components (pilot inputs, flight control computer, control surface servos, primary flight displays, etc) from existing suppliers who already have experience in this space. In the meantime, we are also learning a lot from flight test of our experimental proof-of-concept airframes, which we can feed into this process. We've already built one experimental eVTOL under a previous design, and then two proof of concept airframes with a design much closer to the type certified design that we're now working on; so while we go through the process of breaking down the requirements per 4754A and doing safety assessments per 4761 and working on our final type certified design, we're also learning a lot about operational concerns and refining our design on our experimental proof of concept airframes. As for the V22 line, well, our aircraft can't autorotate, but it's not bad as a glider. That was a primary design criterion; it should mostly be a glider that happens to have electric power. We recently did a cross-country trip halfway across the country on our PoC aircraft in CTOL configuration (https://evtol.news/news/beta-alia-250-prototype-flies-cross-country-to-arkansas https://evtol.news/news/beta-alia-250-prototype-flies-cross-...), with a number of stops on the way, of course, our range is limited by physics, and the majority of that trip was done such that there was an airport within the glide cone of our aircraft; we did allow for a few legs in which that wasn't always the case, and we would need to ditch in the event of power failure, but for most of the trip there was an airport within our glide cone if we had a power off event.