Will The Lilium Jet Work? A Deep-Dive Into The Physics Behind EVTOL Aircraft

The Lilium Jet is a proposed eVTOL (electric Vertical Take Off and Landing) aircraft that the German company Lilium GmbH has claimed it will bring to the market ‘soon’, which would made it the first eVTOL aircraft in the world to enter into commercial service. As anyone who has any experience with VTOL knows, it’s a tricky subject to engineer, let alone when you want to do it fully electric. In a deep-dive video on the Lilium Jet and eVTOL in general, [John Lou] goes through the physics behind VTOL take-off, landing and flight, as well as range and general performance.

It is clear that Lilium’s presented aircraft concept has many issues, some of which are due to new and unproven technologies, while others seem to be founded in over-promising and likely under-delivering. With Lilium having signed a number of contracts to deliver the first Pioneer Edition Lilium Jets and commercial service promised by 2025, it’s hard to ignore that the first full prototype of the 7-seater Lilium Jet is supposed to fly this year.

Although as [John] points out in the video, eVTOL is not an impossible concept, it is important to remain realistic about what is physically possible, and not seek to push the boundaries. When the UK introduced its first mass-produced VTOL jet in the form of the Harrier, it too faced an uncomfortable time as bugs got ironed out. As these eVTOL aircraft would be carrying real human passengers, it’s a good place to realize that although you can pick a fight with physics, you will never come out on the winning side.

Hopefully Lilium realizes this too, and these sleek, battery-powered aircraft will truly take to the skies in a few years.

26 thoughts on “Will The Lilium Jet Work? A Deep-Dive Into The Physics Behind EVTOL Aircraft

  1. I was excited about evtol 5-7 years ago. With nothing really flying, just an occasional demo flight and everyone promising next year they will be certified commercial flying aircraft (does FAA part 25 even cover these things). Regulations have to be adapted, someone has to prove these vehicles. Next year ain’t gonna happen. It will take hundreds of d flying hours to get commercially certified.

    Then there is the experimental category they could do a bunch of learning in. But little is happening there either. The eHang and others were relatively small craft, but now look at volocopter and Joby they are monsters! Evtol, huh, no Joby is a winged aircraft with lifting motors. Aircraft this sized take 3-5 years to be certified.

    Now you want to fly these of the top of every building in town. It might work on clear weather days, but get some flukey weather, and good luck. Tipping over in a strong gust of wind will be fatal to everyone in the vehicle.

    The wings give a limited plan b should something catastrophic happen (failed ground). There will need to be some forward speed when close to the ground to recover the vehicle. Some use parachutes for plan b, but between 20 and 100 feet from the ground and it won’t work.

    Now cool your vehicle is perfect, no plan b needed. You can carry 4 passengers for a 20 minute flight, then charge for 20-60 minutes. That makes no everything sense. You’ll need dozens of flights to move 100 people.

    So your neighbor needs a ride to the airport, and books a flight on one of these. It comes to your neighborhood at 6am, wakes everyone up. These are not quiet. They are moving a lot of air. Nevermind the trees and power lines they can’t fit around. These well need to land at airports or vertiports not in you cul-de-sac.

    I might be wrong.

    1. Thanks for bringing up the certification aspects of UAMs. My employer is just starting to become a supplier in this industry, and I can tell you the FAA still does not have a classification (not sure about EASA). There is a circular coming out soon, but that’s all. As with all government agencies, they have been slow and incompetent when it comes to UAM. In my mind it is a no brainer since they fly in the same airspace, carry passengers, and even worse – fly around buildings. The classification should be at a minimum the same as general aviation (part 25). Probably should be higher due to flying around buildings.

      1. I don’t know if I would say, ok maybe I would, it is government and all, incompetent. There are safety rules because people have died with no or improper regulations.

        Last night I was lazy and said part 25, that is for transport category fixed wing, I should have said part 29. (Part 23 is for GA). Part 29 talks about lots of plan b stuff, like autorotation, that UAM systems can’t do.

        Maybe the manufacturers are getting “no” from the regulators for a good reason, and rather than saying we need to change the manufacturers are saying the regulators are idiots.

        There is lots of room in experimental category aircraft. Put a million hours on your craft, price it works, then go get it certified.

        Certification is expensive. Rumor is the eclipse jet spent a $billion to get certified. This is a whole new category, people should expect some pushback.

        1. You have to understand business and government have completely different objectives.
          Business is to make profit, and in general they have to provide a service or good that people want to do that.
          Government is all about not making mistakes, ensuring fairness, and being completely accountable if necessary.

    2. I work at an eVTOL company. You are right about much of this, but I think you might underestimate how much opportunity there is despite these challenges.

      Right now, there isn’t a regulatory framework for certifying eVTOL; but that doesn’t mean they can’t be certified, it just means that there needs to be considerable negotiation between the manufacturer and FAA on what the certification basis will be, on what rules apply. For the most part, that involves picking between part 23 (normal category airplanes) and part 27 (normal category rotorcraft), based on what makes the most sense for the design in question and expected use. This is more work, risk, and negotiation than if there were already regulations, but it doesn’t make it impossible to certify. And in parallel, the FAA is working on those regulations, informed in part by some of these early certification efforts.

      eVTOL manufactures are already flying experimental prototypes. There aren’t many selling experimental aircraft, because there are a ton of restrictions on selling experimental aircraft; most of them need to be kit-built by the customers, and there’s just not that big of a market, it’s not nearly enough to justify the development expense. But we are doing extensive flight testing on our own experimental prototypes.

      You’re absolutely right that the “fly from every building in town” or “fly from your neighborhood” are pipe dreams. These are going to be flying from dedicated vertiports; at first mostly existing ones that are used by helicopters, and later there will likely be more built. They will still be noisy; a bit less noisy, as the piston or turbine engine does add to the noise profile, but yeah, the bulk of the noise is in the propellers/rotors. They will, however, likely be quieter than helicopters, which use the lift rotor throughout flight while these will transition to the wing, which tends to be quieter. On the approach, they will mostly be gliding down with their thrust propellers windmillng; it is only at the very end when they transition back to vertical thrust that it will become noisy again with their lift kit. And unlike combustion engines, they don’t need to keep idling on the ground. So while there will be noise and you probably don’t want a vertiport right next to your house, the amount of noise and amount of time they are making noise will be a lot less than conventional helicopters or even airplanes, leading to more flexibility in siting of vertiports.

      As far as catastrophic failure, these are being certified to a high standard; in Europe, EASA is demanding the same 10^-9 chance of catastrophic failure per flight hour that larger passenger jets are certified to, if they are going to fly over urban areas. The FAA is being a bit more lenient currently, allowing 10^-8 which match other small commercial aircraft. eVTOL designs meet these standards by having redundant lift; loss of a single, or in some cases even 2 or 3 lift motors shouldn’t be catastrophic, and standard safety analyses are used to ensure that the changes of multiple failures that add up to a catastrophic failure are low enough to meet those standards. And while we are going with sufficient redundancy to avoid a catastrophic loss of lift, we’ve also done piloted simulations of a loss of lift that we consider catastrophic, and found that we can get back on the wing with only about 10 feet of lost altitude, and below 10 feet you can put it down with a slightly hard landing.

      The same considerations apply to helicopters; helicopters have H-V diagrams showing a keep-out region where autorotation would not be possible in time, so you generally need to keep some forward momentum to keep enough energy to safely enter autorotaion. Likewise eVTOL won’t be doing extended hover, but coming in on a glideslope and only transitioning to vertical lift at the very end.

      Anyhow, all of these mean that some of the first opportunities for eVTOL are cases where helicopters are already used for transport. An eVTOL will be an order of magnitude cheaper to operate than a helicopter; helicopters are very inefficient and guzzle gas, and have a huge amount of mechanical, loaded, wearing parts, which require extensive maintenance. So helicopter operators like Blade or Bristow, or medical transport between airports and hospitals, already have all of the vertiport infrastructure, and can immediately save money with eVTOL for trips that are within the range of an eVTOL.

      And that order of magnitude reduction in operating costs also opens up new markets, that wouldn’t previously use vertical transport; like cargo. Getting packages from the airport to the distribution facility in congested areas can take a very long time; adding a vertiport to a distribution facility is a lot easier than a residential neighborhood, and obviously the airport already has the required infrastructure.

      A lot of what these can be useful for are moving between two places that makes ground based transport very slow; lakes, mountains, cities, bays, etc. There are a lot of places where there are fairly short trips, which require very long transit on he ground, but adding the third dimension helps a lot, and vertiports are much smaller and easier to install than airports, and eVTOLs economic to operate for those kinds of trips.

      Are these opportunities enough to support all of the different eVTOL startups? Probably not. I think there is a bit of magical thinking about how quickly the industry will scale up. There will probably be a big contraction as a number of startups wash out. But I do think that there is a real market, which will develop, a bit slower than some of the wildest projections, but also faster than what the naysayers are predicting. There really is real progress, there are a lot of hurdles but a lot of good work being done to get over them, and there are some really big opportunities for niches that conventional aircraft just can’t fill, or can’t fill economically.

        1. Yes. The usual architecture is to have two busses; each bus supplies an independent set of motors/inverters, such that the loss of an entire bus would be survivable. And each bus has multiple battery packs in parallel, where the failure of a single pack would reduce the total capacity available but still be able to supply power. A cross-tie between the busses may be present to allow balancing of load across the packs during normal operation, but able to be severed in case of failure in a pack or bus. This is fairly similar to how fuel systems in conventional aircraft work, where there are left and right tanks and a selector which can select left, right, or both. On many aircraft you normally fly on both, but may select just one if a leak on one side starts sucking fuel out to prevent it from sucking the fuel out of the good side.

          Of course the cross tie is protected by both circuit breakers that can be reset (in case of a transient failure) to prevent a short on one side from damaging the other, along with fuses that will sever the connection permanently in case of failure of he circuit breaker.

          And there’s a whole safety team to collect all of the failure modes of the different systems, including their severity (catastrophic down to minor), which uses predicted failure rates of different components to develop a probability model of combinations of failures which could lead to the different possible outcomes, with thresholds established for the different severities. The current thinking is that for FAA certification, accepted predicted failure rates will be one order of magnitude lower than accepted for similarly sized conventional aircraft, to account for the novelty of eVTOL aircraft, while for EASA it will be on par with commercial transport category aircraft, due to the expected use in lower altitude flight over urban areas.

          And of course, we do lots and lots of testing to failure of the different components. We have spent a lot of time deliberately blowing up battery packs, motors, and inverters, by inducing failures, learning a lot about how the failures manifest, how to contain them, how long you have to land in various failure conditions, and how much reduction in thrust and control authority there is in different failure modes.

  2. Which would be less weight a very large weight distributed lithium battery pack or a fuel cells (I’m guessing Hydrogen). Because I can imagine this eVTOL requiring massive amounts of energy.

    1. The issue with VTOL has always been the engine size and efficiency to produce the vertical thrust component compared to that required to counteract drag for level flight.

      Like any other engineering challenge, the laws of physics mean that you either have a vehicle that excels at VTOL but has lower efficiency with level flight (i.e. helicopter/quadcopters etc.) or you have vehicles that excel at cruise (airliners, conventional aircraft etc.) which then cannot achieve VTOL.

      The Harrier and F22 etc. are edge cases where the efficiency losses to enable VTOL have been accepted with an aircraft design that is compromised: bigger-than-necessary engine lugged around all the time for the short amount of time that VTOL occurs within the operational envelope.

      1. I’m pretty sure they lug around big engines because they’re fighter jets – the energy budget needed at their highest operating speeds are enormous. Now extra trust vectoring nozzles or ducted fans… Sure, those are extra.

        That said, for most small VTOL it makes sense to optimize for glide, with inefficient hover performance – the amount of time spent taking off/landing is to be minimized anyway.

      1. Linked wikipedia article says it has propeller engines, so it’s not a jet. They are calling it one because it sounds cooler and the propellers are hidden from sight.

    1. A jet: an engine that produces motion as a result of the rearward discharge of a jet of fluid.

      “Jet” refers to the discharge of fluid (air) in a narrow stream, not to the means of achieving it.

  3. “As these eVTOL aircraft would be carrying real human passengers, it’s a good place to realize that although you can pick a fight with physics, you will never come out on the winning side.” – yes, we’ve seen that on the other side of the transportation spectrum – deep diving.

  4. All this semantics stuff is fluff. The engineers at Lilium are serious men with serious goals. They must, and do, follow the physics, and the physics here are not that complicated. In the subsonic range, the factors are pretty straight forward – blunt force (thrust) must accomplish two things in an EVTOL: overcome weight to go straight up, then overcome drag to fly horizontally, transferring the fan disc loading to wing loading for efficiency in getting from Point A to Point B. The challenges in accomplishing this are numerous: getting the weight down as much as possible for VTOL, getting enough battery density to provide any kind of realistic range, achieving enough propulsive efficiency to accomplish both, and safety in the form of redundancy of propulsion, control systems and BRS.
    IMO, the engineers at Lilium are doing a commendable job in optimizing all those factors while waiting for battery solutions that just aren’t here yet. If they can build a machine that is commercially viable, acceptably quiet in an urban environnment, and that provides a comfortable experience to customers, they can survive, and only improve as battery technology matures. I hope they can pull it off.

  5. The capacity to develop, produce, certify and safely operate an aircraft is challenging but doable especially today with advanced tools and equipment and competent engineers.

    So I agree with the previous comments and hope Lilium will pull it off.

    One exception – clearly this post was from someone retired with a 1980s mindset with his egregious reference to engineers who are “serious men”.

    Wow. My sincere apologies and support to all the amazing FemSTEM engineers and team members at Lilium. Ignore the ignorance and stupidity and know that you too are fully recognized for your accomplishments and achievements !!

  6. Great video. He hits nearly all the important parts. Glossing over the forward flight L/D as unrealistic is a bit of a missed opportunity to talk about distributed propulsion.

    Suffice it to say that Lilium isn’t obviously violating any laws of physics. They’re just optimistic about how unproven technologies like high performance electric power trains and distributed propulsion will pan out.

    The most unrealistic part of Lilium’s marketing is that they’ll be certified by 2025. That’s a joke.

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