Technology Behind the Lilium Jet | Hacker News Reader

Technology Behind the Lilium Jet

lilium.com

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I was a core engineer in a different manned eVTOL project, so have spent a lot of time thinking about this technology and watching the space. Lilium raised some eyebrows recently with their SPAC IPO valuation of over $3B, without having released footage of a single manned flight. The prototype is unique and aesthetically appealing, but my engineering mind recoils at the complexity of the design. The variable pitch blades, the adjustable exhaust nozzles, the tilt-wing vectored thrust system, etc. With complexity comes cost, maintenance, and exponentially increasing failure modes. Safety is paramount in aviation, so all flight-critical failure modes require redundant backup systems, which further increase cost and complexity. It's a slippery slope.

The eVTOL space is currently seeing a Cambrian explosion of different design concepts, but the first to see economic viability I think will be the simpler ones.

One of my senior projects in college was designing a ducted lift fan UAV. One of the things that was apparent from digging through the old research was the monstrous total complexity even of conceptually simple designs that had 1-2 ducted fans embedded in the body or wings. Computers make the complexities of controlling these things tractable, which was a much bigger problem in the 1960s. But even with relatively modern technology e.g. Lockheed has had a hell of a time with the F-35B.

Does the turbulence from the fan downwash interfere with the wings when near the ground?

yes.

When near the ground and moving slowly the wings are stalled and not providing any lift; they're essentially just dead weight. A bigger risk near the ground with older turbine engine VTOL designs was ingesting hot exhaust gasses back into the intake, thus causing a sudden loss of thrust. The F-35B mostly resolved that by using a shaft driven lift fan in the forward fuselage.

> The eVTOL space is currently seeing a Cambrian explosion of different design concepts, but the first to see economic viability I think will be the simpler ones.

Mind expanding a bit on this? And curious where you are now instead of this space... is it because you see the tech too far out?

My original interest began with the SkyCar 400 from Moller. The same design theory of a lot of eVTOLs today; just with a petrol motor. The petrol motor reduced responsiveness of the control system, but I always thought it would work out in software.

eVTOLs are wonderful, but I think any eVTOL without a 'hot swap' battery is going to be DOA in terms of the personal transport revolution.

I ended up moving on for personal and geographic reasons, nothing to do with the tech. The tech is all there for short-haul flights with batteries as the only limiting factor for range and speed.

There are various paths to economic viability for an eVTOL aircraft but I don't see how Lilium's current prototype fits into any of them. Unlike an ultralight design, it will require a certified pilot to chauffer passengers around until the day when full automation moves into the aviation space. Other than selling as an extravagant toy, this limits it to air taxi service. The design is complex so will be very expensive to buy and to maintain, so I don't see it as an improvement over existing helicopters beyond the 'green' energy source and claimed lower noise.

I am familiar with the Moller SkyCar. Lots of promises were made and broken, and lots of money burned on that project. Many modern eVTOL concepts suffer from the same basic design flaw: forward flight hanging from small propellers is inefficient. If you care about range, you need the lift of a large airfoil. Moller also had stability issues, but the responsiveness of electric propulsion and the advances in IMUs have solved this problem.

I think you can have a viable eVTOL product without a hot-swappable battery in the same way that electric cars have shown.

Their marketing has been all about taxi services for years. It's unclear if you'll ever he able to purchase one.

A landing pad for something like the Lilium would get a lot less NIMBY attention than a helicopter pad though, which is often a major factor in decision-making.

For air taxi service do you think the battery can be recharged in the time it takes to unload passengers, service the cabin, and load new passengers? If there's a long delay for charging that kind of kills the economics.

They could swap the batteries out, maybe.

The whole thing hinges on battery energy density more than anything else. The planning document says 20 seconds of hover at landing, with a 60 second reserve. The demo flight has 45 seconds of VTOL mode/hover at landing. That's cutting it close. You don't get a second chance at landing. No go-around.

Regard this as a bet on improved batteries. With current batteries, it's a nice demo.

This was my take as well - and was wondering what is Fig. 11 based off of? We can predict, but we don't know how the market for batteries will form in the next decade or so. I.e. the COVID disturbed all sorts of markets bad enough that price of many commodities went up X-fold and haven't gotten down yet.

Sounds basically like the Apollo lander...only today we have better computers and control systems.

The Apollo Lunar Module had the ability to abort landing by dropping the descent stage and firing the ascent engine. So if they had run out of fuel while searching for a safe landing site they still had a safe alternative option.

On the other hand, Apollo had no option than to abort to orbit. Just ditching somewhere and hoping for the best wouldn't have been an option on the Moon, but it's commonly done in aviation on Earth.

Yes, and it's a bet they will probably lose, if they really need 500 Wh/kg in three years time. Their battery graph is a fairytale at best.

The current state-of-the-art batteries that researchers build in laboratories that last 10-20 cycles can put out about 400 Wh/kg. You're simply not going to achieve first a 20% improvement over that, then a development of the tech to increase lifetime to something useable, and then scaling that to widespread commercial availability, within three years time.

If we speak about what's available in laboratories, Lithium–sulfur battery gives around 550 Wh/kg with up to 1500 charge-discharge cycles ([1]). Not available commercially, but there are a few companies that try to commercialize them ([2], [3], [4]).

I would not bet that these batteries arrive fast enough to save Lillium.

1. https://en.wikipedia.org/wiki/Lithium%E2%80%93sulfur_battery

2. https://lyten.com/

3. https://oxisenergy.com/

4. https://www.thebatt.com/news/lithium-sulfur-battery-startup-...

Yeah, I'm kind of assuming Li-S batteries won't work out. Sony announced in 2015 they were going to have commercial production in 2020. No news since then. Many others in that field have gone into bankruptcy, including one of those you link to.

Nice catch about one of the company being bankrupt.

I've taken a deeper look at Lyten and it does not seem to be a savior here either:

1. They need to produce graphene at scale and they don't yet have a solid plan to do that.

2. Lyten batteries are only 200 cycles.

Agree their chart is a fairytale based on current chemistries. It's basically a bet on commercially viable solid state batteries, which are targeting somewhere in that 500 Wh/kg range, but are probably a couple more years out than Lilium needs. I guess they could drop their viable range quite a bit at the start to compensate...

I recall Vaclav Smil saying batteries are consistently about 2-3% per year over the long haul. It's not a Moore's law scenario, but a very slow crawl. "Miracles might happen," he says, "but would I bet on a miracle happening? No."

This link is to the point in the video where he says this, but the whole talk is quite interesting. https://youtu.be/gkj_91IJVBk?t=1705

Edit: here's a more recent video of Smil talking about energy density of batteries and the problems there, specifically mentioning the unlikelihood of flying on battery anytime soon. https://www.youtube.com/watch?v=91Cs-ZkAjCg

First Law of Aviation: Take-off is optional, but landing is compulsory.

when you say "simpler", do you mean something somewhat more conventional that is a multi-rotor/tilt-rotor design like the joby evtol?

From a safety perspective (ignoring cost, though which is also critical for viability), I mean minimizing the number of 'Jesus bolts' in the design - single components that if they fail, you are in a world of trouble. In the Lilium for example, what if the tilt mechanism for one of the main wings locks up while in cruise, and you only discover this while coming in for a vertical landing? This could be due any one of many possible reasons: SW bug, electrical failure, mechanical failure, debris jamming the mechanism, etc. You've got one wing producing forward thrust and the rest producing vertical thrust which is not likely to end well. You need to either prove that every part of this particular system is extremely reliable or you need a redundant backup system in place. Due to the design, neither of these options are easy here.

Obviously you need to assume there will be motor-out situations in any eVTOL design and the system needs to handle this with full payload over the flight envelope needed for a safe landing. Lilium has the advantage of many propulsion systems to take over the lost thrust, but many other eVTOLs are questionable here.

The same thinking has to be applied throughout the entire aircraft design (electrical and mechanical) and then you need to test, test, test. Each failure mode, using full payload over the full flight envelope, stability testing under differing weather conditions, the list goes on. To give you a rough idea, having a flying prototype is great but once you are comfortable enough with the design to put in a test pilot under controlled, monitored, and ideal conditions, you are maybe 20% of the way towards something you can responsibly put into service. If you want to see a simple design, check out the Opener Blackfly.

Is an airframe parachute essentially a requirement for a vehicle like this to be viable?

It certainly increases the safety factor, but should never be relied upon as the sole redundant backup system. There are situations where it will not do you any good such as hovering at low altitude. If the aircraft does not have a reasonable glide slope, most people would feel a lot more comfortable with one.

The Lillium aircraft has no glide slope at all. It has wings, but there are no control surfaces to maintain a glide in case of power loss.

More cost.

An airframe parachute is included in the Lilium design. At least it was last time I checked.

Agree re simplicity. Ehang has been using what is essentially a consumer drone design for some time, and has demonstrated success with many flight tests. Although they aren’t using the most efficient design, I wouldn’t be surprised if they dominate the short range air taxi field.

The Ehang AAV does fly, but it seems like a commercial dead end. They aren't going to get type certification to carry paying passengers under the aviation regulations that apply in most countries. The reserve endurance is too low, and the technology for reliable autonomous operations is way too immature.

Meh.

The US is somehow singularly incapable of implementing decent public transport. Thus venture capitalists are throwing stupid amounts of money at exotic technology that could potentially alleviate some of the problems arising from this stupidity.

First we had the "full" self-driving cars and robotaxis coming any day now. Then there's Musk's attempts at tunnel digging and Hyperloop that have gone nowhere. And now eVTOLs that will go nowhere.

It's kinda fun to watch from afar though.

Don't worry, EU has money to throw around on stupid projects as well. Even worse, it's public money.

https://seenews.com/news/croatias-rimac-to-get-200-mln-euro-...

Oh sure, and countries in Asia as well, we don't hesitate to join in on these things.

I'm just saying, if you trace these ideas back to their original motivation, it's the lack of decent public transport in California specifically and the US generally.

Roads required public money to build though.

With trains you don't need self driving cars!

Rather, why don't we imagine a differently configured society? If you have streetcar suburbs, then it's possible to have a lot of last mile delivery delivered by light rail.

Besides the high-tech hype, there's also the economic absurdity behind all these proposals. The average American household already spends 1/6th of its income on (car) transportation. These VTOLs are cars on steroids, with even more modes of failure and imaginably more regulation. I assume they would do a ride-sharing / taxi business model, but even then... the average person won't be able to afford a ride in one of these because the average American can't even keep up with ordinary car payments.

It's depressingly comical to see these futuristic pitches when the reality is there won't be any customers left because the middle class will be completely gone in near time. People need to have money in order to spend it.

All air taxi services are targeted toward affluent customers, not the middle class. That's just the nature of the industry. That doesn't necessarily mean it's a bad business plan — Ferrari does pretty well targeting those customers. But even if the industry is viable that doesn't mean Lillium will succeed.

Exactly. And a seven-seater gets to pay for this.

The V-22 was how many years late? How many billions over budget?

Bell also basically had this same exact dream of tilt-rotors when they were designing and building the V-22. This was a huge part of the sales pitch of titlrotor development in general at the time.

I’m still highly skeptical of where this kind of vehicle, even if you can make it very quiet, fits in to reality. There are so many hurdles to adoption in the regulatory aspect alone that it seems intractable, but the economic aspects feel like the true nail in the coffin.

I don't think that Lillium uses variable pitch blades. Also, using multiple fans will lower the chance of catastrophic failure due to loss of one or more fans. For example, A system with only four fans would mean loss of a single would be truly catastrophic.

If Lillium uses multiple battery packs and redundant wiring and controls, they could achieve excellent redundancy; to a level much lower than many competing in this space.

I get a Magic Leap One vibe from this company. Promises, promises, promises, then reality doesn't quite meet expectations.

"Simple by design, there are no ailerons..." then goes on to describe a vectored-thrust control scheme consisting of essentially 36 blown elevons. Clearly we mean something very different by the word "simple."

I know very little about aerospace, but from a manufacturing perspective it seems like having a single part or unit that you need to manufacture and test would be simpler

but I imagine that this offloads complexity to the flight control software, though – is that what you are referring to?

Moving to an electric ducted fan instead of an internal combustion engine with a variable pitch propeller definitely simplifies things, though replacing that ICE and prop with 36 ducted fans mounted to elevons seems like you're just swapping one set of complexities for a different set, though gaining complexity through redundancy isn't really the worst thing.

My biggest concern with this system is what kind of glide ratio this has in a power loss scenario.

My biggest concern is how you maintain directional control in a power loss scenario.

That too. As far as I can tell, this thing has all the control and glide capabilities of a brick. At least the brick has the inherent safety feature of not being mistaken for a passenger aircraft.

ballistic parachute?

Maybe. Not sure I've seen one that will work on a 7 seater, especially with the extra mass of batteries.

Design for hot-swap batteries, and eject the batteries (with their own smaller chute or chutes)?

Forgive me if I don't consider dropping lithium batteries from several hundred meters up a safety feature.

A parachute on this system is a lot of extra mass to compensate for a problem solved when the Wright brothers were around.

There is a ballistic chute on the cirrus SF50 (their single-engine jet). It's a 7 seater IIRC (6 minimum), and roughly the same weight (2.7T for the cirrus, 3.1 for the lilium).

I mean will a 7-seater helicopter autorotate sufficient to provide control in a power-loss situation?

Yes of course, why wouldn't it?

Here is a video of a Mi-26T performing an autorotation: https://www.youtube.com/watch?v=O27T07bIQB8

That beast can fit 90 troops or 20t cargo. Autorotation can scale to large helicopters.

A way around this risk is to come up with a design for which a total power loss is extremely unlikely (i.e. it would take multiple simultaneous non-correlated failures in a short span of time). This could be achieved through a distributed battery system where each motor has its own battery physically and electrically separated from the others, and critical flight electronics have their own backup batteries. A ballistic parachute is always nice as a last-resort hail mary.

Redundancy does go a long way, but that also introduces a lot of extra weight and complexity into an already heavy aircraft and can screw with your weight distribution.

The parachute can work since there are ones for air dropping tanks, but at the same time I'm not sure I've seen anyone try to use a parachute as safety equipment on an aircraft this heavy.

Agreed, the distributed battery system does not seem practical for the Lilium design but you could get at least part way there by segmenting the central battery into parallel modules with separate safety disconnects. Not sure about the feasibility of the parachute system, it may be impractical due to the aircraft weight as you mention.

Would it be enough for the safety batteries at the motors to be only as big as to provide power for the aircraft for an emergency landing? Like for a 30 sec full thrust?

That might not bother the weight distribution that much.

Maybe? Honestly my weight distribution comment is a tad silly since complexity and weight are far bigger concerns, and two that just don't seem to have any avenue to go away.

Most batteries don't have the power density to output their entire capacity in 30 (or 60, or 120) seconds though, if I'm remembering correctly.

Cirrus Airframe Parachute System

https://en.wikipedia.org/wiki/Cirrus_Airframe_Parachute_Syst...

Unless the eVTOL up for discussion is significantly heavier than the above, looks like a fair approach to last-resort failure recovery

It's fly-by-wire. Uncontrollable in a power loss scenario.

Like most modern jets. Fly by wire is quite common. Also, in many recent cars, the steering column is no longer mechanically coupled to the wheels. Power loss scenarios are bad, which is why people mitigate against such things with all sorts of redundancies.

And of course mechanical connections can fail too.

Most modern airliners have batteries and ram air turbines in order to power the flight control systems even if the engines are out and fuel is exhausted.

“It’s simple! We KILL the Aileron!”

If you work at Lilium, you need to make some tweaks to your website. This was an incredibly frustrating experience, to the point that I gave up.

* Scroll jacking - looks pretty, impossible to use.

* Cookie consent - It's both too large and takes to long to find the right action.

I never saw any cookie prompt since I block cookiebot.com. Things seem to work just as good/bad without it, try blocking that domain and see if it works for you - not just on this site but all others which use these "services".

If you're on desktop there is no spacebar hijacking for scrolling, so I switched to that since the article was interesting anyway. But I agree with the sentiment.

This paper "Electric VTOL Configurations Comparison" [ https://www.mdpi.com/2226-4310/6/3/26/pdf ] presents a pretty decent energetic analysis of a few of these eVTOL projects (including Lilium) and is very generous to them.

Lilium is vaporware. Ask anyone who has worked in the space. Or just look at it. Or their promotional materials.

Ducted fans huh... meh. I don't think they have an interest in people understand how fly-y/hover-y things actually work.

I do applaud the investors for their vigorous desire to increase the velocity of money throughout society, it is important econometrically. Somehow I doubt they'll be happy to see their currency shredded by 36 rc brushless motors struggling to thrust an anemic wing into the sky though (before its glorious ~250km/h cruise to destination [of course])--and then ~15s landing hover.

What exciting times to fly in.

No doubt the whole eVTOL/AAM space will lead to positive and long lasting developments for aviation, but the amount of money chasing specifically the air taxi/uam space (Joby, Lilium, Archer, Volocopter, etc.) is staggering given how utterly nonsensical the business plans and projections are.

Right now everyone is in a race to certify. I guarantee that will be the easy part. The whole list of issues this community is not thinking through and addressing dwarfs those that they are. Energy density is just the start.

For instance: Power requirements for these business models at scale will require a sudden turnaround and mass embrace of nuclear. Infrastructure build will require government investment and widespread use of eminent domain. City laws and ordinances allowing for uam flight need to be rewritten. I could go on…

Btw I am in the industry.

There's a lot of money floating in this sector for sure. But it's also a potentially huge market that might make it worth the risk.

The value proposition is simple: get from A to B quickly at the cost of a few KWH. This plane has a 300KWH battery. That's about 5x a Tesla battery (the big 75KWH ones). The expensive rate for charging a Tesla is about 15$ (give or take). So this, 7 seat plane would cost about 5x that (at the expensive rate, you could get a better rate). 75$. Lets call it 10$ per passenger. And that would be for the maximum range. For short hops, think in terms of a decent coffee at your local coffee shop. Maybe two cups. Just the energy cost of course, there's more cost than that.

But that's great value in a world where short 20-40 mile distances in dense urban environments can take an hour+ to travel and where people routinely do that by taxi, uber, or public transport. The premium segment of that is thousands of people paying north of 50$ routinely for relatively long journeys every day. That kind of is the market for this thing. Charge 50-100$ per ride at a cost that is likely closer to 10$. Nice fat margins.

The unit price of a single plane is probably going to be around 3 million (initially). So, selling a thousand of those would amount to about 3 billion revenue. Likely prices will come down over time. But basically the first few companies that manage to produce vehicles like this in very modest volume are looking to rake in billions of revenue. The early adopter market is going to be amazingly profitable. The long term market when cost drops, production scales, and the novelty wears off is still going to be quite nice. Fundamentally these machines are not vastly more complicated to build than a premium EV and their energy cost is similarly low.

That's why these companies are so hot right now. There will be a lot of companies that fail and a few that might end up printing money. Lilium has a great sales pitch. The only question is whether they can deliver.

This simple value prop fails to take into account the physics of flying, the operational costs of aviation, the societal disruption and associated costs of building and managing support infrastructure, etc, etc. Its nonsense.

i want to agree with you but then again i find myself thinking how even moderately rich ppl use helicopters to move around in dense metropolitan areas basically for decades.

rich single nerds will make this market work imo.

I am not at all an expert in this field, but is "jet" the right word?

I thought that required combusting fuel and using the flaming exhaust to propel something. This is propellors in tubes.

Also this vehicle is about a third of the speed of what most people think of as a "jet plane".

'Jet' has been used to describe many different types of propulsion system. The first jets were what you'd call rocket engines. Then we got 'turbojets' (mostly on aircraft), followed some time later by 'turbofans'. Most confusingly, ducted propellers on submarines are called 'pump-jets'.

I would agree with you that this seems better-described as a ducted fan (which the author also says), but this use of 'jet' is not beyond the pale.

Jet propulsion involves ejecting a jet of fluid [1] and does not require combustion of that fluid.

[1]: https://en.wikipedia.org/wiki/Jet_propulsion

There is also waterjet and jet stream. Not uncommon dictionary of jet is narrow,forceful stream of fluid or the nozzle producing it. Taking that into account difference between jet and propeller in tube is matter of speed and compression. It's just that fuel combustion is one the most practical energy sources including for producing jets of gas.

That's a very unconvincing video clip on that site.

Here's a better one: https://www.youtube.com/watch?v=5ukmS9ZJm40

Note that the video you linked doesn't show their latest "demonstrator". Here is a video of their latest one: https://www.youtube.com/watch?v=jqvMXoVZfDw

Can we make sustainable public transport attractive again - an stop hyping gadgets for the rich?

Those are not mutually exclusive endeavors.

Please be aware though, that there's always going to be a large population segment that will desire to proceed directly from point A to B in comfort.

Unless by "public transport" you're including "government owned autonomous taxis", public transport will never be attractive to that population segment.

> Please be aware though, that there's always going to be a large population segment that will desire to proceed directly from point A to B in comfort.

How large do you consider this population segment to be?

Order of magnitude as large as the group that commutes by car today.

Quite large, certainly a good chunk of the population that currently drives. There's also a segment of the public transit riding population that would prefer personal transit, but do not have the option.

People generally prefer to a) be comfortable, and b) proceed directly from point A to B with no interstitial bullshit.

You almost never get b) with public transit as commonly realized today.

A high-speed train can connect inner cities just as fast as this gimmick jet. It is orders of magnitude more efficient and scalable, and much more safe. A good train could easily be more comfortable, if there is a market for that. Do you think that jet will have dining rooms, sleeping cabins, ample room for luggage? Do you think it will even have a toilet?

Just. Build. A. Train.

High-speed trains are awesome. We absolutely should be building more of them for the people who'd like to use them.

But if someone wants to go directly from point A to B in a "gimmick jet", why should we try to prevent that?

I'll reiterate with your example. Building high-speed trains, and developing "gimmick jets" are not mutually exclusive endeavors. It's clear that you are not personally interested in a "gimmick jet", but there are people who would be. Why exactly should we be stopping those people?

I would not want to stop these people. As an engineer, I find the technology very interesting and cool. And it is electric, so it is relatively less harmful than normal jets and helicopters.

I do think that there are big challenges in people transport (mainly around cars and planes) and this does not solve any of those challenges, as it does not scale. So I see it as a gimmick: cool tech to show off.

If we put a higher regulatory burden on some means of transport than others, we'll distort the market.

I'm all for letting everyone with an idea try it. But we need to apply our safety etc. standards consistently. A lot of these new transport startups are really just regulatory arbitrage where they're relying on not being regulated because they don't fit existing categories. Make it legal to build high-speed trains with the same (low) safety, noise pollution, interoperability etc. standards as gimmick jets, and we'd see a lot more practical transport getting built.

I like trains too (and short-haul aircraft frighten me), but it's much easier to find an unobstructed path for a small aircraft than a high-speed train. There's also more latitude for experimentation with these piecemeal private sector experiments. I would expect a high-speed rail line between San Jose and San Francisco, for example, to cost far more than Lilium's current market cap. I'm not even sure where you'd put the track, without killing an existing train line.

Lilium Jet has 600 employees. What do you think they could contribute to sustainable public transport that the millions of people employed across the globe directly involved in public transport can't do?

Pretty sure NovaBus has a comparable number of employees. So does Lion iirc.

Lilium, as I see it, has by far the most secure design of all air carrier designs we have seen recently. And there have been a lot including some really nice!

Lilium is the only company going for this distributed propulsion system with so many rotors, adding the the safety of the whole thing. Safety should be paramount! A couple of deadly accidents can literally ground a fleet otherwise.

Volocopter also tries a similar approach, but the lack of eVTOL probably makes that bird much slower than the Lilium product.

However, Volocopter seems to be further with their product development, while there are serious doubts if the Lilium concept works at all.

What do you mean that their concept might not work at all? They have flown several iterations of their prototypes already but maybe you have something different in mind, e.g. doubts about the possibility to scale the concept up?

Here are videos of recent test flights:

https://www.youtube.com/watch?v=0idI3sJJKZw

https://www.youtube.com/watch?v=SqMEFdbuzQY

https://www.youtube.com/watch?v=jqvMXoVZfDw

Cool, but the event is not unmannedly flying around in a circle for 1 minute but to transport up to 7 people over several hundred miles.

From those videos, it's difficult to be sure those are full size prototypes and not scale models.

It’s all unmanned unfortunately. Still waiting for them to demo at least a dummy payload like Joby did a few months back.

Also, Joby, Ehang, and Volocopter are much further ahead in the sense they have done point to point or are building out the factory (same time last year Ehang got hit by being called a scam by Citron, however).

From what I read, rumor is the big issue is battery density which is holding Lilium from being viable. We shall see!

Also, Joby is most likely the one to look at to track the progress of this air taxi segment since they have just Spac’ed and their aircraft is the most conventional.

> Safety should be paramount

Interestingly, safety was not even one of the stated "aircraft requirements" from this article:

"So, the Lilium aircraft requirements can be summarised as follows:

    Zero operating emissions
    Highly efficient cruise phase
    Vertical take-off & landing for inner city accessibility
    Low noise for high frequency inner city flight operations and customer acceptance
    High seat capacity to achieve attractive unit economics and affordable pricing over time
    Scalability whilst maintaining ground footprint and low noise."

Since the jet needs certification, safety doesn't necessarily be mebtioned. Without safety no certification. They could have mentioned it so.

why are all these new variants of sport aircraft appearing at the moment? There must be something in the water because I've seen so many re-interpretations of the flying car in the past year or two. They look cool in movies but how are they at all a practical method of transportation outside of being toys for the ultra-rich?

I could see these being a more cost effective option than helicopter transport. Especially in island areas.

I think everyone is trying to replicate the Tesla formula where the first Roadster was a toy for the rich, then followed by real products and now $Trillion in market cap. But there are a few steps in there that are probably hard to copy, like being able to sell carbon credits to other manufacturers, federal tax credit for buyers, etc. Oh, and one Elon Musk.

Be aware that you are usually looking at prototypes, not available products.

Currently its mostly another set of startups to invest in. They might look attractive to VCs because they're ambitious, require lots of capital, and the technical feasability of a specific approach is complex enough to find merits in. The jury is still out if any of them are actually viable from a technology and economic perspective.

Lilium's approach specifically has been publicly critized by aviation experts to not be make sense when looking at the basic physics.

Electric cars have increased the availability of mobile electric power systems: experience with batteries, motors, controllers etc in cars is transferable to aircraft. It's "in the air" so to speak.

Edit: Also the mass market adoption of small quadcopters; these "distributed electric propulsion" flying car type things are basically the same concept: flight control by altering the speed of an array of propellers.

Also a lot of university teaching/research on both of the above.

One of the main backers of Lilium is Frank Thelen (https://en.wikipedia.org/wiki/Frank_Thelen), a VC from Germany, best known for his participation in the German version of Shark Thank ("Die Höhle der Löwen").

He seems to believe that eVTOL are the future of mass transportation, like an airborne version of taxis. I personally don't find that plausible at all, but maybe I'm just not creative enough to imagine a future where this would work.

I really hope that's not the case. Mass transit that's just cars that are even more noisy and can drop on people from the sky?

I mean I love the sci-finess of the concept also I doubt it'll ever "fly".

But I will bet money that we'll see self driving electric Ubers on the road before we see those air-taxis.

And it's going to be incredibly hard to compete with those. Especially as city will be becoming less crowded as more and more people realize they don't need a car anymore.

I live in Bangkok where Taxis are extremely cheap compared to owning a car. I would never bother getting my own car, it's just so convenient to let someone else drive in the city.

There are two axis here:

- are the air taxi concepts technologically viable? I'd say yes, after all they are flying one way or the other. Besides being electric concepts like Joby and Volocopter are proofen. Lilium is techologically more challenging, but still feasible with existing tech. Not sure robo taxis will beat them there.

- Commercial viability. We know there will be a market for robot taxis, they are taxis without a driver. There UAM is the big bet, is there a market, and if so how big is it? There robo taxis might be faster.

Computerized controls and CFD have made these multi-thruster aircraft practical to design and fly. These things are promised to be way easier to fly than conventional helicopters, with stuff like autolanding since fly-by-wire has become almost a commodity. The flowfields from having 10+ fans interacting with a body was also impractical to analyze in previous generations, but now CFD tooling has caught up and we can get good results without physical tests (of course you still need testing at the end).

It's not so much that I thought them impossible - but what problem are they actually solving?

Controllability, safety, carbon emissions, NOISE, better forward flight performance in some cases. Flying a helicopter is VERY hard and that is a huge barrier to public acceptance, but you need a VTOL craft for a practical everyday air transport. What problem does OS X solve over raw FreeBSD? This is a vastly smoother experience.

What Brilliant Web Designer thought it was a good idea to pulse the text? Are you trying to give me a seizure? SMH WTF

And that 2px per full spin of the scroll wheel ensures I really had time to absorb the content.

"fifth evolution of our technology over our six year history" reads: "it still doesn't work, we aren't sure if it can, but need more investor cash"

Their rosy projections of how they expect battery systems to magically materialize and double in density over the next few years to make this device actually work are also quite funny.

Lilium and other electric vtol would benefit greatly in my eyes from a cable assisted starting. Just have 500 m of powercord on take-off and once you are over that initial peak you just drop the cable. Not as sci-fi, but alot of kilometers in ex-change.

These are som pretty impressive goals. If they can truly achieve a hover state as quiet as 60db, and get the efficiency they've outlined, these could truly change the way we look at short-haul travel.

The idea of a 2 hour drive from a small town to the big city being reduced to a 1hr flight (with no airport - yay helipads) is already a market where helicopters are making money. Doing that in even less time, in more comfort (ugh - choppers are loud), and at lower cost per hour/mile will change things dramatically.

Yea, 60 decibels at 100m would be pretty crazy to achieve.

60dB @ 100m is 77db @ 15m

For comparison, in Washington state, the legal limit for an automotive exhaust is 72db@15m.

Still an achievement, but definitely not something you want your neighbor to land at 3 am.

Can we please use dBa (a measure of absolute sound pressure) rather than dB (a relative term used in many domains)?

Obviously aircraft are louder than cars. It seems like this should be compared to a helicopter since it uses the same pads.

They should be compared to vehicles that occur in the space that these "air taxis" are meant to take off and land in. If they're relegated to airports, fine, compare to helicopters. If they're meant to take off and land in my neighborhood, compare them to cars.

It might actually be too good to be true. The German magazine Aerokurier is citing experts that seem to have doubts about feasibility [1].

The category is called air taxis, other competitors are Volocopter and Archer, with different concepts.

[1] https://topgear-autoguide.com/category/sports-car/futuristic...

Experts always have questions about disruptive technology.

They are often not right, since disruptions are not incremental change, but tons of nuanced ones that lead to a significantly better overall system.

Edit: "Often" does not mean always, or the majority of the time. Lilium is clearly not a scam, and seems to be making steady progress towards their goals.

...but physics isn't something that cares If you want to disrupt.

I think this is such an important point to differentiate. Yes, incumbent experts can be caught flat-footed when a new technology comes along that is essentially just much better at what they do. Best example in my mind is the iPhone, where essentially the big old-school cell phone execs totally poo-poo'ed Apple's ability to create a compelling cell phone in 2008: "A computer company doesn't know what they're doing."

But that is fundamentally different than pointing out real physical limitations of an idea, and the danger is you get exactly the OP response, "The old guard always dismisses you until you change the world!" yada yada. This is basically exactly what happened with Theranos - anyone in the actual field of microfluidics knew it was all bullshit.

Of course, but it seems like the gap between what is physically possible and what has been demonstrated is pretty large when it comes to aircraft. Has anyone made a specific argument that Lilium violates the laws of physics?

How energy efficient is this concept? Can't this air taxi thing be done with better public transport and urban design?

Air taxis are unlikely to become as common as the industry hopes.

But helicopters are already viable. Increase reliability, drop upfront/operating costs an order of magnitude, and make it autonomous to remove cost of a pilot; then you have a lot more cases where it would be a viable option.

> Experts always have questions about disruptive technology. They are often not right

They're usually right, because most technology that seeks to be disruptive doesn't end up working.

"Futuristic air taxi: is the Lilium jet a blow job?"

Is "blow job" a literal translation of a German phrase?

That whole article looks and reads like an automatic translation. I’m guessing the original says “Ist der Lilium Jet eine Luftnummer?” Or perhaps “Luftpumpe”. A “Luftnummer”, an “air number”, is “hot air”. A “Luftpumpe” is an air pump but can also mean someone is a BS artist.

There’s also a bit of a bad pun hiding in there if it was “Luftnummer”, because “eine Nummer schieben” means to have sex, colloquially. If you really wanted to preserve that … you’d reach for the “blow job”.

Aerokurier has an axe to grind with Lilium. That their initial coverage caused quite a stir up just motivated them to continue down that road.

Doesn't mean Lilium is actually a guaranteed success, just that Aerokurier isn't the most balanced source.

How is 2h to 1h any „revolution“? And wouldn‘t simple (bullet) trains be better for that?

New train lines won't be built ever again in the United States

> New train lines won't be built ever again in the United States

New train lines are being built now in the United States.

There's construction ongoing on new train lines in the United States. Whether they're being built is up for debate.

A small town doesn’t have the density for a HSR link, see eg China’s HSR debt.

A 120 mile link at 14 million a mile is more than a billion dollars.

I really doubt that at any appreciable scale eVTOL is nearly as cost effective as HSR, no matter how expensive that is

A bullet train is better if there's one that already happens to go from your location to your destination. If they manage to make these VTOL planes safe and cheap enough, they'll be far easier to deploy than any train system. Also while there's overlap between short haul aviation and train markets, the infrastructures don't have to be mutually exclusive.

You are thinking the classic hub and spokes transport network where you have to travel to a major city with an actual bullet train station travel to another big city at high speed and then do an uber/taxi/whatever to get to and from the station on both sides. In the extreme case, this would be more of a point to point kind of thing. Walk to the nearest flat surface where one of these things can land and wait a few minutes to be picked up. Your local super market's parking lot would be big enough. On the other side, you land at a similar venue and then walk to your destination in a few minutes.

Most cities could end up with hundreds or thousands of suitable locations for these things to take off and land long term. So, about similar to the density of subway stations or better. But a lot cheaper to build.

I would say, at least in the near/medium future, the fact you'll never hit a traffic jam for a 1 hour commute is a big plus.

But then again, weather probably will ground it more often.

The ultra rich in London, NYC, and LA: you can live 2 hours as the crow flies by car (which would be 3-6 hours depending on traffic) and cut it to an hour or less.

Or you can plop your floating megamansion offshore and commute to the city from there.

Bullet trains are point to point, and you have transit to station overhead. This is more direct above and beyond an absolute speed advantage.

My take was actually the opposite. I can't see this changing anything.

Since this thing won't be able to take off from my driveway or land at my destination, I figure it will still take 2 hours to go door to door. Once you arrive, you'll still need your own transportation too.

Unless this is somehow cheaper and more comfortable than driving, I can't imagine ever using something like this.

Interesting stuff. Haven't heard of the Lilium specifically, but have seen other distributed electric propulsion concepts here and there. DEP is such a nice idea, eliminating a lot of the problems with helicopters/fixed wing aircraft (noise, space requirements etc.) to carve out a new category of aircraft that is perfectly suited to an emerging technology (high energy density batteries).

If we ever do get flying cars, they will use distributed electric propulsion.

They could test the market for these in Norway, the Scottish Islands or the Greek Islands. Places where 15 min flights with low passenger numbers over a distance of 50-100km would have a high value compared to a 1-2hr ferry crossing. Wouldn’t need to go anywhere near max range and would still mostly over land not much other air traffic to bump into.

Multiple questions for aerospace folks here:

Why are designs with multiple wings no longer developed, not just bi/tri wing planes with wings stacked, but why aren’t their rows of smaller wings, or multiple wings? Do the vortices of the trail of a leading wing fuck up themlift potential on wings behind it?

—-

We simple golf balls for greater distance through the air, and Wales see an advantage to the barnacles that grow on the leading edge of their flippers which have shown to create eddys behind the tail edge of the flipper which aids in greater vortices and greater propulsion..

Why don’t we dimple wing surfaces? Or plane bodies?

Or helicopter blades?

What happens when we simulate the leading edge barnacle bumps on the leading edge of a wing, or more aptly, the leading edge of a blade on a helicopter?

What software is used to model the airflow over a wings surface, such as solid works…

—

How much overlap is there between fluid hydro and aero dynamics? (Specifically the observation of the impact to flow/eddy creation over a surface?)

I always loved the look of Burt Rutan's dragonfly, there's a list of issues mostly related to conventional (non-vtol) flying - https://www.nestofdragons.net/weird-airplanes/tandemwings/qu...

What stands out to me about all this eVTOL/air taxi vehicles are that they operate in 3D space instead of 2D like cars.

This means, even though more inefficient than cars, you can have more “lanes” to operate in.

On roads, you have to have traffic lights and stop streets, traffic itself, and cars driving inconsistent speeds or braking when turning.

In the air this can all fall away, so people can live further away from city centres, streets can be left for cycling, and less concrete/asphalt infrastructure is required.

I've been watching this space for quite a long time and this company seems to have the engineering and backing to make it happen. From a safety perspective, this design has a potential for massive redundancy over several other designs being promoted out there.

Considering that their marketing is not to desperate, and their releases of footage is slow and stead, I predict that they are very confident in their designs.

If I had a few million to throw around, I would definitely be investing in this company.

They're public and shares are $9, so you can throw any amount of money in them.

> Lilium’s greatest breakthrough is not accepting that traditional approach to trade-offs but rather getting to first principle understanding of the physics and then systematically innovating and optimising each subsystem in an integrated and coherent manner such that the resulting aircraft achieves all stated performance objectives.

You cannot "innovate such that ...", you can only "innovate and hope to achieve ..."

There is too much assertiveness in this post.

"The landing gear is fixed and there are no hydraulics."

Will this cause issues during landing? I've seen helicopters land and it can be a bumpy/sketchy experience.

Fixed gear means non-retractable, not necessarily unsprung.

I was thinking hydraulics as in shocks, not as in retracting mechanisms. Thanks!

I have to wonder about the glide ratio. All those fans will probably create a ton of drag. If for some reason you have a complete power/engine(s) failure, that thing is likely to become a lawn dart.

At least with a helicopter you can autorotate if you lose your engine. Unless this plane will get an airframe parachute, I would not fly in it.

The glide ratio of an auto-rotating helicopter is not great; not a very hard target aerodynamically. The glide ratio is not going to be great if all 36 engines are out but I'd expect crash landing as a glider it should be totally doable and probably easier than auto rotating a helicopter (nose down to keep the speed up, flare at the right moment). And of course installing some parachutes on it is probably going to be a thing as well.

Of course that scenario is not great for either this thing or a helicopter in a dense city.

> Zero operating emissions

Could it be clear to everybody that this thing will require energy and thus will, in a way or another, emit somthing (either CO2 from electricity production or CO2 from the construction of means of electricity production).

Besides, why not just invest in trains ? (or lighter alternative if any)

I don't doubt this concept could ferry people around. But its safety margin is razor thin. 60s reserve? That is barely time for an operator to figure out where to ditch.

If I'm reading it correctly, they're claiming 60s of reserve at hover. That would translate to roughly 10m of reserve in level flight.

On a windy day or two, I've done multiple go-arounds in a fixed wing aircraft. In low weather, missing an approach to land is also relatively common.

"Don't worry, your pilot has 60 seconds of reserve with which to land safely" is terrifying rather than reassuring.

I have a helipad on the building next to mine. Being that it can get fairly windy and rainy in the tropics, those guys have balls of steel anyway.

But those approaches do take more than 60s for sure.

There is one fundamental error in this blog; the battery energy density graph. Battery tech does not follow Moore’s law. Batteries are not transistors..

“So, the Lilium aircraft requirements can be summarised as follows…

•High seat capacity to achieve attractive unit economics and affordable pricing over time”

devolves to

“Yes, we are building a 7-Seater!”

What is missing from this blog:

Where is the safety margin? i.e. what happens if something goes wrong? Can the aircraft make a safe landing if the motors fail? Do the limited control surfaces give it enough maneuverability to make an emergency landing in a tight field, especially in bad weather?

tl;dr great blog, thank you for the technical insight, it sounds like you've designed Lilium for a perfect world. Popular aviation needs to have basic safety margins and there are no safety margins evident here.

Why not use hydrogen instead of heavy batteries?

Why not use jet engines instead of propellers?

Not only is this company obviously a scam, if their product actually did exist it would be extremely expensive as well as louder and more dangerous than a regular helicopter.

Quite a bold statement to make with no substantial backing. If you have any experience with drones or following this air taxi movement, then you’d see many working prototypes out there.

This stuff is not so far fetched.

For fun, go look at Peter Sripol’s santa sled that he built: https://youtu.be/L-iWUu2HAWQ

How do the engines point down for take off?

Off topic, sorry, but: Why do web designers think that they can provide a better scrolling experience than the default system behavior that the user expects in every other interaction?

By the way, hijacking scrolling behavior often breaks other things, such as searching for text in a page. Go ahead, try it, search for a term in that article.

Yea, it's atrocious on this article – makes it unreadable tbh as scrolling muscle memory is completely messed up.

I hate scrolljacking too, but to answer your question, overriding the default system behavior in general isn't about providing a better experience but rather a more consistent (from the POV of the designer) experience.

The downside of course is that it looks less consistent with other apps and webpages from the user POV.

I just wanted to add that scrolling works normal on mobile. Including searching. It's only loading a bit too long.

I was about to say that it's quite smooth for me on Safari - then I searched for something as per your suggestion and the whole page was vertically offset and effectively broken... oh dear.

Edit: The scroll hijacking makes more sense in the context of the home page, which actually uses it to reasonable effect (not endorsing the technique, but the home page is attractive at least).

I think the mistake was leaving it on for heavy content pages like the one linked where it adds very little but breaks other basic behaviour.

It's quite smooth with a trackpad. With a mouse scroll wheel, it's basically non-functional.

The sexier the blogpost, the less believable it is for me. Make this page in Times New Roman or Computer Modern typeface, preferably in a PDF format, and I am automatically convinced :-). It just means that they have better things to do than aesthetics!

What is missing from the presentation is anything about recharging. The reasonable conclusion is they have no good story, there.

Most of the profitability of an air transport, when there is any, comes from quick turnaround. Having to wait to charge for an hour eliminates any possibility of that. An alternative is to have removable batteries, so when you land, you drop off dead batteries and slot in charged ones; and charge at the airport.

A better design would have an aft liquid-hydrogen tank and fuel cell, instead of batteries, for longer range and much lower mass. LH2 would be produced at the airfield during minimum energy spot prices, and stored underground.

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