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Comment by schiffern | Hacker News Reader

  >Additionally, a horizontal stabilizer in the back needs to be pitched down relative to the wings, creating downwards lift, pitching the plane up.

Naturally, this is a fundamental source of inefficiency.

This is something I appreciate about the Lilium aircraft: they use canards to avoid this problem. Their latest design places the rear wing slightly above the canard[1], minimizing the downwash disadvantages[2] inherent in many canard configurations.

[1] https://www.youtube.com/watch?v=qZ73PftBfFg&t=273

[2] https://aviation.stackexchange.com/questions/83584/are-canar...

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> > Additionally, a horizontal stabilizer in the back needs to be pitched down relative to the wings, creating downwards lift, pitching the plane up.

> Naturally, this is a fundamental source of inefficiency.

It's also not correct. The stabilizer in the back (or more generally: the wing in the back) needs to have a smaller angle of attack than the wing in front. In the common case (wing in back much smaller than wing in front) the wing in the back often is designed with negative angle of attack to create enough margin, but it's not strictly speaking necessary.

There are planes (like the Lilium you mention, or the planes designed by Burt Rutan) that have a small wing in the front (a canard) and a large wing in the back, providing most of the lift. In that case the wing in the back obviously needs to have a positive angle of attack to create the lift for the plane to stay in the air.

That may look like a special case, but it is not, aerodynamically speaking. The rule is the same for all types of planes: for stability, the back needs a lower angle of attack than the front, and that does not necessarily mean it needs to be negative. It also means the plane's center of gravity does not necessarily have to be in front of the front wing.

See https://www.av8n.com/how/htm/aoastab.html#sec-basic-stabilit..., and more specifically https://www.av8n.com/how/htm/aoastab.html#sec-canard-same.

I don't really know whether canard configurations solve the inefficiency problem of horizontal stabilizers with negative angle of attack: canards necessarily have a high angle of attack, which also creates drag.

(Note that all of the above is only relevant for planes with classic static stability (which is almost all of them); planes with relaxed static stability and fly-by-wire are kept stable by computer instead of aerodynamics.)

"See how it flies" that you linked is a wonderful resource for understanding the physics of aircraft and the skills needed to fly one.

Another, more math heavy treatment, of why airplanes fly that still aims to gain a conceptual understanding is "Understanding Aerodynamics" by Doug McLean: https://www.amazon.com/gp/product/B00B9QLBH0

I have that book! I bought it some time ago; I don't remember why I bought exactly that book, I guess it was recommended somewhere online much like you did just now.

Unfortunately I haven't found the time/energy to start reading it.

I'm a huge fan of the Opener Blackfly, which has been physically designed for VTOL-like performance while providing a very simple flight mode during the transition from level to VTOL:

https://evtol.news/opener-blackfly/

The angle of attack of the lifting surfaces is intentionally offset so that, when the Opener reaches certain speed limits, aerodynamics take over and the plane switches modes automatically, without requiring much pilot input.

I can't wait to see these things buzzing around the skies - imho, this is the closest to 'the flying cars promise, fulfilled' so far ..

The Lilium Jet does not appear realistic to me, most VTOLs barely fly in the best of circumstances. It seems more like a scam to me. Not my field so I could be wrong but the added efficiency of a canard is small change compared to the other challenges involved.

All of these electric VTOL urban air mobility things are hopeless. Various tech companies including google have taken a stab at them over the past two decades and none are anywhere close to entering service. Google shut theirs down. I really want them to work, but nothing gets past the physics/economics of batteries having piss poor energy density relative to fossil fuels and the massive inefficiency of small rotors. I think Boom has a better chance of flying than Wisk. Meanwhile, Uber Copter is up and running in NYC with regular helicopters.

Perhaps VTOL is entirely impractical and has no sizeable niche, but it appears Lilium is the least wrong out of all the existing VTOL physical layouts.

  - No additional draggy/heavy structures for the VTOL components, they reuse the existing wings

  - No large exposed props, which have noise and hazard concerns

  - Propulsors are synergistically combined with the wing upper surface, enhancing lift during cruise

  - VTOL mode actuators are synergistically combined with ailerons/elevators, reducing part count

  - They have wings for efficient long-distance flight (surprisingly some don't!)

  - Contingency ability for runway landing if the battery is too depleted

  - The aforementioned canard advantage

If any of the VTOL schemes are workable (which is admittedly an open question!) it will be Lilium.

It appears you’ve drunk the coolaid. I looked into it more and can now confidently say it’s a total scam. The stuff you’re talking about is window dressing.

Maybe that's true! If you can elaborate on your confident saying with facts, sources, or explanations, I would certainly give it my full consideration. It wouldn't be the first time I've been wrong!

What makes you think it's a "total scam"?

My prior for eVTOL these days is scam, but I googled ‘Lilium Jet scam’ and hit this;

https://news.ycombinator.com/item?id=29651504

What I was looking for is someone to say it can’t work because of the math XYZ and for a person to say back that math XYZ is wrong because of ABC and that second part never happens.

The stuff they talk about is window dressing and doesn’t answer questions like, where are your 500wh/kg batteries?

I remember when Ballon Boy happened and I took one look at that Ballon and it was instantly obvious it wasn’t carrying a kid, but apparently others were still expecting to see a kid when the balloon landed.

  >My prior for VTOL these days is scam

That doesn't answer my question. It just hides your 'scam' claim within your prior.

_Why_ is your a priori expectation for all VTOL to be a scam? And does that generalized reasoning in fact apply to the specific case of Lilium?

  >where are your 500wh/kg batteries

That's probably the least speculative out of all their bets. Moore's Law for microchips may have ended, but a similar (slower) scaling law for batteries seems to be holding for decades now.

My prior is that batteries will continue on the same curve.

You can also check out their stock price. Down from peak 92%, so at least to investors it’s looking less likely they’ll deliver instead of more likely.

I gave a link in which others listed their rationale for why they don’t think it will work.

Their availability estimates for battery capacity is double the long term rate of improvement - which seems unrealistic. But yah know with enough wh/kg just about anything will fly.

Imagine thinking stock price pegged to peak price indicates company value. :-/

  > I gave a link

From your link:

  > 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...

The variable nozzle is still present[1], but Lilium long ago dropped variable pitch[2]. "Tilt-wing" is also inaccurate, since they only move the (already moving) aileron and elevator surfaces, not the wing and its associated structures.

Hopefully this can help clear up these (hastily-Googled) concerns.

  >Their availability estimates for battery capacity is double the long term rate of improvement

This is an interesting claim that I'll look into, thanks. Do you have a source? I found a few slide decks but I couldn't immediately find this part, so any help would be appreciated.

I do expect in the nominal case that Lilium will take perhaps twice as long to come to market, so in the end the delays may roughly cancel out. Time will tell.

Note that battery density will effect all eVTOL startups equally, so it's not really a competitive disadvantage for Lilium per se, but rather an overall industry challenge. And I agree, there are many challenges facing the industry!

Good deep discussion, thank you. Cheers mate

[1] https://lilium.com/newsroom-detail/technology-behind-the-lil...

[2] https://lilium.com/newsroom-detail/youve-never-seen-anything...

What you have shown me has not convinced me to change my opinion and I'm not trying to change yours. The people that I know in the industry focus on certification timeline being overly optimistic which while most defiantly true is less interesting to me than the battery tech, or even the sociology as to why people get so attached to this - like that Nuclear-Powered Sky Cruise concept that many people shared unironically.

No doubt a lot of amazing things can happen with an large increase of wh/kg which is why I think it's incredibly weird that so much effort was put into things that are not that, Lilium has special investments in battery tech with Ionblox, the only interesting question to me is 'is that paying off?'. Not how much % of efficiency can be saved with a canard design - that's window dressing. Also, if they have this amazing battery tech then isn't that the most valuable thing they have and is eVTOL really the best application for it, why not double the range of electric cars instead. Even if they think eVTOL is the best use case then why not run on a skeleton crew until the battery tech arrives, or at least properly test their designs with onboard generators.

Waiting for battery tech is like waiting for engine tech and since so many engines end up vaporware so do all the nice aircraft designs dependent on them.

But sometimes new engines do deliver. The RED Aircraft V12 engines are amazing and should enabled the Otto Celera 500L to work really well. The DeltaHawk engine but they might actually end up delivering a really nice reliable engine. Part of that is a combination of long term stagnant general aviation engine technology and reduced tooling costs for manufacturing with CNCs. So there was a lot of low hanging fruit waiting to be picked.

Obviously Lilium Jets initial claims were wildly unrealistic, I think their current claims remain unrealistic, perhaps by the time they 'deliver' they've scoped it down to small hops. If they plan on selling something that can carry 7 people (they've already sold 20?) then perhaps I would believe it more if they demonstrated something working with 1 person and a whole lot of performance to spare.

>If any of the VTOL schemes are workable...

That reminds me of the "V/STOL Wheel of Misfortune", a diagram of 45 different V/STOL designs, of which four made it into service.

https://www.cmlx.co.uk/hawkerassociation/hamain/wheelmisfort...

On a similar note, the Aviation week ranking for current Advanced Air Mobility projects is named a "reality index"!

https://aamrealityindex.com/aam-reality-index

(Lilium is in the upper middle of this, FWIW)

On that case, past performance is absolutely not a guide for future results.

The weight/power ratio of electrical motors is so different from combustion engines that it's a qualitative difference already. Just because nobody has ever been able to solve that problem, it doesn't mean that a lot of people won't easily solve it now.

Joby aviation seems the furthest along.

Joby has completed many test flights that mimic its real-world route distances and energy-altitude flight regeimes.

Its flight avionics package (essential for VTOL fly-by-wire) has received FAA certification.

Related to Lilium and noise, Joby has a lower overall emitted dB energy and, specifically, a much lower dB signature in the human hearing range.

Huh. What about Volocopter? Appears to be entering market.

https://www.volocopter.com

Well a scam that is flying and more or less working as advertised at this point. You can watch testflights on youtube. Same for the Archer, Joby, Beta Alia, and a few others. Several of those are now doing manned flights and shipping prototypes to early customers.

In short, they fly just fine. What makes you think things are a scam?

While most of them fly just fine, the economics need still to be proven.

That usually comes after certification. Which some of these things are getting close to. I hear Delta is eager to start shuttling passengers from their terminals with Archer. At this point, you can start making some pretty informed statements about operational cost too. Battery life span and cost is a known factor. The cost of electricity is a known factor. There might be some nasty surprises with components, manufacturing, or scaling up production volume of course.

But mostly these things are starting to look like they are getting there.

Well, what Lilium is flying is a demonstrator, not a prototype. The difference is that a prototype is close to the final product and test flights can be used in the early phases of the certification path, demonstrator flights cannot. Or to put harsher: all Lillium has is two model aircraft that have close to nothing in common with the 7 seater they are selling.

And one could call it a scam, when tuh product you sell has nothing to do with the product you show (and no, mock-ups at airshows don't count at all), and the product you sell has, so far, no clear timeline until certification. The aerospace version of vaporware. Whether or not it amounts to an actual scam woupd be for courts to decide. Right now it looks a lot like Nikola, without the option to use a hill to fake the product demo.

As a sidenote regarding test flights: last time I checked, those were unmanned, with a demonstrator and not a prototype and no longer than 6 minutes. Which is as far from what serious people in the field call a test flight of it could be. Good for PR and investors so, it looks cool.

Also, one can make everything fly, if you put enough thrust to it. Doesn't mean you have product that can sustain a business.

That prototype/demonstrator (let's not get silly about words here) looks like it's actually flying properly though. Transitions to horizontal and vertical flights and all. They are planning to have the first manned flight end of this year. A scam would be intentionally misleading people about the ability of this thing to fly at all and then grabbing the money and run.

scam would also involve disgruntled investors trying to sue and getting their money back. There have been a few such cases about investors wanting their money back. But the headline is that Lilium is continuing to raise lots of money and making steady progress to getting their products launched. And those court cases seem to be going nowhere so far. The nature of VC funding is of course that things don't always go to plan.

Just because this company isn't satisfying your need for instant success and instead is following an entirely reasonable path to certification, which is slow for any airplane, doesn't mean it's a scam. By that logic anything is a scam until it emerges fully designed and manufactured on the market. That's not how things work in the real world.

This thing has investors, prospective customers with letters of intent, and flying prototypes.

Nikola is actually shipping trucks at this point too. Yes, they got caught with a non driving prototype running downhill and they got punished for that and the CEO might do some jail time for that. But the thing works now and they are selling lots of trucks that actually move cargo around. A scam would have been if the thing proved to be vapor ware. As it turns out, it wasn't. It was just running a bit late.

>A scam would be intentionally misleading people about the ability of this thing to fly at all and then grabbing the money and run.

There's a fine line between a scam and a business plan which bakes in assumptions that are unrealistic.

I haven't been following those, but the GP's question wasn't about any of the things you enumerated.

The big question is: have they demonstrate a loaded plane flying through a useful distance while keeping enough reserve energy for satisfying the safety requirements?

Their videos are very well produced explanations about everything but this. There's some stuff about a few changes that reduce the reserve requirements, but still, I couldn't find anything about range.

If this argument were sound, tailless designs (i.e. without a separate horizontal stabilizer either before or behind) would be optimal, but what matters to efficiency is the overall lift-to-drag ratio within all the other feasibility constraints. No-one obsesses over lift-to-drag ratios more than sailplane designers and their customers, and the fact that the medium- to high-performing sailplanes are neither tailless nor canards, despite there being airworthy examples of both canard and tailless gliders, is telling us something, at least in the range of Reynolds numbers relevant to glider flight.

> If this argument were sound

Are you saying that having a horizontal stabilizer is not a source of inefficiency? This isn't an argument, it's a basic fact about airplane design. They necessarily contribute to overall drag.

In practical airplane design, there are considerations other than drag that make horizontal stabilizers a worthwhile compromise.

Firstly, 'having a horizontal stabilizer is a source of inefficiency' is not an argument, it is a fact (one that might be a premise in an argument, but see below.) As you know this, how did you get from seeing that I said a certain argument is unsound to supposing that I am disputing this fact?

Maybe you think it is the only way that argument could be unsound, which brings us to the second point: the argument I am commenting on is not 'having a horizontal stabilizer is a source of inefficiency, therefore canards are more efficient', which would not even be valid. It is, instead, the argument that canards are more efficient because the conventional horizontal stabilizer usually produces a downwards force (incidentally, this is not always so [1].) While this may seem an obvious conclusion at first sight, it tacitly presumes a sharp separation of concerns which does not hold in practice.

The argument 'having a horizontal stabilizer is a source of inefficiency, therefore tailless designs have greater efficiency' (which was not made in the post I was replying to, but which is sometimes alluded to) does not hold up any better, on account of the compromises in making a stable and controllable tailless airplane (at least without active stability augmentation.)

[1] For some conventional airplanes, with the GofG near its aft limit, the horizontal stabilizer will produce an upwards force at low speeds (without being unstable as a consequence.) This happens to be the case for many gliders. A while back (and possibly now lost - at least, I have not been able to find it), there was an interesting article (by Wilhelm Dirks - co-founder of DG Aviation, I believe) explaining why, in practice, this cannot be exploited to get a little bit more performance out of a sailplane.

Three issues with pusher-prop "tail-less" designs:

1) While they are more stable in nominal flight regimes, they are far harder to recover to stable flight from perturbations. It turns out, it's (overall) much safer to have a plane that will BOTH stall easier (more predictably) and recover easier than one that is less likely to stall in the first place, but difficult to recover from. One analogy I often use is the difference between a mid-engine car and a front-engine layout. While the mid-engine car has a greater overall theoretical "handling" performance ceiling, a front-engine car behaves more predictably (less twitchy) at the limits.

2) They are more susceptible to CG/balance issues so they have less practical cargo capacity because just a weee bit of pitch/yaw/roll trim results in a drastic drop-off in the aforementioned stellar lift efficiency.

3) They have much longer take-off and landing runway requirements due to less ground-effect and much less overall wing efficiency at near-stall speeds.

Could you explain the inefficiency in terms of energy transformation?

Potential energy change is zero (or negative)?

I get there may be sound or heat from air friction though would put that minimal. Also zero (negative) acceleration around axis due to rotation.

I'm curious how it's inefficient analysing through energy transformation.

Because of the negative lift of the horisontal stabilizer, the main wing needs to provide more lift than the weight of the airplane. This increased lift requires flying the wing at higher angle of attack, which always comes with increased drag (https://en.wikipedia.org/wiki/Lift-induced_drag). Increased drag means lower efficiency.

Analyzing energy transformation is not so useful, because most drag ultimately ends up heating the air. (A little will heat the airplane skin.) There are several different mechanisms that makes that happen, and no easy way to figure out how large it is, but it's definitely not minimal. It's why (most) airplanes need an engine to stay up...

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