5 ms·

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

>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 https://www.youtube.com/watch?v=qZ73PftBfFg&t=273 [2] https://aviation.stackexchange.com/questions/83584/are-canards-actually-more-efficient https://aviation.stackexchange.com/questions/83584/are-canar...

cjbgkagh 2y ago
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.
- carabiner 2y ago
  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.
- schiffern 2y ago
  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.
  - cjbgkagh 2y ago
    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.
    - schiffern 2y ago
      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"?
      - cjbgkagh 2y ago
        My prior for eVTOL these days is scam, but I googled ‘Lilium Jet scam’ and hit this; https://news.ycombinator.com/item?id=29651504 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.
        schiffern 2y ago
        >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.
        2y ago
- jillesvangurp 2y ago
  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?
  - chinathrow 2y ago
    While most of them fly just fine, the economics need still to be proven.
    - jillesvangurp 2y ago
      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.
  - hef19898 2y ago
    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.
mannykannot 2y ago
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.
- inoffensivename 2y ago
  > 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.
  - mannykannot 2y ago
    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.
throwaway211 2y ago
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.
- lutorm 2y ago
  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 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...
boffinAudio 2y ago
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/ 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 ..
roelschroeven 2y ago
> > 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-stability 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 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.)
- lutorm 2y ago
  "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 https://www.amazon.com/gp/product/B00B9QLBH0
  - roelschroeven 2y ago

cjbgkagh 2y ago

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.

carabiner 2y ago
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.
schiffern 2y ago
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.
- cjbgkagh 2y ago
  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.
  - schiffern 2y ago
    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"?
    - cjbgkagh 2y ago
      My prior for eVTOL these days is scam, but I googled ‘Lilium Jet scam’ and hit this; https://news.ycombinator.com/item?id=29651504 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.
      - schiffern 2y ago
        >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.
        2y ago
jillesvangurp 2y ago
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?
- chinathrow 2y ago
  While most of them fly just fine, the economics need still to be proven.
  - jillesvangurp 2y ago
    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.
- hef19898 2y ago
  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.

mannykannot 2y ago

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.

inoffensivename 2y ago
> 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.
- mannykannot 2y ago
  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.

throwaway211 2y ago

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.

lutorm 2y ago
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 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...

boffinAudio 2y ago

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/ 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 ..

roelschroeven 2y ago

> > 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-stability 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 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.)

lutorm 2y ago
"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 https://www.amazon.com/gp/product/B00B9QLBH0
- roelschroeven 2y ago