[HN Gopher] The physics of airplane flight
___________________________________________________________________
The physics of airplane flight
Author : luu
Score : 155 points
Date : 2024-06-30 04:54 UTC (1 days ago)
(HTM) web link (10maurycy10.github.io)
(TXT) w3m dump (10maurycy10.github.io)
| nvy wrote:
| >A common misconception about wings is that they need to have the
| classic airfoil shape to work. In reality, just about any surface
| can create lift and function as a wing
|
| Left unsaid is _why_ aircraft wings have airfoil-shaped cross
| sections with cambered (concave-down) shapes: they produce more
| lift for a given wing loading and angle of attack.
|
| This is why aircraft have flaps, as well. They increase the
| camber of the wing so that the pilot can fly slower without
| pitching the nose up, which is important for example when
| maintaining sight of the runaway on landing approach.
| hyperpape wrote:
| > You will notice that past a certain angle, the lift starts to
| decrease, and is replaced by a lot of drag, a force trying to
| slow down the wing. This is called a stall, and it limits how
| much lift a wing can create at any given speed. A proper
| airfoil geometry can generate more lift before stalling, and
| creates less drag for a given amount of lift, which is why most
| airplanes use them.
|
| I'm a bit of a novice, so it sounds like this is the same
| explanation you're giving. What is the article leaving out?
| Nition wrote:
| Speed. You'll always stall if you go too slow to get enough
| lift. Putting the flaps down increases lift and drag and
| lowers the stall speed.
|
| As for why the article mentions decreased lift whereas flaps
| down increases lift, that's maybe a bit more complicated.
| Lift vs. Angle Of Attack is a curve that tops out at about
| 15deg.[0] Flaps often go to steeper angles than that, but I'm
| not sure if lift actually starts to decrease again in that
| scenario. Certainly adding a curve to the back of the wing
| (flaps) isn't quite the same as changing the angle of the
| entire wing.
|
| [0] https://commons.wikimedia.org/wiki/File:Lift_curve.svg
| nvy wrote:
| .
| carabiner wrote:
| No, flaps increase CL.
| https://www.researchgate.net/figure/Figure-1-4-Effect-of-
| fla...
|
| You might be thinking of total lift on a 3D wing, which is
| something you alluded to re: wing loading. But 3d wings vs
| 2d airfoils are really different concepts that you seem to
| be mixing up. Airfoils are advantageous regardless of wing
| loading (which is an inherently 3d concept) because of
| better L/D and stall characteristics.
| nvy wrote:
| You are correct, I had a brain fart. Not enough sleep the
| last few days.
| exabrial wrote:
| Great explanation! It also decreases the stall speed too; which
| allows a plane to switch to 'low speed mode'
| tonyarkles wrote:
| It's a really fun set of tradeoffs to be honest. You get more
| lift at a given speed/AoA, which lets you go slower without
| stalling. But you get even more drag, which means the engines
| have to work harder to keep you flying at that slower speed.
| lutorm wrote:
| Increasing drag is actually _useful_ when landing. too.
| Trying to land an airplane with very little drag at a given
| spot is difficult because if you 're just a little fast
| they'll float a long time.
| tonyarkles wrote:
| Definitely! Flaps are one of the few "almost free lunch"
| things in aviation where almost everything else has harsh
| tradeoffs to consider.
| mannykannot wrote:
| Except in dire circumstances, airplanes are not struggling to
| produce sufficient lift. What matters is to produce it with
| reasonably low drag, up to all the other constraints that go
| into making a practical airplane for its intended purpose.
|
| There are some airplanes that can be in a situation of flying,
| but unable to climb or even maintain altitude, with flaps fully
| lowered. In such a situation, only carefully raising the flaps
| will allow it to climb away.
| rootusrootus wrote:
| Given how straightforward the physics are, what is the limiting
| factor that kept us from developing planes sooner? Insufficient
| propulsion power to overcome the drag of an inefficient wing?
| bumby wrote:
| My understanding is that it wasn't power-to-drag but power -to-
| weight.
|
| Although the Wright brothers deservedly get a lot of credit, I
| think they made progress once they enlisted the help of Curtis
| due to his knowledge of lightweight internal combustion
| engines.
|
| The book _Bird Men_ is a god read on this subject.
| rootusrootus wrote:
| > The book Bird Men is a god read on this subject.
|
| Thanks for that recommendation, I'll add that to my book
| reading list!
| Stratoscope wrote:
| For anyone who has trouble finding this book like I did, the
| actual title is _Birdmen_ (no space):
|
| https://www.amazon.com/dp/B00GL3MGTI
| foobar1962 wrote:
| Weight. A typical steam train has enough propulsion power to
| overcome an enormous amount drag, but its power-to-weight ratio
| is low.
| imoverclocked wrote:
| The basics are straightforward while having the hindsight of
| all the development we have enjoyed in the last hundred years.
| _Stable flight_ was a hard nugget to crack. Better mechanical
| design, understanding of fluid dynamics, lighter materials,
| engines with higher power-to-weight ratios, etc all brought us
| to where we are today once the Wright Flyer was demonstrated.
| blt wrote:
| Yes, the wright flyer came fairly soon after internal
| combustion engines became light enough.
| mcarmichael wrote:
| Control was an almost completely unaddressed issue before the
| Wrights took it up, even though it is also crucial to useful
| glider development.
|
| This video has a nice delineation of the collection of
| breakthroughs needed:
| https://youtu.be/EkpQAGQiv4Q?t=391
| KineticLensman wrote:
| Yes. AIUI, as bicycle riders the Wright brothers understood
| the need for banking in turns rather than thinking of
| maintaining a constant roll orientation throughout a turn
| nothacking_ wrote:
| Almost none of this is intuitive without hundreds of years of
| hindsight. The more subtle aspects of stability, such as
| avoiding oscillation mostly had to be determined
| experimentally. Then there is also the matter of actually
| constructing a plane, if you want it to be useful, it's going
| to need to be a lot more then some folded paper.
|
| Thrust was definitely also a problem, a glider is not
| particularly useful unless it has a huge lift-to-drag ratio,
| which is only possible with modern materials and a solid
| understanding of airfoil design, which is a whole other can of
| worms.
|
| Even things that seem so basic that we don't even think about
| them, like high were not at all obvious: just look at Sir
| George Cayley's gliders.
| moralestapia wrote:
| Paper planes have been a thing for a long while, but I guess
| no one thought that could scale?
|
| It's like fire, super obvious in retrospective, and yet took
| thousands of years to become a technology.
| mr_toad wrote:
| It's not easy to scale materials and retain the necessary
| rigidity. Like if you tried to scale up a primitive kite to
| support a human it would either be too heavy or too bendy.
| You need better materials and better construction
| techniques.
| NovemberWhiskey wrote:
| > _Paper planes have been a thing for a long while, but I
| guess no one thought that could scale?_
|
| ... and they were right, because aerodynamics is not scale-
| invariant.
| moralestapia wrote:
| Well, airplanes _do_ exist now, so that did definitely
| scale.
| ahazred8ta wrote:
| Basically, understanding airfoils. Back in the 1840s they put
| miniature steam engines in model planes and got them to fly.
| There were powered ground-effect planes in the 1800s that had
| more powerful engines than the Wright Flyer, but the Wrights
| were the first people to invent the wind tunnel, so they had a
| better understanding of lift-to-drag and control surface
| issues. They adapted the flow-pattern analysis tanks used by
| ship designers.
|
| As mcarmichael said https://youtu.be/EkpQAGQiv4Q?t=391
| cm2187 wrote:
| Airplanes were developed almost as soon as piston engines
| became available. Steam engines were too heavy for a plane. And
| even the early piston engine had a bad power/weight ratio. For
| french speakers, there is a video of a retired Dassault plane
| designer where he mentions (among other things) that
| developments in plane design basically followed developments in
| engines.
|
| https://www.youtube.com/watch?v=lu4V9zn0wZ4
|
| Also highly recommend his previous video about the design of
| the Rafale, it's basically a lecture in plane aerodynamics.
|
| https://www.youtube.com/watch?v=XrpsptSLt90
| jillesvangurp wrote:
| Leonardo Da Vinci lacked good propulsion. But otherwise people
| have been toying with replicas of his designs and getting them
| to fly. Compact, petrol engines did not emerge until late 19th
| century. And they weren't very light. So, that was indeed a
| factor. Putting a steam engine on a plane was not going to fly.
|
| Though, they might have invented some kind of functional glider
| and used a steam powered winch. But, I'm guessing the aero-
| dynamics weren't that widely understood yet. And making stiff
| lightweight air-frames also took some time to happen. The
| contraption that the Wright brothers flew was a bit flimsy. And
| it barely flew. It was more of a proof of concept. Things like
| thermals and other updrafts that make gliders work weren't that
| well studied probably.
| schiffern wrote:
| >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...
| cjbgkagh wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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.
| schiffern wrote:
| >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.
| cjbgkagh wrote:
| 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.
| schiffern wrote:
| 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...
| cjbgkagh wrote:
| 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.
| kens wrote:
| >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/wheelmisfor
| t...
| notahacker wrote:
| 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)
| marcosdumay wrote:
| 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.
| calmbonsai wrote:
| 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.
| renegat0x0 wrote:
| Huh. What about Volocopter? Appears to be entering market.
|
| https://www.volocopter.com
| jillesvangurp wrote:
| 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 wrote:
| While most of them fly just fine, the economics need still
| to be proven.
| jillesvangurp wrote:
| 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 wrote:
| 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.
| jillesvangurp wrote:
| 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.
| NovemberWhiskey wrote:
| > _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.
| marcosdumay wrote:
| 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.
| mannykannot wrote:
| 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 wrote:
| > 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 wrote:
| 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.
| calmbonsai wrote:
| 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.
| throwaway211 wrote:
| 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 wrote:
| 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...
| boffinAudio wrote:
| 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 ..
| roelschroeven wrote:
| > > 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.)
| lutorm wrote:
| "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
| roelschroeven wrote:
| 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.
| carabiner wrote:
| > Additionally, when plane speeds up, more drag is produced,
| slowing it down.
|
| Wait, is it speeding up here or slowing down? Slowing down means
| deceleration, speeding up is acceleration, and it can't be doing
| both at the same time.
|
| > When the plane slows down, it produces less drag, allowing to
| to pick up more speed.
|
| Same deal?
|
| I think what he's getting at is that drag increases with the
| square of speed, but it's a very confusing way of explaining it.
| mansoor_ wrote:
| A common equation you will find in aerodynamics texts is:
|
| Drag = 1/2 * fluid density * velocity^2 * C_d * Ref. Area
|
| It approximates the drag experienced by objects as they move
| within a fluid (atmosphere). You can see that drag is
| proportional to the square of velocity, so going twice as fast
| induces 4 times the drag.
|
| Ergo, when you speed up, you produce a lot more drag. This will
| slow you down until you reach an equilibrium between thrust and
| drag (unless you apply more thrust).
| calmbonsai wrote:
| Probably the best practical flight explanation website:
| https://www.av8n.com/how/
| iamgopal wrote:
| Side note, but are there any international aircraft racing held ?
| For efficiency ?
| lutorm wrote:
| There are efficiency races for general aviation aircraft, yes.
| Check out https://en.wikipedia.org/wiki/CAFE_Foundation
| upofadown wrote:
| I suppose the Open category of the World Gliding Championships
| could be considered such a contest.
| dguest wrote:
| Do we still teach kids that planes fly because of Bernoulli's
| principle?
|
| I remember learning about it and wondering why newton's 3rd law
| wouldn't suffice. It's pretty obvious that the wings push air
| down and it's not that difficult to understand (even as a kid)
| that newton's 3rd law works.
|
| The essence of the Bernoulli argument is that the top of the wing
| is longer -> air has to move further -> faster air has lower
| pressure "because Bernoulli" -> pressure imbalance means lift.
|
| Ok, cool, but the "Bernoulli principle" I got as a kid was
| "faster air is lower pressure", which is both empirically wrong
| (the air in a compressor hose is obviously moving faster than the
| air in the workshop) and logically inconsistent (speed is
| relative, after all). You add in a half dozen qualifiers and it
| becomes true, but I wonder if this is more complicated than "the
| wings push air down, the air pushes the wing up".
| cm2187 wrote:
| You can experience faster air is lower pressure when you are
| trying to breath in strong wing (like sky diving or by an open
| window on a car on the motorway). It makes you usually gasp for
| air.
|
| But yeah I was taught planes fly that way in the 90s.
| elygre wrote:
| My mental model is that you can push hard on a wall while
| standing still, but the faster you run along the wall, the less
| you are able to push it.
| lupusreal wrote:
| The Newtonian explaination of lift is partially but not
| completely correct. It only explains some of the lift which is
| empirically observed. Particularly the "push air down" model;
| the tops of wings also pull air down along themselves (assuming
| there isn't flow separation, e.g. a stall) and direct it down.
| To really explain that flow you need fluid dynamics.
| krisoft wrote:
| > Do we still teach kids that planes fly because of Bernoulli's
| principle?
|
| Not just for kids, but it is also in pilot training materials.
| I distinctly remember that it was how lift was explained in my
| PPL book.
| JumpCrisscross wrote:
| > _distinctly remember that it was how lift was explained in
| my PPL book_
|
| TBF, the Handbook of Aeronautical Knowledge [1] does a better
| job.
|
| [1] https://www.faa.gov/regulations_policies/handbooks_manual
| s/a...
| mncharity wrote:
| Chapter 4, PDF:
| https://www.faa.gov/sites/faa.gov/files/06_phak_ch4_0.pdf
| rcxdude wrote:
| A lot of the difficulty of explaining lift of airfoils is that
| generally explanations try to follow a neat chain of cause an
| effect. But with the wing there isn't really a clear one. All
| these statements:
|
| - There is an upwards force on the wing
|
| - The pressure above the wing is lower than the pressure below
| it
|
| - The air around the wing follows a curved path downwards
|
| - The air above the wing travels faster than the air below it
| (NB: not in equal time!)
|
| - The air behind the wing has a downward momentum
|
| are related to all the others, but not straightforwardly: they
| all imply each other to some extent, both caused by and causing
| some of the others. So basically all explanations try to follow
| some path through the tangled web, but by doing so they always
| cause some oversimplification. The only top level chain is:
| shape of wing and angle of attack -> ????? (tangled mess of
| fluid dynamics few people fully understand) -> lift!
| lutorm wrote:
| Exactly, this is why understanding fluid dynamics is so
| difficult. You can't look at some physical laws and assume
| that the right hand side "causes" the left hand side. They
| all represent relations and it so happens that the fluid
| configuration that fulfill all the relations (and that the
| world adopts) is the one that causes lift. Just trying to
| talk about cause and effect is a misunderstanding.
| NovemberWhiskey wrote:
| My favorite example of this was in the air-data computer I
| was working on for a fighter trainer. I was just on the
| software side rather than the aerodynamics, but it was
| notable that the corrections to angle-of-attack and angle-
| of-sideslip measured by the multifunction probes (which are
| way up at the nose of the plane) included terms related to
| the position of the flaps (which are way back at the
| trailing edges of the wings).
| bquinlan wrote:
| That's awesome!
|
| I'm not surprised about the angle-of-attack needing
| correction. The angle-of-attack is defined as the angle
| between the average chord (an imaginary line running from
| the leading edge of the wing to the trailing edge of the
| wing) and the relative wind. Since changing the flap
| position changes the position of the trailing edge, the
| angle-of-attack will also change.
| HPsquared wrote:
| It's easiest to understand as a black box, or a "control
| volume". Consider the air coming in the front (horizontal)
| and the air going out the back (velocity is deflected
| downwards). Momentum change, needs a force to keep things in
| balance. Simple! Fluid mechanics is all about that kind of
| thinking.
| NovemberWhiskey wrote:
| OK, but that's just saying "the aerodynamic force exists,
| because we can observe the plane goes up and the air goes
| down", isn't it?
| sanarothe wrote:
| Classic phenomenological analysis. "This is not reality,
| but as a model it's good enough for a first pass design
| analysis"
| HPsquared wrote:
| It's control volumes all the way down. Look up "Finite
| Volume Method" in CFD.
| rcxdude wrote:
| This is exactly one of the common pathways through that
| middle section, which is nice and simple but doesn't really
| explain anything (why is the air deflected downards?).
| heavenlyblue wrote:
| Because the plane has speed and trading that speed for
| lift
| mncharity wrote:
| > explanations try to follow a neat chain of cause an effect.
| But [...] there isn't really a clear one. All these [...] are
| related to all the others, but not straightforwardly
|
| There seems a pattern of misattributing pervasive failures of
| science education content design, to physical system
| complexity and student deficiency. A favorite of mine was a
| PhD thesis "We taught grade G young students common
| incoherent nonsense about atoms. Surprisingly, that's didn't
| work out well. We draw the obvious conclusion: students in G
| are developmentally incapable of understanding atoms." Which
| might even be valid... for a "regurgitate incoherence"
| definition of "understand atoms".
|
| Here, I wonder if an atomistic explanation might work better?
| Could one craft a nicely accessible, coherent, transferably
| powerful, molecular superball mosh pit story of wings? The
| confusion and disagreements here sound a bit like "It's net
| molecular motion! No, surface impacts! No, differential
| surface impacts!". An abstraction/model fail, rather than
| underlying irreducible system complexity.
| jimmaswell wrote:
| If I'm not wrong, it seems dead simple when you put it like
| this:
|
| - Imagine the jet moves the wing forwards some small distance
| in some small amount of time.
|
| - Due to the shape of the wing, there is now a temporary
| vacuum above the wing as air particles have yet to rush in
| and occupy the space where the wing used to be.
|
| - There is now an unbalanced pressure around the wing
| sufficient to overcome gravity and give lift.
|
| No Bernoulli, no math, just visualizing a bunch of particles
| getting pushed around.
|
| If you think about air this way it also becomes obvious why a
| helium balloon moves in the direction of acceleration inside
| a car. Car moves forward, air in the rear of the cabin is now
| squished while the air in front is stretched out as it hasn't
| caught up to the car yet, pressure gradient sends the balloon
| forwards.
| fransje26 wrote:
| > It's pretty obvious that the wings push air down
|
| The air being pushed down is actually a side-effect of the
| lift-creation process, not the cause of it.
|
| A nice "counter example" is a wing in ground effect (flying
| very close to the ground), where there is less downwash,
| because of the ground, and yet the wing produces more lift.
| It's an effect that can make high aspect-ratio airplanes tricky
| to land.
| JumpCrisscross wrote:
| > _air being pushed down is actually a side-effect of the
| lift-creation process, not the cause of it_
|
| The turning of the gas is absolutely what causes lift. (Where
| the Newtonian explanation is misleading is in "neglect[ing]
| the physical reality that both the lower and upper surface of
| a wing contribute to the turning of a flow of gas" [1].
|
| Put another way: if you know the mass and acceleration of the
| gas about the wing, you can calculate lift. (This is
| impractical for many reasons.)
|
| > _a wing in ground effect_
|
| VTOL aircraft also experience ground effect due to the
| fountain effect.
|
| [1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocke
| t/a...
| HPsquared wrote:
| Curvature of streamlines is related to pressure gradient
| across said streamlines.
| JumpCrisscross wrote:
| > _Curvature of streamlines is related to pressure
| gradient across said streamlines_
|
| Sure. Ultimately just considering pressure or mass
| deflection doesn't work without elaborate workarounds.
| Because neither describes the reality of an airfoil
| turning a moving viscous fluid.
| fransje26 wrote:
| > The turning of the gas is absolutely what causes lift.
|
| No. What causes lift is the differential in pressure
| between the top and the bottom surface of the wing. The
| rest is broadly speaking a side effect.
|
| If the turning of the gas was the necessary mechanism for
| lift, planes in supersonic flight would fall out of the
| sky.
|
| Instead of relying on an airfoil shape for lift, you could
| fly by sucking air from the top of your wing and dumping
| out the back of your plane.
| dan_hawkins wrote:
| > No. What causes lift is the differential in pressure
| between the top and the bottom surface of the wing.
|
| How do you explain airplanes that can fly with wing with
| symmetrical cross-section profile?
|
| How do you explain airplanes flying upside-down?
| bbojan wrote:
| > How do you explain airplanes that can fly with wing
| with symmetrical cross-section profile?
|
| > How do you explain airplanes flying upside-down?
|
| Angle of attack is what causes lift. If you have a
| surface angled against the relative wind, it will produce
| lift.
| dan_hawkins wrote:
| I know all of that. I wanted to provoke parent commenter
| to let them see that the Bernoulli effect doesn't explain
| my two examples.
| bdamm wrote:
| The Bernoulli effect only contributes to making wings
| more efficient. It isn't fundamentally why lift occurs.
|
| You can make almost anything fly if you have enough power
| and a tail. But how efficient will it be? Not as
| efficient as an airfoil that takes advantage of all the
| fluid motion properties.
| dan_hawkins wrote:
| I think you wanted to respond to the parent comment. My
| questions have been a lead to debunk myth that the major
| contributor to the lift is the Bernoulli effect.
| fransje26 wrote:
| To create lift with a symmetrical airfoil, you are going
| to need a non-zero angle of attack. You can see the
| effect of a varying angle of attack on a symmetric NACA
| 0012 airfoil here [0].
|
| The following plot shows the pressure distribution over a
| wing at 3 different angles of attack [1]. As you can see
| from the first plot, some lift is created at -8 degrees
| AOA, but clearly a lot less than the +10 AOA example, as
| that airfoil is optimized for positive angles of attack.
|
| [0] https://www.youtube.com/watch?v=8uMlDL9HiaY
|
| [1]
| http://avstop.com/AC/FlightTraingHandbook/imagese0.jpg
| dan_hawkins wrote:
| Explanation based on Bernoulli effect requires longer
| path of air taking on top than on the bottom of the
| airfoil to create speed/pressure difference. With
| symmetrical airfoil both paths are the same regardless of
| the angle of attack. So when you mention AoA you
| implicitly lead to the explanation that lift, in
| majority, is not based on the Bernoulli effect.
|
| I've read excellent article debunking the Bernoulli
| effect and lift many years ago, I'm not sure I can find
| it again...
| fransje26 wrote:
| Explanations based on the Bernoulli effect are trying to
| explain a speed differential by pretending that two
| particles that were separated on the leading-edge of an
| airfoil, to then travel one above the airfoil, one below,
| would then rejoin at the trailing edge of the airfoil.
| And so, if you were to change the upper-camber of the
| airfoil, the flow on the upper part would need to
| accelerate to be able to join the trailing edge at the
| same time. And that would create a lower pressure,
| therefore lift.
|
| The nonsensical part of this model is that a particle on
| an upper streamline has anything to do with a particle on
| a lower streamline and that it is trying to keep up with
| it. Not so of course.
|
| But the lift created by a pressure difference due to a
| locally faster flow still holds.
|
| > So when you mention AoA you implicitly lead to the
| explanation that lift, in majority, is not based on the
| Bernoulli effect.
|
| For a NACA 0012, you'll need an AoA, to have a faster
| flow on the upper part of your airfoil, as it it
| symmetric. Other airfoils are perfectly fine creating
| lift at 0 AoA.
| JumpCrisscross wrote:
| > _If the turning of the gas was the necessary mechanism
| for lift, planes in supersonic flight would fall out of
| the sky_
|
| Why would pressure (Bernoulli out of Euler) propagate
| supersonically while momentum (Newton) does so
| subsonically?
|
| > _Instead of relying on an airfoil shape for lift, you
| could fly by sucking air from the top of your wing and
| dumping out the back of your plane_
|
| Wings (and the other bits that contribute to lift) are
| bigger than engines. That's the leverage you get with a
| lifting body: you move more molecules than your thruster
| alone.
|
| The correct answer here is unintuitive. But the very
| wrong answer is pressure alone. (As the article we're
| commenting on clearly shows with its brilliant flat-
| cardboard example. You don't need camber to have a
| lifting body, just angle of attack.)
| fransje26 wrote:
| > Why would pressure (Bernoulli out of Euler) propagate
| supersonically while momentum (Newton) does so
| subsonically?
|
| I'm sorry, I didn't understand the question.
|
| But in supersonic flight, with a flat plate, you don't
| have any rotation in the game, as illustrated here [0].
| And yet you will be producing a lot of lift.
|
| [0] https://image.slideserve.com/251762/supersonic-flow-
| over-fla...
|
| > But the very wrong answer is pressure alone.
|
| No, it really is the pressure alone. And viscous drag, if
| you want to be pedantic. Those are the only forces at
| play, the rest is only a side effect of those forces.
| JumpCrisscross wrote:
| > _you don 't have any rotation in the game, as
| illustrated here_
|
| The arrows literally moved down!
|
| > _it really is the pressure alone_
|
| NASA, pilots and aerospace engineers would disagree with
| you. But yes, you _can_ construct a working model of
| flight with just pressure. Same way you _can_ make a
| Copernican model match our observations of how the stars
| and planets move.
| stonemetal12 wrote:
| What causes the pressure differential then? It is my
| understanding that displacing\turning of the gas is what
| creates the pressure differential.
| fransje26 wrote:
| Good question!
|
| The pressure differential, in essence, is created by a
| faster airflow over the airflow. As the total pressure in
| your flow stays constant, if you increase the local
| dynamic pressure (with a faster flow), the local static
| (measurable) pressure decreases.
|
| So if you manage to shape your airfoil so that one
| surface experiences a faster flow (on average) than the
| other, you can create a pressure difference, and
| therefore lift.
|
| And in effect it is true that the gas will most probably
| need to be turned and displaced, but that is really the
| airflow adapting locally to the obstacle (airfoil) it
| encounters. The nose of the airfoil, where the
| acceleration is high, can be a place where a lot of lift
| is created, but it is not necessarily so.
|
| You can see example pressure distribution plots below:
|
| http://avstop.com/AC/FlightTraingHandbook/imagese0.jpg
|
| https://i.sstatic.net/UGurv.png
|
| https://agodemar.github.io/FlightMechanics4Pilots/assets/
| img...
| ljf wrote:
| Don't compare the pressure of the air in the workshop to the
| fast moving air in the nozzle - compare the air in the system
| of the compressor.
|
| In an air compressor, the lowest pressure air is the air moving
| through the hose and out the nozzle - the highest pressure air
| in the system is the 'still air' in the cannister. Think of an
| inflated balloon that you blow up and let go of, the highest
| pressure air is in the balloon, the lowest pressure air is
| immediately next to the mouth of the balloon, despite being the
| fastest moving.
|
| It might feel surprising, but the air that moves faster across
| the top of the wing is lower pressure than the slower moving
| air below the wing. That both the air below and above the wing
| are higher pressure than 'all the rest of the air in the sky'
| is inconsequential to the the plane - we only need to consider
| the air directly interacting with the wing. (though this is not
| to deny the impacts of angle of attack etc etc.)
| greenbit wrote:
| The thing I never found satisfying was this notion that the
| air over the top moves faster because it has _further to go_
| - in what way does the length of a path that lies in the air
| 's future have any effect on its speed _now_? As if the air
| over the top somehow has to match up with the air it was next
| to before the wing split it away below? What mysterious force
| would account for that?
|
| The best I could arrive at was that the forward motion of the
| wing causes the back side of the curved wing top simply to
| pull away from the air in that region, reducing the pressure
| there, and incidentally (because Bernoulli) that air then
| moves faster as a result.
| ljf wrote:
| The speed of the wing is what causes the air to move around
| the two faces of the wing. The air has to move around the
| wing as it is being pulled through it.
|
| Imagine pulling a fixed walled tube though the air, the air
| will move through the tube at roughly the speed that the
| tube is pulled through the air.
|
| Now imagine pulling a funnel that starts off large and gets
| smaller. The same air will now have to move faster to get
| through the funnel (higher pressure at the mouth of the
| funnel, lower at the end).
| zardo wrote:
| > As if the air over the top somehow has to match up with
| the air it was next to before the wing split it away below?
|
| It's not a good explanation intuitively because it's not
| clear why that has to happen, and it's just wrong, because
| that doesn't happen.
| ljf wrote:
| Imagine I fill a bathtub full of marbles - and I pull a
| solid semi circle through the marbles. The marbles that
| flat side moves past will barely have to move, the
| marbles that are displaced by the round side will have to
| 'move further'. They won't come out exactly at the same
| time, but they will have had to move further and move
| faster as the semi circle moves through the bath.
| digdugdirk wrote:
| This is a great example, and the first time I've heard it
| phrased this way. Thank you, I'll file this away for
| later.
| bumby wrote:
| How different is this when the "marbles" are
| compressible?
| stonemetal12 wrote:
| >The thing I never found satisfying was this notion that
| the air over the top moves faster because it has further to
| go
|
| On the one hand I agree that it is a stupid way to phrase
| it. On the other hand if the air doesn't "make it" then
| there is nothing where the wing just was aka a vacuum. The
| low pressure area that forms above the wing sucks the air
| along making it faster. Why doesn't all the air rush to
| fill the low pressure area? Well for air below the wing
| there is a wing in the way, air above the air flowing over
| the wing does rush down to fill the void providing lift,
| air behind the wing does as well creating some drag.
|
| Same for angle of attack it deflects the air that would
| normally be above and behind the wing down (providing some
| lift),making a low pressure area form above the wing which
| the air speeds to fill.
| throw310822 wrote:
| > air above the air flowing over the wing does rush down
| to fill the void providing lift, air behind the wing does
| as well creating some drag
|
| Just a nitpick, but these forces are never pulling, only
| pushing. The air rushing to fill the voids is not pulling
| the wing, is the air below or in front if the wing that
| pushes (and doesn't find an equal push on the other
| side).
| Retric wrote:
| > the air that moves faster across the top of a wing.
|
| Except absolutely flat wings also work where the air _is_
| traveling the same distance. They aren't nearly as efficient,
| but still produce lift.
|
| Wings shape relates to skin effects, vortexes, turbulence,
| and drag. There's a lot of complex interactions involved
| which don't simplify to faster moving air creates lift.
| pmontra wrote:
| Does that flat wing work with a zero angle of attack (that
| is, parallel to the ground) or does it have to point
| upwards?
|
| Race cars use downward pointing wings to generate the
| opposite of lift, to push the car into the ground. Of
| course even car wings have evolved into more efficient
| shapes, because there is a competition to win those races.
| Retric wrote:
| All wings need a positive or negative when upside down
| angle of attack to generate lift. People often draw the
| cord line incorrectly because the flat part of a wing
| isn't zero and wings are mounted with a positive angel of
| attack so aircraft can be level in flight even with a ~15
| degree angle of attack.
|
| Car aerodynamics is complicated. People talk about
| spoiler downforce without really considering the details.
| If you push down on the rear spoiler of a toy F1 car the
| front end lifts up because it's located behind the rear
| wheel. The goal is specifically downforce on the rear
| tires.
|
| Similarly the rotational force on an axle wants to lift
| the front end. There's another torque from the tires
| being located below the force of drag which again wants
| to lift the front of a car.
|
| For strait line dragsters they accept the front wheels
| having reduced contact with the road for improved
| acceleration because they don't need to turn. Where Indy
| and F1 uses front wings, but winged sprint cars pushed
| the classic spoiler forward on top of adding a wing for
| additional control. In racing it's all about different
| trade offs for each sport.
| isthatafact wrote:
| > "All wings need a positive or negative when upside down
| angle of attack to generate lift."
|
| That would be true for symmetric wings, but is not the
| whole point of an (non-symmetric) airfoil or frisbee
| shape to generate lift while horizontal?
| Retric wrote:
| I should have said to generate lift in level flight. Drop
| anything with air resistance and it's technically
| generating lift. However it's important to separate the
| angle of attack relative to the airstream vs angle of
| attack relative to the ground for falling objects.
|
| Anyway non-semmetric airfoils are about efficiency when
| the aircraft never flies upside down. Unfortunately you
| occasionally see mislabeled diagrams where the angel of
| attack seems to be zero when the wing is laying flat
| rather than the leading and trailing edge being level
| which creates a great deal of confusion.
|
| PS: A frisbee shape is largely a question of grip as
| rings can fly further, but they both need positive angel
| of attack to achieve significant distances.
| https://web.mit.edu/womens-
| ult/www/smite/frisbee_physics.pdf
| NovemberWhiskey wrote:
| If Newton's third law sufficed, then the shape of the upper
| surface of the wing wouldn't be important. In fact, it matters
| a lot.
|
| There is no single explanation for why airfoils generate lift
| that works at a grade school level.
| upofadown wrote:
| The turbulence caused by a sharp leading edge of something
| like a flat board causes momentum transfer to the top of the
| wing. The problem isn't in explaining how a conventional
| airfoil works, Newton's law works well enough for that. The
| problem is in explaining what happens when things go wrong.
| Turbulence has been a problem for physicists for a long
| time...
| NovemberWhiskey wrote:
| The Newton's third-law explanation is "air bangs into a
| bottom of airfoil and pushes it up". Without having the
| concepts of boundaries layers, laminar and turbulent flow,
| flow separation and (more generally) the entire Navier-
| Stokes toolbox you don't have the tools for explaining
| _why_ turbulent flow is a problem, for example.
| upofadown wrote:
| The explanation based on Newton's laws of motion is more
| to the effect that the wing interacts with the air in
| such a way as to accelerate some of the air towards the
| ground. The reaction force is upwards.
|
| The Navier-Stokes equations merely model fluid flows.
| Understanding them provides no understanding of the
| behaviour of such flows. That behaviour is emergent from
| the interaction of a great many particles.
| NovemberWhiskey wrote:
| > _The explanation based on Newton 's laws of motion is
| more to the effect that the wing interacts with the air
| in such a way as to accelerate some of the air towards
| the ground. The reaction force is upwards._
|
| But that doesn't have any _explanatory_ power at all. If
| we assume Newton 's laws hold, then obviously if there's
| a force upward on the airfoil then there's a reaction
| force downward on the air.
|
| It'd be like explaining the combustion engine by saying
| "the drive shaft from the engine rotates this way, and
| the reaction force - because the engine is more-or-less
| rigidly mounted to the frame - is resisted through the
| suspension by the wheels being in contact with the
| ground". OK, sure, but I still don't know how the engine
| actually works.
| upofadown wrote:
| I dunno. If I look at even a very simple diagram of the
| flow of air around a wing I see air deflected downward on
| the bottom and air accelerated around a curve on the top.
| Both would be expected to produce a downward reaction
| force.
|
| Added: Or more Newtonish (no action at a distance), there
| is more upward vertical force contributed by the
| particles in both cases than downward force.
| bruce343434 wrote:
| If I stick out a flat board from a moving car window, and
| hold it at an angle, it will "lift up". So indeed, airfoil
| shape does not matter. Angle of attack matters more, because
| that dictates the path of least resistance.
|
| Planes fly by slicing through a lattice of air, with blades
| (wings) that only slice easily in directions that lie on a
| single plane. Orthogonal tail fins means that the vehicle
| doesn't go from side to side as easily, so it mostly keeps
| flying on a line. Take a `+`-shape and elongate it so you get
| a "dentastix" like shape, then hold that out the car window.
| It will go in whichever direction you point it.
|
| Same idea of a boat rudder. And yet with boat rudders, we
| don't say "force of lift". The angle of attack changes, which
| means it now cuts through the water in a different direction
| (and the rudder piece wants to go straight in the direction
| it is pointing, since that way has comparatively little
| resistance in the water), which changes the way the rear of
| the ship moves which ultimately steers the ship.
| NovemberWhiskey wrote:
| > _So indeed, airfoil shape does not matter._
|
| ... what do you mean by that? If you mean "you can
| demonstrate the aerodynamic force using a flat plate", then
| yes you can do that. If you mean "a flat plate is a good
| tool to explain the aerodynamic force", then that's much
| less true. If you mean "in the real world, airfoil shape is
| irrelevant to aerodynamics" that's obviously false.
| bruce343434 wrote:
| I was being a bit facetious, sorry. It matters, but I
| think what I was getting at is often simply overlooked in
| favor of airfoil shape and the pressure difference
| explanation. The situation of gravity no longer being a
| factor such as with vertical rudders seems often missed.
| Then, it's suddenly called "rudder force". Even though
| it's the same thing as "lift". It seems the field of
| physics has trouble with isolating this
| concept/phenomenon and coming up with an apt name for it.
|
| Rudders are symmetric, i.e. don't have camber to create
| high/low pressure on one specific side all the time, and
| yet they work in redirecting (the relative) flow and
| thereby through Newtons 3rd redirecting the vessel!
| NovemberWhiskey wrote:
| The pressure difference explanation _is_ the basic
| explanation, though. The name of the force is "the
| aerodynamic force", and there's not really any confusion
| on that point.
|
| The difference of shape between hydrofoils and airfoils
| is determined by the properties of the masses in which
| they move, explained by the same theories of fluid
| dynamics, rather than any fundamental difference.
| bruce343434 wrote:
| That's just because when you angle the sheet, more
| molecules of air hit one side imparting part of their
| kinetic energy, and fewer molecules on the other side to
| counteract this. I do realize I'm explaining pressure on
| a molecular level here, but to me it's still "slicing
| through" and "pushing against" a lattice of molecules.
| AnimalMuppet wrote:
| For just a Newton's third law analysis, you have to have the
| air moving downward behind the wing. Doesn't the shape of the
| upper surface matter a lot in order to get the air moving
| downward?
| procflora wrote:
| Depends what we mean by grade school. For young kids (and
| honestly most adults) I don't think you need much more than
| this: "A wing, or anything that sends air moving past it down
| toward the ground will cause some lift (a push toward the
| sky), but also some drag (a push on your front toward your
| back). How much of each depends on the shape of the wing and
| how it's moving through the air. Really good wings cause a
| lot of lift without a lot of drag, which is good for not
| using a lot of fuel to get where you're going or for going
| really fast."
| NovemberWhiskey wrote:
| From my perspective, a much superior explanation would be
| something like: "A wing causes an aircraft to fly because
| its shape, and the angle at which it moves through the air,
| creates regions of higher air pressure under the wing, and
| lower air pressure above the wing. This causes an upwards
| force on the wing, and a corresponding reaction force
| downwards on the air itself."
| AnotherGoodName wrote:
| It's still Newton's third law; Push air down and minimize
| pushing air sideways or in swirling vortices because pushing
| air the wrong way wastes energy as per newtons law.
|
| That's what the aerofoil does. It pushes air down but
| mimimizes wasting energy on drag. It's still newtons law.
| HPsquared wrote:
| The qualifier is "along a streamline".
| criddell wrote:
| In school when our physics teacher explained how the shape of
| an airplane wing creates lift and allows the plane to fly, I
| asked how it is that airplanes can fly upside down? I got the
| classic "that would be a great thing for you to research on
| your own time".
| ultrarunner wrote:
| This is actually really cool, because an upside down airfoil
| will still create a high pressure ridge toward its leading
| edge. This causes air that would ostensibly flow along the
| bottom (high pressure) surface to sort of reverse and end up
| being pushed to the upper (low pressure) surface. The
| separation point is further down the leading edge than would
| be intuitively expected. This means the top stream of air
| _still_ goes further, and faster, than the bottom stream of
| air.
|
| So inverted wings still fly, just less efficiently.
| tzs wrote:
| The Newton's 3rd law explanation and the Bernoulli explanation
| are both reasonable approaches and both work, very similar to
| the way that one can explain the path of a thrown ball both by
| Newton's laws and by the principle of least action.
|
| NASA has a good explanation here [1]. Here's a brief summary.
|
| The gas flow has to simultaneously conserve mass, momentum, and
| energy.
|
| If you analyze lift by considering conservation of momentum you
| get that there are velocity differences in the flow at
| different parts of the wing. Integrate those around the whole
| wing and you find a net turn of the flow downward. Conservation
| of momentum (Newton's 3rd) requires an opposite upward force on
| the wing.
|
| If you analyze lift by considering conservation of energy you
| also get velocity differences which lead to pressure
| differences. Integrate pressure over the whole wing and you get
| a net upward force on the wing.
|
| [1] https://www1.grc.nasa.gov/beginners-guide-to-
| aeronautics/ber...
| bumby wrote:
| This doesn't really explain why those velocity variations
| occur in the first place, or am I missing something?
|
| It sounds like "We observe velocity variations on the wing
| and these correspond to pressure variations that create lift
| due to conservation of energy." But it leaves the question on
| what is causing the velocity variations in the first place.
| pc86 wrote:
| Not only kids but pilots. I got my license ten years ago and
| the answer to "How do planes?" was "Bernoulli."
| ultrarunner wrote:
| Not only that, but depending on the particular FAA designated
| examiner you get, failing to tell him Bernoulli can result in
| a disapproval. I've heard of it happening.
|
| Fortunately, none of this has ever mattered in the least for
| actually flying a plane, and there are plenty of sane
| examiners out there.
| kjkjadksj wrote:
| I don't think I learned anything about flight as a kid.
| Frictionless cars, yes.
| hydrogen7800 wrote:
| Published in 1944, _Stick and Rudder_ [0] by Wolfgang
| Langeweische has this to say:
|
| >Forget Bernoulli's Theorem
|
| >When you studied theory of flight in ground school, you were
| probably taught a good deal of fancy stuff concerning an
| airplane's wing and just how it creates lift. As a practical
| pilot you may forget much of it. Perhaps you remember
| _Bernoulli 's Theorem_: how the air, in shooting around the
| long way over the top of the wing, has to speed up, and how in
| speeding up it drops some of its pressure, and how it hence
| exerts a suction on the top surface of the wing. Forget it. In
| the first place, Bernoulli's Theorem does not really explain-
| the explanation is more puzzling than the puzzle! In the second
| place, Bernoulli's Theorem doesn't help you in the least bit in
| flying. While it is no doubt true, it usually merely serves to
| obscure to the pilot certain simpler, much more important, much
| more helpful facts.
|
| >The main fact of all heavier-than-air flight is this: _the
| wing keeps the airplane up by pushing the air down._
|
| >It shoves the air down with its bottom surface, and it pulls
| the air down with its top surface... In exerting a downward
| force upon the air, the wing receives an upward counterforce-
| by the same principle, known as _Newton 's law of action and
| reaction_, which makes a gun recoil as it shoves a bullet out
| forward...
|
| >Say that the wing is basically simply a plane, set at a slight
| inclination so as to wash the air down... But it was early
| found that the drag, lifting, and stalling characteristics of
| such an inclined plane can be improved by _surrounding it with
| a curving, streamlined housing_ [emphasis mine]; hence our
| present wing "sections". The actual wing of an airplane is
| therefore not simply an inclined plane; it is a curved body
| _containing_ an inclined plane.
|
| [0]https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?ed
| ...
| phkahler wrote:
| >> The essence of the Bernoulli argument is that the top of the
| wing is longer -> air has to move further -> faster air has
| lower pressure "because Bernoulli" -> pressure imbalance means
| lift.
|
| That argument doesn't hold up (no pun intended). Just because
| the distance is longer does not mean the air will go faster, it
| could just take longer to get there.
| quietbritishjim wrote:
| Newton's third law doesn't explain stalls - or, at least, not
| their suddenness.
|
| As angle of attack increases (or speed decreases) there comes a
| certain point where the lift suddenly drops in a dramatic way
| that wouldn't make sense from a naive application of Newton's
| laws. What's really happened is that the airflow has separated
| from the wings and Bernoulli's principle no longer applies.
| That's when you get a stall, and the plane starts falling
| rather than flying.
| bilsbie wrote:
| The Venturi effect does show that faster air is lower pressure
| though.
| quibono wrote:
| > With a simple rectangular wing, the center of pressure is 1/4
| of the way along the wing from the leading edge
|
| Is there a nice way to derive this? I find it interesting it's
| not the exact center though I guess it makes sense given the
| angle of the surface.
| kwertzzz wrote:
| I was asking myself the same question. I would love to see a
| derivation from e.g. the Navier Stokes equation for this. I
| think, intuitively, when you draw the streamlines under the
| rectangular wings, the applied force should be related to the
| curvature of the streamlines (which is larger at the beginning
| of the wing).
|
| I made some simple 2D Navier-Stokes solver here where you can
| use the mouse to draw a section of a wing:
|
| https://alexander-barth.github.io/FluidSimDemo-WebAssembly/
|
| The color represents the pressure (I should add a proper color
| bar!)
| JumpCrisscross wrote:
| > _Is there a nice way to derive this?_ "It has been found both
| experimentally and theoretically that, if the aerodynamic force
| is applied at a location 1 /4 chord back from the leading edge
| on most low speed airfoils, the magnitude of the aerodynamic
| moment remains nearly constant with angle of attack. Engineers
| call the location where the aerodynamic moment remains constant
| the aerodynamic center (ac) of the airfoil" [1].
|
| As you go faster, it goes from quarter chord to half. For a
| rectangle, the chord length is equal to the airfoil length.
|
| Why quarter? It comes from thin-airfoil theory [2].
| (Unintuitively, it holds across atmospheres.)
|
| [1]
| https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
|
| [2] http://aero-
| comlab.stanford.edu/aa200b/lect_notes/thinairfoi...
| taegge wrote:
| https://m.youtube.com/watch?v=edLnZgF9mUg I recommend this MIT
| opencourseware video to anyone interested in this topic
| jcynix wrote:
| Airplanes can do rolls and barrel rolls, i.e. they can fly upside
| down. Good luck with simple explanations ...
|
| https://www.scientificamerican.com/video/no-one-can-explain-...
| nothacking_ wrote:
| Easy, the natural angle of attack is determined by the elevator
| position. Flying upside down is totally possible with the right
| inputs.
| bobtheborg wrote:
| Related, very deep, and amazing work:
| https://ciechanow.ski/airfoil/
| https://news.ycombinator.com/item?id=39526057
| vain wrote:
| I remember being taught that Bernoulli's principle causes lift. I
| was skeptical--how does the air on top know to reach the other
| end at the same time as the air at the bottom? I think I did ask,
| and I was just told this is how it works, and that's the correct
| answer for the exam. This was before the internet, and I couldn't
| just look up the correct explanation.
|
| I parked it in my brain as something I didn't really understand
| and forgot about it. This was until not so many years ago when I
| found a satisfactory answer on YouTube. It was criminal to have
| been raised in an era without the internet.
| EncomLab wrote:
| I've flown hundreds of hours using single-surface hang-gliders
| that effectively have little to no "flat plate" effect, and they
| make huge amounts of very draggy, slow speed lift. I've flown
| hundreds of hours using double-surface hang-gliders that make
| much less slow speed lift, but far less draggy lift at moderate
| to high speeds. As in all things with aerodynamics - you optimize
| the design for the performance you want.
| alexb23 wrote:
| For a more in-depth resource that is still very approachable at a
| high school level, I highly recommend John S. Denker's book, See
| How It Flies, full text online https://www.av8n.com/how/
|
| Edit: added book title
___________________________________________________________________
(page generated 2024-07-01 23:01 UTC)