9 ms·
Where did the false "equal transit-time" explanation of lift originate from?
- smallbugfound 1y agoNo idea why you would provide the equal transit time theorem to students, it makes such a low amount of sense that you're inevitably going to get your students extremely confused if they are paying any attention at all. "Why does the air have to transit in the same time period?" "But _why_ is the air moving over the top faster? Weren't you going to tell me how a wing works?" Etc etc etc It is the worst kind of lie-to-children (and adults) in my opinion, it's not a simplified true answer it's a whole cloth fabrication that vaguely gestures in the right direction, partially, if you are being generous. The idea that people get tested on regurgitating it for a pilots license is crazy. It's up there with those ridiculous tounge maps with taste regions on them.
- namaria 1y agoI think a lot of people pattern match on text throughout education because that's least action. Mathematics has been my way to avoid that. And I'm quite inept at mathematics. But if I can develop an abstract intuition of a problem I feel like that takes a lot less space in my head than trying to hold words to that effect in memory. Or as old man Cato said "Grasp the matter, words will follow"
- dguest 1y agoAt the same time, it gives me some hope that humanity can still get smarter. Progress in science and technology oscillates between breakthroughs and consolidation: new ideas are exciting but the repercussions and formalism need some time to sink in. If you don't give it enough time you are left with overly elaborate and confusing frameworks. Usually the academics sort this kind of thing out before it goes into mass market education but it's never too late to simplify.
- jstanley 1y ago> Why does the air have to transit in the same time period? Because otherwise it would leave holes in it where one side moves too fast before joining back up. It makes more sense if you imagine air to be incompressable.
- agos 1y agoair being incompressible makes even less sense, given how daily the experience of compressing air is
- ejolto 1y agoBut the air does move faster above the wing without joining back up with the air moving below the wing, see the video jgord shared https://www.cam.ac.uk/research/news/how-wings-really-work https://www.cam.ac.uk/research/news/how-wings-really-work
- FabHK 1y agoNot at all. Imagine two roads, parallel mostly, but then one takes a detour, like this: ____ ____/ __ \____ -> _____/ \______ _______________ -> _______________ Now imagine them full of cars, bumper to bumper. Now imagine the cars move, at the same speed, on both roads. Same number of cars will come in on the left as go out on the right. The cars will be bumper to bumper, both on top and below (there'll just be more cars on top). Why should cars that come in at the same time on the left exit at the same time on the right?
- jstanley 1y agoThe air was already there, the wing is being pushed into it, it seems to me that when the wing has passed it needs to end up all touching the same pieces it was before otherwise the wing will leave gaps in the air.
- IanCal 1y agoI've always thought it should work the other way around too, people show diagrams where the lines get squished at the top then tell me the pressure had gone down which isn't intuitive.
- bsder 1y ago> it makes such a low amount of sense that you're inevitably going to get your students extremely confused if they are paying any attention at all. Normally because you encounter this with the Bernoulli experiment about blowing over paper causing lower pressure, or the experiment where a spoon or ping pong ball gets pulled into running water. Both of which also turn out to not be Bernoulli's principle (it's Coandă effect)! The first question I asked as a kid was "So then why can planes fly upside down?" and flummoxed the entire room.
- marcus_holmes 1y agoI recently encountered this in a sailing class when the teacher was explaining how "pull-mode" works in sailing, where the wind is coming from ahead of the vessel and pulls the sails rather than pushing them. I knew this theory to be debunked and yet couldn't work out the answer from non-debunked physics (and certainly didn't want to disrupt the class by arguing physics with someone who's been sailing for 50+ years - if it worked for him, it'll probably work for me, even if debunked). Modern sailing vessels always sail into the wind, because they're always going faster than the wind blows. I do find the physics of this fascinating.
- jabl 1y agoFor the physics of sailing see http://gentrysailing.com/pdf-theory/A-Review-of-Modern-Sail-Theory.pdf http://gentrysailing.com/pdf-theory/A-Review-of-Modern-Sail-...
- crooked-v 1y agoSailboats that sail into the wind are also a bit different than planes because it's the boat being "pinched" between the wind-caused lift and the counteracting force from the water that provides the forward propulsion. It's analogous to something slippery shooting out from between two hands if you squeeze it.
- deleted 1y ago[deleted]
- kuhewa 1y ago> Modern sailing vessels always sail into the wind, because they're always going faster than the wind blows. Maybe state of the art hydrofoiling boats, cats and some quite large monohulls, but maybe that's what you meant by modern. Most sailboats built today have a pretty low top speed (due to hull speed limitations) relative to wind speed - monohulls often max out around 5-6 kts aren't going to be faster than the wind pretty much ever
- marcus_holmes 1y agoyeah, sorry, I should have specified modern racing sailing vessels
- dguest 1y agoGlad to see this posted more! I was asking about this 9 months ago and it started quite a thread [1]. I remember learning about this in grade school, finding it pretty confusing, and wondering why a simple "newton's third law" wouldn't suffice. That's incomplete but at least not wrong. [1]: https://news.ycombinator.com/item?id=40835223 https://news.ycombinator.com/item?id=40835223
- jabl 1y agoNewton's third law (as in, thinking of air molecules hitting the wing underside, disregarding any interactions between air molecules) would work for something like a spacecraft in the initial process of reentry, where one may assume that the atmosphere is so thin that air molecules don't interact much with each other. Lower down that assumption is no longer valid, and then applying Newton's laws to a continuum of interacting particles gives you fluid mechanics (Navier-Stokes etc).
- dr_dshiv 1y ago“Just like the "explanation" of seasons by the Earth's changing distance to the Sun, or textbook pseudo-explanations of history, like the "crisis" over the discovery of irrationals, or Maxwell's "mathematical" reason for adding an extra term to Ampere's equation.” Love it! Anyone have any others?
- topspin 1y agoSimplified explanations of how electrical current flows in conductors comes to mind. Water analogies, for example.
- IshKebab 1y agoThe water analogy is great. Not only are the basic equations exactly the same but it's easy to visualise. I don't see why you wouldn't like it. The basic equations are actually identical for a lot of domains, including rotational motion which gave rise to the Spintronics game.
- zokier 1y agoI agree that water analogy doesn't belong in this category because technically it works, but disagree on it being easy to visualize or pedagogically useful. Unless you have really solid understanding of hydraulics/fluid mechanics you are just trading one poorly understood subject for another.
- zihotki 1y agoIt's quite easy to get the solid enough understanding of hydraulics/fluid mechanics since it's something you can see/touch and easy to visualize. Kids love to play with water, a few experiments at school/home and they quickly understand it.
- IshKebab 1y agoYou can feel pressure and see flow rate. They're way easier to understand than voltage and current. Hydraulic accumulators are a real thing. There's even a freaking hydraulic equivalent of a boost converter! It's an amazing analogy. I'd say even calling it an analogy sells it short - it's not "like" electricity; it's exactly the same but in a different domain.
- nonrandomstring 1y agoFluid mechanics has always seemed so complex that scientists joke about it. Supposedly [0] Werner Heisenberg and/or Horace Lamb quipped that when they died they'd ask God about relativity, quantum electrodynamics and turbulence... and they didn't expect he would have an answer for the last. [0] https://boards.straightdope.com/t/did-heisenberg-really-say-this-about-what-he-would-ask-god/631918 https://boards.straightdope.com/t/did-heisenberg-really-say-...
- zihotki 1y agoOn one hand it's very complex, on the other hand it's rather simple if you don't go too deep. It's all about the scale of the map you work with.
- jgord 1y agoHandy video showing relative speed above and below the curved wing surface, and non-equal times : https://www.cam.ac.uk/research/news/how-wings-really-work https://www.cam.ac.uk/research/news/how-wings-really-work
- HPsquared 1y agoBicycles are another commonly-misunderstood system covered on cam.ac.uk. A lot of people seem to think they stay upright by gyroscopic effects. It's more a case of the rider making corrections, and when there is no rider (or hands-free) it works by a feedback loop involving the geometry of the forks. https://www.cam.ac.uk/research/discussion/opinion-how-does-a-bike-stay-upright-surprisingly-its-all-in-the-mind https://www.cam.ac.uk/research/discussion/opinion-how-does-a... Gyroscopic effect is very minor unless you're going very fast, i.e. motorbike speeds and motorbike weight wheels/tyres.
- federiconafria 1y agoThat becomes very evident once you consider how slow a bike can move and still be very stable.
- thatcat 1y agoYou can hold a bike wheel in your hand vertically and spin it, when you try to move it to a horizontal orientation you can feel the extent of the gyro force. It's significant.
- HPsquared 1y agoIt's enough to keep a loose rolling wheel upright by itself, but nowhere near strong enough to keep an entire bicycle plus rider upright. I don't think rim mass has much effect on handling - it's all about the geometry.
- thatcat 1y agoThe force is proportional to speed, you can countersteer a bicycle just the same as a motorcycle. There are 2 separate steering techniques, one for high speed (countersteering) and one for low speed (steering) on motorcycles and bicycles - that results from gyroscopic wheel force. The geometry doesn't change, only thing that changes as speed increases on the bike is the gyro force.
- artemonster 1y agoSo what is correct explanation? Is there a TLDR?
- tromp 1y agoI checked the top hit [1] for googling "how do wings really generate lift" which, being a NASA webpage, instilled some confidence they would have the correct explanation. But alas: > Airplane wings are shaped to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air. Then I read [2] which corrects that view: > “What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” clarified Babinsky, from the Department of Engineering. “This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.” This is still not very satisfying, as it fails to show HOW curvature causes lift. Maybe there is no simple explanation... [1] https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... [2] https://www.cam.ac.uk/research/news/how-wings-really-work https://www.cam.ac.uk/research/news/how-wings-really-work
- nandomrumber 1y agoI don’t see how the first and second paragraphs you provided are incongruent. Aren’t they saying exactly the same thing? And isn’t that Bernoulli’s Principle?
- fanf2 1y agoThe NASA page does not make the false equal time assumption.
- tijsvd 1y agoTLDR is that the full correct explanation is not simple. The Wikipedia article on lift makes a good effort. https://en.m.wikipedia.org/wiki/Lift_(force) https://en.m.wikipedia.org/wiki/Lift_(force)
- samuelfekete 1y agoIt’s still the explanation on NASA.gov https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso...
- seanhunter 1y agoThat isn’t the equal transit time explanation and the information on nasa.gov doesn’t look wrong to me. It’s entirely consistent with the Babinsky “How wings really work” video and experiment shown in a sibling post.
- porterde 1y agoNASA.gov has multiple pages on lift. Maybe one will explain it accurately. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/lift-3/ https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/lif...
- deleted 1y ago[deleted]
- lordnacho 1y agoOk so let's see if HN can put something together: - The wing deflects the air down, so that's one way of creating lift, but most wings are not just flat - An airplane can fly upside down - It's a bad idea to take off behind another plane - Modern wingtips have special shapes that makes them more efficient - Answer has something to do with vorticity, but what exactly? Hopefully we can get something better than whatever AI uses to explain these. I haven't asked it yet, but I get the feeling it would produce something plausible sounding that I won't be able to easily refute, ie it would trick me into thinking I understood it.
- bowsamic 1y agoI remember being taught this in A level physics case study of lift alongside a better third law explanation, with absolutely no acknowledgement or justification for one over the other. It caused a bit of a scandal among the class. Gave me a very sceptical view of physics education in general which I took to my physics undergrad and PhD
- deleted 1y ago[deleted]
- magicalhippo 1y agoWilliam Fraser over on YouTube has a video[1] on aerodynamic lift which I found interesting. In it he briefly touches on the equal transit time explanation, and how the steady-state snapsot presented doesn't really have enough information to tell how the flow field developed. He's been writing a particle-based simulator which he wants to use to show how lift develops from that perspective[2], still a work in progress. Just sharing as I found them interesting and cleared up some confusions I had. [1]: https://youtu.be/ZUBwc67c5_Y https://youtu.be/ZUBwc67c5_Y [2]: https://youtu.be/IVLpbOQUdqU https://youtu.be/IVLpbOQUdqU
- bobbyraduloff 1y agoIf you google “how does a plane generate lift”, the first result you get is a link from nasa.gov which claims that lift is generated this way. Kind of funny considering the SE question includes a screenshot of another NASA resource claiming this is false.
- bandrami 1y agoNASA's (correct) position is that the dynamics of heavier than air flight are complicated and do not admit of a simple heuristic explanation
- metalman 1y agofunny, there are films, of elaborite demonsrations of foils in a flowing fluid, where they can inject die in nice neat parallel bands accross the flow, and they detach at the foil accelerat and deform on the top, and reatch at the trailing edge saw them over on homebuiltairplanes.com, where there is a bunch of top shelf engineers, who hold court, and the perenial topic of lift is disccused and joked about, ie: lift is caused by money... films are old black and white...mit or bell labs, maybe naca....old....well done
- bandrami 1y agoWhat if we put the airplane on a treadmill though?
- financetechbro 1y agoTruth is we don’t fully understand how lift happens