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Where did the false "equal transit-time" explanation of lift originate from?

by IdealeZahlen on 4/23/25, 6:11 AM with 67 comments

  • by smallbugfound on 4/23/25, 6:51 AM

    No 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.

  • by jgord on 4/23/25, 7:19 AM

    Handy 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

  • by dguest on 4/23/25, 6:59 AM

    Glad 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

  • by dr_dshiv on 4/23/25, 7:03 AM

    “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?

  • by bowsamic on 4/23/25, 7:39 AM

    I 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

  • by bobbyraduloff on 4/23/25, 8:19 AM

    If 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.

  • by magicalhippo on 4/23/25, 8:13 AM

    William 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

    [2]: https://youtu.be/IVLpbOQUdqU

  • by marcus_holmes on 4/23/25, 6:57 AM

    I 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.

  • by nonrandomstring on 4/23/25, 7:15 AM

    Fluid 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-...

  • by samuelfekete on 4/23/25, 7:29 AM

  • by bandrami on 4/23/25, 8:49 AM

    What if we put the airplane on a treadmill though?

  • by financetechbro on 4/23/25, 3:13 PM

    Truth is we don’t fully understand how lift happens

  • by artemonster on 4/23/25, 7:27 AM

    So what is correct explanation? Is there a TLDR?

  • by lordnacho on 4/23/25, 7:33 AM

    Ok 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.