Where did the false "equal transit-time" explanation of lift originate from?
59 comments
·April 23, 2025jgord
HPsquared
Bicycles 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...
Gyroscopic effect is very minor unless you're going very fast, i.e. motorbike speeds and motorbike weight wheels/tyres.
federiconafria
That becomes very evident once you consider how slow a bike can move and still be very stable.
smallbugfound
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.
dguest
At 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.
namaria
I 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"
IanCal
I'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.
jstanley
> 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.
FabHK
Not at all.
Imagine two roads, parallel mostly, but then one takes a detour, like this:
____
____/ __ \____
-> _____/ \______
_______________
-> _______________
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?
ejolto
But 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
agos
air being incompressible makes even less sense, given how daily the experience of compressing air is
dguest
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.
jabl
Newton'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).
bobbyraduloff
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.
bandrami
NASA's (correct) position is that the dynamics of heavier than air flight are complicated and do not admit of a simple heuristic explanation
metalman
funny, 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
dr_dshiv
“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
Simplified explanations of how electrical current flows in conductors comes to mind. Water analogies, for example.
nandomrumber
Problem is electricity is kinda three different things, as far as models go.
1) DC; 2) low frequency AC; 3) Radio frequency circuits
With DC, the water in pipes analogy is close enough to be useful.
With low frequency AC, like mains electricity at around household voltages, there are additional considerations to make the water in pipes analogy to be useless enough we generally require people to be licensed to work with it.
Radio frequency electronics is indistinguishable from magic.
tlb
The pipes analogy gives people a wildly unrealistic idea of the speed of electron movement in a wire. They only move a few mm/s at max current in copper wire. Electrons take minutes to go down the power cable from your PSU to your GPU. At GHz frequencies, they move less than an atom diameter.
IshKebab
The 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
I 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.
exitb
There's a discussion on why ice is slippery that has similar vibes to the origin of lift.
blueflow
"/usr stands for UNIX System Resources"
And less technical:
"The rainbow has 7 colors"
"Sun is the day, moon is the night"
So many things, i wish i had a name for these kind of traditional beliefs.
Kim_Bruning
https://en.wikipedia.org/wiki/Lie-to-children
short version:
https://en.wikipedia.org/w/index.php?title=Lie-to-children&o...
dr_dshiv
Love this Sussman quote: “ In fact, this is an engineering model, in the same way that, [for an] electrical resistor, we write down a model V = IR—it's approximately true, but it's not really true; if I put enough current through the resistor, it goes boom, so the voltage is not always proportional to the current, but for some purposes the model is appropriate”
jobigoud
They are called "Common misconceptions" Wikipedia has a full page of them: https://en.wikipedia.org/wiki/List_of_common_misconceptions
magicalhippo
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.
marcus_holmes
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.
crooked-v
Sailboats 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.
jabl
For the physics of sailing see http://gentrysailing.com/pdf-theory/A-Review-of-Modern-Sail-...
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kuhewa
> 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
yeah, sorry, I should have specified modern racing sailing vessels
throwaway290
Is it debunked or just renamed from "pull" to "lift"? Lift much more confusing when you think of a boat instead of a plane so "pull mode" probably works better in practice. Since no one has a full understanding of lift you're probably fine...
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samuelfekete
It’s still the explanation on NASA.gov
https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso...
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porterde
NASA.gov has multiple pages on lift. Maybe one will explain it accurately.
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/lif...
seanhunter
That 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.
bandrami
What if we put the airplane on a treadmill though?
bowsamic
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
nonrandomstring
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-...
zihotki
On 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.
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