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Where did the false "equal transit-time" explanation of lift originate from?
by IdealeZahlen
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.
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.
> 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.
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"
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.
> 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.
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?
The 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.
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
air being incompressible makes even less sense, given how daily the experience of compressing air is
Handy video showing relative speed above and below the curved wing surface, and non-equal times :
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.
That becomes very evident once you consider how slow a bike can move and still be very stable.
You 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.
It'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.
The interrupted smoke is a great visual! I'd like to see that with a zero angle of attack.
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.
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).
“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?
Simplified explanations of how electrical current flows in conductors comes to mind. Water analogies, for example.
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.
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.
How come? It seems it is very analogous to water in a pipe. Let's say I have a 30 m long garden hose. The water in it moves (in the order of magnitude of) 10 m/s in it. If the water is turned off, and I hold my thumb at the end of the hose and turn the tap on, I feel the water pressure almost immediately (the speed of mechanical waves in water). So it is obvious that there are two speeds. One is the speed of propagation and one is the speed of the medium, and they are way different.
All models are wrong, some are useful.
If you go on to study electronics / electricity beyond junior schooling level, you’re going to be exposed to progressively less wrong models.
Or watch three YouTube videos, there was some action from the big guns a couple years ago.
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.
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.
It'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.
You 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.
There's a discussion on why ice is slippery that has similar vibes to the origin of lift.
"/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.
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”
This is only sensible for models that are useful for something.
They are called "Common misconceptions" Wikipedia has a full page of them: https://en.wikipedia.org/wiki/List_of_common_misconceptions
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
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.
NASA's (correct) position is that the dynamics of heavier than air flight are complicated and do not admit of a simple heuristic explanation
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
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.
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.
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.
For the physics of sailing see http://gentrysailing.com/pdf-theory/A-Review-of-Modern-Sail-...
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> 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
yeah, sorry, I should have specified modern racing sailing vessels
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...
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-...
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.
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It’s still the explanation on NASA.gov
https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso...
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.
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NASA.gov has multiple pages on lift. Maybe one will explain it accurately.
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/lif...
What if we put the airplane on a treadmill though?
Truth is we don’t fully understand how lift happens
So what is correct explanation? Is there a TLDR?
Layman's guess: wings push air down, air pushes wings up.
An everyday experiment is sticking your flat hand out the window of a moving car. With slight wrist rotations, you'll find even slight deviations from neutral (parallel to the ground) cause your "wing" to rise or fall, with a force that seems proportional to the angle.
We can hypothesize that a symmetric wing, with zero angle of attack, should experience no lift:
That's part of it. But an asymmetric airfoil will in fact generate lift with a 0° angle of attack, so it's not all Newton either.
I 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/dynamicso...
[2] https://www.cam.ac.uk/research/news/how-wings-really-work
The NASA page does not make the false equal time assumption.
I 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?
The very short answer is that the wing pushes the air down, thus (by Newton's 3rd law) the air pushes the wing up.
That's incomplete though; an asymmetric airfoil with 0° angle of attack does generate lift. The problem is the "equal times" -- the times are not "equal", but the air passing over the curved side does go faster.
Sure. By deflecting the air down.
> Cambered airfoils generate lift at zero angle of attack. When the chord line is horizontal, the trailing edge has a downward direction and since the air follows the trailing edge it is deflected downward.
But the air on the leading edge is deflected upwards when it hits
TLDR is that the full correct explanation is not simple. The Wikipedia article on lift makes a good effort.
Oh god, I love the diagram https://en.m.wikipedia.org/wiki/Lift_(force)#Explanations_ba... just desperately trying to get it's point across.
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.
Crafted by Rajat
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