Where did the false "equal transit-time" explanation of lift originate from?(hsm.stackexchange.com) |
Where did the false "equal transit-time" explanation of lift originate from?(hsm.stackexchange.com) |
"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.
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.
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.
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"
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.
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?
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.
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.
Love it! Anyone have any others?
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 basic equations are actually identical for a lot of domains, including rotational motion which gave rise to the Spintronics game.
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.
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.
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.
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
[0] https://boards.straightdope.com/t/did-heisenberg-really-say-...
https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso...
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/lif...
- 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.
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:
> 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
Aren’t they saying exactly the same thing?
And isn’t that Bernoulli’s Principle?
All of the lift can be explained by Newton's laws, but explaining why the air moves down in the first place can't be explained by Newton's laws.
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.
> 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.