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

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

#41

Post by agos »

jstanley wrote: post #29
smallbugfound wrote: post #2

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…

> 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.

air being incompressible makes even less sense, given how daily the experience of compressing air is

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

#42

Post by federiconafria »

HPsquared wrote: post #35
jgord wrote: post #9

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

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 ve…

That becomes very evident once you consider how slow a bike can move and still be very stable.

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

#43

Post by tlb »

nandomrumber wrote: post #28
topspin wrote: post #6

Earlier quoted context omitted.

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 wi…

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.

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

#44

Post by blueflow »

dr_dshiv wrote: post #5

“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?

"/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.

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

#45

Post by nandomrumber »

tlb wrote: post #43
nandomrumber wrote: post #28

Earlier quoted context omitted.

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 wi…

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.

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.

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

#46

Post by bandrami »

bobbyraduloff wrote: post #39

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

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

#47

Post by bandrami »

6figurelenins wrote: post #33
artemonster wrote: post #14

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 li…

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.

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

#48

Post by bandrami »

FabHK wrote: post #30
artemonster wrote: post #14

So what is correct explanation? Is there a TLDR?

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.

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

#50

Post by Avshalom »

tijsvd wrote: post #27
artemonster wrote: post #14

So what is correct explanation? Is there a TLDR?

TLDR 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)

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.
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