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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?

#31

Post by marcus_holmes »

kuhewa wrote: post #25
marcus_holmes wrote: post #3

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 -…

> 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 wi…

yeah, sorry, I should have specified modern racing sailing vessels

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

#32

Post by IanCal »

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…

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.

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

#33

Post by 6figurelenins »

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 lift:

https://aviation.stackexchange.com/a/35139

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

#34

Post by 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.

[1]: https://youtu.be/ZUBwc67c5_Y

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

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

#35

Post by HPsquared »

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 very fast, i.e. motorbike speeds and motorbike weight wheels/tyres.

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

#36

Post by throwaway290 »

marcus_holmes wrote: post #3

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 -…

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...

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

#37

Post by nandomrumber »

tromp wrote: post #26
artemonster wrote: post #14

So what is correct explanation? Is there a TLDR?

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 differe…

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?

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

#38

Post by zihotki »

zokier wrote: post #24
IshKebab wrote: post #16

Earlier quoted context omitted.

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.

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

#39

Post by 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.

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

#40

Post by zihotki »

nonrandomstring wrote: post #8

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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