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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?

#61

Post by fanf2 »

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…

The NASA page does not make the false equal time assumption.

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

#62

Post by blueflow »

dr_dshiv wrote: post #60
Kim_Bruning wrote: post #53

Earlier quoted context omitted.

https://en.wikipedia.org/wiki/Lie-to-children short version: https://en.wikipedia.org/w/index.php?title=Lie-to-children&o...

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.

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

#64

Post by thatcat »

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…

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.

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

#65

Post by IshKebab »

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.

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.

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

#66

Post by karmakaze »

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

The interrupted smoke is a great visual! I'd like to see that with a zero angle of attack.

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

#67

Post by HPsquared »

thatcat wrote: post #64
HPsquared wrote: post #35

Earlier quoted context omitted.

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…

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.

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

#68

Post by bsder »

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…

> 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.

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

#69

Post by jstanley »

FabHK wrote: post #52
jstanley wrote: post #29

Earlier quoted context omitted.

> 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 them full of cars, bumper to bumper. 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…

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.

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

#70

Post by thatcat »

HPsquared wrote: post #67
thatcat wrote: post #64

Earlier quoted context omitted.

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 force is proportional to speed, you can countersteer a bicycle just the same as a motorcycle. There are 2 separate steering techniques, one for high speed (countersteering) and one for low speed (steering) on motorcycles and bicycles - that results from gyroscopic wheel force. The geometry doesn't change, only thing that changes as speed increases on the bike is the gyro force.
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