Comment by slavak | Hacker News Reader

Comment by slavak | Hacker News Reader

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The experts weren't _wrong_ in their understanding. Bernoulli (creating lift by way of pressure differential) and Newton (reaction to redirection of the flow downwards) are different ways of describing the same thing; integrating either the pressure or velocity vector of the airflow around the wing will give you the correct results for lift.[1]

Whenever people argue about which interpretation of lift is correct I think back to this (https://xkcd.com/895/) comic about teaching how gravity works in general relativity. Only in the case of lift the explanations are actually _correct_, albeit somewhat circular. ("So the air above the wing sticks to the surface, which redirect it downwards. But _why_ does the air stick to the wing?!")

Also in no way do hummingbirds violate any known laws of physics, although they do have a pretty impressive way of harnessing them.[2]

[1] https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

[2] https://phys.org/news/2005-06-hummingbird-flight-evolutionar...

I commented elsewhere that there is no physical reason why the Bernoulli effect would cause the upper streamline and the lower streamline to reach the back of the wing at the same time - and to my knowledge there is no experimental evidence that it does. I may be wrong about that but I have never seen an adequate rebuttal.

You're right- there isn't, and it doesn't. In fact a parcel of air moving over the top of the wing will beat its counterpart moving below it to the trailing edge. This[1] video has a very good demonstration of this. (Relevant part begins around 0:25.)

So saying that air on top of the wing moves faster, creating a pressure differential and thus lift, is absolutely correct. The problem begins when some people try to come up with an intuitive explanation for _why_ the air would need to speed up. "Because this is the lowest energy state that conserves energy, momentum, and mass" isn't a very satisfying answer; neither is "because this system of PDEs say so"; so they came up with the "equal transit time" explanation, which is simple, intuitive, and completely wrong.

Hopefully aeronautical engineers at P&W didn't actually believe that last bit?

[1] https://www.youtube.com/watch?v=UqBmdZ-BNig

The streamlines don't need to meet for Bernoulli to apply.

That statement, that the upper airstream flows faster because it has to meet up with the lower airstream, is wrong and easy to disprove experimentally.

So, putting aside that specific statement, re-read the post you're replying to.

The airstream above the aerofoil does travel faster, and there is a lower pressure region there (commensurate with the effect described by Bernoulli), and it turns out that this is a valid way to model the forces involved just as it is valid to model them as a redirection of the airstream.

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