[carabiner]

It's the "same amount of time" part that is blatantly wrong. Yes geometry has an effect but there is zero reason to believe leading edge particles, at the same time point, must rejoin at the trailing edge of a wing. This is a misconception at the level of "heavier objects fall faster." It is non-physical.

The video in the Cambridge link shows how the upper surface particles greatly overtake the lower surface flow. They do not rejoin, ever.

[timr]

Again, you're not wrong, it's just irrelevant for most audiences. The very fact that you have to say this:

> Yes geometry has an effect but there is zero reason to believe leading edge particles, at the same time point, must rejoin at the trailing edge of a wing.

...implicitly concedes that point that this is subtle. If you gave this answer in a PhD qualification exam in Physics, then sure, I think it's fair for someone to say you're wrong. If you gave the answer on a marketing page for a general-purpose chatbot? Meh.

(As an aside, this conversation is interesting to me primarily because it's a perfect example of how scientists go wrong in presenting their work to the world...meeting up with AI criticism on the other side.)

[carabiner]

Saw you were a biologist. Would you be ok if I said, "Creationism got life started, but after that, we evolved via random mutations..."? The "equal transit time" is the same as a supernatural force compelling the physical world act in a certain way. It does not exist.

[timr]

I am a biologist (biochemistry, but close enough). I don’t have a problem with what you wrote.

It’s not the same thing at all, though. We don’t know what “got life started”, and that’s the realm of faith.

This is more like saying that “evolution is due to random mutation”, which is technically wrong, but close enough to get the point across.

[adgjlsfhk1]

right, the other is that if you remove every incorrect statement from the AI "explanation", the answer it would have given is "airplane wings generate lift because they are shaped to generate lift".

[timr]

> right, the other is that if you remove every incorrect statement from the AI "explanation", the answer it would have given is "airplane wings generate lift because they are shaped to generate lift".

...only if you omit the parts where it talks about pressure differentials, caused by airspeed differences, create lift?

Both of these points are true. You have to be motivated to ignore them.

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

[tatjam]

But using pressure differentials is also sort of tautological. Lift IS the integral of the pressure on the surface, so saying that the pressure differentials cause lift is... true but unsatisfying. It's what makes the pressure difference appear that's truly interesting.

Funnily enough, as an undergraduate the first explanation for lift that you will receive uses Feynman's "dry water" (the Kutta condition for inviscid fluids). In my opinion, this explanation is also unsatisfying, as it's usually presented as a mere mathematical "convenience" imposed upon the flow to make it behave like real physics.

Some recent papers [1] are shedding light on generalizing the Kutta condition on non-sharp airfoils. In my opinion, the linked papers gives a way more mathematically and intuitively satisfying answer, but of course it requires some previous knowledge, and would be totally inappropriate as an answer by the AI.

Either way I feel that if the AI is a "pocket PhD" (or "pocket industry expert") it should at least give some pointers to the user on what to read next, using both classical and modern findings.

[1]: https://www.researchgate.net/publication/376503311_A_minimiz...

[ummonk]

The Kutta condition is insufficient to describe lift in all regimes (e.g. when the trailing edge of the wing isn't that sharp), but fundamentally you do need to fall back to certain 2nd law / boundary condition rules to describe why an airfoil generates lift, as well as when it doesn't (e.g. stall).

There's nothing in the Navier-Stokes equations that forces an airfoil to generate lift - without boundary conditions the flowing air could theoretically wrap back around at the trailing edge, thus resulting in zero lift.

[timr]

The fact that you have to invoke integrals and the Kutta condition to make your explanation is exactly what is wrong with it.

Is it correct? Yes. Is it intuitive to someone who doesn’t have a background in calculus, physics and fluid dynamics? No.

People here are arguing about a subpoint on a subpoint that would maybe get you a deduction on a first-year physics exam, and acting as if this completely invalidates the response.

[ummonk]

How is the Kutta condition ("the fluid gets deflected downwards because the back of the wing is sharp and pointing downwards") less intuitive to someone without a physics background than wrongly invoking the Bernoulli principle?