> For an airplane wing (airfoil), the top surface is curved and the bottom is flatter. When the wing moves forward:
> * Air over the top has to travel farther in the same amount of time -> it moves faster -> pressure on the top decreases.
> * Air underneath moves slower -> pressure underneath is higher
> * The presure difference creates an upward force - lift
Isn't that explanation of why wings work completely wrong? There's nothing that forces the air to cover the top distance in the same time that it covers the bottom distance, and in fact it doesn't. https://www.cam.ac.uk/research/news/how-wings-really-work
Very strange to use a mistake as your first demo, especially while talking about how it's phd level.
Source: PhD on aircraft design
“This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”
But like you say flat plates can generate lift at positive AoA, no curvature (camber) required. Can you confirm this is correct? Kinda going crazy because I'd very much expect a Cambridge aerodynamicist to get this 100% right.
I've always been under the impression that flat-plate airfoils can't generate lift without a positive angle-of-attack - where lift is generated through the separate mechanism of the air pushing against an angled plane? But a modern airfoil can, because of this effect.
And that if you flip them upside down, a flat plate is more efficient and requires less angle-of-attack than the standard airfoil shape because now the lift advantage is working to generate a downforce.
I just tried to search Google, but I'm finding all sorts of conflicting answers, with only a vague consensus that the AI-provided answer above is, in fact, correct. The shape of the wing causes pressure differences that generate lift in conjunction with multiple other effects that also generate lift by pushing or redirecting air downward.
(Also, once you've got the 'moving faster' you can then tell a mostly correct story through bernuolli's principle to get to lower pressure on the top and thus lift, but you're also going to confuse people if you say this is the one true story and any other explaination, like one that talks about momentum, or e.g. the curvature of the airflow causing the pressure gradient instead is wrong, because these are all simply multiple paths through the same underlying set of interactions which are not so easy to fundamentally seperate into cause and effect. But 'equal transit time' appears in none of the correct paths as an axiom, nor a necessary result, and there's basically no reason to use it in an explanation, because there's simpler correct stories if you want to dumb it down for people)
There is no requirement for air to travel any where. Let alone in any amount of time. So this part of the AI's response is completely wrong. "Same amount of time" as what? Air going underneath the wing? With an angle of attack the air under the wing is being deflected down, not magically meeting up with the air above the wing.
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
The "wrong" answers all have a bit of truth to them, but aren't the whole picture. As with many complex mathematical models, it is difficult to convert the math into English and maintain precisely the correct meaning.
They spout common knowledge on a broad array of subjects and it's usually incorrect to anyone who has some knowledge on the subject.
> “What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” clarified Babinsky, from the Department of Engineering. “This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”
The meta-point that "it's the curvature that creates the lift, not the distance" is incredibly subtle for a lay audience. So it may be completely wrong for you, but not for 99.9% of the population. The pressure differential is important, and the curvature does create lift, although not via speed differential.
I am far from an AI hypebeast, but this subthread feels like people reaching for a criticism.
That doesn't matter for lay audieces and doesn't really matter at all until we try and use them for technical things.
If I lay out a chain of thought like
Top and bottom are different -> god doesnt like things being diffferent and applies pressure to the bottom of the wing -> pressure underneath is higher than the top -> pressure difference creates lift
The video in the Cambridge link shows how the upper surface particles greatly overtake the lower surface flow. They do not rejoin, ever.
People seem to overcomplicate what LLM's are capable of, but at their core they are just really good word parsers.
Most of the phd’s I know are studying things that I guarantee GPT-5 doesn’t know about… because they’re researching novel stuff.
Also, LLMs don’t have much consistency with how well they’re able to apply the knowledge that they supposedly have. Hence the “lots of almost correct code” stereotype that’s been going around.
I was using the fancy new Claude model yesterday to debug some fast-check tests (quickcheck-inspired typescript lib). Claude could absolutely not wrap its head around the shrinking behavior, which rendered it useless for debugging
It’s very common to see AI evangelists taking its output at face value, particularly when it’s about something that they are not an expert in. I thought we’d start seeing less of this as people get burned by it, but it seems that we’re actually just seeing more of it as LLMs get better at sounding correct. Their ability to sound correct continues to increase faster than their ability to be correct.
Meanwhile the demo seems to suggest business as usual for AI hallucinations and deceptions.
This is the problem with AI in general.
When I ask it about things I already understand, it’s clearly wrong quite often.
When I ask it about something I don’t understand, I have no way to know if its response is right or wrong.
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
In fact I'd classify it as downright strange.
An LLM doesn't know more than what's in the training data.
In Michael Crichton's The Great Train Robbery (published in 1975, about events that happened in 1855) the perpetrator, having been caught, explains to a baffled court that he was able to walk on top of a running train "because of the Bernoulli effect", that he misspells and completely misunderstands. I don't remember if this argument helps him get away with the crime? Maybe it does, I'm not sure.
This is another attempt at a Great Robbery.
It goes on:
> At this point, the prosecutor asked for further elucidation, which Pierce gave in garbled form. The summary of this portion of the trial, as reported in the Times, was garbled still further. The general idea was that Pierce--- by now almost revered in the press as a master criminal--- possessed some knowledge of a scientific principle that had aided him.
How apropos to modern science reporting and LLMs.
Post-training for an LLM isn't "data" anymore, it's also verifier programs, so it can in fact be more correct than the data. As long as search finds LLM weights that produce more verifiably correct answers.
And I might be wrong but my understanding is that it's not wrong per-se, it's just wildly incomplete. Which, is kind of like the same as wrong. But I believe the airfoil design does indeed have the effect described which does contribute to lift somewhat right? Or am I just a victim of the misconception.
This is an LLM. "Wrong" is not a concept that applies, as it requires understanding. The explanation is quite /probable/, as evidenced by the fact that they thought to use it as an example…
I asked ChatGPT for help with Wordle the other day, by asking for a 5-letter word that contained P, M, K and Y. It said:
> Yes, the word skimp contains the letters P, M, K, and Y
Would you say that wrong is not a concept that applies to this answer?
A quite good example of AI limits
This seems like a reasonable standard to hold GPT-5 to given the way it’s being marketed. Nobody would care if OpenAI compared it to an enthusiastic high school student with a few hours to poke around Google and come up with an answer.
These are places where common lay discussions use language in ways that is wrong, or makes simplifcations that are reasonable but technically incorrect. They are especially common when something is so 'obvious' that experts don't explain it, the most frequent version of the concepts being explained
These, in my testing, show up a lot in LLMs - technical things are wrong when the most language of the most common explanations simplifies or obfuscates the precise truth. Often, it pretty much matches the level of knowledge of a college freshman/sophmore or slightly below, which is sort of the level of discussion of more technical topics on the internet.
>In fact, theory predicts – and experiments confirm – that the air traverses the top surface of a body experiencing lift in a shorter time than it traverses the bottom surface; the explanation based on equal transit time is false.
So the effect is greater than equal time transit.
I've seen the GPT5 explanation in GCSE level textbooks but I thought it was supposed to be PhD level;)
Common misconceptions should be expected when you train a model to act like the average of all humans.
https://jimruttshow.blubrry.net/the-jim-rutt-show-transcript...
> “What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” clarified Babinsky, from the Department of Engineering. “This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”
So I'd characterize this answer as "correct, but incomplete" or "correct, but simplified". It's a case where a PhD in fluid dynamics might state the explanation one way to an expert audience, but another way to a room full of children.
The hilarious thing about this subthread is that it's already getting filled with hyper-technical but wrong alternative explanations by people eager to show that they know more than the robot.