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Yes, it is completely wrong. If this were a valid explanation, flat-plate airfoils could not generate lift. (They can.) Source: PhD on aircraft design
by peterdsharpe 1y ago
Yes, it is completely wrong. If this were a valid explanation, flat-plate airfoils could not generate lift. (They can.)
Source: PhD on aircraft design
- ge96 1y agoWhat is the actual answer? I know the "skipping stone" idea is wrong too, thinking it's just angle of attack
- base698 1y agoWeight of the air deflecting downward. Plain ole Newtonian equal and opposite reaction.
- datadrivenangel 1y agoBut also pressure providing force. It's complicated.
- rtkwe 1y agoIt's both lower pressure above the wing (~20% of lift) and the reaction force from pushing air down (give or take the remaining 80% of lift). The main wrong thing is that the air travels faster because it has to travel farther causing the air to accelerate causing the lower pressure that's double plus wrong. It's a weird old misunderstanding that gets repeated over and over because it's a neat connection to attach to the Bernoulli Principal when it's being explained to children.
- boombapoom 1y agoa classic example of how LLM's mislead people. They don't know right from wrong, they know what they have been trained on. Even with reasoning capabilities
- rtkwe 1y agoThat's one of my biggest hang ups on the LLMs to AGI hype pipeline, no matter how much training and tweaking we throw at them they still don't seem to be able to not fall back to repeating common misconceptions found in their training data. If they're supposed to be PhD level collaborators I would expect better from them. Not to say they can't be useful tools but they fall into the same basic traps and issues despite our continues attempts to improve them.
- casualscience 1y agoHow can you create a pocket of 'lower pressure' without deflecting some of the air away? At the end of the day, if the aircraft is moving up, it needs to be throwing something down to counteract gravity.
- ummonk 1y agoExactly. The speed phenomenon (airflow speeding up due to getting sucked into the lower pressure space above the wing) is certainly there, but it's happening because the wing is shaped to deflect air downwards.
- rtkwe 1y agoThe point isn't about how the low pressure is created just that the low pressure is a separate source of lift from the air being pushed down by the bottom of the wing.
- ummonk 1y agoNo, what still matters (when explaining why the wing is shaped the way it is) is how the low pressure is created. In this case it's being pulled down by the top of the wing.
- bilsbie 1y agoAngle of attack is a big part but I think the other thing going on is air “sticks” to the surface of the top of the wing and gets directed downward as it comes off the wing. It also creates a gap as the wing curves down leaving behind lower pressure from that.
- qq66 1y agoAir pushes on the wing. The control surfaces determine in which direction.
- dist-epoch 1y agoTL;DR; - it's complicated https://www.youtube.com/watch?v=CT5oMBN5W5M https://www.youtube.com/watch?v=CT5oMBN5W5M
- IshKebab 1y agoIt's really not. The wing is angled so it pushes the air down. Pushing air down means you are pushing the plane up. A wing can literally be a flat sheet at an angle and it would still fly. It gets complex if you want to fully model things and make it fly as efficiently as possible, but that isn't really in the scope of the question. Planes go up because they push air down. Simple as that.
- rcxdude 1y agoIt's both that simple and not. Because it's also true that the wing's shape creates a pressure differential and that's what produce lift. And the pressure differential causes the momentum transfer to the wing, the opposing force to the wing's lift creates the momentum transfer, and pressure difference also causes the change in speed and vice-versa. You can create many correct (and many more incorrect) straightforward stories about the path to lift but in reality cause and effect are not so straightforward and I think it's misleading to go "well this story is the one true simple story".
- casualscience 1y agoHow can you create a 'pressure differential' without deflecting some of the air away? At the end of the day, if the aircraft is moving up, it needs to be throwing something down to counteract gravity. If there is some pressure differential that you can observe, that's nice, but you can't get away from momentum conservation.
- Tadpole9181 1y agoThe pressure differential is created by the leading edge creating a narrow flow region, which opens to a wider flow region at the trailing edge. This pulls the air at the leading edge across the top of the wing, making it much faster than the air below the wing. This, in turn, creates a low pressure zone. Air molecules travel in all directions, not just down, so with a pressure differential that means the air molecules below the wing are applying a significant force upward, no longer balanced by the equal pressure usually on the top of the wing. Thus, lift through boyancy. Your question is now about the same as "why does wood float in water"? The "throwing something down" here comes from the air molecules below the wing hitting the wing upward, then bouncing down. All the energy to do this comes from the plane's forward momentum, consumed by drag and transformed by the complex fluid dynamics of the air. Any non-zero angle of attack also pushes air down, of course. And the shape of the wing with the "stickiness" of the air means some more air can be thrown down by the shape of the wing's top edge.
- deleted 1y ago[deleted]
- gilbetron 1y agoHere's a relatively quick read (press the "next" buttons under the "Guided Tour" section to get to the other 3 slides) from NASA. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html 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.
- timr 1y ago> 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. Exactly. The comments in this subthread are turning imprecision in language into all-or-nothing judgments of correctness. (Meanwhile, 80% of the comments advance their own incorrect/imprecise explanations of the same thing...)
- nilsherzig 1y agoLooks like OpenAI delivered on the PhD response
- antisthenes 1y agoGPT-6 will just go on forums and pretend to be a girl that needs help with homework.
- nxobject 1y agoBy "PhD", do they mean "overconfident first-year grad student"?
- deleted 1y ago[deleted]
- Tadpole9181 1y agoSorry, I know nothing about this topic, but this is how it was explained to me every time it's come up throughout my life. Could you explain a bit more? 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.
- deleted 1y ago[deleted]
- andoando 1y agoIm quite sure the "air on the top has to travel faster to meet the air at the bottom " is false. Why would they have to meet at the same time? What would cause air on the top to accelerate?
- FeepingCreature 1y ago(Layman guess) Pressure? The incoming split air has to go somewhere. The volume of air inflowing above and below is roughly the same.
- Tadpole9181 1y agoI did a little more research and explain it above. The fundamentals are actually right. The leading edge pressurizes the air by forcing air up, then the trailing edge opens back up, creating a low pressure zone that sucks air in the leading edge back. As a whole, the air atop the wing accelerates to be much faster than the air below, creating a pressure differential above and below the wing and causing lift. The AI is still wrong on the actual mechanics at play, of course, but I don't see how this is significantly worse than the way we simplify electricity to lay people. The core "air moving faster on the top makes low pressure" is right.
- deleted 1y ago[deleted]
- zombiwoof 1y agoBut we live in the world of Trump where facts don’t matter. If GPt 5 says this is how it works, that’s how it works and Fox News will back it up
- WithinReason 1y agoAnd flying upside down would be impossible
- Tadpole9181 1y agoCambered wings produce negative lift upside down, which is compensated by increasing the angle of attack. Lift comes from multiple sources.
- timr 1y agoExcept it isn't "completely wrong". The article the OP links to says it explicitly: > “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.
- ttoinou 1y agoI would say a wing with two sides of different length is more difficult to understand than one shape with two sides of opposites curvatures but same length
- carabiner 1y agoIt'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 1y agoAgain, 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.)
- deleted 1y ago[deleted]
- ActionHank 1y agoThis sort of tracks for my experience with LLMs. They spout common knowledge on a broad array of subjects and it's usually incorrect to anyone who has some knowledge on the subject.
- synapsomorphy 1y agoIt appears to me like the linked explanation is also subtly wrong, in a different way: “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.
- kqr 1y agoYes, it is wrong. The curvature of the sail lowers the leading angle of attack which promotes attachment, i.e. reduces the risk of stalling at high angles of attack, but it is not responsible for lift in the sense you mean. It could be argued that preventing a stall makes it responsible for lift in an AoA regime where the wing would otherwise be stalled -- hence "responsible for lift" -- but that would be far fetched. More likely the author wanted to give an intuition for the cuvature of the airflow. This is produced not by the shape of the airfoil but the induced circulation around the airfoil, which makes air travel faster on the side of the far surface of an airfoil, creating the pressure differential.