3 ms·

What is the actual answer? I know the "skipping stone" idea is wrong too, thinking it's just angle of attack

by ge96 1y ago

base698 1y ago
Weight of the air deflecting downward. Plain ole Newtonian equal and opposite reaction.
- datadrivenangel 1y ago
  But also pressure providing force. It's complicated.
- rtkwe 1y ago
  It'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 ago
    a 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 ago
      That'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 ago
    How 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 ago
      Exactly. 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 ago
      The 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 ago
        No, 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 ago
Angle 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 ago
Air pushes on the wing. The control surfaces determine in which direction.
dist-epoch 1y ago
TL;DR; - it's complicated https://www.youtube.com/watch?v=CT5oMBN5W5M https://www.youtube.com/watch?v=CT5oMBN5W5M
- IshKebab 1y ago
  It'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 ago
    It'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 ago
      How 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 ago
        The 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 ago
Here'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...)

What is the actual answer? I know the "skipping stone" idea is wrong too, thinking it's just angle of attack

by ge96 1y ago

What is the actual answer? I know the "skipping stone" idea is wrong too, thinking it's just angle of attack

base698 1y ago

Weight of the air deflecting downward. Plain ole Newtonian equal and opposite reaction.

datadrivenangel 1y ago
But also pressure providing force. It's complicated.
rtkwe 1y ago
It'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 ago
  a 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 ago
    That'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 ago
  How 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 ago
    Exactly. 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 ago
    The 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 ago
      No, 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 ago

Angle 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 ago

Air pushes on the wing. The control surfaces determine in which direction.

dist-epoch 1y ago

TL;DR; - it's complicated https://www.youtube.com/watch?v=CT5oMBN5W5M https://www.youtube.com/watch?v=CT5oMBN5W5M

IshKebab 1y ago
It'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 ago
  It'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 ago
    How 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 ago
      The 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 ago

Here'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...)