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I’ve never understood how this battle could possibly persist, considering that planes can fly upside-down. The fact that Bernoulli’s Principle creates a low-pre

I’ve never understood how this battle could possibly persist, considering that planes can fly upside-down. The fact that Bernoulli’s Principle creates a low-pressure zone on the top of the aerofoil just gives you laminar flow at higher angles of attack. It really isn’t that mysterious, unless you insist that only a single physical law is allowed to come into play.

mlyle 3y ago
> I’ve never understood how this battle could possibly persist, considering that planes can fly upside-down. That's orthogonal. Planes flying upside down still have air flowing faster over the new "top" of the wing than the bottom. Though, a common misunderstanding is that air must be taking the same "time" to go over the top and the bottom. This is not true. You can build a pretty good mental model of flight with either simplified mechanism, but you end up underestimating performance by a fair bit.
- blagie 3y ago
  > You can build a pretty good mental model of flight with either simplified mechanism, but you end up underestimating performance by a fair bit. I've never hear of a good mental model built on Bernoulli's Principle. They are either: - Wrong - Give no intuition The key question is /why/ air moves faster on one side, and once you get away from the misconception of same time, I've never heard an intuitive argument. Redirecting air down: - makes complete sense - can be demonstrated by sticking your hand out the window of a car - works with flat airfoils (sails, kites, etc.), and explains those well too And the shape of the wing comes in from wanting the leading edge parallel to incoming airflow, and the other edge parallel to outgoing airflow. On a sail, I can adjust the shape. On a steel wing, I can't much, so I make a shape which works across different angles of attack. All of the other mechanisms, you can gradually work in from there to get accurate models. Toss in air moving to the low-pressure area, and having momentum, and you get why helicopters are less efficient than planes, as well as vortex shedding. It all builds up. As a corollary, Bernoulli tells you where air moves faster, but that follows from lift. Not the other way around. At least in any model which fits the human brain.
  - ddalex 3y ago
    Why are you benchmarking models to human intuition? If you apply the same reasoning to quantum mechanics you'll have a bad time. I'm sure models can accurately compute how much of the wing lift comes from the suction effect on the top of the wing versus the push on the bottom of the wing.
    - blagie 3y ago
      Because it's literally what I'm responding to: "You can build a pretty good mental model of flight with either simplified mechanism" For computational models, neither of these are great starting points. When I last looked, finite element methods were where the action was. And if you don't apply the same to quantum -- looking for intuition -- you'll have a bad time. Quantum computing algorithms came from human insight.
  - Linosaurus 3y ago
    > The key question is /why/ air moves faster on one side, and once you get away from the misconception of same time, I've never heard an intuitive argument. Since the air follows a curved surface instead of straight ahead, you could perhaps apply your intuition for centrifugal force. Momentum sort of pulls the air away from the surface. I’m not even sure this explanation is wrong.
    - blagie 3y ago
      That's interesting. Counterpoint: A wing with the opposite curvature would work too, just much less efficiently. Your explanation has an element of truth, but an element of untruth. It'd take more analysis to tease them apart.

mlyle 3y ago

> I’ve never understood how this battle could possibly persist, considering that planes can fly upside-down. That's orthogonal. Planes flying upside down still have air flowing faster over the new "top" of the wing than the bottom. Though, a common misunderstanding is that air must be taking the same "time" to go over the top and the bottom. This is not true. You can build a pretty good mental model of flight with either simplified mechanism, but you end up underestimating performance by a fair bit.

blagie 3y ago
> You can build a pretty good mental model of flight with either simplified mechanism, but you end up underestimating performance by a fair bit. I've never hear of a good mental model built on Bernoulli's Principle. They are either: - Wrong - Give no intuition The key question is /why/ air moves faster on one side, and once you get away from the misconception of same time, I've never heard an intuitive argument. Redirecting air down: - makes complete sense - can be demonstrated by sticking your hand out the window of a car - works with flat airfoils (sails, kites, etc.), and explains those well too And the shape of the wing comes in from wanting the leading edge parallel to incoming airflow, and the other edge parallel to outgoing airflow. On a sail, I can adjust the shape. On a steel wing, I can't much, so I make a shape which works across different angles of attack. All of the other mechanisms, you can gradually work in from there to get accurate models. Toss in air moving to the low-pressure area, and having momentum, and you get why helicopters are less efficient than planes, as well as vortex shedding. It all builds up. As a corollary, Bernoulli tells you where air moves faster, but that follows from lift. Not the other way around. At least in any model which fits the human brain.
- ddalex 3y ago
  Why are you benchmarking models to human intuition? If you apply the same reasoning to quantum mechanics you'll have a bad time. I'm sure models can accurately compute how much of the wing lift comes from the suction effect on the top of the wing versus the push on the bottom of the wing.
  - blagie 3y ago
    Because it's literally what I'm responding to: "You can build a pretty good mental model of flight with either simplified mechanism" For computational models, neither of these are great starting points. When I last looked, finite element methods were where the action was. And if you don't apply the same to quantum -- looking for intuition -- you'll have a bad time. Quantum computing algorithms came from human insight.
- Linosaurus 3y ago
  > The key question is /why/ air moves faster on one side, and once you get away from the misconception of same time, I've never heard an intuitive argument. Since the air follows a curved surface instead of straight ahead, you could perhaps apply your intuition for centrifugal force. Momentum sort of pulls the air away from the surface. I’m not even sure this explanation is wrong.
  - blagie 3y ago
    That's interesting. Counterpoint: A wing with the opposite curvature would work too, just much less efficiently. Your explanation has an element of truth, but an element of untruth. It'd take more analysis to tease them apart.