5 ms·

What mistake? The classic mistake is to assume air travels faster over the top than the bottom because the top path is longer and they must take equal time. In

What mistake? The classic mistake is to assume air travels faster over the top than the bottom because the top path is longer and they must take equal time. In fact air does travel faster over the top, quite a bit faster than the equal time fallacy would indicate.

jordanb 9y ago
I wonder if there's a word for this; where people learn that one bit of common knowledge is a fallacy and then "overcorrect" by assuming related things are also wrong. Perhaps "meta fallacy?"
- mannykannot 9y ago
  I do not know if there is a name for it, but it comes from not fully understanding why the original fallacy is fallacious.
- ithkuil 9y ago
  Cargo cult fallacy?
- theophrastus 9y ago
  The legal term, which presupposes the fallacy, is: Falsus in uno, Falsus in omnibus[1] [1] https://en.m.wikipedia.org/wiki/Falsus_in_uno,_falsus_in_omnibus https://en.m.wikipedia.org/wiki/Falsus_in_uno,_falsus_in_omn...
21 9y ago
The mistake would be that this is what provides the lift force, as stated in the quote. If this was so, birds wouldn't need to flap, which is the actual lift generator (pushing air down).
- mikeash 9y ago
  The pressure difference pushes air down. It does provide the lift force. Birds flap to create thrust, not lift. Surely you’ve seen birds flying without flapping?
  - 21 9y ago
    > Surely you’ve seen birds flying without flapping? Isn't that when they just "ride" the winds? The wind pushing against the back of their wings which is hold angled thus pushing it higher (like a boat sail).
    - mikeash 9y ago
      Sorry, I don’t really understand what you’re saying here. Birds and airplanes work on the same principles. Airflow over the wings produces lift. That airflow also produces drag, which must be counteracted with thrust to maintain altitude. The only difference is that birds produce thrust by flapping, and airplanes produce thrust with engines. A non-flapping bird is equivalent to an airplane with the engines shut off.
      - 21 9y ago
        I was referring to this theory, which NASA calls wrong: https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html Most interestingly, NASA also calls wrong what you seem to defend, or maybe there are some subtleties that I'm missing: https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
        mikeash 9y ago
        No, that second link describes the equal time fallacy, which I called out as wrong in my original comment.
    - mannykannot 9y ago
      There is no equivalent, in flying (other than balloons), of a sailboat running before the wind. A gliding bird is always facing into the relative airflow.
- mannykannot 9y ago
  In general, birds do not need to flap; they can glide, and flapping is more about propulsion than lift. In the case of hummingbirds hovering or birds launching into the air from the ground, I believe it is correct to say that vortex formation is an essential part of the process. In fact, the analysis of lift generation by airfoils treats the flow as a vortex superimposed on a linear flow, so in that sense, the explanation of lift generation is both inseparable from and dependent on vortex formation, though I am not sure the article's author had this in mind.
  - vlehto 9y ago
    "the explanation of lift generation is both inseparable from and dependent on vortex formation" Are you sure about this? There are of course high lift devices and wingtip vortices are unavoidable. But I'm pretty sure you can (at least theoretically) generate lift with an airfoil in a wind tunnel while having completely laminar flow.
    - mannykannot 9y ago
      I am referring to the Kutta–Joukowski theorem [1]. I don't pretend to understand it in detail, but here it says "the fluid flow in the presence of the airfoil can be considered to be the superposition of a translational flow and a rotating flow." Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases. [1] https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem (I have seen another version that I think offers a more intuitive, though less rigorous, presentation, and if I can find it I will add a link.)
      - icegreentea2 9y ago
        The rotating flow / vortex in that sentence is probably referring to the 'circulation' of the air flow, which has some mapping to the lay idea of vorticity, but isn't fully it. It's basically a measurement of deflection through a closed boundary.
cryptonector 9y ago
Air going over the top of a wing more quickly than the air below is NOT what causes lift. What causes lift is "entrainment" plus the wing shape redirecting entrained air downwards at the trailing edge. By Newton's third law of motion, the acceleration of air downwards by the wing implies the acceleration of the wing upwards (and with the wing, the airplane).
- dilyevsky 9y ago
  False. Both Bernoulli Principle and Newton's 3rd Law explanations are correct - https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html
  - cryptonector 9y ago
    Correct, but the biggest factor is the entrained air.
- mikeash 9y ago
  You can’t redirect air downward without a pressure differential. You can’t have a pressure differential without a speed differential. “Air goes faster on the top of the wing,” “pressure is lower on top of the wing,” and “air is pushed downward” are physically equivalent statements.
  - cryptonector 9y ago
    That's not how it works. It's almost all entrainment and very little Bernoulli effect. The proof lies in symmetric wings, which generate no lift but still entrain air. Einstein famously designed what he thought should be a very good wing based on the Bernoulli principle, and it turned out to not work, and if you look at it[0] you'll see why... [0] https://www.google.com/search?&q=einstein+wing+design https://www.google.com/search?&q=einstein+wing+design
    - mikeash 9y ago
      What do you mean, symmetric wings generate no lift? There are plenty of lift-generating symmetric airfoils out there. Your link indicates that Einstein fell victim to the equal time fallacy. While the equal time fallacy supposes that lift is generated because air moves faster and generates a pressure differential, air moving faster to generate a pressure differential does not necessarily imply the equal time fallacy.

jordanb 9y ago

I wonder if there's a word for this; where people learn that one bit of common knowledge is a fallacy and then "overcorrect" by assuming related things are also wrong. Perhaps "meta fallacy?"

mannykannot 9y ago
I do not know if there is a name for it, but it comes from not fully understanding why the original fallacy is fallacious.
ithkuil 9y ago
Cargo cult fallacy?
theophrastus 9y ago
The legal term, which presupposes the fallacy, is: Falsus in uno, Falsus in omnibus[1] [1] https://en.m.wikipedia.org/wiki/Falsus_in_uno,_falsus_in_omnibus https://en.m.wikipedia.org/wiki/Falsus_in_uno,_falsus_in_omn...

21 9y ago

The mistake would be that this is what provides the lift force, as stated in the quote. If this was so, birds wouldn't need to flap, which is the actual lift generator (pushing air down).

mikeash 9y ago
The pressure difference pushes air down. It does provide the lift force. Birds flap to create thrust, not lift. Surely you’ve seen birds flying without flapping?
- 21 9y ago
  > Surely you’ve seen birds flying without flapping? Isn't that when they just "ride" the winds? The wind pushing against the back of their wings which is hold angled thus pushing it higher (like a boat sail).
  - mikeash 9y ago
    Sorry, I don’t really understand what you’re saying here. Birds and airplanes work on the same principles. Airflow over the wings produces lift. That airflow also produces drag, which must be counteracted with thrust to maintain altitude. The only difference is that birds produce thrust by flapping, and airplanes produce thrust with engines. A non-flapping bird is equivalent to an airplane with the engines shut off.
    - 21 9y ago
      I was referring to this theory, which NASA calls wrong: https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html Most interestingly, NASA also calls wrong what you seem to defend, or maybe there are some subtleties that I'm missing: https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
      - mikeash 9y ago
        No, that second link describes the equal time fallacy, which I called out as wrong in my original comment.
  - mannykannot 9y ago
    There is no equivalent, in flying (other than balloons), of a sailboat running before the wind. A gliding bird is always facing into the relative airflow.
mannykannot 9y ago
In general, birds do not need to flap; they can glide, and flapping is more about propulsion than lift. In the case of hummingbirds hovering or birds launching into the air from the ground, I believe it is correct to say that vortex formation is an essential part of the process. In fact, the analysis of lift generation by airfoils treats the flow as a vortex superimposed on a linear flow, so in that sense, the explanation of lift generation is both inseparable from and dependent on vortex formation, though I am not sure the article's author had this in mind.
- vlehto 9y ago
  "the explanation of lift generation is both inseparable from and dependent on vortex formation" Are you sure about this? There are of course high lift devices and wingtip vortices are unavoidable. But I'm pretty sure you can (at least theoretically) generate lift with an airfoil in a wind tunnel while having completely laminar flow.
  - mannykannot 9y ago
    I am referring to the Kutta–Joukowski theorem [1]. I don't pretend to understand it in detail, but here it says "the fluid flow in the presence of the airfoil can be considered to be the superposition of a translational flow and a rotating flow." Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases. [1] https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem (I have seen another version that I think offers a more intuitive, though less rigorous, presentation, and if I can find it I will add a link.)
    - icegreentea2 9y ago
      The rotating flow / vortex in that sentence is probably referring to the 'circulation' of the air flow, which has some mapping to the lay idea of vorticity, but isn't fully it. It's basically a measurement of deflection through a closed boundary.

cryptonector 9y ago

Air going over the top of a wing more quickly than the air below is NOT what causes lift. What causes lift is "entrainment" plus the wing shape redirecting entrained air downwards at the trailing edge. By Newton's third law of motion, the acceleration of air downwards by the wing implies the acceleration of the wing upwards (and with the wing, the airplane).

dilyevsky 9y ago
False. Both Bernoulli Principle and Newton's 3rd Law explanations are correct - https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html
- cryptonector 9y ago
  Correct, but the biggest factor is the entrained air.
mikeash 9y ago
You can’t redirect air downward without a pressure differential. You can’t have a pressure differential without a speed differential. “Air goes faster on the top of the wing,” “pressure is lower on top of the wing,” and “air is pushed downward” are physically equivalent statements.
- cryptonector 9y ago
  That's not how it works. It's almost all entrainment and very little Bernoulli effect. The proof lies in symmetric wings, which generate no lift but still entrain air. Einstein famously designed what he thought should be a very good wing based on the Bernoulli principle, and it turned out to not work, and if you look at it[0] you'll see why... [0] https://www.google.com/search?&q=einstein+wing+design https://www.google.com/search?&q=einstein+wing+design
  - mikeash 9y ago
    What do you mean, symmetric wings generate no lift? There are plenty of lift-generating symmetric airfoils out there. Your link indicates that Einstein fell victim to the equal time fallacy. While the equal time fallacy supposes that lift is generated because air moves faster and generates a pressure differential, air moving faster to generate a pressure differential does not necessarily imply the equal time fallacy.