4 ms·

Nor do we know how airplanes work. Or whips.

by whiddershins 13y ago

speeder 13y ago
What parts we don't know about these?
- rayiner 13y ago
  I don't know about "don't understand" but did you know that there is a lot more to why a wing creates lift than the "pressure differential" explanation commonly put forward: http://en.wikipedia.org/wiki/Lift_(force) http://en.wikipedia.org/wiki/Lift_(force). In particular, the typical assertion that the air at the top has to speed up so it arrives at the trailing edge at the same time as the air from the bottom is false.
  - speeder 13y ago
    I see... The speed explanation is not incorrect, except the REASON for it is. I learned on school that the air on top of the wing move faster to catch up with the one in the bottom (this never made sense to me...), the air on top indeed move faster, we only don't know why. Damnit, I should be a physicist, not a programmer :P
    - sebbi 13y ago
      Lower pressure = air can move faster ...
    - claudius 13y ago
      If you were a physicist, you’d likely also have to be a programmer – the inverse, unfortunately, does not hold.
      - michael_h 13y ago
        Public service announcement for physicists fancying themselves as programmers: Learn the following topics before going any futher: * BLAS (please) * LAPACK * memset * memcpy * 'Segmentation Fault' is not a synonym for 'exited successfully'. Those who maintain your code will thank you for it. Sometimes it might get you chocolates.
        CamperBob2 13y ago
        'Segmentation Fault' is not a synonym for 'exited successfully'. Next, you'll tell me that 'Thank you for playing Wing Commander' is not a synonym for 'EMM386 exception 006A at 00B8:3128'.
    - jasonwatkinspdx 13y ago
      > the air on top indeed move faster, we only don't know why We know exactly why. The mass of air above the wing is moving into a larger physical volume, so it spreads out to a lower pressure. Bernoulli showed that pressure and flow velocity are always related (for non viscous flows), so where you get a low pressure you'll also get increased velocity. The "to catch up" part is wrong, as well as the idea that it's the "longer path" that makes the air speed up. A symmetrical airfoil still generates lift as long as it has some angle of attack. Now, there is one complexity here in that air isn't totally inviscid. It does have some viscosity, and this creates some complex boundary layer effects and turbulence. So the Bernoulli explanation is a bit of an approximation. The "real" math is Navier-Stokes, but that's not useful to most people for understanding flows based on looking at formulas (it's the math that underlies computational simulation of fluids but nearly useless with pen and paper). A better "highschool textbook" explanation is the concept of circulation, which captures most of the important details without requiring you to understand tensors and vector calc.
  - rypskar 13y ago
    I think the part about how airplains fly was good described in https://news.ycombinator.com/item?id=3379928 https://news.ycombinator.com/item?id=3379928.
  - dylangs1030 13y ago
    Addendum: Amazingly, this is still the standard explanation given in textbooks for pilots, and the answer you can expect from a pilot (and I've asked).
- sp332 13y ago
  Well, there's a common incorrect explanation: Those who believe the "popular" explanation are wrongly insisting that any parcels of air divided by the wing's leading edge must meet again at the trailing edge. This is incorrect. It doesn't occur: experiments easily show that the air above a wing far outraces the air below, and parcels never meet again. But the real answer is known: http://amasci.com/wing/airfoil.html http://amasci.com/wing/airfoil.html
psidebot 13y ago
The operation of airfoils is non-intuitive and many (event most pilots) hold misconceptions about how they work. That does not mean that the physics is not well understood by those who design them. There maybe a few gaps (particularly for supersonic flight) still being explored, but in general it's inaccurate to claim that we don't know how airplanes work.

Nor do we know how airplanes work. Or whips.

speeder 13y ago

What parts we don't know about these?

rayiner 13y ago
I don't know about "don't understand" but did you know that there is a lot more to why a wing creates lift than the "pressure differential" explanation commonly put forward: http://en.wikipedia.org/wiki/Lift_(force) http://en.wikipedia.org/wiki/Lift_(force). In particular, the typical assertion that the air at the top has to speed up so it arrives at the trailing edge at the same time as the air from the bottom is false.
- speeder 13y ago
  I see... The speed explanation is not incorrect, except the REASON for it is. I learned on school that the air on top of the wing move faster to catch up with the one in the bottom (this never made sense to me...), the air on top indeed move faster, we only don't know why. Damnit, I should be a physicist, not a programmer :P
  - sebbi 13y ago
    Lower pressure = air can move faster ...
  - claudius 13y ago
    If you were a physicist, you’d likely also have to be a programmer – the inverse, unfortunately, does not hold.
    - michael_h 13y ago
      Public service announcement for physicists fancying themselves as programmers: Learn the following topics before going any futher: * BLAS (please) * LAPACK * memset * memcpy * 'Segmentation Fault' is not a synonym for 'exited successfully'. Those who maintain your code will thank you for it. Sometimes it might get you chocolates.
      - CamperBob2 13y ago
        'Segmentation Fault' is not a synonym for 'exited successfully'. Next, you'll tell me that 'Thank you for playing Wing Commander' is not a synonym for 'EMM386 exception 006A at 00B8:3128'.
  - jasonwatkinspdx 13y ago
    > the air on top indeed move faster, we only don't know why We know exactly why. The mass of air above the wing is moving into a larger physical volume, so it spreads out to a lower pressure. Bernoulli showed that pressure and flow velocity are always related (for non viscous flows), so where you get a low pressure you'll also get increased velocity. The "to catch up" part is wrong, as well as the idea that it's the "longer path" that makes the air speed up. A symmetrical airfoil still generates lift as long as it has some angle of attack. Now, there is one complexity here in that air isn't totally inviscid. It does have some viscosity, and this creates some complex boundary layer effects and turbulence. So the Bernoulli explanation is a bit of an approximation. The "real" math is Navier-Stokes, but that's not useful to most people for understanding flows based on looking at formulas (it's the math that underlies computational simulation of fluids but nearly useless with pen and paper). A better "highschool textbook" explanation is the concept of circulation, which captures most of the important details without requiring you to understand tensors and vector calc.
- rypskar 13y ago
  I think the part about how airplains fly was good described in https://news.ycombinator.com/item?id=3379928 https://news.ycombinator.com/item?id=3379928.
- dylangs1030 13y ago
  Addendum: Amazingly, this is still the standard explanation given in textbooks for pilots, and the answer you can expect from a pilot (and I've asked).
sp332 13y ago
Well, there's a common incorrect explanation: Those who believe the "popular" explanation are wrongly insisting that any parcels of air divided by the wing's leading edge must meet again at the trailing edge. This is incorrect. It doesn't occur: experiments easily show that the air above a wing far outraces the air below, and parcels never meet again. But the real answer is known: http://amasci.com/wing/airfoil.html http://amasci.com/wing/airfoil.html

psidebot 13y ago

The operation of airfoils is non-intuitive and many (event most pilots) hold misconceptions about how they work. That does not mean that the physics is not well understood by those who design them. There maybe a few gaps (particularly for supersonic flight) still being explored, but in general it's inaccurate to claim that we don't know how airplanes work.