3 ms·

> In the light of evidence that conflicts with your statement, why should we believe you? Can you clarify what you mean? Which evidence? >In particular, we kn

> In the light of evidence that conflicts with your statement, why should we believe you? Can you clarify what you mean? Which evidence? >In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong. These two perspective are in fact congruent. I decided to explain lift by reasoning about the circulation generated by the wings (https://en.wikipedia.org/wiki/Circulation_(physics) https://en.wikipedia.org/wiki/Circulation_(physics)). In this picture, the motion of the air in the wake of the wings has a net rotation (clockwise when the wind is coming from left to right) that generates an upwards resultant force on the wing. This is analogous to the pressure argument. I feel this perspective provides a clearer understanding. With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is. As I stress, it really is a simple and well understood problem. Complexity with fluid dynamics arises from problems like turbulence, multi-phase flow and computational modelling.

ColinWright 5y ago
> With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is. That's true. However ... > ... the circulation generated by the wings ... You haven't explained why there is "circulation generated by the wings", you have just stated that there is. Note, if you take the full flow over the wings and subtract the vector field that is the undisturbed average flow then you are left with a circulation. The two are effectively equivalent, so "explaining" by saying that there is "a circulation" is basically saying that the air flows faster over the top and slower underneath. It's a non-explanation. I do have an explanation as to why the air flows faster over the top, but this discussion is too small to contain it. PS: I agree that all the modelling and equations work very well, and from that point of view its "well understood". However, the discussion here shows that for many people, even those who are technically capable, it is not "well understood".
- mansoor_ 5y ago
  I think we have turned this into a point scoring contest rather than clarifying the issue. To summarise: Any statement to the effect of "aircraft lift generation is complex" is probably wrong. It is wrong because a high-level picture given by Newton's third law (momentum conservation) is correct and adequately explains lift generation. You do not need to go into the details of velocity or pressure distribution on the wing. You can step back and look the overall airflow and reason about why lift was generated. > You haven't explained why there is "circulation generated by the wings", you have just stated that there is. I only mention circulation, to state that both the arguments are similar, just from a different perspective. In my initial post, I reference only Newton's third law (the conservation of momentum). Naturally the source of the circulation is the same as the source of the velocity/pressure distribution. As stated earlier, you do not need these details to explain lift. You do need them to explain phenomenon like stall or drag.
  - ColinWright 5y ago
    I suspect we are really not far from each other, but we might have different ideas of what is "intuitive", or "obvious", or "complete", or "well understood". I've spoken with several aerodynamicists over many years, and they all say that the "conservation of momentum" idea, and the "downwash" explanation are only part of the story, and when you run the numbers the lift you get is not completely explained by those alone. But at this point I need to go back to work, and I'll come back when I can to see how the overall discussion is going. It does feel unlikely that I'll have anything useful to add.
    - mansoor_ 5y ago
      Ok. To clarify the physics, the "conservation of momentum" idea explains all of the lift, not just some of it. If you draw a bounding box around the wing and measure the momentum at either end, the lost momentum will be because of the wing, some of that will be lift generation (there is also drag + other turbulent losses).
      - 5y ago

ColinWright 5y ago

> With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is. That's true. However ... > ... the circulation generated by the wings ... You haven't explained why there is "circulation generated by the wings", you have just stated that there is. Note, if you take the full flow over the wings and subtract the vector field that is the undisturbed average flow then you are left with a circulation. The two are effectively equivalent, so "explaining" by saying that there is "a circulation" is basically saying that the air flows faster over the top and slower underneath. It's a non-explanation. I do have an explanation as to why the air flows faster over the top, but this discussion is too small to contain it. PS: I agree that all the modelling and equations work very well, and from that point of view its "well understood". However, the discussion here shows that for many people, even those who are technically capable, it is not "well understood".

mansoor_ 5y ago
I think we have turned this into a point scoring contest rather than clarifying the issue. To summarise: Any statement to the effect of "aircraft lift generation is complex" is probably wrong. It is wrong because a high-level picture given by Newton's third law (momentum conservation) is correct and adequately explains lift generation. You do not need to go into the details of velocity or pressure distribution on the wing. You can step back and look the overall airflow and reason about why lift was generated. > You haven't explained why there is "circulation generated by the wings", you have just stated that there is. I only mention circulation, to state that both the arguments are similar, just from a different perspective. In my initial post, I reference only Newton's third law (the conservation of momentum). Naturally the source of the circulation is the same as the source of the velocity/pressure distribution. As stated earlier, you do not need these details to explain lift. You do need them to explain phenomenon like stall or drag.
- ColinWright 5y ago
  I suspect we are really not far from each other, but we might have different ideas of what is "intuitive", or "obvious", or "complete", or "well understood". I've spoken with several aerodynamicists over many years, and they all say that the "conservation of momentum" idea, and the "downwash" explanation are only part of the story, and when you run the numbers the lift you get is not completely explained by those alone. But at this point I need to go back to work, and I'll come back when I can to see how the overall discussion is going. It does feel unlikely that I'll have anything useful to add.
  - mansoor_ 5y ago
    Ok. To clarify the physics, the "conservation of momentum" idea explains all of the lift, not just some of it. If you draw a bounding box around the wing and measure the momentum at either end, the lost momentum will be because of the wing, some of that will be lift generation (there is also drag + other turbulent losses).
    - 5y ago