3 ms·

That's a very bold statement, so I'm compelled to ask: In the light of evidence that conflicts with your statement, why should we believe you? In particular,

That's a very bold statement, so I'm compelled to ask: In the light of evidence that conflicts with your statement, why should we believe you? 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. [0] Even faster than the (incorrect) equal transit time idea would imply.

simonh 5y ago
The statement only referred to the net motion of the vehicle and the deflected air. The fact that during that process the air here of there had higher or lower pressure is incidental, and not contradictory to this explanation. The air might have other things changed about it by the process, such as turbulent motion, but again those are incidental and are not contradictory to the general statement.
mansoor_ 5y ago
> 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.
bondarchuk 5y ago
Bernoulli goes both ways and in this case I think the other way makes more sense. Force of air on the wing is net upwards, therefore the pressure at the bottom of the wing is higher than at the top, therefore, by Bernoulli's principle, the airspeed at the top is higher than at the bottom. Since we are trying to give higher order explanations for lower level phenomena you can probably dissect this a thousand different ways and conclude "it's not that simple", but I don't believe this explanation is wrong at the granularity at which it functions, and it really is quite simple. (edit) Furthermore the detail of the air speed difference is not even relevant for the explanation mansoor_ gives, so I would hesitate to call it "conflicting evidence". Of course you can demand of a high level theory that it explains a fact which is not necessary for the high level theory to function, and then say that it's not as simple, but I don't think that's fair.
wbeaty 5y ago
His statement is right, but it requires that tip-vortices and downwash-plumes exist. The air behind the wing is carrying momentum, and is left with downward motion. (In typical 2D airfoil diagrams, this cannot happen.) Also, his statement is only true if we completely eliminate Bernoulli (since any wing which flings air downwards, is also performing net work, and injecting energy into air-parcels. If the net energy of parcels is changed, then Bernoulli concepts cannot be employed.) "Simple" is the problem. It's simple to describe wings verbally, but the math is not simple at all. Helicopters are the same: they generate a column of downwash, but this involves performing net work, and involves vortex-shedding, so simple Bernoulli concepts are utterly forbidden. Simplified: wings are an example of propulsion, and fluid propulsion always involves vortex-shedding, which falls under the heading of Turbulence. It's easy to verbally describe simplified elementary turbulence. It's just a vortex which moves around, and involves work performed on the air as a whole. But jeeze the math involved!

simonh 5y ago

The statement only referred to the net motion of the vehicle and the deflected air. The fact that during that process the air here of there had higher or lower pressure is incidental, and not contradictory to this explanation. The air might have other things changed about it by the process, such as turbulent motion, but again those are incidental and are not contradictory to the general statement.

mansoor_ 5y ago

> 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.

bondarchuk 5y ago

Bernoulli goes both ways and in this case I think the other way makes more sense. Force of air on the wing is net upwards, therefore the pressure at the bottom of the wing is higher than at the top, therefore, by Bernoulli's principle, the airspeed at the top is higher than at the bottom. Since we are trying to give higher order explanations for lower level phenomena you can probably dissect this a thousand different ways and conclude "it's not that simple", but I don't believe this explanation is wrong at the granularity at which it functions, and it really is quite simple. (edit) Furthermore the detail of the air speed difference is not even relevant for the explanation mansoor_ gives, so I would hesitate to call it "conflicting evidence". Of course you can demand of a high level theory that it explains a fact which is not necessary for the high level theory to function, and then say that it's not as simple, but I don't think that's fair.

wbeaty 5y ago

His statement is right, but it requires that tip-vortices and downwash-plumes exist. The air behind the wing is carrying momentum, and is left with downward motion. (In typical 2D airfoil diagrams, this cannot happen.) Also, his statement is only true if we completely eliminate Bernoulli (since any wing which flings air downwards, is also performing net work, and injecting energy into air-parcels. If the net energy of parcels is changed, then Bernoulli concepts cannot be employed.) "Simple" is the problem. It's simple to describe wings verbally, but the math is not simple at all. Helicopters are the same: they generate a column of downwash, but this involves performing net work, and involves vortex-shedding, so simple Bernoulli concepts are utterly forbidden. Simplified: wings are an example of propulsion, and fluid propulsion always involves vortex-shedding, which falls under the heading of Turbulence. It's easy to verbally describe simplified elementary turbulence. It's just a vortex which moves around, and involves work performed on the air as a whole. But jeeze the math involved!