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  1. #41
    Senior Member Evan's Avatar
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    It's almost like [a], except that the science is not Lift but Mechanics of Fluids.
    And I am not sure what is the difference with [b]. Science IS basically empirical observation, theoretically modeling these observations, confirming with more observation, making predictions with the model, and then testing to confirm if the predictions hold true.
    I guess I would say that known science (meaning scientifically "proven" fact) has withstood all reasonable scepticism. Is there room for doubt or scepticism in the prevailing explanation of airfoil-induced lift? If not, then I would consider it a known science. But I get the impression that there is room, and that the explanations for lift are very scientific postulations conceived to explain an observed phenomena—that they are the product of abductive reasoning—(that lift was discovered by physical experimentation long before the postulation was deduced (or induced?) to explain it).

    But this is what I'm asking you, because I really don't know the answer.

    I also really don't know that I could accurately explain how a wing creates lift to a five-year-old. I could say it's due to the plane of the wing deflecting the air downward and Sir Newton requiring an equal but opposite upward force. I could explain that the airfoil creates an area of lower pressure above the wing also resulting in an upward force.

    But this is how a five-year-old (god bless them) would have me:

    Me: The airfoil creates an area of reduced pressure above the wing, so the higher pressure below the wing pushes it upward (Bernoulli).

    Kid: Why?

    Me: Because the airstream above the wing is induced by the airfoil to accelerate and this reduces the local air pressure.

    Kid: Why?

    Me: I don't know kid... I think I hear your mother calling you.

    So what would you tell a (relatively smart but pre-Gabriellian) five-year-old to answer that question?

  2. #42
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    I guess I would say that known science (meaning scientifically "proven" fact) has withstood all reasonable scepticism. Is there room for doubt or scepticism in the prevailing explanation of airfoil-induced lift? If not, then I would consider it a known science. But I get the impression that there is room, and that the explanations for lift are very scientific postulations conceived to explain an observed phenomena—that they are the product of abductive reasoning—(that lift was discovered by physical experimentation long before the postulation was deduced (or induced?) to explain it).
    Are you serious or you forgot the blue font? That's how Science works!!! Did Newton came up with the General Theory of Gravity before it Gravity was observed? No, Newton observed Gravity, modeled it, and that's the General Theory of Gravity (and, basically, all Newtonian Mechanics). Did Einstein come up with the Theory of Relativity and concluded that the speed of light was constant? No, he modeled it around the discovered fact that the speed of light is constant. Now, the good thing about Science is that the models and theories developed to explain nature often make predictions of things that had not been observed so far, and by testing them you can strengthen the theory or destroy it. To name only 2 examples, gravitational waves (Relativity) and winglets (Aerodynamics). Winglets were not observed in nature, they were reasoned out from the ramifications of the theory/model and THEN created, tested and found to work as predicted.

    A Theory is NEVER "true". It is just a better (or worse, or useless) approximation to nature.
    You can put the explanation of airfoil-induced lift right there with evolution, germ theory of disease, relativity, gravity, and quantum mechanics. None of them are perfect of the ultimate truth. But that is how much doubt or skepticism there is around it.

    Now, in fact, in the very end, everything is particles Physics modeled under Quantum Mechanics and General Relativity (still lacking a unified theory). But I am not going to explain how lift works at that level because I don't know how, and nobody knows how. So we have macroscopic models that assume, for example, that the air is acontinuum instead of a bunch of sub-atomic particles and quantum fields. And because we do the same when we talk about biology, chemistry, car dynamics or whatever a little more complex that a few sub-atomic particles.

    If that's not "proven enough science" for you, I can't help you.

    But this is what I'm asking you, because I really don't know the answer.

    I also really don't know that I could accurately explain how a wing creates lift to a five-year-old. I could say it's due to the plane of the wing deflecting the air downward and Sir Newton requiring an equal but opposite upward force. I could explain that the airfoil creates an area of lower pressure above the wing also resulting in an upward force.

    But this is how a five-year-old (god bless them) would have me:

    Me: The airfoil creates an area of reduced pressure above the wing, so the higher pressure below the wing pushes it upward (Bernoulli).

    Kid: Why?

    Me: Because the airstream above the wing is induced by the airfoil to accelerate and this reduces the local air pressure.

    Kid: Why?

    Me: I don't know kid... I think I hear your mother calling you.

    So what would you tell a (relatively smart but pre-Gabriellian) five-year-old to answer that question?
    I guess that if a kid asks you how clouds are formed you are going to start to explain how the air is a mixture of gases, gaseous water being one of them, and how the solubility of the water in air depends on the temperature so when the temperature goes down to a point called "dew point" it can't hold the water dissolved anymore and, since the temperature is less than that needed to keep the water gaseous by itself, it will form little droplets that is what we see as clouds. I, instead, will show him water vapor coming from bathtub filled with water and make a nice analogy with the sea and the clouds.

    No, I would not go the way you did for lift. I would say:

    Put your rollers on and I will put mine. Ok now push me hard. Hey, why did you move back? Push me hard but avoid moving back. No, grabbing the chair or using the brake is cheating. Come on, push me. Why why are you moving back again? You can't help it? That's fine. You'll see, you cannot push something without being pushed yourself in the opposite direction.
    Now, air is difficult to see, so let's play with water. Take that spoon and hold it hanging lightly from the tip of the handle, now put the round part under this stream of water from the tap. Slowly. SO what did you see? Correct! The shape of the spoon gets the water is deviated, pushed to one side, and the spoon is pushed to the other side. Now kid, get in the car.
    Ok, we are doing 60 miles per hour in this solitary highway and we are in the 1st lane next to the shoulder so go ahead and stick your hand out of the window. Extend your hand and put it horizontal, like this. Now tilt it a little bit up, like this? Fell the force pushing your hand up? Want to guess what is happening? Correct! Wile we cannot see it, the air is being deviated, pushed down by your hand in that angle, same that happened with the water and the spoon. And since your hand is pushing the air down, the air is pushing your hand up, just like when we pushed each other with the rollers. And that is the same way how the wing pushed the air down, so the air pushes the wing up, and planes can fly.


    The problem is that you want a 5-years-old-friendly explanation as to how lift is produced an the intimate level of the interaction of the the infinitesimal elemental volumes of air between themselves and with the airfoil. I don't know how to do that but that is MY fault, not that the theory is not there, is not fully understood, there are doubts or skepticism, ambiguity, lack of consensus among experts about it or whatever. The theory is and works perfectly fine, whether I can explain it to a 5-years-old kid or not.

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  3. #43
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel
    That's how Science works!!!
    To varying degrees. I guess the issue will always be an epistomological one, but I'm not trying to go that far for the sake of argument here. You can ask me what causes a rainbow and I can give you the answer in one or two sentences using science that is very nearly beyond any scepticism. You can ask me what the nature of gravity is and I will need to give a more equivocal answer that still includes a significant degree of scepticism.

    I'm under the (perhaps false) impression that 'air-turning' aspect of lift does not account for the total lift generated by the airfoil, and that the entire understanding of causal factors behind an airfoil creating lift is not as agreed upon and beyond scepticism as, for instance, the science of optical refraction. That is not to say that 'known science' demands absolute certainty (which may not be possible in anything), but to inquire as to whether the entire theory of lift is considered 'known science' on the level of things that are universally considered to be resolved as factual by the scientific community. Is the current prevailing explanation of the mechanics of lift at that level of factual resolution or do problems remain that leave it open to scepticism and thus alternate theories and relegate it to the theoretical realm rather than the factual one?

    The theory is and works perfectly fine, whether I can explain it to a 5-years-old kid or not.
    The fact that a simple enough explanation cannot be given for the non-obvious part of the equation (the part that doesn't involve sticking your hand out the car window), that even I can understand, causes me to become sceptical...

  4. #44
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    To varying degrees. I guess the issue will always be an epistomological one, but I'm not trying to go that far for the sake of argument here. You can ask me what causes a rainbow and I can give you the answer in one or two sentences using science that is very nearly beyond any scepticism.
    Are you sure of that? Do you want to see who many steps down the WHY ladder we can go before you face skepticism? Because sooner rather than later we are going to reach the point of "why does the light travel slower in water than in the air?" And good luck with that. Sooner or later, with anything, you are going to reach the point where you will be faced with 2 options: a) Because God did it this way and don't ask me why again, the Lord works in mysterious ways" or b) "There is no why, that is HOW it works, and there is no hidden will or intention behind it. Get over it".

    Science explains HOW nature works not why.

    I'm under the (perhaps false) impression that 'air-turning' aspect of lift does not account for the total lift generated by the airfoil,
    Yes, that's a false impression. The shoving of the air down (if that's what you mean by the "air-turning" explains 100% of the lift, and it could not be in any other way unless there was something else beyond the air generating the lift, which there isn't. Any force that the air makes on the wing will be accompanied bu an equal and opposite force that the wing makes on the air. If the force on the wing is up, the force on the air is down. And because the air was happily at rest in equilibrium buoyancy in air, the sum of forces was already zero before the wing came by, so whatever force the wing makes on the air will cause an acceleration in the same direction.

    and that the entire understanding of causal factors behind an airfoil creating lift is not as agreed upon and beyond scepticism as, for instance, the science of optical refraction.
    Again, false.

    That is not to say that 'known science' demands absolute certainty (which may not be possible in anything), but to inquire as to whether the entire theory of lift is considered 'known science' on the level of things that are universally considered to be resolved as factual by the scientific community.
    Yes, it is resolved, except that we are waiting for the winner of the million dollars who discovers a closed solution for the Navier Stokes equation. But the concepts behind the most complete and universal Mechanics of Fluids theory is in fact quite straight forward. We do know all the forces that act on an infinitesimal volume of air. Pressure, fiction, inertial forces and external forces. We know how much is the mass of an infinitesimal volume of air (volume times density). We know how the density, volume and temperature interact (Pv=nRT, does it sound familiar?) That's it. End of the theory. Yes, we are assuming that the air is continuum and a perfect gas. These are approximations. Where these approximations break down, the theory breaks down. As does the refraction of photons on water if you shoot one photon on one molecule of water.

    Is the current prevailing explanation of the mechanics of lift at that level of factual resolution or do problems remain that leave it open to scepticism and thus alternate theories and relegate it to the theoretical realm rather than the factual one?
    It is as factual as any scientific theory. As factual as what causes a rainbow if you like.

    The fact that a simple enough explanation cannot be given for the non-obvious part of the equation (the part that doesn't involve sticking your hand out the car window), that even I can understand, causes me to become sceptical...
    There is no non-obvious part that is not at play when you stick your hand out of the car. 100% of the lift is wing shoves air down so air reacts pushing wing up. Get over it.

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  5. #45
    Senior Member 3WE's Avatar
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    Default F.A.O.: Gabriel

    I know I enjoy the debate and learning and razzing and am sometimes "part of the problem".

    BUT...

    ...when you have significant training in aerodynamics, and you go to some effort to explain things, but someone argues incessantly...and I really mean incessantly

    Do you feel the need to ask if they have any demonstrable training or experience or credentials in aerodynamics?, or even physics?...Not that you are "asserting" your qualifications, but maybe a "reminder" that you might have some valid insight that should be considered instead of incessantly dismissed...because of your qualifications?

    (PS: I also enjoy ironing and seeing the foot on the other hand (as they say in Airplane!)).
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  6. #46
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel
    Get over it.
    Get over it? I'm just beginning to get it in the first place. You must be aware that MOST non-engineers have an incorrect understanding of the function of an airfoil and the mechanics of lift, largely because the 'pressure-differential' explanation is the MOST popular one and is still being widely taught. I am just trying to learn the truth from our resident aeroengineer, but I can't always follow his Vulcan explanations.

    I'm not arguing the point. I'm using a bit of Socratic method to extract a simpler explanation from you and to pin you down on a few key points (perhaps we should designate green font for this purpose).

    I understand now that lift is entirely the result of an air mass being turned by the angle of attack of the lower wing surface and that the shape of the wing chord is the result of aerodynamic issues related to the location of the stagnation point and factors that effect the useable AoA range and minimize drag. I understand boundary layer separation issues, viscosity and compressibility issues (but I need my understanding to focus on low speed flight first).

    What still puzzles me is the role (if any) of the upper wing surface to actually generate a lift force. You now have me under the impression that the speed of the flow OVER the wing and thus the local pressure there has NO lift-generating function. Correct?

  7. #47
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    What still puzzles me is the role (if any) of the upper wing surface to actually generate a lift force.
    I am an air molecule sitting there minding my own business. I am probably an N2 (related to plant fertilizer no less). A thin flat sheet of cardboard slices by BELOW ME at a downward angle...there is now a "vacuum" underneath me as the molecules (mostly N2, but the occasional O2 and a sprinkling of other stuff, including H2O) that used to be below me have gone further below me.

    That vacuum = reduced pressure = net lift.

    Now, I do not recall if Gabbiee addressed that "rough rule" that 60% of lift comes from OVER the wing, 40% below...I almost think he has dismissed it as not true. I did read where Gabieeee almost totally destroyed that the curvature generated lift.

    Yes, me and moles of my friends above the cardboard sheet do move downward to "fill" the vacuum, but nevertheless at a reduced pressure.

    Now- do you ever:
    -Ride a bicycle?
    -Drive a car?
    -Ride an airline?
    -Have any pilot ratings?


    I will side with Evan on this one thing....the Carburetor jet sprays fuel in right at the narrowest part of the venture tube....We outsiders visualize that CRAMMING AIR TOGETHER, NOT PULLING IT APART....

    Of course, that isn't really the mechanism..the mechanism is that cylinders suck "air in" and generate a vacuum that runs all the way up to the top of the throttle plate...even with wide open throttle, you have the venture tube generating some extra suction....I can see the tippy top of that narrow space as having maybe maximum vacuum....nevertheless a good place to sneak some gas in to be stirred up by turbulence.
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  8. #48
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Get over it? I'm just beginning to get it in the first place. You must be aware that MOST non-engineers have an incorrect understanding of the function of an airfoil and the mechanics of lift, largely because the 'pressure-differential' explanation is the MOST popular one and is still being widely taught.
    And it is not incorrect. I mean, it can be an incorrect version of it, but the pressure differential is how lift happens.
    I can almost see you saying "What? Didn't you just said that the lift was 100% the result of action and reaction when shoving air down?"

    Yes. The wing pushes air down and the air pushes the wing up. But how is this push manifested?
    There are 2 and only 2 ways how the air (or a fluid) can exert a force: pressure over an area and viscous friction over an area.

    So the fact that the air is pushing the wing up means that it exerts a pressure and friction distribution over the wing that, when integrated over the wing area (both upper and lower surface), results in a total net force. We call LIFT to the component of that net force that is perpendicular to the free stream and DRAG the component parallel to the free stream. [1]

    It's like what I said before about a free-falling object. Can the speed gain be 100% explained by the acceleration due to gravity? Yes Can the speed gain be 100% explained by the conversion of gravitational potential energy into kinetic energy? Yes. They are not different things. They are different perspectives of the same thing. It's not 2 different explanations, it is the same explanation just approached differently. And one can be derived from the other. One means or implies the other.

    I'm not arguing the point. I'm using a bit of Socratic method to extract a simpler explanation from you and to pin you down on a few key points (perhaps we should designate green font for this purpose).
    I am good with that, I love the Socratic method.

    I understand now that lift is entirely the result of an air mass being turned by the angle of attack of the lower wing surface
    Then you didn't understand. It's not just the lower surface. It is the whole airfoil (if you are in 2D, or the whole wing in a real 3D wing). The presence of the wing in the air current makes the air do what it does, otherwise it would not meet the necessary conditions of separating at the trailing edge and not penetrating through the wing.

    If you take 2 airfoils with exactly the same shape of the lower surface and the same "angle of attack of the lower surface" but different shape of the upper surface, the lift can and probably will be different. The fact that the air needs to go around the upper surface (vs flowing through it) and not separate until it reaches the trailing edge affects hoe the air behaves.

    In other words, it is not the lower surface pushing the air down bu the airfoil deviation the air down. Both the upper and the lower surface. Go ant try the experiment of the spoon that I explained to the 5-years-old kid in my previous post. I mean it, go and try it. See how just the upper surface can generate lift (in a spoon under a stream of water where yo can have the water flowing just over the upper surface, something that cannot be replicated with a wing immersed in air, so you will have both surfaces participating always).

    What still puzzles me is the role (if any) of the upper wing surface to actually generate a lift force. You now have me under the impression that the speed of the flow OVER the wing and thus the local pressure there has NO lift-generating function. Correct?
    Nope, not correct, as explained above. Sorry if I my previous explanation gave that impression. But I included the flow o the upper surface in my explanations starting with the thin flat plywood. I even explained how you can have LOW pressure mostly around ALL of the airfoil, both the upper and lower surface, and how the pressure on the upper surface would be even lower than on the lower surface thus generating a net upwards force.




    [1] By the way and as a side note, the lift is almost perfectly explained taking into account just the pressure (but you need to be aware that the friction is necessary for the formation of the boundary layer and its separation at the trailing edge, although you can impose this condition artificially in the model and then forget about friction), while for the drag you need to take into account both pressure and friction.

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  9. #49
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    I know I enjoy the debate and learning and razzing and am sometimes "part of the problem".

    BUT...

    ...when you have significant training in aerodynamics, and you go to some effort to explain things, but someone argues incessantly...and I really mean incessantly

    Do you feel the need to ask if they have any demonstrable training or experience or credentials in aerodynamics?, or even physics?...Not that you are "asserting" your qualifications, but maybe a "reminder" that you might have some valid insight that should be considered instead of incessantly dismissed...because of your qualifications?
    Nope. Not at all. If I felt that need I would have done it.

    Now, there are cases where I use my credentials. That is when people take things I am saying lightly or not even reflecting on them and just contesting or neglecting them them without any consideration.
    "Hey John, I have these credentials, so supposedly perhaps you should listen more carefully what I say when we are talking about aerodynamics than, I don't know, Malaysian cuisine.".

    But Evan seems to be making an honest effort here, so all is right.

    By the way, Evan, I am not perfect even in Aerodynamics, so I can be wrong.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
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  10. #50
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Are you sure of that? Do you want to see [how] many steps down the WHY ladder we can go before you face skepticism? Because sooner rather than later we are going to reach the point of...
    If you read my post carefully, I said that my use of the term 'known science' was a matter of degree, a high degree of certainty, not absolute certainty, and that I wasn't interested in going down the rabbit hole of epistomology here. I'm hoping of for a theory-of-evolution level of certainty. What I'm trying to establish is exactly what is left open to scientific scepticism, what, if any, unresolved 'problems' still exist with the prevailing theory.

    My original point though, is that most pilots probably couldn't tell you how a wing generates lift (i.e. How Airplanes Fly), including members of this forum, at least prior to this thread. There is quite a bit of misconception out there and the pressure-diferential explanation** seems to be the most widely cited one.
    I also concur that pilots do not need to know this, but generally agree with you that pilots that know better are better pilots.

    I'd love an HONEST admission of how many had it wrong or learned something new here. I'm #1.

    ** The one that states that the split upper and lower airflow must exit the trailing edge at the same time.

  11. #51
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    Now, I do not recall if Gabbiee addressed that "rough rule" that 60% of lift comes from OVER the wing, 40% below...I almost think he has dismissed it as not true. I did read where Gabieeee almost totally destroyed that the curvature generated lift.
    Yes, I've heard that many times but no, you never saw me saying such a thing, basically because for me the proposition itself doesn't make sense in concept, regardless of the fact that not all airfoils are the same and that the pressure distributions change with the AoA.

    How do you measure the contribution of one surface independently from the other? If Po id the pressure of the free-stream, and in the lower surface you have on average 90% of Po and on the upper surface you have 80% of Po, how much is the contribution of each surface? And if the lower has 1.1 Po and the upper 0.8 Po?

    But ok, say that somehow you can establish that in some specific case the upper surface is contributing to 60% of the lift, whatever that means. Would the upper surface still be making 60% of the lift if there was no lower surface? Or if the lower surface had a different shape? (because a different shape in the lower surface will change the pressure distribution in the upper surface). So how much of the 60% contribution of the upper surface is caused by the lower surface?

    In my opinion, nonsense.

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  12. #52
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel
    Then you didn't understand. It's not just the lower surface. It is the whole airfoil (if you are in 2D, or the whole wing in a real 3D wing). The presence of the wing in the air current makes the air do what it does, otherwise it would not meet the necessary conditions of separating at the trailing edge and not penetrating through the wing.
    Let's just contain it to this: there is a wind-tunnel video online, used to refute the 'erroneous theory' of which I've spoken, that sends short bursts of smoke over an airfoil at perfect intervals to show the speed of the airstreams. Clearly, the upper stream accelerates and leaves the trailing edge well before the lower one. What causes this to happen?

  13. #53
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    Let's just contain it to this: there is a wind-tunnel video online, used to refute the 'erroneous theory' of which I've spoken, that sends short bursts of smoke over an airfoil at perfect intervals to show the speed of the airstreams. Clearly, the upper stream accelerates and leaves the trailing edge well before the lower one. What causes this to happen?
    The lower stream is SLOWED because this board is plowing through it and PUSHING it down (and forward just a tad).

    The upper stream is accelerated because a 'vacuum' PULLS it down and back and into the low pressure area that even goes behind the wing.

    I dunno, but would seem to make a little sense.

    I also get the idea of the air arriving at the same time (the fallacy)...one's simple thought process would be "why not"....but as you think a little more, it transitions to why would it?
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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Let's just contain it to this: there is a wind-tunnel video online, used to refute the 'erroneous theory' of which I've spoken, that sends short bursts of smoke over an airfoil at perfect intervals to show the speed of the airstreams. Clearly, the upper stream accelerates and leaves the trailing edge well before the lower one. What causes this to happen?
    The fact that the air is accelerating more around the upper surface than around the lower surface and that this difference is is bigger than the the difference between the length of both paths.

    I know, this answer does not satisfy you, but that's because you (well, not YOU, but you all out there that come with this) are putting the proposition upside down.
    PLEASE READ THE WHOLE COMMENT.

    There is NO CLAIM or assumption in the lift theory that the air going up will leave at any particular time compared to the air going down. Not at the same time, not before, not after.

    The burden of the proof rests on those making the claim. So the the people that expect that the air would leave together at the trailing edge I ask... Why do you expect that?
    - There is no known mechanism that would make this happen, so this is an extraordinary claim, and extraordinary claims require extraordinary evidence (an aphorism created by Carl Sagan and repeated ad-nausum by Bill Nye).
    - So let's go and try it, and as you saw, it fails. That's it, whatever was the reason for that hypothesis, you can forget it. it is FALSE.
    - But even before trying the experiment, if you really think about it, not only that there are no good reasons to expect that this would happen, but in fact it is easy to see that it doesn't need to happen. You can very easily come up with a mental experiment where it is clear that 2 element of air that are flowing one exactly next to the other and are separated, take different path, and their PATHS rejoin later on, will be out of sync.

    Check at this. This is what I myself came up with, well before I attended any Mechanics of Fluids course, when confronted with the notion (found in a book of aerodynamics for pilots that I was reading when doing my PPL) that the particles divided at the leading edge had to rejoin at the trailing edge. Do you think that particles 1 and 2 will meet again at the other end? Now THAT would me magical!!!

    Click image for larger version. 

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    Last but not least. So ok, you found a video that shows how, from the air separating at the leading edge, the air flowing above the airfoil reaches the trailing edge before the air flowing below. Have you asked yourself if that NEEDS to be the case? Have you even consider whether that is just one particular example or a generic rule? Let's explore that.

    Let's take the same airfoil than in the video and let's put it at a negative AoA, negative enough that the airfoil produces a negative lift. What would you expect to happen? If you expect that now the air flowing UNDER the airfoil will reach the trailing edge first, you will be most likely right.

    See? The air flowing above the airfoil doesn't NEED to reach the trailing edge first.
    I know, I know. The better proposition would be that the air flowing on whichever side is the side where the lift force is pointing, will reach the trailing edge first.

    So let's explore that....
    At the first positive angle of attack, generating positive lift, the air flowing over the airfoil reached the trailing edge first.
    At the second negative AoA, generating negative lift, the air flowing under the airfoil reached the trailing edge first.
    This means that there has to be some AoA in between these 2 where the air flowing above and below the airfoil will reach the trailing edge together. Do you agree with that conclusion?
    So let's say that we play with the AoA until we find the one that makes that happen. What do you expect the lift to be at that AoA"? Zero?

    Well, then you are wrong, unless the airfoil is a symmetrical one. Otherwise, the airfoil in that condition will still produce a bit of positive lift. Why? Ok, take what I say with a grain of salt, but just for the sake of visualization, let's say that the air flowing above the airfoil will still have to travel a longer distance than the air flowing under the airfoil due to the longer geometry of the upper surface, and since now both particles of air are taking the same time to make their trips (not because that NEEDS to happen but only because we adjusted the AoA until we found the one that makes it happen), the air flowing above will need to go faster which means lower pressure as per Bernoulli.

    But wait, it gets better.

    Now I have the airfoil making a bit positive lift and with the air flowing above and below reaching together the trailing edge. For sure there will be an AoA smaller but close enough to the current one where the lift will be lower but still a little bit positive. What will happen with the flow going above and below?
    YES SIR: THE LIFT IS POSITIVE YET THE AIR FLOWING BELOW THE AIRFOIL WILL REACH THE TRAILING EDGE A LITTLE BIT EARLIER!!!!

    What? Why? How is is that possible?
    WRONG QUESTION. The lift doesn't depend on which side reaches the trailing edge first. There is no assumption, claim or conclusion regarding that. So why would it not do it?
    Asking that question if "sort of" asking "How can the wing produce positive lift if the name of the pilot is Charles?" It is a nonsensical question.

    The real question is, why would anybody expect that for lift to exist the air flowing above the airfoil should reach the trailing edge at any particular time compared to the air flowing below the airflow? Because whatever that reason is, it is wrong.

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  15. #55
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    The lower stream is SLOWED because this board is plowing through it and PUSHING it down (and forward just a tad).

    The upper stream is accelerated because a 'vacuum' PULLS it down and back and into the low pressure area that even goes behind the wing.

    I dunno, but would seem to make a little sense.

    I also get the idea of the air arriving at the same time (the fallacy)...one's simple thought process would be "why not"....but as you think a little more, it transitions to why would it?
    Exactly. If I had seen your post before writing mine, perhaps I would have just said "Exactly" and wait to see Evan's feedback before posting my comment.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

  16. #56
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Gabriel View Post
    Exactly. If I had seen your post before writing mine, perhaps I would have just said "Exactly" and wait to see Evan's feedback before posting my comment.
    Glad to know I'm not the only one who misses reading things.

    Unfortunately, plants cannot use N2. The actually need NH4...Although NO3 generally transforms quite nicely...but there's this crazy symbiotic bacteria that changes N2 to NH4...you get into all sorts of diffusion dynamics and energy balance...crazy stuff...

    NH4NO3 is also good at going boom when mixed with an oil source.

    And, ATL Crew, the biggest problem with manure is that the NH4 tends to change to NH3 and diffuse off- causing the plants to miss out on much of the nitrogen fertilizer value.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  17. #57
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Gabriel View Post

    Click image for larger version. 

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    Indeed....but why so few humps...seems like three or four would be even better. And how about the carburetor vernturi?
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  18. #58
    Senior Member Evan's Avatar
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    Ok, there are what I feel are a few assumption here:

    Quote Originally Posted by Gabriel
    So let's say that we play with the AoA until we find the one that makes that happen. What do you expect the lift to be at that AoA"? Zero?

    Well, then you are wrong, unless the airfoil is a symmetrical one. Otherwise, the airfoil in that condition will still produce a bit of positive lift. Why? Ok, take what I say with a grain of salt, but just for the sake of visualization, let's say that the air flowing above the airfoil will still have to travel a longer distance than the air flowing under the airfoil due to the longer geometry of the upper surface, and since now both particles of air are taking the same time to make their trips (not because that NEEDS to happen but only because we adjusted the AoA until we found the one that makes it happen), the air flowing above will need to go faster which means lower pressure as per Bernoulli.
    So, are you saying that, at zero AoA, the airstreams depart the trailing edge at the same time?

    Quote Originally Posted by Gabriel
    Now I have the airfoil making a bit positive lift and with the air flowing above and below reaching together the trailing edge. For sure there will be an AoA smaller but close enough to the current one where the lift will be lower but still a little bit positive.
    So, are you saying an AoA below zero (negative AoA) is capable of generating positive lift?

    Quote Originally Posted by Gabriel
    The real question is, why would anybody expect that for lift to exist the air flowing above the airfoil should reach the trailing edge at any particular time compared to the air flowing below the airflow? Because whatever that reason is, it is wrong.
    Because it served to explain how the asymmetrical shape of the wing chord FORCES the air to accelerate IN ORDER to meet that requirement. And the acceleration explains the pressure differential (And, yes, we know this is not true). But, as you pointed out, this science is based on conclusions drawn from observation and deduced logic, and certain problems remain if you go far enough down the path.

    Because the question that remains unanswered (or unsatisfactorily answered) is WHY does the airflow across the top of an assymetrical wing chord accelerate?

    Perhaps this is the answer I'm questing for?

    Quote Originally Posted by 3WE
    The upper stream is accelerated because a 'vacuum' PULLS it down and back and into the low pressure area that even goes behind the wing.
    What is causing this vacuum? The air displacement from the leading edge? The friction along the boundary layer that pulls it back into place?

  19. #59
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    So, are you saying that, at zero AoA, the airstreams depart the trailing edge at the same time?
    Did I say zero? No, I said that if the upper flow wins at one AoA and the lower flow wins at another AoA, there HAS to be some AoA somewhere between these 2 where the 2 flows arrive together. I am not making any claim of what AoA would that be, it will be different from airfoil to airfoil and, while most likely it will be a small AoA, it doesn't need to be and in general it will not be zero (unless it is a symmetrical airfoil). This particular AoA can be found, for any given airfoil, experimentally and theoretically. This is another prediction that the theory is capable to make with very good accuracy.

    So, are you saying an AoA below zero (negative AoA) is capable of generating positive lift?
    No I am not saying that. I am saying that an airfoil that is not symmetrical, at the previously found AoA (that doesn't need to be zero), will typically still generate a bit of positive lift. That means that there will be an AoA even lower than that where the lift will be zero, and any AoA in between these 2 will have positive lift with the lower flow winning the race to the trailing edge.

    That said, I am telling you NOW (well, I've already told you a few posts ago, in the long explanation of airfoils) that, if you define the AoA in the conventional way (the angle between the free stream and the chord line of the airfoil), then yes, most asymmetrical airfoils have positive lift at some smallish negative AoA (or, the AoA of zero lift is negative), because they are cambered, and the camber displaces the lift vs AoA curve upwards.

    Because it served to explain how the asymmetrical shape of the wing chord FORCES the air to accelerate IN ORDER to meet that requirement.
    Yes, but it is a false explanation.

    Because the question that remains unanswered (or unsatisfactorily answered) is WHY does the airflow across the top of an assymetrical wing chord accelerate?

    Perhaps this is the answer I'm questing for?

    What is causing this vacuum? The air displacement from the leading edge? The friction along the boundary layer that pulls it back into place?
    We can have several didactic explanations, none of them completely accurate. 3WE gave you one. I gave you another one (for the one AoA where both flow reach the trailing edge together, the air flowing above will still have a longer path and hence will need more speed to reach together).

    But the accurate explanation may be not satisfactory for you. It is the only thing the air can do to change its constant speed profile of the free stream (with all the molecules moving in straight parallel lines at the same velocity) to a profile that goes around the airfoil and separates at the trailing edge. If the air did anything else it would be violating F=m*a for the individual elements of air, or it would not be going around the airfoil.

    But let me make an analogy (also imperfect). Imagine that you have train tracks follow a straight line but at some point they have to deviate to ago around a mountain and then come back to rejoin the original straight line. Let's say that there is no friction and the track is horizontal so the velocity will remain constant. However, the speed vector will need to change its direction because the train NEEDS to remain on the track, which changes direction. Now you go and measure the force between the train and the track at each point during the path around the mountain and ask... Why is the force precisely what it is at each point? There is no "outside" reason for the force being what it is. It is what it NEEDS to be to remain on the tracks and comply with F=m*a at the same time (and it can be predicted theoretically using F=m*a and compared with the experimental result, and you will find a close match). If the force wasn't exactly what it is at each point along the tracks, the train would either not follow the tracks or violate F=m*a.

    The same happens with the air around an airfoil, with 2 caveats.
    - The force applied on the air for it to deviate around the airfoil is a pressure. But pressure, by it own nature, acts in every direction. You cannot thus, in general, just deviate a flow but keep its velocity constant. The same gradient of pressures that makes it turn around the obstacle will also make it increase or decrease the speed.
    - The train doesn't "know" that there is a curve ahead in the tracks. The air (subsonic) does know that there is an obstacle ahead because the pressure disturbances propagate at the speed of sound. That's why the stream lines react to the airfoil ahead well below reaching the airfoil itself. You have variations of pressures before the air reaches the airfoil. The tracks are actually the stream lines, not the airfoil itself (although for the air actually touching the airfoil, the stream line needs to match the contour of the airfoil when going around it).

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

  20. #60
    Senior Member BoeingBobby's Avatar
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    It's not like this is something new here folks. https://en.m.wikipedia.org/wiki/Bernoulli's_principle

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