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  1. #101
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel
    Evan, I will address several of the points in your latest posts. I tried to be brief but I failed miserably. There was too much to cover and I tried to make longer but simpler explanations than highly condensed super-technical ones.

    But there is ONE SINGLE BIG point that I think is the key, and that is the way that you try to separate the different effects in different components or contributions, when in fact we don't have neither different effects nor different component or contributions. It is one complex, intertwined system where you have variables interacting with each other in a way that cannot be described as a one-way cause-effect and where also different variables are sometimes the expression of the same situation from different points of view which are in fact equivalent and not competing nor collaborating.
    I want you to know that I am absorbing everything you are offering here, as well as everything else I can consider reliable.

    But first, let's take a moment to review the scenario driving my inquiry:

    ----------------------------------------------------

    I'm in middle-row. As if that isn't bad enough, there's a smart, endlessly inquisitive eight-year old future Gabriel next to me in the window seat. And in the aisle seat is a deadheading 50,000 hour airline pilot.

    The kid turns to the pilot and says, "How does the wing work?"

    The pilot tells him (as he learned long ago in his pilot training), that the wing shape creates a pressure differential that causes lower pressure on the top of the wing and so the higher pressure on below the wing pushes it upward.

    "Why?" The kid asks him.

    "Because the curved distance on the top is longer than the less curved distance on the bottom and thus the air must travel faster across the top to rejoin the same air at the wingtip, and that lowers the air pressure above the wing.", the pilot confidently replies.

    After nearly choking on my Vodka and Xanax, I interject, "Uh, no disrespect to your lengthy credentials or your veteran airmanship, but that is actually incorrect".

    "Oh really?", the pilots snaps back at me, "So then, please explain to us the correct answer!"

    The kid looks at me with much suspicion.

    Ok, now I'm in a predicament. If I recite Gabrielesque posts #33 and #95, I'm going to cause the pilot to change his seat and cause the kid to give up hope and we are going to be closing our tray table and returning our seats to an upright position before I get halfway through it.

    I need an answer that can at least compete, in terms of general comprehension and brevity and satisfactory logical appeal, with Mr. 50,000 hour Pilot's answer. It has to be no longer than a short paragraph.

    That is my goal here. You seem to be saying that it can't be done. Your short answer, that the "Wing pushes air down, air reacts pushing wing up" is the equivolent of saying, "Go away kid"; it doesn't provide satisfactory logical appeal. And your more descriptive answers are, let's just say, not brief.

    So maybe it just isn't possible. If that's the case, than so be it. I'm not quite ready to accept that though.

    -------------------------------------------------------------

    A lot of what you wrote here I understand quite well now (Having learned quite a bit since post #5). A few things continue to puzzle me.

    • Circulation

    My—apparently flawed—understanding is that this 'circulation' is a closed-loop, bound vortex, which exists to satisfy the Kutta Condition by providing a force that will relocate the rear stagnation point from the upper wing surface to the tip of the trailing edge, and that this vortex is the product, via the Magnus Effect, of another initial 'starting' vortex that is created aft of the wing when the wing first begins to push through the air (or with changes in AoA or airspeed). Now, I visualize this 'bound' vortex in my mind as being a clockwise (when viewed with the leading edge to the left) flow above the boundary layer that mixes with, and thus alters the velocity of, the main oncoming airstreams. (One question this raises in my mind is how a closed loop vortex as described can turn around the trailing edge).

    Now, in terms of chicken-and-egg (where does the cycle start, hence, how can it be explained cyclically) The—apparently flawed—order of events I gleaned from Denker is:

    1. Airfoil is stationary in still air
    2. Airfoil begins to move forward / airflow is introduced to airfoil - stagnation point is on the upper aft wing surface / causing cleaved airflow to produce an inital 'starting' vortex aft of the wing.
    3. Airflow increases / 'starting' vortex induces an opposing 'bound' vortex encircling the wing / 'bound' vortex + airflow = the amount of force necessary to relocate the stagnation point to the trailing edge / lift is now possible / 'starting' vortex is left behind.
    4. Bound vortex produces forces that a) increase the airflow speed across the top and b) reduce the airflow speed across the bottom (note the diagram with the black line showing where undisturbed air would have reached vs the upper and lower 'cleaved' airstreams from the airfoil. The upper flow is has been sped up but the lower flow has been slowed down.)
    5. And from there, Bernoulli and Newton take over, and increases in AoA restart the cycle to produce a stronger 'bound' vortex circulation, thus a more pronounced pressure differential and thus more lift.


    Is that way off?

  2. #102
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Is that way off?
    Yes!!! Again you are taking individual concepts that are ok and making a fruit salad of them. So some loose ideas:
    - I am repeating myself here but the circulation is a speed profile, not a force. Only pressure gradients can make the air accelerate and bed its path.
    - The air doesn't has upwash, downwash, accelerates above and slows down at the bottom BECAUSE of circulation Circulation is THE NAME THAT WE GAVE TO this distribution of speeds.
    - The circulation is NOT a vortex. The circulation is the distribution of speeds around a vortex. A vortex has rotor. Around the vortex however the rotor is zero ("rotor" is a vectorial magnitude of a vectorial function (like a speed field), it can be understood as how fast would a ball rotate around its own axis if you hold it in one position (not free to move but free to rotate). Around a 2D airfoil (or around a spinning cylinder), outside of the boundary layer, the rotor is zero.
    - The bound vortex is not induced by the starting vortex, they are created simultaneously because vorticity shall be conserved (like energy or momentum).
    - In a 2D airfoil (that can be though as a wing of infinite wingspan) there is no starting vortex. The vortex is considered to continue infinitely along the infinite span and you never see the vortex bending back near the wingtip and joining back at the starting vortex. You can think that the wingtips and the vortex ends meet in the infinity (think that they form a circle of infinite diameter) and such a closed vortex has voticity zero (like a smoke ring) so you are not creating or destroying vorticity by increasing or reducing the lift. But you don't have "one vortex". You have a system of vortexes that add up to a finite vorticity (finite in 2D, it becomes zero if you extend it to infinity) and that can be replaced by a single vortex that will work very well to model the lift force but not to model the lift around the airfoil.
    - In a 3D wing, the situation is much more complex The figure that I think you have in mind, like a long rectangle where one off the short sides is basically the wing (or the bound vortex), the other short side is the starting vortex, and the long sides get longer and linger as the wing moves forward (with the bound vortex) and the starting vortex remains still is a gross simplification, useful for some things, but not to describe in detail the flow of air around a wing.
    - Both in the 2D and 3D versions, if you use the single vortex model, you can explain and measure accurately many things including lift, induced drag, induced angle of attack in the tail, ground effect... but you will have air flowing through the walls of the airfoil. Reality is more complicated than "a vortex". If you want I can go to the more complete explanation.
    - In any event, vortexes don't occur in potential flow because the rotor in potential flow is always zero (but potential flow ha solutions for cortices introduced artificially in the model as singularities, and you don;t even need to establish the vortex itself, you can establish the circulation caused by it, like with the Kutta condition). In reality, all the vorticity (not the circulation) happens in regions where the viscosity cannot be neglected and hence the flow cannot be modeled as potential, like the boundary layers and the core of the vortex systems (like the core of the wingtip vortexes). That is complicated to do (it can be done, computers do it all the time) so it is useful to artificially out vortexes here and there as singularities in the potential flow model, with that vortexes having a similar effect to the vorticity that takes place in those regions.
    - And now you want to deal with non-stationary aerodynamics? With an airfoil that is not moving steady in a steady stream of air but that is changing airspeed an d angle of attack? And that will make the explanation more clear?

    Evan, before trying to explain the lift it terms to your satisfaction, please try to explain to me, in terms to your satisfaction, why does the tracks of the train in a curve make the exact force that they make on the train?

    And by the way, even if your explanation was right, I don't see how that would satisfy neither the pilot nor the 8 years old.

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  3. #103
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Yes!!! Again you are taking individual concepts that are ok and making a fruit salad of them. So some loose ideas:
    - I am repeating myself here but the circulation is a speed profile, not a force. Only pressure gradients can make the air accelerate and bed its path.
    - The air doesn't has upwash, downwash, accelerates above and slows down at the bottom BECAUSE of circulation Circulation is THE NAME THAT WE GAVE TO this distribution of speeds.
    - The circulation is NOT a vortex. The circulation is the distribution of speeds around a vortex. A vortex has rotor. Around the vortex however the rotor is zero ("rotor" is a vectorial magnitude of a vectorial function (like a speed field), it can be understood as how fast would a ball rotate around its own axis if you hold it in one position (not free to move but free to rotate). Around a 2D airfoil (or around a spinning cylinder), outside of the boundary layer, the rotor is zero.
    - The bound vortex is not induced by the starting vortex, they are created simultaneously because vorticity shall be conserved (like energy or momentum).
    - In a 2D airfoil (that can be though as a wing of infinite wingspan) there is no starting vortex. The vortex is considered to continue infinitely along the infinite span and you never see the vortex bending back near the wingtip and joining back at the starting vortex. You can think that the wingtips and the vortex ends meet in the infinity (think that they form a circle of infinite diameter) and such a closed vortex has voticity zero (like a smoke ring) so you are not creating or destroying vorticity by increasing or reducing the lift. But you don't have "one vortex". You have a system of vortexes that add up to a finite vorticity (finite in 2D, it becomes zero if you extend it to infinity) and that can be replaced by a single vortex that will work very well to model the lift force but not to model the lift around the airfoil.
    - In a 3D wing, the situation is much more complex The figure that I think you have in mind, like a long rectangle where one off the short sides is basically the wing (or the bound vortex), the other short side is the starting vortex, and the long sides get longer and linger as the wing moves forward (with the bound vortex) and the starting vortex remains still is a gross simplification, useful for some things, but not to describe in detail the flow of air around a wing.
    - Both in the 2D and 3D versions, if you use the single vortex model, you can explain and measure accurately many things including lift, induced drag, induced angle of attack in the tail, ground effect... but you will have air flowing through the walls of the airfoil. Reality is more complicated than "a vortex". If you want I can go to the more complete explanation.
    - In any event, vortexes don't occur in potential flow because the rotor in potential flow is always zero (but potential flow ha solutions for cortices introduced artificially in the model as singularities, and you don;t even need to establish the vortex itself, you can establish the circulation caused by it, like with the Kutta condition). In reality, all the vorticity (not the circulation) happens in regions where the viscosity cannot be neglected and hence the flow cannot be modeled as potential, like the boundary layers and the core of the vortex systems (like the core of the wingtip vortexes). That is complicated to do (it can be done, computers do it all the time) so it is useful to artificially out vortexes here and there as singularities in the potential flow model, with that vortexes having a similar effect to the vorticity that takes place in those regions.
    - And now you want to deal with non-stationary aerodynamics? With an airfoil that is not moving steady in a steady stream of air but that is changing airspeed an d angle of attack? And that will make the explanation more clear?

    Evan, before trying to explain the lift it terms to your satisfaction, please try to explain to me, in terms to your satisfaction, why does the tracks of the train in a curve make the exact force that they make on the train?

    And by the way, even if your explanation was right, I don't see how that would satisfy neither the pilot nor the 8 years old.
    Rabbit hole...

    Help me understand this sentence:

    Then imagine that a headwind springs up, a steady overall wind blowing in the nose-to-tail direction (left to right in the figure), giving the parked airplane some true airspeed relative to the airmass as a whole. At each point in space, the velocity fields will add. The circulatory flow and the airmass flow will add above the wing, producing high velocity and low pressure there. The circulatory flow will partially cancel the airmass flow below the wing, producing low velocity and high pressure there.
    Denker is clearly referring to two separate things here: the airmass flow and the circulatory flow. I assumed by 'add' he meant force. But you say "circulation is a speed profile, not a force". What is he adding here? Mass? Or what...does...he...mean?
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  4. #104
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Rabbit hole...

    Help me understand this sentence:



    Denker is clearly referring to two separate things here: the airmass flow and the circulatory flow. I assumed by 'add' he meant force. But you say "circulation is a speed profile, not a force". What is he adding here? Mass? Or what...does...he...mean?
    He is adding speed vectors. As stated there: At each point in space, the velocity fields will add

    Note that this is a mathematical model. In reality, you don't have one flow that turns around the trailing edge (top) and another flow that circulates in closed paths around the airfoil -including around the trailing edge but int he opposite direction- (mid) which meet and combine to generate a combined flow that, oh by chance, leaves the airfoil smoothly at the trailing edge. (bottom). The flows in the top and mid and mathematical constructions, and are impossible in practice because they have singularities in the trailing edge with infinite speed and zero pressure that a real fluid cannot achieve. The top is what the potential flow model predicts. Pure potential flow, without imposing the Kutta condition. The mid is the potential flow of the circulation that we add to generate the Kutta condition that we know happens in reality. The only real flow is the bottom one (and that is still an approximation, a very good approximation outside the boundary layer, that cannot be modeled by the potential flow).

    (wow, that was short )

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  5. #105
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    He is adding speed vectors. As stated there: At each point in space, the velocity fields will add

    Note that this is a mathematical model. In reality, you don't have one flow that turns around the trailing edge (top) and another flow that circulates in closed paths around the airfoil -including around the trailing edge but int he opposite direction
    So this is what is confusing to me. Also, the NASA site seems to refer to an interaction of a separate 'induced' 'circulatory' flow (a bound vortex) and a free stream flow:

    Quote Originally Posted by NASA
    So the free stream flow over the top of the cylinder is assisted by the induced flow; the free stream flow below the cylinder is opposed by the induced flow.
    This is in reference to a spinning cylinder:

    https://www.grc.nasa.gov/www/k-12/airplane/cyl.html

    But is then conformally mapped as a 'bound vortex' onto an airfoil shape:

    https://www.grc.nasa.gov/www/k-12/airplane/shed.html

    And that last link shows the interaction as required by the conservation laws (Navier Stokes) to achieve a net vorticity of zero between the bound vortex and the shed (trailing) vortex.

    When I have time I just have to go back and bang my head on all this a bit more. There has to be a concise conceptual explanation hidden behind all this complexity...

  6. #106
    Senior Member Evan's Avatar
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    By the way, this is NASA's simple child-friendly explanation of how an airfoil works. The problem is that it does not actually explain how the wing shape makes the air move faster (or how a barn door does):

    Quote Originally Posted by NASA
    Airplane wings are shaped to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air.
    If you click on the word lift, it gives you this:

    Quote Originally Posted by NASA
    For a body immersed in a moving fluid, the fluid remains in contact with the surface of the body. If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body.
    And this is found on numerous pages of NASA's site:

    Quote Originally Posted by NASA
    Lift is a force generated by turning a flow.
    Can we combine the three and arrive at a simple explanation?

    Well, here's what NASA has to say about that:

    Quote Originally Posted by NASA
    The real details of how an object generates lift are very complex and do not lend themselves to simplification.

  7. #107
    Senior Member 3WE's Avatar
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    The real details of how an object generates lift are very complex and do not lend themselves to simplification
    Other than that "the largely flat (but nicely streamlined shape and somewhat stall-improving shape) is angled so as to shove air downwards", I'm ok with it and was ok with it many pages ago.

    I am guessing you still are not.

    By the way- what is it that is shoving the plane forward when it's gliding with the engines at idle (or not available in a true glider)
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  8. #108
    Senior Member Evan's Avatar
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    Quote Originally Posted by 3WE View Post
    Other than that "the largely flat (but nicely streamlined shape and somewhat stall-improving shape) is angled so as to shove air downwards", I'm ok with it and was ok with it many pages ago.
    This is a terrible explanation of how an airfoil creates lift.

  9. #109
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    This is a terrible explanation of how an airfoil creates lift.
    Discouncur...a flat surface angled to shove air downwards is an incredibly good description of a very powerful airfoil.

    You might talk to some folks in Alabama about the recent performance of some really nasty draggy and frequently flat airfoils- many of which might exceed critical AOA's and be stalled with respect to smooth airflow....All at unremarkable airspeeds comparable to most commercial aircraft. Very significant levels of lift were achieved.

    https://www.youtube.com/watch?v=Ld6fAO4idaI
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  10. #110
    Senior Member Evan's Avatar
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    Quote Originally Posted by 3WE View Post
    Discouncur...a flat surface angled to shove air downwards is an incredibly good description of a very powerful airfoil.

    You might talk to some folks in Alabama about the recent performance of some really nasty draggy and frequently flat airfoils- many of which might exceed critical AOA's and be stalled with respect to smooth airflow....All at unremarkable airspeeds comparable to most commercial aircraft. Very significant levels of lift were achieved.

    https://www.youtube.com/watch?v=Ld6fAO4idaI
    Ok, Gabriel has had it up to here with me. I'm continuing to unravel the mystery of 'circulation', as some of the other sources I am finding seem to be at odds with post #102 (which is not to say it is inaccurate, at least not at this point).

    But thus far, my observation is:
    1. An easily understandable and fairly complete explanation of how an airfoil generates lift does not seem to exist.
    2. The most popular theory outside of the 'expert' community is the incorrect one about a pressure differential due to the streaming molecules rejoining at the same time at the end of the airfoil.
    3. That theory, while incorrect, gets across the most important thing: lift is caused by pressure differentials that are the product of the airfoil shape moving through the airstream.
    4. So, since most people don't really seem to care about the 'whys' involved, that explanation is probably sufficient for most people, including most commercial airline pilots.
    5. I would still like them to have a better one.
    6. NASA says that the real details of how an object generates lift are very complex and do not lend themselves to simplification. And they went to the moon, so they might be right.

  11. #111
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    So this is what is confusing to me. Also, the NASA site seems to refer to an interaction of a separate 'induced' 'circulatory' flow (a bound vortex) and a free stream flow:



    This is in reference to a spinning cylinder:

    https://www.grc.nasa.gov/www/k-12/airplane/cyl.html

    But is then conformally mapped as a 'bound vortex' onto an airfoil shape:

    https://www.grc.nasa.gov/www/k-12/airplane/shed.html

    And that last link shows the interaction as required by the conservation laws (Navier Stokes) to achieve a net vorticity of zero between the bound vortex and the shed (trailing) vortex.

    When I have time I just have to go back and bang my head on all this a bit more. There has to be a concise conceptual explanation hidden behind all this complexity...
    There is one important difference between the spinning cylinder and the Joukowski airfoil: The spinning cylinder is... spinning. The airfoil is not. Let me quote some other material I found in the Internet:

    Quote Originally Posted by #23
    This is a mathematical result that you obtain by making some assumptions and then calculating how the flow would behave around a cylinder exposed to a transversal wind, and how that flow changes and how a force perpendicular to both the cylinder and the airfoil increases, when the cylinder rotates along its axis at increasing speed, and then make a coordinates transformation to make this cylinder look like an airfoil (Google Joukowsky airfoil or Joukowsky transform). And this mathematical results very closely match reality, which is surprising since some of the assumptions are quite crazy and even contradictory. For the model to work you need to consider that there is no viscosity (the formulas are for what is called potential flow) but that there is viscosity (for the rotating cylinder to drag the air around with it).
    Quote Originally Posted by #33
    The air will just do what it needs to do. [...]. It needs to separate at the trailing edge [...] This is is a tricky [assumption], it is something that doesn't come automatically in the equations of potential flow but that you have to "impose" in the model it is called the Kutta condition, super-genius-aerofluodynamicist-of-the-past-millennium took a while to figure this out and, without this condition, lift simply cannot exist. In the same way as the stagnation point where the flow splits may not be exactly at the leading edge but a bit behind on the lower surface, the potential flow model predicts that the "re-joining" point will happen not at the trailing edge but a bit ahead of it on the upper surface, with the air going from below to above around the trailing edge, and no force shows up in the model. This led to several super-ge...-past-millennium to assert that heavy-than-air flight was not possible, as if gliding birds did not exist. Remember I said that they were working with the potential flow model, which is an ideal flow with no viscosity. And this trailing edge separation just would not happen in a potential flow, unless you impose it. In reality, the Kutta condition happens because under the potential flow the air would accelerate to infinite speeds when turning around the sharp trailing edge, which would not only violate Einstein's speed limit but also breaks the approximation of no viscosity. With such huge speed gradients viscosity becomes non-negligible-at-all, the air cannot accelerate to the infinite speed required to turn around the trailing edge, and separates instead. In the potential flow model the Kutta condition is obtained by adding circulation, which is like having the airfoil contour made of "treadmill" and having that band "circulating" around the airfoil. Then you start the treadmill and speed it up more and more until the separation happens at the trailing edge, and now you have a mathematical model that closely matches the flow around the real airfoil. Circulation is what makes lift. Remember the model of the cylinder spinning that I talked before?
    NASA also says (in your second link): "The flow turning that occurs in the creation of lift also creates bound vorticity within the airfoil."

    Note how they say that the flow turning CREATES the bound vorticity. Not the other way around. I am not endorsing that statement, but also I don't endorse you statement that the bound vortex creates the lift and flow turning.

    In my view, it is the Kutta condition (in the mathematical model of the potential flow) which creates all the field of pressures and speed vectors (including the vorticity, the turning of the flow, and what not). That's why, in the Joukowski transform, you add vorticity exactly as needed for the separation to happen in the trailing edge. This is because, in reality (not modeled by the potential flow) the fluid DOES separate at the trialing edge because it is NOT potential (i.e. there is viscocity, which cannot be neglected when the air tries to turn at infinite speed around the sharp trailing edge, the flow cannot sustain that, and it separates). So it is the viscosity which causes the air to separate at the trailing edge, and that separation causes all the rest at once (the full pressures and speeds fields). In the spinning cylinder, it is also the viscosity that we neglect what causes the circulation. Because if there was no viscosity the cylinder would spin happily without dragging any air around and for the air it would be the same whether the cylinder is spinning or not. So as you can see, both for the airfoil and the spinning cylinder, viscosity is what causes the circulation and lift. No viscosity = no circulation and no circulation = no lift. The genius thing is that the old guys came about a way to still use potential flow even when they could not neglect viscosity, which was done by summarizing the effect of the viscosity as the Kutta condition, imposing that condition to the model, and forgetting about viscosity.

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  12. #112
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Ok, Gabriel has had it up to here with me. I'm continuing to unravel the mystery of 'circulation', as some of the other sources I am finding seem to be at odds with post #102 (which is not to say it is inaccurate, at least not at this point).
    Please expose the contrast. It can be just an apparent contrast, or I might have explained myself not clearly, or the other explanation is wrong, or my understanding is wrong (in which case I would love to fix it).

    But thus far, my observation is:
    1. An easily understandable and fairly complete explanation of how an airfoil generates lift does not seem to exist.
    2. The most popular theory outside of the 'expert' community is the incorrect one about a pressure differential due to the streaming molecules rejoining at the same time at the end of the airfoil.
    3. That theory, while incorrect, gets across the most important thing: lift is caused by pressure differentials that are the product of the airfoil shape moving through the airstream.
    4. So, since most people don't really seem to care about the 'whys' involved, that explanation is probably sufficient for most people, including most commercial airline pilots.
    5. I would still like them to have a better one.
    6. NASA says that the real details of how an object generates lift are very complex and do not lend themselves to simplification. And they went to the moon, so they might be right.
    1. Fair. Compared to an explanation of what other phenomenon that is at the same time easily understandable and fairly complete? Why don;t you try this?
      Evan, before trying to explain the lift it terms to your satisfaction, please try to explain to me, in terms to your satisfaction, why does the tracks of the train in a curve make the exact force that they make on the train?
      I asked you this question several times. I would really like to have your answer.
    2. Yes, the most popular explanations of may things is wrong. It is a shame indeed.
    3. Yes, but that is, in my opinion, a lame excuse to tech it or use it (if anybody uses this "utility" excuse).
    4. No additional comment.
    5. I agree.
    6. (Psts! Sending rockets to the Moon has almost nothing to do with the generation of lift) Other than that, go back to 1.


    It looks to me that you resist the idea that, once you (you = reality) impose the conditions of impenetrability of the wing and the separation in the trialing edge, then there is just nothing else that the air can do other than what ti does in other for the individual parcel of air to comply with F=m*a. That's why I would like to see how you explain the force that the rails make on a train in a curve without resorting to something similar to that.

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  13. #113
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    It looks to me that you resist the idea that, once you (you = reality) impose the conditions of impenetrability of the wing and the separation in the trialing edge, then there is just nothing else that the air can do other than what ti does in other for the individual parcel of air to comply with F=m*a. That's why I would like to see how you explain the force that the rails make on a train in a curve without resorting to something similar to that.
    I don't resist the idea. I don't even think it's an idea. It seems to me to be a statement of fact. The train must follow the rails because they are there, But is it an explanation of how and why?

    I don't know much about vorticity but I understand that Stokes requires a 'starting vortex' must have an equal and opposite one, which is the 'bound vortex', which is the circulation, and that, while these vortices are created simultaneously, the 'starting vortex' appears to be the result of initial air movement across the airfoil where the rear stagnation point IS on the upper aft surface of the airfoil and is moved toward the trailing edge. I also know the the 'starting vortex' is an physical thing (not just a mathematical expression) because it has been observed. So logic tells me that the 'bound vortex' is also a physical thing, apart from, but added to, the main airsteam flow, to produce the actual resultant flow of air about the airfoil.

    I don't know much about vorticity, but I know a 'bound vortex (the vortex-line runs spanwise and the airfoil is entirely enclosed by the vortex)' must flow around both stagnation points and around both the leading and trailing edges of the airfoil. We know the main airstream flow cannot do this. So I interpret this to mean the bound vortex is a separate, turbulent air disturbance occurring far enough from the boundary layer to allow it to flow around the entire airfoil and is altering the speed vectors of the main airstream flow due to viscosity, thus increasing the speed of the upper flow and diminishing the speed of the lower flow.

    That's the best sense I make of it at this point.

    Read the 'circulation' part of this link:

    http://www.boundvortex.com/ReadArtic...x?ArticleID=53
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  14. #114
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  15. #115
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    Quote Originally Posted by Evan View Post
    I don't resist the idea. I don't even think it's an idea. It seems to me to be a statement of fact. The train must follow the rails because they are there.
    That was not the question. I am taking as a given that the train follows the track. Trains follow tracks, that's what trains do. Air doesn't penetrated aluminum or composite but flows around them, that's what air does.

    But remember your original question, what causes the exact speeds and pressures distribution in the vicinity of an airfoil that makes, for example, that parcels flowing above the airfoil reach the trailing edge not together (as the common misconception has it) but earlier. I was wondering how would you explain something similar but easier and more tangible, to a level of completeness, accuracy, easiness and graspability that meets the same standards you expect for such an explanation of the generation of lift. The question was:

    Evan, before trying to explain the lift it terms to your satisfaction, please try to explain to me, in terms to your satisfaction, why do the tracks of the train in a curve make the exact force that they make on the train?
    Would you attempt to do that?

    I don't know much about vorticity but I understand that Stokes requires a 'starting vortex' must have an equal and opposite one, which is the 'bound vortex', which is the circulation, and that, while these vortices are created simultaneously, the 'starting vortex' appears to be the result of initial air movement across the airfoil where the rear stagnation point IS on the upper aft surface of the airfoil and is moved toward the trailing edge. I also know the the 'starting vortex' is an physical thing (not just a mathematical expression) because it has been observed. So logic tells me that the 'bound vortex' is also a physical thing, apart from, but added to, the main airsteam flow, to produce the actual resultant flow of air about the airfoil.
    First of all, let me state that I am reaching the muddy boundaries of my knowledge on the subject. I knew about the starting vortex, and its existence (including its creation whenever the angle of attack changes) is even used in the equations of dynamic longitudinal stability and control. But I don't remember having read or having being taught the details of how the starting vortex is created, and I never look it up by myself. Not that it's not interesting, but the question never occurred to me. I guess that was satisfied with just the conservation thing. I knew that vorticity has to be conserved and I knew that the airfoil needed vorticity to generate lift and that the model of the airfoil with the needed vorticity to enforce the Kutta condition matches the fields of pressures and speeds extremely well. That was enough for me I suppose.

    As an analogy, have you ever wondered classic example of the ice skater spinning and increasing the angular speed as he brings his body vertical and his arms and legs closer to the body? Easy, right? Conservation of angular momentum. But conservation of angular momentum is not a force, or a moment. Take a simpler example of the skater spinning with his body already vertical but his arms extended horizontally, as he brings his arms to the body, he spins faster due to conservation of angular moment: as the moment of inertia goes down, the angular speed goes up to keep the product of both constant. But wait a second... His head has not conserved the angular moment. It has the same moment of inertia than before, and now it is spinning faster, it has increased its angular momentum. The same than all his body except his arms. The conservation of angular momentum cannot explain that by itself. There has to be some mechanism, with forces and moment, that explains this. And there is, and it is not that hard to explain, but it is not easy either. Something similar happens with the prcession (the classing experiment of the bicycle wheel spinning and hanging only from one end of the axis). In this case it can be easily explained with variation of the angular momentum. But again, that by itself doesn't explain how 2 individual VERTICAL forces (the weight of the wheel and the tension in the cable from where the wheel is hanging) make the wheel and its axis accelerate and displace horizontally. And in this case the explanation is not so easy. I analyzed it by myself, it is something that it is not taught, and I don't remember it anymore, but I do remember that it took me analyze the force on the simplest possible wheel (a massless but rigid T with 2 dimensionless masses on the tips of the horizontal bar) to figure it out. I am confident that there is no simple, accurate, accessible and complete explanation for that.

    Do we need to go to the individual particles of air to explain the deep mechanics of the generation of the starting vortex? Maybe, I don't know.

    The flow separating ahead and above the trialing edge is a nice start (pun not intended). But why does that happen? I don't know, but perhaps this is a hint (from your latest link):

    Photographs of the airflow over a wing made visible by such techniques as smoke show that the initiation of lift creates the starting vortex, just as the theory shows.
    Wait a second... the starting vortex induces the bound vortex which is responsible for the lift whose initiation creates the starting vortex? Isn't that kinda' circular?

    I don't know much about vorticity, but I know a 'bound vortex (the vortex-line runs spanwise and the airfoil is entirely enclosed by the vortex)'...
    Caution here, little details matter at this level. The airfoil is not enclosed by the vortex, it is enclosed by the circulation that RESULTS from that vortex. Actually, from one of your NASA links (emphasis is mine):

    The flow turning that occurs in the creation of lift also creates bound vorticity within the airfoil. For a general shaped airfoil, there is some distribution of vorticity which we can think of as small vortices. For the simple Joukowski airfoil, shown in this figure, there is a single vortex present at the center of the generating cylinder. The flowfield from this generating cylinder has been conformally mapped into the airfoil, but the vorticity has been maintained.
    I should add that, a property of any conformal mapping or transform, any originally closed curve remains closed and point inside a closed curve in the original coordinates remain inside the conformed closed curve, which is to say, the single vortex remains inside the airfoil (don't quote me on this but I think it was on the mean line at 25% of the chord). I will come back to this. Now let's return to your interrupted quote.

    [I know a 'bound vortex'...] must flow around both stagnation points and around both the leading and trailing edges of the airfoil. We know the main airstream flow cannot do this.
    What exactly does that mean? The "main airstream flow" far ahead of the airfoil is a flat profile of constant and parallel speeds and it would remain as such if it wasn't for the airfoil. It would not even generate an airfoil shape or any stagnation points without the airfoil. It would just flow through the imaginary contour of the airfoil. Now, saying that the main airstream flow, in the presence of the airfoil, would move around the airfoil but cannot do the circulation is... let me think the right adjective, ridiculous. What else could the main airstream do Not separate at the trailing edge and turn around it at infinite speed? The presence of the airfoil imposes the 2 conditions: impenetrability of the wall and Kutta condition.

    So I interpret this to mean the bound vortex is a separate, turbulent
    Wow wow wow.... Caution again with the terms. Turbulent? That is a totally different animal. Even WITH viscosity and a boundary layer, you don't need turbulence to explain lift or any of the vortexes. Turbulence is unsteady and chaotic fluid movement, unpredictable and almost indescribable except in statistical terms. A nice, well defined vortex and the circulation it generates is not turbulent at all. Please please PLEASE leave turbulence at a side. That IS a part of fluid dynamics that is not fully understood or very well modeled by pure theoretical models (you need to bring experimental results to simulate it). And lift can happen in perfectly laminar flows.

    So I interpret this to mean the bound vortex is a separate air disturbance occurring far enough from the boundary layer to allow it to flow around the entire airfoil and is altering the speed vectors of the main airstream flow due to viscosity, thus increasing the speed of the upper flow and diminishing the speed of the lower flow.
    Where would you put the center of the bound vortex so that the disturbances induced by it are far enough from the boundary layer but in a way that it induces the speeds that it does induce? Also, the speed contribution credited to the bound vortex is highest on the first streamline next to the airfoil and reduces as you move away from the airfoil. How do you explain that? In the flow created by a vortex, the speed increases as you near the center.

    The mathematical models that use potential flow model the flow of air not only AROUND the airfoil, but also INSIDE it. And the bound vortex is not outside the airfoil, it is inside it. Do you think that that is real? It isn't. Now, the circulation IS real, the vorticity IS real, and it IS equivalent to having a vortex inside the airfoil. Not only that, but in fact, except in the simplest model of the Joukowski airfoil (which is based on a spinning cylinder), there is not ONE bound vortex but a system of vortexes or, more properly, as NASA said, a distribution of vorticity (you can think of it as infinite bound vortexes all infinitely small band infinitely close one from the other in a way that you have a vorticity density distribution, pretty much in the same way that you can think of the pressure acting on the airfol as infinite many infinitely little forces infinitely together).

    Again, in summary, my position on the subject is not that one thing causes the other (things = vortexes, circulation, speeds, pressures). They are all caused by the physical restrictions imposed by the presence of the airfoil (impenetrability and impossibility to turn around the trailing edge).

    --- 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. #116
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    Default An important aereoknotikal breakthrough...

    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    Well I'm confused. That image could be one of two things:
    1) An illustration of why a cheeseburger provides no lift at an AoA of approximately zero.
    2) An experiment by a large restaurant chain to find an efficient means of adding smoke flavor to food.

    Please clarify.
    Be alert! America needs more lerts.

    Eric Law

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