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Thread: A photograph of Boeing Bobby discovered on the WWW

  1. #21
    Senior Member 3WE's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Actually, just about the same.
    Unfortunately you are correct as my strategy to the cockpit rests mostly on the bad-fish scenario...nevertheless, you have never seen my stick and rudder skills and in my cockiness, I understand automatic braking systems as well as the maximum available hydraulic PSI, and do not see myself doing 38,000-ft nor localizer-capturing Q-400 stalls...I'm thinking it could be done.
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  2. #22
    Senior Member 3WE's Avatar
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    Quote Originally Posted by elaw View Post
    Allow me: it's mágico.
    Caultrons perhaps?
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  3. #23
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    Please explain in plain English OR Espanol BUT NOT Engineer-speak how shoving air up over the front of a wing (and having a bigger hump on top) adds lift.
    Maths would be shorter, but ok, you requested English so English you will have.

    Take ANY airfoils of the same chord (with big, small or no hump, with things protruding to the front or not), increase the AoA by 1 degree, and the lift will increase by the same amount.
    Let me be even more clear:
    Any airfoil will have an AoA where the lift is zero. Let's define that that AoA is zero degrees (i.e. let's start to count the angle of attack from the zero lift reference).

    Take a 727 airfoil in landing config. The basic airfoil has a complicated profile, with a "nose shape"in the front, a bigger hump above, a smaller hump below, and a sharp edge in the back. But on top of that we have a slat (mix is slot and flap but used only for leading edge devices) extended in the front and a very complex triple-slotted Fowler flap extended in the trailing edge. There will be one AoA for this airfoil where the lift is zero (a certain "nose down" angle). Hold it there.

    Take a "flat thin plywood" airfoil with he same chord. There will be one AoA where the lift is zero for this airfoil too (as for any Symmetric airfoil, this zero-lift AoA will be with the mean line, or the line in this case, parallel to the free stream flow). Hold it there.

    Now take both airfoils and increase the AoA by say 5 degrees. Both airfoils will go from zero lift to exactly the same non-zero lift.

    How much the lift coefficient of an airfoil increases with the AoA is the same for every airfoil (as long as it is away from the stall, i.e. flow separation, and they are quite more long than they are thick).
    Not only that, but if you measure the angle of attack in radians, the coefficient of lift increases by 2*Pi per radian of AoA. (see note [1])

    So what's the fuzz with camber, leading edge radius and camber? (the 3 things that give the airfoil its shape and humps). Why don't we stick with the flat thin plywood?
    Well, 3 things:
    - Maximum AoA and stall behavior.
    - Minimun drag and lift at which the minimum drag happens.
    - Room to put stuff (spars, mechanisms, fuel...)

    You'll see, fluids don't like flowing around tight corners smoothly, they tend to separate (that's bad for the leading edge because stall, and good for the trailing edge because if they didn't we would not have lift).

    Let's take our plywood and put it at a small AoA. You will see a couple of things:
    - The stagnation point (the point on the plywood contour that divides the air that will flow over from the air that will flow under the airfoil) is not at the trailing edge but a little behind it on the lower side.
    - That means that the flow that goes over the airflow will need first to move a little bit to the front on the underside and then go around the leading edge before being able to move over the upper side. But the leading edge is sharp and what did we say about the fluid going around sharp corners? The fluid will separate at the leading edge, even at very small AoAs. Stall? Nope, the fluid will re-adhere to the top surface a little bit after the leading edge, creating a "bubble" of separated flow (by the way, and I don't want to go into the details, I don't know if you can visualize this but this bubble will be a low pressure zone, and it is above the stagnation point that will be a high -highest in fact- pressure zone, low pressure above, high pressure below... did anybody say lift?). As you increase the AoA this bubble will grow until it is no longer stable and suddenly you will have separation over the entire upper surface. Now yes, stall. And it will happen not a very small AoAs but yes at AoAs that are much smallers that we used to when we think of "normal" airfoils stalling.

    Let's "bend" the leading edge down so the stagnation point happens exactly at the leading edge and we have a radius that the airflow can contour without separating. That is great and it works, but it works perfectly only at one specific AoA. If you are at a higher AoA, the stagnation point will still happen behind the leading edge so the air will still have to go around the still sharp leading edge and will separate, but all this will be delayed and the stall will happen at a higher AoA. And if the AoA is lower than the optimum one you will have the bubble in the underside behind the bent-down leading edge, which is not a big problem because under positive lift this bubble will always re-adhere do to the increased pressure on the underside, but on negative AoAs / lift it will stlal sooner than the straight plain airfoil.
    A byproduct of this. What is the AoA now? If we measure it taking the flat part as a reference, nothing changes. If we measure it taking as reference the straight line that connects the now dropped leading edge with the trailing edge (which is how the AoA of an airfoil is measured, just by convention), we can see that all the flat part behind the trailing edge is at an angle and that the air will be deflected down. So we have lift at zero AoA. It not a symmetrical airfoil anymore. We have "camber" (the max distance between the chord line and the mean line of the airfoil).
    So a couple of lessons learned here:
    - Curving the leading edge down increases the max AoA and max lift that the airfoil can sustain without stalling (but it will stall sooner with negative AoAs)
    - Camber increases the lift at zero AoA.

    Note, also, that this shape creates a hump on the top but a sag in the bottom.

    What can we do better than just bending the leading edge down, which works especially well only for one specific AoA?
    What if we make a round leading edge? SO the air will NEVER encounter a sharp edge to go around no matter where the stagnation point is?
    Great. Now I hope that it is intuitive that having an airfoil that looks like a pencil with a ping-pong ball on the end is not a good idea. It's much better to take half circle (let's say the left half, like an inverted D) and continue with horizontal tangent lines at the top and bottom than then smoothly curve towards each other and meet in a sharp trailing edge. We have just invented thickness and created a symmetrical airfoil.

    We have several good things here. This shape will have the ability to produce lift over a quite wide range of AoA without any flow separation. Not only that, but it provides room for structure, fuel, mechanisms, etc. Note that we also have humps, of the same size, on the top and the bottom. This airfoil is great and you can find it for example in the vertical and horizontal tail of many airplanes (including Cessna and Pipers), but it has zero lift at zero AoA.

    Wait a minute.... Let's take that airfoil and still drop the leading edge (with the circle and all) and add some camber.

    Well, now you have an airfoil with a big hump on the top and a small hump on the bottom, that has a positive non-zero lift at zero AoA, and that will reach a higher stall AoA and higher lift before stall.

    Done? Almost. Drag.

    I will not go in details here, but you want to minimize drag, ok? And the drag of the airfoils depends on the angle of attack. There will be one angle of attack where the drag will be minimum. And this AOA happens when the stagnation point is exactly at the leading edge. In airfoils with more camber this will happen at higher lift coefficients and in airfoils with low camber at lower lift coefficients, because this happens more or less in the middle of the usable AoAs for the airfoil (from negative stall to positive stall). In symmetrical airfoils, the stagnation point will be on the leading edge when the AOA and the lift is zero.

    You would normally design the plane, and select the airfoil, so it spends most if the operational time close to that optimum AoA. So you don't use super-cambered airfoils that have the ability to reach super high lifts at super high AoAs, because at cruise (where you don't need so much lift coefficient) they would be making quite more drag that a competing, more reasonable airfoil.
    But at the same time you do want a super-lift airfoil to reduce the stall speed and be able to take-off and land at reasonable speeds. What can we do?

    Well, remember the first improvement that we did to the flat plywood airfoil? When we bent the leading edge down? Imagine that instead of bending the existing leading edge down we added an accessory to the leading edge that was basically and extended leading edge curved down. It's basically the same, right? Well, this trick (also the simple bending down) works with any airfoil. Even with the one we selected for minimum drag at cruise. And any similitude with a slat is NOT a coincidence. The slats typically have 2 positions: sealed (used for take-off in combinations with a little bit of flaps) and slotted (used for landing in combination with a lot of flaps). The 1st one, sealed, is basically what we just described. The 2nd one, slotted, is on one hand a further and more down extension of the first (so it is good for even higher AoAs) but also reveals a slot between it and the rest of the airfoil that lets high-pressure (i.e. high energy) air from below leak to the low pressure top of the wing. This injects energy to the boundary layer which further retards flow separation, but I will not go into that. As I will not go into flaps either because the question was not asked, but suffice to say that it increases even more the camber and, if they are Fowler flaps, also increase the sing area. And if you ever say a transport category jet landing I think it is quite intuitive how a wing with the flaps extended deflects more air more down than a clean wing.


    Note [1] 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, and you know, cylinder, circle, hence Pi (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).

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  4. #24
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Gabriel
    [...Explanation...]
    Thanks.

    Quote Originally Posted by 3BS
    All the curvature and hump businesses does little to GENERATE lift, it just keeps things working better.
    Just to be an a$$, I am going to push the issue as to why when they make the final clean up which is a little bit of flap and a WHOLE LOT of slat, they nose up a little bit.
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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by 3WE View Post
    Just to be an a$$, I am going to push the issue as to why when they make the final clean up which is a little bit of flap and a WHOLE LOT of slat, they nose up a little bit.
    Because of: Flaps, flaps, slats and slats.

    1- Flaps 1: Flaps increase the lift an any angle of attack (if we keep the angle of attack reference line fixed to the wing/airplane when we extend the flaps). So when you retract them the lift goes down and to keep 1G flight you need to compensate increasing the AoA.
    2- Flaps 2: In all airliners the flaps don;t go just down, but they extend back, effectively increasing the wing chord and hence its area. And, not shown in the previous equations, the lift is also proportional to the wing area (in Lift = k * r *V^2 * AoA, the k contains the wing area S, or the chord C if you are working 2D just with the airfoil, and in that case the k = Pi.C/2). So, again, a retraction of flaps means a reduction in wing area which means a reduction in lift that you have to compensate increasing the AoA.
    3- Slats 1: Most of the leading edge device is not just a dropped leading edge (these devices exist too) but something that was not there that now is there exposed to the wind,and that something also has an area. In other words, just like the flaps, they "extend" and increase the wing area. So read Flaps 2.
    4- Slats 2: I don't discard that, even ignoring the previous point, the salts (even a simple dropped leading edge) can have some small effect on the lift vs AoA curve, other than moving the stall to higher AoAs. My explanation in the previous was of course a simplification and there was no specific airfoil or slat design described, rather a generic, conceptual, "fundamentals" explanation. Not all the airfoils and not all the slats behave EXACTLY the same, which is why you have so many airfoils and slats designs.

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  6. #26
    Senior Member BoeingBobby's Avatar
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    Quote Originally Posted by Gabriel View Post
    Because of: Flaps, flaps, slats and slats.

    1- Flaps 1: Flaps increase the lift an any angle of attack (if we keep the angle of attack reference line fixed to the wing/airplane when we extend the flaps). So when you retract them the lift goes down and to keep 1G flight you need to compensate increasing the AoA.
    2- Flaps 2: In all airliners the flaps don;t go just down, but they extend back, effectively increasing the wing chord and hence its area. And, not shown in the previous equations, the lift is also proportional to the wing area (in Lift = k * r *V^2 * AoA, the k contains the wing area S, or the chord C if you are working 2D just with the airfoil, and in that case the k = Pi.C/2). So, again, a retraction of flaps means a reduction in wing area which means a reduction in lift that you have to compensate increasing the AoA.
    3- Slats 1: Most of the leading edge device is not just a dropped leading edge (these devices exist too) but something that was not there that now is there exposed to the wind,and that something also has an area. In other words, just like the flaps, they "extend" and increase the wing area. So read Flaps 2.
    4- Salts 2: I don't discard that, even ignoring the previous point, the salts (even a simple dropped leading edge) can have some small effect on the lift vs AoA curve, other than moving the stall to higher AoAs. My explanation in the previous was of course a simplification and there was no specific airfoil or slat design described, rather a generic, conceptual, "fundamentals" explanation. Not all the airfoils and not all the slats behave EXACTLY the same, which is why you have so many airfoils and slats designs.
    "In all airliners" You sure about such an absolute statement?

  7. #27
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    "In all airliners" You sure about such an absolute statement?
    You are right. The DC-3 is also an airliner after all. Let me correct:

    In all Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer transport category jets.

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  8. #28
    Senior Member Evan's Avatar
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    Questions:

    Quote Originally Posted by Gabriel
    Stall? Nope, the fluid will re-adhere to the top surface a little bit after the trailing edge, creating a "bubble" of separated flow.
    we can see that all the flat part behind the trailing edge is at an angle
    1) Did you mean leading edge there or am I just not getting it?

    2) You've given us a nice explanation of leading edge aerodynamics and wing camber and you've already brought Newton into it on the lower wing surface (airflow 'turning'), but... how does the upper surface of the wing contribute to generating lift?

    Quote Originally Posted by Gabriel
    4- Salts 2: I don't discard that, even ignoring the previous point, the salts (even a simple dropped leading edge) can have some small effect on the lift vs AoA curve, other than moving the stall to higher AoAs.
    3) Are we talking magic salts?

  9. #29
    Senior Member BoeingBobby's Avatar
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    Quote Originally Posted by Gabriel View Post
    You are right. The DC-3 is also an airliner after all. Let me correct:

    In all Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer transport category jets.
    Still wrong! Blanket statements will do that to you. Convair 880/990 NO LED's, many EMB's and Bombardier's, NO LED's. So even leaving out all of the older piston engine aircraft, you are still wrong. And Evan, are you EVER going to answer my original question?

  10. #30
    Senior Member 3WE's Avatar
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    Edit: Aircraft without LEDS.


    DC-9-15 (and -20 I think)
    Many turboprop airliners.
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  11. #31
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Quote Originally Posted by Gabriel
    You are right. The DC-3 is also an airliner after all. Let me correct:

    In all Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer transport category
    Still wrong! Blanket statements will do that to you. Convair 880/990 NO LED's, many EMB's and Bombardier's, NO LED's. So even leaving out all of the older piston engine aircraft, you are still wrong. And Evan, are you EVER going to answer my original question?
    Convair? I said Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer.

    LED's? I said FLAPS (which are TED's).

    Quote Originally Posted by Gabriel
    2- Flaps 2: In all airliners the flaps don't go just down, but they extend back
    Is there any Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer transport category jets that don't have flaps that extend back? Not that I know but maybe, in which case I will stand corrected again.

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  12. #32
    Senior Member BoeingBobby's Avatar
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    Quote Originally Posted by Gabriel View Post
    Convair? I said Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer.

    LED's? I said FLAPS (which are TED's).



    Is there any Boeing, Airbus, McDonnell Douglas, Bombardier and Embraer transport category jets that don't have flaps that extend back? Not that I know but maybe, in which case I will stand corrected again.
    Whatever. I have had about as much fun with you all as I can stand. What is that old saying? No good deed goes unpunished? Think I am going to start to enjoy my retirement. Evan, still waiting for your answer!

  13. #33
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    1) Did you mean leading edge there or am I just not getting it?

    3) Are we talking magic salts?
    Fixed both in the original posts. Thanks.

    2) You've given us a nice explanation of leading edge aerodynamics and wing camber and you've already brought Newton into it on the lower wing surface (airflow 'turning'), but... how does the upper surface of the wing contribute to generating lift?
    This is a question that makes no sense four us super-genius-aerofluodynamicist-of-the-millennium (ok, them, not us). There is no "contribution" of one surface vs the other.

    What makes lift is the distribution of pressures around the airfoil, which will depend on the airfoil design and angle of attack.
    You will even find cases where, in the majority of the airfoil, the pressure is lower than the atmospheric pressure, both the upper and lower surface. Just that on the upper surface it is even lower than on the lower surface.

    You will even find cases of similar airfoils that have the same camber geometry and where the thickness distribution of one is just a scale-up of the thickness distribution of the other. For example, NACA 2408 (8% thick) and 2415 (15% thick). Both airfoils have virtually the same lift-to-AoA plot (especially away from the stall zone), meaning that they make the same lift for the same AoA, for any AoA away from the stall.

    Yet, the pressure distributions are different. What you will find is that the pressures are lower in the 2415 all around, both in the upper and lower surface.
    But when you plot the DIFFERENCE between the pressures in the upper and lower surface for each point along the chord, you will see that both plots look almost the same (you'll see, away from the stall, the DIFFERENCE in pressure between both surfaces is more a function of the geometry of the mean line. i.e. the camber, than the thickness distribution).

    In this way, even if you have a "low pressure" in most of the lower surface, you can still have positive lift due to an "even lower pressure" on the upper surface.

    You before "explained" how the airfoil creates a "constriction" on the upper surface which makes the airflow accelerate as it needs to flow through a "narrower" pipe, and how this increases speed means lower pressure due to Bernoulli.

    It can be argued different, we can say that the airflow accelerates the airflow (and the pressure goes down due to Bernoulli) and that acceleration makes the channels to constrict because, due to conservation of mass, more velocity will mean smaller cross section (you don't need a physical barrier that narrows, the same will happen with a stream of water in free fall, the speed will increase due to the gravity and the stream will become narrower, would you say in that case that the water accelerates because of a constriction)?

    But there is still a third way that it can be argued. We can say that, due to the boundary conditions (explained later), the airfoil forces the air to reduce the pressure, this generates an increase in speed (due to Bernoulli) and the constriction.

    So which of them is correct? It is a little bit of each? No, it's none and and all. There are not "individual contributions" of these effects, are different ways to express the same phenomena.

    It's not that the airfoil does this to the air or that the air does that to the airfoil. There is an interaction between the 2.
    The air will just do what it needs to do.

    That is, the air flowing around the airfoil needs to meet a certain 3 conditions:
    1- Being a continuum, it needs to keep a smooth gradient of speeds and pressures.
    2- It cannot penetrate walls (i.e. the flow that is immediately in contact with the airfoil must be tangent to the airfoil).
    3- It needs to separate at the trailing edge*

    In fact, you can mathematically simulate an airfoil by pacing on the boundary or inside the airflow an infinite number of infinitely small singularities called sources, sinks and rotors Make these sources many but not infinite and small but not infinitely small and you have a numerical model of the style of finite elements that lets you approximate the airfoil, or whole wing, or whole airplane, with an arbitrary accuracy as long as you have sufficient computing power and patience. What you do is set up a number of "checkpoints" on the contour of the airfoil (one of the HAS to be the trailing edge) and calculate the speed field by adding just a constant speed field (that the free stream) to the field generated by each source, sink and rotor. The speed vector of the air at each point of the field (that is, at any point anywhere, including inside the airfoil) will be a function of the intensity of each source, sink and rotor. Then calculate the speed vector at each checkpoint, which, again, each of them will be a function of the intensity of each source, sink and rotor. And finally, you make a quite simple system of linear equations where you FIND the intensity of each source, sink and rotor by making:
    - Component of the speed vector perpendicular to the contour of the airfoil at checkpoint A = 0 (and the same for every checkpoint).
    - Separation happens at the trailing edge.
    - Sum of the intensity of all source and sinks = 0 (i.e. we are not creating or destroying air)

    And then, when you run the simulation of a constant air flow coming from the left and all these sources, sinks and rotors more or less where the airfoil would be, it is amazing to see how the flow flows around an airfoil that doesn't physically exists and how you have a second flows that goes from the sources to the sinks that remains inside the imaginary airfoil and doesn't mix with the external flow.
    The next thing you can do is calculate the pressure of the air at each point in the external flow that is immediately next to where the airfoil would be (i.e. the points where both the external and internal flow are in contact but neither crosses the boundary), and then you can fo further and integrate all the pressures on the top surface on one hand and all the pressures on the bottom surfaces on the other hand. What you will find is that, except in a very narrow zone around the front stagnation point, all the pressures in the contour are lower than the free stream pressure (i.e. atmospheric pressure), but it is even lower on the top surface than in the lower one. Again, it is just what the air NEEDS to do to separate at the trailing edge, avoid penetrating the airfoil, and keep a smooth gradient of pressures and speeds.

    * Condition 3 is is a tricky one, 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. 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 "re-joining" point under potential flow 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. 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. Also the "no walls penetration" is something that you need to impose in the model otherwise the air would just flow across the airfoil without being disturbed, but this condition is obvious. 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 inpentrability of walls happen because, well, walls are impenetrable, and 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?

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  14. #34
    Member ATLcrew's Avatar
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    Quote Originally Posted by BoeingBobby View Post
    Evan, still waiting for your answer!
    You'll be waiting a while. He's still to answer the much more benign question of what airlines he rides on as a passenger. In all these years he made one peep about being on Air Berlin (RIP) once.

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Foxed both in the original posts. Thanks.
    Outfoxed, you might say.

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    Quote Originally Posted by Evan View Post
    Outfoxed, you might say.
    Shot!!! (see? another typo)

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel
    So which of them is correct? It is a little bit of each? No, it's none and and all. There are not "individual contributions" of these effects, are different ways to express the same phenomena.
    I do appreciate the technical lessons you provide, but I'm afraid I'm finding this one as impenetrable as an LH-B747 post. Is it possible to give a layman's answer to how the entirety of factors introduced by an airfoil create lift?

    Let me put this to you as a multiple choice question. Which of the following is true?:

    [a] Lift is a known science, the mechanics of which are absolutely proven to be known.

    [b] Lift is an observed phenomenon, for which the mechanics are empirically observed and theoretically deduced to form a single universally-accepted* theory.

    [c] Lift is an observed phenomenon, for which the mechanics are empirically observed and theoretically deduced to form a number of competing theories.

    * By the super-geniuses of aerofluodynamics

  18. #38
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    Quote Originally Posted by Evan View Post
    I do appreciate the technical lessons you provide, but I'm afraid I'm finding this one as impenetrable as an LH-B747 post. Is it possible to give a layman's answer to how the entirety of factors introduced by an airfoil create lift?

    Let me put this to you as a multiple choice question. Which of the following is true?:

    [a] Lift is a known science, the mechanics of which are absolutely proven to be known.

    [b] Lift is an observed phenomenon, for which the mechanics are empirically observed and theoretically deduced to form a single universally-accepted* theory.

    [c] Lift is an observed phenomenon, for which the mechanics are empirically observed and theoretically deduced to form a number of competing theories.

    * By the super-geniuses of aerofluodynamics
    It's all PFM.

  19. #39
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    I do appreciate the technical lessons you provide, but I'm afraid I'm finding this one as impenetrable as an LH-B747 post. Is it possible to give a layman's answer to how the entirety of factors introduced by an airfoil create lift?

    Let me put this to you as a multiple choice question. Which of the following is true?:

    [a] Lift is a known science, the mechanics of which are absolutely proven to be known.

    [b] Lift is an observed phenomenon, for which the mechanics are empirically observed and theoretically deduced to form a single universally-accepted* theory.

    [c] Lift is an observed phenomenon, for which the mechanics are empirically observed and theoretically deduced to form a number of competing theories.

    * By the super-geniuses of aerofluodynamics
    It is definitively not [c]

    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.

    We have a full physical and mathematical model of how fluids work. The model is found to work (it closely matches reality). And generation of lift is just part of that.

    But there are no competing theories. What we have is akin to asking "Why does an object accelerate when it falls down? Because it converts potential gravitational energy into kinetic energy so 1/2*m*v^2=m*g*h or because the force of the weight imposes an acceleration given by F=m*g=m*a?" It's not one or the other. It is both. "Ok, so how much is the contribution of each of these effects? Or are they different competing theories and we don't know which of the 2 is the correct one?" Nonsensical question, they are not different contributions or theories, they are different ways to view the same thing. In fact one equation can be mathematically derived from the other.

    The governing theory is the Navier Stokes equation, which is basically F=m*a applied to an infinitesimal element of fluid, and where the forces are normal forces (pressure), share forces (viscosity), inertial forces (depending the frame of reference you choose) and external forces (like gravity or those made on the elementary volume of fluid by the walls of a container).

    The full Navier Stokes equation has very few assumptions (like assuming that the fluid is a continuum, which it is not, and that the Newton equations are correct, which they are not, but both are damn good approximations), and no known solution (there is a 1 million dollar for whomever finds a solution to the full Navier Stokes equation).
    However, do not dismay, because it can be tackled numerically and specific cases can be simulated and solved with arbitrary precision.

    Also, the Navier Stokes equations have known closed solutions if you add more assumptions (things like incompressibility, no or constant viscosity, laminar flow, etc...)

    For example, the Bernoulli and hydrostatic equations, for example, were conceived long before the Navier Stokes equation. But they can also be derived from the full Navier Stokes equations by adding assumptions (line all speeds and accelerations are zero for Hydrostatics, or laminar non-viscous and constant speed flow for Bernoulli, which is called potential flow).

    The generation of lift can also be derived from the Navier Stokes equations taking the same assumptions than for Bernoulli and adding the boundary conditions (cannot penetrate the airfoil and must separate at the trailing edge). The assumptions are very good away from the stall and out of the boundary layer (where the flow is viscous it is typically turbulent too so bye-bye Bernulli), yet luckily the boundary layer is a very thin layer around the airfoil, so the shape of the airfoil+boundary layer is basically the shape of the airfoil. The Kutta condition (separation at the trailing edge) was added first empirically, based on observations that the flow does separate at the trailing edge (something that the potential flow equations did not predict), but then it was understood when it was seen that the assumptions of the potential flow equations predicted an impossible infinite-speed singularity at the trailing edge (if you don't force the separation there) and that the boundary layer that was being neglected (where the viscous processes take place) could not theoretically sustain that and it would separate, as it does in practice as the boundary layer theory predicts.

    This simplified theory of potential flow with no flow separation except at the trailing edge is capable of explaining lift, pressure drag and induced drag. But not parasite drag. The boundary layer theory can predict the parasite drag, however it needs some numbers to be "forced" from outside, like where the transition from a laminar boundary layer to a turbulent boundary layer happens. There are theories to predict that too on a perfect surface. Add rivers, bugs, paint texture etc and it becomes quite unpredictable, so you "force" it by hand (like the Kutta condition).

    There are better models based on finite elements that can predict virtually anything by using fewer assumptions and dividing the "space" into a lot of little (but not infinitesimal) volumes and "checkpoints" at the boundaries to enforce the boundary conditions. These equations are so good that, with current computer power, the test pilots of a new transport category jet can fly it in the simulator in all the envelope before the first flight takes place, and even before wind tunnel testing. And the results are not perfect but very close.

    So, like every science, Mechanics of Fluids is based on observations, the observations are transformed into laws of equations with assumptions, and the equations are then used to make predictions, that are then tested and found to hold true confirming the power of the theory.

    As said: We have a full physical and mathematical model of how fluids work. The model is found to work (it closely matches reality). And generation of lift is just part of that.

    --- 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. #40
    Senior Member 3WE's Avatar
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    Quote Originally Posted by ATLcrew View Post
    It's all PFM.
    No magico.

    The lift comes from the pilot pulling on the stick or yoke, and then the engines pull/push the plane up.

    By the way- as to the most recent exchange. Evan seems unable to comprehend that wings “grab” passing air and shove it down...which creates a reaction called lift.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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