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  • Gabriel
    replied
    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.

    Leave a comment:


  • BoeingBobby
    replied
    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?

    Leave a comment:


  • Gabriel
    replied
    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.

    Leave a comment:


  • 3WE
    replied
    Originally posted by Gabriel
    [...Explanation...]
    Thanks.

    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.

    Leave a comment:


  • Gabriel
    replied
    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).

    Leave a comment:


  • 3WE
    replied
    Originally posted by elaw View Post
    Allow me: it's mágico.
    Caultrons perhaps?

    Leave a comment:


  • 3WE
    replied
    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.

    Leave a comment:


  • BoeingBobby
    replied
    Originally posted by 3WE View Post
    Indeed.

    Better chances of driving a 747.
    Actually, just about the same.

    Leave a comment:


  • elaw
    replied
    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.
    Allow me: it's mágico.

    Leave a comment:


  • Evan
    replied
    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.

    As for now, I agree with Evan, the YouTube, the Wright Brothers and my rather flexible human hand held flat out the car window at 70 MILES per hour (and I don't care what that is in KPH, but it's roughly 60 knots).

    Thanks in advance.
    Let me have a whack at this:

    Because something causes the airflow over the wing to speed up, resulting in a lower pressure above the wing, causing the higher pressure below the wing to LIFT the wing upward (That's the Bernoulli part).
    That something is either the shape of the wing itself being present there or a result of the shape of the wing itself resulting in something else that causes the airflow to speed up.
    One theory I've read is that the shape of the wing distorts the fluid tubes in which the air flows above the wing, causing them to constrict, and Bernoulli showed that when you constrict a tube, the fluid speed increases. Counter-argument: When looking out the overwing window, I've never seen a fluid tube and it sounds a lot like magic to me.

    I think Gabriel can solve this one for us.

    Leave a comment:


  • Gabriel
    replied
    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.

    As for now, I agree with Evan, the YouTube, the Wright Brothers and my rather flexible human hand held flat out the car window at 70 MILES per hour (and I don't care what that is in KPH, but it's roughly 60 knots).

    Thanks in advance.
    I will but later. Stay tuned.

    Leave a comment:


  • 3WE
    replied
    Originally posted by Gabriel View Post
    F = m*a
    m:
    m = k1 * V * r
    a:
    a = k2 * V * AoA
    L = F = m * a = k1 * V * r * k2 * V * AoA = k * r * v^2 * AoA
    Lift = k * r *V^2 * AoA
    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.

    As for now, I agree with Evan, the YouTube, the Wright Brothers and my rather flexible human hand held flat out the car window at 70 MILES per hour (and I don't care what that is in KPH, but it's roughly 60 knots).

    Thanks in advance.

    Leave a comment:


  • Evan
    replied
    Originally posted by 3WE View Post
    I have read that 60% of lift is "suction" from the top of the wing...40% because the bottom side shoves air downward.
    So a little bit of Bernoulli, a little bit of Newton.

    L = B + N * V * AoA* ^ M (M= Magic)?

    Leave a comment:


  • Gabriel
    replied
    Originally posted by 3WE View Post
    It accelerates a mass of air downward in a rather direct relationship to the AoA.
    Correct but incomplete.

    F = m*a

    m:
    The mass of air accelerated is the volume times the density.
    The volume is proportional to how much air is flowing around the wing, hence speed.

    hence m = k1 * V * r (V= speed, r = density)

    a:
    The intensity of the acceleration is proportional to the angle that the air is deflected which is proportional to the AoA.
    But is also proportional to the speed: If you deflect air down from 0 degrees to say 10 degrees down, the vertical speed at 10 degrees will be 17% of the horizontal speed.

    Hence, a = k2 * V * AoA

    Finally, L = F = m * a = k1 * V * r * k2 * V * AoA = k * r * v^2 * AoA

    Lift = k * r *V^2 * AoA

    [/splitting-hairs geek]

    Leave a comment:


  • 3WE
    replied
    Originally posted by Evan View Post
    Fixed.

    Honestly, I'm torn between Gabriel's up quark explanation and vastr magic one. However, I think the book every pilot should be reading is How Airplanes Don't Fly.
    Modifications noted...

    Why having something of a hump on top works...why those BIG ASS leading edge slats that shove air UPWARDS and over the wing work...Shoving air UP sure as hell doesn't make lift.

    I have read that 60% of lift is "suction" from the top of the wing...40% because the bottom side shoves air downward.

    Magic, indeed- but from reading Wolfgang Langewiesche, it really doesn't matter.

    Leave a comment:

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