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Fundalmentals of Lift?

  1. Jul 13, 2006 #1
    Is lift essentially created because of higher pressure at the bottom of the wing compared to the top of the wing, causing a postive net upward force?

    With PA=F

    And the difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it, achieved by the unique shape of the wing?
     
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  3. Jul 13, 2006 #2

    Danger

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  4. Jul 13, 2006 #3

    Gokul43201

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  5. Jul 13, 2006 #4

    Danger

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    Good one, Gokul. I hadn't seen that before. Thanks.
     
  6. Jul 14, 2006 #5
    I am not after any detailed analysis. From this thread it seems that what I have posted is largely correct. I just need a confirmation of that and maybe some concise criticism if need be.
     
  7. Jul 14, 2006 #6

    Danger

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    While the Bournoulli (sp?) effect has a part in it, most lift is in fact a result of air being deflected downward with the Newtonian reaction of the wing being deflected upward. Your best bet is to read the links whether or not you want to. You can't come to a serious science forum and then ignore the science behind the answer to your question.
     
  8. Jul 14, 2006 #7
    I provided one scientific explanation for why lift exits and it seems to be a very worthy candidate. Your explanation of air being deflected downward hence wing deflected upward is completely complentary to (what I suggested) the Bournoulli principle because higher pressure (compared to the top of the wing) at the bottom of the wing results in more force per area. Hence more air molecules being deflected downward (then molecules deflected upward on top of the wing) after they hit the wing resulting in a net upward motion of the wing.
     
  9. Jul 14, 2006 #8

    Danger

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    What I was trying to say is that the thing will lift even without a curved upper surface. That proves that the pressure differential isn't that important. I readily admit that when I was taking my flight training, they cited the Bournoulli principle. I didn't question it at the time, because I didn't care what held the damned thing up as long as I could play with it.
    Anywho... if you've ever seen a hydroplane or F1 race car get airborne, you can see that it wasn't because of having a curved upper surface.
     
  10. Jul 17, 2006 #9

    rcgldr

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    I created this thread as well:

    https://www.physicsforums.com/showthread.php?t=107565&highlight=wings+lift

    Yes, when generating lift, the pressure below is higher than the pressure above.

    No, lift requires an effective angle of attack to accelerate air downwards, a flat board will generate lift. You can stick your hand out the window of a fast moving car and angle it so it produces lift. The unique shape of the wing just improves the lift versus drag ratio for an intended range of air speeds and wing loadings. In some cases, like civilian aircraft, efficiency is traded off for ease of manufacturing, which is why near flat bottom wings are used so often.

    The speed of the air is relative to a frame of reference. Relative to the surrounding air, the air above a wing is traveling slower than the air below it.

    As already mentioned, lift occurs because air is accelerated downwards, and the total force is simple force = sum of the mass of air molecules affected by a wing passing by, times the average acceleration of each air molecule.

    At moderate AOA (angle of attack) even for a flat board, most of this downwards acceleration occurs from above the wing.

    A simple explanation is that as a wing passes through the air with a moderate AOA, it deflects air downwards from below, and introduces a void as it passed through the air from above. This mostly downwards moving void creates a low pressure area that draws air towards it from all directions, except air can't flow upwards through the wing, so there's a net downwards acceleration of air towards this moving void.

    Since the pressure above the wing is lower than the pressure below the wing, some of the air stream is "stolen" by the low pressure area, lowering the seperation point of the air stream in front of the wing. This reduces the amount of air flowing below a wing, and adds some downwards component to the lower stream so that the wing has less deflection to do. The air that moves upwards from the seperation point had to be accelerated downwards even more to fill in that void from above the wing, and this requires even more air from further above to help "push" the air into that sucking void. These are the main reasons that most of the downwards acceleration of air occurs from above a wing, even in the case of a flat board, as long as AOA is not extreme.

    Getting back to air speeds above and below a wing, the speeds are different because air is being accelerated towards the low pressure area, and away from the high pressure areas. The speeds aren't constant either, but instead, the air is being accelerated, towards low pressure areas, and away from high pressure areas.

    The classic Bernoulli theroem is a case of conservation of energy. The total engergy of a volume of air is it's kinetic energy (which is frame of reference relative), pressure, and temperature. A wing passing through the air peforms work on the air, accelerating the air mostly downwards and some forwards, and therefore it's changing the kinetic energy of the air. Using either the air or the wing as a reference, the downwards flow represents an increase in kinetic energy. Relative to the air, drag flow represents an increase in kinetic energy, and relative to the wing, a decrease.

    The most efficient airfoils used for gliders have curved upper and lower surfaces. If you draw a line from the leading edge the the trailing edge of a cross section of a wing, where the line is in the middle of the wing (equi-distant from upper and lower edges), the (downwards) curvature of this line is called camber. Gliders, both full scale and high end models is where most of the work in wing design is done these days. Wings are described as a basic airfoil shape, (the basic shape independent of camber), the thickness (% of the thickest part compared to the chord (distance from leading to trailing edge)), and the camber.

    If you refer to the thread I posted, I included quite a few links on this subject.
     
    Last edited: Jul 17, 2006
  11. Jul 17, 2006 #10

    FredGarvin

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    A glider's efficiency comes not so much from the aerfoil shape (it does play a big role though), but from the very high aspect ratio.
     
  12. Jul 17, 2006 #11

    rcgldr

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    My point was that most of the current work is being done on gliders, specifically radio control gliders, since lift to drag ratio is important, especially for contest models. For powered models, it's not that important, many just use symmetrical airfoils, and in the case of full scale civilian aircraft, with a few exceptions with the "experimental" or kit aircraft, there's just not a lot of work being done, because new models aren't created that often.

    As I mentioned, there is a NACA air foil replacement for the Cessna 182, that is cambered above and below, has a shorter wing chord, better lift to drag ratio, and more speed range, but it costs more to make than a flat bottomed wing, so it's rarely used except for experiements.

    There's a lot of work being done on air foils, espeicially contest radio control gliders (since new models are made much more often than full scale models due to the cost). I refer to some of the airfoil designers, some of which are still very active, from a quote in my other thread below. I had the impression that the difference between a 50:1 glide ratio and a 60:1 glide ratio for a cross-country model has a lot to do with the airfoil and overall design of these gliders, it's gone beyond just aspect ratio.

    The HQ airfoils by Dr. Helmut Quabeck are one example; he also makes actual rc contest gliders (most of this time is spent making the wing molds used to manufacture hollow modled composite wings). Selig / Donnivan teamed up to create the SD series of air foils, SD7037 is a popular air foil for rc gliders. Michael Selig / Ashok Gopalarathnam teamed up later to create the SA series of air foils. Rolf Girsberg made the RG airfoils, RG15 and the faster RG14.
     
    Last edited: Jul 17, 2006
  13. Jul 17, 2006 #12
    You mentioned 'that most of the downwards acceleration of air occurs from above a wing'. If that is the case than there is there would be a net downward acceleration of air making the wing go downwards and not upwards.

    Is this statement correct "The difference in pressure is created from lower wind or fluid speed at the bottom of the wing compared to the top of it created because of the angle of attack."?
     
    Last edited: Jul 17, 2006
  14. Jul 17, 2006 #13

    russ_watters

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    No: action-reaction. Stand on a skateboard and throw a bowling ball in one direction and you go in the other. Throw air in one direction and you go in the other. It is easiest to see with a fan.
    You have to be a little careful about how you define angle of attack. Effective angle of attack has the zero lift point as zero angle of attack, but geometric angle of attack (drawing a line from the tip to the trailing edge) is often negative at zero lift due to camber.
     
    Last edited: Jul 17, 2006
  15. Jul 17, 2006 #14

    rcgldr

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    The air that's being accelerated downwards from above the wing is well above the wing, and is being accelerated towards a low pressure area immediately above the wing. As mentioned, the air is being accelerated towards this low pressure area from all directions but upwards, since the presence of the wing prevents upwards flow, so the result is a net downwards acceleration of air.

    Without a change in velocity (or more accurately, a change in total energy), the air doesn't generate any force and/or peform any work, so it's not the velocities, but instead the change in velocties that matter. In the case of a normal wing, most of this change in velocity is downwards (lift) and a bit forwards (drag).

    To summarize: a wing with an effective angle of attack travels through the air. This results in the creation of pressure zones that are different than the surrounding air, and the pressure above the wing is lower than the pressure below. Depending on the shape of the wing, a pressure zone may be created above the wing, below the wing, or both. Air is accelerated from higher pressure areas towards lower pressure areas. The presence of the wing prevents upwards air flow through the wing, so a net downwards acceleration occurs, corresponding to lift. The wing also prevents horizontal flow, so a net forwards acceleration of air occurs, corresponding to drag.

    Afterwards, the air recovers by eventually returning to the velocity and pressure of the surrounding air, but relocated from where it started. In a closed system, a flying model will increase the pressure differential within the closed system, so that the net downwards force created by the pressure differential versus altitude within the closed system will be exactly the same as the weight of the air and the model within the closed system. (This assumes that there is no net vertical component of acceleration of the center of mass of the closed system.)

    For an example of a wing that mainly produces lift from below, imagine a flat bottom wing inverted and traveling backwards, almost all of the lift is due to deflection, and the drag factor is high. NASA once considered a high drag lifting body with this type of shape as a space return vehicle.
     
    Last edited: Jul 17, 2006
  16. Jul 18, 2006 #15
    Are you refering to the fact that the air pushes from below the wing causing an upward force (causing lift of the wings) and downward force on the air molecules.
     
  17. Jul 18, 2006 #16

    FredGarvin

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    I never said it was just aspect ratio. There are a lot of aspects that are involved. However, the glaring, number one thing that all high efficiency aircraft have in common is a very large aspect ratio in an effort to reduce induced drag. Tip effects/losses tend to dominate on wings. I am sure that people are playing around with different sections along the spans and probably with varying wing twists as well. However, no matter what one does with the cross section, there is a reason why the U-2, Voyager and Global Flyer all have the designs they have.
     
  18. Jul 18, 2006 #17

    rcgldr

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    I think what I left out is the fact that in the typical case where most of the lift is due to air accelerated downwards from above the wing, that the air flows from above (and a bit in front) of a wing, and then flows downwards behind the wing as it passes by. There's a downwash of air at the trailing edge of a wing.

    The "upwards" push occurs because the pressure above a wing is less than the pressure below a wing. It's possible for both these pressures to be less than the pressure of the surrounding air, or for the pressure below the wing to be the same as the surrounding air. All that is required is a difference in the pressures above and below the wing to produce lift.

    Take the case of a flat bottom type wing. There' is a specific effective angle of attack where there is a low pressure area above the wing, but no change in pressure below the wing. If I remember correctly, a Cessna 182 can't fly this fast (200 to 300 knots, not sure), but a high powered radio control model with a flat bottom wing can.

    So the result of this special case is a low pressure area just above the wing that moves with the wing, and no pressure change below the wing. Air accelerates from all directions towards this moving low pressure area, from above, from in front, from behind, from the sides, but hardly any flow from below, because the wing prevents upwards flow through the wing, except for the flow that goes around the wings edges. So the accelerations cancel except since the upwards flow is mostly blocked, the result is a net downwards acceleration of air from above the low pressure area, corresponding to lift. The wing also prevents some of the front to back flow, so there is also a net forwards acceleration of air, corresponding to drag.

    As I just mentioned, the net flow starts from in front and above the wing, and ends up behind and below the wing. At the trailing edge of the wing, there's a downwash (lift) and some forwards flow (drag). At the leading and side edges, there's some upwards flow (which reduces lift somewhat). There's also an inwards flow from the sides, but these mostly cancel each other out.

    The amount of air involved is huge. I've read that the mass of air affected per second by a small plane traveling at 100 knots is 2.5 to 5 times that of the plane. How much air is affected is somewhat subjective. Should you count the air that is only accelerated by .001g or less?

    Now you could try to go through all sorts of complicated math, but Newtons laws aren't going to be violated, so we know that in level flight, lift will exactly equal the weight of an aircraft, and that the sum of the vertical component of forces from mass times acceleration of all the air molecules affected by the passing of the aircraft must be equal to the lift.
     
    Last edited: Jul 18, 2006
  19. Jul 18, 2006 #18

    rcgldr

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    I didn't word my post clearly.

    Generally in a contest glider, full scale or model, lift to drag ratios and adjustable camber for a range of air speeds is something that people involved are willing to pay for, regardless of the cost to manufacture such wings. This in turn, creates a lot of research activity into such airfoils.

    In the case of most powered aircraft, the cost to manufacture is an important aspect of an airfoil, sacrificing some of the airfoils effeciency, with the exception for some experimental or special purpose aircraft, where again the cost to manufacture isn't as much of an issue. So most powered aircraft end up with airfoils that aren't as areodynamically efficient as they could be, because of the cost to manufacture aspect.

    This is the reason I mentioned glider airfoils, because there is more research activity for them than there is for powered aircraft airfoils.
     
    Last edited: Jul 18, 2006
  20. Jul 18, 2006 #19

    rcgldr

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    In a typical aircraft, at normal cruise speed, angle of attack is high enough that there is significant higher pressure and downwards deflection from below a wing. However, the difference between this higher pressure and the surrounding air isn't as much as the difference between the surrounding air and the low pressure area above the wing. Part of this is because the air flows around the leading edge from below the wing to above the wing very close to the surface of the wing, but mostly because the low pressure area above the wing draws the air from in front of and below the wing, in effect stealing some of the air flow that would otherwise go below the wing, and the high pressure area below the wing, resists the air flow towards it, deflecting it upwards in front of the wing, directing even more of the air flow to over the wing.

    So there's a significant upwash in front of and at the leading edge of a wing, which is the reason that most (but not all) of the lift is due to what is happening above the wing. In order for a wing to generate lift, it has to end up producing more downwash behind it than the upwash in front of it.

    Other tidbits. Symmetrical airfoils are more efficient than cambered air foils at very low angles of attack, in other words at very high speeds. Cambered airfoils are more effcient at lower speeds. Since gliders generally fly at relatively slow speeds, they use airfoils with more camber for increased efficiency. One thing I'm trying to find again, is an aritcle that pointed out that cambered airfoils at low speeds reduced the upwash effect at the front of the wing by moving the center of pressure (differential) further back on the wing, increasing the distance between the pressure zones and the seperation point of the air flow in front of the wing; ovbiously moving the center of pressure back helps, I'm not sure if it's the camber or if it's a more complicated aspect of airfoil design that accomplishes this.
     
    Last edited: Jul 18, 2006
  21. Jul 19, 2006 #20
    So essentially the difference in pressure between the top and bottom of the wing is a result of the angle of attack of the wing?


    I see what you are getting at here. But I like to think about things from a slightly different persepective. I like to think that the fact that the air below the wing is blocked by the wing, the air molecules push against the wing hence creating lift (the air give the wing upward momentum). This way of thinking helps explain the mechanism behind lift and is more intuitive at least for me.
     
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