Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Forces on a wing lift come from the pressure difference

  1. Jul 5, 2003 #1
    right from what i understood from the brain teaser question about forces on a wing lift come from the pressure difference between the two sides of the wing (faster air on top, u get the picture). i know there are many explenations for lift, but in this one why is it assumed that the air on top of the wing should miraculously increase in speed just because it has further to travel (wrt the air underneath the wing).
     
  2. jcsd
  3. Jul 5, 2003 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Rather then thinking of the air moving past the wing, think of the wing moveing through the air. Now as the front edge of the wing separates the air, it pushs the upper layer up, while the bottom layer travels straight across the wing. Since the wing is curved the upper surface is longer then the bottom. When wing has passed the air crossing the top surface has traveled further then the air crossing the bottom surface, but the time is the same, therefore the upper layer MUST be moving faster.
     
  4. Jul 5, 2003 #3

    I didn't have a clue on the Q either - I was just thinking "well there are only four forces" but I guess this was more specific.

    Hey INTEGRAL - that symbol, the curved line, in your av - I have seen it alot in math that I have never had to deal with, what does it mean, and how come in my geomatry alg and calulus and physics I never had to see it?
     
  5. Jul 6, 2003 #4

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That is the symbol for integration (an Integral, hence the name!) I am surprised that you have been able to get very far in Calculus without running into it. Generally the 2nd term or semester of a college level Calculus course is pretty much dedicated to it.
     
  6. Jul 6, 2003 #5
    Well I only had to take precalc as I am a biology major....

    ...I may end up taking calc of one variable but I don't need to..

    I shoudl state that, as precalc courses usually contain absolutely NO calculus!
     
  7. Jul 6, 2003 #6
    I'm afraid that the time it takes for layer of air above the wing to travel to the tip is not the same as the time it takes for the lower layer of air to reach the tip. In fact, the upper layer reaches the tip before the lower layer does, making it travel even faster then you expected. Still, with no offense meant, your reasoning becomes flawed because of your assumption that the time is the same.

    Now, I myself am not an expert in the field, but if you take a look at the preview to Understanding Flight by David F. Anderson and Scott Eberhardt at Amazon.com, you can view the following excerpts:
     
  8. Jul 6, 2003 #7
    Same time?

    Perhaps the following might explain:

    If the time taken for air to flow over the top and bottom of the wing is not the same, then as the wing moves through the air, air approaches the top and bottom at the same rate. But if air moves over the two surfaces at different rates, then u should expect air density to increase/decrease indefinitely near one surface of the wing. This leads to a contradiction. So the time taken should be the same if we assume steady state to set in.

    If this is true, then we still need to explain why the relative acceleration between the air path.
     
  9. Jul 6, 2003 #8
    On 2nd thought, it still could not explain why the time is the same.(different times does not neccessarily lead to accumulation/depletion of mass)
     
  10. Jul 6, 2003 #9
    i didnt think it was easy, it definitely has something to do with the trailing edge tho because they are important (damage them and your in big trouble). when i've gone through uni in 4 years time and i'm a master in aeronuatics i'll try and explain.
     
  11. Jul 7, 2003 #10

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The time for the two flows to reach the trailing edge of the wing is -in general- NOT equal. That is one of those "bad science" explanations of lift. They give an easy to understand explanation which is unfortunately incorrect.

    I'm planning on writing an article for www.physicspost.com later in the summer about this (once I finish getting situated in my new house), but I'll post a watered down version here.

    The reason that the pressure is lower on the top of the wing is due to the bernoulli effect - as speed of the flow goes up, the local pressure goes down. However, the bernoulli effect itself does not explain WHY the speed goes up, and that is a cause for much confusion (especially on the internet...)

    The reason why the flows speed up has to do with the compressibility of the air. If you put air in a plastic bottle and squeeze, the air will compress. If you put water in the same bottle, it will not. Water is -for the most part- incompressible.

    The thing with air is that if it has the option to move out of the way instead of compressing, it will (there is always some compression, but for speeds less than Mach 0.3, it can be neglected). If you take a piece of cardboard and move it through the air, the air will flow around the cardboard much easier than it will compress.

    Picture a set of streamlines (example: http://oldsci.eiu.edu/physics/DDavis/1150/11FldMtn/flow.html ) running straight left to right. If you place a disturbance in the way (like a wing), the streamlines will get deflected. Since the same amount of mass needs to go through each streamline, they either need to speed up in the constricted space or become more dense. Since the density doesn't increase (low speed incompressible flow), that means that the flow must speed up.

    I hope that makes sense. It'll be easier to understand once I get some pictures drawn up to accompany it.

    EDIT: I are not a English major
     
    Last edited: Jul 7, 2003
  12. Jul 7, 2003 #11

    russ_watters

    User Avatar

    Staff: Mentor

    Ironically, these "troubles" are other misconceptions about flight. They don't take angle of attack into consideration. They have nothing to do with that equal transit times misconception.

    1. An inverted plane with an asymetric wing will *NOT* stay airborne at 0 aoa.

    2. A symetric airfoil at 0 aoa produces zero lift.
     
  13. Jul 9, 2003 #12
    Weird. To force air to flow faster above wing, it must be first be compressed. Where compression grows, there must be pressure growth. Faster flow or not, but if air pressure is higher above wing than below, then lift is unlikely. Okay, wing shape is such that after initial quick compression front, decompression occurs at back edge.

    Whatever. Air pressure is 1 atm, and thats about max difference you could ever get, and when there is vacuum above wing should be max lift. I'm not sure, but I recall its called a stall condition, drastic loss of lift.

    I've understood from some debates that pressure difference is pretty least important thing. Explanation I got was that angle of attack is the key, and that lift is produced more by air mass diverted up or down, working like impulss, rather than Bernoulli. Air is compressed under the wing, and goal is to have it compressed above wing aswell at back edge. For that front shape of wing attacks air, shock compression that is via special shape of wing kept as uniform as possible down to back edge, where it together with drag cause the pressure to pop up there also. On the way air mass is diverted as per angle of attack, and thus vertical thrust.

    So, I understood that asymetric wing is not for creating lowpressure condition above wing, but solely to make compressed air pressure as uniform as possible there, accounting for air mass inertia. Symetric wing with attack angle would tend to reach vacuum stall condition sooner than asymetric wing, therefore it isn't preferred. But sheet of paper can offer almost as good lift with right aoa.

    Does this make sense?
     
  14. Jul 9, 2003 #13
    Does this also applie to Jet Fighters? It appears to me from the pictures that i have seen that they wings on fighter planes arent shaped the same(or to the same degree) as that of a large commercial plane. Im guessing that this is because of the smaller mass of the fighter relative to that of a larger plane.
     
  15. Jul 10, 2003 #14

    russ_watters

    User Avatar

    Staff: Mentor

    As stated above, at low speed (below ~220mph), compressibility effects are negligible and you can ignore them. The delta P due to the faster flow is all that matters.
    A stall doesn't need to be a vacuum, its just a condition where airflow is detached from the wing surface and extremely turbulent. If the air isn't following the shape of the wing, the shape of the wing can't have the desired effect on the air. Thus the drastic loss in lift (and drastic increase in drag).
    Well, no. At 0 aoa, an asymetrical wing will produce lift due to the principles discussed above.
    No. It is. The benefit of an asymetric wing is that it creates lift at 0 aoa.
    The only time a sheet of paper is better than an airfoil is when your goal is to produce no lift: at 0 aoa, a piece of paper (flat) produces no pressure drag. At any aoa, the sharpness of the leading edge will disrupt airflow.

    You're right that they are shaped differently, but the difference is due to speed, not size. Most fighter planes are designed for speed above mach 1, so their airfoil shapes are a compromise between subsonic and supersonic airfoils. As said above, a sharp edge is terrible for subsonic flow, but for supersonic flow, a (very) flattened diamond shape with sharp edges is best. The general compromise is that most of the airfoil is the same shape except that the leading edge is sharp.

    Ideal airfoil changes again when you get into hypersonic speeds. At hypersonic speeds, the primary concern becomes frictional heating, so you design the airfoil to do whatever possible to keep air from flowing smoothly over the wing. This is accomplished by going back to the blunt leading edges similar to subsonic craft (picture the space shuttle - blunt nose, very round wing leading edge). A sharp edge has a cone shaped (or wedge shaped) shock wave attached to its leading edge. A rounded edge pushes air in front of it creating a barrier to airflow and a curved shockwave. Hypersonic planes/airfolis are often called "waveriders" because they literally ride on their own shock waves.
     
  16. Jul 10, 2003 #15

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Current hypersonic vehicles (we need to use rockets to go that fast ATM) have blunted noses for a specific reason. Russ mentioned it, but didn't make the reason clear. The sharper the leading edge, the less energy gets translated to the environment in the form of heat.

    All spacecraft which return to Earth have blunt noses because they need to get rid of their huge kinetic and potential energies which they have when in orbit. They do this by 'aerobraking' and dissipating the energy into the air.

    Current concepts for hypersonic scramjets have leading edges just like supersonic fighters, only more so. Their leading edges are razorsharp (we're talking accidental decapitation sharp), and their entire fuselage is built to be part of one big wing which deflects the shockwaves just below the engine compartment.
     
  17. Jul 10, 2003 #16
    Cool thanks for that russ and enigma, cleared that up for me as best as i could have hoped.
     
  18. Jul 11, 2003 #17
    hihi, I'm sorry to disappoint you, but that is also misconception. Think - difference in length between top and down of airfoil is few percent. Max pressure difference you can possible get in airspace is 1 atm. Thus, based on that alone your dP can be only few percent of 1 atm.
    But I was pretty in the weeds too. :smile: In reality things are quite abit more complicated. See at the end.

    When air is detached from wing, then what is replaceing it there?? vacuum as extreme case. When above wing is vacuum and below is 1 atm, isn't that condition for maximum lift?

    At 0 aoa ANY wing will produce zero lift. Problem with asymetric wing is that its aoa is not what you visually would think it is.

    Do you know that asym wing and "barn door" produce equal lift at upto 12 aoa? Do you know that asym wing in inverted flight produces SAME amount of lift at 12 aoa? The only difference is that cambered wing has higher stall aoa than flat wing, and THAT makes it preferred wing and eventually better overall lift capacity, but in only normal flight.

    But to better back up, I searched alittle. Here is short summary from Nasa site:
    http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

    "The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems"

    The real details are best (of what I found) covered here: http://www.av8n.com/how/htm/airfoils.html

    Please read it, it clarifies misconceptions very well and shows off real complexity of wing lift.

    "The misconception that wings must be curved on top and flat on the bottom is commonly associated with the previously-discussed misconception that the air is required to pass above and below the wing in equal amounts of time. In fact, an upside-down wing produces lift by exactly the same principle as a rightside-up wing."

    Each of the following statements is correct as far as it goes:
    The wing produces lift "because" it is flying at an angle of attack.
    The wing produces lift "because" of circulation.
    The wing produces lift "because" of Bernoulli's principle.
    The wing produces lift "because" of Newton's law of action and reaction.
    We now examine the relationship between these physical principles. Do we get a little bit of lift because of Bernoulli, and a little bit more because of Newton? No, the laws of physics are not cumulative in this way.

    There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli's principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton's laws."


    Bottom line is that most misconceptions are due to overeager simplifcations.
     
  19. Jul 11, 2003 #18

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, not exactly. The pressure on top of the wing cannot go below 0 atm (obviously), but the pressure on the bottom of the wing can (and does) increase.

    It's not that air is detatching, it is that the airstream is detatching. What is left is stagnant air which gets dragged along with the wing. Stagnant air (due to Bernoulli... no velocity) has the same pressure as the surrounding airmass.

    This is absolutely not true. AoA is defined as the angle between the freestream velocity and the leading edge-trailing edge chord line. The symmetry has nothing to do with it.

    Almost all wings produce lift at 0 AoA. You actually need to specially design an asymmetrical wing to produce 0 lift at 0 AoA.

    Do a search for NACA airfoil Lift Coefficient/AoA charts, and you'll see.

    No they don't. Lift varies greatly by airfoil crosssection.

    No, they don't. Where are you getting this information?

    Again, absolutely not true. If it were, then commercial jets (which do not usually go above 6 to 10 AoA would have flat wings (because they're cheaper to build), and fighter planes would have the fat cambered wings.

    Please note that it does not say that an upside down wing produces the exact amount of lift. The principle is the same, yes, but the numbers are not.

    Like the brain teaser pointed out: the only fundamental contributors to lift are pressure and friction. That's it.

    The angle of attack is not a "because". Angle of attack increases lift because it slows down the air on the bottom of the wing, increasing pressure.
    Circulation is not a "because". Circulation is a way to come up with a number for lift without first obtaining pressure distributions and integrating across the surfaces of the wing.
    Bernoulli's principle is not a "because". Bernoulli just shows why the pressure drops when the speed increases. (or vice-versa)
    Newton's Laws are also not a "because". They are another possible way to find out how much lift is generated if you know how much downwash is created.

    I'll agree wholeheartedly here.
     
    Last edited: Jul 11, 2003
  20. Jul 11, 2003 #19

    russ_watters

    User Avatar

    Staff: Mentor

    Obviously we're arguing on the same side here for the most part, but could you explain that? How can a symetric wing produce lift at 0 aoa? The pressure profiles will be exactly the same on the top and bottom surfaces (by definition).

    I'm guessing you didn't read his link, enigma....

    wimms, the title of that website is "A New Spin on the Perceptions, Procedures, and Principles of Flight," ie much of what you find there is *NOT* conventional aerodynamics. Some of the things on the site such as the definitions of chord line and camber are the accepted definitions, but this should be noted:
    And HERE is his justification. It would seem he is using what he would consider more convenient for pilots. Though this definition will work (you'll have to redo pretty much all of the calculations and wind tunnel tests ever made on lift, but hey, to each his own), it is *NOT* the generally accepted definition. The generally accepted definition is "The acute angle between the chord of an airfoil and a line representing the undisturbed relative airflow." www.dictionary.com Like I said, the other way would sorta work, but it would complicate life greatly. And oh, by the way, the AOA indicator on an airplane uses the generally accepted definition - so this guy will confuse pilots more than he will help them.

    Also, I have skimmed the site and thats the only glaring innacruracy I see. There is some info on a barn door (for example), but though he explains that the MECHANISM is similar, he does *NOT* say that the resulting lift is the same. If you have a link to that 12 deg aoa thing, I'd like to see it.

    One other point: Nowhere does he say that there is a vacuum on the upper surface of the wing during a stall.

    wimms, I really recommend you read some ACCEPTED explanations on aerodynamics. Though most of that site is ok, the differences are enough that they will cause you lots of headaches trying to reconcile what he is saying with what everyone else says.
     
  21. Jul 14, 2003 #20
    Guys, if you go to laugh at me or argue me out, thats ok with me. But if you go on and laugh at someone who has enough insight into the subject to write a book for pilots, then excuse me, I don't trust you. Such attitude sounds like "anyone who doesn't precisely follow my textbook is a crackpot".

    I admit I have inadequate understanding in the subject, but I've seen enough debates on that subject to at least know this: physics of aerodynamics is not finished and is not explained away by bernoulli alone.

    Are you actually pilot to say such things? He covers his definition here:
    http://www.av8n.com/how/htm/aoa.html#sec-raoa-aaoa

    "..we are free to choose how the angle-of-attack reference stick is aligned relative to the rest of the wing.
    Throughout this book, we choose to align the reference with the zero-lift direction. That means that zero angle of attack corresponds to zero coefficient of lift. According to the standard terminology, the angle measured in this way is called the absolute angle of attack.

    Some other books try to align the reference with the chord line of the wing. The angle measured in this way is called the geometric angle of attack.

    If you try to compare books, there is potential for confusion, because this book uses ``angle of attack'' as shorthand for absolute angle of attack, while some other books use the same words as shorthand for other things, commonly geometric angle of attack. To make sense when comparing books, you must avoid shorthand and use the fully explicit terms.

    Also note that are many possibilities, not just absolute versus geometric; the choice of reference is really quite arbitrary. It is perfectly valid to measure angles relative any reference you choose, provided you are consistent about it. (Aligning the reference stick with the fuselage is useful in some situations)

    Using the chord as a reference works OK if you are only talking about one section of a plain wing. On the other hand:
    On typical airplanes, the chord of the wing tip is oriented differently from the chord of the wing root. Which one should be considered ``the'' reference?

    When you extend the flaps, the chord line changes. (See section 5.4.3 and section 5.5 for more on this.) Most books that choose to measure angle of attack relative to the chord line violate their own rules when the flaps are extended, and continue to measure angles relative to where the chord of the unflapped wing would have been. That is illogical and creates confusion about how you should use the flaps. This is one of the reasons why it is advantageous to think in terms of absolute rather than geometric angle of attack.

    Thinking about geometric angle of attack would be advantageous if you were building an airplane, or conducting wind-tunnel research on wing sections. Engineers can look at a wing section and determine the geometric angle of attack. "

    Hope that clarifies confusion I caused in this thread. I took his definition of absolute aoa as conventional. Enigma, some of your objections were to points from the book and came from this confusion.

    As to commercial airplane aoa vs fighters, com-planes have cambered wings to reduce drag for more comfort. fighters don't have high aoa, at mach-1 that would cause pilot loose it and excess stress on craft, doesn't it? They need least air resistence for max speed. And besides, they have to have same stall performance upside down. no?

    http://www.av8n.com/how/htm/airfoils.html#sec-inverted-camber

    "At small angles of attack, a symmetric airfoil works better than a highly cambered airfoil. Conversely, at high angles of attack, a cambered airfoil works better than the corresponding symmetric airfoil. An example of this is shown in figure 3.14. The airfoil designated ``631-012'' is symmetric, while the airfoil designated ``631-412'' airfoil is cambered; otherwise the two are pretty much the same.11 At any normal angle of attack (up to about 12 degrees), the two airfoils produce virtually identical amounts of lift. Beyond that point the cambered airfoil has a big advantage because it does not stall until a much higher relative angle of attack. As a consequence, its maximum coefficient of lift is much greater."

    Seems like I jumped to unwarranted generalisation from symmetric airfoil to 'barn door'. Should have stayed at symmetric wing.
    Yes, that was purely my own imagination. (but nowhere does he even talk about stall physics). My reasoning was that airstream detaches because air mass of stream is unable to follow the wing shape due to air inertia, leaving volume of depressurised air between wing and detached airstream. Still we may find thin vacuum between airstream and stagnant air.

    I'm not into highend aerodynamics. This site came up just to show off that wing lift isn't simple to express in terms of bernoulli alone. There are many opposing 'theories' around that are all in some way wrong, "pressure difference alone matters" included. As far as I understand, nothing in this book is NOT ACCEPTED explanations on aerodynamics.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Forces on a wing lift come from the pressure difference
  1. How wings generate lift (Replies: 11)

Loading...