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Behavior of the air when the angle of attack increases

  1. May 26, 2014 #1
    I have to write 150 to 250 words on the behavior of air when the angle of attack of a wing increases.
    I have 2 graphs, one is on the Cl and the other on the Cd

    This is for the drag coefficient:
    http://www.plaatjesdump.nl/upload/89b3970a883a30f9fb840a6982d2859c.jpg

    This is the graph of the lift coefficient:
    http://www.plaatjesdump.nl/upload/71ab849935f91c7361c56ac0b7a9d0e3.jpg

    I can see that the lift coefficient decreases rapidly after the 10° mark and the drag increases more rapid.
    But honestly, I have no clue on how to write the 150 to 250 words on this or why the air behaves like that. I have no idea where to start.
    If someone could help me get into the right direction, I would appreciate it very much.
     
    Last edited: May 26, 2014
  2. jcsd
  3. May 27, 2014 #2

    verty

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    Homework Helper

    I take it you know how a wing generates lift, just search online for this information.

    Now look at images 1 and 4 http://web.utk.edu/~cnattras/Physics221Spring2013/modules/m9/turbulence.htm [Broken]. Notice in picture one that the air molecules passing over the top of the cylinder stay in contact with the cylinder, the air molecules have a relatively stable speed, try to understand this, this is very important. No air molecules are misbehaving, it is all very orderly.

    Now look at picture 4, the air above the cylinder is not staying in contact and air is getting dragged along behind the cylinder. Argue that the factors that generate lift start working in other ways (explain how) to create the effects you see in the graphs.
     
    Last edited by a moderator: May 6, 2017
  4. May 27, 2014 #3
    The main problem was that I do know how a wing generates lift and after a certain point generates more drag than lift but I didn't know how to write that down.
    I have made alot of progress and I have actually finished it.

    The air will deliver with every degree of increase in the angle of attack the same amount of lift. This is observable in the graph in the lineair line, from -5° to 10°.

    After the angle of attack reaches it's critical angle of 10° and the angle of attack keeps increasing, the liftcoefficient reduces drasticly. This is because of flow separation, the air flow can't follow the wingprofile.

    This separation is already occuring at low angle of attacks but the air not separated remains dominant over the separated air.

    When the angle of attack increases, the air that separates from the wing will also increase. After a certain angle of attack, the separated air will become dominant. The air flow will produce less lift but will produce vastly more drag.

    This is clearly observable on both graphs. The liftcoefficient reduces drasticly and the dragcoefficient will increase exponentially.

    So between certain angle of attacks, the air flow produces more lift than drag but after it's critical angle has been reached, which was 10° in this wing profile, the air flow will produce less lift but produce more drag.

    From an angle of attack of 10° does change of air flow occur around this profile.
     
  5. May 27, 2014 #4

    rcgldr

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    As a wing travels through the air with some angle of attack, the upper surface of the wing moves downward with respect to the air, and the air follows that surface to fill in what would otherwise be a void. Below the critical angle of attack, most of this flow follows the surface of the wing from front to back. Above the critical angle of attack, the air starts to form vortices where the bottom of the vortice involves forwards air flow, reducing lift and increasing drag. Sufficiently beyond the critical angle of attack, it's almost one very large vortice. Do a web search for youtube wind tunnel stall to see examples like the one bekiw. Note that there is significant separation below critical angle for this wing at the speeds used in this example.

    https://www.youtube.com/watch?v=xW63SZ1LAqo
     
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