Aerodynamics thread

Discussion in 'Air Force' started by siegecrossbow, Mar 12, 2012.

  1. PhilFYW
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    PhilFYW New Member
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    upload_2019-3-30_21-49-52.png
    This graph from Dr Song's paper shows that a lower aspect ratio wing is capable of producing a greater max lift coefficient compared to a higher aspect ratio wing.

    I believe this is the graph you referred to when you say that the paper shows that a smaller wing would generate more efficient vortexes.
     
  2. PhilFYW
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    Just a question - why is yaw used so infrequently in ACM and dogfights? It seems that most media and news represent maneuverability as turn rates and roll rates, and yaw is frequently left out of the equation.

    Shouldn't yaw be an important part of maneuverability since it allows faster response time (pitch + roll vs yaw)?

    Thanks in advance!
     
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  3. Air Force Brat
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    an interesting question? which may leave you with more questions than answers? so let me take a stab at giving you an answer.

    In straight and level flight aerodynamic forces on the aircraft are balanced, or "trimmed"... you are in "trim".....

    If you apply aft stick to increase pitch, you increase lift as well as drag, that will result in a climb and a reduction in airspeed if power is not increased....

    forward stick decreases pitch, hence decreases lift and drag, resulting in a descent and increasing airspeed if power is not reduced and drag increased by adding flaps or lowering landing gear.....

    if you apply left stick, you decrease angle of attack on the left wing, and increase angle of attack on the right wing, resulting in a "roll to the left".....

    if you apply right stick, you decrease angle of attack on the right wing, and increase angle of attack on the left wing, resulting in a "roll to the right".....

    if you step on the left rudder, you will yaw the nose left, you have increased the angle of attack of the vertical stabilizer, pushing the tail of the aircraft to the right....

    if you step on the right rudder, you will "yaw" the nose of the aircraft to the right, you have increased the "angle of attack on the vertical stabilizer, pushing the tail of the aircraft to the left.....
     
    #1113 Air Force Brat, Apr 8, 2019
    Last edited: Apr 8, 2019
  4. Pmichael
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    Pmichael Junior Member

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    This is literally impossible. And pretty sure the paper doesn't claim it.

    What the paper mostlikely does that a low aspect ratio design is for fighter aircraft, especially in transsonic and supersonic regimes, the better option. That fact is basically the bread and butter why there are so many delta fighters out there.
     
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  5. Air Force Brat
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    Yep, you're right..... but its easy to get confused as you try to sort all this out, especially from papers and articles that may not always be as clear as we mean to be....
     
  6. Air Force Brat
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    So now that we have that out of the way, pitch and roll are natural functions when flying the airplane and make coordinated flight possible, the rudder is able to contribute by negating the effects of adverse aileron yaw...

    if you pedal left or right, you will "yaw" the airplane, which causes a "skid" or uncoordinated turn... it feels very awkward, and unnatural, especially to your passengers which will find it disconcerting...

    NOW, back to ACM, when we are at very low speed, or even post stall, in a 5th Gen fighter, or even a 4+ Gen with OVT, its possible, even productive to make a "pedal turn" where you yaw the aircraft or point the nose where you want it... with the rudder pedals....

    This is possible with the amazing new aerodynamic characteristics of newer, more docile aircraft... if you tried that in an F-4, F-14, F-15, you would "depart" the aircraft and no doubt lose control in your "departed" state, you would have to "recover" the aircraft by "unstalling" the wing in order to return to the fight...

    if you were to do this at low altitude in an older jet fighter, you would be creating a "smoking black hole", and if you and your RIO were unable to "punch out", you would both be DEAD! as in Terminated!

    hope that helps answer your question....
     
    #1116 Air Force Brat, Apr 9, 2019
    Last edited: Apr 9, 2019
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  7. latenlazy
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    latenlazy Colonel

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    It’s entirely possible with the right configuration. The idea here is that what you lose in lift coefficient from a lower aspect ratio you gain more in lift coefficient from optimizing vortex coupling interactions, which is what’s explain in Song Wencong’s design paper. There’s probably a limit to how much you can increase lift coefficient in this way, but the effects are, according to the paper, experimentally proven. If the wing were interacting with the free stream by itself without any vortex generating devices then reducing aspect ratio wouldn’t have this effect, but that’s not what’s being looked at here.

    Edit: Here’s the citation from the study.

    https://defence.pk/pdf/threads/translation-of-j-20-article-by-dr-song-wencong.165231/
     
    #1117 latenlazy, Apr 9, 2019
    Last edited: Apr 9, 2019
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  8. latenlazy
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    latenlazy Colonel

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    Yes. I mistakenly attributed the effect to a smaller wing because for the same sized body a lower aspect wing usually means a shorter wingspan, but the aspect ratio is what’s important here.
     
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  9. Pmichael
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    Pmichael Junior Member

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    That quote doesn't claim what was said in the post. In fact the paper just repeated old aircraft design wisdom.
     
  10. latenlazy
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    “Under high AOA conditions, liftbody LERX canard configuration aircraft concentrate lift on the body and inner portions of the wings so moderately lowering the aspect ratio will not only not lower the max lift coefficient but raise it (see figure 10).”

     
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