Aerodynamics and aeroelasticity

Discussion in 'Flight Test Data' started by drgondog, May 24, 2008.

  1. drgondog

    drgondog Well-Known Member

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    Soren you have explained to us that I don't know what I am talking about when I say the study of Aeroelasticity was only an art during WWII.

    Here is the place to debate it

    You also have said you know a great deal about thiis subject and explained it many times to other members of the this forum.

    Here is is the place to demonstrate this.

    First, I would suggest that you explain the two or three characteristics of an airframe that are of most importance to an airframe structures guy that the Science of Aeroelasticity is close to yielding analytically today and approximately when in time the art started transitioning to a 'science'?

    To refresh your memory, here are questions for you to start the ball rolling:

    Explain
    1. Aeroelasticity - How would you model an airframe system to yield analytical results for a.) natural frequency of the system and components with at least one degree of freedom?

    2. How or why you believe that the designers of the Fw 190 (or ANY Airframe Designer - ANY nation) understood aeroelasticity and used that knowledge in their designs

    3. Give an example of an airframe Model' period, which was developed during WWII -or even afterward.. and explain how those analytical model results, from any approach in preliminary airframe design used during WWII, were close to actual results under loads.


    We will keep it cordial. Maybe I know stuff about this that you don't - maybe you know more. We can learn?
     
  2. DerAdlerIstGelandet

    DerAdlerIstGelandet Der Crew Chief
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    I for one am looking foward to this discussion.
     
  3. Glider

    Glider Well-Known Member

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    Me too, I was taught the basics but nothing compared to this level of detail
     
  4. buzzard

    buzzard Member

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    Me too :)

    ...let's get ready to ruuuuuuuumble!

    I'll be back as soon as I can find my Kevlar jock.
     
  5. drgondog

    drgondog Well-Known Member

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    Guys - I am going to wait one day to see if Soren answere the questions about aeroelasticity before we 'dabble' - and I do mean dabble.

    I know the theory, I know structures and I know a lot about aero - but I called this an arcane art in WWII for a reason.
     
  6. renrich

    renrich Active Member

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    I am looking forward to this. I hope I can figure out what youall are talking about.
     
  7. Arsenal VG-33

    Arsenal VG-33 Member

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    I've read that high speed aircraft like the SR-71 and the civilian Concorde would "stretch" while in flight. I've never understood exactly how this phenomenon occured, or how their designs took this into account. Is this related to the question? If it is, I'm interested.
     
  8. buzzard

    buzzard Member

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    I'm not familiar with the Concorde 'stretch' but I presume it had the same cause as with the SR-71...heat expansion.

    The SR-71 had several design features to accomodate this problem. One was the longitudal corrugations of the wing surfaces. The hi-flash-point fuel (JP-7?) was also used as a heat sink. The a/c also leaked fuel while on the ground due to the designed loose fit of some of the structure.

    JL
     
  9. Glider

    Glider Well-Known Member

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    Heat was the cause of the Concorde 'stretching' this impacted the C of G and was compensated for by fuel transfer in three trim tanks. Center of lift was impacted by her speed and the tanks were used to compensate for this.

    One interesting difference between the SR71 and Concorde. The SR71 was painted black as that was the best way of getting rid of excess heat, whereas Concorde used special white paint.

    Same problem, totally different solution.
     
  10. pbfoot

    pbfoot Active Member

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    I believe all aircraft stretch after a while . I'm aware of a C47 and a F101 that stretched over the years
     
  11. drgondog

    drgondog Well-Known Member

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    You are right about thermal expansion, and with the very high heat range experienced by this a/c, the allowable stresses are reduced significantly for the structural materials. IIRC at Mach 3., 60,000 feet the surface temp is around 500 degrees F and the stagnation temp much higher (i.e leading edge of the wing.

    Aluminum didn't cut it so both Titanium and stainless steel were introduced to solve the 'stress problems' (lightweight, higher stress/wt and higher yield points)

    Two more complications for something like an SR-71

    At the surface (outer skin) to inner caps (stiffeners) to webs (connecting caps) to internal and shielded from High Temp surface - there is a temperature gradient going from high to low.. this temp characteristic has two effects - One introduce thermal stresses within the materials, and 2.) lowers the rigidity of the structure introducing Creep... simply 'stretch'

    All of these associated 'strecthes or deformations' have a tendency to alter both the airfoil and the dihedral of the wing.

    The top of the wing for example in high G manuever is at lower temp than bottom of wing (Higher CL, lower pressure/higher velocity on top surface) resulting in introducing 'more' dihedral' to wing and possibly negative camber to airfoil - all altering handling characteristics.

    Creep under these conditions and under repeated loads will introduce factors similar to fatigue

    I think I have this right - it has been a long time
     
  12. syscom3

    syscom3 Pacific Historian

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  13. drgondog

    drgondog Well-Known Member

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    Syscom - Good post - it actually pretty much got basic definitions and avenues of investigation right, at least nothing in my knowledge base over rides it. I note with amusement that they aren't delving into analysis techniques.

    I'm also not entirely certain of the date, as described, because certain aspects of aeroelasticity were investigated earlier when it was possible to isloate the system under analysis from the overall airframe.

    Von Karman, for example figured out alternating vortices on bridge suspension cables were the complex frequency inputs derived from a cross strong wind - leading to resonance, Departure and failure of a bridge (in WA state? 1930's??)

    Flutter on a P-38 elevator, as symmetric, would reduce to analysis with the math in hand in WWII, ditto the 109 elevator when they went to pure canteliver on the 109F.. trying to figure out the failure mode - which I believe was resonance/departure.

    I know when i was fooling around with it in late 60's/early 70's there was a long way to go - and frequently impossible to do in Preliminary Design.

    Thanks for the post - everyone can take a look - including Soren if he needs it.
     
  14. Soren

    Soren Banned

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    Christ :rolleyes:

    Bill when will you sieze with the pissing matches?? Don't you think I know what this is all about ?? Could you answer every single of those questions above Bill ?? No. Also when did I ever become an a/c designer Bill ? Have I ever claimed to be one ?? All you want is a fight, you have no intentions of keeping this cordial.

    It's also very convenient that you avoided all other of my questions, and the reason is clear: You can't support your claims, your bold claims that the P-51 109 are close in terms of turn performance and that the field of aeroelasticity was considered witchcraft by aerodynamicists during WW2 being perfect examples.

    Now I can tell you what aeroelasticity is and what its effects are on an a/c (Although wiki covers allot of it), I can also tell you that it was in no way witchcraft during WW2 which you claimed it was and that even the Soviets had Scientists specializing in this field, namely M.V. Keldysh, in the early 1930's. That having been said we get better at each field within science as time goes by, and ofcourse aeroelasticity is better understood today, and also A LOT easier to guard against because of the ability construct and test an airframe in sophisticated computer simulations before ever deciding to actually build it. During WW2 the methods were crude by comparison and the most reliable results were achieved by conducting test flights. One method used was carefully examining the wing profile under heavy loads while at the same time establishing the maximum load factor of the wing itself.

    Finally I asked you to wait until Crumpp came on the scene, why did you ignore this Bill??

    Anyway following your next reply I'll consider wether it is at all worth participating in this thread..

    All in all I consider myself friendly not to just ignore this thread..
     
  15. drgondog

    drgondog Well-Known Member

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    Soren, here is a fundamental problem. You do not know much about either fundamenta Aerodynamics (the underlying physics of fluid mechanics) or airframe structures and the disciplines of modelling an airframe along different approaches depending on which problems you are trying to solve.

    If you would stick to what you know a lot about, we wouldn't get into these 'pissing contests'

    Virtually every equation you have used, including CLmax, are mathmatical approximations based on different boundary conditions and the model you are trying to solve.

    The classic airfoils in the tables are always 2D, 'infinite' wing, constant cross section and no twist. Then get ready to modify the crap out of it based on twist, based on tip/chord ratios, based on tip geometry, based on wing plan from, based on 'cleanness' of surface (nacelles, bomb racks, dirt, wavy surfaces)..

    But you swoop in on someone who may know less than you about boundary conditions and start pontificating.

    Fine - sit out. I can debate and I can learn - what have you got to teach me?
     
  16. Soren

    Soren Banned

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    And in the same test a heavy draggy Fw-190 Jabo turns with the Mustang Mk.III, not only proving that the Bf109 tested wasn't even close to being pushed to the limit, but also proving that the Fw-190 fighter turns better than the Mustang.

    LoL! All you have done Bill is quoted Rall and then relied blindly on dubious Allied tests. I on the other hand have not only cited the comments from many LW aces test pilots, including LW chief test pilot Heinrich Beaviaus, but also from all modern Bf-109 pilots (Even forwarding an interview on video), and then backed it up with undisputable aerodynamic facts.

    And Finally Gene made a very nice comparison which clearly showed the Bf-109 to be a greatly superior turn fighter compared to the P-51, and on top of that proving the fact that the Fw-190A was a slightly better turn fighter than the P-51. But ofcourse you did, as always, try to refute this because it simply didn't suit your opinion on things, what a surprise..

    The excellent comparisons posted on this forum by Gene (Crumpp):

    Bf-109 K-4 1.98ata
    [​IMG]

    Fw-190 D-9 C3
    [​IMG]

    P-51D Mustang
    [​IMG]

    So Bill, would you care to present us all to the analytical approach of your claim that the P-51 Bf-109 are close in terms of turn performance ??? Now THAT would be interesting!

    No you don't and below is the proof:

    Sit out ?? Bill you started this thread with questions direct at ME! You have no intentions of either keeping it cordial or learning anything. Even what Gene says you insist upon refuting! But I've had enough, cause you'll never admit when you're wrong!

    Oh and Bill, in the future don't think you can dictate what other people should do, cause you can't and you don't decide who shall answer which questions first. And trying to do so only keeps people from wanting to participate at all!
     
  17. kool kitty89

    kool kitty89 Well-Known Member

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    Soren he obviously created the thread to get this ongoing argument (as well as some of the side comments on aerodynamics, incliding mine on Elliptical wings) off of unrelated threads (namely the Spitfire vs 109, La 7, Yak 3 thread) and get it a bit more organized.

    No offence, but assuming it's a personal attack seems a bit paranoid.
     
  18. Soren

    Soren Banned

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    It doesn't if you read his posts in which he AGAIN insults me to try and pour fuel onto the already enormous fire.

    Also keeping in mind all the threads he has sidetracked I have every reason to believe this is just another attempt at creating a fight.
     
  19. kool kitty89

    kool kitty89 Well-Known Member

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    However the biggest point Bill seems intent on arguing currently was your (Soren) statement about the Fw designers planning on the elastic deformation under G. But you admitted you were wrong already after Gene corected you.
     
  20. kool kitty89

    kool kitty89 Well-Known Member

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    At least this way you have a specific thread to discuss this rather than (either of you) derailing other topics Soren.
     
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