Rear Bear Vs. The Russian Bear

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by pie_row, Dec 2, 2009.

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  1. Dec 26, 2009 #361

    Starman

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    Okay fellas, I think I owe everyone an apology and an explanation for saying that the way most people think is a form of insanity. I was trying to dump on Pie face but ended up dumping on myself.

    Anyway, this is how thinking is viewed from the perspective of some people who have been practicing stillness meditating for a long time:

    First off, the kind of thing you do with math and engineering analysis is what is referred to as reasoning as opposed to thinking. It is thought about a specific problem with a specific and physical concrete goal in mind. That's perfectly fine and good and it isn't crazy.

    What is meant by thinking is the trains of words that go through most people's heads almost continuously. The trains of words usually keep coming around in loops, over and over, and they are frequently a type of self brainwashing, if you will. For example, even when a person realizes that these trains of words are wrong or even meaningless in the context of their understanding they often do not have the power to stop these trains of words. I think you'll agree that this is a little bit crazy.

    Then there is another aspect to thinking as it refers to our actions in interpersonal relationships. Often the way we deal with others are automatic responses that arise on their own, whether they be with friends or unfriends. In other words, they pop out from our unconscious programming that we learned as children. It is usually after the fact, or during the act, that people rationalize their actions, but the reasons that we come up with will frequently be a type of ego self defense and not the 'true' reason for the reaction. I hope we can agree that these that these are things we all do, and that also is a type of craziness.

    Now, after some time, months or years, of meditation, we can sometimes put the brakes on and prevent these spontaneous actions from arising, at least prevent some of the self damaging ones some of the time. Some people have more native ability with this type of restraint even without meditation and others are more 'broken'. Frequently the pre-meditation response that people will have to their own negative reactions is to blame the other person, and that's a little bit crazy too. Going by the definition that if a person continues to repeat an action that does them no good then that is a bit of crazy.

    So perhaps it would be best to think of it in terms of levels of dysfunctionality, and although it may have looked like it, I did not mean to imply that you are a bunch of raving lunatics if you can't stop your thinking, I'm there too sometimes as well, it's just that I've seen the other side enough to know about the difference.

    OK? I hope that helps.

    Peace to you and may your new year be filled with health and prosperity.

    Oh yes, and Santa brought me a 3D design program. :ban:
     
  2. Dec 26, 2009 #362

    Mac790

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    There were also special prepared versions of the Spitfire to fight with German high altitude reconnaissance Ju-86 P2, one of those fight took place at 15 000m.

    Seb
     
  3. Dec 27, 2009 #363

    lr27

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    You'd be better off milling it out of styrofoam! (and putting some other stuff on the outside.) Titanium is heavy. Therefore, you'd have to use very thin sections. These sections would buckle. You'd have to put styrofoam (or probably something a bit stiffer than that) inside to prevent that. Now, if you have a really high load in a small space, it might be just the thing to use. Airliner wing spar?

    Also, I understand that titanium is pretty hard to machine.

    Straight strength to weight is far from the only consideration.

    One of the nice things about, say, unidirectional carbon composite, is that it's very stiff but not heavy. At least it's stiff in one direction. In that direction, it's about as stiff or maybe even a bit stiffer than titanium. (Depends on the exact type, but that's typical.) However, it's only a bit over a third as heavy. A section that was the same thickness as the titanium would obviously be comparably stiff, but just over a third the weight. Or the same weight and twenty (I'm not kidding) times as stiff. Assuming the load is all in one direction. If not, of course, you have layers in other directions too, although it would probably still come out ahead. By comparision, aluminum that was the same weight as titanium would be a bit under 3 times as stiff, and aluminum of the same stiffness would weigh 70 percent as much. Wood does even better, assuming you have room for it.

    But something like styrofoam lets you work with really thick sections without much weight. Of course, there are exotic foams with better performance.
     
  4. Dec 27, 2009 #364

    JimCovington

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    1) That just may (maybe, perhaps!) have been a tongue-in-cheek post about milling it from titanium.

    2) Stiffness and strength are not the same.
    Panes of glass are stiff but not strong.
    Aramid fibers are strong but not stiff.

    3) Styrofoam (expanded polystyrene) generally sucks as a building material because most of the "good" adhesives dissolve it.
     
  5. Dec 27, 2009 #365

    Autodidact

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    Actually, it may have been (at least partially) tongue-in-cheek, but the fact that I know NOTHING about titaniun other than the fact that people with lots of money and/or lots of titanium like to use it, needs to be factored in there.
    Is it titanium's thermal properties that engineers like the most? But that can't be all; the russian aerobatic planes tend to use it as landing gear leg material, don't they?
     
  6. Dec 27, 2009 #366

    JimCovington

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    For the same weight, it's stronger than steel but not as stiff.
     
  7. Dec 28, 2009 #367

    Autodidact

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    Very interesting; titanium has a modulus of elasticity of slightly more than half that of steel and a yield strength of not quite twice that of steel. I think this means that titanium "stretches" more than steel but also takes almost twice the stress before deforming plastically?
     
  8. Dec 28, 2009 #368

    lr27

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    Titanium is great when you need:
    -strong material that resists corrosion
    -you need something stronger than aluminum but that builds up a thicker section than steel for the same weight
    -stands up to heat (I forget the properties, but I have the impression that it holds its properties to a higher temperature)
    ---------------------------------------
    When you're designing something mechanical, you balance density, stiffness, and strength. If a material is light but weak, and you have enough room, you can make up for that by making it thicker and therefore stiffer and stronger. (The stiffness will go up by a larger factor than the strength.) You could make those landing gear legs strong enough with wood, but then they'd be really bulky, and way too stiff. You could make them strong enough with steel, but the airplane would probably sag too much unless you made them a lot stronger than you needed. Or hollow. Titanium may have been just right. Or perhaps it was a marketing ploy.
    ------------------------------------------
    There are epoxies and polyurethanes which work just fine with styrofoam, plus you can hot wire it. Or you can use something like some of the urethane foams, but don't hot wire them!
    -------------------------------------------
    Just for yucks, I just did a back of the envelope analysis based on a wing half the size of the one proposed, which was engineered by someone who really knows what he's doing, plus he provides us many of the properties. AND I've seen this wing flown many times including violent launches which probably match the specs given.
    http://www.charlesriverrc.org/articles/bubbledancer/PDFs/bd_V3.pdf
    Anyway, we have a wing weighing about 1.1 lbs, 117 inches span, 13.5 aspect ratio, can take 150 lbs load, which is good for 90mph full up elevator. We double the size of everything, double the section thickness to 18 percent(to take advantage of increased Reynolds number), switch from balsa sheeting to 1mm plywood, and double the cross section of the spar cap (already 4X). The ply is at 45 degrees and is FAR stiffer in torsion than the balsa was. I get that the wings weight is now around 15 (!) pounds, but I've identified some pessimistic errors of mine which mean it might be less. On the other hand, perhaps we should double that to 30 to account for ailerons. (This wing has only spoilers as is.) It should be good for about 2400 lbs ultimate load, which is not bad for a proposed airplane that would probably have a gross weight of something like 300 lbs! If the max Cl is 1.5, stall speed is probably around 50mph. Resonant frequency in torsion is something like twice as high as the small wing, assuming the large wing weighs 30 lbs and weight is distributed similarly. Resonant frequency in bending is something like 0.7 or 0.8 times the small wing's. I don't know the relation between chord, stiffness, and flutter speed, however. I'm guessing that as the chord goes up, you'd need higher airspeed to get the same flutter frequency. If necessary, it wouldn't take much more material in the caps to stiffen them up. A couple of pounds. If I was pessimistic about aileron weight, then it's already available.

    I suppose I could go over to "Basic Glider Criteria" and use their calculation, but I'm getting tired.
     
  9. Dec 28, 2009 #369

    lr27

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    Steel is a variable target when it comes to yield strength. There is steel which is good for 200,000 psi or more. For that matter, I think titanium is a variable target as well. I know with steel you can get a yield anything from 30kpsi to over 200kpsi depending on the alloy and heat treating, and any possible work hardening.

    For the set of properties you dug up, you're right about the stretch. I used to have a glider with both titanium and steel wing rods. The steel rod was kind of lousy so it was around the same strength as the titanium. Both were undersized and would take a permanent bend if I didn't watch it. But with the titanium rod the bending in the elastic region was much more obvious, so it was easier to back off the winch in time. Even so, the designed rod was undersized and I used to have a collection of 3 or 4 bent titanium wing rods.

    BTW, the styrofoam comment was a bit tongue in cheek too.
     
  10. Dec 28, 2009 #370

    Autodidact

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    lr27, thanks for that! I take shots in the dark but when it results in an informative post like yours, I feel that it's worth the risk of embarassment. This is not only helpful to me, but to other neophytes as well. And yes it was just "structural steel", apparently architectural type (I-beam) and titanium "alloy" whatever that meant.
    Interesting about using the elasticity of the titanium spars as a sort of "gauge"; that sort of experiential mental picture provides almost as much information as the dry specifications.
     
  11. Dec 28, 2009 #371

    autoreply

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    I said that for low drag you want the widest point pretty far forward. Turbulent low drag that is. Laminar flow is an illusion when your prop is completely messing up your nicely laminar flow, hell, they don't even get it to work past a 1/64" scratch in a glider...
    Well, the difference is the material, composites can be extremely smooth (when polished more than you like) which is the difference with the Mustang that by the way achieved significant laminar flow, just not past the point with negative pressure gradient.
    Well, the fact that Pie_Row doesn't bother about reality makes things slightly harder ;-)


    As for Pie_Row's design; 100 lbs (or 40 lbs airframe) is possible, no doubt, all other things compromised, so AR=4 Delta, very light construction, tailless, horizontal pilot position.

    That's the point however, Pie_row aims at 12.5 aspect ratio. That's simply not feasible for such a weight and wing area, only your spar will be over 40 lbs. (it's a ridicuously thin wing, and both LR27 and Pie_row still have half of the spar caps outside of the airfoil, completely neglecting buckling and such)
    Just some remarks:
    *That model doesn't have a 250 lbs load in between the spars, this one does.
    *When upscaling; dynamic pressure goes up with the square and weigth with the cube, so flutter with the 5th (!) power... Any clue why the torsional section usually is more than half of a gliders wing weight?

    Can a 50 lbs airframe, capable of carrying 180 lbs pilot+fuel and a 50 lbs engine be build? Yes, while challenging it can be done for sure. There's no point in doing it however since a bit more weight will lower the required power and raise all other specifications while also lowering cost..

    I suggest Pie_row to simply build this wing, just to show us we're wrong ;-)


    As for titanium, the very reason the Russians use it for their aerobats is energy density, it can absord much more energy per unit of mass compared to aluminium and considerably more than steel.
     
  12. Dec 28, 2009 #372

    JimCovington

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    The aircraft you just described is totally and completely impossible. It will never work and it will kill the pilot if it ever does manage to get off the ground.

    -Lord Kelvin



    (For those of you who are still hitting the xmas eggnog, or who have begin 2010 celebrations a bit early - the above post is a poke at this whole thread, including me.)

    There are two links in this post. FOLLOW THE LINKS!


     
    Last edited: Dec 28, 2009
  13. Dec 28, 2009 #373

    autoreply

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    [/FONT]
    Source
    Get rid of the flaps, landing gear (one wheel is enough), canopy and install lighter engines and you're there :)
     
  14. Dec 28, 2009 #374

    pie_row

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    With an 8 lbs engine and a 15lbs landing gear and very minimal instrumentation more like 60 lbs for the air frame.



    I've made some assumption That will tend to help things along in the light wing department.
    A 3:2 taper ratio
    A 20% thickness root profile tapering to an 11.111% at the tip (root section is 3.8 inches deep.)
    I did indeed make a math error in my calc and thank you for pointing that out. I'm now calculating a spar cap size needed as 0.25sq inches. For the compressive loads at 9G and 1.5 factor of safety.



    I'm not trying to carry a 180lbs pilot I weigh about 150~155lbs. And I am most definitely not trying to carry a 50lbs engine. DA-150 weighs in at less than 8 lbs bare and if it weighs more than 16lbs installed I'll.... At a minimum flying speed of 60 mph I really don't think that I'll be needing a re-rive either.
    Well I'm thinking that it should come close to going 100mph on 1gl/hr. Or 100 mph at 100 mpg. That is a reason for doing it if ever there was one.



    3.6” is a more reasonable spar depth. Under the assumption that I'm making. This should cut your weight guess for the spar in half.
     
  15. Dec 28, 2009 #375

    pie_row

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    yip
     
  16. Dec 28, 2009 #376

    pie_row

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    Autoreply So what is the flutter like on a high aspect ratio wing at 400mph? and how many Gs?
     
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  17. Dec 28, 2009 #377

    JimCovington

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    It's like this:

    [video=youtube;kQI3AWpTWhM]http://www.youtube.com/watch?v=kQI3AWpTWhM[/video]

    NOT 400mph.
     
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  18. Dec 28, 2009 #378

    pie_row

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    You changed what you wrote and what you quoted of me after the fact. And so this statement of yours is a lie. And in doing this you abused you moderator privileges.
     
    Last edited: Jun 22, 2011
  19. Dec 28, 2009 #379

    JimCovington

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    I guess I'd have to know how much the model's pilot weighed to answer that. :)

    What I'm saying is that you haven't scaled the 150lbs dead weight for yourself. It makes a big difference.
     
  20. Dec 28, 2009 #380

    Mac790

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    Yes you are right, I should write a little bit longer sentence. For CFD analysis usually surface models are used. Those models don't incorporate things like rivets, gaps between a fuselage and doors, etc. So it might be some differences between your virtual model and real object (it's really hard to create perfect smooth surface, always you will have some kind of gaps, etc). So even if you create a perfect laminar shape, it doesn't mean you will get it in real world.

    About Avanti, I don't know how close is it to the laminar shape, I haven't seen one, so I can't tell how smooth surface is. Did they really try to design laminar fuselage, or maybe they "achieved" it accidentally. I would really like to see analysis for it.
    There are so many questions without answer, so I really don't want assume things.

    But two thing are sure, it looks nice, and it's fast.

    edit. it also seems that the main target was to create "extra" lift
    http://en.wikipedia.org/wiki/Piaggio_P.180_Avanti

    Seb :)
     
    Last edited: Dec 28, 2009

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