A challenge to you all

Discussion in 'The light stuff area' started by stan40353, Sep 11, 2011.

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  1. Jun 25, 2012 #621

    PTAirco

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    For this kind of unconventional design, I would certainly build myself a single panel and test it to destruction, in a properly controlled set-up. That usually answers any lingering doubt as to what is exactly is going on under load. And it's cheap enough to do that and gives you a chance to practice sticking the skin on without mishaps.
     
  2. Jun 25, 2012 #622

    Autodidact

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    I'm not an engineer either, but I have been reading about skin buckling lately. The force that the skin will react to that can cause buckling is the stress times the skin cross sectional area that is subjected to that stress and, since the skin is very thin, the force is not that great even though the stress is. To find the stress, you slice the wing at a section and calculate it's area moment of inertia. Then you plug that into the beam stress formula and it gives the stress at any distance "y" from the neutral axis. The spar has a much larger cross section and, while it is under less stress, its cross section times its stress yields a much larger force that it reacts to and it has the lions share of the bending moment. The problem is that, while the skin does not contribute greatly to the bending strength of the wing, it is under a lot of stress and if it is not supported by the ribs at narrow enough intervals, it will buckle and the formula to determine its buckling strength with respect to its unsupported length (between ribs) is based on the Euler column buckling formula. The problem with the Cessna 172 skins buckling is that the rivet spacing is not small enough to prevent inter rivet buckling and this could be for numerous possible reasons such as 1) an engineering miscalculation, 2) corner cutting due to cost, 3) corner cutting due to the desire to keep the weight low, 4) rivets working loose so that end fixity of skin segments is lowered, 5) etc., etc....

    Can't we all just get along?
     
    Last edited: Jun 25, 2012
  3. Jun 25, 2012 #623

    Head in the clouds

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    Much as I sometimes have difficulty with Gordo's manner I usually find that what he has to say is logical. I like this concept of wing building but, as presented, it has problems which have been tried and tested many times over here and no-one has been able to make it work.

    In essence it's pretty mad to try and mix spars which deform under load and skins which cannot conform to that. If you want to have a flexwing then it's no accident that wings of that kind are invariably fabric covered. And of course that requires a more substantial internal structure, particularly to resolve the drag/anti-drag loads and also the torsional loading, hence twin struts (rather than single), or wire bracing. In this proposal the skins are being required to carry the drag loads and, in combination with the absolutely inevitable oil-canning regardless of close rib spacing (even if the wing has a solid foam core - if it has a tubular spar) there will, under high lift conditions and close to the stall, be a substantial 45 degree rippling of the top surface skin which will tear it away from foam ribs/core. This is not supposition it is well proven and documented and has happened here numerous times. Basically you cannot mix tubular spars with metal skins, and most particularly not with rigid foam ribs. Additionally the use of struts with tubular spars is far from ideal unless you also have jury struts to limit the bending between the fuse and the strut attach point, another reason why wire bracing would be superior for this type of wing structure.

    Also - I know personally quite a number of people who retrofitted aly D nose skins (intending to improve the foil shape under flight loads) to apparently stiff wings which had front and rear tubular spars with two point (each) wire bracing and the aly skins were buckled and torn within an hour or two of flight. The prototype of my Macro series of aircraft Macro* Mac Ultralight* Mahaonas initially had Klegecell PVC foam ribs bonded with acrylic adhesive to the aly skins and the ribs tore (not the bonding) after the first stall test even though the spar was an I beam and no apparent bending could be induced into it. The future Macros had beaten aly ribs and substantial drag spars and that solved the problem.

    Having said that, in the case of the Macro I think the lack of a drag spar was the major issue, rather than the foam ribs, and that is my major concern (other than the tubular spar) with Gordo's proposal.

    I wonder whether it wouldn't be better to get rid of the (potential) problem altogether and furnish the wing with a front C section spar similar to the rear one. There would be very little extra work in making the ribs two-piece instead of one-piece, and bonding them to each side of the front spar, and adding a drag spar from the outboard strut attach point to the rear spar fuse attach point (using a strut would then make sense). Also the C spar would then be near the wing centre of pressure and eliminate the risk of the spar, complete with some epoxy, tearing away from the ribs rotationally in shear when subjected to high torsional loading.


    I'm surprised to hear this, is this the case in Canada particularly or in USA also? Because I've been generally surprised at how few ultralights have broken inflight over here. We have had a couple break because of flutter issues and a Drifter lost a blade and cut the tail off but generally our crashes have been through pilot stupidity of one kind or another - bad weather, tiger country, fuel exhaustion, low level aeros, rising terrain etc.
     
  4. Jun 25, 2012 #624

    autoreply

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    You don't have to be an engineer to understand it, as you've just shown us....

    Bottom line, with your numbers, the skin will see twice the shrinkage/elongation of the spar caps. If we ignore buckling, if the cap and skin are the same size, the skin will take 2/3rd of the load (twice as much as the spar cap). If we bring skin buckling to the table, you probably have to reinforce your spar by orders of magnitude to keep the skin below the buckling load, highly dependent on how you restrain the skin from span wise buckling of course. Basically you make the spar stiff enough to avoid buckling of the skin.

    Now take a strip of alu cap material and hang a car on it. Now put that same flat sheet and compress it a bit to get a feel for how low a buckling load is. Two orders of magnitude oversized spar caps might be required. BBerson and you are right...
     
    Last edited: Jun 26, 2012
  5. Jun 25, 2012 #625

    bmcj

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    Just a wild thought with absolutely no engineering analysis, but what if you went with a flexible corrugated aluminum (somewhat like the Ford Trimotor)? It would be more material and ore weight, but the ridges could flex with the spar (so long as the spar is carrying the entire bending load) and the corrugation may even enhance the torsional rigidity.
     
  6. Jun 25, 2012 #626

    Head in the clouds

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    That's a nice bit of lateral thought but the (chordwise) corrugations would prevent you from curving the skin over the cambered surface and also you would have removed any drag-load support that the skins would provide if they were flat. So it'd be a kind of belt and braces approach in reverse.

    There really isn't any getting away from it, either you build a flexwing or a rigid wing, and flexwings need a flexible covering, rigid skins need a rigid spar...
     
  7. Jun 26, 2012 #627

    BBerson

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    Gordo,
    By my count Davidb, PTAirco, Autodidact, Head in the clouds, Autoreply, bmcj all agree with my comments.

    I have already determined that commenting on your ideas is a waste of time for you because you already know everything about airplane engineering.
    My comments are for the benefit for those with less experience that visit this forum.

    I will NOT respond to you directly if possible, but I WILL post comments that concern matters of safety as I see fit in the future because this thread and this forum is not just for you.

    Take care Gordo.
     
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  8. Jun 26, 2012 #628

    Hot Wings

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    The BD-5 wing is of the configuration being discussed. The skins wrinkle. Fixes tried:

    Double the number of ribs. Skins still wrinkled.

    Thicker skin and double ribs. Skin still wrinkled.

    "I" beam internal spar stiffener. Skins oil can but no permanent deformation.

    Just one data point, but real world experience to build upon.
     
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  9. Jun 26, 2012 #629

    topspeed100

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    Oh ...so you notissed that too. Some people are possibly born to know everything beforehand.
     
  10. Jun 26, 2012 #630

    gordonaut

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    Last edited: Sep 25, 2012
  11. Jun 26, 2012 #631

    gordonaut

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  12. Jun 26, 2012 #632

    davidb

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    Gordo, I'm a wood guy but I really like your idea because the simplicity of construction makes me think I could build with aluminum as you describe. The adhesive tape is a bit expensive but well worth it to avoid a thousand rivets. The buckling discussion probably only came up because we don't see the struts in your sketch. Will you post a sketch of the struts and how it all attaches to the fuselage? I'm having trouble visualizing how you address torsional loads with a single spar.
     
  13. Jun 26, 2012 #633

    gordonaut

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    Last edited: Sep 25, 2012
  14. Jun 26, 2012 #634

    gordonaut

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  15. Jun 26, 2012 #635

    topspeed100

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    Can someone in layman terms explain me why this aeroplane is under 50 kg empty weight ?

    BTW: Is McCulloch M101 10 hp engine only ? Or the same 12,5 hp as in Whing Ding II of 122 lbs weight. http://en.wikipedia.org/wiki/Hovey_Whing_Ding
     

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  16. Jun 26, 2012 #636

    litespeed

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    I am still a little confused about your calc's and saying no wing buckling will occur.

    You say the wing will only have two feet unsupported after the strut and will flex 1 inch. Given that the skin must move 1inch in 24 inches of skin, I would expect buckling- /the top skin is under compression and the bottom tension. This very thin skin will have to move somehow, given the short distance for this 1inch to be taken up- I would think buckling is still going to be a issue. I accept you calculated for 9G but as you said it is quite easy to attain that.

    Phil

    Why don't we look at having a c spar instead and remove this issue, as has been suggested.
     
  17. Jun 26, 2012 #637

    Head in the clouds

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    You've got your sums wrong somewhere.

    Your very inaccurate remarks makes one wonder whether you've ever actually built or owned a tubular spar wing. I have, several of them.

    There is no such thing as a VERY STIFF tubular spar, regardless of how you brace it, let alone doing so with a strut at a single attach point.

    The Oz certified Drifter has triple sleeved spars, almost full length not just 18" and about 3ft tip cantilever, with double bracing points on both front and rear spars and the tips can easily be deflected with one hand by a couple of inches. In flight they visibly move several inches and that is at probably less than 2Gs.

    I built a single seat midwing aircraft with tubular spars which had full length double sleeving and similar bracing and tip cantilever (similar to the Drifter), one of the simple tests we used to do on those types was to flex the tips by partially lifting the empty airframe to observe the deflections between the bracing points so as to make an assessment of the bracing balance i.e. the tendency for lift loads outboard of a bracing attach point to deflect the spar inboard of it and so affect the inner wire tensions. You could easily deflect the tips by 2 inches with around a 100lb load.

    The point about a tubular spar is that it deflects very easily at first and then stiffens up as the load and deflection increases, however the damage will have been done to the skins by that time.

    As for your 1" tip deflection at 9G, you could be quite close in the example you give because your 70% half span attach point will be reasonably close to the noble point of the spar's length and the aerodynamic loads upon it, although allowing for tip losses I would bring the bracing point in a bit. However, just because your tip deflection could even be zero, if you truly selected the noble point and correctly compensated for reduced outboard load due to tip losses, that doesn't mean that the spar would be undeflected, in fact it would be in a shape resembling a sideways 'S'
     
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  18. Jun 26, 2012 #638

    Head in the clouds

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    Yes, and there's probably no problem with the strength of the skins/box in that application but the problem you have is that the load path from the skins to the spar is just a ring of polystyrene glued to the spar. As your Cp moves fore and aft (and you haven't mentioned that above) the torsional load could be very high and I wouldn't bet my life on the shear strength of polystyrene...
     
  19. Jun 26, 2012 #639

    gordonaut

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    Last edited: Sep 25, 2012
  20. Jun 26, 2012 #640

    topspeed100

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    Stan since I also have been on this subject now for a bout 6 years I can show you two really small and relatively cheap to manufacture types. Other is a pusher and the other a tractor design...and both btw would be the world smallest aeroplanes if built.

    Wings off they could without a prop fit any closet ( cables loosened ) !

    Bigger is more practical...electric as well...very economical to operate ( for trade secret purposes some features are not presented here ).

    Smaller one's tail volume has not been checked ( looks a tad too small ).
     

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    Last edited: Jun 26, 2012
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