sailplane spar joining methods

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by handprop, May 28, 2009.

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  1. May 28, 2009 #1

    handprop

    handprop

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    Hey guys

    Could any of you point me in the direction of methods used in joining sailplane wings. Iv'e been spending each night studying structural design of wings and I am starting to look ahead at how I want to join my aluminum wings in the steel tube fuselage for my Doodlebug project.

    The Sonerai is a similar type set-up as far as aluminum wings and steel tube fuselage. The method used here is a steel box where each spar slips in the box at the 1/2 way point and then pinned top and bottom on each spar at the root.

    For the Doodlebug I was thinking about studying how some of the sailplane folks do it where the spars overlap but I cannot seem to find much information on how they do it. Does anyone know how this is done on aluminum wings or know of any sailplane websites where I can generate some ideas for some calculations?

    Mike
     
  2. May 28, 2009 #2

    bmcj

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    SuperGlue?
     
  3. May 29, 2009 #3

    ultralajt

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    It depends strongly on the wing type. Is it cantilever or strut supported, it is one or multi spar...ect...

    In cantilever wing design, a one of options is the central wing yoint on uper and at lower spar caps as it is done at Monerai and Moni gliders. See attached photo and drawing. Top joint shown. Equal allso is mounted on bottom spar cap. The bolts that connect "hinge" with spar cap are stressed to shear, and allso the main pin that connect both joints is stressed to shear.

    If wing is supported by struts, the attachment hardware is easier to build as they are much les stressed.

    If wing attachment is made on a fuselage side structure, the internal fuselage structure must be strong and build according to spar carry trough loads.

    Mitja
     

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  4. May 29, 2009 #4

    ultralajt

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    Another option on design where both wing spars goes trough a box in the fuselage, can be seen on a Cri-Cri plane.
    The Cri-Cri wing spars are connected trough a tunnel made in the fuselage.

    Mitja
     
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  5. May 29, 2009 #5

    bmcj

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    Some of the glass sailplanes have a joiner box in the fuselage through which tapered spar ends are inserted and pinned. The spar end on one wing tapers forward and the spar end on the other tapers aft so that the two mate tightly in the box. Both spar ends retain their full vertical height. The taper is only fore or aft as seen from above. Forgive the crude drawing; it was a very quick MSpaint creation.


    [​IMG]

    Bruce :)
     

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  6. May 29, 2009 #6

    lr27

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    I hadn't played with the CAD system in a while, so I did a fancier picture. See attached if it doesn't come up in line.
     

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  7. May 29, 2009 #7

    Norman

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    Here's one similar to the one Mitja showed but designed for a wood spar. If you're doing a plastic spar the one Bruce showed is probably better, not as many holes in high stress areas and fast to assemble. This page has a picture of a variation on the tapered ends that has one spar fork and the other fits into the forked piece. If you're going to use carbon spar caps don't drill holes through them. Either do a fiberglass bearing plate like Marske's system or the tapered friction fit
     

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  8. May 29, 2009 #8

    Norman

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    Almost. You've [got] them tapering in both dimensions. They should be full depth (or nearly so) all the way to the end. i.e. the caps taper but the webbing dosn't
     
    Last edited: May 29, 2009
  9. May 29, 2009 #9

    George Sychrovsky

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    That is not true , Flight design CT’s use taper just like shown in the drawing above , I don't have a pic at this moment but others use it also

    http://www.hpaircraft.com/hp-24/update_30_march_09.htm
     
  10. May 29, 2009 #10

    handprop

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    Thanks fellas, interesting..... My wings will be all aluminum with a steel tube fuselage so maybe the drawing Bruce whipped up for me makes the most sense.

    I believe my thought process is correct, wherein a spar that overlaps vs. spars that but up on the ends would be a stronger method, all things considered.

    It would be easier if I could keep the verticle height the same height and taper one dimension only like you guys were saying.

    The box also makes sense. I have a drawing of the Cri Cri spar joint method but it's way more complicated than it needs to be. I would imagine the box would have to be perfect in order for the two spars to fit tight and then pin. I bet that would make one heck of a strong assembly.

    Dihedral would make things a bit more complicated :shock:.

    Any ideas on which method of spar building would best allow me to somehow build taper in the ends (built up strips vs. angle extr.)? Anyone have any experience with this or have seen it done?

    Mike
     
  11. May 29, 2009 #11

    handprop

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    George, just missed your post. The drawing like Bruce had is what your talking about right? I guess i'm a little confused.

    If I had a box to slip the spar into the verticle depth should remain straight I think?? Yes-no?

    Mike:)
     
  12. May 29, 2009 #12

    handprop

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    Normans method(link) is interesting as well. To use just one pin would be pretty neat. With one pin the box structure might require some built-in tapered pieces to mate with the tapered spars.

    Huh, I suppose it could be pin-less at the spar joint and have two tabs, one at the wings LE and one near the TE to keep it rigid.

    Arg!!! :)

    Mike
     
  13. May 29, 2009 #13

    bmcj

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    There is no rule that says the bayonette ends of the spars have to retain their full vertical depth. However, a full depth allows your carry-through box to mate up with and support the spar across the full length of the insert. If you go with a vertical taper, then you must rely on the pins to carry the bending loads because the spar caps will not bear against the carry-through box (unless you want to make a more complicated box structure with tapering receptacles). Also, the deeper the shear web, the greater the bending load they can carry.

    Bruce :)
     
  14. May 29, 2009 #14

    Norman

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    Don't do that, Mike--

    The LE and TE of the wing are nearly useless in bending. The spar caps are what has to be firmly connected to the fuselage. You will also need a third point to keep the wing from twisting, that'll most likely be a compression spar at the control surface hinge line.
     
  15. May 29, 2009 #15

    bmcj

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    I second what Norman says here.

    Bruce :)
     
  16. May 29, 2009 #16

    handprop

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    Thanks Norman, I see what your saying.

    Now that I think about it, using no pins in the spar results in the box taking the stress whereas if overlapped and fastened, the loads are more concentrated into the spars themselves.

    Hmmm.... a lot to think about here.:)

    Mike
     
  17. May 29, 2009 #17

    Norman

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    Thanks for the link. Another page on that site shows more pictures of the spar layup.


    Notice that I said "or nearly so". Ir27's drawing looked like it had too much taper to me. Structurally you don't really gain much, if anything, by tapering the root end of the spar but it does complicate the mold so if you don't have to why bother. Now bending the bottom cap would allow you to set a dihedral angle so that would be a good reason for taper but just a few degrees.
     
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  18. May 29, 2009 #18

    lr27

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    That picture was just conceptual. Th taper merely saves you some weight, I think, and some expensive material, perhaps.

    Just for yucks, I ran the crippled FEA that goes with the CAD, and it showed the max. stress down near the base of the stub. So if the taper is excessive, it's not by much.

    The FEA had some simplifying assumptions that make it less than definitive.
     
  19. May 29, 2009 #19

    BBerson

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    Here is a G109 motorglider spar stub photo.
    The Schweizer 1-26 or 1-23 is worth looking at for a metal spar stub end.
    BB
     

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  20. May 29, 2009 #20

    handprop

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    The photos and links you guys posted are interesting. Here are two photos of a glider spar and pocket I found so far. Thanks for the tip BBerson. Mike
     

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