sailplane spar joining methods

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

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  1. May 30, 2009 #21

    BBerson

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    That looks like a late model metal skinned Schweizer 1-26 wing photo. To get a good look, just help somebody assemble one sometime at the airport and see how it works. The Pilatus B4 metal glider has a real neat wing spar attach also.
    BB
     
  2. May 30, 2009 #22

    handprop

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    Thanks BB, I'm going to try and look up the Pilatus tonight and see what I come up with. There are some guys I know with gliders so I should be able to get a first hand look.

    Mike
     
  3. May 30, 2009 #23

    ultralajt

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    Pilatus B-4 wing yoint looks pretty similar as you show on your two photos above. Maybe a bit different look but the same principe.

    Mitja
     
  4. Jun 2, 2009 #24

    George Sychrovsky

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    This is what I’m talking about. Contrary to your statement “They should be full depth (or nearly so) all the way to the end” they taper down almost to one third, from 6 inches to 21/4 to be exact.
     

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  5. Jun 2, 2009 #25

    Norman

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    OK, George, it can be done either way. I would still rather keep the extreme fibers as far away from each other as possible.
     
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  6. Jul 6, 2009 #26

    ultralajt

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    Well, I find the wing root atachment to the fuselage of Pilatus B4.

    Mitja :ban:
     

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  7. Jul 6, 2009 #27

    orion

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    You really don't need to nor, in some instances, is it even advised. If you draw yourself a free-body diagram of the loads and the resolution of same into the mounting pins, what you'll find is that the pin at the end is really only in shear. In other words, the only load on the inner-most pin is a vertical in-plane shear (no bending). As such, the material surrounding the pin must be able to take the localized vertical load and of course the resultant bearing stress caused by the pin and its bushing.

    Moving outboard from that "base" mounting point you then have the vertical shear load acting further and further away from the area of interest - as such, the vertical shear load times the distance then equals a moment and it is for that moment that you're trying to increase the distance between the spar caps. For that reason can the caps can be tapered down at the end (there's no moment there).

    And the maximum strength needs to be at the point of the other pin since that immediate area sees the maximum root bending moment of the wing.

    You can keep the caps apart as you say, but then you also need to design in the proper shear mechanism into the web since that too is a potential failure source.

    There's a few design variables to balance here so the end configuration will depend on a few specifics of the actual layout. For instance, if you have a very high bending moment it is unlikely that you want to put a kink in the spar cap that's usually under compression. But in an aerobatic application it may be beneficial to avoid the kink in both caps. But if you have a kink, then you also need to stabilize the cap there for the resultant out of plane load. This means more external structure capable of handling the proper amount of shear.

    Choices, choices.....
     
  8. Mar 13, 2011 #28

    planebuilder

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    So the important thing to remember here is there must be a complete understanding of all the wing reactions. The design of this sailplane wing root is similar to the ASW 28. Notice the steel pin at the nose of the root rib? There is also a similar pin near the trailing edge. This wing joint is designed to react wing bending internally by pinning both spars together. This joint is made independant of the fuselage, in other words there is not a common bulkhead that this pin would pass through. The other wing load reactions that need to be considered are Vert and Fore/Aft shear, drag and accelerated stall condition. All of these latter conditions are reacted through the two wing root pins. Each pin has a receptor socket in the fuselage root rib, these sockets each have a compression tube that attaches to its opposite mate on the other side of the fuselage. All of the sockets are capable of reacting vert and fwd/aft shear. The drag and the accelerated lift loads are reacted as a couple between these two sockets. Look at the zero g dive condition, there is a drag load that is trying to bent the wing aft. This load is reacted by placing the rear sockets in compression. The aft shear, is again reacted by both pins and sockets. The much heaveir load that needs to be accounted for and reacted is due to the acceperated stall condition. During a sharp pull up at V Max, the wing lift vector changes direction and a componant of this vector is reacted by the wing trying to bend FWD, this load and shear again is reacted by placing the FWD pin in compression and again, both FWD and AFT pins react the shear. When you add self connecting controls, the brilliance of this design is really apparent. I really like this design because it only requires two pins to connect both wings. I am in the process of redesigning my Robin Robin Ultralight wing to this configuration. I am using a blade/fork design rather than two tapered spars. The redesign also allows me to correct an oversight on the prototype wing and that is the lack of a wing walk. I have to reinforce the root ribs to the point where a pilot walking on them is not a design load condition
     
    Last edited: Mar 13, 2011
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  9. Jun 6, 2011 #29

    Lucrum

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    Pilatus B-4 wing root.jpg



    I am considering a wing attach method similar to the above diagram. Except the spar and caps will be one continuous piece (laminated wood) and the pins going through the upper and lower caps would 1) be in the same vertical plane and 2) would be located at the side of the fuselage. I had planned on a, as yet to be determined, welded and or bolted steel truss of sorts to pin the wing to.

    1) Am I in LA LA land? IOW would this be a generally accepted/satisfactory method?
    2) Is there a reason/benefit to having the upper and lower pins in different vertical planes, as in the diagram?
    3) I'm assuming this arrangement would be considered a "Pinned" wing?
     
  10. Jun 6, 2011 #30

    autoreply

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    Everything can be made to work.
    Not that I know, but there're a couple of good reasons not to do this. The bending moments are reacted to the fuselage in an almost scary way. Search for the Ka6 and Ka2, ASK 13 gliders. If I understand you correctly they have exactly the same method which works fine (same materials too).

    Why the asymetrical idea (pins on the side of the fuselage)?
     
  11. Jun 6, 2011 #31

    Lucrum

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    Raymer, "pinned" wing attachment. I'll search for those other gliders, when you say they have the same method do you mean the same as shown in the diagram or what I was asking about?
     
  12. Jun 6, 2011 #32

    autoreply

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    What you described with the joint in the middle.

    So basically like in post 3, but in wood and with the attachments to the fuselage at the rear and forward part of the root ribs.
     
  13. Jun 6, 2011 #33

    Lucrum

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    I might not have described what I had in mind very well.
    Center Section Crack Locs.jpg
    I was thinking along the lines of the above pic except with two bolts on either side for a total of four. With the bolts going through the upper and lower caps as opposed to the center of the spar as depicted above. (I'm currently planning on a laminated wood I-beam spar)
     
  14. Jun 6, 2011 #34

    orion

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    That looks sort of scary since the bottom connection does not have a straight load line to the other side. But if it was done in a more structurally sound manner, I don't see any reason why this would not work. As far as reason behind it? I'd be willing to bet it was because this could be assembled by a single person. You lift the wing up and position the outer (lower) pin in place. You then pivot the wing up and support it, after which point you simply slide in the center pin (ok, may need two people) and the aft pins. But it is simple.
     
  15. Jun 7, 2011 #35

    caribeanbound

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    The Janowski J2 with a cantilever wing uses a tapered( fore/aft) spare end that works out pretty simply. Just like a lot of sailplanes.You might find drawings on the Yahoo Janowski aircraft site. I believe that one of the group posted them.I have a set but my camera isn't here to download the drawing.
     
  16. Jun 7, 2011 #36

    autoreply

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    Why put something "between" the spars? You're looking at two loads, internal bending moment in your spar (huge) and the weight of the fuselage (times the max g-loading, not so huge). Common practise is to deal with both in their own way. One continous spar, either cut in the middel or with overlapping spars:
    [​IMG]
    Then, hang the fuselage from some hardpoints, usually at the leading and trailing edge of the root ribs. To prevent the wings from folding during the violent deceleration of a crash, you add a simple bar between the two root ribs, usually between both rear and fwd ones.

    @ Orion; this way too you can assemble them alone (if they weren't so #$%^^ heavy). Lay the wing on the fuselage and connect the rib attachments. Lay the other wing on the fuselage too. Lift one wingtip and support it, lift the other and support it too.

    A tensioning-bar (to pull the upper spar caps together) makes it a truly one-man operation.

    Here the Ka6:
    [​IMG]
    Spar caps joined by 2 pins, fuselage hinged by attachments of which one is visible.
     
  17. Jun 7, 2011 #37

    Lucrum

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    Scale Attachment.jpg

    1) I really appreciate the time and effort spent by some of you more knowledgeable guys answering my questions.
    2) Above is a 1/6 scale wing I put together with scrap materials using the construction method I currently intend. ("one piece" wing, laminated spruce I-beam spar, three solid wood ribs per wing [root, 1/2 b and tip]with lightning holes, foam core, 1/16" ply skin covered with a single layer of S-glass to protect the wood and allow for some sanding)
    3) While I don't want wing R&R to be a major headache I don't plan on doing it often. (I hope)
    4) The bolts on the model wing are larger than scale and I didn't drill holes for the lower spar cap bolts, AFAIK I don't even need the lower bolts but I would feel better with them
    5 ) I haven't worked out details of the fittings/structure on the fuselage where I had planned to attach the wings yet, hence the sheet metal bolted to the spar in the photo.
    6) Hopefully this will give you a more accurate representation of what I was trying to describe.

    One of my original questions was is there any reason not to have the upper and lower attach bolts in the same vertical plane,directly above/below each other?
     
  18. Jun 8, 2011 #38

    highspeed

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    Watch your bearing loads with the wood. Attention needs to be paid to the grain direction to eliminate splitting of the wood under load. For concentrated loads it is advisable to use high density bearing material to prevent crushing. Check out ANC-18, it's got everything you ever needed to know about wood structures.
     
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  19. Jun 15, 2011 #39

    autoreply

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    No, unless geometry dictates so (no place for both bolts aligned vertically).

    Ow, and a subtle kick for this topic ;-)
     
  20. Jun 15, 2011 #40

    Lucrum

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    Thanks, although I hadn't forgot about it I hadn't looked at it yet either.
    After having looked at it Instead of four bolts, two on either side through the upper and lower caps, I'm considering only one larger bolt on either side running through hardwood bearing blocks wedged between the upper and lower caps. Any thoughts or suggestions on this configuration are most welcome.
     

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