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Rear spar hole flanging

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  • Rear spar hole flanging

    The plans call for most of the holes in the 0.032" rear spars of the LSA to have a 3/8" flange. Are most builders doing this? Does it really help strength? Does it cause any distortion of the spar? I have also been warned about the 2024T3 cracking when this is done. Any thoughts?

  • #2
    I was worried about that when I did mine but it ended up to be no bid deal. I tried using the same "rubber press" method I used for flanging the ribs and found that wasn't going to work. I made a set of flanging dies and used my vise to press them in and it went just fine. No distortion and no cracks.

    I have a thread in the LSA plans build forum that has a link to a PDF of my wing build. The process I used is in it.


    • #3
      fyi, the link is:


      • #4
        Thanks. I had not seen that pdf. I do not have a vice that large but I have the steel to make a hydraulic press and a wood lathe to make the hardwood dies.


        • #5
          Today I got my home made hydraulic press set up and flanged the outer section of one rear spar. The flanging went very well and it looks great. But much like I suspected, it bowed the spar in two plains, across and length wise. In the length, it is about 1/4" bow. Clecoing a cross angle to the spar will not remove the bow in that plane. My parts look almost exactly like yours except they are made of hardwood.


          • #6
            Not sure what was different. Perhaps the hydraulic press applied more pressure or the wood gave a bit more then the bench vise/aluminum dies and so allowed distortion of the material just outside the flange ares. I.E the actual dies distorted a bit outside the flange area and imparted this distortion to the material where it shouldn't have been formed or the flange wasn't set consistently around the diameter..

            Like I said, I was worried at that point after all the work that had gone in to match drilling all the strips, brackets, etc. but mine didn't seem to distort at all.

            Note I tried the rubber forming process as was used for flanging the ribs and that did not work.

            The flanged edges of the spars need to be straight/parallel as that sets the top/bottom wing surfaces and cannot be "adjusted" as the wing is framed. If you lay a straightedge from root to tip on the flanges where the skin will attach they must be straight. I would be surprised if that relationship was impacted.

            A bit of a lengthwise(I.E. from root to tip) bow in the plane perpendicular to the wing skin wouldn't be as troublesome as once you put the spars in to the assembly structure, you will see they can sag quite a bit and this is worked out as you piece in the ribs and skin. You will be using some temporary jigging to hold the front and rear spars parallel to each other at the correct distance and a 1/4 inch bow(not a localized kink but a gentle curve) over the length of one half of the spar (around 8' or so) likely would be easily taken care of..

            The bow in the "vertical" perhaps could be adjusted out with a reasonably large padded tube running the length of the spar in the center and some strategic "pressure" to bend the outer edges down.

            Message me a phone number if you want to discuss.


            • Bcone1381
              Bcone1381 commented
              Editing a comment
              I will validate that more pressure gives better results. This is not from personal experience, rather an Aluminum Rib Building lecture at Oshkosh 2015. Also, the Larger the Hole the Greater the Pressure required to limit the distortion. The Presenter liked to use hex head bolts, and commented that on larger holes the Bolts will get distorted over time due to the high torque applied to them. I thought an air impact wrench work well.

          • #7
            As I think on this a bit more, I believe the issue may be with the die flexing a bit and therefore the flange would vary slightly around the diameter.

            I think I was OK because I ended up with a pretty stout die set, not out of forethought, but simply because of the material I had laying around.

            I think pulling the two halves together with a centered through bolt and sturdy backup washers would ensure a consistent flange as the die would distort evenly around the outside. Pressing the two halves together with a vise could set up a distortion where the area backed up directly by the vise jaws is a bit more stretched/formed that the area not directly under the jaws if the die set distorts slightly..

            I suppose if the backing plates used in the press(or vise) were beefy and covered the entire surface of both die halves things should be fine as well.

            Don't know but sometimes it is good to be lucky.


            • #8
              Could someone please tell me the distance between wing spars on the LSA? By that I mean the length of the main ribs. Thanks.