in the enclosed picture are three examples of types I'm speaking of. In the top picture, the lift struts are attached via a single eye bolt. The eye bolt is obviously vertical through the wingspar

I looked at the rudimentary engineering drawing elsewhere on this site showing the forces acting on the wing spar, and get that the wingspar is tending to make an S curve. Let's assume that the wing spar in question has an internal sleeve doubler to effectively increase wall thickness to resist compression and prevent the collapse of the tube.

In the middle drawing, the most common method method. Two straps fore and aft, attached via a bolt horizontally though the spar. I have an elementary understanding that this second, commonplace method puts drilled holes through what I believe is the "neutral axis" of the spar.

finally, the bottom illustration of the "texas parasol" type of fitting. It shares a commonality with the eye bolt in that there are holes drilled through the compression and tension axis vertically through the spar and attaching a u shaped bracket to the bottom of the spar into which the lift strut is through bolted. Two holes, albeit smaller in diameter than the single but larger hole of the eye bolt.

enter image description here

  • $\begingroup$ welcome to the site! I just rotated the image so it can be read better $\endgroup$
    – Gypaets
    Commented Mar 21, 2020 at 17:37
  • $\begingroup$ @ToddMcLauchlin Please have a look at the tour to find out more on how this site works. I had to reject an edit to John K's answer twice already. Please use comments under the answer instead of edits to ask for clarification. $\endgroup$
    – Bianfable
    Commented Mar 24, 2020 at 7:30

1 Answer 1


Vertical bolt = less optimal. The reason is the tube walls in the 12:00 and 6:00 locations (top and bottom areas) are effectively the primary tension and compression members of the tube, and the 9 - 3:00 areas are the shear zones.

In other words, if the tube was an I beam, you'd be putting holes right in the cap strip flange of the I beam where the tension and compression loads are highest. You're like the plumber who drills holes in floor joists too close to the edge and severely weakens the joists.

The horizontal bolt is much better because the holes are in the neutral axis of the tube, the middle of the shear zone, and the bolt/nut serve to prevent the tube from ovalizing (squashing flat) under bending load. So the horizontal bolt actually strengthens the tube overall, because the hole has little effect on the upper surface's compression strength and the lower surface's tensile strength, and the bolt and nut support braces the tube against flattening (by preventing the sides from spreading), so the upper and lower surfaces can get closer to their actual stress limit potential.

So the middle picture, pound for pound structure wise, is the best method. However you can do it the other way if you size the material to compensate for the strength lost to the holes at the top and bottom, so either way will work.

And in fact, on something like an ultralight with wire braced wings and a spar supported at multiple points along the span, a designer may find a vertical eye bolt like the top sketch to be the most elegant solution in the greater scheme of things, and it's worth while to do it that way and compensate as required with heavier meat adjacent to the holes than would be needed with a horizontal hole.

But if you tested the single vertical bolt configuration against the single horizontal bolt configuration, with identical hole sizes and material thicknesses, the vertical one would almost certainly fail first.

enter image description here

  • $\begingroup$ John K thank you for your reply. I think I need to post a new question as to how to proceed to address the issues you have identified. $\endgroup$ Commented Mar 24, 2020 at 7:56
  • $\begingroup$ @ToddMcLauchlin you need to find a book on the design of tubes as beam spars. You need to know how to do the stress analysis to select and size the material, then when you have a design you have to build it and do a static load test on it (set it up upside down on a test fixture and pile on the sand bags) up to the intended load limit to validate the analysis. That's if you actually intend to bet your life on the structure in the air. If you don't plan to do all that, don't bother. Just build a Goat from the free plans available on-line. $\endgroup$
    – John K
    Commented Mar 24, 2020 at 15:08
  • $\begingroup$ Wouldnt the vertical connection also have unwanted bending stresses on the strut from trying to meet the wing at a fixed angle? $\endgroup$ Commented Mar 5, 2021 at 12:42
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    $\begingroup$ The lift strut in his diagram with the vertical bolts still has a pin joint horizontal bolt where the strut connects to the bracket, so deformations in the spar tube under load won't transfer any movement in the bracket into the strut tube, so no. The only real problem with it is the inefficient connection with the spar tube itself where the bolts are both removing meat from the critical tension and compression axis, and also promoting flattening of the tube when bending is applied, further weakening it. $\endgroup$
    – John K
    Commented Mar 5, 2021 at 17:29

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