# How do I reinforce a tubular spar that has been drilled vertically for an attachment bolt?

Previously posted a question about lift strut attachment to tubular wing spar with a bolt vertically through the spars. See drawing below.

I tried using Bruhn's analysis book to formulate a a reinforcement to the tension and compression areas of the spar. I'm not an engineer, and need guidance.

I don't know enough or what formula to calculate the loss of strength due to the 3/8" bolt hole in the tension and compression surfaces of the spar.

I can provide the following data for calcualtions if that's helpful, the site does say to provide as much inofrmation as possible.

The eyebolt the strut attaches to is 101 in from the root, and the moment of lift at the strut attachment point from the outboard portion of the wing was calculated to be 10015. WIth an OD of 2.00 and and ID of 1.75, the moment of inertia at the strut attach was .303. This gave a stress developed at this point as 33,052. Increasing the OD to 2.125 as in solution B increases the moment of inertia to 0.533, and reduces stress to 29940. and OD to 2.25 in solution A to increases moment of inertia to 0.742 and reduces stress to 26994. These were calculated with a safety factor of 1.5.

Does solution A create stress risers at the ends of the 2.25in external sleeve? Does the solution B lack sufficient compensation for the loss of material?

@robert DiGiovanni... in this added drawing, the middle presentation shows the original design, the single bolt horizontal through two straps which connect to the lift strut.

• I feel we are veering off of Aviation, and into structural engineering.
– Federico
Mar 24 '20 at 12:38
• @ToddMcLauchlin I'll repeat a comment I added to your other post... 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. Mar 24 '20 at 15:13
• Well, John K, I'm trying to avoid having to REPEATEDLY design something only to have it fail repeatedly. That's the point of all this...guidance, like, yeah you're on the right track, or no, your solution fails to compensate for.... but it that's too much for you and your friends here, then you're really not all that helpful, are you? I have a book...I did some calcs... isn't the purpose of this site to globally share wisdom and experience? I really don't think your condescension is at all helpful. But thanks for the tip on the goat.... I'll send it to you when I get it fiinished. Mar 24 '20 at 21:28
• The sleeve in the middle drawing has potential. As always, it's strength vs weight penalty. When in doubt, overdesign (or get good advice). Mar 24 '20 at 21:40
• @ToddMcLauchlin I wasn't trying to offend, just pointing out reality in the event you are intending to fly your design. I'm not actually a stress guy, just someone with some practical knowledge and experience, and I don't know if there actually are any on this particular forum that can give you that sort of detailed guidance. You'll probably have better luck on a structural engineering forum since the questions you have could really be applied to anything using tubes as cantilevered beams. Mar 24 '20 at 22:41