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Andrews Web Site
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Date: 7-4-2020
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Number of Hours: 6.00
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Manual Reference:
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Brief Description: Bending Outer Spar Webs
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I started building the outer spars. First step is to bend a piece of 0.040 2024-T3 that is 83 inches long. The size of this material is a challenge for my sheet metal brake in terms of getting an accurate bend. I did quite a bit of set up work. The issue is, that the brake flexes enough to change the final bend in the spar in terms of "warp" (not in the bend direction). Using small set-up pieces does not flex the brake the same as full size parts, so it is hard to get things perfect without using a lot of material. Image 1 shows my brake with the first spar ready to bend. The first spar was bent after a number of 3 x 85 inch test parts. As it turned out, the full width spar had different warp than the test parts. Image 2 shows that when I placed the first spars edge to edge, you can see a gap at mid-span. When measured, each spar had a mid-span warp of about .06 inches. If I were to build a wing with these, the wing would be thinner at mid span than at the ends. So, back to the drawing board....., I had to readjust the bending brake to accommodate. This was quite a process of adjusting the preload tension in the bed, leaf and clamp. More of an art than science since you are trying to match tool and part flexing.
However, in the end, the second set of spars I built are shown in image 3 and are absolutely perfect...or at least as near as I can measure. Also, as far as the flange placement, the final spars were within 0.01 of the correct dimensions as bent, so when I final trimmed the spars, I removed 1 inch from each end of the 85 inch as bent spars to arrive at the required 83 inch final length. A bit frustrating overall, but this was just another of those steps that you have to get right or you cannot move on.
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Bending first spars on brake
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Showing mid-span warp on both first spars
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Second spars are flat and acceptable
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