Main wing spar Issue 4 - interference between spar caps and spar core fold radii

An interference was detected between the spar caps and the fold radii of the main spar core. This interference was deemed serious, for while the total thickness of the stacked spar layers is 23.3mm some portions of the stack were measured to have a thickness of 24.1mm, a full 0.8mm thicker.

This interference was observed in both wings, both top and bottom, and results in a gap as large as 0.4mm between the spar core and the immediately contacting spar cap. My concern with this is that the high points of the interference may wear down over time resulting in a loss of AN bolt clamping force.

This varying thickness may also be an issue when bolting the wings into the spar carry-through. The gap in the spar carry-though is 25.3mm, which is about 2mm greater than the spar stack itself. I do not want to stress the carry-through by trying to close a gap this large with brute force. Rather, this gap can be closed with a shim (2mm 6061-T6, maybe 1.6mm) readily if the gap is fairly constant. But if the spar stack itself increases to 24.1mm, then the use of a shim becomes much more problematic. If I use a 1.2mm shim so that it will fit, there will still be regions where the gap approaches 1mm which will have to be closed by brute force.

Edit 22 March 2021 I have just received the main spar carry-through with the fuselage kit. The slot in this carry though is closed onto a spar "stump" consisting of a stack of aluminium layers identical to that used in the main spar. This gap is measured to be 23.3-23.5mm, NOT 25.3mm as above (this figure was supplied by another builder). This means that 1) no shimming will be needed, and 2) this interference removal is absolutely essential in order to be able to assemble the wing.

I can avoid all of this by taking action now.

On the bottom side interference is concentrated at the edge in the vicinity of the most outboard AN6 bolt and extends about 50% along the inboard edge adjacent to the AN3 bolts.

On the top side the interference is concentrated along the inboard 50% of the inboard edge adjacent to the AN3 bolts.

Note that the photo showing the interference is taken with the AN6 bolts in place and tightened but not the AN3 bolts. If the AN3 bolts are in place there is possibly a twisting action on the spar cap because the interference is only on the exterior edge which might bring them out of plane, possibly partly obscuring (curing?) the matter, but certainly confusing matters. TAF have stated a limited amount of intereference is ok, but gave approval to chamfer.

My solution to the interference is to chamfer the spar caps appropriately, then treating the final result with alodine.

Given the goal is elimination of the interference, this underlying cause is shown up most clearly with some of the AN6 bolts clamped and the AN3 bolts removed. This avoids confusing the picture due to flexing of the spar cap.

I proceeded iteratively, removing a minimum material, looking for high points (looking for shiny spots on the sanded surface, etc), reclamping (with the AN6 bolts only) then measuring. In this way I was able to reduce the clamped tickness of the spar cap, at all points, to 23.3-23.4mm.

In the event I made a planning error and removed material further along the edge of the top-side the necessary. This happened also on the bottom side to a lesser extent (do to iteration). Also, more material than necessary was removed right at the edge, due to difficulty in controlling the chamfer radius (all done by hand without a jig). I have presented this to TAF - they approve the result however.

I have flipped the wing structures over, to show the top side. Actually it does not look too bad at all - the flash exaggerates things a bit - but it rankles. My initial feeling was to try again and order new caps. The cost is moderate and it would mean a few more items in the corner-of-shame. I am thinking about it. There is no opportunity to revisit this later.

If I do repurchase then I would construct a jig that is shaped according to the bend radii, and process each cap separately to completion, to a greater precision with no stuff-ups

I have a further opinion on this chamfering - the excess material removed should not be a problem. However as these caps are a small part costwise of an order of additional parts (and some replacement skins) where I am paying a large amount to get airfreighted I decided to include them. It was more hassle to think about it any further. I will re-evaluate with replacements in hand.