Resolution of fit issue with CF-SKN-004 - part 1

The front bottom CF-SKN-004 supplied was Revision1, despite the packing list stating Rev 2. Similarly for the floor skin CF-SKN-011. The issue discovered here was the separation between the rows of 4mm hole intended to match the spar carrythough bottom flanges is 78.8mm. However the separation between the rows of bottom flange holes is actually 76.6mm. Worse, it was found that when the spar carrythough was fully clecoed into position with the CF assembly and the side skins, and rib 5. this mismatch error between the flange and skin hole was approximately evenly distributed between the rear and front rows. I did consider upsizing and recentering to 4.8mm avinox rivets, but the evenness of this split was a little uncertain, and even when balanced was beyond the upper limit of what could be corrected, even with some recentering of the holes. Furthermore, with the skin already referenced to the side skins, CF assembly etc this matching would have to be done inverted, a nearly impossible thing to do accurately.

Sling did offer to replace the skin with the correct revision 2 (great service), but the separation of these rows (now 77.8mm) would still be incorrect, and also the rear row of holes out of position by approx 1mm. I did not ask, but I am sure Sling would have produced a custom skin with these holes missing. But even that would not have solved the problem because the presence of all the other holes would reference the skin, requiring inverted and inaccurate match reaming and measurement work. And there would be the wait.

The only solution is to start with blank sheet. This blank sheet must not be cut exactly to shape, as the reference to the side skins and CF assembly is not yet estabished. Instead the stategy must be to establish the two initial rows of holes to match the flanges, then cleco the carrythough-skin combination into place with the side skins and CF assembly. At that point all the attachment holes with these parts can be marked and match drilled. Only then is the new skin cut to exact size.

I could only get 0.64mm 6061-T6. The original skin was 0.5mm, but Sling ok-ed this.

I did cleco the bottom skin to the side skins, along the bottom edges - alignment was perfect, confirming the relative positions of the side skin and the bottom skin, as well as the carrythough are all correct.

The Rev 2 skin has a 4 degree crease, absent in the Rev 1 skin. The crease is not really necessary, and serves no structural function (confirmed by Sling). In fact about 50% of the flex imposed on the sheet occurs forward of the shoulder of the flange.

It was found that the rev 1 skin rear holes are about 1.0mm aft of the carrythough rear flange holes, the front skin holes are about 1.2mm forward of the carrythough flange holes - consistent with the 2.2mm total mismatch observed above.

I did not examine the outermost flange holes, these are slightly perturbed in position, by about 0.1mm due to a bending artefact.