Dealing with slight misalignments between the ribs and main spar mounting brackets

The rib mounting brackets of the main spar have been found to be subject to a slight tolerance. This is easily missed in most cases, and it was generally possible to easily rivet the ribs fully in place. But on doing this a mismatch was then found between the rib flange holes and the skin. In some cases some careful clecoing would overcome this, albeit making it slightly difficult to insert the skin rivets without forcing. In a few cases the mismatch exceeded 0.5mm and trying to force these resulted in inevitable damage to holes when de-clecoing, and opening the possibility of causing skin oil-canning by placing the latter under considerable stress.

A lot of builders refer to this, but I cannot tell how bad their situation is compared to mine. I have decided to address the ultimate cause of the problem.

In one case left rib 3 the mounting bracket tolerance error is sufficiently great to cause the rib to point upwards several degrees (if the aft flange is not attached to the rear spar channel).

I have found that by derivetting the spar attachment brackets from the ribs, while leaving the ribs generally rivetted in place elsewhere, a perfect skin match is easily achieved in almost all cases with no forcing of clecoes.

The tolerance of the spar attachment brackets then reveals itself by the misalignment with the rib holes.

This problem has been found to be significant in

Left wing: ribs 2, 3, 4, 5, 6, 7

Right wing: ribs 3, 8

How big is the mismatch? For most of these the mismatch is around 0.5mm, sometimes greater. In these cases I have approval from TAF to upsize/recentre these holes to 4.8mm rivets. The exception is the top- and bottom-most holes where for ribs 2-6 there is not enough hole-centre-to-edge clearance to make upsizing advisable according to the 2x rule. In this case TAF advises the 4mm rivet be maintained and the rib hole be elongated just enough to compensate for the misalignment. This is a compromise, but an acceptable one as the elongation of the rib hole is in the opposite direction to the predominant loading of interest of this connection (i.e. to pull the rib away from the spar). In any case TAF have indicated that when this problem arises in production they simply elongate all the holes - they do not upsize at all.

In this sense my fix is superior to that used in production.

In one case - left rib 3 (the one that was pointing up) - the misalignment is small at the bottom thought the midplane and amenable to upsizing but increases to 1.2mm at the top hole (with the original 4mm rivet size). Again the elongation is in the same sense as above, making elongation of the top hole an acceptable compromise. Upsizing is also acceptable, but one or two locations may result in a rib hole that is not perfectly round. Given TAF's acceptance of elongation in many contexts to a degree well beyond what I have tolerated in this build, I am not too concerned about this compromise.

TAF have given approval of placing additional 4mm rivets equi-distant between the existing rivets, along the original rivet line, If I so choose.

I am also bearing in mind a major function of the additional more-densely placed ribs 2, 3, etc - this is in the step-on region of the wing.