Tail spread of 4mm and 4.8mm Avinox rivets

I have a number of skin fit issues in the wings. There are two causes of this 1) drift in the hole positions in the skins themselves; and 2) slight displacement of the holes of the forward facing main spar flanges affecting mostly the top flange.

Only issue 2) is addressed here. It causes a hole mismatch with the fuel tank skins, up to 2mm in places is a simple cleco from the inboard end is performed with the existing holes. Whether giving full priority to the inboard rivet connections, or in some way blending the mismatch, some hole elongation would be necessary at some point along the top of the fuel tank skin. My plan is to avoid this as much as possible (any remaining elogated hole existing only at or near the outboard end) by recentering and upsizing the 4.0mm rivet holes in the forward facing spar flanges to 4.8mm.

SB14 calls for some of these rivets to be stainless steel, specifying Avinox 0BE61-00512 4mm rivets which have a grip range of 3.0-5.0mm. I have obtained 4.8mm equivalents 0BE61-00614 which have a grip range of 3.5-6.0mm. This is not only compatible with the fuel-tank-skin spar connection (3.2+0.64=3.84mm), but also any connections on the rear facing spar flange if needed, and stronger. They pull force is also at the operating limit of my pneumatic rivet gun.

A concern is the size of the expanded tail of these rivets. In many cases the clearance hole in the inner flange of the spar is offset - sometimes maximally - with respect to the rivet hole itself. TAF have tested 4mm Avinox rivet setting under this circumstance and claim the strength is unaffected, as reported in Pascal Latten's blog. My tests below show that even when centred the expanded tail of a 4.0mm rivet will form an interference fit with the 1/4" diameter of the clearance hole. If the clearance hole diameter is left unchanged, the tail expansion of a 4.8mm rivet will likely be compromised, and especially so if the clearance hole is offset. I wish to avoid entering this untested territory, so my plan is to counterbore all clearance holes to a size that will allow the 4.8mm rivet holes to expand fully.

My tests show the 4.8mm rivet hole expands to 7.7mm diameter when setting the spar flange and fuel tank skin, slightly less if there are additional layers (such as might be the case if these are used anywhere on the rear facing flange). So I shall counterbore to 5/6" = 7.93mm with a #12 pilot after upsizing and recentering. This will eliminate all offsets. Even in some extremal cases, the forward facing spar flanges are so wide that the 2D rule is easily met for this inner flange, when the clearance holes are enlarged (and also recentered) to this size.

I will also take the opportunity to counterbore the 4mm holes on the rear facing spar channel with a 1/4" = 6.35mm counterbore with #21 pilot to recentre the clearance holes, restoring the 4mm Avinox rivets to nominal operation. Some clearance issues with rib flanges and stiffeners impede access - in those cases I will take advantage of the TAF tests and allow any offset to remain. My tests of the tail expansion of the 4mm Avinox rivets show 6.5mm max in context, an interference fit to be sure but not so bad. The spar inner flange is not so wide so counterboring to 5/16" could impinge on the 2D rule, and be a case of rapidly diminishing return and embody a possibily unfounded assumption that the rivet setting is in any way negatively affected by such a small degree of intererence.

Great service from TAF - they are supplying replacement fuel tank skins with the forward-facing flange holes omitted. I will match drill when fitting, using a strap doubler. This eliminates the need for upsizing to 4.8mm and counterboring to 5/16". I may counterbore to 1/4" in some cases where the clearance hole is badly offset from the hole position.