Survey of off-centre main spar inner flange clearance holes

Some of the clearance holes are substantially off-centre. These 1/4" = 6.35mm are sufficient for proper tail expansion of Gesipa 4x10mm Aluminium rivets (5.8mm) and very slightly undersized for Avinox 0BE61 4mm stainless steel rivets (6.5mm). In the latter case there is probably some leeway, and in any case upsizing this clearance hole to say 5/16" will erode engineering margins on the rear facing flanges. In this case the 2D rule would be broken with a ~8mm diameter hole as the centre-edge distance is only 13mm. Not such issue exists for the forward facing flanges, as the centre edge distance is 18mm. TAF have indicated this "2D rule" is not an absolute, and could be relaxed to 1.5D.

However I am choosing to stick with the existing 1/4" = 6.35mm holes for both stainless steel and aluminium rivets. Note that there witll be no upsizing of any holes on the main spar to 4.8mm, on account of TAF supplying replacement fuel tank skins.

The complication is many of these clearance holes are off centre, some so much so that the edge of the rivet hole coincides with the edge of the clearance hole - i.e. a displacement of 1.2mm

This offset greatly disturbs the proper expansion of both the Avinox and Gesipa rivets. TAF in tests (from Pascal Latten's blog) have shown that the shear and tensile performance of the rivets is unaffected. In the application here, shear performance is most important, and it is easy to understand why the shear performance would be maintained, even with the tail highly distorted.

However I have decided to recentre the 1/4" clearance holes with a 1/4" counterbore, with #21 (4mm) pilot wherever I can get access. Depth control is by use of a 1/4" drill stop. I will apply lubricant to remove friction between the pilot and the 4mm hole it sits in. I have obtained both a 90 degree drill chuck to enable access.

A survey of clearance holes so affected:

Right wing:

forward top flange - all fully offset

forward bottom flange - 3 outboard moderately offset,

rear top flange - well centred

rear bottom flange - well centred

Left wing:

forward top flange - 2 outboard moderately offset,

forward bottom flange - well centred

rear top flange - all moderately offset

rear bottom flange - well centred