First full test skinning and exploration of rivet hole mismatch

This is the first full test skinning of the rear fuselage to discover more precisely how much rivet recentering and resizing is required. A certain amount of blending can be done in practise to shift error around such that fewer rivets need to be changed. This process can also reduce the amount of upsizing to the full 4.8mm rivet size, making 4mm rivets suitable in almost all cases. It has been found that while almost all of the longeron 301 rivets aft of rib structure 4 must be upsized to 4mm, only a few in the vicinity of the rib 5 structure need be upsized to 4.8mm. The appropriateness of this conclusion has been checked with Sling.

It is essential - via upsizing as needed - that all layers at the longeron 301 skin-skin connection are aligned and properly filled by the rivet chosen. Hidden elongated layer holes will lead to fretting and or skin slippage. The skin is the structure and this tubular structure is subjected to sizeable torques from rudder and vertical stabiliser side forces which act though a line above the fuselage. As the rear fuselage gets narrower the tangential forces on the skin become greater, increasing the shear loads on the rivets at the skin-skin connection. Any elongated hole layers (with the possible exception of the interior longeron 301 layer due to the rivet's ability to swell significantly at the tail) will not be able to withstand those forces to the same degree as a properly formed hole. Fretting and twist deformation of the rear fuselage structure is likely to result.

Even with properly formed holes, such upsizing of rivets to 4mm has the added benefit of reducing the pressures on the rivet body, further reducing the likelyhood of fretting due to deformation of the soft rivet material itself.

The centre-edge distance of the upsized holes in particular is somewhat less than the ideal 2x or even 3x, but the forces described above are acting along longeron 301 rather than transverse to it. In any case the centre-edge distance of many of the 4mm skin holes along longeron 301, particularly near the front, are also marginal in this respect. If any of this proves a problem in the field, doubler plates can be inserted under the skin in these regions and addtional rows of rivets installed.

If more than a very few holes must be upsized beyond 4.8mm the option of replacing a skin must be considered. This skin would be custom, with the offending holes omitted. Isolated cases would be ok, particularly if I can restrict the elongation to the rearmost (longeron) layer at the rivet tail, where some swelling capacity exists.

I hope that with further joggling, and small adjusments of flanges etc I can further reduce the misalignments making this latter possibility unlikely.