Upsizing of rear fuselage rear bottom skin holes

Between ribs structure 4 and 6 on the left hand side there was an approx 0.8mm drift of the 3.2mm holes in the rear bottom skin. After clearing with Sling, it was decided to recenter and upsize to 4.1mm and use 4mm rivets.

It was also decided to upsize the corresponding right hand rivets to 4mm, despite the main error being absent. However there was some minor mismatches at the interface between the rear side skin and side skin which required blending and upsizing anyway.

The motivation for general upsizing of both sides, aft of rib 4, was to make both of these connections a bit stronger. It has been noticed on a club aircraft that the 3.2mm rivet connections along longeron 301 aft of rib structure 5 show some signs of fretting, with noticeable fracturing of the paint around the rivet. This has been attributed to the twisting forces on the tailcone resulting from the side loads on the vertical stabiliser and rudder. Such forces impose a torque on the tailcone which translates to fore-aft shearing forces on the surface of the tailcone structure. These forces become larger at the narrower end of the tailcone.

Skin misalignments are going to necessitate upsizing along longeron 301 aft of rib 4. Such upsizing has the added virtue of making the connection stronger against such forces. But since the shearing forces affect the entire surface of the tailcone then such forces will also be imposed on all other skin joins, in particular the bottom skin joins discussed here.