Fitting of rear fuselage skins

Previously a plan of attack for dealing with the serious mismatches between longeron 301, rear top skin and rear side skins was developed, involving selective upsizing and recentering of holes to either 4mm or 4.8mm. It had been concluded that in almost all cases the skin-skin and skin longeron mismatches could be eliminated.

This plan was put into effect. With everything was clecoed in place, it was found necessary in the event to upsize all the rivets aft of rib 4 along longeron 301 to 4.8mm

A) Between ribs 4 and 5, upsize and recentering to 4.8mm to absorb the vertical mismatch between the bottom skin / longeron 301 holes (which match well) and the rear top skin holes. Such upsizing/recentering was in almost all cases able to fully absorb the mismatch. The only exception was in the three rivet locations forward of rib 5, where some elongation of the bottom skin hole, by about 0.7mm, along the bottom edge remained. This is thought to be unproblematic, as the stresses between the sheets (due to tailplane forces) will be mostly forward/aft in nature. Any forces tending to pull the sheets apart in the azimuthal direction will bear on the perfectly formed upper edge of the bottom skin hole. In any case on a few out of many rivets are involved. As a precaution I will set these rivets with JB-Weld

B) Between ribs 5 and 6, upsize and recenter to 4.8mm to absorb the horizontal mismatch between the rear top and bottom skins (which match quite well) and the longeron 301. Such upsizing/recentering was in all cases able to fully absorb the mismatch between the skins and the longerons. In a few locations just aft of rib 5 a very slight elongation (<0.2mm) of the longeron holes remained. This elongation will be fully filled by the swelling of the rivet tail.

Further significant mismatches were present between the longeron 401 holes and the rear side skins, up to 1mm. This large mismatch was due to rear side skin tolerances, not the longeron itself. All these holes were upsized and recentered to 4mm rivets, fully absorbing the mismatch. In a few locations just aft of rib 601 a very slight elongation (<0.2mm) of the longeron holes remained. Again, this elongation will be fully filled by the swelling of the rivet tail.

Some mismatches were also present between the rib 601 holes and the rear top skin. These were upsized to 4mm, and in one case 4.8mm

One hole on each side of rib 901, which mates with longeron 401, was upsized to 4.8mm to absorb mismatches with the longeron hole, largely an avoidable stuff-up.

All the 3.2mm rivets holes connecting the side skins and rear side skins to rib 5 were upsized to 4mm. These holes matched well, in their original form, however the wear of repeated clecoing of these locations, together with some admittedly small mismatches of 0.2-0.3mm, led me to upsize. Futher rationale was a desire to upsize these connections for greater strength.

Longerons 601 was a perfect match to the both the side and rear side along the entire length - partly by design.

The other longerons all had random hole position drift of up to 0.2mm, causing mismatches of this order with the skin holes. These were cleaned with a 3.3mm chuck reamer, elongating the hole in the longeron and not the skin. Reliance on the ability of the rivet tail to swell prevents any problems arising.

Very pleased with the result. Sling have indicated they would deal with this mismatch by upsizing to 4mm rivets mostly, maybe some 4.8mm rivets. I found that it was not possible to achieve aligned round holes by only upsizing to 4mm. However at this level the alignment is improved over the original situation. Perhaps this is Sling's approach. In fact at 4mm the alignment between ribs 5 and 6 is quite good. Also, perhaps they pull the top skin edge downwards a little more, bringing the holes between ribs 4 and 5 more into alignment. I chose to avoid this, noting that this would induce oil-canning in the top skin. It is all technique, and it certainly involves quite a bit of it to pull off this remedy.

The straightness of the fuselage - absence of twist - was verified with a 2-axis laser level

Photos to follow