Strengthen wing skin 3-4 connection - part 6 - match drill new skin 3 holes

The next step is to match drill the new skin 3 holes, with the help of the the drill jig described in the previous part. To avoid interference with the skin 4 rivets now in place, this jig was trimmed back.

These new skin 3 holes will pass through: skin 3, the new shim, and finally the doubler. Before drilling, the shims were slipped into place between the doublers and skin 3, ensuring that they were tightly butted against the edge of skin 4.

These remaining rivets will be set when the top skins are finally assembled. This will happen after the bottom skins are rivetted.

Conclusion - a really worthwhile improvement in my opinion with no perceptible downside. All done "at own risk" of course. Since this modification has been done during the construction phase on issuance of the E-AB CofA it will be frozen in place into the design of this airframe and validiated during the stage 1 and 2 testing, like all other customisations. This modification could not be made subsequently without it being deemed a "major modification" requiring much paperwork, maybe additional testing, or the issuance of a Service Bulletin.

In particular, this change has been verified to cause no change to the aerodynamic shape of the wing top surface, and to result in no lumpiness in the wing surface (based on extensive test clecoing at several trial stages, only partially documented here)

Both left and right wings were given identical treatments.

With a total of 15 additional 3.2mm Gesipa rivets on each of two new skin-doubler transitions then with a shear breaking load of 720N one can expect an improvement of the skin 3-4 connection strength by 15 x 720N = 10800N = 1100kg. This represents an approximate doubling of the original strength, not counting the factor that is possibly compromising the original connection, namely the small hole-edge distance.

If this strength needs to be further increased in the future by, say, upsizing the new rivet rows to 4mm then ample (13mm) edge-hole centre distance has been provided in the doublers.