Service Bulletin #4 is included in Revision C of the plans. This requires manufacturing 8 skin doublers which will be installed on the outside of the skins over the flap track guide arm rib locations on the upper and lower sides of the flaps. The plans include a full sized template for laying out the cuts on the 12" x 12" .050" sheet aluminum. This allows for the prescribed number of rivet locations across the nose ribs, spar and aft ribs at each rib pair. These are the same rib pairs that received the reinforcement angles.
I used the edges of the sheet aluminum for the more critical edges and cut the remaining lines on the band saw. Cleaned the lines up with a file and then de-burred the edges and radius'd the outside corners by eye on the bench grinder. I drilled the interior radius corners where the flap track and deployment arms protrude with 3/16" and 1/4" bits, then filed each with a circular file. Interesting to note the flap skins have different interior radii for each side of the slot. Not sure why but the skin doublers are built to match.
The manual says the template covers the maximum size of the rivet hole locations for both upper and lower skins, and they can be cut down to suite the actual rivet locations. Not so sure I will do that. I may just keep them all consistently (slightly) over-sized.
Before installing these onto the skin, they will be curved over a 3" metal pipe to conform to the curvature of the upper and lower skins, and match-drilled from the interior of the skins. That step comes just prior to final flap assembly and riveting. Because I plan to polish all aluminum surfaces, but I still want corrosion protection in place, I will probably just self-etch prime the interior side of the doublers as a barrier to the bare aluminum skin on the flap skins.
