One of the things I have to do in order to make my KR2 more inspectable is to make an inspection hatch at the tail end of the fuselage. This is necessary to check the elevator bellcrank and cables and the undercarriage bolts. Actually, without this hatch, it would be possible to check their condition (using a borescope or even a mirror), but not to remove them, or to tighten the tailwheel bolts. If you are still at the boat construction stage, I would recommend you make one now. It would probably be easier than what I have to do now.
What I wanted to do was to make a ready-made doubling plate, complete with threaded holes; then, cut the opening in the existing skin (which is thin plywood with one layer of fiberglass); then slip the doubling plate inside, and glue it; then, use the removed skin as the door. The first thing I did was to determine the opening shape and size. The shape was easy: since the doubling plate has to slip through the hole, it has to be some sort of oval. So, I made a racecourse shape, with the straight part long enough so that the length of the opening (skin cutout) is the same as the width of the doubling plate. I made a template with Microsoft Powerpoint, printed it in various sizes, and selected the smallest size that would allow me to slip my hand into the opening. This ended up being 74x104mm (2.91" x 4.09"). So, the drawing consists of three concentric racecourses: the inner is the opening, the middle one is the skin cutout, and the outer one is the outline of the doubling plate. I made provisions for eight threaded holes for M4 screws (that is 4mm diameter, about 0.15"). The template also has plenty reference lines, so as to ease positioning on the aircraft skin later. I first took a piece of 2.8mm plywood (0.11") (which proved a serious overkill afterwards) , and glassed it on one side with one layer of BID. Then, I printed the Powerpoint template on a sticker label, and glued it on the other side. I then drilled countersunk holes to the size of M4 Rivnuts. The idea was not to rivet the Rivnuts, but rather to Epoxy them in place. I prepared the Rivnuts by sanding them, sawing a key in the rim (to prevent any rotation),and filling the threaded hole with candle wax. I slipped the Rivnuts in their holes,removed the template, and covered the second side with a second layer of glass. Once it was cured, I stuck another template label, and marked the centers for the two half circles of the racecourse. I screwed the plate on a piece of plywood. Then, uning a flycutter, I drilled the two inner half circles off, then finished the shape with an electric file. The last tasks were to cut the outline of the doubling plate with a bandsaw, to remove the wax and open the screw holes in the fiberglass, to smooth the edges of the opening, and to seal the cuts with Epoxy. The result is very neat, although a bit heavy: it weights 50 grams (1.76 oz) complete with Rivnuts, but without the screws. I probably made the plate twice as thick as it should be. In France, there is a rule of thumb in aircraft construction that says that if the doubling plate has at least the same quantity of material than you cut out from the skin, then you're fine. I guess the difficult part will be to cut the skin. This is where the pneumatic hacksaw will help a lot... By the way, if anybody is interested in the Powerpoint template, just shoot. Serge Vidal KR2 "Kilimanjaro Cloud" Paris, France
