Starting the fuel tanks

I've been working on the fuel tanks this week. The tanks seem to me to be a little more parts intensive. I started by getting the ribs for both tanks final drilled, fluted, and deburred. The z brackets that will hold the tanks to the spar web come as one long piece of aluminum formed into the z shape. The brackets have the holes punched in them and have nearly all the material between the individual brackets milled out, leaving just a couple tabs to be cut thru to separate them. After cutting them all free the brackets must be deburred and then countersunk to rivet on the nut plates that will take the AN3 bolts thru the spar web. I really really hate countersinking after the hundreds of holes on the spar flanges, but with only 36 holes per set it wasn't too bad. The z attach brackets are one of the few parts that get primed on the fuel tanks as well.

One set of z attach brackets and the inboard and outboard tank ribs

Once the ribs and z brackets were all set I started to cut the skin stiffeners. The bottom of the tank ribs do not have any cutouts in them, so instead of the full length stiffeners used elsewhere on the wings, the tank bottom gets two rows of short stiffeners that only span a singe rib bay. I imagine the ribs are stronger this way, and the breaks prevent fuel (and more importantly water) from getting trapped behind a full length stiffener. The stiffeners also come as several long pieces of formed aluminum with slots and holes to guide your cutting. After cutting them all free, and there are a bunch of them, I deburred them all and dimpled them. The top stiffener is cut full length from a piece of j channel and match drilled from the skin. Because of my cutting error earlier in the build I had to buy a couple of new pieces of j channel to fabricate these.

All of the tank bottom stiffeners after deburr and prior to dimpling

I went ahead and clecoed the left tank together to match drill it's stiffener, this was not an easy task. The tank skin does not want to take the shape of the ribs and required more than a little persuasion. I even broke out the dead blow hammer to get some of the ribs to line up. I have read other build logs that talk about spending pretty significant amounts of time for each rib when doing the final construction with the tanks sealant, and had wondered why it would take so much time. I don't wonder anymore. Tomorrow we'll be messing with the assembly trying to figure out a good way to get things to fit together easily before trying it with wet sealant everywhere.

 

Left tank clecoed together to match drill stiffener, note the dead blow hammer