Tuesday, May 28, 2013

Left tank progress

Progress on the left tank has been pretty good and working with the tank sealant isn't nearly as bad as I had thought it would be. The stuff certainly makes a mess of everything but if your careful and watch what your doing it's manageable. I bought a quart of acetone and a bag of rags to use for clean up. The rags will be plenty for both tanks, but I should have bought a gallon of acetone. The recommended solvent for clean up is MEK but that stuff seems really awful for you, and is hard to find in CA. The acetone has worked very well as long as you don't let the stuff fully set prior to cleaning.

Left tank rib installation with sealant bead

We tried to get 2 ribs done each work session, and the long stiffener done with the tank filler and drain flanges. The inboard most rib and forward attach bracket were done Saturday prior to leaving for the holiday weekend. All the contact surfaces were scuffed up and cleaned well prior to starting, with a final cleaning of the surfaces planned for that day right before beginning. Our process was to mix up the sealant, place three good beads of sealant material on the skin, and then 100% cleco the rib into place. Sealant would squeeze out of the sides and the rivet holes and this sealant would be used to create a continuous fillet of sealant around the edges of the rib. In a few cases the squeezed sealant was not enough to create a fillet large enough for my satisfaction so I added some and spread it out.

Left tank prior to placing inboard end rib
Once we were satisfied with the fillet we riveted the ribs into place. Rivets were also cleaned in acetone before use. We would remove a single cleco, place a small dab of sealant on the hole, and then place the rivet. This process left a small ring of sealant around each rivet. There are a few rivets that stand just a bit proud because of the sealant, but I would prefer that to having a leaky tank cause paint blisters later. Finally we spread sealant on the shop head side of the rivets to ensure coverage there as well.

Outboard tank bay with fuel tank filler flange

We put a lot of excess sealant on the long stiffener since we were a little worried that sliding it in from the side would squeegee all the sealant from the inboard end. Turns out this fear was unfounded and we ended up with crazy fillets and plenty of excess to cover the shop heads as well. One rivet on the filler flange will have to be drilled out and replaced. I forgot to install the vent line clip on the aft side as called out in the plans. I thought about just drilling a new hole aft of the flange and using a rivet specifically for the clip, but decided that it would look funky.

The vent line will have to be installed prior to dropping the back baffle and completing the left tank. I ordered up the required tool to create the flare in the tubing, so this will probably get done this weekend. Then it will be time to cross our fingers and pressure test the tank. In the meantime I'm getting all of the parts for the right tank ready. All in all I don't think that doing the tanks is too bad of a project, certainly not as bad as countersinking the spar flange. That said, I will be glad to get them done.

Sunday, May 19, 2013

Introduction to Fuel Tank Sealant

 Been working on the fuel tanks all week but haven't accomplished a whole lot. The tanks seem to take extra time, and I'm not sure why. Originally I figured that without having to prime any of the tank interior parts the prep would go rather quickly, but that's not the way it has worked out so far.

 I started out last Friday by breaking a tool which slowed me down some as I waited for a replacement to be shipped. The plans have you countersink (my favorite build activity) the tanks rear baffle. This baffle is the last item to go on, and as such you have no access to form a fillet with the fuel tank sealant. The process used is to layup a pretty healthy bead of sealant on the skin, and then drop the baffle straight down. This should push the bead of sealant ahead and create a good seal. Obviously having to spread the skins to allow skin dimples to pass and then seat could cause a problem, so we countersink. Anyway, I had finished one side of the skins and decided that I could use a break for a few minutes. I knew that I would need to countersink the fuel cap flanges for skin dimples so I decided to knock these out. While countersinking the final hole my hand steadying the flange slipped allowing the flange to tilt and snapping the pilot off of my countersink cutter. I ordered up two this time so that a future breakage wouldn't slow me down.

Fuel tank cap and flange. The countersink cutter broke while cutting the incomplete countersink on the right.

 I wasn't comfortable placing any of the ribs without all of the countersinks done on the skins, so there was no rush getting everything preped. Over the next several days I dimpled the left tank skin, got all of the contact surfaces for the left tank roughened and then cleaned well, and continued on parts prep for the right tank.

This past Friday we got our first taste of working with the tank sealant. With the short stiffeners ready to go, and relatively easy to do, we decided to go for it. The first step was to clean the contact surfaces very well so that the sealant would be able to make a good bond. The plans called for using MEK but it seems that it's hard to get in California, so I used acetone instead. After the acetone had evaporated I masked the areas where I didn't want to get sealant, leaving enough space for a good fillet seal around each stiffener. I didn't mask the area where the ribs will go later, thinking that I would just clean those spots afterwards. I'm not really happy with the way that turned out, and I will be masking the rib spots on the right tank for sure.

Left tank skin with the stiffeners riveted and sealed and the masking tape still in place

With everything clean and masked we mixed up a batch of sealant. The sealant is a two part system and claims to have a working time of about 2 hours. I had heard that this stuff smells bad, and they weren't kidding. When I opened up the can of base it smelled like raw sewage, in fact if it weren't for the white color and thick consistency you could easily think you had a can of sewage. After mixing in the accelerator either the smell got a little better or I was used to it. I wasn't sure how much would be needed for each stiffener so we mixed up about 35 grams in the first batch. In the end we would need another 35 gram batch to finish up.

Left tank skin with the masking tape removed

 At each stiffener location we ran about a 1/16" bead all the way around the rivet holes and then a small blob between the interior holes. We then laid the stiffener and placed a cleco in every rivet hole. The clecos did a good job pulling the stiffener in tight and sealant squeezed out of all sides. I then used my finger to form a fillet all the way around each stiffener. There were only a couple of places where we needed to add a little sealant to form a solid fillet.

Once all of the stiffeners were clecoed on we moved the skin into the cradle and began to rivet. The process was to pull a cleco, place a small dab of sealant in the hole, push the rivet thru, and then drive the rivet. This should have created a good seal around the rivet dimple and hole, and after driving there was a small sealant ring around each rivet head.

Rivet shop heads sealed

After all the rivets were driven we used the remaining sealant to seal the shop heads of each rivet. This will hopefully form a complete seal of each stiffener. I then cleaned the excess sealant from each location where ribs will pass thru, and then pulled the masking tape. I feel like the fillet seal may have been compromised while cleaning for the ribs so I'll be sure to place extra sealant during rib placement. When we get to the right tank I'm going to be sure to mask the rib locations as well.

I also received the new countersink cutter Friday, so I was able to complete all of the required countersinking today. Tomorrow we'll start with the left tank ribs. All in all I don't think our first run with the sealant was too bad, certainly not the disaster others have made it out to be. The sealant certainly smells bad and is a sticky mess, but if you take your time and watch what your doing it's alright.


Thursday, May 9, 2013

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

Friday, May 3, 2013

Right wing structure riveted

Went on a riveting spree the last couple days, and got the right wing top skins riveted on as well as riveting the outboard leading edge together and to the spar. Now both wings are completed to the same stage and are ready for the fuel tanks. 

Right wing with top skins and leading edge riveted on
 I'm out of town for the weekend but I will start preparing all the parts for the fuel tanks next week. Hopefully it isn't as bad as it seems and I can construct leak free tanks on my first try.

Left wing structure