THE LH FUEL TANK
After studying the plans for the wing assembly, I decided to assemble the fuel tank prior to putting the nutplates on the spar as the plans called for. The predrilled holes in the tank skin were smaller than the pre-drilled holes in the spar and I felt that the best way to keep these holes aligned was to assemble the tank, bolt it to the spar via the “Z” angles and back drill the skin attach holes.
The upper left photo shows the tank baffle with the ribs attached sitting on the spar. I had actually drilled the “Z” angles to the spar and the baffle per the plans at this point. A trial fit of the tank assembly revealed that the skin was off vertically and laterally. I discarded the “Z” angles and changed my technique. I marked a center line on each flange of each “Z” angle, full length of each flange. Then I made a line perpendicular to this line in the center of each flange. I drilled a #30 hole through the center mark on one flange of each “Z” angle and cleco’d the “Z” angles to the baffle. Next, keeping my lines exactly in the middle of the existing rivet holes, I drilled the remaining 4 holes in the flange. I then attached each rib to the baffle with clecos through the rib, through the baffle and then through the “Z” angle. I cleco’d the outer tank skin to the ribs and baffle and placed the entire assembly on to the spar again. I made sure that the holes in the tank skin were lined up with the holes in the spar laterally. I then placed a cleco in several of the holes from the inside of the spar through the tank skin (upper right photo). The clecos were installed this way because as previously mentioned, the holes in the tank skin are smaller than the holes in the spar. I then made sure that all of the “Z” angles were touching the spar. I then checked to make sure that the center marks on the “Z” angles were close to the center of the bolt holes on the spar. As you can see in the photos below, these holes were aligned pretty well. I drilled the holes out to a number 12 and disassembled the tank.
The photos below show the tank attach (“Z”) angles during the various stages of locating them to the tank and the wing spar.
After the “Z” angles were properly located to the baffle (tank spar?), the ribs were cleco’d on and the outer skin was attached. The skin attachment was very difficult initially, because I attached it with clecos while forcing the skin, which is fairly stiff, to the contour of the ribs. The plans call for a simple fixture which I thought was simply a storage device. This plywood cradle only took about 2 hours to construct. See the two photos below. It is actually a tool which holds the skin in the proper to their final size. Mostly, the enlarging of the holes was without incident. There was one hole at the top of each rib that was in the skin and not the rib. Also the forward edge of each rib was very tight in the leading edge of the skin. I did some re-forming of each rib because of this problem. Speaking of re-forming the ribs, I left out a step in this narrative. Prior to the preliminary assembly of the ribs to the baffle and then to the wing spar, the ribs had to be straightened and fluted. Every rib in this kit has to undergo this process. I did not take any photos on the fuel tank ribs, unfortunately, but it is the same process as the empennage ribs. It is a tedious job that must be done on a flat surface, but if care is taken, somehow almost like magic, all of the holes always line up. It amazes me that someone has figured out how to align holes that are punched in a rib prior to forming it or at least the final forming of it and then align them to a skin formed in a different fixture. It works very well though.
After the holes were enlarged the tank was taken apart. The protective coating was removed around the holes in the skin where the stiffeners and the ribs attach RH photo below). Note the masking tape to keep the soldering iron straight during this process. This photo also shows how far the skin has to be moved to attach it to the ribs.
The next item to accomplish is to locate the fuel cap (LH below), and the tank drain (RH below). The fuel cap bracket flange is bent to the contour of the upper wing skin as received from Vans Aircraft. Care has to be taken to ensure that the flange is orientated to the skin properly, otherwise the skin could be distorted during the attachment process. The drain valve fitting is pretty much of a no-brainer. The fitting does have to be aligned to the pre-drilled hole in the skin though.
After protective coating was trimmed, it was time to dimple the skin. The dimpling tool from Cleaveland Tools is certainly a handy device (below left). The RH photo below shows the tank skin all dimpled and ready for assembly. What I did not take a picture of was the rib dimpling process. There was no particular season, I just forgot. After all the holes were dimpled, I took a nylon wheel and roughened the surface where the ribs and the stiffeners will be attached.
As soon as the ribs and the tank surfaces were properly roughened, I mixed sealer and started the stiffener installation. The right photo above shows the first two stiffeners installed. The left photo shows my sealer mixing technique. I purchased this battery powered scale from Office Depot for about $30.00. I cut a paper cub (Don’t use plastic or Styrofoam!) down to size. I put 2 ounces of sealer in the cup and add .2 ounces of hardener. This particular brand of sealer uses 10 parts of sealer to 1 part of hardener by weight. Mix thoroughly. If this stuff is not stirred properly it will never set up. I put a liberal coat of sealer on each stiffener and installed them using the back riveting technique. I squeezed the rivets in the drain valve fitting. I clean all of the tools including clecos very frequently during this operation. Sealer is hard to remove when it is cured. MEK takes it off until it is cured.
These two pictures (below) show the completed stiffener and drain valve fitting installation.
At this point I was almost ready to install the ribs in the tank. Before the rib installation I needed to wrestle with the fuel quantity installation. I had decided to use the capacitance fuel quantity indicating system from Vans. This system installs aluminum plates with insulating spacers on two ribs in the tank, the second rib from each end. It is a lot easier to drill the attach holes in these ribs prior to installing the ribs in the tank. The lower left photo shows one of the plates clamped to a rib for drilling. There are three 3/16” holes in each plate. The photo at right shows all of the insulating spacers, four per screw. A nutplate gets installed for each screw on the plate for installation of the plate after the ribs are installed. There will be more on the installation of this system later.
It is now time to install some ribs. The ribs need to be installed wet with sealer. Sealer is messy and I try to minimize the mess.
The first thing that I do is to put masking tape on the inside flange of each rib (above left). This will keep sealer from oozing through the rivet holes until after the rib is installed. I do this in the hopes that the cleanup process to follow the rib installation will be minimized. I spend little extra time in the beginning to save time at the end. Above right is a photo of the rib ready for installation. The other advantage of taping the inside flange is that each dimple can be completely filled with sealer. Any fuel leaks at a rib will be at a rivet hole. With the rivet hole dimple completely filled with sealer prior to riveting, chances for a leak are drastically reduced. A fuel leak at a rivet hole with the tank closed up would be difficult to correct.
This photo (above center) was taken shortly after the installation of the rib. Notice that this is not the first rib I installed this day. Before I describe a procedure, I like to make sure that it is going to work. I have started to pull the tape at this point. Notice that the only sealer on the inside flange is coming through the rivet holes. Once the riveting starts there will be more sealer on the inside flange but not an excessive amount. The sealer will ooze out between the rib and the skin. This is where the sealer is needed. Sealer that is smeared all over everything else is nothing more than a mess. Notice the other rib that was previously installed. There is no sealer on the web of the rib or the inside flange. The excess sealer was relatively easy to clean because care was taken to put sealer only where it was needed. One more thing I should mention is that I installed all of the ribs in the tank with clecos prior to riveting. I only sealed the one I was working at the time. This was done to keep the tank from twisting which would make it difficult to install.
The photo above left shows the tank with the five internal ribs installed. The two end ribs will be installed later. How do I know that the tank is not twisted at this point? The photo above right shows that all of the fuel transfer holes still line up.
At this point by looking at the date on these pictures, it is plain to tell that I installed all five internal ribs in one day. This is misleading. I started at 7:00 AM and finished after 10:00 PM. I mixed sealer for two ribs at a time at the most. It was a long day but a productive one. I am one year into this project at this point and so far I have not needed any one to help me install rivets. I am enjoying this project up until now. I just wish I had more time to work on it.
There are some details to complete prior to the installation of the two end ribs. The fuel cap fitting needs to be installed. The vent line needs to be installed. And last but not least the fuel quantity plates (probes?) must be finished and installed. I decided to tackle the fuel cap first. Notice from the date on the photo at left above that it is still June 15. This was one of my most productive days on the project so far. If I could have about 75 days like this in a row I would be done. The bracket at the top of the fitting is for the vent line. This does not come with the kit. I made it from 2024 T3 aluminum, .025 thick. This is thicker than the plans cal for (.020). The photo at right shows the fuel cap fitting installed on the skin, vent line bracket and all. As with all the other parts that could potentially leak, the fitting was installed wet with sealer.
That was all for today. A little over 13 hours total. The next item on the agenda will be to seal each of the rivets inside the tank. I decided to take a one day break from the job first.
Two days later I started this task. I modified an acid brush (See photo above center). I mixed sealer and had a ball. This is a tedious process. The bright side is that it allowed me to check my rivet installation. Most rivets were Acceptable. I did find a few that were not good, either too flat or bent over or not driven long enough. I fixed all the bad rivets before I sealed them. You can see sealer sticking out from under the flange of the rib in the photo above. I had previously cleaned the excess with MEK. The protective coating on the skin is still in place and that made the cleaning process easier.
The fuel vent line was the next thing to install. The most difficult part of this operation was getting the line to go through the bracket on the fuel cap fitting (Upper left). Looking at the other photo (Upper right) you can see a wire wrapped around the vent line. This is a fuel quantity wire which is attached to the probes.
As I stated earlier I would get into more detail on the fuel quantity system later. Here it is, later. The left photo shows the capacitance fuel quantity "probes" as I will call them from now on, next to their drawing. The right photo shows a close up of one of the plates where it must be notched to get clearance with a tank skin stiffener.
Probes are attached to ribs in these photos. The wire wraps around the vent tube and is sealed to it in several spots. I had to take the plates back off to seal the wire terminals. Note that there are four insulators per screw, three between the probe and the rib and one beneath the screw head.
Here are two photos (above) of the outboard rib after it was installed. Looking at the left photo you can see that I plugged the tooling holes with rivets. These are 3/16” holes but I squeezed a 5/32” rivet in them and it tightened up very nicely. I then sealed over them. The forward one had to be removed later because a stiffener plate had not been installed. No photo was taken of the plate. Notice the bead seal on the rib inside the tank. The next item on the agenda was the inboard rib. Lots of things had to happen prior to the installation of this bad boy.
Next a large access hole gets cut in the rib. Here I am with a drill press and a fly cutter cutting the access hole. These are the first pictures that I took with me in the photo. I'd just read an article that mentions that the builder should be shown building the airplane in the pictures. I will make it a point to do this occasionally from now on.
Above left is a shot of the inboard rib with the re-enforcement plate cleco’d in place. Right photo shows the re-enforcement plate sealed and riveted on. The strap that is on the rib is to keep the inverted fuel or “flop” tube from hanging up on the rib. Notice the difference between the dates in the photos. I wish I could work on this everyday but that is impossible.
I am now running down the home stretch as far as closing the left fuel tank is concerned. The parts in the picture below are the baffle (or tank spar), inboard rib, attach angle and the seven “Z” angles that attach the tank to the wing spar. Before I primed these parts I etched and alodined them. I masked off the side of the rib and baffle that are on the inside of the tank So that I could prime the side that is exposed. I wish I had done the same thing on the outboard rib. This would have been the best time to prime that rib also. (Continued on LH Fuel Tank Page 2.)