THE RH FUEL TANK
As with the left 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. I decided to try a little different approach this time, keeping in mind that I want to assemble the tank prior to drilling and installing the nutplates on the spar. The first thing I did was to lay the “Z” angles on the spar in the direction that each one should face per the drawing (16A). I numbered the angles 1 through 7, starting from the inboard end. I drew a centerline on one leg of each angle and drilled a #12 hole on this line centered from each end. I cleco’d the angles to the spar with 3/16 (gold) clecos, right photo below. After I made sure the centerline was centered in each attach hole in the spar, I drilled the other 2 holes in each angle.
Next, I installed three ribs in the center of the tank skin. I attached this assembly to the tank baffle and then I placed it in position on the spar (photo below left). I made sure that the holes in the tank skin were in the center of the corresponding holes in the spar. I then installed several #30 (copper) clecos from inside the spar through the tank skin (photo below right).
With the baffle held in place via the leading edge skin cleco’d to the spar I drilled the “Z” angle brackets through the pre-existing holes in the baffle where the last two ribs (which were not installed yet) on each end will be located (photo below left). I then removed the leading edge skin and drilled the three center “Z” angle brackets through the pre-existing holes in the baffle (below right).
At this point I cleco’d all of the ribs to the leading edge to make sure that I did not have any fit problems (above left). There are two holes in each rib that are not pre-drilled for some reason. This is the proper time to drill these holes. I don’t remember if I have brought this up before but any 3/32 dimpled holes that I have to add are drilled with a #41 drill bit. This is the same approximate size as the pre-existing holes. I do not enlarge the holes to #40 prior to dimpling. I feel that the holes will be a little tighter after dimpling minimizing the chances for bending the rivets while setting them. The right photo above shows me removing the protective coating in anticipation of dimpling the holes. I left the protective coating between the ribs and stiffeners in place until I was ready to install the baffle. Be careful with this type soldering gun. it is designed for intermittent duty and you need to let it cool off periodically. I caught one on fire already.
After the protective was removed I roughed up the area where the sealer would be. I did this after dimpling on the left tank and it was more difficult to do. This is the time to rough up the skin, prior to dimpling. This is also the time to locate the fuel cap retainer to the wing skin (above right). After the fuel cap retainer was located, I placed the leading edge skin in the stand and started to work on the stiffeners ( below left).
The stiffeners come in two long strips. The plans clearly tell you how to cut the individual stiffeners from the strips. After all of the stiffeners were cut and shaped per the plans was time to dimple the fuel tank skin. The picture at lower right shows the dimpling in process. After the leading edge skin was dimpled I dimpled all of the holes in the stiffeners. In preparation for sealing the Stiffeners, I roughened up the bottom flange of each of them. On this fuel tank I decided to etch and alodine all of the parts of the fuel tank prior to sealing and riveting.
The photo at lower left shows the right hand fuel tank parts after I etched and alodined them. I did not etch and alodine the area where the protective coating is. I decided to leave the protective coating in place until the fuel tank was completely assembled so that the protective coating would help keep the sealer off of the fuel tank inside skin while I was working. Thus I had less work to do when it was time to close the fuel tank by installing the rear baffle. The next three photos show the stiffeners riveted to the lower fuel tank skin. The photo below right shows the first six stiffeners riveted in place. I put fuel tank sealer on each stiffener prior to riveting this them in place.
The photo below left shows all of the stiffeners in place. It also shows that I placed sealer on each rivet head. To do this I made a tool out of a piece of 1/4in. aluminum tubing approximately 1/2in. long. I placed the tool into the wet sealer to fill it with sealer. I then placed the tool over a rivet. This process went fairly quickly. Hopefully I will not have any fuel leaks. The picture below right shows all of the stiffeners after riveting. You can see my back riveting bucking bar to the left of the skin in this photo. I back riveted all of the rivets on the stiffeners.
If you have already read my assembly of the left fuel tank you will know that I have decided to use a capacitance type fuel quantity system. The “ probes” are simply two aluminum plates connected with some wiring that are attached to the second rib from inboard and the second to last rib outboard. The photo below left shows these two ribs with the drawing that came with the system. As with the left fuel tank the holes in these two plates were transferred from the plates to the ribs. The holes in the plates are 3/16 of an inch in diameter. After the holes are drilled into the ribs, they are enlarged to 1/4in. in diameter (below right). The tool I am using is a Unibit. Why did I put tape on it? So that I wouldn't forget to stop after 1/4".
Three nutplates must be installed on each “probe.” It is not a good maintenance practice to counter sink material this thin so I dimpled the “probes” and then the nutplates. See photo at left and right below. Any material that is less than .032” in thickness is too thin to countersink. Besides, it doesn’t take any extra time to dimple these holes instead of counter sinking them and I don’t have to worry about chattering a counter sink in material that is too thin. Generally speaking a counter sink chatters when the full height of the pilot penetrates beyond the backside of the material.
The photo below left shows the number two and the number six ribs and the two fuel “probes” ready for assembly. There is some trimming required on the outboard “probe” but I will go into that a little later. The photo below right shows the first rib that I installed. Notice the masking tape on the rivet line. I put sealer on the ribs prior to installing them and the masking tape is to keep the sealer off of the inside flange so that cleaning the excess sealer will be a little easier later. This is a messy job and I tried to minimize the mess.
The photo above left shows three ribs riveted in place. The tape has been removed and the excess sealer has been cleaned off. The photo above right shows five ribs riveted in place (to the bottom skin only at this time). Notice the masking tape on the inboard side of each rib. After the ribs are riveted on completely, I intend to place a bead of sealer along the inboard edge of each rib to help prevent fuel leaks. The masking tape will help keep sealer off of the face of the rib thus making cleanup easier when the sealing is done.
Photo above left shows a typical rib riveted in place. Notice that the tape is still in place on the other side of the web and that the excess sealer has been removed. The next process will be to cover each rivet head with sealer like I did on the stiffeners. The photo above right shows three ribs completely riveted in place, top and bottom, while two are still in work on the top. Notice how many photos have the same date. This was a very long day that started at about 4:00 PM and ended at about 2:00 AM. It was long but productive.
The photo at left below shows all of the internal ribs after riveting. The excess sealer has been removed. Each rivet head has had sealer put on it as I did on the stiffeners. I have cleaned all of the excess sealer using MEK and a good paper towel ( not the cheap towels available at the “quick shop”). I use “Rags in a Box” by Scott Towels available at where else? Lowe's. In the photo below right you can see that I have applied the bead sealer to one of the ribs. I have cleaned off the excess sealer and have removed the masking tape from the face of the rib. Note that the protective coating is still in place at this time. Notice the date of these two photos. It was about two and a half weeks from the time the riveting was completed until the bead sealer was installed.
Below are two pictures of the fuel drain fitting installation. This step is straightforward and doesn’t require much explanation.
Above are two photos of the fuel cap fitting after it was installed. The left photo also shows that the vent line is installed. I forgot to take pictures of the bracket holding the vent line that is riveted to the fuel cap fitting (left photo) as I was making it. The bracket is similar to the one in the left fuel tank. Notice the dates on these four photos. Nearly two and a half months had elapsed since I had managed to work on this fuel tank. I had a very busy summer and I didn’t get to work on the project as much as I would have liked. However, I am working on this project as a hobby with no particular deadline in mind for its completion. It will be done when it is done.
The photo at left above shows the outboard and rib after a small stiffener was riveted to it. There are no rivets in a space of about 2in. around the leading edge of each rib so a stiffener is needed on the outboard and the inboard ends to keep the rib from flexing which might break the sealer loose. Also with no stiffening a crack could possibly develop at the leading edge of the rib although that is unlikely. I am assuming that this is why the engineers put the stiffener on these two ribs. (It also could be that they felt that I had not had my hands in wet sealer enough.)
The photo above left shows the fuel tank to fuselage attached bracket as it is being cut from a section of aluminum angle, 2 X 2 ½ X 3/16 per drawing 16A. It would have been nice if they had drawn this fitting full scale. Then you could have used the drawing as a template. It wasn’t actually that hard to do but they had to draw it anyway so why not full scale? Photo above right shows the finished fitting in place on the rib ready to be drilled. The drawing gives you a dimension from the leading edge of the rib to the aft end of the fitting. The only problem is that the drawing does not give you any dimensions that would help locate this fitting in the vertical plane but that is a problem that will be solved during the wing installation. For now I’m just guessing where the vertical reference line should be.
This rib also gets an internal stiffener. The stiffener is just slightly larger than the fitting. The photo above left shows a red arrow pointing to two holes here. The larger of the two holes is the “jig” hole that is used in the manufacture of the rib. I point it out because one of the rivet holes is very close to this hole. This is my own fault because the rivet pattern is my own. The plans call out to put "several" rivets here but they do not give you a pattern. Picture below right shows another view of this situation. The large hole in the rib the fitting and the stiffener is for the fuel line fitting which will be installed later. At right above you can see a wire and a fitting attached to the rib. The wire is for the fuel quantity system. It is soldered to a BNC bulkhead connector and sealed. The fitting is for the fuel vent line to be installed later. It is also sealed.
The photo above left shows the stiffener attached with three AN426AD4-4 rivets. The stiffener is counter sunk and the rib is dimpled. Wet sealer is put on the stiffener prior to riveting. Notice that in squeezes through all the rivet holes. This rib must now be installed on the fuel tank before the sealer sets up. That is because the tank attach fitting can’t be installed until the rib is riveted in. It is a bad maintenance practice to rivet over dried sealer so the rib must be riveted in now.
Before the rib was riveted in I put a heavy bead of sealer in the leading edge of the skin where the rib would sit (below left). This is an extra precaution I took in an attempt to keep tank from a leaking in this area. Naturally The rib itself was installed wet with sealer as well. The photo at right below Shows inboard end with all of the riveting complete and the fuel line fitting installed. I put a red “dust cap” on the fitting to keep dirt out. I also put a cap on the vent line (not shown). I didn't go into much explanation about the brace that you see across the access opening. This is basically the same type of set-up that the left fuel tank has. Its purpose is to keep the "Inverted Flop Tube" fuel line from getting stuck against the rib. I didn't invent it. It is on the drawing - honest.
The photo below left shows the fuel quantity wire attached to the vent line via some “zip ties and some sealer. The plans tell you to seal the wire to the vent line which I did. I used these ties temporarily to hold the wire until the sealer dried. After the sealer dried I removed the ties (below right). The RH photo also shows the flop tube assembly (bottom arrow) and the brace to keep the tube from getting caught on the skin stiffener (top arrow). The photo also shows the retainer that keeps the flop tube away from the access door (no arrow). Notice that the protective coating has also been removed and that there is very little sealer on any of the parts.
The photo above left shows the little trap door that I call a “flapper” door. This assembly is designed to slow the retreat of fuel out of the fuel pickup bay. It is very easy to make. One thing that the plans don’t tell you is that you must put a shim under the upper half of the hinge the same thickness as the “flapper” or the “flapper” will not rest flat on the rib. I drilled the forward Portion of the hinge with a #60 drill bit and put .025 safety wire through the hole and twisted it. (It is hard to see this in the photo.) This will keep the pin from sliding out forward. The baffle will keep it from sliding aft. I put the right hand photo in to fill the space. It shows the outboard bay just prior to closing the tank.
The two photos above show the inboard and the outboard fuel quantity probes after installation. Notice that the outboard probe has the notch cut in it. This is to maintain clearance with the stiffener. The minimum clearance is 3/16” per the plans. The inboard probe did not have a problem. There are a total of four hard insulators on each attach screw. Further the screw has an insulator that is made from 1/4in. O.D. tubing which insulates it from the rib. I certainly hope that this system works because it will be a pain in the rear to open this tank back at up if there is a problem. We shall see.
Two photos above both show the fuel tank completed. The right hand photo shows that the rivets that were used to install the “Z” angles have been sealed externally. All of the rivets in the face of a baffle are blind. I used aircraft quality blind rivets P/N CR3213-4-2 instead of the ones supplied in the kit. Up to this point I have only used aircraft quality blind rivets on this project. I installed each rivet wet with sealer and then applied sealer externally as well. Hopefully it will not leak.
The next step was to bolt the fuel tank to the spar (upper left photo). There is a mistake shown in this photo. Do you know what it is? There should not be nutplates on the inboard attach angle. The nutplates are on the spar for these three bolts. I will address this error a little later. The upper right photo shows the 1/8” copper clecos holding the skin to the spar from the inside. It is interesting that the holes in the spar are 5/32” and the holes in the skin are 1/8”.
The photo above left shows me installing a 5/32 cleco through the spar into the skin after I enlarged that hole with a #21 drill bit. I drilled from the inside out on each hole with a 90° drill motor. The right hand photo shows 5/32 clecos installed. I reamed all of the holes between the 1/8 clecos with a #21 drill bit and then I removed the 1/8 clecos and reamed those holes with a #21 drill bit. If you look at the two photos closely, you can tell that I had two bolts in each attach angle to keep the fuel tank in place. When I first laid out the tank parts in the first stages of building the fuel tank I made sure that the fastener holes in the tank were centered on the holes in the spar. When I bolted the tank back on to the spar to enlarge these holes, I made sure that the holes in the tank skin were still centered on the holes in the spar. They were (whew!) so I proceeded.
The two photos above show the mistake that I mentioned earlier. The photo on the left shows three nutplates that should not be there. These nutplates should be attached to the spar. I drilled the nutplates off. I then took six rivets and cut them as short as I could with a rivet cutter. I squeezed them into the nutplate holes. Problem solved. Obviously when I drilled the nutplate holes in all of the attach angles, I forgot that one attach angle does not get nutplates. Looking back, what I should have done was to mark that attach angle or put tape over the nutplate holes.
Picture above left shows the fuel tank nearly completed. All that is left is to install the access panel and leak check the tank. The photo above right shows the attach nutplates on the spar. Notice that there are no chatter marks on the counter sunk holes. I go into this in depth in the ribs and spar section. Let us now move to that section.