9.5.0 Sump Tank and Fuel Lines

The first step is to get the darn thing to fit properly in the fuselage. It gets installed on the floor in back of the whale tail (part of the keel). I had already sanded down about an 1/8 of an inch to get it to fit. However, according to the factory, that’s not enough! It turns out that since I’m one of the first XL builders installing a full pushrod system for aileron control, I needed to chop off close to an inch to make clearances available for the pushrods. Yikes! That seemed like a lot. But I did it. Since I already ground down about a sixteenth to an eighth, I decided to cut about 3/4’s of an inch off since I knew I’d be sanding it even more to get it to lay flat on the cover. So, in the end, I guess I’m close to having chopped off the 1” requested by the factory.

The next step is to install hard points for the vent line and the sump drain. I added another hard point just in case I ever need to run a fuel return line to the sump. (Lycoming engines, which I’m planning on installing, don’t require a return line but Continental engines do.) I ended up installing the vent and return line hard points on the top (shown here at the bottom of the photo). Many other builders recommend venting directly from the top, but in addition to that, I discovered that the landing gear bulkhead, which this butts up against, is higher and covers up the normal location for the fuel connections. The factory said I need to grind down some of that bulkhead to create a clearance for those fuel lines. Putting some of the lines on the top eliminates having to grind down even more of that bulkhead.

The factory installed bulkhead is shown on the back wall of the sump tank (lower back) in this photo. I still need to drill and tap that hard point for the two fuel lines coming from the wing tanks.

Fuselage - Fuel Sump Tank

I've added hard points for the vent line (bottom right), the sump drain (middle top) and an extra hard point (bottom left) should I ever need it for a fuel return line.

The fuel tank cover also needs to have hard points installed - one for the low fuel sender (that’s the top hard point in the photo below) and for the fuel output feed to the engine (the lower hard point in the photo). And this is where some of the controversy comes in. The factory says to install 3/8” lines for everything fuel-related. But some builders had trouble maintaining proper fuel flow with a 3/8” line feeding the engine. They recommend 1/2”. So, that’s what I’m doing, since I’m looking at installing a 300 hp Lycoming. Too big isn’t going to hurt anything and I’d rather not have to “fix it” later if there’s a problem. The two fuel lines coming from the wing tanks remain at 3/8”.

Some of these hard points came pre-tapped, which is good and bad! Some of the tap sizes are huge - the taps themselves are also expensive. But how do I epoxy these in place without getting glue in the threads? Well, I covered the backside with duct tape and filled the pre-tapped insides with grease. Once the epoxy and the lay-ups cured, I pulled the tape off the back and scooped the grease out. That worked pretty well but I still had to chase a few threads and mess around with the outside where the fittings attach to get them to fit properly.

Fuselage - Fuel Sump Tank Cover

And here are the hard points added to the sump tank cover for the low fuel sender (top) and fuel feed to the engine (bottom).

Some of the hardpoints came pre-tapped from the factory, but those that weren’t needed to be tapped by me. All of the fuel fittings use NPT threads (National Pipe Thread), which means that the threads taper as they are cut. I quickly discovered that cutting NPT threads is a bit trickier than standard machine screw threads. And these taps are huge! Check out the size of the tap wrench I had to buy for this! I also use a special thread tapping oil to help lubricate the tap while cutting.

Fuselage - Fuel Sump Tank - Tapping Fuel Ports

That's a big tap handle - and tap! Tapping these holes took a long time!

Once all of the threads are cut it’s time to seal up the tank. The factory recommends a special EZ-Poxy mixture to seal the tank. Some people use what’s called “Jeffco” (although it has a different name now). Apparently either product will work. I decided to stick with EZ-Poxy since I’m used to working with that. An additive called Cabosil is mixed in to create a thick, gooey, paste-like substance that smooths out the interior nicely. When adding the 2nd coat to both the tank and the cover, I placed the tank on top of the cover to let the epoxy ooze into the seam. It created a nice, tight fit. The rocks are used to weigh down the cover during cure.

Fuselage - Fuel Sump Tank - Sealing the Tank

After coating the internal parts of the tank with a special mixture of EZ-Poxy and Cabosil, it's time to attach the cover using the same mixture.

Fuselage - Fuel Sump Tank - Cover Trim

Once the epoxy cures, it's time to trim the excess material off of the cover in preparation for external lay-ups.

After the epoxy cures and the tank cover is sealed, I applied a couple of layers of BID around the perimeter to make 100% certain that the tank is sealed. You can see the BID lay-ups in the photo below. Once that cured, it’s time to make some attachment flanges. This actually turned out to be more difficult than I thought it would be. The manual recommends a 2-BID “bottom” lay-up (which would be the top of the tank as shown in the photo below) to be attached to the two layers of triax attached to the side of the tank. I didn’t at all like the way that was working, so I attached the two layers of triax first, let that cure, then flipped the tank into the position you see below to add the two layers of BID. Doing that gave me more control over the process and I ended up with a nice, tight BID lay-up, as you can see in the photo below.

Fuselage - Fuel Sump Tank - Attachment Flanges

The external lay-ups are finished so the next step is to make some flanges for attaching the sump tank to the bulkhead.

Once everything cured, I cleaned up the flanges and drilled mounting holes in preparation for mounting in the fuselage. By the way, the reason all of the ports are covered with duct tape it to keep dust and other stuff out of the tank. Trying to keep it clean inside!

Fuselage - Fuel Sump Tank - Trimmed Mounting Flanges

The tank is just about ready to install now that the flanges are cured and drilled.

And finally, here is the installed tank along with all of its fittings and the tank’s fuel level sensor. The tank took longer than I thought it would to build, mainly because of the cure times for the various processes, but in the end it came out nicely! You can also see my brake lines (white NylaFlow tubing) routed around the tank. I used some cable clamps to hold them in place, making for a nice, clean layout.

Fuselage - Fuel Sump Tank - Installed

The completed tank with all of its fittings is now installed firmly to the bulkhead.

Installing the sump tank was the last item on my list of things to do before “topping off” the fuselage. The process for putting on the “top” is actually way back in Chapter 5 of the manual, but most builders put that off until everything that needs to be installed in the lower fuselage is completed. It’s much easier to do all of that mechanical work with the top off. If you’d like to skip back to Chapter 5 to see the process for installing the top, click here. Otherwise, the next step in Chapter 9 is to install the strakes, which hold the main fuel tanks. But I can’t do that until the top is on and finished, so…  More to come later!

ui© John Trautschold 2018