Fuel Tank

Last night and tonight I leak tested the fuel tanks using balloons, a bike pump, and lots of soapy water. I didn’t detect any leaks, but I’m going to test it again using a nanometer, and let it sit for 24 hours.

The tanks are tested before being shipped out, but I have installed a couple of additional fittings and plenty of people have found leaks in quick build tanks, so I wanted to be careful. I taped over the fuel caps, attached the hose and valve, and used a rubber band to hold the balloon in place. There’s no easy way to measure that low of an air pressure (1psi), but the balloon gives you something of a gauge. The air pressure is just to ensure that air would be pushed out any holes, causing the soapy water ssolution to form bubbles.

I tested it on the balloon and saw how a small leak causes an in-missable amount of bubbles.

I didn’t detect any leaks, but I also couldn’t test the back side of the tank while it’s still attached to the wing. I don’t want to drop the tanks unless I have to, so I will probably just rig up a manometer and monitor the air pressure over a day or two. If there is any indication of a leak, I’ll investigate further. If not, I’ll call it good.

Tonight I installed most of the fittings into the fuel tanks. The only fitting I didn’t install was the drain on the left tank. Since that wing is sitting on the bench, and the drain port is on the bottom, I’ll wait until I put it back in the stand to install.

I spent time researching exactly how much torque these fittings need, and how to apply the sealant. Couple of useful videos on the EAA website, and more specific info on Vans Airforce. These (like all fuel fittings) are NTP, tapered threads. Torque values are not really applicable, since the jamming of the threads is what provides the seal. It’s not a complete seal, since there is a spiral pathway that fuel could flow out of the fitting, so some sealant is required. Over-torquing will damage the threads, since these are aluminum fittings and relatively soft. Vans recommend installing by tightening until the first sign of resistance is encountered, then turning 1.5-2 turns beyond. The sealant recommended by both the EAA and Vans Airforce members is called EZTurn, which is a sticky, but workable fuel-resistant sealer. Tape is not recommended, due to the risk of contamination of the fuel system with chunks of tape. For the same reason, any sealant should not be applied to the first 2-3 threads on fuel fittings.

I started by cleaning out the fitting ports, using some rubbing alcohol on a lint-free rag. Despite taping over these ports, there was a surprising amount of dirt in the threads. Then I cleaned the fitting threads and applied a coating of sealant, skipping the first 2-3 threads.

I got all of the fittings in, except the left tank drain port, which I can quickly add once the wing is in the stand. I’m going to keep it on the bench until my landing lights arrive on Thursday, so I can fit them. The last step in the tank work is testing for leaks, which I will tackle next weekend once the proseal has fully cured.

I’ve been waiting on some more tank sealer so I could install the fuel senders. UPS claimed they delivered it on Wednesday, but it didn’t show up. On Thursday it turned up with another package, so I was able to install the senders tonight.

I took time doing prep work; scuffing the surfaces, cleaning the surfaces, checking that my multi-meter was working, checking that the screws installed in the nutplates, checking that I had the right size Allen Key, etc. Finally I was ready to mix the sealant and get started.

I had one wing on the bench and one in the stand. The wing in the stand was in a slightly awkward position, as the stand itself was interfering with the placing of the sender into the hole in the end of the tank. With a bit of adjustment I got it to work ok, and that’s the wing I started with. In hindsight, I should have started with the easier wing, so I could build some experience. Oh well.

On the right wing, I could have used more sealant. I’m sure I got a good seal in the end, but I don’t think I hit the goal of 0.7mm between sender and rib. This space is to allow easier removal of the sender in the case of future tank maintenance. With a slight gap, a blade can get in there to cut the sealant. I have some gap there, but I think it’s less than desired. It’s also tricky to get an even torque on the screws around the flange of the sender, it’s probably not exactly square. I checked that the plate is grounded, and it seems fine.

Then it was onto the easier-to-access left wing. This was quick, clean, and came out great. I used more sealer, and ended up with a nice even bulge of seal around the sender flange. Looks to be just right. Hopefully if I have to do maintenance on tanks, it’ll be the left one!

Next up I’ll be installing the tank drain fitting, then pressure testing the tanks for leaks.

Last night and tonight I bent the fuel sender float arms to the right shape and test fit them into the tanks. It’s a bit hard (impossible?) to tell if the floats are exactly where they are supposed to be, since the tanks are already assembled and sealed. Perhaps if I had a tiny borescope I could peek inside the tank drain and see.

These came out ok, and from careful testing, I can tell they are just off the bottom of the tanks (plans call for at least 1/16 from the bottom), and not interfering with the vent lines. I have no way of testing if they are staying clear of the top of the tank.

I can’t do the final install on these until I get another shipment of proseal, hopefully this week.