Starboard Wing Nearly Ready for Closeout

After nailing on the rear skin support, the wing is ready to accept a fuel tank and upper skins

I am at a good point to pause on building the starboard wing so that I can finish the fuel tank. I completed a quick task before switching gears though – I wanted to add some protection to the wire running through the lower stringer so that it doesn’t have the possibility of being frayed where it passes through the ribs. I bought some fancy edge grommet from Aircraft Spruce called Spring-Fast® Composite Grommet Edging. It’s much more expensive than nylon caterpillar edging, but it’s a better design that can stay in place without glue.

Adding in a small length of edging to the rib to protect the wire running through the channel
This is the other place I wanted the grommet to protect the wires for the landing lights
Front of the Kuntzleman LTR landing light

I did some of the preliminary work a few months ago so I am ready to start the stinky job of sealing the tank. I started with closing the tooling holes on the end ribs and adding the rivnuts to rib 101, which holds the cover for the fuel level sender. The instructions say to put on the AN-style fittings for the fuel pickup and return at this step, but they were a pain to work around when shooting the rivets into the ribs on the other fuel tank so I’ll install and seal the fittings later.

Lightly sanded the areas where sealant will be applied. I also had to enlarge the holes for the fuel pickup and return fittings – 9/16” and 7/16”, respectively. The SEM Solve stuff I use for cleaning the metal before priming is also a great degreaser to use before sealing
Everything I will need is within reach
Mixing up a small batch (33g) of sealant, which is more than enough to fill a 10mL syringe
This stuff is really stinky – respirator needed!
I applied a small bead of sealant to go between the rib and this flange plate. The holes in the plate were not drilled to the proper size for the 4mm rivnuts, so I used a step drill to open them up to 7/32”, and then a 6mm drill to get the final size.
Finished product

Attached left fuel tank

Fresh back from my work trip to Germany, I was ready to attach the fuel tank to the wing. First, I wanted to check that all the bolts are torqued correctly on the fuel sender cover, but I wasn’t too sure about the correct torque value. The screws provided in the kit are M4 x 16mm hex cap, and the parts list calls them stainless steel, zinc-coated. This doesn’t seem to be a normal alloy/finish combo, so I couldn’t find what torque to use. I chose a value of 20 in-lbs of torque, somewhere between class A2 and class 8.8 from this helpful chart. This seems about right, since M4 is similar in size to an AN3 bolt, which should be torqued 20-25 in-lbs.

After setting the torque, I applied a little dab of paint (nail polish) to each bolt head so that I know they’re already torqued.

Attaching the tank is a little tricky. The z-brackets / channel brackets must bolt to the main spar, but alignment is not guaranteed, and the AN3 bolts don’t self-center. I had to borrow a trick from Craig Maiman, which was to grind down the end of the bolt to give it a taper, so that it could guide itself into its mating nutplate. I think floating nutplates would have been a better design choice here.

Looks are deceiving! It’s actually the skin underneath that’s misaligned.. the fuel tank skin actually lined up pretty well with the main spar cap

Once the tank was attached, I found some pretty bad misalignment of rivet holes on the bottom skin, so I spent quite some time filing and then reaming these holes so that I could get the clecos and rivets in.

Bottom side of the fuel tank, getting ready to rivet the skin onto the main spar cap

The wing is looking pretty good with the tank attached. I’m not quite finished putting in the rivets – tomorrow I’ll finish the bottom side and then flip the wing back over to the top to finish riveting.

Fuel tank leak test passed

After the required minimum 3 days elapsed to allow the sealant to cure, I was able to perform leak tests on my left fuel tank this past weekend. I closed off the vent and fuel supply fittings, and then used a bicycle pump tethered to the fuel return fitting via 1/2″ ID tubing to pressurize the tank. The instructions say no more than 1 bar of pressure, which is a little less than 15psi – pretty low to register on the pressure gauge on the pump, so I just gave it a few pumps. You can see the skins start ballooning up right away, so I knew I was getting air into the tank!

Typical floor pump with built-in pressure gauge
No bubbles!
Drats – lots of bubbles

All of the surfaces along the rivet lines were bubble-free, but my first attempt to close the fuel level sender cover didn’t work. I really didn’t want to apply sealant to the bolts for this cover, so that it will be easier to service in the future. My first attempt involved using nylon washers on the M4 bolts in place of stainless washers, but it just wasn’t enough of a seal.

I tried adding small Viton o-rings on the screw, at the end of the threaded shaft. In theory this will work well – but the problem with this idea is that the o-ring gets squished too much as the screw is torqued down, and eventually causes the o-ring to tear. I tried chamfering both the washer and the cover surface, to give the ring some place to expand into as the bolt is tightened, but there was still too much tearing.

My third idea was to make flat washers out of the same material as the custom gasket between the cover and the rib, but this material is also pretty squishy (75A durometer) and wants to tear before getting to the nominal 50 in-lbs of torque. There is a company that makes sealing washers called Seeloc, but they don’t appear to be fuel resistant, and at $2 a piece they are quite expensive. Their solution appears to work because the washer and seal are designed correctly so that the seal can squish down and form a tight seal without tearing.

My fourth and final idea was to apply a short section of heat-shrink tubing to the last third of the bolt shaft closest to the head, so that a seal is formed as the bolt is tightened. The heat shrink I’m using is made from polyolefin, which I learned is resistant to fuel. The idea is similar to using Teflon tape, but without the worry of bits of tape tearing off and potentially clogging something in the fuel injection system.

Small bit of 3.7mm diameter heat shrink tubing on the M4 bolts for the cover

The heat shrink tubing seems to have done the trick – I can feel the resistance of the tubing as it starts biting into the threading of the rivnut, and it torques down fine. I backed a screw out to make sure it doesn’t tear apart and leave debris, and all looks fine. Since this little piece of tubing does not extend past the end of the rivnut and into the fuel tank, I feel confident that I won’t have debris entering the fuel system.

One more pressure test, and this time no bubbles! So it looks like my heat shrink tubing solution will work.

No bubbles!

I’m on another business trip this week to Germany, so I won’t be able to make any more progress this week until the weekend. I had hoped to finish up the other tank before my sealant expires at the end of the month, but I don’t think I’ll be able to. I wonder if it’s still safe to use the sealant? I’m using the Flamemaster CS3204 sealant recommended by TAF, and it’s been in my cool dry garage / basement all winter.

Starting Right Fuel Tank

Since I have all the fuel tank build steps fresh in my mind, I’m building the other one now. It took me about 14 hours to build the first one, not including leak testing and remediation (if needed). Let’s see how long it takes the second time?

Closing up the fuel tank

This weekend I made it my goal to close up the left fuel tank. After one last inspection of the tank’s inside for debris, a final cleaning of all the mating surfaces, and staging all the rivets, screws, brackets, and tools, I mixed up a 66g batch of sealant and got to work. It’s a lot of work to complete within the 2 hour working time of the sealant.

Bead of sealant applied to all of the surfaces that mate with the back cover

I ran into one problem that I didn’t quite anticpate – the last U-shape bracket that attaches the tank to the main spar is too tight to get a rivet tool into – so I have a mix of rivets facing opposite directions. But one in particular, just above the 90° fitting for the vent, had no clearance on either side to get in a rivet puller. I have no idea how you’re supposed to be able to set a rivet in this location! Some quick thinking led me to a solution to use an M4 screw with nyloc nut in place of the 4.0mm rivet.

A nyloc nut and M4x10mm screw instead of a rivet

After setting all the rivets for the brackets, it’s a race against time to temporarily mount the tank to the wing to set the rivets that secure the back wall of the tank. Of course, I had some trouble getting the tank lined up correctly to the wing, but once I did, the rivets went in pretty easily.

Finally – a complete leading edge!
Looking inside to the fuel level sender cover

Now the tank must sit like this for 3 days for the sealant to cure, then I get to take the tank off again and perform a leak test.

Misc Fiddling with Fuel Tank

I’m back at it with the fuel tank. I was fiddling with the fuel sender cover and decided to add a ground terminal by drilling out one of the closed rivets, and then putting it back in but also adding a little metal tab. My concern is that with all the various gaskets and seals I’m using, there isn’t a good ground path for the fuel level sender, so I could get erroneous readings. It’s temporarily held on with some tape for now until I mix up another batch of sealant.

I also riveted this end rib into place. As you can see in the picture, I had my fluke meter out testing the resistance on the fuel sender as I moved the float arm up and down – seems to be working fine, I measure between 3-185 ohms.

I started pulling off the blue tape I had put over the fuel line fittings so they wouldn’t get gobbed up with sealant. I’m surprised the fuel drain is so close to the fuel pickup – the rivets are just a hair width away from touching the filter mesh!

Misc Fuel tank tasks

I spent Sunday and Monday prepping a few miscellaneous parts: fuel tank cap, fuel drain, and brackets that attach the tank the the main wing spar. My kit was missing one of the brackets, but Jean D sent me replacements a few weeks ago. Here’s the replacement part with my name on it, how’s that for personalized service?!

The various brackets need to have anchor plates attached, held in with tiny 2.4mm (3/32″) flush rivets. Strangely my kit did not include enough of these rivets – I have 24 anchor plates, and each needs 2 rivets = 48 rivets, but the kit only included 34. This size in countersunk-style is a little hard to find, I had to go to a speciality rivet supplier (Jay-Cee) to find some to buy.

After mixing up some more of the smelly sealant, I attached the fuel cap and drain. One of the rivet mandrels broke on the drain, doh! Normally I’d just drill it out and replace, bit I’m afraid to drill this one out because of course it’s the one closest to the fuel pick-up screen and I don’t want to damage that! Luckily it looks like the rivet body formed ok so I’ll just file down what’s left of the mandrel.

The fuel cap is pretty nice, it sits flush with the skin and opens with a key. I was a little surprised though that it’s only held in place with the sealant.

Fuel tank ribs sealed

The five ribs that make up the fuel tank are now installed and sealed for the left tank. Very messy, stinky work! By far, my least-favorite part of this build so far. And there’s still more to do on this tank. And I still have a whole other tank to build!

I’m glad I masked off the areas on the skin first, it made things look much cleaner once the tape was removed. I think all my fillet and fay seals look good, but it looks like a few rivets didn’t seat properly as I was rushing to shoot them in – I’ll have to drill those out and try again.

I used 80g of part A + 8g of part B fuel tank sealant, and that seemed to be enough to complete this part of the job. It was also as much sealant as I could apply within the 2 hour working window. Luckily you can safely plan to clean up excess oozing sealant after the 2 hours, it was still easy enough (though not easy) to remove with acetone and a rag.

The instructions say to install the fuel line fittings on rib 101 and 105 prior to installing the rib in the skin – I don’t agree with this, as it made it much more difficult to accomplish the fillet seal, especially on rib 101 with the fittings that are very close to the edge of the rib. I’ll change up the order a bit when I work on the right fuel tank. I did make sure to protect the fittings with tape so that they don’t get plugged up with sealant – I’ve read several accident reports involving loss of engine power due to fuel starvation, caused by blockage. Here’s one. And here’s another infamous one involving an RV-10. So I’m trying to be very careful.

Not too many in-process photos to share since my gloves had sealant all over, but here’s some decent before-and-after pics. In the interest of time, I just used my finger to shape the fillet seals this time – I’ve done that before with silicone caulk and it seems to work just fine.

I taped off the fittings so that they don’t get plugged up

Slowly building the fuel tank

I guess I’ve been distracted a lot lately, as I’m making very slow progress on this fuel tank! I think it’s partially because I’m afraid to work with the sealant, which is messy and has a 2 hour working time once mixed. I’ve also been doing a lot of reading about best practices to seal and chatting with fellow Sling builders, since I don’t want to ever have to deal with leaks.

I found a helpful diagram on a long thread on Van’s Air Force, I think it makes it very clear:

There are some slight differences since I’m using blind rivets instead of bucked flush rivets, but still very applicable.

The TAF manual makes a handy recommendation to heat up the tip of a large syringe with a heat gun and then squash it with pliers in order to make a nozzle that dispenses a ribbon – that was a good suggestion! That creates the “fay” layer in the figure above.

A few more tips I’ll note here that I didn’t recall seeing in the build manual:

  • Put down blue painter’s tape to keep the sealant fillets clean (inspired from Pascal Latten’s build)
  • To achieve the fillet, I used a smaller 10mL syringe to run a bead along the edge and then used a popsicle stick edge to press in the sealant and shape the fillet
  • I used just a few clecos to tack the stringer in place and immediately started riveting –as opposed to clecoing it in, waiting for sealant to cure, and then coming back days later to remove clecos and replace with rivets, re-sealing the rivets as I go
  • After setting the rivets, I applied sealant around the body of each rivet to encapsulate it per the figure. To reach around to the back side of the rivet body (hard to do inside the U-shaped stringer), I used a little hook-shaped wire to reach around to the back side of the rivet to spread the sealant evenly.

Anyway here’s a few pictures of attaching a stringer to the upper side of the skin and sealing it. It took me the full 2-hour working time to do just this stringer, but now that I have the process practiced, I think the ribs will go faster.

A 60mL syringe filled with fresh sealant, ready to dispense

The hook is fashioned from a piece of 14AWG solid copper electrical wire

My helper for the day – our dog Kooper!

Fuel tank breather lines

Today’s task was to create the breather lines for the fuel tank. I’ve had the tubing and some of the tools for a while, but I needed a smaller tubing bender, which I just recently received.

Here’s what I used:

The flaring tool is a bit specialized; 37° flares are needed for AN fittings, whereas I think 45° is more common for other style fittings. The tubing bender is nothing special, but I needed a 1/2″ bend radius, whereas the generic tool you’d find at Lowes is more like 1″, too big.

As you can see in the pictures, the results came out pretty good using the right tools… however it’s a bit annoying to have to buy $150 worth of specialized tools to form $10 worth of aluminum tubing. Guess I’ll need to come up with ideas for things I can make with the flaring and bending tools and sell on Etsy! 😬

For the U-shaped part that goes inside the fuel tank, I used a piece of tubing 4.5″ long. For the tube that goes down from the tank, I used a piece 210cm long as per the instructions.

Be sure to put the fittings on the tube before you bend and flare it!