Skinned the Elevator

I’m continuing with closing out empennage tasks; like the rudder skin issue I described last week, I also had a problem with one of the elevator skins from the original kit I bought. The skin was missing a row of pre-drilled holes for a stiffening channel, and I couldn’t see an easy way to match-drill in the right place, so TAF sent me a replacement skin a few months ago. After prepping and priming the skin Tuesday night, I clecoed it together Wed night. So far, so good, but I have to pay attention to the alignment of the left and right ends to make sure they align perfectly – I’ve seen other builders use a laser level and/or a digital inclinometer to achieve this, and I’ll do the same.

These empennage parts go together so much faster than the fuel tank! With that being said, I’m anxious to get back to the wing. I’m ready to complete the close out of my left tank, after which it must be attached to the wing so that the sealant sets in place correctly. We’ve had a long stretch of cold weather in NH lately so it’s too cold to work in the garage right now. The data sheet for the Flamemaster CS3204 sealant doesn’t list an acceptable temperature range for curing, it just says that it cures at room temperature and will take longer at colder temperatures.

Skinned the Rudder

After a long period of working on the fuel tank, I’m at a step where it needs to be attached to the wing. Unfortunately it’s ridiculously cold in NH right now, which makes working in the garage unpleasant. So, I decided to take a break from the wing and finish up some empennage tasks.

I had to request a new rudder skin from TAF a while ago, as the original one that came with my kit was damaged. The replacement skin arrived after I had already started working on the wing, so I didn’t get back to finishing the rudder until now.

It felt nice to quickly finish something in a day! The only thing that slowed me down was having to modify some rivets to shorten them to fit on the aft edge of the skin. Below is the (almost) complete rudder – the only thing left to do is attach the fiberglass cap, which I’m purposely waiting to do until I decide on which recognition strobe I’m going to use. I guess I also have the task of filling the rivet heads at some point.

Next up is the elevator, which also had a skin problem, but I have the replacements in-hand.

Newest piece in my kit, manufactured in 2018. Most of the other parts I have were made in 2016.
Clecoed Up
Done! I may try attaching it to the vertical stab to see how it looks

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!

Sealing Day #1

The instructions say to attach and seal all the various fittings to rib 101 before sealing up the main tank, so I did that today. I mixed up a very small batch of sealant, enough to put into a syringe for dispensing. I used a digital scale accurate to 1/2 a gram to measure out 20g of part A and 2 grams of part B. I had little 10ml syringes on hand already from another project, but the 14 gauge needles were a bit too small for how thick this sealant is. Nonetheless I successfully got everything gooped up. It ain’t pretty, but I as long as it doesn’t leak I’m happy.

I also bought some 60mL syringes for the main tank work, which I’m planning to do tomorrow.

Before wrapping up for the evening, I took the fuel level sender out of its box and was surprised to find it comes attached to a nice round cover and Viton (!!) seal, which I guess I have to remove and throw away to instead use TAF’s custom bracket that attaches to TAF’s custom cover. Wonder why they did this? Well, if nothing else this reinforces my decision to use a Viton seal that I made as I described in my last post.

As you might be able to tell from the photos, I’m not working in my garage anymore – it’s been so cold lately (8°F this morning!!) that I had to move someplace warmer. Luckily, I have a finished & heated basement, so at least for the cold winter months I’ll be working there. The downside is that my finished space isn’t big enough to fit the whole wing, so I’ll be working on smaller pieces for a while, and maybe sneak back to the garage if we get some more moderate weather.

I think when this plane is finished I’m gonna point my plane south and won’t look back!

Readying to Seal Fuel Tank

I am getting ready to start sealing up the fuel tank.  There are a number of fittings that get attached to the skin and ribs, and all these fittings need to be sealed.

For the fuel level sender, which is attached to rib 101 via an access cover, I decided I want to have a removable seal. The kit is designed to use a cork seal for this cover, but I starting looking for other materials since I know from experience that cork degrades over time. 

Another option is to throw away the cork and just use the same sealant that’s used for the ribs and skin – other Sling and Vans builders have done this.  While this results in a good seal, I think this will also make it very hard to service in the future.

My search for a gasket material led me to two choices – one called Buna-N and another called Viton, which is a trade name for a synthetic fluoropolymer elastomer that is very resistant to fuel and oil.  It is commonly used for this kind of application – here’s an example of one for application on Piper and Cessna Aircraft.   Here’s a brief comparison of the pros and cons of each material.

I bought a 1ft² sheet of 1/16″ thick Viton rubber sheet, and designed a cutting template using my Silhouette Cameo – an plotter marketed for arts & crafts, but equally as useful for stuff like this.  The Cameo isn’t really designed for cutting thick rubber sheet, but I was able to do it using their deep-cut blade (good for materials up to 2mm thick).  The software has an option to run over the same spot multiple times with the blade since the cutter head doesn’t push down very hard.  I found I had to use 10 passes for the cuts to be deep enough to remove the cutout from the sheet.

Here’s a short video of the cutter in action – you can see the gasket shape if you look closely.

Custom Viton gasket for the fuel quantity sender

The end result came out pretty good and fits perfectly.  For the bolts that hold down the cover, I bought some Viton o-rings to fit over the bolts that came with the kit. 

I won’t know how well this works until it comes time to do leak checks – but switching to plan B (using sealant liquid) will be very easy if the rubber seal doesn’t work.

Here’s a link to the Viton seal product I bought:

Also sold via Amazon.

Closing up VS Skins

I attached the VOR antenna using 20mm length M4 screws with nyloc nuts and then began closing up the skin.

Unfortunately the antenna is inaccessible once the skin is riveted on, so for now I think I’m leaving one side open just in case – will just use a few clecos to hold it closed.

VOR Antenna Provisions

In addition to the doubler plate, I also need to modify the skin to accommodate the antenna elements.  I notched the skin so that I can slide it up around grommets at the base of each arm.  Pictures are better than words in this case…

Antenna attached to rib 4. The arms on this antenna aren’t removable so I had to slide one element through the hole
Notches I cut into the skin. On one side, the notch is exactly centered using one of the existing rivet holes; on the other side, I had to make a new hole with a 1/8” pilot drill. I used a step drill to open up both holes to 9/32”, and then used shears to create the notch. The notch is slightly narrower than the 9/32” hole so that the grommet “snaps” into place and doesn’t slide around
Testing of grommet fit in the notch
Sliding the notch over the antenna and grommet
Perfect fit!
View from the top side
Both sides fit nicely. Notice that this antenna has elements that are offset from each other, hence why the notches on the skin had to be in different places