After trying to adjust the skins to eliminate the “banana” curve at the end of the right skin, I discovered other builders had also run into this, and it was because the skin was bent incorrectly. Worse, you can’t really fix this either.
As it turns out, I had another right skin that came with my original kit that I rejected for missing a line of rivet holes for a stiffening channel (it looks like my kit had a Sling 2 skin instead of a Sling 4 skin, since the Sling 2 doesn’t have this stiffening channel). I decided to test fit this skin to see how the end lined up. As luck would have it, this one lines up much better than the replacement skin.
So, I decided to measure where the missing holes needed to be, and simply drilled the original right skin.
With that problem out of the way, I was able to finish clecoing and riveting up the skin. I finished one side and will complete the other side next.
At the moment, I have the elevator temporarily attached to the horizontal stabilizer with some 1/4″ bolts, and I’m using clamps on the ends to keep the elevator horns lined up perfectly while I rivet the skin.
One of my fellow builders (Thanks Pascal!) recommended temporarily attaching the elevator to the HS to make sure the “horns” (end part of the elevator) line up perfectly with the HS skins before nailing the elevator skins down. The correct hardware to attach these two parts is AN4 bolts, which I don’t have yet, but I used some common 1/4″ bolts I had on hand. I then used some clamps to line up the horns to the horizontal stab.
I found that I need to adjust the skin to make things fit right. One of the folded ends of the skin doesn’t look quite straight, it looks like the bottom-left diagram in the picture below. The manual says to remove the clecos, adjust skin and try again. I’m not quite sure how this will help though – the holes are drilled pretty tight, and don’t leave much room for adjustment. I’m thinking that I will use a reamer to open the holes up to #31 size and then try again.
Here’s an in-process photo. This thing starts to look much bigger when you put the parts together!
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.
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.
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.
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 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.
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:
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.
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!
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!