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

VS Structure Assembled

Vertical stab parts are put together, minus the skin. I’m putting in an optional antenna for VOR/Localizer/Glideslope reception, so I had to make a doubler plate for the top spar where the antenna will be mounted to. To make the plate, I used a sheet of 0.032″ 3003 aluminum that I bought from Amazon a while ago, and simply traced the pattern from the rib.

I added wires and grommets for the antenna and for a light that will go on the top of the rudder. Since I had to pass the wire through two of the lightening holes, I attached some nylon caterpillar grommet material to the edge and secured it with silicone sealant.

(Left) Standard rib 4 as supplied in the kit (Right) Custom doubler plate made per kit instructions

All riveted together

Probably not what’s typically used for glue in an airplane, but this GE silicone has a wide temperature range and remains flexible, so I think it will do the trick for holding the caterpillar grommet in place

Vertical Stabilizer Parts Prep

I decided to take a break from working on the wings as I await a few things I need for sealing the fuel tanks.

A few weeks ago I got in a shipment of goodies from Jean at TAF, including missing brackets for the VS, so now I can build that up as well as finish off the rudder and elevator.

Today I prepped the VS parts and primed them in just under 3 hours. I still have to prep the skin, but next steps are to start putting the structure together and figure out how to mount the combo glideslope/localizer antenna I bought back in the spring.

Cold, wet, rainy weather has returned to New Hampshire so it’s back to using my “paint booth” and respirator mask

Left Fuel tank started

I created a jig for assembling the fuel tank using the templates included with the plans. After transferring the profiles to a sheet of 1/2″ MDF, I cut the shape by hand with a rotozip-style tool, then filed and sanded as needed to smooth it out.

Next I followed the plan instructions which say to cleco the left side ribs, stringer and skin together before attempting to seal them. Parts mostly went together ok – there are one or two holes I’ll have to modify.

Since it was a nice day, I hopped on my bike and took a ride over to the little airport in town – Brookline NH (NH16).


It’s one possible option for a place to keep my plane when it’s complete. NH16 is a cute little private airfield, with some homes and hangars directly off the runway. The runway is pretty short though, just under 2000′ long, with tall pines at one end… I’ll have to take a look at performance numbers for the Sling 4, especially the 50′ obstacle clearance run!

Finished up Lower Skins

I was able to get in a few good 5-hr sessions this weekend and finish up fitting and riveting up the lower skins. I had to ream quite a lot of holes to make everything line up ok. But the end result looks good.

I had some nice weather on Saturday, so I took care of priming the nose skin, since that can be attached after the lower skins are in place.

Since I was putting the nose skin on, I also assembled the landing lights. I still need to do the electrical connections on the back.

As I mentioned in earlier posts, my lower skins didn’t really line up well with the spar caps and ribs. TAF advised me to elongate the holes with a drill/reamer, which I did – but in a few cases the alignment was pretty bad so I opted to up-size a few holes to a 4.8mm rivet.

Next I think I’ll start building the fuel tanks.

Progressing with wing bottom skins

Continuing with fitting bottom skins, and elongating holes where needed to fit properly on the spar cap.

I’m using a round file to remove most of the needed material, and then using a #20 reamer to clean up the hole and ensure it’s roundish.  So far so good – I’ll do this to both bottom skins before I start pulling the rivets.

Left two holes filed down a bit, before reaming. Right hole is unmodified.
Finally have most of the rivet holes filled in, just a few more holes to fix.