Closing Up the Vertical Stabilizer

Final preparation, fitting and riveting of the skin to the VS structure was done today. Everything went together beautifully. I’d done extensive research, detailed review and careful pre-fitting to be certain that I had confidence in a process that would – and did – produce my intended result.

First EAA Tech Counselor Visit

I was privileged to have a visit from a fellow EAA Chapter 430 member and Technical Counselor – Harry Cook. Together we reviewed my build environment and progress. I was pleased to receive positive feedback relative to my understanding of what I’m doing and workmanship involving the vertical stabilizer to the point immediately prior to covering.

VS Structure, Antenna and Cables

Over a couple of sessions, I’ve installed additional m4 rivnuts, permanently riveted together the VS structure and mounted the VOR antenna base to Rib 004 – previously reinforced with its doubler.

Edge grommets have been applied to lightening holes of the rear channel and rib 003. I’ve elected to replace the factory-supplied rubber grommets with Heyco 0.375in nylon snap-bushings – to protect the coax and strobe light wiring where it passes through the VS structure. I enlarged the forming holes to 9.5mm, as directed. The forming holes at the front of ribs 002 and 003 were similarly enlarged to accommodate the bushings for my (optional, build-specific) VOR antenna coax cable. Routing the coax near the front of the structure is intended to keep the cable clear of the ‘works’ for the elevator.

After a bit of experimentation with my newly acquired rotary coax stripper, I installed the Amphenol BNC male connector to the RG-400 coax cable. As mentioned in a earlier post, I’ve decided to use M27500-20TG4T14 for the strobe – rather than the factory-supplied PVC jacketed wire. Some heat shrink tubing bulks up the cables as they pass through the bushings and the edge grommets. A few dollops of aluminum-compatable gasket-forming RTV locks down the bushings and the cables to the structure. I don’t want any movement from vibration.

VS Skin – Prep and Test Fit

I spent several work sessions to make sure I can expect good results when it comes time to close up the VS by riveting the skin to the underling structure. By that time – the VOR antenna, its RG-400 coax cable and the tail strobe wiring must be in place and will be expected to last the lifetime of the aircraft. No pressure!

Holes around the edges of the skin were typically large enough to accommodate 3,2 x 8mm domed rivets, but the holes in the skin at the interfaces with the ribs were smaller and needed to be enlarged with a #30 straight flute chucking reamer. All holes in the skin were carefully deburred. Overall, the concentric alignment of holes in skin and structure were pretty good, but a few will need to be match-reamed during the final fit, immediately prior to the riveting.

The antenna base required 4 recesses be machined into the plastic-like material that allow shortened rivets to fit without interference. My trusty Dremel Tool did both jobs handily – shortening the rivets and creating the recesses.

Pulling Rivets – We’re Really Having Fun Now!

I’ve had the empennage sub-kit since late July. Today – finally, in November – I’m very pleased to have pulled my first rivets.

It’s taken me this long to get my act together to the point of seeing my way clear to assemble a few parts. As is happens, the VS rear channel is a good starting point.

If things weren’t exciting enough for one day – I opened an email from Barry Jay (TAF) and was surprised to find first pictures of my factory quick-build. The fuselage is built up with evidence of many alodined parts and even some gray paint on interior spaces.

VOR Antenna Mount Fabrication

Incorporating a VOR antenna keeps my options open. The antenna gets built permanently into the VS structure. I have to commit to the antenna choice now, but not the avionics. The Rami AV-520 happens to be an ideal size and has removable whiskers. If I eventually decide that I don’t want or truly need VOR/LOC/GS capability, the whiskers come off and the VS of the bird is clean.

Optimum mounting the antenna requires a little custom fabrication. A doubler plate must be positioned and the mating rib drilled for rivets. I ordered the doubler base-plate part number from TAF. The top VS rib gets modification to accommodate the whiskers and provide access to set-screws.

The fabrication process I undertook was a variation of what is described in the Empennage Construction Manual, pp. VS4. Eventually, small holes also need to be located and created in the VS skin to accommodate the whiskers and tool access to the set-screws. Careful positioning of the antenna base allowed me to minimize impacts to the VS top rib and the skin.

Rivet clearance areas need to be machined into the (hard plastic-like) antenna base. Several rivets also need to be shortened. The general process for rivet shortening is in the construction manual. There are places where space is tight and a bit of finesse helps to achieve good fit.

Rivnut Mounting – Step 1 Complete!

I’ve finally gotten to the point of acomplishing the very first step on page VS1 in the Empennage Construction Manual! I’d been worrying about it. Getting the rivnuts securely mounted calls for a rather deliberate process with good technique. It all went very well. What a relief.

Having an effective tool – Astro Pneumatic Tool ADN14 Rivet Nut Drill Adapter Kit – and practice made the day. I’ve seen a couple of other builders recommend this particular tool. Because it relies on a cordless drill motor and identification of an appropriate torque (clutch) setting, I felt there would be a good chance of getting consistently good results. This seems to be the case.

Practice! I got an assortment of metric rivnuts and used the tool to work with them until I knew exactly what to expect. Now I have 14 perfectly mounted rivnuts!

BTW – the KAI mentions using high strength Loctite to insure that the mounted rivnuts don’t slip in their holes. I say – learn how to mount them firmly and they won’t slip. No Loctite is needed. Because IMO – if they’re not tight enough to stay put, then they’re not satisfactorily mounted.

Metal Prep and Priming

Over the course of 3 days, I got my feet wet with metal preparation and applying self etching primer. Deburring seems pretty straight forward. I got great results on small holes using the Avery Tools Speed Deburr and Countersink tool. I started out using a file to smooth edges, but I as I transitioned to my Scotch-Brite cut and polish wheel (C/P 7A-Med), I gained confidence and proficiency.

Next, I did an initial degrease step outdoors with acetone and paper towel. This also seems to get much of the material identification printing off of the surface. Then, I did a second cleaning with Simple Green Extreme Aircraft in 3:1 solution with distilled water, rise with water and dry. From this point, nitrile or white cotton gloves handling is in play.

My etching and conversion coating results are probably adequate, but not anywhere near as lustrous as others have masterfully achieved – notably those by Des Howson for his Sling 2 Taildragger build.

Working with Bonderite C-IC 33 Aero (Alumiprep) and M-CR Aero 1201 (Alodine) proved a little more challenging for me than deburring. I have to work outdoors with a mortar tub, garden hose and cold well water. It’s chilly too. I’m economy minded, so I’ve opted to use a [silicone basting] brush application technique with solution concentrations called out by the technical process bulletins published by Henkel.

The tiny batch of PTI epoxy primer I got from Aircraft Spruce was completely unusable, with clumps thicker than clay and no luck trying to thin/reduce. I had intended to use it to prime the rear channel parts of the vertical stabilizer. To avoid further delay, I just went with self-etching primer.

Finally, I got several VS parts primed.

Spray Booth – Economy DIY

It’s autumn now, and already too cool to prime parts outside. I need a place in my shop to do it. I tried a cardboard box on a workbench, but quickly decided that a ventilated booth is required.

With about $8 worth of 1×2 wood strips and $20 of plastic sheeting, a $12 dryer duct, and an existing space heater (w/fan) — I was able to quickly build a 4′ x 5′ x 9.5′ tall spray booth.

I only need to work with relatively small parts. The longest pieces in the empennage are only a few feet long and can be suspended with a vertical orientation. Ventilation is minimal. Hopefully it will be enough.

EAA Workshop – Sheet Metal

There was lots of good information and practical hands-on work at the EAA SportAir Workshops this weekend. The Museum of Flight Restoration Center at Paine Field (Everett, WA) makes for a great venue.

Sling builders were well represented, with 3 active projects and at least a couple prospective Sling TSi-leaning workshop attendees. It was nice to see and chat with fellow builder Philip Rueker, taking part in the Electrical course.

I didn’t quite finish my project, but they gave me a certificate anyway. I’m so glad I chose a Sling 2 kit with pulled rivets. I would be absolutely terrified to undertake a project with solid rivets and needing to conjure sufficient skills to wield a pneumatic rivet driver and bucking bar to achieve 14000-plus decent – let alone perfect – results. OMG!