Tag Archives: fitting

LH Fuel Tank Fitment – Success!

I took a bunch of photos and texted them to Jean d’Assonville. After several days of phone tag – one or the other of us were busy – we connected for a brief chat. That’s all it took.

It turns out that I was in pretty good shape after all. My hell was all in my head. Having quite a few rivets that were hand-fitted at various places around the fuel tank flanges is a good sign. The thing that really set me free was to hear that it is acceptable to dress stubborn rivet holes with a chucking reamer or drill. The same thing is true for Z-bracket holes on the spar. You have to do what you have to do. I just needed to hear the guidance. I’ve learned from experience – it doesn’t usually pay to be impetuous.

A shim is necessary under the top of the Z-bracket at the root of the tank. The other Z-brackets fit well enough. Other builders have needed to shim several brackets. Mine were flat on the spar. Another good thing.

Over a period of several days I evaluated fitment and planned my procedure. Then, I carefully fit the tank in place, one last time, as I’ve done twenty times or more by now.

I made a little shim from material I had on hand. A washer would have worked too, but I thought aluminum against aluminum might be better than against steel. Slightly more surface area is probably not such a bad thing either.

I fitted as many rivets as I could – by hand. The remaining 4,0 mm holes around the flanges were match-reamed with a #20, straight-flute chucking-reamer and deburred. Clecos and additional rivets were hand-fitted.

A mix of AN3-4A and 3A bolts, with washers, were threaded through the spar web and into the anchor nuts on the Z-brackets. (I began with the 4A length for 2 outer brackets, but then worried that the length might turn out to be slightly long.) At the root Z-bracket, two AN3-13A bolts, with washers and nuts and with the shim in place – were finger-tightened.

Finally – today, I mixed up a tiny bit of Flamemaster CS-3204 B-2, as recommended by Sling Aircraft to deter galvanic corrosion between the aluminum and the stainless steel rivets that are called out for use at the wing root – top and bottom.

Dipping the SS rivets in the sealant – one-by-one, I placed them and then pulled them with my trusty Milwaukee M12 lithium battery-powered tool. Rivet pulling continued with for the balance of 4,0mm aluminum rivets. Lastly, the 3,2 x 8mm rivets around the leading edge at RIB-105 were pulled. The Z-bracket bolts were all torqued. The 4A and 3A lengths are both fine. It’s all good. The fuel tank is mounted!

LH Fuel Tank Fitment, Plan B – Vertical Stands

With a seat-of-the-pants concept, a circular saw and a box of screws, I’ve managed to fashion a pair of Sling 2 custom vertical wing panel stands. Poof! It all came together.

With the LH wing panel on the stands, I’ve got much better access. Hopefully this will be the day I get the tank mounted.

Unfortunately, most of the same fit and alignment issues persist. This is starting to feel like Fuel Tank Fitment Hell.

Before I do something that’s un-recoverable, I’ll reach out to Jean d’Assonville at Sling Aircraft (TAF) USA before it gets any later in the day. It’s Friday and hopefully I can get out of this hell before the weekend. Stay tuned.

LH Fuel Tank – Final Fit (or Not)

The LH wing panel assembly has been back up on the workbench for several days. I can’t see any reason not to tackle permanent mounting of the fuel tank.

Well – I found a few reasons to not mount the tank today. I’m struggling to fully fit the tank and to align the rivet holes. I don’t have convenient access to the bottom surface of the wing panel. Gravity doesn’t seem to be helping either.

There’s a curious mix of places where rivets fit easily and squarely and other places where they won’t fit squarely, or at all. I’ll have to shift to Plan B. I think that means building some sort of stand(s) to hold the wing panel in a vertical leading-edge-up orientation.

Rudder Pedals – Tubes, Brackets and Bearings

Work in the center fuselage continued with trial fitting of the rudder pedal tubes. It’s looking good. Initially, the pilot-side (LH) mounting brackets were easily positioned and the 4,0 x 10mm rivets dropped easily into most of the holes. And, after clearing a bit of paint in the holes, the rest of the rivets fit as well.

The flight control linkages rely heavily on composite Vesconite bushings, or bearings, depending on how you want to think about their purposes at different places around the airframe. I’ve known for some time, after reading accounts and watching videos posted by other Sling builders, that getting smooth, friction-free action of the controls takes some care. Some folks use the expression – black magic.

These bushings are supposedly designed to be self-lubricating. That’s all well and good, but I know some builders have resorted to supplemental lubrication. I’m trying to avoid greasy, oily, dirt-collecting areas inside the cabin if I possibly can. I’m having some luck – so far.

Two key factors need careful attention – clearance and alignment. Having enough, but not too much clearance, makes alignment slightly less critical. Buttery smooth operation, without additional lubrication, seems to be achievable.

Opening up the U-shaped retainer areas in the floor brackets and the top caps, so that no squeezing of the captured bearings occurs, makes all the difference. I used a small sanding drum on my Dremel tool at a low RPM setting. Eventually, I was able to capture the bearings in the brackets without causing any pinching of the bearing around the pedal tubes.

Next, I used some fine sandpaper around a piece of dowel to relieve a small amount of material from the bearing’s inner surface, particularly at the edges of where a saw had cut them in half. They were once circular and then cut into halves. Some cuts were better than others, but it’s not unusual to find a slight overhang from one or both halves that narrows the bearing bore at the seams where the halves meet when they’re captured in the brackets. Just a slight amount of narrowing can cause binding.

After repeated cycles of fitting and filing, the result is smooth, friction-free operation.

The rudder pedal tubes came nicely coated with gray primer. Areas on the tubes were masked from paint where the bearings ride. Except – one of the masking areas is misplaced by 1cm. I’d read about this, and sure enough, when I measured I found the off-by-1cm error too. The Vesconite bearings are designed to ride directly on the steel. Relatively soft paint would likely gum up the bearing and defeat the self-lubricating properties. I put some protective masking tape around the tube and used a strip of fine sandpaper to precisely remove additional paint.

Last, but not least – I can see how the Sling-branded rudder pedals are going to look. I think it’s much cooler than the plain T-bars. It took almost 6 months after my quick-build kit was delivered to finally get all of the pedal parts. That was a full year after I’d placed my quick-build order, which included the option. All’s well that ends well.

The Sling-branded pedals are essentially the same as the ones for toe-brakes, except that they are mounted on the standard pedal tubes and the hand-brake configuration is used. The toe-brake option has different pedal tubes, different brackets on the floor and no hand-brake. I know because I got a bunch of those parts. I worked with the factory and eventually got all of the parts I actually needed for my pedals.

RH Flap – Ribs and Skin Assembly

With a now ample supply of 4,8mm rivets in both 15 and 10 mm lengths, finishing the lefthand flap could proceed. I did, however, have to make a decision about how to address hole misalignment involving the short ribs of the hing-rib subassemblies. The solution I chose was hole enlargement and larger 4mm rivets.

I’ve learned that perfect factory bends are required in order to get relaxed fit and freedom from structure twists and wags on the trailing edges of control surface skins. Knowing what to look for during inspection is essential. It had been months since I’d received the quick-build wing kit components and done my inspections. I was reasonably confident the skins were good, yet there was a huge sense of relief to see them actually fitting very nicely.

For the flaps and ailerons, it is common practice to initially rivet only the bottom surface of the skins to the ribs and brackets. The top surface and the row of rivets at the leading edge of the control surface remain free until they are fitted to and the trailing edges are perfectly aligned with the each other and the wing.

Elevator – Counter Balance Skins and Composite Tips

Finishing the elevator was accomplished over a period of about 3 weeks. The composite tips needed repeated sessions of fitting, filling, sanding and priming to achieve a satisfactory appearance. The interface between metal and fiberglass part was dramatically improved from what it would have been, had I left the fiberglass parts untouched.

The fiberglass parts were built up, especially around the leading edge, with Poly-Fiber SuperFil epoxy filler to reduce unsightly gaps. It takes a day for the filler to cure before wet sanding with 400 grit 3M paper, followed by Rust-Oleum wet-sandable automotive (rattle can) primer and the better part of another day for that to dry. Patience is the key

Once I was happy with the fit of the tips, it was time to match drill the parts against the holes in the counter balance skins. That was quickly and easily done by hand with my lithium-powered hand drill and a #30 bit. I’d reviewed numerous discussions about how others attached their tips and decided to simply follow the construction manual, using the ordinary 3,2 x 8 mm domed rivets that were supplied with the kit. Done and done.

The elevator tips took a while to complete, but I didn’t get carried away. All-in-all, the results look rather nice – me thinks.

Elevator – Structure Assembly and Covering

The elevator presented itself as the most daunting of the empennage sub-assembly phases. It’s a lengthy piece – over 8 ft (2.5M) long, with ample potential for unwieldiness, twisting and treachery. Yet, after several weeks of thoughtful and careful steps, the thing has come together nicely.

Since the main spar channel was assembled, many of the remaining elevator fabrication and assembly tasks were accomplished during the last half of April and the first week of May.

Simple wooden supports were clamped and strapped to my workbenches. The structure merely rested on three points. Alignment was assessed with a laser-level, before and during fitting, and again after assembly. It all seems to be spot on.

I’ve talked much about it before. Vertical orientation seems to allow the structure to be established and then remain naturally true and relaxed, throughout the entire sequence of tasks – at least for the Sling 2 kit. It’s easy to work from all sides, with a minimum of manipulation. Gravity feels like it’s been working more for me, rather than the dark forces of Twist and Distortion.

The main surfaces of the elevator are covered with two skins. Each skin covers both top and bottom. There is a critical bend at the trailing edge. If the TE bend is not perfect, you’ve got trouble. Out of the box, my elevator skins were good. I’ve had skins for other components that weren’t. Believe me – it is absolutely futile to attempt assembly with an improperly fabricated skin. I know what to look for [now]. Also, the EL skins are extremely delicate – especially the LH one, where there are only a few inches of highly vulnerable material between the top and bottom panels of the skin. Great care in handling is essential.

The leading edges of the elevator are formed by factory bends that wrap the skin around the main spar channel, to overlap and join with a single row of rivets. Some builders have used a roll-forming tool to “break” the edge of the overlapping (top) skin. I have the tool, but didn’t use it. In my unpracticed hands, the potential to make things worse, not better, presented itself as I experimented on a few (not enough) scraps of aluminum sheet. A man’s got to know his limitations. The LE bends of the factory fabrication were better on one skin than the other. As you might expect, the resulting LE overlap was better on one side than the other. The LE seams are out of sight. It’s fine. Done and done.

Closing Up The Rudder

With a good rudder skin, I was able to prepare, fit and secure the skin to the structure. Using techniques that I’ve learned from building similar components, the rudder went together smoothly. It’s a fine result.

There are still several tasks remaining to complete the rudder – fit the skin for the aero counter-balance, mount the internal counter-balance weight, run the strobe wire, fit the strobe to the rudder tip, and then finally, finishing the fiberglass and mounting of the rudder tip.

The great people at TAF USA worked hard to support me and promptly get me a rudder skin that I was satisfied with. The one that came with my empennage kit wasn’t bent properly on the trailing edge. It simply did not fit happily on the structure. A second skin was unfortunately damaged from moving about inside the crate during shipping. The third one was the charm.

It is crucial that the skins are fabricated perfectly. This plays a huge part in the resulting components being true. Precise fabrication is a critical element of the pre-punched and bent parts that allows them to assemble into a component that is uniform and free of twists, even without the use of jigs. The design of Sling aircraft absolutely depends upon the accurate fabrication of the parts. If yours aren’t right, work with TAF to get ones that are. Don’t mess around.

Closing Up the Horizontal Stabilizer

Completing the horizontal stabilizer went smoothly and turned out beautifully – ultimately. The fine folks at TAF, now Sling Aircraft, were super-supportive. Without going into great detail, I’d found that the leading edge bends of both HS skins were not on centerline – root to tip. They just wouldn’t fit properly – imposing significant stress and twisting of the structure. Sling Aircraft stepped up to quickly provide replacement HS skins and that saved the day.

I’ve been impressed with how precisely the holes in the skins match with the assembled structure. I’ve made a very conscious effort to take advantage of the kit precision. I try to get the skins initially positioned with very few clecos. I want to be able to move the skin slightly, until I can see that nearly all of the holes in the skin and structure are concentrically aligned. Starting the fitment process with fewer clecos makes that easier – possible. In practice, I’ve found that good overall initial skin position, relative to the underlying structure, allows the great majority of rivets to drop in – effortlessly. Once I’ve got the skin in place, I can further anchor things down with alternate clecos and hand-inserted rivets for the entire HS assembly.

I used a vertical HS working orientation that allowed me to evaluate skin fitment on top and bottom surfaces at the same time. Once the skin was in place, very few rivet holes needed attention – and then, only the slightest dressing with a chucking reamer in my lithium battery-powered drill. With a relaxed final fit of skin, rivets and structure – I’ve seen that when the rivets are pulled, nothing really moves. The permanently fastened skins are remarkably free of surface deviations. I’m quite pleased and anticipating that the contours of the final painted surfaces will be excellent.

HS Structure Assembly

The horizontal stabilizer assembly started by fitting together 2 sections of rear spar channel with the center section sandwiched by doubler plates – front and back. The result is over 8ft long. Appropriately sized cleco fasteners temporarily hold the parts together. A laser level helps to confirm that the channel is true – straight and free of twist.

The rear channel components were permanently fastened with a combination of 4.0 x 10mm and 3.2 x 8mm pulled rivets. Assembly continued with ribs joining with the front spar channel and clecos hold the front components as they are fitted and fastened in a similar fashion as the rear.

I’ve found that when 2 or 3 parts are sandwiched together with many rivets, it can be a little tricky to get a relaxed fit. Many overlapping holes must align precisely, in order for the rivet shanks to fit through all of the layers easily. I try to take whatever time is necessary to get the best concentric alignment of as many holes as possible, so that little or no reaming is necessary. The kit parts are punched very precisely and overlapping holes will likely line up, given the chance.

My assembly process starts with just a few clecos, while test fitting rivet shanks in many or most of the holes. Then I loosen and reset those few clecos until there is good natural alignment of as many holes as can reasonably be achieved. Eventually, a majority of the holes will line up perfectly, leaving only a very few that may need a little reaming to easily accept a rivet. Straight-shank chucking reamers seem to do a great job. Use the exactly right sizes. #30 and #20 are common.

The HS structure, without skins, is somewhat delicate. I’ve used a couple of stiffeners, from a Vans Aircraft workshop (skills practice) kit, clamped to the innermost HS ribs to provide support while the entire structure is riveted.

The HS structure is symmetrical. At some point, a decision must be made as to which side will be the top and the other side, therefore, the bottom. For my HS, continuity of how the rib flanges relate to the spar channels has turned out to be somewhat better on one side than the other. The side with the best potential for smooth skin support was chosen to be the top. I used a black permanent marker to make indications inside the front spar channel, where they can be seen during the various assembly phases.

With the HS top chosen, left and right HS panels become apparent. 2 Heyco 0.375in snap-bushings have been placed in the rear forming holes of the 2 innermost left-side ribs and anchored with some dabs of gray RTV. The nylon snap bushings are intended to protect the pitch trim servo wire (cable) as it passes through the ribs. I’ve elected to use nylon snap bushings instead of the rubber grommets supplied with the kit.