Tag Archives: fit

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.

CF – Control Tubes, Brackets and Bearings

Mounting the control tubes in the center fuselage is very much like the process for the rudder pedals. Once again, clearance and alignment prove to be the main variables.

A couple of important discoveries made a big difference. The first discovery was that the elevator and flap torque tubes are not perfectly straight. They’re close, but they are not absolutely perfect. With an ever so slight bow and four bearing points, over a meter of distance, the centers of the bearing bores present different impacts, depending on rotational position of the tube. The variations are tiny, but if the bearing clearances are too tight – you get a surprising amount of binding.

The other discovery was that not all of the bearings have identical inner diameter. It’s a long story, but I ended up with a few extras and happened to find one that has good bit looser fit on the elevator torque tube.

The bearing with the looser fit proved to be ideal, when I put it at the LH end of the elevator torque tube. Careful dressing of the fuselage brackets and the retainer brackets with the Dremel sanding drum resulted in perfect fits everywhere – and fantastically smooth action.

Careful observation of how the bores of individual bearings center around the tube allowed me to ever slightly shift those centers in good directions as I opened the U-shaped bearing capture areas to eliminate pinching of the bearings around the tube. Each custom fit retainer bracket and bearing is marked for a specific location and orientation. I’m confidently optimistic that there will be just enough clearance for friction free operation after the retainers are permanently riveted.

At the moment, the flap torque tube movement is good, but it isn’t quite a limber as the elevator tube. It’s not of great concern. The flaps are driven by a linear actuator and won’t be anything I notice in the hand controls, which will be effortlessly smooth.

The two main control stick tubes are each supported by a pair of bearings along the roll axis. The front bearing is captured by brackets with U-shaped retainers, top and bottom, that are rigidly riveted to the structure at the front of the main wing spar carry-through.

The upper retainer should have been riveted before the carry-through structure was mated with the surrounding center fuselage members. This is another instance of the factory failing to comply with documented assembly details.

I can’t really get good enough access for confident pulling of 4,0 x 10mm rivets, so I’m going to use screws and elastic stop-nuts for the uppermost holes and (probably) use rivets for the balance of bracket mounting holes. I’ve got to end up with good alignment and enough clearance so that the bearings don’t bind when the brackets get pulled down. I’m certain that the retaining brackets are going to shift slightly from their un-riveted positions. I might use screws after all. That would make it reasonably straightforward to take it all apart and re-tweak the final fit until I get it right. Absolutely no binding will be tolerated. Smooth, smooth, perfectly smooth – that’s how it will be. Period.

LH Aileron – Finishing Up

The LH aileron had been languishing under the bed in my guest bedroom with the skin cleco’ed in place. It just needed the outboard rib with the reworked hinge brackets.

Riveting the skin on the bottom went well. Now it’s back under the bed again, with and in the same state as its buddies. Final riveting of ailerons and flaps will wait until I’m in the mood to fit them on the wing assemblies. That might even wait until I have the wings joined to the fuselage. We’ll see. I think that decision depends on what I decide to do about painting. Fly first or paint first? Yes – another excellent opportunity for inspirational procrastination.

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.

Main Landing Gear Assembled

I thought it would be very challenging, but the main landing gear bolted to the fuselage without much of a fight. I’d spent days wondering how it could be done. As it turned out, I found I was able to do it single-handedly. Good preparation was a key factor.

Initial fitting revealed several things that needed to be addressed. First, there was space between the composite landing gear and the heavy steel brackets where it attaches to the fuselage with M10 Class 12.9 bolts – two on each side. I confirmed with the factory what I quickly suspected – too much gap. I found out that the gap, if any, must be 0.5mm or less. I was seeing just under 3mm. The factory offered to make custom laser-cut stainless steel shims. I measured and provided details about the LH and RH gap distances and received the shims in just a few weeks. Beautiful.

Next, the long M10 bolts needed to fit easily though concentric holes in the sheet metal channel and heavy steel brackets in the center fuselage. The brackets were perfect, but holes in the channel were a little bit tight. The slightest attention with a tapered reamer made it all just right. It was good to realize that this should be done before struggling to fit the bolts through the landing gear without sufficient clearance.

With a pair of tapered pins I’d made from spare 10mm bolts, inserted front to back, I was able to set and then hold the landing gear in position and slip in the actual mounting bolts. The bolts went in from the back, through the fuselage, shims and other mounting hardware and pushed the pins out the front. The elastic stop nuts were tightened to 25 NM, as specified in the Sling 2 LSA Maintenance Manual. (There’s treasure trove of important Sling 2 assembly procedures and details in that manual.)

All Sling Aircraft models are designed and manufactured in South Africa. Measurements are specified in millimeters and most of the fasteners are metric. There is, however, key structural hardware that is AN aircraft hardware – much more typical for aircraft designed, built, flown and maintained in USA. Detailed fastener documentation for my aircraft will be very important to anyone performing maintenance and they will need to be alerted about this beforehand. Spare fasteners may not readily at hand at a typical shop. I expect to keep some spare hardware in the aircraft for repairs.

Ailerons – Fab, Rework and Pre-Assembly

Aileron assembly has been delayed by ignorance and procrastination. It’s amazing how long it took me to decide to lever $20 out of my pocket for a tool. There’s an anchor nut that gets attached to a rib with a couple of stainless steel rivets that have a 120 degree countersink. I was reluctant to spring for a 120 degree, #40 pilot cutter. This left me pondering various alternative ways I might proceed to attach the anchor nuts. The door was left open because the construction manual doesn’t say anything about it. But, I did have reference examples – other builder’s and identical anchor nuts mounted in my quick-build fuselage. I finally ended up getting the stupid pilot cutter and then mounted the anchor nuts as I knew they should be from the very beginning.

Another self-inflicted setback has been in play. Sometime earlier, I’d riveted one of the aileron hinge bracket and rib sub-assemblies together. Unfortunately, something I’d noticed, but dismissed during initial fitting, had to be corrected. The bolt holes on the inner and outer aileron hinge brackets were not in alignment. To compound the problem, I reasoned that it would probably be ok to ream the bolt holes a little – make them oblong – and somehow that work out ok. Wrong! The result was better alignment, but at the cost of precision (proper) fit.

Sloppy fit for the outer aileron hinge just isn’t going to cut it. What could I do? Eventually, I did what I I should have done in the first place – ask the factory for guidance. I sent an email and got an overnight response directly from Mike Blyth – designer of all Sling Aircraft models. The outer bracket just needs to be bent a bit more. So simple! That absolutely did not occur to me. Sadly, I’d ruined (by reaming) the inner and outer brackets for one aileron and needed new ones. TAF USA rushed me replacements. Fantastic service!

With new brackets in hand, I slightly increased the bends on the outer brackets for both ailerons, removed the old brackets from one of the ribs and riveted all of the sub-assemblies together. Beautiful! I can sleep again. No more worries thinking about how I would try to rationalize wobbly ailerons to myself, the DAR, my technical counselors and everyone else.

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 – 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.