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.

LH Landing Light – Lens Retainer Strips

Several months ago, I’d taken a look at the landing/taxi light lenses and what it might take to mount them nicely. The LH wing panel was sitting horizontally on a workbench at the time.

About the first thing I noticed was that the mounting holes around the perimeter of the lens did not all correspond to the holes in the wing where the lights and the lens go. The holes lined up (pretty well) with the lens on the outside of the skin, but not when I put the lens in the opening, behind the skin – where it really belongs.

The other thing wasn’t really so much something I noticed, but rather, I realized that I needed to find a better solution for mounting the lens than the factory provided for – sheet metal screws through the skin and into the plexiglass lens. That’s just not going to cut it.

Now I had another classic opportunity for inspirational procrastination. I put the time to good use. It took weeks, but once again – procrastination paid off! The idea of retainer strips with anchor nuts came early. I also found that I’d likely use #4-40 hardware, because metric MK-1000 nut plates are absurdly expensive and challenging to source in the US. I hate mixing hardware standards on this bird, but that’s just how it goes. The blind anchor nuts on retainer strips behind the lens stuck in my head as an obvious solution.

What was not obvious to me at the time, was how to hold the retainers in place so that the lens could be fitted and fastened with the little screws. I made a prototype with a hand-cut strip of 0.020 aluminum and held the anchor nuts with AN426AD-3-3 solid flush rivets. The strip was flimsy and I attempted to hold it against the backside of the lens with – if you can believe it – sewing thread. Once I got the screws started, I’d pull the thread out. I was too unwieldy.

Weeks went by. Then it hit me – the same basic idea, but with 0.5 x 0.025 stainless steel strips, held to the plexiglass lens with little #4 CSK screws and ny-lock locknuts. I had to make new holes in the bottom half of the lens to match the wing, made six retainer strips and mounted them to the lens. Now I have a lens that is easily installable and removable. I’ve come to believe that this is what the factory does for the Sling TSi and I might have seen it if I’d looked at the TSi construction manual. Oh well. I got there. I’ll repeat the fitting and fabrication process for the RH lens assembly.

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.

Airplane Noises

As far as I can tell, once an experimental aircraft build project gets to a certain point, there’s a right-of-passage that must be embraced — sitting in the thing. Today is the day.

I made a work platform from wood. It fits inside the fuselage on either side. I can sit on it, if and as needed.

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.

Rework Outer Seatbelt Brackets

Any work on and in the center fuselage is part of a big step forward. In this case, the work is actually rework, owing to one of several shortcomings introduced during the factory quick-build of my fuselage – which evidently was a bit hasty. TAF admittedly rushed to ship in the last hours before a three week holiday shutdown. Let’s just say there were a few shortcuts taken.

There was a general class of shortcut that I’ve found several examples of – riveting places and parts that shouldn’t have been until other steps were completed. The most vexing examples were the seatbelt anchor brackets being mounted without the AN5-5A bolts and washers in place. I’d noticed this very early on after delivery and have been troubled ever since about how I’d be able to get things as they should be.

As I mentioned in my last post, I finally overcame a mindset that I’d have to drill out and re-rivet the center brackets to place the bolts. With courage and great care I was able to flex my way to a happy result. The outer brackets demanded the drill and re-rivet approach. That has proved very doable, especially since I’ve got the fuselage solidly down on the floor where I can work on it without concern about toppling. Yay again!

I’ve gained some valuable experience with drilling out the big 4,8mm rivets from my rework of the aileron and flap hinge bracket sub-assemblies. With a #12 bit in my lithium battery-powered drill I was able to get the outer brackets off cleanly. And when the time comes, I’ll be able to rivet them back on – with the bolts in place. Glad I don’t have to worry about this anymore.

Fuselage Sitting on Main Landing Gear

It may not seem like a big deal, but having the fuselage safely off of the workbench, where it had been (somewhat precariously) perched, is a great relief and a meaningful milestone. It’s now in a position where I can work inside the center fuselage – comfortably and confidently.

The fuselage spent months sitting on the wooden structure that held it inside the sea shipping container. Then it sat for weeks on a workbench and up on some blocks that I was constantly concerned might cause great damage if the structure should somehow roll forward or backward on those blocks.

With the main landing gear in place and some help from my wife and also from my friend Charlie, the three of us were able to remove another wooden shipping structure at the tail, clear the blocks and the workbench from underneath the center fuselage and set the whole business down on the floor. The tail is supported by a padded sawhorse. Joy! Joy!

I’ve been waiting for months to work inside the center fuselage. I did the first bit of work right away – installing the center seatbelt anchor bolts into the brackets where they should have been placed by the factory – before the brackets were riveted to the CF structure. It’s clearly called out in the construction manual, but the step was forgotten.

At first, I thought I would have to unrivet both center brackets in order to get the bolts into position. But, I eventually became hopeful that I could flex the brackets just far enough to slip the bolts in. Yes! It worked. Good thing too, because it really would have been nearly impossible for me to re-rivet the brackets with conflicting structure in the way. I’m so glad I went ahead and attempted positioning the bolts as I did. Done!

I have the same factory-forgot-the-bolts issue with the outer anchor brackets, but at least there’s ample space to drill out and re-rivet. There’s no way I can flex those outer brackets enough to get the bolts where they need to be. The structure in that area is far too stiff.