Category Archives: Wings

Wingtips – More Rework

After a gallant effort on the original RH wingtip, I ordered and received a replacement RH wingtip. The new part still had too much length and thickness at the trailing edge, but the workmanship of the layup was much improved and it had noticeably better overall shape. It has still taken lots of work to get the new RH tip to fit. But, it’s coming along and it’s going to work out well.

The LH tip is proving to be huge headache. Despite extensive rework, it’s now clear that the LH wingtip isn’t close enough to the proper cross section to fit and it would take extraordinary effort to get it in the ballpark.

Just like the RH wingtip, the length of the LH part at trailing edge is too long and the trailing edge profile (taper) is too thick to fit the trailing edge wing skin. But the kicker is the overall height. It’s just too narrow – top to bottom. It’s almost 2 cm short, at the widest point.

Trying to stretch the height of the LH tip requires too much force. The LH wingtip has no imbedded reinforcement strip on the bottom edge. The top reinforcement is out of position. Without proper stiffness, the tendency for waviness between the rivet holes would be profound.

To continue with the original LH wingtip, I’d have to section the part and almost re-manufacture it. I’ve decided that it’s much too much time and effort, although I have seen one other builder go to such lengths and spent over 130 hours on just trying to get his wingtips to fit. I’m at more than 40 hours on wingtips and I’m going to cut my losses and get a better baseline part.

I went ahead and negotiated a new LH wingtip from the factory, as I did for the RH side. They’ve kindly agreed to send me one. Kudos to Sling Aircraft for standing by their product. Stay tuned.

Wingtips – Rework in Progress

The fiberglass composite wingtips are needing a lot of rework. I don’t really believe it was intended be this way. What’s a simple amateur home-builder supposed to think?

The biggest area needing attention seems to be where the inside trailing edge of the wingtip is supposed to fit into the narrow skin at the tip of the wing. Several issues are being dealt with. Overall length is too long – by about 2cm (almost 1/2 inch). And, the fiberglass structure doesn’t match the contour of the wing profile near the TE. The taper of the fiberglass needs to become quite narrow. Plus – it’s all got to look pleasing and with and both wingtips ending up to be nicely matched – both aesthetically and aerodynamically.

I’ve decided that some sort of struts are needed to help the wingtips fit the contour of the wing. The kit instructions indicate that the wingtips are just match drilled and riveted. But, just letting the rivets pull the fiberglass shape seems like it’s going to lead to unwanted waviness from dips between the rivets. I fabricated simple struts to take some of the tension load off of the rivets. We’ll see how that goes.

I’m making progress, little by little – but it’s slow. Rework involves epoxy and with winter temperatures, each cycle needs a full 24 hours or more to cure. There’s plenty of sanding and shaping to do. Check fit. Then repeat.

I expect building an airplane to take some work, so I’m not going to complain too loudly. I fill terrifically privileged to be doing this project.

Wingtip Lights

Making a commitment to a specific model of wingtip light turned out to be quite the adventure. I did it. I think I’ll be happy with the choice. Time will tell.

Early on, I considered FAR ยง 91.209 Aircraft Lights and advisory circular AC 20-30B. That led to:

a. Advisory Circular AC 20-74, Aircraft Position Lights and Anti-collision Light Measurements.
b. Advisory Circular AC 43.13-2A, Acceptable Methods, Techniques, and Practices, Aircraft Alterations.
c. Technical Standard Order ('.ISO) C30b, Aircraft Position Lights.

I began to wonder how much time and effort I would have to put in to end up with demonstrably legal and adequate day and night, VFR and IFR lighting on my Sling 2 experimental. Man! This could get involved!

Just like my Warrior, I expect to have a red beacon on top of my VS and 3-in-1 lights on the wingtips. What and how much do I have to do – and prove – to achieve the same thing on my Sling 2? I asked questions on several builder groups and forums. Would 3-in-1 lights on the Sling 2 wingtips be ok? Will they be visible? Will I need a white tail light? Are non-TSO’d lights even legal at night? On and on.

I eventually decided that I was overthinking the whole business. I’d chosen the red LED tail strobe months ago. That’s a done deal. There’s a place on the wingtips for teardrop-shaped lights – and dammit! – they’re going to be cool-looking 3-in-1 LEDs. There!

It simply came down to which light? I’ve got Aveo for the tail. For sync compatibility, I’ve found I have to stick with Aveo on the wings too. They have a TSO’d product. They also have an experimental version of that same light. Even that experimental one is pricey. Aircraft Spruce had a sale on Aveo PowerBurst NG DayLite, w/white base – 300 bucks-a-pair. That’s more palatable than $780/pair for the nearly-almost-TSO’d experimental ones.

So there you have it – the tipping point! Price! Decision made. Hopefully the DAR will be happy when the time comes. I like ’em.

Another thing that gave me pause was how to mount the lights to the wingtips. The KAI no longer matches the composite parts that came with the kit and it hasn’t been updated in a long while. There is a callout for a plate with rivnuts to be mounted by some [unstated] means to the inside of the wingtip, under the site where the light will sit. That area is quite rough and I didn’t see how I was going to make a plate to fit and to get it attached and aligned.

Fortunately, I got some inspiration from a post on the FB Sling Builders group. Epiphany! Rivnuts and epoxy-glue is the way to go. Brilliant! The lights have mounting holes perfectly suited for #6-32 screws. M4 is too big and M3 is too small. Here we go mixing standards again – but what can you do?

I used the rubber base-gaskets provided with the light as a template to mark where holes go in the wingtips. I did some careful drilling and enlarged the hole for the wire bundle with a step-drill.

I needed the screws to be square to the mounting surface. I put each #6-32 x 1.25″ screw into its hole and threaded on a rivnut. I observed some amount of gap under the edges of the rivnut and used a wood-carving bit in the Dremel Tool to machine a small, flat surface for each rivnut to seat against, inside the wingtip. I cleaned up the areas with solvent on a paper towel. I positioned the rivnuts, wide flange to the mounting surface, and secured them gently with their screws. Then I carefully applied [original] JB Weld epoxy to the fiberglass and built up a mound around the rivnut, just below where the screw emerges.

Once the epoxy was cured I had perfectly aligned and ruggedly attached blind anchors for the the lights. This worked so nicely and was so easy to do I can hardly stand it! The rivnuts are not “set” as they normally would be. They’re just glued in place. For this application, the light mounting screws will never be tightened to the point where the rivnuts might compress.

Composite Wingtips – Take 1

It didn’t take long before I realized that I’m in for a battle. The fiberglass wingtips, as supplied with the kit, simply don’t come close to fitting the wing. They’re obviously hand-made parts and are nowhere near identical. Frankly, I expected better. But, they are what they are.

I don’t have much hope that if I push the factory for new parts, I’ll get anything [much] better. I’ll count myself fortunate if I can get satisfactory results with less than the 130 hours another Sling 2 builder has put into his wingtips. Jeez – that’s a lot of time!

Right off the bat – the overall length is far too long to fit into the end of the wing panel. The airfoil shape cross-section is decidedly too flat. The up-sweeping trailing edge scallops are oddly different shapes. The lack of alignment at the point where the tapered wing skin is supposed to accept the trailing edge of the wingtip is unfortunately grotesque. Cutting and reforming will be necessary. Ultimately, the wingtips will be permanently mounted with 3,2mm multi-grip blind rivets. I haven’t settled my mind as to how I will mount the wingtip lights.

I decided to make a simple wing-shaped jig from a 2 x 4 foot section plywood. This jig is much less elaborate than others I’ve seen, but it will hopefully result in a useful tool and a reasonably consistent reference I can use to evaluate and correct the various eccentricities of these fiberglass parts.

I’ve been fortunate to be able to see what other builders have encountered and done with their wingtips, and so, I’ll share my adventures too. For the Sling 2 builders, we all seem to be – more or less – in the same boat.

RH Wing – Fuel Tank and Inspection Covers

Practice makes perfect they say. Well, maybe not perfect. But the going seems a little easier when you’ve been there before. So it was for mounting the second fuel tank. It actually fit slightly better than the other one. I knew what to do, and on it went without issue.

I did the same basic steps as I did for the first tank. I felt confident, and the work went quickly. I had to carefully dress a number of the overlapping holes in the tank and spar with the #20 chucking reamer in the lithium drill. I knew what to expect and there were no surprises. I also had to touch the holes in spar web to align with the outermost Z-bracket so I could get the AN3 bolts through and threaded into the anchor nuts. Once again, I used a length of cord to pull on the Z-bracket while the tank was being fitted for the final time. That worked like a charm.

There are several stainless steel rivets, top and bottom of the spar near the root. Those are treated with fuel tank sealant for corrosion protection. The rest of the rivets are aluminum multi-grip. Riveting goes pretty fast when the pieces are fitted in place.

Having both fuel tanks mounted is a big milestone and yielded a rather wonderful sense of satisfaction that was sweetened with a measure of relief. Success! Good result. Yay!

The M4 rivnuts for the inspection panels went in without a hitch. I had to enlarge the holes in the bottom wing skin to #A – the perfect size (I’ve found) to accept the rivnut prior to setting them with the drill-mounted tool I use. Screw holes in each of the cover panels had to be enlarged for close clearance of M4 stainless steel button-head hex screws.

LH Wing – Inspection Covers

Compared to mounting a fuel tank, enlarging holes and setting M4 rivnuts seemed easy and definitely less stressful. Still, it was no time to be careless. The wing skin is easy to damage.

It took several steps to get the factory punched holes enlarged to letter number #A diameter, a perfect size to accept M4 rivnuts. I double checked my references and reviewed the process I would use. A series of increasingly larger straight-flute chucking reamers in my trusty lithium-powered drill did the trick.

All of the holes in the wing skin and the inspection covers were in the correct places and the alignments matched perfectly. The drill mounted rivet setting tool worked flawlessly. All rivets are consistent and tight. The screw clearance holes in the covers are ideal. Everything is fits together nicely.

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.