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.
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.
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.
Assembly of the adjustable seats has been straightforward. They’re made up of 2 hinged panels that have a simple channel structure, sandwiched with identical skins – top and bottom. Inside the seat base is spring-loaded lever and cable mechanism for the slide locking pins. I opted to adapt some clevis pins instead of using the kit-supplied (large) solid rivets to assemble the linkages. The rivets proved difficult to deal with. It took a few days of pondering, but I eventually realized that custom fabrication of clevis pins were the way for me to go.
The only metal preparation I did was deburring and scuffing with a Scotch-Brite pad. I may or may not paint the seats as they’ll be almost entirely covered by the upholstery. The structures and panels fit perfectly and went together quickly. Sling 2 seat design has apparently changed over the years. The recently manufactured parts I had didn’t exactly match the construction manual, but understanding and dealing the differences was not difficult.
There are now at least 2 ways that the piano hinge can be mounted between the seat base and back, so that it can folded forward to access the luggage compartment area. The deciding factor seems to be how far beyond perpendicular to the seat base the seat back will naturally recline. The construction manual shows the hinge on the surfaces, riveted across step transitions where the side channels overlap the skins. The hinge, mounted to the back and bottom edges of the seat panels just seems more appropriate and allows for about 21 degrees backward and no restriction (until the panels meet) in the forward folding direction. That’s perfect. Seat recline angle is set by side-straps anchored to brackets at the edges of the seat panels.
The seats slide on rails mounted to the center fuselage. Clearances are pretty close, but appear to be perfectly aligned. Finding that helps to confirm that the center fuselage is built straight and square. Oh let me tell you that’s good news!
Finally — the fuselage has been freed from the heavy wooden stand that secured it within the sea shipping container. My small but mighty workforce – buddy Charlie, my wife Mary Ann and I – managed to lift the fuselage off of the stand and place it on blocks, supported by a workbench. Hooray!
In hindsight, I probably should have cut down the legs of the stand at the tail – before dealing with the front. The blocks keep the front and rear level and about the same height as it had been. It’s a little precarious, but it’s only supposed to be for a short time – until I get the main landing gear attached.
Getting the landing gear into position wasn’t too difficult, even though the landing gear is rather heavy. I was able to use a hydraulic floor jack to lift it into place and insert a couple of through-bolts – all by myself.
There’s a tip in the construction manual about the factory using “pointed” bolts to lead the way for the the actual mounting bolts. I ground down the threads to make rounded and somewhat tapered tips on two extra bolts I happened to have. This made it relatively easily to just push the bolts through the steel brackets and the landing gear. That’s very encouraging and I’m hopeful that inserting all 4 mounting bolts will be doable.
Unfortunately, there is more gap than I think there should be between the landing gear and the faces of the steel mounting point brackets where four 8 x 175mm through-bolts will pull everything together. Measuring the thickness of a stack of scrap aluminum sheet inserted into the gap, I was able to determine that it’s 2.5 to 3.0mm.
I checked with the factory and they say the gap should be 0.0 to 0.5mm. This excess-gap issue is apparently not uncommon, yet somehow remains undocumented. They offered to make me custom laser-cut shims. I appreciate that. Hopefully it won’t take too long to get them. Meanwhile, I’m reluctant to do much while the fuselage is perched on the blocks. I wouldn’t be happy if it were to roll forward or backward. Crunch!
Not having the fuselage sitting solidly on the landing gear is going to hold me back from working in the center-fuselage area to mount controls and linkages. I don’t really have the inclination and wherewithal to build a fuselage “rotisserie” like some aircraft builders do. And, I don’t have ready access to enough warm bodies to muscle the fuselage around the shop, putting it on it’s side, et cetera. At this stage, I still have some other things to work on.
As I near the end of dealing with fitting skins to structure, my confidence was pretty high that this would go well for the flaps (and eventually the ailerons). I’ve learned important lessons about how to inspect skins for proper fabrication – especially bends.
As I discovered from the building the empennage, lengthwise bends (folds) of the skins must be very close to perfect or else entire structure will be pulled out of true alignment when preparing or attempting to close up the final assembly.
There has proven to be considerable lead time in the process of securing replacement parts and the earlier a problem is discovered, the better. Almost immediately after the main QB kit was delivered in February, I looked over the flap and aileron skins – very carefully – and determined that they’d likely be acceptable.
Outdoors metal preparation with Alumiprep 33, Alodine 1201 and then rattle-can primer is much more convenient and pleasant with the warm summer weather. I opted to use NAPA 7220 gray self-etching primer, as none of the surfaces would be exposed. I had the stuff on-hand, but find that I don’t like it as well as the Rust-Oleum product, if for no other reason than the performance of the spray can. The any-angle can from Rust-Oleum is superior, even though I paid considerably more for the 7220 primer. (As I’ve mentioned before, if I do another build, I may well forgo alodine and primer altogether. With my budget and facilities it has been a huge time sink and perhaps not worth the effort. Even at my tender young age, I’ll be pushing up daisies before corrosion would be an issue with an untreated airframe.)
Due to a shortcoming with the listed shipping quantity in the wing kit packing list (KPL), I received only enough 4.8 x 15mm rivets to assemble one flap. I also found that one-size-fits-all — didn’t. It turns out that the overall thickness of one parts stack-up was very slightly less than the recommended grip length of the 15mm rivets. Even though there was no mention of this issue in the assembly instructions, it became obvious that a 10mm length would be better.
I ordered more rivets – both 15mm and 10mm lengths – twice. Once from TAF USA and then from a supplier of Gesipa rivets in UK. TAF sent a big batch of 10 and 15mm rivets to me overnight. Bravo! Great support effort! Thank you!! But, the rivets were not to my liking. They are some alternate brand, different design, slightly larger diameter (didn’t fit) and not nearly as well finished as the Gesipa product. I ordered the real deal, but it took 2 weeks to get them in-hand.
Hooray – the fuel tanks seem to be sealed! That’s fantastic because if they weren’t, just about any rework scenario would be ugly. Thankfully, I don’t have to go there.
Having an active build community that shares experiences is so incredibly valuable. I think it’s absolute essential for the growth and long-term success of any kit manufacturer. Fortunately for home-builders of Sling Aircraft, there’s a steadily increasing number of builders and contributions to the knowledge base. That’s where I found the details of employing a water-manometer for safely and confidently testing the integrity of the fuel tanks. Thank you fellow Sling 2 builder — Pascal Latten: Sling2 Fuel Tank Leak Test
My test apparatus was not anywhere near as well-done as Pascal’s, but it worked – once I got the apparatus itself to not leak. At first, I was testing the apparatus. It failed miserably. Once I eliminated all of the leaks caused by clamped hose connections, I was eventually able to get to the point of testing the tanks themselves.
Over a test period of several days, I logged tank pressure vs. temperature and local barometric pressure readings. In theory, you can then compute a leak-rate value that can be compared to an established value (found in a reference table) that is acceptable for whatever you’re wanting to keep in your tank. In this case it’s gasoline.
I ran into a bit of a snag with my barometric pressure readings. After being astounded how little the barometric pressure readings were changing over the test period, I discovered that my home weather center was more for decoration than practical use. The old – thump on the glass trick – revealed that the mechanical pressure gauge was sticky. That pretty much trashed my data. But, all was not lost.
I’d casually kept eye on local barometric pressure, through local weather reports, and it really wasn’t changing a whole lot. It stayed within a rather narrow range. Temperature went up and down – and so did the tank pressure readings – quite a lot! It was somewhat frightening. Do I have I leak, I wondered? But throughout the tests, and ultimately, the ending tank pressure matched starting pressure at the same temperature. I felt good about that – perhaps even better than I might have felt about the subjectively assessing the relative size of an inflated balloon or nitrile glove. In the end, I didn’t bother to compute a leak value for either tank, but I have confidence that it’s all good to go!
With help from my buddy Charlie, I was able to get the left wing out of the stand and onto the workbench. It’s the first time I’ve had a chance to take a good look at the factory quick-build wing panel since it was unloaded from the shipping container back in February.
Wing jigs are not normally supplied with the QB wings, as their attachment points are (partially) occupied by late-stage structural attachments – especially for the outboard (wingtip) area. Nevertheless, I requested (and received) a set of jigs from TAF USA and was able to make effective use of them.
All-in-all, the wing looks pretty good. That’s a big relief. Wires for taxi and landing lights, pitot tube electrical and eventual 3-in-1 wingtip lights are there. Vinyl tubing for pitot airspeed and AOA sensor are in place as well.
Next, I’ll set up for pressure testing the fuel tanks with a homemade water manometer. The integrity of the factory-assembled fuel tanks must be assured before more time passes. What happens if there’s an issue? That’s a complete unknown. I will keep my fingers crossed and hope for the best. Positive thoughts!
Never underestimate the amount of procrastination required to get something done.
As usual, parts preparation takes most of the time. The fiberglass tip, as supplied in the kit, was a bit rough. There were quite a few voids and other imperfections in the layup. The trailing edge was too fat to fit nicely with the skin. Cutting and re-gluing with a bit of glass cloth and West 105 epoxy resolved that. The contour of the tip leading edge needed building up and shaping – requiring several passes. Epoxy takes hours to cure, so each step takes a day. Epoxy filler and wet-sandable primer attends similar time-sinking characteristics. Along the way, test fitting and match drilling of the mounting (rivet) holes was accomplished.
I didn’t really like the way the construction manual prescribed M4 rivnuts for the aluminum doubler that serves as the mounting base for the strobe. My concern is that rivnut installation might crush the fiberglass. I opted instead to make a new part that uses MK1000-06 anchor nuts and is riveted in place with AN426-3 solid flush rivets. Having the patience to eventually arrive at the decision to do this and then actually fabricating the mounting plate demanded all of the procrastination I could muster.
Copious foot-dragging precipitated the decisions about wiring and method of tip attachment. For some reason, I just didn’t want to shorten the (rather stiff) wire bundle of the Aveo Mini Max LED beacon. At the same time, I didn’t want the splice to be at or near the point where the wire exits through the bushing in the rib. A loop seemed the answer. And so it was. Final fitting of the tip to the rudder and pulling of the 3,2 x 8 mm rivets went well. I’d long struggled with the temptation of making the tip removable, à la Pascal Latten, by installing dozens of anchor plates, flush rivets and #4-40 screws, but my steadfast procrastination eventually paid off and the scales tipped in favor of just pulling rivets and being done with it.
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.