The composite engine cowling is one of the key contributors to the look of a Sling aircraft. I love the way Slings look. There are other models available from Sling Aircraft now, but it’s the Sling 2 that started it all. Now my Sling 2 is starting to look like it should.
There are a few pointers in the plans about the cowling, but mostly it’s up to the builder to figure out how to get the cowling to fit – properly and securely. I’ve complicated the process by introducing a custom spinner assembly. The factory-supplied spinner, while likely adequate, has to be built up from parts. The spinner I ordered through the propeller manufacturer – Whirlwind – is beautifully made from carbon fiber, comes finished and ready to paint. It’s a slightly different look, but still very, very Sling-like.
The 9.75″ Whirlwind spinner is notably different from the factory unit. The backing plate flange faces backward instead of forward. This will mean that the cowl will have to fit somewhat farther back on the fuselage than it otherwise would for the standard spinner.
To get the cowl aligned and with the proper setback, I’ve made a fixture that mounts to the prop hub extension. The hub extension is a standard part of the kit. Repeated fitting and trimming will hopefully arrive at a near perfect position and alignment. More to come on that.
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.
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.
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.