One things leads to another. A question about the factory-installed rudder cables got me started down the road of test-fitting the vertical stabilizer. I was very pleased to find that it was easy to do and the fit appears to be excellent.
I hadn’t really expected to do this step just now. But, in order to evaluate much about the rudder cables, the entire control mechanism for the rudder and steerable nose-wheel needs to in place and adjusted. I learned this during a customer support exchange with Sling Aircraft’s Jean d’Assonville. I’d called him because I was concerned that the rudder cables may have been installed improperly during factory the quick-build of my fuselage. Jean assured me that it was very unlikely that the cables were wrong.
I was wondering because the KAI talks about one cable being slightly longer than the other. Somehow they seemed to be the other way around. Jean said that the only way to properly evaluate the setup would be to assemble everything. It only takes 15 minutes, he tells me! LOL. But, he really meant it!
It’s going to take me hours and hours, over days and days, to get the entire rudder control mechanism in place. That’s not only because I’m slow and plodding, but also because I’m not ready to install the nose-wheel yet. It’ll all just have to wait until I get the avionics rack, LRU’s and harness in place while the wheels are off and the fuselage is low and as easily accessible as it can be.
So then. I still don’t know for certain that the rudder cables are installed properly. But, I did get inspired to get the VS out from storage in the house and get it fitted on the fuselage. Technically, that’s progress! The nice fit between the fuselage and vertical stabilizer is satisfying too.
Trial fitting revealed that clearance between the arms on the pedal tubes and the hands of the travel-stops, bolted to each opposite arm, was nil. I could actually hear a squeak sound where there was contact, as I worked the pedals back and forth with my hands.
I pondered what to do about it, using my tried and true procrastination skills. As I have no proper equipment for clamping and bending anything heavier than light sheet metal – removing a bit of material from the the lower hand of each travel-stop came to my mind as the answer.
The pedal tubes were removed from the center-fuselage and the travel-stops unbolted from their respective arms. The Scotch-Brite wheel on my bench grinder did a fine job and I was easily able to remove about 1.5mm of material from the lower hands of the travel-stops to open up a satisfactory clearance.
The parts were reassembled and the pedal tubes reinstalled on the floor of the center fuselage. This time around, I installed the upper retainer brackets differently than I had for the trial fit. By now, I’ve noticed what several other builders and the factory are doing. For each bearing, there are a pair of top retainers. Rather than both retainers being together on the same side of the lower bracket, I’ve put one on either side, with space between. That seems to be the way it’s supposed to be. I like it.
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.
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.
Building an experimental aircraft from a kit is more than just a paint by numbers affair. And, with so much information available online – finding several ways to accomplish a task is not unusual, especially if you look. As it happens, I spend hours and hours searching for and looking at how others are doing things to build the same model, as well as similar types – or just general whys and hows of related skills or techniques.
Sometimes I come across an idea that just seems better than what I see in the kit construction manual. (20 years of aircraft ownership and maintenance have shown me that [most] aircraft designers and manufacturers do not actually walk on water.) My kit instructions describe a process that may possibly go beyond what the servo manufacturer – Ray Allen Company – anticipated as an acceptable way to mount their T2-7A servo.
My kit instructions call for enlarging 4 holes on the servo mounting rails (of the composite housing) from their original 0.125 (1/8) inch size to 0.2340 (#A) inch, and then setting an M4 steel rivnut into each hole. The documentation for the servo indicates that the holes may be enlarged to approximately 0.1440 (#27) inch, just enough to clear a #6-32 screw. The M4 rivnut approach seems like it risks the servo. Apparently other builders have had similar concerns and pursued alternatives.
For the Sling 2, the pitch trim servo sits flat on a tray that is riveted to the structure, inside the elevator. 4 screws pass through holes in the bottom elevator skin, the tray and the servo rails – and then must thread into something. I expect that #6 washers and elastic stop-nuts would be just fine. But, they may be just a bit fiddly to work with under the circumstances. Space inside the elevator is tight.
A fellow Sling 2 builder came up with what I thought was a great way to go – fabricate a pair of 0.0625 (1/16) inch thick aluminum straps with 6-32 (K2000-06) nut-plates attached with solid flush (AN426) rivets. The straps not only accept the screws, they also capture the entire length of the mounting rails on either side of the servo. When I first saw it, the solution immediately struck me as simple and solid.
Elevator assembly is straightforward, but you have to carefully study and understand several details, to avoid pitfalls.
The current version of the Sling 2 Empennage Construction Manual leaves much to the imagination of the builder. Build sequence details are very important. The written steps are basically in the proper order, but the labels (numbered bubble call-outs) are not to be relied upon. I had to cross-reference several pages to figure out exactly what parts were referenced in each written assembly step.
If one is not very careful, it is easy to rivet together parts prematurely and/or to occupy holes that need to be left open for later steps. Even the factory has trouble with this. I have more than a few rivets to drill out and remove on my QB fuselage, in order for me to rework factory build issues. Take time to understand what exactly has to happen to achieve the correct result.
Spring weather is here with luxuriously warm sunshine. I was able to get all of the smaller elevator parts Alodined and primed. The main channel parts had been done while my paint booth was still set up, before the QB delivery.
Once again, I’ve demonstrated that shortcuts don’t pay off. This time, I tried to skip scuffing the parts. I degreased, rinsed, applied Alumiprep 33 and rinsed again. But, my brush application of Alodine didn’t produce any measure of satisfaction. The results were blotchy and left places that just seemed bare. So, I went back, scuffed every square millimeter of every part with my trusty (red) Scotch-Brite pad. Then more degreasing with diluted Extreme Simple Green Aircraft, rinsing, Alumiprep, another rinse, more Alodine and a final rinse. Better this time.
Brush application of Alodine simply does not compare with dipping, but as a base for priming, it seems fine for good paint adhesion. If I was going to leave the Alodine treated aluminum un-primed, I think I’d have to go with dipping to get a more uniform “golden” appearance. There’s also un-tinted Alodine. I haven’t tried it. It might be hard to tell how effective the application is, especially given the primitive conditions and minimalist process I’m using. Stick with what you know.
My shop is full of wings and fuselage and my paint booth is now the great outdoors. It works well. I can paint more and in less time. There’s the added bonuses of not having to wear a body suit, a respirator or mess about with the ventilation fan. Good old Rust‑Oleum Self-Etching Primer in a rattle can is easy and effective.
I suppose it may seem silly to devote so much discussion to this topic, but I have spent more time on metal preparation than anything else – by far. It’s been terrifically time consuming. I think perhaps a future me might skip Alodine and priming of any next project. It’s certainly proving to be a lot of work. For this build, I’ve already come this far. Plus, the QB wings and fuselage were Alodined at the factory. Possibly, the effort will add an extra bit of long-term value. It’s satisfying, in any event.
There are several points in the build where short rivets are required. Sling Aircraft (TAF) has chosen to leave this task as an exercise for the builder, to adapt some from the ordinary ones supplied with the kit. Here I’m shortening 3,2 x 8mm rivets to about 5mm. These will be used to fasten the piano hinge and control horn to the elevator trim tab.
For some steps, I use a small box end wrench and a socket to provide support around the entire head of the rivet, as I drive the mandrel out or back in using a hammer. I use a cutoff-wheel in the Dremel Tool to trim the rivet body.
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.