Tag Archives: fabricate

FFW and CF Tasks

While the engine mount is off, I’m getting firewall forward and center fuselage tasks done that would be more challenging to do later.

I’m not keen about how the factory seems to expect the stiff-wire push-pull cable to go from the instrument panel, through the firewall and then to the heat box on the firewall. As with other firewall penetrations, I’m not content to just stuff the cable through the firewall with a grommet. I’m also not going to settle for having the cable penetrate the firewall at the absurd angle needed to even have a chance of getting to and working the heater box vane. Instead, I’ve designed and fabricated a jack-shaft bellcrank arrangement as an alternative. I’m still going to use the factory-supplied cable assembly – straight through the firewall, a bowden cable clamp and then attaching the wire to a nylon control horn. Another control horn, at the other end of a shaft, translates the push-pull control motion approximately 90 degrees – to be in line with the action needed to work the arm on the heater box vane. I made a couple of brackets out of aluminum angle and mounted the mechanism on the firewall.

Since my QB airframe was built and delivered, the factory has rethought how and where the ELT antenna goes – to just ahead of the vertical stabilizer. It’s too late for me. The structure was changed to accomodate a new mounting bracket and I’m not going to attempt a retrofit. The old location for the ELT antenna was inside the cabin. I’ve designed and fabricated a bracket to mount the antenna inside, just ahead of the rollover structure on the RH side of the fuselage.

Now that I’ve got my hands on the main battery – EarthX ETX 900, 16AH, LiFePo4 – I’ve been able to build and connect 4 AWG cables from the battery terminals to the 12V contactor and to the airframe ground lug. The high current cables are short and tidy.

I’ve had to acquire [standard AN] replacement hardware for re-mounting the engine mount, but this time, along with the front cables for the ballistic parachute. Longer bolts are needed to pass through the heavy cable-attachment tangs. Initially, I didn’t have the tangs. I eventually got those, along with a bunch of other factory parts that should have shipped with the main kit. The cables and the engine mount are on! Good deal.

With the engine mount in place, I’ve mounted the nose gear strut. Some months ago I accomplished fitting of the bushings, retainers and bolts. That made is super easy to just bolt it all together and connect the push-pull rods to the rudder pedals linkage.

I’m still waiting to put the wheels on because the fuselage is that much lower to the ground, making the inside of the center fuselage (CF) somewhat more accessible than it would be with it higher. I’m taking advantage of the easier access while I dress and secure the wiring and prepare parts of the control linkages and autopilot roll servo.

I’m pretty happy with my approach to securing wire harness bundles as they pass through various openings in the CF structure. I found a source for AN743-13 aluminum angle brackets. These brackets are just right for supporting insulated (Adel) clamps around the wire bundles. It was very challenging to drill holes and rivet the brackets at this late stage of the build. I didn’t have the luxury of doing it while the structure was open, sitting on the bench. Nevertheless, the brackets and clamps are in place and they’re pretty nice. I’ve also put some edge grommet in a few places, just for peace of mind.

I’ve previously tested the flap actuator with temporary connections, but now I’ve made the connections permanent with crimped butt-connectors and various layers of insulation and protective armoring. I’ve done checks to insure that the wiring will be clear of moving mechanisms. It all looks very promising and I’m feeling happy about the work.

Another thing I’m pretty happy about was my purchase of a simple jig for drilling nice cross-holes in the control tubes. Beautiful!

More Antennas and Fittings

After giving things a lot of thought, I finally summoned enough resolve to drill some holes in the aircraft skin, and most, where none had existed before. You’ve got to do what you’ve got to do. Antennas, static ports and bulkhead connectors need holes. Now I’ve got them.

Determining the location of antennas is where most of the thought went. I tried to get the transponder blade antenna pretty close to the fuselage centerline – right in the middle as it turned out. Airframe shading and interference between antennas and other equipment are key concerns. The COM 2 antenna is also on the bottom of the fuselage, just behind the rear wing spar carry-through. I tried to keep it as far away as I could from the the transponder antenna and GMU 11 magnetometer. I didn’t get all of the separation distance called for in various installation documents, but what could I do? This is a tiny little airplane — not a King Air. Time will tell if it’s enough. Build on.

All of the antennas are made by Rami and came along as part of the package with my panel, harness and avionics. Some builders have made rather large doublers. For the moment I’m using only the doublers that came with the antennas. I may eventually go beyond those, but for now, everything is in place.

Of the 14 holes that were already in the ELT mounting tray, none were where I wanted them to be. I made 4 more in the tray that allow 4.0 x 10mm rivets, through the fuselage skin and into the tray, without interfering with the plastic case of the ELT itself.

I made an airframe ground lug from a 5/16″ brass bolt and installed it at the location where the KAI called for a 12 AWG avionics ground bus wire to be passed through a grommet in the firewall on it’s way to the negative battery terminal. I’ve decided to have a 10 AWG cable for the avionics ground bus to the lug (inside) and a 4 AWG cable from the same lug, on other side of the firewall, to the battery. I plan to attach a 6 AWG engine [starter] ground cable to the lug as well.

Rubber fuel lines penetrating the firewall through ordinary grommets, as the kit assembly instructions (KAI) call for, didn’t float my boat. I’ve gone with AN6 bulkhead fittings for the fuel supply and return lines. These fittings need smaller holes than the ones pre-punched in the firewall by the factory. I designed and fabricated a doubler out of steel and riveted over the original holes. The fittings are mounted there.

A set of step-drills is absolutely essential for making holes in sheet metal. I’ve got a cheap set but they’ve worked well.

I’ve set M6 rivnuts (with some JB Weld epoxy for good measure) in the fuselage rib, behind the parachute compartment, where the Rotax ECU goes. Getting the 3 large engine wire bundles and connectors in place, given the minimal space remaining with the LRU rack mounted, is going to be super tight. Fingers crossed.

As long as I had rivnuts and JB Weld handy, I decided to rework the Andair Duplex Fuel Selector valve to receive M4 rivnuts and then set them in place. These steps are called out in the KAI.

My factory quick-build fuselage came with not one – but two – static ports in the rear fuselage. However, the factory now says not to use them. They’ve also said they haven’t officially decided where the new location(s) should be either. They think it’s going to be finalized “soon” and anticipate that it’s going to be just in front of the fresh air NACA duct near the front of the fuselage. They sent me a picture and I’ve gone with that location.

The outside air temperature probe found its home in the LH fresh air NACA duct.

IBBS Mounting

The navigation avionics and engine management systems each have a TCW Integrated Battery Backup System (IBBS). The backup systems provide essential power in the event that one or both alternators in the Rotax 912iS engine should fail. There are 2 battery power units that have to be mounted someplace. I’ve found a spot on the fuselage rib, behind the parachute compartment that is within comfortable reach of the associated wiring harness connectors.

I thought that the batteries could use more support than they would get, were they attached directly to the rib. I designed and fabricated doubler plates to reinforce the rib. The batteries are fastened to the doublers.

I’ve had to consider that one day the battery units will have to be replaced. It won’t be easy, but I will be able to get to them from the front, after I remove the pilot side display and the remote LRUs and vertical rack that sit behind it.

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.

Ailerons – Fab, Rework and Pre-Assembly

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.

Seat Assembly

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!

Rudder – Composite Tip and Beacon

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.

EL – Pitch Trim Servo Retainer

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.