Category Archives: Fuselage

CF – Control Tubes, Brackets and Bearings

Mounting the control tubes in the center fuselage is very much like the process for the rudder pedals. Once again, clearance and alignment prove to be the main variables.

A couple of important discoveries made a big difference. The first discovery was that the elevator and flap torque tubes are not perfectly straight. They’re close, but they are not absolutely perfect. With an ever so slight bow and four bearing points, over a meter of distance, the centers of the bearing bores present different impacts, depending on rotational position of the tube. The variations are tiny, but if the bearing clearances are too tight – you get a surprising amount of binding.

The other discovery was that not all of the bearings have identical inner diameter. It’s a long story, but I ended up with a few extras and happened to find one that has good bit looser fit on the elevator torque tube.

The bearing with the looser fit proved to be ideal, when I put it at the LH end of the elevator torque tube. Careful dressing of the fuselage brackets and the retainer brackets with the Dremel sanding drum resulted in perfect fits everywhere – and fantastically smooth action.

Careful observation of how the bores of individual bearings center around the tube allowed me to ever slightly shift those centers in good directions as I opened the U-shaped bearing capture areas to eliminate pinching of the bearings around the tube. Each custom fit retainer bracket and bearing is marked for a specific location and orientation. I’m confidently optimistic that there will be just enough clearance for friction free operation after the retainers are permanently riveted.

At the moment, the flap torque tube movement is good, but it isn’t quite a limber as the elevator tube. It’s not of great concern. The flaps are driven by a linear actuator and won’t be anything I notice in the hand controls, which will be effortlessly smooth.

The two main control stick tubes are each supported by a pair of bearings along the roll axis. The front bearing is captured by brackets with U-shaped retainers, top and bottom, that are rigidly riveted to the structure at the front of the main wing spar carry-through.

The upper retainer should have been riveted before the carry-through structure was mated with the surrounding center fuselage members. This is another instance of the factory failing to comply with documented assembly details.

I can’t really get good enough access for confident pulling of 4,0 x 10mm rivets, so I’m going to use screws and elastic stop-nuts for the uppermost holes and (probably) use rivets for the balance of bracket mounting holes. I’ve got to end up with good alignment and enough clearance so that the bearings don’t bind when the brackets get pulled down. I’m certain that the retaining brackets are going to shift slightly from their un-riveted positions. I might use screws after all. That would make it reasonably straightforward to take it all apart and re-tweak the final fit until I get it right. Absolutely no binding will be tolerated. Smooth, smooth, perfectly smooth – that’s how it will be. Period.

Airplane Noises

As far as I can tell, once an experimental aircraft build project gets to a certain point, there’s a right-of-passage that must be embraced — sitting in the thing. Today is the day.

I made a work platform from wood. It fits inside the fuselage on either side. I can sit on it, if and as needed.

Rudder Pedals – Tubes, Brackets and Bearings

Work in the center fuselage continued with trial fitting of the rudder pedal tubes. It’s looking good. Initially, the pilot-side (LH) mounting brackets were easily positioned and the 4,0 x 10mm rivets dropped easily into most of the holes. And, after clearing a bit of paint in the holes, the rest of the rivets fit as well.

The flight control linkages rely heavily on composite Vesconite bushings, or bearings, depending on how you want to think about their purposes at different places around the airframe. I’ve known for some time, after reading accounts and watching videos posted by other Sling builders, that getting smooth, friction-free action of the controls takes some care. Some folks use the expression – black magic.

These bushings are supposedly designed to be self-lubricating. That’s all well and good, but I know some builders have resorted to supplemental lubrication. I’m trying to avoid greasy, oily, dirt-collecting areas inside the cabin if I possibly can. I’m having some luck – so far.

Two key factors need careful attention – clearance and alignment. Having enough, but not too much clearance, makes alignment slightly less critical. Buttery smooth operation, without additional lubrication, seems to be achievable.

Opening up the U-shaped retainer areas in the floor brackets and the top caps, so that no squeezing of the captured bearings occurs, makes all the difference. I used a small sanding drum on my Dremel tool at a low RPM setting. Eventually, I was able to capture the bearings in the brackets without causing any pinching of the bearing around the pedal tubes.

Next, I used some fine sandpaper around a piece of dowel to relieve a small amount of material from the bearing’s inner surface, particularly at the edges of where a saw had cut them in half. They were once circular and then cut into halves. Some cuts were better than others, but it’s not unusual to find a slight overhang from one or both halves that narrows the bearing bore at the seams where the halves meet when they’re captured in the brackets. Just a slight amount of narrowing can cause binding.

After repeated cycles of fitting and filing, the result is smooth, friction-free operation.

The rudder pedal tubes came nicely coated with gray primer. Areas on the tubes were masked from paint where the bearings ride. Except – one of the masking areas is misplaced by 1cm. I’d read about this, and sure enough, when I measured I found the off-by-1cm error too. The Vesconite bearings are designed to ride directly on the steel. Relatively soft paint would likely gum up the bearing and defeat the self-lubricating properties. I put some protective masking tape around the tube and used a strip of fine sandpaper to precisely remove additional paint.

Last, but not least – I can see how the Sling-branded rudder pedals are going to look. I think it’s much cooler than the plain T-bars. It took almost 6 months after my quick-build kit was delivered to finally get all of the pedal parts. That was a full year after I’d placed my quick-build order, which included the option. All’s well that ends well.

The Sling-branded pedals are essentially the same as the ones for toe-brakes, except that they are mounted on the standard pedal tubes and the hand-brake configuration is used. The toe-brake option has different pedal tubes, different brackets on the floor and no hand-brake. I know because I got a bunch of those parts. I worked with the factory and eventually got all of the parts I actually needed for my pedals.

Rework Outer Seatbelt Brackets

Any work on and in the center fuselage is part of a big step forward. In this case, the work is actually rework, owing to one of several shortcomings introduced during the factory quick-build of my fuselage – which evidently was a bit hasty. TAF admittedly rushed to ship in the last hours before a three week holiday shutdown. Let’s just say there were a few shortcuts taken.

There was a general class of shortcut that I’ve found several examples of – riveting places and parts that shouldn’t have been until other steps were completed. The most vexing examples were the seatbelt anchor brackets being mounted without the AN5-5A bolts and washers in place. I’d noticed this very early on after delivery and have been troubled ever since about how I’d be able to get things as they should be.

As I mentioned in my last post, I finally overcame a mindset that I’d have to drill out and re-rivet the center brackets to place the bolts. With courage and great care I was able to flex my way to a happy result. The outer brackets demanded the drill and re-rivet approach. That has proved very doable, especially since I’ve got the fuselage solidly down on the floor where I can work on it without concern about toppling. Yay again!

I’ve gained some valuable experience with drilling out the big 4,8mm rivets from my rework of the aileron and flap hinge bracket sub-assemblies. With a #12 bit in my lithium battery-powered drill I was able to get the outer brackets off cleanly. And when the time comes, I’ll be able to rivet them back on – with the bolts in place. Glad I don’t have to worry about this anymore.

Fuselage Sitting on Main Landing Gear

It may not seem like a big deal, but having the fuselage safely off of the workbench, where it had been (somewhat precariously) perched, is a great relief and a meaningful milestone. It’s now in a position where I can work inside the center fuselage – comfortably and confidently.

The fuselage spent months sitting on the wooden structure that held it inside the sea shipping container. Then it sat for weeks on a workbench and up on some blocks that I was constantly concerned might cause great damage if the structure should somehow roll forward or backward on those blocks.

With the main landing gear in place and some help from my wife and also from my friend Charlie, the three of us were able to remove another wooden shipping structure at the tail, clear the blocks and the workbench from underneath the center fuselage and set the whole business down on the floor. The tail is supported by a padded sawhorse. Joy! Joy!

I’ve been waiting for months to work inside the center fuselage. I did the first bit of work right away – installing the center seatbelt anchor bolts into the brackets where they should have been placed by the factory – before the brackets were riveted to the CF structure. It’s clearly called out in the construction manual, but the step was forgotten.

At first, I thought I would have to unrivet both center brackets in order to get the bolts into position. But, I eventually became hopeful that I could flex the brackets just far enough to slip the bolts in. Yes! It worked. Good thing too, because it really would have been nearly impossible for me to re-rivet the brackets with conflicting structure in the way. I’m so glad I went ahead and attempted positioning the bolts as I did. Done!

I have the same factory-forgot-the-bolts issue with the outer anchor brackets, but at least there’s ample space to drill out and re-rivet. There’s no way I can flex those outer brackets enough to get the bolts where they need to be. The structure in that area is far too stiff.

Main Landing Gear Assembled

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.

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!

Fuselage – Off of the Shipping Stand

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.