Rear Floor Assembly

I started fitting all the ribs I primed this past weekend to the rear spar carry-through assembly and floor skin. The fuselage is beginning to take shape!

I’m fitting the clecos from the top side, even though rivets go through bottom. Seems easier to do it this way rather than working upside down!
Turning the assembly on its side so that I can rivet from the bottom side

Center Fuselage Parts

In and around holiday events this past weekend I was able to put in a few hours on center fuselage parts. Not much to show, other than a bunch of floor ribs that have been cleaned and primed.

Speaking of priming – I found that I spend a lot of time on this step, so I’ve been looking for ways to speed this up. I am still deburring and lightly sanding parts as before, but I’ve switched up the cleaning routine. Rather than using a combo of simple green, water, acetone, and isopropyl alcohol, I’ve started simplifying to a one-step product that’s recommended by the manufacturer of the primer I’m using (SEM self-etching primer). The cleaner is called SEM Solve, which comes in a spray can and wipes off with a clean towel. So far the results are great – the primer is adhering well and the finish looks good. SEM doesn’t give guidance on how much of the cleaner you will need, but I’m finding that I need one can of SEM Solve per 4-5 cans of the self-etching primer at 3 coats of primer.

I switched to the gray primer color for the fuselage, for no other reason other than aesthetics — there are likely to be primed parts that are visible even after putting in the interior, and I think it would be nicer to see a neutral gray color instead of green.

Preparing the rear floor panel
Various ribs to attach to the floor panel
Rear spar carry-through assembly, to which the ribs above will attach to
Attaching an autopilot servo bracket to the main spar carry-through assembly. Access was really tight — I had barely enough room to get this relatively slim rivet gun into place.
Completed autopilot servo bracket

I have to put in an order to Aircraft Spruce soon, among the items I need to buy are:

  • Garmin GMU 11 Magnetometer plus connector kit
  • Garmin GSA 28 servo (x2) plus connector kit
  • Shielded twisted pair wire for the CAN connection to servos and magnetometer, as well as some normal straight wire for power & returns
  • Soundproof insulation for the interior skins (I want something more substantial than what is provided in the kit)

Anyone have guidance on how much length of wire I need to order, especially the shielded wire for the servos & magnetometer? There may be more wire I need at this point, like antenna wire for comms, ELT, etc., but I need to figure that out.

Rear Fuselage Progress

I was able to log quite a few hours this past weekend, working mainly on the rear fuselage. First, I finished assembly of the tailcone, which is fairly beefy since the empennage control surfaces attach to it. Some rivets were a little tricky to set due to limited room.

I needed to ream this outside hole slightly, but I needed some help from a flex shaft attachment to get enough clearance
I couldn’t get a 4x10mm rivet to sit flat here, due to a formed rivet already directly behind it, so I decided to modify a rivet a little per the next picture.
I removed about 3 mm from the rivet body to fit in the hole above – that was enough to successfully set the rivet.
All done! Looks interesting

Next, I cleaned and primed parts for the larger rear fuselage ribs, and assembled ribs 2, 3 and 5. Unfortunately I had to skip rib 4 for now, as I discovered a crack in one of the formed parts. Rib 2 didn’t quite come together as nicely as ribs 3 and 5; despite using the template, there’s some twist in the curved top pieces. I’m not sure if this is a problem yet – maybe the twist will get worked out once the rib is supported by the skin.

Lining up rib 3 using the guide provided with the kit
Perfect fit and flat – rib 3
Rib 5 also went together nicely
Unfortunately I discovered this crack in one of the rib 4 pieces as I was prepping for priming. Replacement part is on order!

I moved onto building up the floor of the tailcone, reinforced with several longerons. I vaguely remembered a warning that the orientation of some of the longerons was incorrectly illustrated in the manual – sure enough, after researching for a good half hour, I found confirmation from reading Peter’s blog. Left and right longeron 902 are swapped in the CAD model used for the manual. By the way, this error is still present in the TSi manual as well, so please be careful and pay attention to the part number left/right digit, and take a look at photo below for proper orientation. If you read on in Peter’s blog, you’ll see that having these incorrectly swapped will cause a headache later on during the center/rear fuselage join.

Longerons 902L/R (short ones), shown in the proper orientation, with the c-channel facing towards the centerline

Last thing I decided to work on was the luggage floor skin. Curiously, my fuselage kit included two skins: one for the parachute option, one for without. Also curiously, the skin for the parachute option was missing a few holes for a support channel specific to the parachute option, so I had to mark and drill those holes.

Non-parachute luggage floor skin on left, parachute on right. I wonder why the inspection ports are in different locations?

I’m holding off on riveting the supports in place, since this skin is supposed to have some foam attached to the bottom for insulation. I figure it will be easier to measure & cut the insulation without these supports in place, so I’m holding off on attaching for now. The kit includes foam, but I’m considering using a denser foam like other builders have opted for.

I masked areas to control where primer gets applied. Outside these areas, insulating foam gets applied instead, and I think the adhesive will stick better to the bare metal. The support channel running across the middle of the skin is unique to the parachute option. I had to match-drill a few holes into the skin that were missing.

I’m not sure what to work on next. I don’t want to do too much more with the rear fuselage, since the next step involves attaching the ribs to the floor skin, and will start to become a fairly large assembly. I want to complete assembly of the right wing before I do that, but I’m on hold waiting for a replacement rear spar. I think I’m going to resume working on the right wing’s fuel tank, which I started in March but then set aside to finish up the left wing assembly.

Right Wing Skins and Starting Fuselage Assembly

I consulted TAF about my damaged rear wing spar, and they advised against repairing it so they will send me a new one. That limits how much I can do on right wing assembly for now, but we had some nice weather this weekend so I decided to prime the right wing skins along the rivet lines so that they’re ready to attach. I already have most other components for the right wing primed and ready for assembly.

I occasionally like to make sure the primer is adhering well, so I perform tape pull tests after the primer has been given a day to dry. I wasn’t too happy with primer pulling up on a recent part I was working on, so I made a slight process change – instead of using Scotch Brite to scuff the surface, I’m now using 400 grit sandpaper. I think the plastic protective sheeting that’s applied to the aluminum leaves behind an adhesive residue that the Scotch Brite doesn’t quite release. Wet-sanding with the 400-grit paper and water seems to work pretty well, so that’s what I’m doing going forward.

Sanding and cleaning the skin. I’ve had some mixed results using Scotch Brite pads, so now I use 400 grit sandpaper only and have had much better results with primer adhesion.
Same skin after cleaning off the sanding residue. First I wipe off the wet sanding residue with a clean cloth, then wipe with a clean cloth with a small amount of acetone, finally wipe with a clean cloth with a small amount of isopropyl alcohol.
All primed and ready – I usually get the desired finish with 3 coats of the primer

I decided to start assembling fuselage components, starting with the ribs of the rear fuselage. Assembly of these components was straight-forward.

Rib 8
Completed rib 8 and rib 9; they come together later on in the assembly.

Fuselage Skin Rack

As I started inventorying the fuselage skins, I realized that I didn’t have a good place to put them. I found some unused space above my garage door and improvised a shelf out of reclaimed wood from the shipping containers for the wing and empennage.

Simple rack made from the 1.5″ squares that made up the frame of the wing shipping container. The skins are pretty light, so I wasn’t too worried about making the rack super strong
Plenty of room for all the big skins, and they’re in a place where they’ll be safe from getting bumped into
The smaller skins and all other fuselage parts are neatly stored away on wire shelves

I’m now complete with the inventory, and I only have a small number of discrepancies to resolve – they mostly seem related to a design change in the parachute box. My fuselage kit pieces didn’t match what’s shown in the Sling 4 construction manual, but I looked at the Sling TSi manual and found my parts match the pictures and part numbers – so it appears to be a running change in the design that is common to both models, but the manual hasn’t caught up yet.

With inventory out of the way, I am resuming work on the right wing. I have the fuel tank partially assembled, but I first needed to address a problem with the rear spar that I forgot about. While in shipment, something punched a small hole into the angled edge that forms the upper cap. I smoothed out the damage, and made a small doubler plate to restore strength.

Damage to right wing rear spar, near where the step ribs attach
View of the same defect from the inside, after I gently flatted the area back down, then sanded and filed the hole smooth
I made a small doubler plate to restore strength to this spar cap

Inventory of Fuselage

There are many, many parts in the fuselage, I can see why this will take a while to build. 5 hours so far just counting and touching all the parts, and I’m just now getting to the bottom of the crate where most of the skins were packed. The process is worth it, though – I think I have some discrepancies to resolve, and that’s much easier to do now than later on when I need a missing part to finish a major assembly!

A few dozen skin pieces

Left Wing Done For Now

I spent 6.8 hours on Sunday installing the wing step skin, and then filling the remaining rivet holes with spackle and sanding them smooth. With these two items done, my left wing is complete for now, minus a few additional tasks (such as mounting the wing tip and wing tip lights). So, my wing is now resting in its new home in the newly-completed wing rack.

I spent the remainder of my time on Sunday completing inventory of the fuselage kit. There are a lot of pieces! I got through most of the packed sub-kits, and the major pieces left to inspect / count are the skins, side channels and longerons.

Wing step skin finally riveted on
For now, I just used the existing holes to secure the flange on the top step skin. It doesn’t seem to need more rivets, despite the holes being somewhat slotted (see my earlier blog posts). I did upsize one rivet near the edge to 4.0mm.
Picture with the flap and aileron installed, and end jig removed

Wing Rack, and Step Skin Solution

I put together a wing rack this week following plans from an EAA article. I had most of the material already, so it didn’t cost much – the straight sections are 2×4, 2×6, or 4×4 pieces of lumber I had laying around, while the gussets were made out of pieces of the wing & empennage shipping containers. I had to buy casters, which I got from Amazon for not much money, and some screws. I’ll move the left wing over to this rack shortly, and then begin assembling the right wing.

On my wing step skin issue, I got feedback from TAF on what to do – their suggestion is to put in additional rivets in new holes to be drilled between the existing holes. Simple enough solution, though I don’t think I need to put in quite as many new rivets as per the picture below. I’m happy to get this resolved and move on, since it’s the last wing task I need to do, minus the composite wing tips.

Finishing Aileron; Wing Step Skin Try 2

I had a heck of a time getting the aileron to line up properly with the trailing edge of the wing and the flap. I wound up using a technique that involved putting some tape on the skin overlap, twisting the aileron by hand while on the wing, taking the aileron back off, then setting rivets. I had to use a reamer to open up some of the holes so that I could set the rivets with the aileron twisted to match the wing. It was very much a trial-and-error process, since I couldn’t find a way to set alignment once and then rivet the skin.

At first I cleco’d the edge and then used blue painter’s tape to hold the skin in place so that I could remove the clecos and check fit
I needed a stronger tape to hold the skin down, so I switched to strapping tape
I marked some black lines over the skin overlap joint to make it easier to see if the skin was still aligned after taking it off the wing
As you can see from looking closer, the holes don’t line up very well after the aileron has been twisted for alignment. I use a reamer to open these holes up.
I guess it was worth the effort; the aileron alignment looks good

Next, I wanted to install the wing step skin so that I could call this wing finished for now. TAF sent me a replacement skin along with the fuselage kit that I received recently. This skin doesn’t seem to fit either; just like in my earlier post, the holes on the flange don’t line up with the holes on the ribs or lower skin, so I can’t get clecos in.

I decided to try drilling the holes, and as I expected, I ended up having what looks like slots. I am considering just going with this skin as-is; I don’t think this section of skin sees much tensile or sheer loads, but I’m checking with the TAF tech support team to see what they say.

Comparing the new skin (top) with the original skin (bottom). On the original skin, match-drilling to the wing results in holes that are above the CNC-cut holes, while on the new skin match-drilling to the wing results in holes that are below the CNC-cut holes. You can see there are some holes that are less than 2D from the edge, a potential problem.
I decided to match-drill the rest of the holes on the replacement skin, because it lines up better with the flap when installed on the wing. The holes I drilled are the lower set of holes, some of which are marked with black arrows. I used a file to open up and smooth the adjoining holes.

This week I am going to build an EAA wing rack; I need to get this wing off my work tables and off to the side so that I can build the other wing, and then begin the fuselage.

Fuselage Kit Arrived

My fuselage kit completed its cross-country trip and it looks like the crate held up well. Not sure yet where I’m going to store the parts, since I still have wing parts all over the garage, but luckily I have the long weekend ahead to sort it out!