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.

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

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.

I Have a Serial Number!

I just found out that TAF has assigned serial # 180k to my kit!

I’m guessing the ‘k’ indicates kit-build, as opposed to factory-build. It’s good to know that there are about 179 other Sling 4’s that are already built or in the process of being built! It’s a pretty active community too, I see posts regularly from other blogs and also on the Sling Builders FaceBook group.

Wing Top Skins Riveting

Last weekend I riveted the upper skins on the left wing. As I did when clecoing the skins in place, I started riveting onto the strongest parts first (4mm rivets into the main spar cap), then ribs, then stringers.

When pulling these blind rivets, it leaves an open hole that should be filled in. I borrowed an idea from Craig Maiman, who in turned got his idea from Kit Planes magazine, to use a light-weight spackle to fill in the rivets. I bought some from a local Sherwin Williams store, and filled a syringe with the material and then injected it into the rivet cavities. It comes out in a creamy/pasty consistency, and then hardens relatively quickly (within an hour) so that it can be sanded smooth.

Short GIF video of filling the cavity – almost like dental work!

I wanted to add the step skins next, but I found an issue with fit of the skin. Specifically, the holes in the top step skin (WG-SKN-003-L-C-) did not line up with the bottom skin & ribs. My assumption was that the skin was bent incorrectly, which was confirmed by TAF technical support. This skin is pretty thick material, so there’s no way I can fix this myself – and besides, aluminum doesn’t like to be re-bent. I thought about drilling new holes, but because the mis-alignment is about 2.5mm, and the holes are 3.2mm in diameter, I would wind up creating a slot instead, and the rivet possibly could pop out over time. Instead, TAF will be sending me a replacement skin.

The bend of the upper step skin tucks between the lower skin and the ribs (which is what this lower skin is attached to with clecos in this pic). But because the holes in the top skin are misaligned in the red circled area, I can’t get any clecos in there.
The red lines help illustrate the misalignment here between the upper and lower step skins (the lower step skin is the correct one)
Pretty clear to see the misalignment in this pic. The red circle is where the rivet hole is on the step skin rib.

Since I had this issue with the left step skin, I decided to check the right side skin, but that one looks to be OK.

While I wait for a new step skin, I’m going to build the ailerons & flaps, and then get started on building the right wing.

Left Fuel tank started

I created a jig for assembling the fuel tank using the templates included with the plans. After transferring the profiles to a sheet of 1/2″ MDF, I cut the shape by hand with a rotozip-style tool, then filed and sanded as needed to smooth it out.

Next I followed the plan instructions which say to cleco the left side ribs, stringer and skin together before attempting to seal them. Parts mostly went together ok – there are one or two holes I’ll have to modify.

Since it was a nice day, I hopped on my bike and took a ride over to the little airport in town – Brookline NH (NH16).

https://www.airnav.com/airport/NH16

It’s one possible option for a place to keep my plane when it’s complete. NH16 is a cute little private airfield, with some homes and hangars directly off the runway. The runway is pretty short though, just under 2000′ long, with tall pines at one end… I’ll have to take a look at performance numbers for the Sling 4, especially the 50′ obstacle clearance run!

3d Printed Wedge

I ordered the little close quarters wedge from Cleaveland Aircraft Tools last weekend, but I’m still waiting for it to arrive. Meanwhile, I thought I’d try making a temporary one out of plastic. Pretty quick work to design the part in Fusion 360 and then send it to my 3D printer.

Wedge getting printed using Simplify3D

25 minutes later!

Looks great, and I bet it’s even pretty darn close to the dimensions of the metal one!

… but it didn’t work so well, it was easily crushed by the force of the rivet puller.

I managed to get most of the tough-to-reach rivets anyway without this aid. My manual rivet puller has an adjustable head, so I was able to get into the tight spaces.

Left Wing Wiring Installed

Time spent: 6.3 hours

Busy day doing various tasks on the left wing. First, I ran the pitot & AoA lines and built the little standoffs per the manual.

Since it was nice outside, I built a little fixture so that I could prime the inside of the wing skins. I used mostly reclaimed wood from the shipping containers that my kit arrived in.

Skin priming fixture

Wing skin #2

Next up was wiring. The kit includes quite a few wires already: 6-conductor cable for wing-tip lights, plus 3x individual 16AWG wires for the landing and taxi lights. To this I’m adding 2x 14AWG wires for pitot heat and a 22AWG discrete line for the pitot heat regulator.

Twisting the power and return lines together on high-draw devices like the pitot heat helps reduce magnetic fields that can interfere with sensors, particularly the magnetometer

Using a drill to twist the wires together

I put these wires all into expando sheathing to protect the wires and keep things neat and organized. Only problem however was that this cable harness was now larger (in diameter) than I could fit in the pre-drilled holes in ribs 12-16, so I had to enlarge the holes and put in bigger grommets. Because these ribs are so closely spaced together, I had to take them out partially, but it wasn’t too bad since they were only held in with a few clecos.

Expando sheathing, some electrical tape at the end to keep the sheathing from unraveling, and then a heat shrink boot goes over it

Looks good with the heat shrink tube covering the transition

Temporarily taking out ribs 13-16 so that I could upside the wiring holes and grommets

Success! Wire harness fits nicely through the ribs

View of the harness routing after flipping the wing over

For my last task of the day, I started attaching the bottom wing skins with clecos for a test fit. Assuming everything fits ok with all skins on, then I get to take all of the skins off again and rivet the ribs to the spars… That’s a lot of clecos to install and remove!

Wing skin #1… So far it’s ok, but it looks like some holes are not lining up right. May have to using chucking reamer to open up a few.

Assorted Bits Arriving

I’ve been busy again with a few other things (planning for a wedding is surprisingly complex!) but that allowed time for some parts to arrive.

First, I realized I needed to buy a pitot tube, since wing skin close-up comes very soon! I have a Garmin GAP 26 on order, it should arrive next week. It was on backorder (!) from Aircraft Spruce, wonder if the recent Oshkosh airshow had anything to do with it? The version I’m getting is heated and regulated, meaning it has a little thermostat keeping the probe at a constant temp rather than just sucking down 10 amps continuously and getting red hot. I’ll have to find a space in the wing to mount this regulator, which has to be within 2 feet of the pitot probe. This particular probe also has an Angle of Attack (AoA) pressure port, so I need to run two pneumatic lines back to the fuselage.

For the pneumatic plumbing I decided to go with flexible 1/4″ color-coded nylon tubing, rather than the clear vinyl tube included with the kit, so I bought some tubing and fittings from my favorite hardware supplier, McMaster-Carr. While I was at it, I also got some nice expando sheathing for wire harnesses, special zip ties, and standoffs for securing the zip ties. I say “special” because most zip ties are made out of nylon that becomes brittle below 32 degs F – well, it regularly gets much colder where I live, so I got ties that are made from a heat-stabilized nylon, are flame-retardant and rated for -40 to +220 deg F. Same with the wire sheathing I bought, it’s made from PET plastic which is also flame-retardant.

The kit doesn’t call for a heated pitot as standard equipment so I had to get some extra wire. The stuff to get is called Tefzel wire, named for the material the plastic jacket is made from. This material won’t smoke or cause hazardous gases if there were a fire, so it’s a good safety measure. 14AWG for the current-carrying wires (white), and 22AWG for a status discrete line (blue).

Tefzel wire – used extensively for aviation and race cars

Various AN-style fittings for the pitot pneumatic lines and upcoming fuel tank build.