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!

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.

Aileron Alignment

As I’ve posted previously, I’m trying to be proactive about ensuring the aileron trailing edge lines up with the rest of the wing. I decided to re-use the same trick I used for the elevator & horizontal stab: assemble the part in-place, using clamps to set the assembly straight. So, I loosely assembled the aileron with a handful of clecos and blue tape, put in a few rivets (some set, others loose), and then attached the aileron to the wing.

Using clamps to hold the aileron in place while I pop the rivets

After popping a few of the rivets on top and bottom going into the rib flanges, I can say that this process helps, but it appears the final twist is really controlled by how you set the rivets on leading edge of the skin (where the top and bottom skin edges meet). Unfortunately, these rivets are not accessible while the aileron is mounted to the wing! So, I will ponder how to hold the alignment while taking the aileron off the wing to set the remaining rivets.

Alignment still looks good after setting most of the rivets that I can get to

Left Wing End Rib

All 3 sizes of rivets to secure the end rib. I also had to use my hand puller for some of the rivets, due to tight clearances.
Checking alignment of the trailing edge – flap looks good. Aileron coming next!

1 hour last night riveting the end rib to the left wing. It’s amazing how much stiffer the wingtip feels after riveting this rib in place.

Pascal L. recommended a trick to check alignment of the trailing edge using a string. I haven’t done the aileron yet, but it looks like I got lucky with the flap!

Yesterday I was also looking at lighting again. Aveo has some updated products on their website, a PowerBurst Daylite NG 3-in-1 light for wingtips, and a MiniMax Ariel for position / recognition. I’m thinking of getting a white Ariel for top of the rudder, and a red Ariel for fuselage belly. The Ariel is works both in steady and flashing mode, which is perfect. I have to find out where I can buy them, since neither are listed at Aicraft Spruce (yet). I will put an inquiry into Aveo and see what they say.

Rivet Filling & Left Wing End Rib

My fuselage kit will be arriving this week, so I spent this weekend finishing up the left wing. Last few steps are to attach the wing tip and control surfaces (aileron, flap). Before taking the jig off the wing tip, I worked on filling in all of the rivets on the bottom side of the wing with Sherwin-Williams Shrink-Free Spackling, as recommended by other kit builders. After injecting the spacking into each rivet using a 10ml syringe, I sand it down with 400-grit sandpaper to a smooth finish, using a little job aid that exposes just the rivet head. The sanding block I use is a detail sanding block kit available at Pep Boys with multiple fine grit sandpapers that stick onto the flexible block. It’s very useful for this task, but I also use this kit for removing scratches.

Sanding block and a job aid to mask off everything but the rivet head. The job aid is made from a scrap piece of aluminum, with a hole drilled to match the size of the rivet head.
Filled and sanded rivet heads

One last task while I had the wing upside down – install rivnuts for the large inspection holes. There are also a number of smaller inspection holes, but I’m not sure how covers are supposed to be attached – there aren’t enough rivnuts included with the kit, so I’m guessing these just get riveted on?

I installed rivnuts on the large inspection hole (right), but there are several smaller inspection holes (left) that I’m guessing will just have covers that are riveted on – probably don’t need to be inspected as often.

With rivet filling complete I flipped the wing back over and removed the end jig, allowing me to attach the end rib. Once riveted in place, I can also attach the composite fairing, but I may hold off on doing that until it’s closer to final assembly time, for a few reasons: (1) I haven’t purchased the wingtip lights yet, (2) I’m considering modifying the tips a little so that the lights will be closer to the trailing edge, similar to the TSi wingtip, and (3) I think the wingtips stand better chance of not getting damaged in transit if I leave them off. I’m planning to store the wings in our friend’s basement, since I don’t have space in my own basement.

End rib attached – still need to feed the wire for the wing tip lights through the small hole in the bottom of the rib
Sling TSi wingtip, as seen at Oshkosh 2018. The upturned edge offers a better place to mount the position/strobe light than the Sling 4 wingtip. I wish I could retrofit these wingtips, but the wing shape on the TSi is different so I don’t think they’ll work on my wing.

I also attached the flap to see how it fits – looks good so far.

Flap fits nicely, and moves freely
Flap held in the ‘up’ position using a little clamp that I improvised out of aluminum angle stock

Next task is to finish up the aileron. I have to figure out how to attach it to the wing such that I can still adjust the fit of the skin, whilst having it held together with clecos. I know from reading other blogs that getting the twist of the skin just right can be tricky.

Clecos getting in the way! But this picture looks cool, the aileron kinda looks like a Fowler flap in the full down position.