Cabin Heater

Since I live in New England, having effective heat in the airplane will be important to me. As I was reviewing the updated fuselage manual for the Sling TSi, I noticed that TAF updated the cabin heater design, from a system that uses ram air flowing through a cuff over the muffler, to a system that uses a heat exchanger coupled into the Rotax cylinder head water coolant loop – which is the same approach used in cars. I think this is a much better design, and greatly reduces the possibility of carbon monoxide and other exhaust gases coming into the cabin. I also think it’s important to be able to send warm air onto the windshield to clear fog.

After a bit of sleuthing, I believe I’ve found the unit that TAF sources, a Siroco Tenere Performa 4D heater. I have an email into TAF to see if they’ll sell the parts to me to put this into my Sling 4. I’m also considering coming up with something on my own. This box weighs about 4 pounds, uses 80W for the blower motor, and will supply up to 20,000 BTUs of heat per hour, which should be more than enough. But it doesn’t really have any way to direct airflow, so the TSi kit also supplies (I think) a bunch of hoses and ducts with butterfly valves to control the airflow.

I’m surprised to see there aren’t many off-the-shelf options for cabin heaters that take advantage of the Rotax engine’s water cooling system. Kitfox offers a very simple device for their planes — it’s really just a case fan bolted onto a heat exchanger. I’ve also read that Vans may have something for their RV-12. But I’d like to see something closer to what’s in cars, with dampers and doors in a blower assembly that directs airflow to your feet, face, or defroster vents.

Right Wing Build Underway, Big Announcement Today at Oshkosh

I received a replacement right wing rear spar earlier this week so now the wing can get built up. Perry from TAF came up with “quite the contraption” (as he called it) to ensure it made its way to NH undamaged – it’s mostly a long piece of 6″ PVC pipe. But it did the trick, as the spar arrived in perfect condition.

Kinda like the tube used for shipping the fuselage longerons, only bigger!
Peeling off the plastic and deburring the holes
After cleaning and priming, assembly begins! Attaching the step ribs and stiffener plates to the rear spar

Sadly I had to skip Oshkosh this year, as I couldn’t take time off this week to go (I was just on vacation last week!). As much as I wanted to go, I am happy to instead be working on my Sling. When I bought the kit last year, there were only a handful of other people in the US building a Sling 4, maybe a total of 5 people. Now, especially with the release of the TSi, there are quite a few more builders and the Facebook Sling Builders group is very busy. It’s pretty exciting to be part of it; almost every day I think about starting test flights and flying out to Oshkosh to show off this beautiful plane!

One big thing I did miss tonight at Oshkosh was the announcement of a high-wing Sling TSi variant at the “Sling Ding” social event. It looks pretty nice, I’m sure it will be popular! To me, the design looks like it took some cues from the Cessna Cardinal, which I know many pilots loved for its swept back windshield/wing, lack of wing struts compared to 172s/182s, and relatively good performance for a 4-cylinder engine. I was looking at buying a used Cardinal before I decided to get the Sling 4 kit. Maybe this will be my next project!

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.