Fuselage Join, GPS antenna bracket, Torque Tube Stop Collars, SB #0018 …

Time:   19:00 hrs

Work was actually done July. 24 th 2020 till July 29th 2020 and Aug. 12th 2020

Before actually doing the fuselage join, I installed a mounting bracket for the diversity GPS antenna. If son / cobuilder Paul ever decided to go IFR with our Sling 4 (I won’t), he will need a certified ADSB transponder with diversity. There are certain requirements for placement of the diversity GPS antenna. The top of the arch, where the RF joins the canopy would be the best place and will meet this requirements.

We can place the GPS antenna here, because we do not have any cables for the parachute passing here (no BRS). Originally I intended to simply use M4 rivnuts with M4 taper head screws, but Midwest Panel Builders have advised not to do so. It is critical that the taper angle of the screws matches the countersink of the antenna, else it can crack the antenna if incorrect.

According to the Garmin manual the GPS antenna requires #8-32 UNC-2A x 1.0 screws, which have a different countersink angle than the M4 taper head screws.

I therefore had to fabricate a mounting/stiffener bracket using appropriate nutplates. Thanks go to RV12 builder friend Wolfgang for the nutplates, the solid rivets and lending me his rivet squeezer/dimpler – couldn’t have done without!

In the last and next picture you can see the conduit I ran up behind rib 2 for the antenna cable. The conduit will be covered with leather or carpet when doing the upholstery work in the cabin on rib 2.

Though the manual mostly finishes the CF before joining the fuselage sections, this would be very difficult for me. The garage I’m building in is very small and I need more space for the upcoming work on the brake lines and on the firewall in the near future.

Also I have heard that TAF USA now does the join before doing the sidewalls and the firewall. With the carpets and the sound insulation mostly installed in the luggage compartment , I decided it was time to join the rear and center fuselage now.

At least with the riveting tools I have, some rivets are impossible to set with the short elevator torque tube (CT-AEL-002-C-C) underneath the back seat in place and I had to remove that once again. I therefore suggest to not rivet it in place until having joined RF and CF.

Another reason to not rivet it in place early in the build are the stop collars. They also are fairly difficult to install with the torque tube in place.

Even with the short elevator torque tube removed, some of the rivets were really difficult to set (at least with my riveting tools). The backseat strengthening channel (CF-CHL-039-C-C-) forward of the torque tube is in the way. Do yourself a favor and also cleco that in place until after the fuselage join.

The stop collars which center the long elevator torque tube (CT-AEL-001-C-C-) forward of the landing gear channel are doable with the torque tube in place, but I did it with the flap torque tube removed (see reason below).

Being at it, I also wanted to install the two stop collars that I still had in my bags. Could only be the flap torque tube, right? Not! I later noticed in the manual that the flap torque tube do not have stop collars, but it was to late. I had already unriveted the torque arms, removed the torque tube and drilled the holes for the collars, so I installed them. With approval of TAF I used the Avinox BE61-0512 (that were used for the Service Bulletin #0014) as a suitable replacement to rivet the torque arms back on.

On July 28th 2020 TAF issued Service Bulletin #0018 concerning insufficient swaging of oval sleeves on the rudder cables.

Checking the crimps on my rudder cables, all measurements were out of range. After contacting TAF I was given the options:
1.) recrimp the sleeves myself using a Nicopress swaging tool
2.) have TAF send me a new set of rudder cables

Option 2.) would have meant to uninstall the cables. This would require to turn the fuselage sideways and undrill the rivets for the cable guides through the small inspection panels on the floor (without actually being able to see the rivets other than with my borescope camera) – not really a thing I was keen to do.

Besides other information about cable crimps, TAF has sent me a copy from a Dec. 2013 Kitplanes article by Dan Horton which advices against using cheap copycat crimping tools, because they do not produce consistent crimps. I was lucky being able to borrow an original Nicopress swagging tool (thanks Josef!) as the originals run at ~350€.

So with the original tool in hand I preferred option 1.) and with the help of my Head Supervisor and World’s Best Copilot this job was done within one hour. When visiting Sling 2 builder friend Alois B. in Munich first week of September, I took the tool with me and we were able to also perform the SB #0018 on his Sling 2.

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Luggage Compartment Sound Insulation and Carpets

Time:   26:00 hrs

Work was actually done July 21st till July 29th 2020

Remembering fellow builders my age “lamenting” what a pain it was to do the carpets in the baggage compartment with the side skins on (or even worse with the cannopy installed), I’ve decided to do the soundproofing and the carpets in the baggage compartment now and install the baggage floor, while I still had good access (RF and CF still apart).

It is definitely much easier to do that job standing comfortably on the ground in front of the luggage compartment and have everything at a fairly comfortable workheight compared to kneeing on the back seat, working in a bent forward, twisted overhead position. I can only advise doing it before joining RF and CF.

I started with first installing the parts that cover the stringers. For this work I used a contact spray adhesive, that has a fairly high temperature range. For documentation purposes I am posting the photos here.

I liked the instant strong adhesion here, as no clamping was needed. Any slight imperfection in positioning the part did not really matter, because it would be either invisible (on the inside of the stringer) or be covered by the next to install soundproofing. An experience I’ve made here: it makes things easier, if you rehearse putting the part in place before actually applying the spray adhesive.
… Sorry, no photos here for that part of work …

After I had the stringer parts in place next was to line the upper area of the luggage compartment with soundproofing. Im using selfadhesive 1/4″ SUPER SOUNDPROOFING from AC which has an FAA certificate that it
“…meets or exceeds. the flammability test criteria that is contained in CFR 25.853(a) APPENDIX F, PT l(a)(l)…”

To make templates for the soundproofing (and carpets – see comment below) I just cut some thin flexible cardboard slightly oversize, placed it on the stringer and squeezed the excess into the corner. To put the soundproofing in place I first did an Aceton wipe, pulled the rear bottom corner of the adhesive backing back, positioned it on the stringer and pressed the rear bottom corner in place. Working upwards and forward I pulled more and more of the backing off.

For glueing the carpet onto the sound proofing I used a PU construction adhesive I could source locally. It did not have the highest temperature range (-30 to +80°C) but it was the only one which explicitly stated suitability for Aluminum. With a fairly thick consistency applying it with a cartrige makes this a “no mess” job even when working overhead ( … as long as you do not to touch anything else with it 😉 …).

I applied the adhesive to both, the soundproofing and the carpet. For the soundproofing I sparingly spread it full surface with a spatula, richer at the edges. On the back of the carpet I laid a roughly 4 cm crosshetch pattern with the cartrige leaving out ~2 cm at the edges. Doing so allowed me to put the carpets in place without making a mess. Except for that narrow overlap at the parachute box I did not have to fixate the carpets.

A word of warning here:
Whatever adhesive you use – do a test first. Do not use contact adhesives on SUPER SOUNDPROOFING.

The producer of SUPER SOUNDPROOFING describes the product as a “… closed cell vinyl/nitrile insulating material, which will not absorb water or oil…” .
I noticed that my spray adhesive will dissolve the top layer – a µm thin foil. It will not attack the foam, just the foil, whose main purpose (I think) is to inhibit moisture intrusion.

When doing my tests it was not really obvious that the foil had dissolved. The contact adhesive spray I am using disperses in fine droplets, which dissolved the foil into small lumps hardly different than the droplets themselves. Just when accidentily touching the glue at some stage I pulled off a patch of foil. I assume the thin foil to be PVC since the contact spray adhesive is not suitable for that (see photo above)

To be able to continue with carpeting Paul next riveted the luggage floor in place. I am glad he did that work, which I have dreaded a little for a while. One can reach back there with only one arm in the narrow gap underneath the floor and there is hardly enough space for the pneumatic rivet gun. It all worked better than exspected.

Since I expect someone climbing back there someday I have added small brackets (red circles in photo above) to stiffen up the area, where the luggage floor stringers rest on the bulkhead. These brackets also made it easier to set the rearmost rivets, which connect stringers and bulkhead.

Once the luggage floor was in place I could finish with what I had left.

I could not find the lock for the luggage compartment door and I don’t have weather proofing material to seal the door yet.

I would like to have a lock that uses the same key as the cabin door. Most likely I will have to fabricate some sort of striker plate for that and therefor did not yet install a strip of carpet above the door.

The grey gaps will not be visible any more once I have the door seal and that last stripe of carpet in place.

Unfortunately, at the time when I ordered my kit, TAF did not consider the fact that I do not have a parachute box. Many of the carpets had the cutouts for the parachute box and I was able to make a couple pieces frome these unusable parts. TAF will send me carpets for the bulkhead and the floor, when I have my cowling shipped in the near future.

Overall I am quite happy with the outcome so far. It’s amazing, what a difference the soundproofing makes.

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RF Top Skins

Time:   24:30 hrs

Once upon a time there was a Sling 4 builder, who wanted to finish the rear fuselage ….

OK, ….. let me start another way:
More than two years ago I wanted to finish the RF by riveting on the top skins, but some rivetlines were off by >3mm and at the rearmost rib (601) not a single rivet hole even overlapped. The details can be found here: RF Testfit of Top Skins

Fast move forward two years:
After discussing the problem with technical, TAF has sent me

  • new RF top skins that were 3mm longer at the overlap with undrilled rivet lines, where there was a mismatch and
  • undrilled longerons 301 and 302 (at the overlap between side and top skins)

I first matchdrilled the undrilled longerons 301 and 302 to my side skins. The starboard side longeron had the taper at the front pointing the wrong direction. After consulting with TAF, I cut the taper on the correct side and fabricated a doubler inside the longeron.

RF Longeron 301

During the very early stages of my build, when browsing through the RF Factory build photos on Craig’s build blog (Thanks Craig – they still are a big help at times!) I noted this small detail.

RF Shoulder Harness Mod (4)

At first I thought I was missing some parts and contacted TAF. They told me, that this was a special order for a costumer, who wanted a shoulder harness but they could send them to me. This mod of course has to be installed before closing the RF.

RF Shoulder Harness Mod (8)

RF Shoulder Harness Mod (9)

Above photo I think gives a pretty good idea, how the mod works. The harness bolts to that eyeplate at the baggage bulkhead.

The standard sling 4 never had a shoulder harness for the back seat passenger safety belts. I have seen the slot for the eyeplate on current TSi build photos, but the eyeplate was not in place, so I not sure, wether this mod is standard on the TSi now.

Once the shoulder harness mod was in place, I started matchdrilling the top skins. Actually I did not matchdrill, but used a modified matchdrilling procedure to match the rivet holes of the top skins, the side skins and the longerons at the overlap area.

With all other rivet lines already drilled in my new skins (and fitting well to my RF ribs), I was able to cleco the new top skins together and to the ribs. But instead  of overlapping the side skins, I slid them inside behind the side skins, marked and drilled 5 resp. 6 holes at the rib positions. I then slid the longerons in place and clecoed them at these holes thus sandwiching the top skin between the longeron and the side skins.

Marking and drilling holes between the ribs then allowed me to cleco the top skin in place firmly sandwiched between the side skins and the longerons. This now allowed me to exaktly mark all the other holes. I drilled those holes slightly high to achieve a tight fit of the skins.

For the missing rivet holes at the rearmost rib (601) I fabricated a little drill jig to dublicate the rivet holes to the top skins.

Match drilling with the top skins clecoed in place resulted in an almost perfect fit when it was time for a testfit. There was only one spot where minute bulging was visible, which may need some attention again.

RF Top Skin Testfit (2)

RF Top Skin Testfit (3)

When testfitting the top skins I noticed that my cabletray was obviously a narrower version, than is currently used with the newer skins.

RF Cable Tray (1)

Though I will not install a parachute (and therefor will cut the cables off for weight), I still will install the cable tray because it does structurally stiffen the top skins significantly.

At first I thought of fabricating a wider version myself (using my old top skins),
but then I decided to try and bend the tapered sidewalls apart.

Bending the sidewalls apart while keeping the rivet lines parallel creates twisted complex 3D compound curves, both on the side walls and the flange. Making twisted flanges, that fit to the top skin while simultaniously achieving the target distance between the parallel rivet lines along their entire lenght on both sides required some patience and time.

RF Cable Tray (3)

But I’m quite happy with the outcome and it still took a little less time, than fabricating a new tray. And I think it even looks better than the original 😉 .

Next I had to take care of  the antenna brackets. Our cocktail bar …ah … parachute box is in the original location behind the luggage bulk head and the antenna behind the box. When I started building the RF, I already have installed the conduit for the RG58 antenna cable delivered with the kit for that position.

TAF has later moved the box and antenna forward and the antenna bracket was made significantly stronger. TAF has later also sent me the stronger version, so I have both antenna brackets now.

The RG58 would do the job for COM, but higher quality RG400 seems to be the standard now. I decided to put the antenna in the more forward position because of the shorter cable run and  better accessability (via the cocktail bar) using an RG400 cable.

Having both antenna brackets, I decided to just leave the conduit and RG58 in place as a reserve. The next photos show the routing of the conduits and  final location of both antenna brackets.

RF COM Antenna Cable Routing (2)

Note: the white cable above is just to document the cable run and will be replaced with an RG 400 later

RF COM Antenna Cable Routing (4)

The top skins act  as a ground plane for the COM antenna. It is important to have a good electrical connection between the antenna mount and the top skin. I therefore did not prime the brackets and skins in that place and alodined them on the inside.

RF Top Skin Primed

Today I did a (DC-) resistance measurement and I am not so sure any more if alodining was a very good idea. The Ω reading was 2 to 5 times higher compared to raw 6061-T6. Nothing like anodized aluminum, which is a good insulator but still. I’m aware this is all very unscientific and the electrical connection may behave much different at VHF.

I’ll leave it at that for now, but it is definitely something to keep in mind, should I ever have problems with transmission or reception quality of the COM. Since I still have access to the antenna bracket via the cocktail bar, I can drill out the rivnuts, sand the alodine off  at the rivnut holes and set new rivnuts.

Before riveting the top skins I also remembered that all fellow builders whose blogs I’m following have mentioned that they have doubled the number of rivets at the joggle where the composite canopy joins the RF top skins. Most of them also have upsized to 4mm rivets at least in the upper part of the arch. I remember that this has been ok’d by TAF. Since it is much easier doing it on the workbench, I drilled all these extra holes now – just 3,2 mm  for now, which I will upsize, when matchdrilling the canopy to the top skins.

RF Top Skin Testfit (4)

After all those preparations the fun part, namely riveting on the top skins was quick and easy.

RF Top Skins Riveted (1)

RF Top Skins Riveted (2)

I could not find the small antenna mounting plate in my parts bags so I could not set the rivnuts yet and had to leave that area unriveted.

RF Antenna Mounting Plate

Observant  readers propably will notice the bulge at the top of the arch immediately. The bulging occured already before  I attached the cable tray, so it definitely is not a side effect of me bending the tray incorrectly. I have tried a little, but any effort to get rid of it would cause wrinkles somewhere else (and would propably drive me crazy).

Again, reading fellow builder’s blogs paid off here. I think it was on Peter C.’s blog, where I read about a trick first when fitting the canopy. Thanks Peter! He mentioned that he firmly had to push the canopy upwards to get a decent fit at the top of the arch – even using some “brute force” methods like using a car jack 😉 .

Remembering this I tried to push the rib upwards at the top by hand and voila –  the bulge was almost gone.

When the skins were riveted, Paul tried to install the luggage door frame the next day.
No matter how he tried to install it, he always had some bulging or wrinkles somewhere.

When I looked into it a day later, it still took me another half hour to find out just by pure luck. The two halves of the door frame spacer were slightly to long at the top (~1mm ; exaggerated for photo sake below).

RF Luggage Door Frame

The oversize was small enough to still allow fitting the frame with clecoes except for one or two rivet holes. But as already said, it would cause bulging or creases – depending on where one started clecoing. Removing just a mm immediatly solved the problem.

I have used Sikaflex to seal the door frame spacer against the skins and the door frame.
Being on my own, working with Sikaflex was a mess here and cleanup took a while.

The luggage door itself is delivered flat and has to be bent to match the curvature of the fuselage. The plans recommandation to ” …hold the door over your thigh or a suitable high density foam roller and press firmly to create a gradual bend. … “ did not work well for me. The door is fairly stiff and it was difficult to get the bend in the right place using this method.

Here’s what worked for me using things I had lying around:

RF Luggage Door Bending

I used different thickness shims to clamp a wood block to the workbench (with  an offset from the workbench edge) thus creating a wedged gap. After shooting the photo I increased the wedge (appr. 10mm gap at front, 2mm at the back end). The edge of the workbench sets the position of a bend, the wedge defines the curvature.

Sliding the door into the gap and evenly  pressing down with flat hands gave very good control about the position and amount of bend. Once I had this fixture it took about 15 minutes and four or five iterations, to get below fit (which I am happy with).

RF Luggage Door Testfit (2)

It’s hard to tell from the photo below, but:
the top corners are slightly overbent and the vertical sides are slightly underbent. After adding a 1-2 mm foam insulation layer for weather proofing to the inside of the door, the door latch will pull the door flush with the skin when closed.

RF Luggage Door Riveted

Overall I am quite happy how the door came out. I still have to install a striker plate for the door latch.

One more thing I did is getting back to the RF luggage floor. That’s been already finished, but I did not like the tie down rings I’ve used very much. When I saw the tie down system, that Jim P. was using (thanks Jim!), I was sold. The system is called L Track or Airline Track Anchor Rail System

On Ebay I was able to source a kit for 55€ consisting of 4x1m rails, 10 tie down fittings, 8 end caps and a glue cartridge. I bought the slim version of the rail (25 mm wide x 10mm high)

RF Baggage Tie Down System

What I like about it is that one can position the tie down rings in the rail anywhere you need them and they are removable. I am installing the rail with 4mm countersunk rivets plus glue. The rail at the back can only be installed after the luggage floor is installed, which I will do after the fuselage join.

RF Baggage Tie Down System (2)

Please note that the line of 6.35mm (sic!) holes in the luggage floor channel is not required, when the backseat mod is applied (those Sling 4 that have the parachute box forward of the baggage bulk head)

RF Luggage Shelf Support Bracket (1)


I have also stiffened up the luggage shelf since I expect someone climbing back there someday to access the “cocktail bar”.

RF Luggage Shelf Support Bracket

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Motivation Photos

Shortly after New Year fellow member of my aero club and RV12 builder friend Wolfgang invited me for a flight in his RV12 (Wolfgang’s RV12), for which he has received the airworthiness certificate last June.

Flying on a cristal clear winter day here in the Austrian Alps is one of the most beautiful flying experience one can imagine, so no way I could refuse this generous offer.

So here are a few motivation pictures:

2020 Flug mit Wolfgang (1)

2020 Flug mit Wolfgang (2)

2020 Flug mit Wolfgang (3)

Thanks Wolfgang for offering this flight!

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Three years after …

Time:   8:00 hrs

Work was actually done Dec. 27th 2019 till Dec. 31th 2019

With aileron and flap aligned, the left wing is mostly finished too. This is almost exactly three years after taking shipment of my kit Nov. 16th 2016

With all the AirVors crew being home for Christmas, I had helping hands to lift the wing off the workbench and put it into storage.

First we had to take the rear fuselage out of storage again to gain storage space for the wing on the rigtht side of the garage. Then we lifted the wing off the workbench, put it leading edge down on soft styro foam and bolted the jig to the far end wall with three wall plugs.

wnl wing Storage (1).jpg

Son Paul had this great idea, which made building a wing cradle or wing dolly unnecessary and also saves some space.

To protect the wing from accidental damage, we again built a blind wall and a small workbench from plywood out of the crates the kit originally was shipped in. Again I painted the blind wall white (with the cheapest wall paint I could find). Though I have bright led ceiling lights the white side walls really improve illumination of the workspace significantly.

We also shortened the legs to lower my two workbenches and make work on the fuselage easier. Lastly son Klaus and his friend helped me to carry the center fuselage upstairs from the basement and carry it to the garage.

Below photos show my rearanged “hangar”. The left wing, vertical stabilizer and horizontal stabilizer are stored behind the blind wall on the left side of the photo, the right wing behind the right.

I could have stored the elevator on top of the right wing, but (with consent of the World’s Best Copilot) putting it on top of the wardrobe in the master bedroom was much easier 😉 (it’s hardly visible).

WRKSHP Wing Storage (3)

WNL Wing Storage (3)

It takes some organizing, but yes, a Sling 4 can be built in single garage – provided it is long enough. This unfortunately (being “only” 5,9m long) is not true in my case.

For the time being I will be able to finish work on the fuselage, but when time comes to hang the engine and fit the cowling, I will have to exchange the window on the back wall for a door. That’s ok for me, as my garage currently does not have a second entrance. I will then let the tailcone protrude out of the door by ~0,5 m.

With the wings now in storage,  open items before paint will be:

    • fill rivet heads
    • install rivnuts for service panels
    • pull pitot and AOA lines

Open items after paint will be:

    • install taxi and landing lights and lens and connect
    • install nav/pos/strobe lightand connect
    • install push rods

Putting the top skins on the rear fuselage and joining the rear and center fuselage will be our next jobs.

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Right Wing Aileron and Flap

Time:  7:00 hrs

Work was actually done Nov. 28th 2019 till Nov. 30th 2019

The method I used on the left wing is somewhat tedious, so I tried another method on the right wing, which was much faster, even though the aileron did not align immediately and also required adjustment (the left aileron did not need any adjustment).

I am therefore describing here the procedure I used this time:

First thing to do is to rivet the flat parts of the aileron. Do not rivet the holes in the rounded areas yet. Then you need to check, how well the aileron aligns at the trailing edge using the prepunched rivet holes.

To lock the twist for the alignment check, I’ve removed the stems from five rivets, pushed the stemless rivets into place and held them in place with Ducktape (or painters tape will do).

As it takes only 1/10th of mm to cause a misalignmnet of 3 or  4 mm at the trailing edge, to my experience just using Ducktape alone does not lock the twist enough, as it leaves some wiggle room for lateral movement.

wnl Align Aileron (1)

Then I mounted the aileron (notice the painter tape, where the five rivets are) …

WNL Align Aileron (1)

… and measured the misalignment using an appropriate drill bit (from my drill bit set).

WNL Align Aileron (3)

I overcompensated here (0,5 mm thicker drill bit) to allow for some springback.

Then I put the aileron on my workbench, removed the five stemless rivets, placed that very drill bit at the oposite end to shim the trailing edge (and thus correct for the misalignment, as measured with this drill bit before).

Weighing down the inbord and outboard end of the trailing edge, I took a look of how much the leading edge rivet line was out of alignment. This was laterally off by only ~0.1 mm, so within tolerances to still use the 3,2 mm rivets.

I therefore reamed out two holes and set these two rivets.  I then mounted the aileron again to check alignment, which was near perfect. So I took it off again, put it on the workbench like before and continued reaming out and setting the other rivets.

WNL Align Aileron (5)

With the leading edge riveted, I installed the aileron again and clamped it at the wingtip end.

Next I prepared the flap in the same fashion: insert five stemless rivets to temporarily lock the twist for measuring the misalignment and flip the flap over to find a shim with correct thickness.

WNL Align Flap (8)

WNL Align Flap (7)

Flip flap back again, clamp at the inbord end and measure how much the alignment is off  (used an appropriate drill bit again).

WNL Align Flap (4)

Flip flap over again, remove the stemless rivets, increase thickness of shim by adding the drillbit, weigh down both ends to set the twist and set one rivet. Then flip back, clamp the flap and check alignment. Continue riveting if alignment is ok, which it was at that point (or drill out rivet and adjust accordingly).

WNL Align Flap (9)

Seems, like I’ve become a little slapdash here and riveted away without checking once more – which I probably should have. The alignment is not as perfect as on the left wing, but it’s not off enough to warant undrilling all rivets again.

In hindsight looking at one of above photos I should have overcompensated also by 1 mm instead of the 0,5 mm, since the flap is much longer than the aileron.

WNL Align Flap (10)

When aligning the aileron and the flap I observed that both, the aileron and the flap have a hump on the bottom side at the trailing edge even though the skins are in line.

wnl aileron Flap Trailing Edge Hump

I was worried about that and contacted technical at TAF. Technical informed me that this will be found on all Sling 2 and 4 and that I need not be concerned about directional stability, drag or stall behaviour as all flight testing had been done with flaps and ailerons like this.

Before putting the wing into storage, I twisted the ldg/taxi light wires and pulled them into the conduit. Remember to add some lenght, as the wire gets shorter by twisting (if memory serves me right, I think I added ~10%).

WNL Twisting Wire


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Left Wing End Rib and Wingtip

Time:  13:30 hrs

Work was actually done Nov. 9th 2019 till Nov. 24th 2019

With the help of my younger son Klaus and some of his friends (thanks guys!) we repositioned the wing front to back and upside down to make work on the wing tip easier.

After removing the wing tip jig I first installed the end rib (WG-RIB-011-R-A-). This required some adjustment of three flanges that were slightly incorrectly bent, but after that the fit was perfect. Like on the right wing I fabricated a bracket for the conduit and pulled the nav/strobe/pos light cable.

WNL Wing Tip (1)

Upon a first testfit of the wing tip the fit and alignment was quite good, but the wingtip was short at the aft end by 8-10 mm. So in the sense of  FAR §21.191 (g) (“….solely for their own education or recreation…”) it was time for my first composite layup.

After sanding off the gelcoat / primer and cleaning with an acetone wipe I built a dam out of stiff plastic.

WNL Wing Tip Epoxy Work (1)

First  I cut thin fiberglass mat which I wetted with epoxy resin using a paint brush. I then layed theese out on a sheet of plastic in a staggered manner to achieve a smooth transition. After adding another ply of plastic I squeeged out the air bubbles and excess resin, removed the plastic again, transfered the layup to the wing tip and weighed it down to get a good bond.

WNL Wing Tip Epoxy Work (3)

WNL Wing Tip Epoxy Work (4)

After two days of curing I turned the wing tip over and made a layup on the other side and let it harden again. Before sanding the layup into a smooth transition I brushed on a layer of resin to get rid of some pinholes.

Making the layups was actually was quite fun and quick. Not so the sanding to achieve a smooth transition, which took two or three hours and another layer of epoxy resin.

The translucent area in above photo shows, who much the wingtip was to short.

Again like on the right wing I added some aluminum reinforcement strips to the inside of the wing tip with Sikaflex 252i (underneath the recessed area, where the tip is rivetted to the wing skin).

WNL Wing Tip (8)

After installing the mounting brackets for nav/position/strobe lights, some sanding of the recess, edge forming and light bending of the wing skin I was quite happy with the fit and rivetted the wing tip in place (again sealing it with a thin layer of Sikaflex).

WNL Wing Tip (5)

Again like on the right wing I could not get perfect alignment along a small area near the leading edge, but a little body work will take care of that, when it comes to painting.

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Left Wing Tank

Time:  42:15 hrs

Work was actually done May 18th 2019 and Oct. 19th 2019 till Nov. 7th 2019

As with the wing structure much of the preparation work and part of building the tank had already been done together with the right wing tank. When we had small Proseal jobs to do on the right wing tank, we also prepared for doing some small jobs on the left wing tank too, so not to unnecessarily throw away partially used Proseal cartriges.

Installing the stringer, tank ribs, flange plates and fittings was actually done May 18th 2019, but I am logging the time here.

WNL Fuel Sender Flange Plate
WNL Fuel Sender Flange Plate

WNL Fuel Tank Ribs Prosealed
WNL Fuel Tank Ribs Prosealed

One fuel tank mounting channel (WG-BKT-010-X-A- shown in the photo below) had caused me some head scratching on the right wing tank. Sure enough I had the same problem on the left wing tank.

Here is the reason:

WNL Fuel Tank Mounting Channels (1)

If one bolts the mounting channel the plans way to the main spar as depicted above, it would let the channel sit on top of the neighbouring leading edge rib (WG-RIB-106-R-A-) causing misalignment issues, when it comes to riveting the tank to the main spar.

Rotating the channel 180° solves the problem.

WNL Fuel Tank Mounting Channels (2)

This is a real mean problem, because at no time one has visibility at the offending area – not even with an inspection camera. Just a wild guess and trial and error let me find this out to be the reason for the misalignment when testfitting the right wing tank.

Another thing to check before closing the tank was, how the fuel tank back plate aligns with the main spar.

WNL Fuel Tank Mounting Channels (5)

The flange of the back plate should transition nicely into the wing skin, because the tank skin sits on top of the wing skin. I had to rebend the flange a little at the top inbord end.

Like on the right wing I also adjusted the travel stops of the fuel sender arm before closing the tank, which would be difficult once the tank is closed. I also shortened the transducer wire and check continuity of the transducer reading.

WNL Fuel Sender (2)WNL Fuel Sender (1)

One last testfit before finally closing the fuel tank …

WNL Fuel Tank Test Fit(1)

WNL Fuel Tank Test Fit (4)

Closing the tank went well. Because the testfit was really good, this time (unlike the right wing) we did not turn the wing vertically for closing the tank. We again used Saran wrap to protect the main spar from oozing out Proseal (which would make removing the tank for the leak test difficult).

After closing the tank (with the temperature being fairly low already) I wrapped the tank with two electric heating blankets and bubble wrap to let the Proseal cure for a week.

WNL Fuel Tank Closed (1)

Closing the tank went well? – well not quite!
Here’s a little riddle: what’s wrong in the next picture! (sorry for the blurry photo)

WNL Fuel Tank Leak Test (3)

The fuel breather tube is on the wrong side! It should be inside the tank! When I wanted to install the ventline to perform the leak test, the breather tube was still in the same bag. I had forgotten to install it before closing the tank.

I strongly suggest to install the breather tube together with the fitting. Should this dumb mistake happen to you too,  contact me and I can talk you through the process how to install it without reopening the tank again.

This was tricky and cost me a couple hours of work, but I have managed to install it through the fuel cap flange after the tank had been closed.

WNL Fuel Breather Liine

To perform the leak test the fuel sender had to be installed. For detail notes please have a look at the Right Wing Tank blog entry. With my not so successful first Right Wing Fuel Tank Leak Test doing the leak test on the left made me a little nervous, but all went well.

WNL Fuel Tank Leak Test (1)

Again I was amazed, how sensitive this primitive manometer is.

Notice how the rising air pressure corresponds with the falling water column.
(Note: I did not care to correct the readings for variation in room temperature)

WNL Fuel Tank Leak Test (2)

Happy with the outcome of the leakt test, mounting the tank to the wing went smooth.

WNL Fuel Tank On (1)

The double row of black clecos are the rivet holes, where Service Bulletin #0014 mandates to use stainless steel rivets now. These shall be installed with some corrosion preventing coating (e.g. Proseal).

Since I am using a sealant gun and Proseal cartriges I did not want to waste a whole cartrige for this little job. Not having an accurate scale with a 0,1 g resolution at hand, I improvised a method to mix small amounts of Proseal at a time.

I took the plunger out of the cartrige and made a scale of  10, both on the plunger and the cartrige.

WNL Proseal Cartrige

Just squeeze out equal parts of the white and black stuff and you automatically have the correct ratio.

WNL Fuel Tank On (2).jpg

Making the fuel tank inner nose rib (WG-RIB-100-R-C- not yet installed in above photo) fit required some patience because the flanges were not bent quite correctly. This same nose rib still needs to be installed on the right wing.

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Left Wing Structure

Time: 112:45 hrs

Work was actually done Aug. 26th 2019 till Oct 16th 2019

With the right wing in storage work was started on the left wing. Since all the prep work (deburring, scotchbriting, priming …) and  riveting the subassemblies had already been done together with the right wing, doing the left wing went much faster than the right.

Of course the experience gained on the right wing like how to best fit the skins to the structure helped a lot too.

I do not remember anything noteworthy doing the left wing structure and I have not taken any pictures. As with the right wing I deviated from the plans and also ran conduits for the cabeling and the pitot and AOA lines.

WNL Conduits (4)

Most of  the brackets, that hold the conduits in place I fabricated as doublers for the mainspar stiffener channels. Here are some photos, how I routed and attached the conduits.

WNL Conduits (6).jpg

WNL Conduits (7)

As visible in above photo I am installing a Garmin GAP 26 regulated heated pitot / AOA. The current G3X installation manual (version AL) specifies a minimum length of 8 inches aluminum tube for the GAP 26 between the probe and any non-metallic tubing connected to it. This is to avoid exposing the tubing to excessive heat. I seem to remember, that an older version mentioned not to cut the tubes at all. I just left the tubes uncut and hand bent the tubes of the probe such that they run straight into the conduits.

WNL Conduits (10)

The next photo shows the bottom conduit traversing to the front of the main spar for the landing and taxi lights and the top conduit for the nav/pos/strobe running to the wing tip.

WNL Conduits (11)

Since the temperature controller must be installed within two feet of the pitot probe, it is not possible to put it upfront behind the instrument panel. I have therefore fabricated a bracket with 4mm rivnuts to hold the controller. This bracket also acts as a doubler for the small stiffener channel attached to the main spar. The temperature controller and the wiring are within easy reach through the nearby service panel.

WNL Conduits (8)

WNL Conduits (9)

The spade connectors are just loosly hooked up for the photo as is the pitot mast that is just clecoed to the pitot mount brackets temporarily; I have removed the pitot mast and pitot head to prevent damage when later putting the wing in storage.

The pitot mast will later be installed using 4 mm bolts and a spacer. Thus  I will be able to remove the whole pitot mast and probe even with the bent aluminum tubes.

The two following (poor) photos from my mobile are the only ones I have of the left wing before mounting the fuel tank.

WNL Skins Final Fit (1)

WNL Skins Final Fit (2)

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Right Wing Aileron and Flap

Time: 24:00 hrs

Work was actually done Aug. 18th 2019 till Aug 20th 2019

One thing Sling 4 builders have to take care of is to get the trailing edges of flaps and ailerons in line with the trailing edges of wingtip to wingroot (I am not sure, wether Sling 2 builders or Sling 4 TSI builder have to deal with that).

Using the prepunched rivet lines at the leading edge overlap of flaps and ailerons will result in ~13 mm flaps low misalignment.

WNR Aligning Aileron Flap (12)

I would like to give credit to Peter C., who I think was the first to document the problem in his 3-5-2016 blog entry. Thanks Peter!

I have also documented the problem here and here.

TAF later added a recommendation in a plans revision to hold off riveting the ailerons and flaps until the wings are completely finished.

When I started building the flaps and ailerorns, I already had that information and now with the right wing finished, it was the time to take the flaps and ailerorns out of storage again.

Paul had removed the blue plastic, scotchbrited and primed the inside of the skins and the rivet lines of  the flap and  aileron. For an initial alignment check, we first riveted the flap and aileron except for the leading edge rivet line and the three to four forwardmost rivets on the top and bottom.

AIL Left Riveting (1)

Since it is not possible, to use clecoes at the leading edge for a testfit, I ducktaped the leading edge skin ovelap of aileron and flap. In hindsight just taping a couple rivets in place with the stem remmoved should work better.

Next I clamped the aileron to the wing end rib, the flap to the wing step and pulled a string between these two point.

WNR Aligning Aileron Flap (5)

WNR Aligning Aileron Flap (3)

I do not have a photo showing the initial (mis)alignment situation. I was lucky here, because the aileron was exactly  in line. So Paul finished riveting the aileron skin including the leading edge (not forgetting to riveted the aileron balancing tube in place first). This makes the aileron very rigid and it will not twist any longer.

Next we again clamped the aileron to the wing end rib as shown above and we also clamped the flap to the aileron, which nicely aligned the two.

WNR Aligning Aileron Flap (8)

Aligning the trailing edge will slightly twist the flap and shift the top and bottom skin against each other creating an offset of  approx. 1mm between the two (exagerated for clarity in the photoshopped picture below)

WNR Aligning Aileron Flap (4_1).jpg

I then marked this area  and flipped the flap over. I weighed it down on the inbord end and shimmed it at the outbord end to exactly show the marked area thus setting the flap twist.

WNR Aligning Aileron Flap (6)

I then mostly filed away the offset area, flipped it back and clamped it again into alignment to check how much more offset needed to be filed away.

After repeating this procedure twice, the upper and lower skin were flush at the overlap in both positions (flipped and clamped), which indicated that the twist was set correctly with the shims.

Now, with the flap folded back in the shimmed position, setting one single rivet close to the outbord end of the flaps was easy. This locks the twist and a check (now without the center clamp) showed the flap and aileron nicely in line.

WNR Aligning Aileron Flap (1)

I strapped the skins together with a couple stripes of ducktape (to get the rivet holes horizontally aligned), reamed out a couple rivet holes to 4mm (because of the lateral offset the 3,2mm holes would have to be elongated to appr. 4mm) and set those rivets.

Checking the alignment again and slightly adjusting the shimming then let me rivet the rest of the flap. There are still some small open items like rivnuts for the inspection panels, pulling the wires for and installing the landing and taxi lights.

At that a mandatory service bulletin (Service Bulletin #0014 Rev. 1- Rivet replacement on centre and wing spars) was issued, which mandates replacing part of the mainspar aluminum rivets with stronger stainless steel rivets.

With the flaps and ailerons aligned and not knowing when I would receive the rivets for the SB I called the wing finished for the time being and with the help of some of Pauls friends we put the wing into storage

WNR Wing Tip (9)

The wing is now stored behind the fake wall on the left side made from the plywood crates in which  the kit was delievered.

I will address the SB #0014 and the couple open items before test fitting the wings onto the fuselage.

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