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.
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.
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.
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.
When testfitting the top skins I noticed that my cabletray was obviously a narrower version, than is currently used with the newer skins.
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.
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.
Note: the white cable above is just to document the cable run and will be replaced with an RG 400 later
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.
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.
After all those preparations the fun part, namely riveting on the top skins was quick and easy.
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.
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).
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:
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).
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.
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)
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.
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)
I have also stiffened up the luggage shelf since I expect someone climbing back there someday to access the “cocktail bar”.
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