Pitot Installation Complete, Electrical Checks OK

Although I’ve had it for a while, I never finished the pitot tube installation on the left wing. It was one of few remaining tasks to do before I close out the wing. I finished up wiring the heater controller module, then took care of the plumbing.

Pitot lines carefully bent using the same tool I used for the fuel tank breather lines. To achieve the gentle radius, I had to bend the pipe a little bit at several points.
Wiring all dressed up and secured. Deutsch DT connector used as a disconnect that will be accessible from the inspection hole. The wiring is staked down to ensure it doesn’t interfere with the aileron torque tube that comes later (with fuselage kit?)

There are a few solutions out there to link together the 3/16″ aluminum tubes from the pitot with the 1/4″ plastic tubing that goes to the fuselage; I chose to use AN style fittings on the aluminum tube, screwed into these nylon fittings that I got from McMaster. BTW, I also bought nylon tubing from McMaster to use instead of the clear vinyl tubing supplied with the sling kit.

I like these fittings because they form an interference fit on the threads, and the screw-on nut includes an o-ring and a metal gripping ring that bites into the tube to form a tight seal.

Although these nylon fittings have NPTF-type “dry seal” threads, the mating threads on the AN fitting are the NPT variety, so a thread sealer is recommended. Unfortunately I can’t use the Loctite 577 that I would normally use (it’s only for metal fittings), so a search for a thread sealer suitable for both plastic & metal fittings led me to a product from Rectorseal, T Plus 2. It’s sold at Lowes for a few bucks.

I applied Loctite 577 to the AN fitting nut, and TP2 to the threads for the nylon fitting. Leak checks were negative, with a balloon staying inflated for 24 hours. I tied together the ends of the blue and green tubing using a short section of the vinyl tubing that was originally supplied with the kit; heating it up with a heat gun made it flexible enough to fit over the stiffer nylon tube.

Soapy water sprayed onto the pressurized tubes, no bubbles
Balloon covering both the pitot inlet and the AoA sensing ports

I did electrical checks next, to ensure wiring to the lights and pitot heater were good. Lights are pretty straightforward to check. Although these Kuntzelman LTR lights are bright, I don’t like how they use a threaded lug for ground/return, which is the same lug that’s used to hold the light in place — this makes the whole wing become a return path, and I’m afraid that could lead to electrical noise problems later. I ran a dedicated return wire for the lights, so I’ll have to figure out a way to keep the lights electrically insulated from the structure. Nonethess, the lights worked fine. I used a bench top adjustable power supply to apply 12V, and according to the ammeter on my power supply, these lights draw about 1.1 amps each, pretty efficient for the amount of light they throw out!

Soon these will light up a dark runway
Bright!

For the pitot, I had to use a different power supply capable of pushing more amps, so I used a DC switching supply rated up to 10A, then I used a Seek thermal camera that snaps onto my iPhone to watch the temp rise on the pitot.

Right after power is applied to the pitot heater – tip temperature 84°F
The Garmin G3X install manual doesn’t state what the regulated temperature should be, only that the regulator “limits the maximum tip temperature to make the probe safer to the touch.”. I saw a max temp of about 124°F, which should be hot enough to prevent ice accumulation while not burning someone touching it
Drok adjustable power supply that I bought from Amazon a while ago – something I had laying around, nothing fancy

Now with these tasks done, I did some final inspections and vacuumed the inside of the wing before putting on the upper skins.

Primer applied along the rivet lines for the inner top skin
Outer skin cleco’d in place
Inner skin cleco’d in place. Holes for the ribs and spar line up fine, but the holes for the two upper stringers don’t line up so well.

Most of the holes line up ok using clecos, but the alignment is not perfect so I’m going to have to use a chucking reaming to open the holes up, which will create more metal shavings.. which means I have to vacuum one more time. So I’m going to do that, pull the skins off one more time to vacuum up the shavings, then I should be ready to rivet the skins down and move on.

Wing skins test fit

Spent a good amount of time over Labor Day weekend making progress on the left wing.  With all the ribs and spars cleco’d together, the next step is to test fit the top and bottom skins.  I ran out of 5/32″ clecos, so the skins are held on mostly with 3/32″ size clecos going into the ribs.

Top skins test fitting

Nose skin added

The skins on the root side of the wing fit pretty well, but at the wingtip end I started to see the holes walking away from the mating holes on wing spar – I think this is due to a joint on the main spar that’s held together with some plates, rather than being one continuous piece of metal.  As a result, I’ll have to ream out a few holes towards the wing tips on both the top and bottom skins.

The manual says the skin fit is good when the skins sit nice and flat on the ribs – which they are.  I wish I could just start riveting the skins on now – but the next step is to take the skins off and start riveting the ribs onto the spars. That leads me to my next challenge – some of the rivet locations for the ribs are very difficult to access!  I am categorizing the difficultly in 3 ways:

  1. Some are difficult because they are in a tight spot, like right next to a wall, and you just can’t get the rivet puller in there cleanly.  Some other builders recommended using this tight-quarters rivet pulling tool ($20) from Cleaveland Tool Supply – I think this thing is mandatory!  Unfortunately I didn’t realize this until now, so I have to wait a few days to receive it in the mail.  I may try to make a temporary one in the interim.  This tool bends the rivet madrel out about 10 degrees, and gives the rivet puller a flat surface to push up against.

    Close Quarter Rivet Pulling Wedge
  2. Others are made difficult by the order-of-operation in the build.  I had the pitot/AoA tubing and wiring all nice and neatly routed through the step ribs on the root of the wing as per pg 11 in the manual, only to realize that this blocks access to the rivets… so I had to take it all out.  Luckily I hadn’t installed the pitot tube yet, or put electrical connectors on the wiring, otherwise this would be a lot more difficult.

    Good luck getting a rivet gun in there with the pneumatic and electrical lines in the way!  You can also see that I’ll need the close quarter tool here since the holes are right next to the wall of the rib, and you can’t access these holes from the opposite side of the main spar.
  3. The last difficulty category is due to me misunderstanding the instructions… or the instructions not being very clear.  Pg 7 and on in the wing manual shows the rear spar skin support attached, but I learned the hard way that this can’t be riveted on until after the ribs are riveted (pg 14), since this support blocks access to the holes for the ribs.  Pg 8 does have some instructions about riveting these holes “last for clearance of the riveting tool,” but it really should say to wait until after the ribs are attached on page 14.

    Can’t get to the circled holes with the rear spar wing skin support (indicated with arrows) in the way!

    I did think about this potential interference when I attached the skin support–but I thought I would be riveting on the ribs from the inside.  So, I had to drill out all these rivets, which thankfully didn’t take too long.

Lesson learned from all this: I have to spend a little more time thinking through the order of build going forward.  I’m also thinking about marking up TAF’s kit assembly instructions and sending it back to them to incorporate into a future revision.  The right wing should go together more easily, now that I know all this.

Cavity for the landing and taxi lights

Since I now have wiring in the wing, I have been giving thought to how to connect everything.  I personally don’t like the cheapo flat crimp connectors you see in retail auto and hardware stores, nor do I want to use solder or wire splices, makes things harder to service.   At work we’d normally use MIL-DTL-38999-style connectors from Amphenol and others, as they’re pretty much the norm for aerospace.  They’re extremely durable with a price to match — one complete male/female set will cost over $50, and the crimping tool is over $500.  After researching, I decided on using Deutsch brand connectors, which are pretty commonly used in the automotive industry.  They are weather-tight housings with solid barrel-style pins that get crimped on 4 or 8 sides, resulting in a very high-quality crimp that should last the life of the plane.  Since they’re so common, they’re also relatively cheap and easy to get – about $7 for one complete male/female pair.  There is still a relatively expensive crimping tool to buy, part # HDT-48-00, but I was able to get an off-brand iWiss tool on eBay for about $150.   I figure I’ll use these connectors all throughout the plane, including the avionics, so I think it’s a good investment.

Deutsch DT series connectors, for 14-16 AWG size wire. They also make a DTM series which is smaller and suitable for 18-20 AWG wires.  This is a 4-position connector that I’ll use for the landing light.

Last item to report: I received a box from TAF with long-awaited replacement parts!  These are parts that were either missing or damaged when I received the kits back in April.  I didn’t need the parts right away, since I had plenty of kit to keep me busy, therefore I was okay with waiting for them to come the slow way.  TAF was really good about sending out new parts without any question/hesitation.  Most of the parts were for the empennage, but I also had two parts from the main wing spars that needed to be replaced as shown in the pic below.

Replacement fuel tank bracket (right) to replace the damaged one (left).

 

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.