Wing skins test fit

Posted On September 4, 2018

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.

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.

   

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.

   

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.

   

   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.

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.

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.