Limited time at the shop this week as I concentrated on some home/cabin maintenance that needed to be done.
Pulled the right wing spar assembly apart again for deburring and prime. Took almost an hour just to scuff everything with Scotchbrite, and clean down with lacquer thinner in preparation for prime:
The self etching primer dries fast, but I decided to let everything cure for a couple of days. When I came back to the shop, I was ready to reassemble the spar.
I cleaned off the bench completely in order to make room for the assembly process.
Like the stab, elevator and flapperons, it's important to have a flat "surface" to assemble on. I placed the steel angles on the bench edges and lined them with painters tape to keep their surfaces smooth. Before adding the cross members again, I started the process of riveting the spar back together:
Acceptable practice for rivet placement is that the head of the rivet should be on the surface of the thinnest material being assembled. The centre spar doubler is 063 and the spar web is 032, so I started by first completing all the pulled rivets called out in the plans for the spar. To do this, I flipped the spar over and pulled the rivets from the aft side. I also attached the wing spar tip while in this position:
The root doubler and spar web are both 032, so it doesn't matter which way rivets are pulled and the plans don't have directions on this. I decided for consistency to pull them in the same direction the driven rivets will be along the spar caps:
Driven rivets are called out in the plans for most of the spar and depending on their location they have different lengths before being formed. The length is dependent on what thickness of materials are being joined. We don't have any rivets in stock that are correct length for some of the spar cap/spar web/centre spar doubler interfaces. There aren't many of them, so instead I used a rivet cutter to shorten a few longer ones (made -9 rivets which we have lots of into -7 rivets).
There are very specific standards with regards to properly formed driven (bucked) rivets. The formed tail of the rivet MUST be 1.5 times the diameter of the rivet tail once bucked.
I originally thought I'd be able to use the hand squeezer to form the rivets along the spar caps but I decided to test that theory first on some scrap material. This also confirmed I had the correct length of rivet for the thickness of the material (trust the plans but verify!). I used a piece of aluminum angle and 032 sheet to simulate the spar cap/spar web and discovered the A5 driven rivets are much too hard to squeeze by hand - I couldn't squeeze hard enough on the tool to get the correct formed head dimensions.
Putting the hand squeezer away, I got the air rivet set out and attempted to drive the first rivet. It went much easier than I expected and once I developed a feel for it, I got good at estimating the amount of time on the trigger to set the rivet correctly. The shank of an undriven A5 rivet is 4 mm in diameter, so the formed head needs to be 6mm in diameter.
It takes a bit more time than pulled rivets, but the evolution is the same. Place the rivet in open holes between the clecos, drive/form with the rivet gun/bucking bar then repeat on the next empty hole. I started with the top spar cap and measured each formed rivet for conformity as I went along. Once I had alternating holes done, I removed the clecos and filled in the rest. Next was the spar doubler and strut pickup angles. These were driven from the other side to respect thickness/rivet standards.
Then I repeated the process for the bottom spar cap:
Laying the spar down on the cross tubes of the bench confirms the spar is completely straight and true - very happy as everything that attaches to the spar is relying on this. It's amazing how stiff the assembly is and that without all the clecos it weighs much less too. The wing attach point still needs to be anodized before it gets riveted onto the spar, I'll be doing that this coming week.
Once I have the anodizing done on the wing and strut pick ups done, I'll add them to right wing spar - these have much longer rivets. Then I'll get all the spar cap holes on the left wing spar upsized to A5. Then the process of disassembly, debur, clean, prime and reassembly begins, followed by doing some more buckin' rivets!
Getting closer every day. Thanks for following along!
A couple of really productive days in the shop this past week, but not many photos to share.
Started the final fit-up of the upper strut fittings. The plans call for the strut fittings to extend 107mm from the lower wing skin. To make fit-up correct, I scribed a line 107mm from the rounded tip:
With the strut pickup held in position against the angle, it's a simple matter of lining up the scribed 107mm line with the bottom of the spar. The skin is 020 here, so the line is very slightly past the spar:
I drilled the strut/pickup interface hole as per the plans, but the plans don't really define the spacing of the mounting bolts that attach it to the strut angle. Base doin what I see in the plans, it appears to be evenly spaced, so that's what I went with. It started with A3 pilot holes in the pick-up:
Clamping the strut pick-up tp the strut angle in the correct position. The camera angle makes it look like the scribed line is inside the spar line, but it is actually where it needs to be. From here, I drilled through the strut pickup and into the angle. Once I had a couple of clecos in place, i removed the strut angle from the spar and took both to the drill press to enlarge the 3 pilot holes up to AN3 bolt size (sorry no pictures). The bolts will be added after everything is deburred/primed.
The next step was to take off the strut assemblies, the web and root doublers and L stiffeners to permit the spars to lay completely flat on the drill press. Without these doublers, the spars are fairly stiff, but they are still long and challenging to move around the shop. Must be careful not to introduce any unwanted twist.
To accomplish cutting of the lightening holes on the drill press, I set the spar on a movable workstand at the one end:
Fly-cutter in the drill chuck. The yellow tape flag has a written note on it as a reminder that the cutter was already set for 95mm diameter, but I double checked anyhow. The spar web sits flat on top of a piece of plywood that fits between the bottom of the cleco pins. It supports the back side of the web as the cutter scribes it's circle:
I used a level on the web between the drill press and work stand to ensure the spar was completely flat for drilling:
Started cutting the lightening holes at one end of the spar, then worked inwards to the next, clamping the spar to the plywood and drill press work area. Cutting with the fly-cutter is always an adventure, but securing the piece, lubricating the cutting head with a bit of WD-40 and using slowly increasing pressure goes a long way to making good clean circles.
There are 5 lightening holes inboard of the web doubler and 4 outboard. With the inboard ones done, I flipped the spar end-for-end and drilled the outboard ones. The I repeated this whole (hole?) processes on the 2nd spar.
I'm really pleased how the process I came up with worked out. It wasn't complicated and went fairly quick, but I always have a certain amount of trepidation when using the fly-cutter. If the cutter jams or grabs the material it could damage the web material beyond repair, meaning redoing the entire spar (a very expensive mistake). Thankfully, I didn't have any issues with either spar. Here they are, back on the bench awaiting deburr and flanging of the holes. You can see in the bottom right that I hadn't trimmed the spar cap of the left wing yet, but that has been done since this picture was taken.
With the lightening holes deburred, I followed the same process as the wing ribs. The flanging die and two large C clamps worked well using the corner of the bench to reach from both sides of the spar.
In order to accommodate the length of the spars, I had to switch corners of the bench. Maggie the shop dog/chief inspector was kind enough to move out of the way for me when needed - even if she doesn't look impressed in this picture :)
Even more so than when the lightening holes are cut, it's important that the spar is lying flat for flanging the holes. I used another piece of thin plywood to support the spar at the opposite end.
I did get and answer about the rivet spacing on the spar web doubler from Zenith - my theory was correct and I can shorten the rivet spacing to 20mm without issue.
With most major fit up and drilling complete on the spars, next up will be pulling everything apart again for final deburring edges and holes and priming of the mating surfaces. Then I'll begin the process of reassembly with clecos in preparation for driving and/or squeezing the solid rivets. I want both spars complete and ready for wing ribs and skins. One spar with the assorted ribs and other parts will go into storage while I work on the other to make room on the bench.
Another item on my list is deciding on the best way to prevent dissimilar metal corrosion (sometimes called galvanic reaction corrosion) on the spar and root pickups. I'm considering some DIY anodizing of the aluminum parts using simple chemistry theory. More on this later :)
Thanks for following along!
Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.