With the unpackaging, inventorying and storing out of the way it was time to get down to building. I was particularly excited because, having purchased a mostly complete tail cone, I did not have much opportunity to assemble stuff. I gained a bunch of confidence by attending Troy Grover’s two-day class but it’s not the same as putting together something that will someday carry you skyward.
In fact, the first thing you start on with the wing kit (Section 13) is preparing the main wing spars. The only real “assembly” is riveting on a bajillion nut plates and a couple other things–but more about that shortly. First things first, though–it was time to “get acquainted” with the spars. The first thing Van’s instructs you to do is ascertain the correct orientation (top/bottom, left/right, fore/aft) so the instructions that follow will (a) make sense and (b) not cause you to ruin what is clearly the single most expensive component in the kits. After working with the spars for awhile the orientation became obvious but initially I made sure to triple check the plans before marking the spars with a Sharpie.
As you can see above, the spars look absolutely gorgeous in their Alodine coating. It’s a shame they get embedded inside the wing and makes you wish you had some transparent aluminum from Star Trek IV (which apparently is a thing now) so you could show them off. One other aspect that might not be so obvious from the photo is that these bad boys are heavy. After tossing around the entire tail cone like it was a shiny rag doll, hefting these thirteen foot girders on and off the workbench definitely substituted for my normal weight room routine*.
*Which is basically walking by the free weights I bought last year now collecting dust in a corner of the basement.
“Ya Know, Maybe It Should Be Longer”
Once you’ve established the correct orientation the first real assembly task is to rivet small extensions to the ends of each spar. This seems like a curious step since Van’s could easily (?) have just made the spars 8 inches longer, saving a bunch of fitting, drilling, deburring, priming, etc., and the crates the spars ship in are long enough to fit the finished product. My guess is that there is a real engineering reason for doing it this way or it helps comply with the “51% rule” since I can now say I, the manufacturer of record, “completed the spar assembly”.
You can also see from the photos that I elected to prime the parts before final assembly. I had debated this for awhile since the previous builder did not prime the empennage. In the end, though, I decided to start priming parts for a few reasons. First, I noticed that, despite my best efforts to be careful, it is nearly impossible not to add the occasional scratch or scuff while working with some aluminum parts and a bit of extra protection seemed like a good tradeoff against the incremental time and weight. Second, trying to work inside the unprimed empennage was challenging due to the reflections off the bare aluminum. It was like working inside a circus funhouse.
J-Stiffener: Hip-Hop Star or Airplane Part?
With the wing spars now at their final lengths the next step was to use them as drill guides for the j-stiffeners that will eventually get laid horizontally inside the wing ribs. This is accomplished by cutting to size and clamping the two pieces (a ‘long’ one and a ‘short’ one, which overlap) against the main spar flange (with a 1/16″ protrusion) and drilling through the skin attach holes.
This is repeated for the upper and lower stiffeners, making for a lot of drilling. Once I got into a groove, though, it went pretty fast, as you can see below:
The only difference between top and bottom is that there are several holes in the lower flange that do not get drilled. This is because they coincide with the access covers and, presumable, get doublers and nut plates later on. I meticulously marked these holes on the spar and, getting in “a groove” then…proceeded to mistakenly drill a couple of holes anyway. Fortunately I caught myself before drilling all the holes. I figure I can affect a repair (such as just filling the hole with a flush rivet) once I get to the wing skin attach phase. Dummy.
That (Counter)sinking Feeling
After completing the j-stiffeners and setting them aside I moved on to a really fun task–countersinking the spar flanges. (And by ‘fun’ I mean tedious and time consuming.) Again, I find it ironic that one of the first few tasks you tackle with the wing kit is to aggressively remove metal from a massively expensive piece of hardware.
The instructions are fairly straightforward and obvious since, at the end of the day, the wing skins must attach using flush rivets (or, in the case of the fuel tanks, screws) to the entire span. The important thing is to countersink to the right depth, according to the fastener being used. Again, this is fairly obvious once you wrap your head around the layout but there was one sentence in the instructions that I swear took me several hours to decipher:
Now, I consider myself fairly adept at the English language but having to parse phrases like “rib to spar flange attach rivet holes” and “inboard of the most outboard” (all with no assistance from punctuation, mind you) had me stumped. Given it was already getting late I gave up for the night and decided to take another run at it in the morning.
The new day didn’t dawn so brightly, at least as far as comprehending this instruction was concerned. Fortunately I got inspired by reading ahead in the plans and, in the end, had to draw a little diagram to convince myself I understood what to do. Here’s how it breaks down:
- Each wing rib has a tab with two holes that line up with the spar flange. Those are the “rib to spar flange attach rivet holes” referred to in the plans.
- Most of these holes get rivets when the skins are attached and the rivets bind together the skin, rib, and flange.
- However, for the ribs that sit aft of the fuel tank, the forward-most holes (those that are “in line with the nutplate attach rivet holes”) get riveted only to the spar flange, since the fuel tank (and the wing skin which attaches to it) needs to be removable. In these locations the fuel tank skins just sit on top of the rib attach rivets.
- Finally, rather than just say, “the ribs that sit aft of the fuel tank”, the plans describe the locations as those being “inboard of the most outboard fuel tank attach nutplate”.
After the rush of adrenaline from solving this mystery subsided I realized that there are only 7 holes that fit the description laid out in the plans on each side of the spar. Why couldn’t Van’s have just pointed these out in the drawings? There are plenty of other places (for example, on the previous page!) where Van’s gives instructions like, “don’t drill here”. They could have just said, “These are the 7 holes we’re talking about!!” Instead they chose a riddle the Sphinx would have been proud of.
So I (carefully, this time) marked the 28 holes that needed this special treatment and got out the countersink cage, but not before reviewing plans Section 5 for proper countersinking technique. Given the importance of the spar to things like, um, keeping the wings attached to the airplane, and that you can’t “uncountersink”, I wanted to make sure I didn’t get sloppy when drilling the several hundred holes. Van’s also recommend you make some dimpling guides out of scrap aluminum sheet so you can judge the required depth of the countersink. Troy had these available during his class and I’d already planned on making them. Now ended up being the perfect time and you can see an example below (picture taken later, after I’d installed a nut plate):
I’ve included a few pictures below of the countersinking process but there’s nothing much to see (other than the amount of aluminum shavings) that get produced.
Nut Plates Galore!
I’m not quite sure exactly how long it took me to finish the (initial) countersinking. I suppose I could check my build log but, in event, it seemed like weeks! Finally it was time to rivet on the (many) nut plates, both for the fuel tanks and access plates. At this point I can’t say enough how happy I am that I opted for the Cleveland Aircraft Tool pneumatic squeezer. Once I got the depth adjusted the tool made short (and perfect!) work of the hundreds of flush rivets required during this step. It was such a nice change from the “countersink, check depth, check cage, blow away shavings” of an earlier process.
Once the nut plates are installed you are instructed to countersink the screw holes to fit the appropriate flat head screw. One item of note is an instruction to “spot prime” the holes created when countersinking the fuel tank attach holes. I’ve seen other builders spot prime all the countersunk holes and so I was curious why the instructions were specific to the fuel tank holes when normally priming is “if/as desired”. My guess is that these holes (a) fairly large, (b) don’t get riveted and (b) are expected to be exposed on occasion during fuel tank fitting and eventual servicing.
Given my penchant for overengineering, I decided to mask the spar flange prior to countersinking and then using this mask to avoid overspray on the spar. I think it turned nice and didn’t obscure the beautiful spar flange (that, as stated earlier, no one will ever see…).
The final bit of “nut plate madness” occurs near the spar root, which requires a handful of these clever devices be riveted onto the spar web. This is mostly straightforward except that the rivets nearest to the step bars makes countersinking and pressing a standard rivet set a bit awkward. In the end I used a manual countersink tool and a back-rivet set to pound the rivets in this tight quarter.
Brackets and Bolts
While it seemed like I had been working on Section 13 for awhile (sometimes a full day on one step) I was still not done. The final tasks involve tapping and drilling some aluminum stock to fabricate tie down brackets and then installing them, along with aileron bell crank brackets.
Cleveland Aircraft Tool offers some excellent brackets with the holes already tapped but what’s the fun in that? I had bought a tap and die set specifically for this project and wanted to give it a go. After watching several YouTube videos on the right technique (and after acquiring some tapping lubricant) I proceeded to tap the holes to required depth.
I was pretty pleased with the results. I was careful to back the tap out after every quarter turn or so to free up the shavings and blew it clean with air. I did manage to slightly bend the flange on the first go because of how I had it clamped in the vise. (You can see that below) I clamped the other bracket horizontally and it turned out fine.
Once the hole is tapped the plans have you drill a pilot hole in one corner and use that to position/cleco the bracket to the spar so the remaning holes (for both AN470AD4/AN426AD3 rivets and AN3 bolts) can be match drilled. I used clecos to position the nut plates until I got the first holed drills. I then removed the bracket, deburred and primed it:
Feeling pretty pleased with myself I came back the next day prepared to rivet on the nut plates and then attach the bracket to the spar. It was then that I realized I had forgotten to match drill the uppermost holes that are sort of hidden underneath the spar flange. A quick detour to rectify that and the brackets went on pretty quickly.
And with that I bid farewell to Section 13, in what seemed like, in the grand scheme of things, not much time at all. I guess I better get that fuselage kit ordered soon!
Building N726AG 