Backriveting rudder stiffeners

With the stiffeners and skins all cured, I started riveting the stiffeners to the rudder skin. This was my first time backriveting, and it couldn't have gone much better than it did. I'll echo what a lot of other builders say - backriveting is the easiest method of riveting that produces the best-looking results.

Rivets and tape in place.

Or should I say, it produces the best-looking results when done correctly:

This is how the stiffeners should look when they're attached...

...and this is how they look when you're not paying attention. This setup would make it pretty difficult to close the rudder skins together, so I don't recommend it! Thankfully it wasn't too difficult to drill out the rivets on that first stiffener and reattach it the correct way.

We had a Halloween party at our house, so I came back outside and continued riveting anytime I got bored or sick of making small talk with other people. Some curious partygoers ventured out in search of the noise and watched me work for a bit. It made for a good opportunity to show off the build so far and to demonstrate how the rivet gun works.

Priming rudder skins and stiffeners

The rest of the rudder parts had more work ahead before they could be primed, but the stiffeners and skins were ready to go.

Since I'll be able to backrivet the stiffeners to the skin well before assembling the rest of the rudder, and since it was a good day to do some priming, I went ahead and primed them. It took a long time to prepare and finally spray the parts so I didn't have much opportunity to take pictures along the way.

Finishing rudder trailing edge countersinking

It's getting close to being time to prime the rest of the rudder parts, so I went back to the trailing edge to finish all the countersinks. I assumed that my earlier method of simply holding the trailing edge with my other hand wouldn't work as well since the holes would be enlarged and wouldn't make for a good guide for the countersink, so I went back to clamping the trailing edge to the smaller edge piece before drilling. It worked alright for a few holes but I started noticing that a couple of the holes were coming out slightly oblong after drilling. I realized that the clamps had started to slip with the vibration of the drill, so I decided to switch back to holding the piece in my hand. As it turns out, the countersink works perfectly even with the enlarged holes as long as you hold the cage tight enough against the piece so the whole thing doesn't move. The rest of the holes came out almost perfect using this method.

The weird lighting in the shop makes the edges of the holes look a little wavy, but they are actually round

Horizontal stabilizer complete! Riveting HS skin

I guess working on both the HS and VS at the same time was beneficial after all - now I can say it only took me 2 more days to finish the horizontal stabilizer!

I started by riveting the HS-707 nose ribs to the HS skins. The skins are pre-bent but still try to force their way open with enough force that clecoing only the ribs in place can be an issue. All of that force is concentrated in just a few holes on that rib, which can both "pop" a crease into the skin and bend the flanges of the rib. Mike Bullock's build log mentions that he used masking tape to help keep the edges of the skin closer together to help prevent this. I used the masking tape, and I also clecoed the HS-708 ribs in place behind the nose ribs (suggested in the next step in the plans to help keep things aligned). I had some trouble getting the bucking bar into place in the tight space near the front-most rivets, and one ended up with a deformed factory head. It had to be replaced, but the rest went into place with some patience.

Before attaching to front spar assembly, I went slightly out of order and riveted on the HS-706 end ribs with the squeezer so I wouldn't have to buck these rivets with the skin on. The VS assembly has you rivet the entire skeleton together before putting on the skin, so I'm not sure why the plans have you putting on the HS skin before the end ribs go into place.

The front spar went into its place rather easily, and I began the process of riveting on the skin using the same technique as with the VS. Starting at the intersection of the HS-706 and 708 ribs, I worked my way to the tip, then back to the root, then straight back to the aft edge of the skin.

The whole thing's a lot lighter and looks a lot better without all the clecos in the middle...

With one side mostly done I could start working on the other side using the same process.

Now we're getting somewhat close to a finished airplane part.

The rear spar assembly went on next. Most of the holes could be reached with the squeezer, but I used the rivet gun and bucking bar on the rivets in spaces too small for the squeezer to reach (i.e. the rivets next to the hinge brackets on top of the spar doublers).

The HS-708 rib is attached to the rear spar using BSPQ-5-4 pop rivets. The ribs had a slight twist in them causing the holes to not completely line up at first, so I took a rivet punch and maneuvered them back into place before inserting the rivets. These went in pretty easily and looked great.

...I went back and fixed that one slanted shop head next to the hinge bracket later on.

This is what the shop heads look like from the inside. Very nice!

The final step: riveting on the end ribs. Thankfully I could reach all of these with the squeezer. Every rivet should be set on the bottom sides of the ribs, but I nearly forgot to leave some of the holes open on the inboard top side for the fiberglass tail fairing that will be installed later. I caught myself just as I was about to make the mistake and saved myself from having to drill out any more rivets than I already had at this point.

Just like that, I have the horizontal stabilizer ready to go!

I couldn't resist propping the VS up along with it just to see how it looked.

There it is – (most of) the tail!

Another sub-assembly down, and many, many more to go. I'll finish up the rudder next, then I'll move on to the elevators. I went ahead and ordered the wing kit since there's an 8-week lead time on every kit except the empennage kit. Planning ahead to avoid any down time is a must. Momentum is the key to success! The wings should be here by early January, giving me just enough time to finish the rudder and the elevators and clean out the shop in preparation for a bigger assembly. Things are starting to get really exciting now!

Vertical stabilizer complete! Dimpling and riveting VS parts

Based on previous experience working on the horizontal stabilizer, I decided to finish the vertical stabilizer first. The VS is a lot easier to work on and will be done a bit faster since it's a smaller part. I still needed to dimple the understructure and the skin since I had elected to wait until after priming to dimple everything. The AKZO is pretty much bulletproof now that it's fully cured, so dimpling now wasn't an issue at all. I was able to get most of the holes using the DRDT-2, but the ribs and the very ends of the spars had to be dimpled with the hand squeezer since the DRDT couldn't reach them without deforming the parts.

I ended up having to re-countersink the bottom holes on the spar doubler since the primer had increased the thickness just enough to start showing a gap between the spar and the doubler when they were put together. The countersink cage was still set from the first time I did this, so this task went by quickly. I clecoed the parts together and started squeezing the flush rivets first, then moved on to the round headed ones. Most I was able to reach with the squeezer, but the rivets on the hinge brackets had to be bucked since the squeezer couldn't fit into the limited space. I was a bit nervous at first since this was my first time bucking rivets on the project itself, but I was both relieved and emboldened when they turned out looking nice.

I did end up having to drill a couple rivets out, namely the long -7 rivets that are really easy to fold over instead of squashing straight down. Using a #40 drill as a pilot hole then removing the rest of the rivet with the #30 bit worked well without enlarging any of the holes.

Those two empty holes at the bottom used to contain some pretty poor rivets. Not anymore!

I had one minor slip-up with the rivet gun when I didn't hold enough pressure on the round rivet head and the gun slipped. I have a couple of minor smileys in the spar now, but I doubt this is going to bring the whole airplane down.

A short while later I had a completed rear spar.

Apart, these pieces are flimsy. Together, they're pretty darn strong. ...there's a metaphor in there somewhere. 

With the rear spar done, I began work on the front spar by riveting the ribs to the spar itself. Attaching the 706 and 707 ribs were easy enough, but the only real challenge is keeping the spar still while putting so much force on the rivet gun and bucking bar. The assembly is so light that I ended up having to clamp it AND put weight on it to keep it from moving when I used the rivet gun.

My makeshift VS spar jig. 

When it came to joining the bottom 705 and 704 ribs at the same time, I was forced to use the offset attachment for the rivet gun. Having to indirectly apply pressure to the rivet head while keeping it square and also having to hold the bucking bar steady while the ribs themselves are in the way the whole time is difficult. Still, I powered through and got it done.

I hadn't yet dimpled any skins at this point in the build, so my DRDT-2 wasn't really set up for it. I've seen builders build tabletop shelves to elevate the skins to the level of the DRDT itself, but my impatience made me decide to build a stand that lowers the DRDT to table level instead.

Just a few spare 4x4s joined with some extra 2x4s. Simple, but sturdy. 

I made the stand at a height that brings the lower dimple die on the DRDT level with the table with carpet on top. This should support the skin while still making it easy to maneuver it around to dimple each hole.

Dimpling skins is where the DRDT-2 really comes into its own. Once it's set to the correct pressure, dimpling is way more easy and consistent than with your typical C-frame/hammer setup. There's no listening for that "right sound" to let you know the dimple is deep enough – just push down on the handle and you're good to go.

The DRDT is also supposed to help prevent a known issue with the C-frame – making extra, unwanted holes in the skin should the skin slip off in between hammer strikes. Of course I, being the rebel that I am, decided that no one could tell me what I can and can't do with my DRDT, so I accidentally punched a hole in the VS skin when I pushed the handle down without looking.

My actual reaction: "Wha– NO."

I had just made a mistake, using a tool designed to avoid making that very mistake.

I immediately rushed to the forums for advice. Did I just waste dozens of hours of work, or will I still be able to salvage my poor VS skin?

Thankfully this seems to be a common issue (one that happens more often on the thin rudder skins than anywhere else). I followed the steps others took: flatten the offending hole, drill/ream it to #40, deburr it, and then re-dimple it and put a rivet in it.

The offending hole, after clean-up.

Since this rivet won't be structural and only needs to fill this hole, I chose the absolute shortest rivet I have: an AN426AN3-3, normally reserved for attaching stiffeners to the thin rudder skins.

The rivet in place.

It's a good thing I didn't punch that hole any closer to the edge of the skin since the shop head was almost touching the rear spar. Any closer and I probably would have had to replace the entire skin. Luckily there was no interference between the new rivet and the spar and no deformation of the skin itself. It's not too noticeable as is, but I should be able to put some filler over it just before painting the aircraft and no one will ever know it's there.

This is later on after all rivets had been set. The extra rivet looks out of place, but at least it doesn't cause any issues.

With that catastrophe avoided I finished dimpling the skin and clecoed it to the skeleton. It was time for the fun part – riveting on the skin. The plans recommend starting in the center of the skin at the intersection of the front spar and the VS-707 rib, then working your way up toward the tip. I ended up drilling out my first attempt at setting the -4 rivet here since I easily flattened the shop head. A -4 rivet honestly seems too short for the 3 overlapping pieces of dimpled aluminum at this hole and I considered stepping up to a -5 length rivet instead, but I was able to set it better the second time so I didn't bother.

The rest of the rivets set with the rivet gun and bucking bar went by a lot easier and faster than I had predicted. Once you get the correct placement of the gun and bucking bar and have a feel for how much you need to feather the trigger and for how long, it's actually easy to get consistent shop heads down the row.

These are looking nice. In pictures there appears to be a slight gap between the rivets and the skin, but they do actually sit flush in the holes.

At first I was pausing to check the shop head of every rivet with the rivet gauge, but as it's been said (and I was skeptical at first) your eye is actually pretty accurate once you know what the proper shop head thickness and diameter are supposed to look like. I quickly got to the point where I would only have to recheck a shop head after shooting multiple rivets, and this helped the process go by much faster.

The bucking bar was scraping off primer in a few places, so I wrapped it in tape to protect the spar.

With the skin riveted to the front spar and the 707 rib I clecoed on the rear spar. The skin is riveted to the rear spar first, then to the top and bottom ribs to close everything out.

I wanted to make squeezing the rivets as easy as possible, so I set the VS with the spar horizontal to make everything easy to get to. I got the idea from one of Jason Ellis's videos. I couldn't use the squeezer on every rivet – the hinge brackets actually don't allow the squeezer to fully contact the shop head of the rivets in that area. I was forced to use the rivet gun and bucking bar, which really wasn't an issue now that I had a lot of practice with it.

While the part was horizontal I went ahead and set the blind rivets that connect the rear spar to the 707 rib. It's a good thing there isn't an abundance of blind rivets in this build since I only have a manual pop rivet puller. It takes a surprising amount of force to get the mandrel to break free of the rest of the rivet, and it ALWAYS makes a super loud, startling pop when it finally does break. I'd consider getting a pneumatic blind rivet gun if there were a lot more of these, but the hand riveter should suffice for this build. If I decide to build an RV-12 down the line (which uses blind rivets for almost everything) then I'll pick one up.

Some of those scratches are from the rivet puller bouncing when the mandrel suddenly breaks. It just happens too quickly to react.

I set the piece vertical for riveting the top and bottom ribs. The forward-most rivets on either side of the top rib were challenging. That area is too small for my bucking bar, and the squeezer didn't seem to be fitting in the limited space. I pondered how I could solve this without ordering another, smaller bucking bar. I then realized that I could take the flatter, wider flush set and put it inside the rib on the shop head while putting the thicker head outside on the factory head. That juuuust barely allowed the squeezer to fit, and I was able to squeeze all of these rivets without any deformation.

And just like that, I have a finished airplane part sitting on my workbench!

Well, technically it'll be completely finished once I get the fiberglass tip installed, but I'm going to wait and take care of all the fiberglass parts at the same time. For now it feels great to have a chunk of my airplane pretty much complete, the first milestone of this long journey. 

The horizontal stabilizer is next...

Priming HS and VS skins, remaining VS parts

I still hadn't primed the horizontal and vertical stabilizer skins yet, so I took the opportunity to do so since it was a nice day to prime. I also primed the remaining spars and the spar doubler for the VS since I wasn't able to prime them with the other HS and VS parts the last time. I left most of the vinyl on the skins up to this point so I was able to cover most of the bare metal with frog tape. I used masking paper to cover the larger areas of exposed skin not covered by vinyl.

Preparation in progress. It's pretty easy to tell which parts are untouched and which are ready to prime.

A few days later, the parts are cured. We have one primed HS skin...

...and the other skins ready to go.

Starting rudder trailing edge countersinking

The trailing edge needed to be removed and both sides needed to be countersunk to accept the dimpled trailing edges on the skins. I ended up purchasing another countersink cage from Cleaveland so I wouldn't have to remove the #30 countersink and reset the cage. For around $30, the additional cage makes sense to avoid having to drill more holes and test the depth in scrap pieces every time I change the countersink.

At first I took the small trailing edge piece from the Van's practice kit and clamped it under the full size trailing edge to square everything off before using the drill. Because the trailing edge is so small I would have to unclamp it, move it down, and reclamp it every couple of countersinks. It was becoming tedious, and I soon realized that this wasn't really necessary. The easiest way to complete this step is to just take the trailing edge in one hand and hold it tight against the countersink cage while drilling it. The countersink bit is held in place laterally by the hole itself, so the countersunk hole will turn out nice as long as the cage is held tight against the surface of the piece.

Turning the piece over and countersinking a few of the holes on the other side of the piece made me realize that the #40 holes get enlarged when both sides are countersunk. I quickly turned to the forums to see if I had made a mistake. From what I could tell the enlargement of the holes not only seemed to be a normal occurrence, but it didn't seem to matter that the holes would be larger than the rivets because the skins on either side are what actually hold the rivets in place rather than the trailing edge piece itself. Once the rivets are set they'll probably expand to fill this space anyway, so it's not a huge deal that the holes get slightly bigger.

Riveting HS rear and front spars

With the primed parts fully cured, I could begin the final assembly of the horizontal stabilizer.

Riveting the rear spar was my first time extensively using the squeezer during the build. The trial and error of setting the appropriate gap between the sets wasn't a difficult process, but I ended up drilling out the first rivet. I had squeezed it probably 10 times to get the proper setting and the shop head was looking pretty deformed. I replaced the bad rivet and continued riveting the spar doublers and hinge brackets to the rear spar pieces.

As stated in the plans, hand-squeezing AN4 rivets takes a bit of grunt. I had to clamp the spar to the worktable and weigh it down on either end to keep it from moving while I exerted pressure with the squeezer. The Cleaveland main squeeze is nice to have here since it claims to require less force than competing squeezers. Considering the amount of effort I still had to use, I'd hate to imagine having to use another squeezer here. The longer rivets (the -7s that go through the spar doublers, spar, and hinge brackets) were a bit of a challenge by themselves since they are liable to fold over if the squeezer isn't held perfectly square.

With all of the rivets set on the rear spar (aside from the holes left empty to later attach the front spar assembly) I bolted the center hinge bracket bearing onto the spar. These bolts will need to be torqued and sealed, but I'll only hand tighten them for now and take care of torquing during final assembly.

The front spar was next. Once again I had to re-countersink the holes in the HS-710 and HS-714 angles since the primer had increased the thickness of the holes. I did my best to squeeze as many rivets as I could, but the 3 inch yoke on my squeezer wasn't quite long enough to fit around the wider HS-714 angle and reach the holes on the HS-710. Out came the rivet gun, which easily took care of those.

The HS-00005/6 ribs were a pain to rivet. The plans suggest "gently flexing" the ribs out of the way to reach these rivets. I was again forced to use the offset rivet gun attachment, of which I'm now very wary. Flexing the ribs out of the way while also holding the gun and bucking bar perfectly square proved once again to be a challenge, and my first attempt at using the offset attachment left me with a nice row of smileys on the flange after it bounced itself off the factory head. After a few minutes of fuming and cursing I went back to it and managed to carefully muscle my way through the rest of these rivets on either side. Hopefully I won't have to use the offset attachment much later on in the build...

My efforts yielded a interesting wall piece.

Priming HS and VS parts, successfully this time

The Critter sprayer arrived a couple days ago as planned, so I attached the regulator from the Harbor Freight gun and an inline filter. Just like that I had a fully-functional sprayer. Testing with water yielded similar results to the water test with the other gun. Everything looked promising so far.

Simple, but effective

It turned into quite the assembly, but at least the weight should help me keep it steady?

Today was finally a good day to prime, so I gave all the parts a quick re-scuffing to remove any aluminum oxide that had formed and I cleaned the parts with soap, water, and acetone. I mixed up the AKZO and brought the parts outside during the 30-minute induction time, then cautiously started spraying. Thankfully, my second attempt at priming went far better than the first.

Despite hours and hours of prep, actually shooting the primer only took about 15-20 minutes for all of these parts.

The resulting finish on the parts, while being ever so slightly heavier and more textured than the finish left by the HF sprayer, is still adequate and provides complete coverage and protection. Success!

The only real criticisms I have of the Critter sprayer are A) the round spray pattern is tight and doesn't cover as much area as the fan pattern of the HF gun, B) the siphon design occasionally causes rogue drops of primer to shoot out and land on the parts leaving unsightly spots, and C) the siphon doesn't reach to the bottom of the jar, meaning every priming session will leave you with around half an ounce of primer remaining in the jar that can't be sprayed. All of that said, the easier cleaning of the gun pretty much offsets all of that since I won't have to buy a new sprayer every time I want to prime some parts.

Attempt #1 at priming HS and VS parts

When priming day had arrived (selected because it was my day off and because the humidity was finally less than 75%) I prepared all of the parts for being primed. When it comes to prepping the parts I see a lot of builders that insist on using chemicals like Alodine and Alumiprep to remove the alclad layer and chemically etch the pieces, but I'd rather not touch that stuff.

The method I use to prep the parts is simple: I scuff the pieces with a Scotchbrite pad to remove the alclad and create a nice rough surface for the primer to adhere, clean the dust off the parts with unscented dish soap and water, then wipe down each part with acetone to remove any oils or other contaminants. I used the same method for the test pieces and they came out nearly perfect, so I really don't think exposing myself and the environment to harsh acids and chemicals is mandatory.

The simple priming prep kit – everything you'll really need.

Prepping all of the ribs and spars for the HS and VS took a while, around 3 hours. I mixed up the AKZO and took all the parts and the compressor out back while waiting for the 30 minute induction time to expire. Once the primer was good to go I poured it through a filter into the spray gun, then took it out back. I connected it to the compressor, ensured the pressure was set correctly at the compressor and at the gun, got close to the first couple of parts to be primed, pulled the trigger, and watched dumbfounded as the sprayer splattered primer all over itself, the ground, and pretty much anywhere except onto the parts. Ah %#$&;!, the gun crapped out on me at the last second!

I only had an hour or so of daylight left (the humidity skyrockets after the sun goes down) as I made a frantic attempt to clean the gun with paint thinner, but the results were the same. My guess is that I didn't clean the spray gun well enough last time, and whatever primer got left behind hardened and blocked parts of the gun and the nozzle. The spraying ability of the gun is most likely permanently reduced. The day was ending, and I felt defeated. I took the parts back in the garage and called it a night. All of that work, and nothing to show for it.

I considered going out and buying another Harbor Freight gun, but I didn't want to run the risk of having another spray gun fail on me because I couldn't clean it properly. These HVLP guns have a lot of parts that need to be disassembled and thoroughly cleaned to avoid ruining the ability of the gun to atomize the paint properly, and I don't want to go through that again only to ruin another one.

Doing some research, I came across some builders that used airbrushes and airbrush-like spray guns to prime everything. Airbrushes are nice because they keep the paint and air components separated until they come together to atomize the paint into a spray. This makes cleanup a lot easier and make the gun itself less prone to malfunctioning after multiple uses. The downside is that most airbrushes have a small paint reservoir, typically only a few ounces. That's fine for sessions with fewer parts, but having to constantly refill the sprayer when priming a lot of parts or priming large parts such as the skins would get annoying.

Eventually I was introduced to the Critter spray gun, which functions like an airbrush but has a much larger paint reservoir in the form of a 16-ounce mason jar. It's typically meant for staining and painting decks and fences, but I figured it'd be worth a shot since I saw it mentioned by another builder on the VAF forums. I went ahead and ordered one on Amazon (free 2-day shipping is a wonderful thing), so I can try it out this weekend and hopefully fix this mess.