Sealing and riveting rudder trailing edge, starting leading edge rolling

Now for the most fun/scary part of the whole rudder construction - the trailing edge!

Leading up to this point was stressful. The trailing edge needs to be back riveted to ensure a flush head on each side. You can't backrivet with clecos in, so you can use either tape or tank sealant to keep the trailing edge locked in place between the rudder skins. The tape method is what's recommended by the latest Van's documents since it's a lot cleaner (no mess) and faster (no waiting days for it to cure). The tank sealant method was the method originally recommended, and a lot of builders still prefer it over the tape. I debated back and forth over which method I wanted to use, and I eventually decided to order both the special 3M double-sided tape and some tank sealant and decide later on.

Regardless of which method is used to keep the trailing edge together, it's recommended to use an aluminum angle to ensure the edge stays straight while your adhesive of choice cures. I took care to find the straightest piece of aluminum angle I could find at Lowe's, and I match-drilled holes into the angle along the length of the rudder so it could be clecoed in place after gluing the edge. I did this after I had already dimpled the rudder skins causing a couple of holes to be slightly enlarged, so I recommend match-drilling the angle before dimpling the skins.

When the day came to do the deed, I decided to try the tape method first. I wiped down the wedge and the trailing edges of the skins with acetone to clean the mating surfaces. With the rudder in place on the bench, I took out the tape and began peeling off the protective paper. Immediately the tape itself began pulling apart in shreds. No matter how carefully I peeled, I couldn't get a clean, smooth strip of tape. I couldn't be confident that this tape would work well, so I decided to use the tank sealant instead.

Yeah, this stuff does stink. Time to break out the odor mask!

Tank sealant definitely isn't the nightmare goop a lot of people make it out to be. It actually turned out to be mostly painless, but smelly. The best way I can describe it is "garbage being burned in hot, fresh asphalt."  I ordered the sealant in the caulk-type container on purpose to make mixing and applying easier and cleaner than with your standard tubs.

With the sealant mixed, the container fits nicely into a standard caulk gun. This makes application so much simpler than just using popsicle sticks.

I went to work applying the sealant to one side of the wedge. I worked in sections, shooting a line of sealant down about a third of the wedge at a time then using a wide popsicle stick to spread it evenly over the width of the wedge. This stuff is super thick and takes considerable effort to spread, so patience is needed here. The viscosity also means that any excess won't easily squeeze out when both skins are pressed together, so I had to be careful to make sure the coating was thin and even along the entire length to avoid any major bulges or waviness along the trailing edge.

With one side coated, I flipped the wedge upside down and carefully set it in its place on the bottom skin. I applied sealant to the other side and lowered the top skin in place, then clecoed the aluminum angle in place on the bottom of the assembly to hold everything straight. 

I temporarily put some weight on the trailing edge to help hold it tight, but the clecos provide enough clamping force on their own that this isn't really necessary.

The next step is patience, since the tank sealant needs time to cure. I ended up leaving it for about a week before coming back to it since it was starting to get cooler in the garage and the sealant sures much more slowly in cold temperatures. When I came back, I unclamped the angle and immediately checked how straight the edge turned out.

It has just a slight bow along the entire length, but it's well within the .1 inch limit the manual recommends. Which means, it's time to rivet!

Backriveting the trailing edge was one of the most stressful experiences of the empennage thus far. I wanted to avoid having to redo the dozens of hours of work I had sunk into this particular control surface, so I made sure to take my time and follow the instructions to the letter. I set the rivet gun pressure lower than normal so setting the rivets into their recess would be slower and more controlled, then I started at the middle of the trailing edge and set every tenth rivet to lock things into place. From there I partially set the rivets halfway between the last ones, then repeated this process until every hole was filled with a partially-set rivet. I then went back over each rivet in the same order and set the rivets almost fully. Throughout the process I kept check to make sure the small amount of bow wasn't getting any worse.

Since I couldn't get the rivets fully flush using the rivet gun alone, I flipped the rudder over and used the mushroom set to buck the rivets against the backrivet plate. This set each rivet flush into its recess and left me with a finished trailing edge.

The trailing edge isn't bowed quite as much as it looks here - the counterweight on the horn tends to lift the trailing edge when the rudder is laid flat on the workbench.

With the trailing edge out of the way the penultimate task prior to rudder completion was to roll and rivet the leading edge, which proved to be a bit frustrating as things progressed. I used the pipe and tape method to start the bends. The smallest section at the top seemed like it would be the easiest section to bend, but getting it into its desired shape required a lot more force than I had anticipated.

In hindsight I probably should have used duct tape instead of masking tape, but I didn't have any issues with it here.

Riveting rudder skins

Next came the more tedious task of installing the counterbalance skin and the the counterbalance weight. It takes many clecos and a bit of force to get the skin in place, then the rivets need to be bucked since they're too far down to reach with the squeezer yoke I have. It's a little tricky to hold the bucking bar steady in that area, but not too bad.

The counterbalance weight won't yet fit into the rib because of the rivets just installed, so it needs to have a little material filed away. This is just trial and error of filing away a bit, checking the fit, and repeating as necessary. Eventually the weight slides nicely into place.

A warning about this part: the top rib isn't in place to secure the skin and the lower counterbalance rib, so putting the weight in with the skeleton upright and without clecos holding the skin to the spar will cause the lower rib to bend down! Ask me how I know...

The lower four rivets were the only ones that really interfered with anything, so I only needed to shave the counterweight in those four places. 

Installing the bolts and nuts to secure the weight in place took a little improvisation. The nuts need to be torqued correctly and the only socket I had that was long enough to reach the nuts down in the counterbalance horn was slightly loose on the wrench. The screws are Phillips head, and of course they began to strip as soon as there was tension. I had to have force on the screwdriver to prevent it from slipping, so I pressed it tight against my worktable while using the wrench to torque the bolts. It wasn't pretty, but I was able to get both nuts torqued. A dab of torque sealant on each and I could finally start installing the rest of the rudder skin.

The top nine holes at the overlap between the counterbalance skin and main rudder skins was a challenge. The six holes closest to the top could be reached with the squeezer, but the lower three holes on each side had to be bucked. Once again, it was awkward having to blindly balance the bucking bar that far down while making sure the rivet gun is straight. A longer squeezer yoke would have been useful here.

I had forgotten to dimple the hole I'm pointing at here. It overlaps with the counterbalance skin, so there's no good way to dimple it once the skins are riveted on unless you're comfortable trying to dimple both skins at the same time.

The top rib slid into place at this point, and riveting the rest of the skin to the skeleton went well. Pretty much every hole can be reached with the squeezer except for the last three or four near the narrow aft ends of the ribs. I decided to leave these open until after the trailing edge is finished to allow easier access back there.

Looking good!

It looked too good to not stand it up again.

Dimpling and riveting rudder spar

With the parts cured, I dimpled everything then started putting the skeleton together. The spar and the spar doublers went together much like they did on the vertical and horizontal stabilizers, but the rudder now has platenuts that will connect rod end bearings for attaching the rudder to the vertical stabilizer.

The platenuts only have two holes, and each had a small gap between it and the spar when only one cleco is installed. To keep the platenuts flush for riveting I used a spare bolt and nut to lightly tighten the platenut to the spar. 

Much better!

The rivets called out for use with the platenuts are AN470AD4-5, which are the same length as the rivets used to fasten the spar doublers to the spar. With the addition of the platenuts' thickness, these rivets didn't seem long enough to provide a thick enough shop head. These are critical components and I wanted to be 100% sure that they wouldn't fail, so I just used slightly longer -6 rivets instead. Looking ahead at the elevator diagrams, the platenuts for those are called out to be one length longer than the rivets used for the doublers. I used that as justification for using the longer rivets here.

The longer rivets were a good choice. The shop heads would have been pretty thin with the shorter rivets...

The ribs needed to be dimpled before the can be attached to the spar. The aft end of the R-904 bottom rib is too narrow for the regular dimple dies to fit, so I had to get creative and use the close-quarters dimpling kit I got from Cleveland tools. 

The female die is fixed to a piece of steel and screws into the table. I put the male die in a spare 2x4.

With the 2x4 in place on top, all I have to do is put the rib between the dies and smack the board with a mallet. The clamps keep everything from going flying when the hammer comes down. It's crude, but it works.

Success!

The ribs got riveted onto the spar and I managed to get the horn brace and lower fairing attach strips on without much trouble. There way no way I was going to get the bottom four rivets with the squeezer or the bucking bar, so I used some LP4-3 pop rivets instead. Problem was, I only had three left. Not a big deal since I can order more once I place the order for some tank sealant. Oh yeah! That part's coming up soon!

This part will be hidden by the bottom fairing, so trying to get solid rivets in these holes probably isn't worth the trouble.

Starting the elevators, trimming and drilling elevator stiffeners

Over the past few weeks I worked on and off on trimming the elevator stiffeners between working on other parts. It's more or less the same tedious process as on the rudder stiffeners, but the elevator stiffeners are presented slightly differently.

Instead of having identical long pieces that each have to be cut down to a specific length, the stock elevator trim pieces are notched differently depending on which stiffeners will be made from a certain piece. One long piece was notched to make five small E-720H stiffeners, while others make A, B, C, D, and H stiffeners.

For the right elevator I had to take six stiffeners that I had already cut out and trim them even more to specific lengths called out on the diagram. I had to be careful here - the measurement is from the aftmost hole, which will become the second aftmost hole after the stiffener gets match drilled to the skin, to the end of the stiffener.

Above is an E-720K.

Eventually I was able to get all the stiffeners done, and I clecoed them to the skins for match drilling.

Priming remaining rudder parts

It was another good day to prime so out came the Critter sprayer, AKZO, and paint racks.

For the counterbalance skin I primed the whole inside, but on the outside I only primed the section that would slip under the rest of the rudder skin. I've seen some builders just prime the whole thing inside and out, but I want to preserve the aluminum finish until I can get the whole airplane painted properly.