Another new skill added to my resume: back riveting. I don’t understand why the builder manual says that back riveting is a two-person job, because at least in this case it was an easy one-person job. First up was getting a good back riveting plate setup. I have a small 8″x4″x1/4″ plate that came with my original tool kit from Cleaveland – and that seemed sufficient until a local builder friend, Jerry, loaned me his 21″x4″x1/2″ plate … and now I wonder how I would have every really enjoyed using that wimpy plate I was originally going to use. “Go big, or go home!”, right?
I wanted the plate in-set in a board which would provide ample surface on each side to support the work, so I purchased a 4’x2’x1/2″ piece of particle board and used the jigsaw to cut out an area to accommodate the plate. It came out nicely, though honestly next time I would have a slightly wider board (there’s barely sufficient board above and below the plate to support the structure, it could break easily when being moved).
Back riveting is just as it sounds: instead of riveting a rivet from the front, you rivet it from the back. This means that instead of using the traditional method of a rivet gun on the front (manufactured head side) of the rivet and a bucking bar on the back side of the rivet, you use a steel plate on the manufactured side of the rivet and your rivet gun on the back of the rivet. Essentially, you’re smashing the rivet from the back side. This requires you to use a special back rivet set in the rivet gun – that set has a spring-loaded sleeve which helps keep the set’s face centered on the shop side of the rivet.
Why use the back rivet technique? It avoids direct impact from the rivet gun on the front (external side) of the skin, and when you’re riveting thin skins (as is the case with the .016″ thick rudder skins), you end up with a prettier riveting job since you’ve eliminated the risk of some minor cosmetic issues resulting from direct impact of a rivet gun on the outside of the skin.
Though not required, the job is far easier if, once you load fresh rivets in the holes, you tape the rivets down (from the outside skin side) – this temporarily holds them in place until you smash them from the other side. Then you simply pull the tape off when you’re done riveting. I decided to experiment with using the fancy 3M low-stick tape that came with my tool kit and also some standard blue painter’s tape. Both worked just as well, so it doesn’t really matter what you use. Though, somebody may argue that the painter’s tape is a better cushion and may prevent some scuffing? (who cares about scuffing if you’re going to end up painting the airplane anyways)
Here are some before and after pics …
I cannot emphasize enough: make sure, every time you put the rivet gun on top of a rivet, that you are ON TOP OF THE STEEL PLATE. All it takes is one attempt at riveting without the steel plate behind the rivet and you’ve irreparably damaged the skin (and have blown many hours of work). I double-check this before I pull the trigger, each time.
Next, the shear clips (R-00914-*) are attached to the front of the stiffeners which were just riveted to the skins. The shear clips are attached with LP4-3 pop rivets. I’m not sure why, as it would have been just as easy to use standard AN470 rivets … but of course I stuck with the build instructions. On the second pop rivet set, the mandrel broke off in the wrong place (despite being careful to keep the pop rivet puller properly aligned and not putting sideways pressure on it). In these pics, look for the red circle.
Since a visual inspection of the back side of the pull proved that the rivet was fully pulled and looks structurally sound, this was merely a cosmetic issue. I used a pair of diagonals to cut the mandrel off at the base – but that didn’t go very well. I ended up putting a very minor dent in the skin when I got a little too aggressive with the diagonals. The dent was pushing outwards with respect to an exterior view of the skin. If you mess up a skin, you start all over. I was very fortunate that with some mild pressure on the outside of the skin, the dent was fixed – very hardly perceptible at this point. However, I did need to address the matching deep scratch on the inside of the skin. So, I cleaned up the inside with a 3M ScotchBrite pad and primered that small section.
Everything is now ready for the final step of closing up the skins. Both skins have stiffeners attached, and shear clips are attached. The TE (trailing edge) is primered and ready to go, and I just received the ProSeal today.
It seems the process of sealing up the trailing edge is going to be a bit of a challenge to get it come out straight and perfect – so, I’m a little anxious about getting that done. I’ll dig into that this weekend with some help from a good buddy who has an Aeronautical Engineering degree and who is one of Microsoft’s big brains … I figure I can’t go wrong with that kind of assistance.
After reading far too many online posts about techniques and approaches to this next step, I gleaned one idea which I think will be helpful: using a L-shaped angle aluminum piece along the TE to help keep it straight while the initial set of ProSeal cures. Below, you’ll see some pics of the angle aluminum that I “match-drilled” to the trailing edge. I was careful not to actually match-drill the holes, since that could possibly enlarge or mess up the existing TE holes … so I used a marker to carefully mark the holes then hand-drilled them. This was not as precise as I had hoped, so I ended up drilling out those #40 holes to #30 size so that they are more tolerant of not being perfectly lined up with the TE. Since clecos will go thru the angle aluminum first, before they engage the TE, I will get away with the enlarged holes without any penalties – hopefully. I’ve put some clear packing tape on the face of the angle aluminum to prevent any ProSeal from sticking to it.