SportAir Workshop
I have read many builder's websites and was already familiar with the build process, but I had never really had a chance to pick up and play with any of the tools of the trade. There are lots of specialized tools involved in building an airplane.
Riveting can be done several ways depending on how much room you have to manuever. Traditional riveting uses an air powered rivet gun and a bucking bar to squeeze the rivets. It definitely helps to have a partner for this type of riveting. Backriveting uses a flat steel plate as a bucking bar. You can also use a rivet squeezer. You can sure make your hands sore using a squeezer, but the resulting rivets look superb and are very consistent.
There are lots of different types of rivets. There are blind pull rivets (aka, pop rivets) for riveting when you can't access the other side of the material. Flush set rivets are set flush to the surface of the material. This requires either countersinking the material if it is thick enough, or dimpling on thinner material. There are a number of ways to dimple. You can use a C-frame that has a large throat to handle wide sheets of material, or you can use the rivet squeezer fitted with a set of dimple dies.
Before you rivet, you have to line up the materials, clamp them together, then drill the holes for the rivets. Van's kits come prepunched, so this process is super easy. You take a cleco clamp and use them to hold together the materials, then you match drill the parts to the proper size hole. The drilling creates a small burr on the edges of the hole, so you have to disassemble everything and "deburr" the edges. Otherwise the material won't lay flat against each other, and the burrs can create small stress fractures in the material, which can lead to larger cracks. Not good to have cracks in your airplane! Needless to say, the deburring process is time consuming. Our small practice project probably had 100 holes, times two pieces of material, with 2 sides each. We got lots of deburring practice.
Our first project was simply to join an angle piece to a small skin with 3 different types of rivets, then connect that skin to another skin using a simple overlap joint with flush rivets. I made my first screwups on this project. Better here than on a real airplane part. I got carried away with the drill and forget that the second row of rivets required a smaller drill bit. Oops. I caught myself after only 4 of the 12 holes. I was able to fill the bigger holes with bigger rivets.
The second practice project looks like a small control surface section. It has 2 skins with stiffeners, 2 end ribs, a spar and a small wedge of aluminum that holds the trailing edge together. The first step was to take the stiffener material (looks like a piece of aluminum angle) and cut it into 4 separate pieces. Then using snips, I had to cut a wedge off of each piece so that it would fit into the narrow side of the airfoil shape. The snips leave a rough edge, so we used a file to smooth out the cuts. Then it was cleco the stiffeners to the skins, match drill the parts, deburr the holes, dimple the material for flush set rivets, reassemble and finally back rivet the stiffeners on to the skins. Next came the spar and end rib assembly. The spar was too thick to dimple and too thin to countersink, so I had to do a little of both. First you use the squeezer to dimple the thick material, then run the counter sink bit in the hole to give it a better profile. There was also a doubler plate on the spar at each end where it connected with the ribs. I was able to squeeze most of these rivets. The next step was to take and attach the skins to the spar/rib assembly. More match drilling, deburring, etc... The hard part was riveting the skins to the spar. You had to have a partner for this, since it required getting your hand down inside the assembly between the 2 skins with a bucking bar and pounding the rivets on the outside with the gun. I tried doing it myself at first and managed to ding up the skin. I got help the rest of the way and things went much smoother. I was able to squeeze all but the last 2 rivets on the ribs. The ribs get really tight at the trailing edge, so you can't get the squeezer in there. We had to use a special thin bucking bar, and even then it was not a very good fit. The last 2 rivets were a bit mangled. I had to drill out a couple of them and try it again. The trailing edge was the next step. It uses a small wedge to hold the angle between the top and bottom skins. You have to double flush rivet the edge so it is smooth on both sides. This was pretty easy to back rivet, but it require a fine touch on the rivet gun to keep the edge straight. The last step was to roll the top and bottom skins together on the leading edge. This makes a nice rounded shape on the leading edge. To do this you have to bend the skin material towards each other. We used a wooden dowl and duct tape. It was actually pretty easy. A final row of pull rivets to secure the two pieces and it was done.
Here is a picture of my completed practice project kit.
It took about 9 hours of work to get this little project completed. Now I need to buy some tools of my own and continue practicing before I commit to buying the aircraft kit. I'm sure now that I can do this. Overall, the class was well worth the time and money. It was also a lot of fun, and I met some really nice people who are also starting on their own projects.
