Back in the shop for a few hours this week, continuing work on the first of four slats.
When we last visited my progress (ha!) I had just completed laying out the first rivet line on the skin where it attaches to the slat support angle (pseudo spar). With that complete, I flipped the skin over on the bench and marked out the second rivet line.
With the slat support angle in correct position, I drilled out the rivet holes to A3 on both edges. The plywood edge of the bench works well to anchor everything. I waited to do the holes where the skin/support/ribs meet to ensure fit. All the holes will be drilled out to A4 eventually.
Flipping the clecoed assembly back over, I began the process of fitting the three slat ribs. The ribs fit as expected so I drilled through from the skin, through the support angle an into the corner of the rib:
I continued the process for all three the same way and the fit well. The rib flanging die works well as a weight to hold the skin upright at the edge of the bench as I double check alignment.
Happy with the alignment, I removed the slat skeleton from the skin and began the process to layout the underside rivet lines of the ribs:
The inboard rib is 30 mm in from the edge of the skin and rivet holes in the skin are laid out, centre punched and drilled to A3. There are 5 rivet holes along the underside on each rib including the one at the spar.. I'll wait to drill the last two near the rear of the slat until the skin is wrapped around the trailing edge and I know where it will meet the skin on the other side.
Redlining the centre of the ribs makes line up easier when drilling through the pilot holes of the skin. This picture makes the rib appear twisted for some reason, it is not. Weird. To do this, I've removed the slat pick-up bracket - it will be added later once the slot in the skin is cut.
With the inboard rib close to the inboard skin edge, it was easy to start forming the curve of the skin. I used three finger clamps to secure the rib using the skin support L, using the redline visible through the pilot holes in the skin as the alignment reference. I further confirmed the alignment using a measuring square. The other two ribs were slightly more difficult to line up as I had no way to clamp them in place for drilling, but the skin was already starting to curve with the first rib so I managed to get them lined up well.
With all three ribs lined up, I drilled through the skin into the rib, starting at the 90 degree corner, gently pulling the skin across the rounded underside of the ribs. The other holes into the support L on the outer ribs will wait until the slat support brackets are installed because the slat support bracket thickness will change the location of any holes drilled now. It's hard to capture the curve of the skin in pictures, but it turned out well. I always pictured in my head that the slats were much narrower - seeing the initial assembly here makes me realize how wide/thick they actually are!
I want to make sure the slot for the slat in the skin is accurate and doing so took some head scratching. How do I mark out where on the skin the slot goes when I can't mount the slat attach bracket until the slot exists? It's compounded by the curvature of the skin and the tapering angle of the bracket. I also happens in two places, the inboard and outboard ribs on each slat section.
What I came up with turned out pretty good. I used a duplicate slat support bracket as an example.
Knowing that I really only need two reference points between any slat attach bracket and slat rib to determine position, I used the original slat brackets to drilled out two matching holes for each of the ribs. Shown marked below on the duplicate - two holes matched for the "G" rib and two holes matched to the "H" rib
The smallest diameter machine bolts I had on hand meant I had to upsize the holes in the example bracket a bit (hence the reason for a duplicate bracket - I didn't want to change the A4 size in my actual bracket).
The bolts are just long enough to make the bracket stand-off the rib to the skin edge so I can then scribe a line back from the edge to the rib. This gives me an accurate start and stop end for the slot I need to cut.
Here is the final picture with the slot cut, the attach bracket through the skin and mounted to the rib. I've also drilled for the rivets through the skin and into the support L. I'm happy how this worked. In hindsight however, this didn't work for the outboard rib as it was too far away from the skin edge to make this work. For the outboard slot, I just duplicated the same length and position as the inboard, slowly lengthening the slot until it fit well. the key concern is matching the rib holes and being equally positioned in reference to the opposite end of the slat section. I've accomplished that doing it this way, so I'll do the same again on the next slat. This picture also shows the curvature of the skin on the underside of the slat.
With the underside of the slat now secured, I began the process of laying out the rivet lines for the top side. This is a single row of rivets as there is no need for a support L on the top side of the slat.
I used a seamstress measuring tape for it's flexibility and made notes of where the centre of each rib flange was as well as where along the length of the spar the rib is. The inboard rib is easiest, as it is only 30 mm in from the edge of the skin. I measured all three and marked out rivet holes.
With the hole locations identified, I drilled the pilot holes from the inside which I will then back drill from the outside when the skin it rolled over the ribs. Again, I'll wait to drill any holes that double through the lower skin once I confirm everything is lined up.
At this point I also made the first bend in the trailing edge with the bender using a 020 shoe to maintain the required bend radius. In this picture you are looking at the top skin which curves over the top of the rib, forms the trailing edge and covers over the tail of the ribs (this will become more clear when I get the skin curved over the rib).
With the help of Ron and a long board, we bent the trailing edge over by hand as far as we could. The aluminum bar stock is taped in place to prevent us bending the trailing edge completely flat and the round rod inside the trailing edge helps form everything.
With the bend close, we needed to figure out a way to make a small crease along the trailing edge. This crease creates the return angle from the trailing edge to where the skin meets the tail of the ribs. We used a thinner board to clamp the skin down tight slightly inside the trailing edge while the rod remained inside the radius to give something to bend against.
I turned out great and I'll continue to take pictures as I move along. Next up will be moving the whole assembly to a specially created "slat box" that Ron built. we'll be able to tighten the top of the slat skin down across the ribs and drill through the pilot holes into the ribs. It should also be ready for the trailing edge rivet holes.
Thanks as always for reading! Stay tuned for more coming soon.
Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.