I started making the fins from the template I got from RockSim. I traced the fin pattern onto some 1/2-inch plywood and cut them out using my scroll saw. Then I sanded them down the outer edge's of the aerodynamic fins. I did this to all three fins. The original plan was for a 54mm motor mount not a 75mm. So the Root edge's needed to be sanded down as well. I set the fins aside for now.
I had bought 6 3.9 to 3.0 inch centering rings for this project. Because the width of the ring was small ( it was 3/8-inch) I used an 8 by 32 furniture nut instead of a T-nut. Using a 15/64 drill bit, I drilled three evenly spaced holes. Then being careful not to split the wood, screwed the nuts in the wood. After they were all in, I used CA glue all around the outside and inside of the wood around the nut and top and bottom as well to secure the nuts into place. This is how it came out.
I had all the parts ready for the motor tube to be assembled. Using the center ring for the motor retention as a guide for the holes, I drilled the 5 remaining centering rings to install three 3-foot 10/24 all thread rods. I had to offset one rod for my rail point attachments, more on this later. I needed to grind down the number 10 washers because they where too tall and would stand above the centering rings when installed. I test fit the rings on motor tube and lined them up. After spacing the ring per the RockSim specs, I used the body tube as a guide to tack the fins to the motor mount tube. This way they would be aligned with the fin slots. Before I did this, I used my small drum sander to put a groove on the root edge of the fin to fit it better to the MM tube. Once I had the fins tacked in place, I epoxied the root edges to the motor mount tube using 15 minute epoxy. I install tape dams at the end to keep the epoxy from running off the tube. After epoxying all fins on both sides, I started to prepare to fiberglass the fin roots to the MM tube. I started by cutting the glass for all the fins. I also at the same time prepared the two LOC tube couplers to be glassed on the inside to make them stronger as well. This allowed me to mix up one batch of epoxy resin for the fin's and the tube couplers. I applied the epoxy to the fin's and MM tube one at a time and then the glass. When I finished with the fin's, I moved to the tube couplers. I applied the epoxy on the inside of the TC, then rolled up the glass and unrolled it outward inside the TC. After the glass was applied, I used balloons inflated inside the TC to push the glass outward. This makes a very smooth fiberglass job. I set everything aside to cure.
Next, I installed all three threaded rods. This took some time and help due to the small clearance of the centering rings. I basically had to put each nut and washer on the thread it down to the next one and do the same thing. Once they where all threaded I was able to tighten them. This is how it looks. Note that the threaded rod extends above the top of the motor tube. These extensions will fit into the bulkhead and attached there for total fin can strength. Next I epoxied all the centering rings on both sides of the motor tube. As mentioned above, I had to offset on rod. Note: the picture makes it look like the rod is bent going from left to right, this is an optical illusion. This allowed me to drill holes into the top and middle) center ring for the rail point attachments. I measured this twice to make sure it was correct. Then I slid the motor tube assembly into the lower body tube and positioned it where it would be mounted. Then I was able to test the rail mounting hole into the upper and middle rings. I pulled the motor tube assembly out to verify the holes where in the middle of the center rings. I finished drilling the holes to accept the 8/32 T-nuts and the body tube. Then I had to grind down the two holes so the T-nuts would not interfere with the body tube as it was being installed. Then for the bottom rail button, I used a scrap piece of 3/16 plywood. I cut it to about an inch by an inch and a half and drilled a hole in the center to the last T-nut and epoxied it into place. Then I installed the motor tube into the body tube and test fit all the rail buttons.
Next I started epoxying the motor tube into the body tube. By injecting epoxy into the body tube right before each ring and pulling back and forth allowed me to get each ring coated with epoxy on both sides. I then verified the rail button holes lined up. After I was sure that I was done with the motor tube, I installed the motor tube and epoxied the bottom rail button into place. I also epoxied all the inner fin fillet's and center rings. At the same time I used tape dams to finish the outer fin fillet's as well. I finished with the last center ring. I put epoxy on the inside of the body tube and outside of the motor tube. Then I slid the bottom centering into place. After the epoxy setup, I put tape on the motor retention bolts and screwed them down to help seal the threads from getting any epoxy in them.
The last construction item I had left was the tube coupler. I needed to sand down the bulkhead to fit into the glassed tube coupler. I then test fit the bulkhead and marked the place for the threaded rod. I then drilled the holes for the threaded rods and glued the bulkhead into place on both side with epoxy. I then finished installing the nuts and washers for the bulkhead and at the same time epoxying the tube coupler to the body tube.
The next item is to glass the fins. I got the fiberglass pieces cut for the fins and mixed up the epoxy. Then I applied the epoxy to all the fins and then applied the glass. This completes this section for the Fin-Can assembly.
