Finally had some time to build last night. I'm always paranoid about my fin alignment, so I attempted to construct a guide from an old piece of hardboard that I had in my garage. I copied the alignment guide from the back of the instructions, glued it to the hardboard, extended the fin lines by tracing around the root edges of the actual fins and then cut it out with a jig saw. It came out ok, though I have to shim the tube in a few places to make sure that it is centered in the guide. Now that I'm finished with the first set of fins, I think I probably could have done just as well by sighting down the fin can like the instructions describe.
I had previously glued the thrust ring to the motor mount tube and the aft centering ring to it. Trying to follow the instructions to the letter, I tacked a centering ring ~1/4" from the front of the motor mount. After sanding the rings a little and roughing up the surface of the MMT, I was able to get the motor mount to fit into the slotted air frame, but the CA didn't hold the front CR on very well. It snapped free as I inserted the MMT. I just stuck a pencil through the fin slots and worked it towards the front of the MMT because it was only there to hold the MMT straight while the fins were attached--it was going to come back out in a little while anyway. I also had to increase the length and width of the slots in the air frame by about 1/16" with a Dremel to get the fins to slide in. (Even then, it was still a little tight. Maybe I should have sanded the fins a little more.)
Next I mixed up a little bit of 5 min epoxy (The instructions called for 15 min, but I only had 5 and 30 min stuff.) and used it to secure the root edge of the fin to the motor mount tube, being careful to not drip any glue that could accidentally adhere the air frame to the fin. This worked quite well, though I should have probably sanded the slots just a little more to allow the fins to slide a little better. My hack-job fin alignment guide worked pretty well too.
After letting the epoxy cure for ~ 15 min, I rotated the air frame 120 deg and repeated the procedure. Later I discovered that I did tack the body tube to the fin in one small spot, but it was easily freed with my knife blade. Overall the process worked well. Like I said earlier, my alignment guide worked well, but I suspect that you could do nearly as well by just putting the printed guide on the floor and sighting down the tube.
Now for the part that made me lose sleep (literally). I cut small tabs out of the airframe to extend the slots to the end of the tube. This worked fine using a sharp Exacto blade. Per Mike's instructions, I saved the little tabs in a Ziplock bag for later. It took a little wiggling, but I then slid the MMT and the fins out of the back of the airframe. Because the front CR was already loose, I had to reach in and pull it out separately.
Then disaster struck! Well not really, but I did gasp when one of my fins snapped off as I pulled the fin can out of the airframe. The instructions say that the fins are only supposed to be tacked into place at this point--the internal fillets are what really provide the strength. But apparently one of the fins had not made good enough contact with the MMT. While this sucks, it's best that I caught it now. Honestly, if I hadn't pulled out the fin can, I would have never known that I had a poor quality bond. I just scrapped the epoxy off the MMT and the fin's root edge, applied fresh epoxy and then re-inserted the fin can into the airframe. I used 30 min epoxy this time to make sure that the sucker formed a strong bond, and I let it sit overnight. (I'm just using the Bob Smith brand of epoxy for now. I ordered some Rocketpoxy on Black Friday, but it hasn't arrived yet.)
The next morning (after dreaming about fillets) I woke up early to ostensibly help my daughter study for a history test. Luckily I was able to multi-task, so while quizzing her on Chinese history (Amazing what they teach now; I'm sure I was never taught that stuff.) I pulled the fin can out again and started the fillets. I slid the CR that had been previously sanded to fit into the airframe to the front of the fins and attached it using a thick fillet of 30 min epoxy. I then systematically applied the internal fillets. One of the fins, the one that had previously broke off, was attached a little worse than the others. This was evident because the epoxy for the fillets leaked through a few gaps in the root edge and ended up on the other side of the fin. I used some blue painters tape on the back side of the fin (opposite the side I was filleting) to keep the epoxy from running through these small gaps. The other fins were fine. I hope that one doesn't come back to bite me.
Thanks for reading this lengthy post. I'll work on the front fins later this week.
Now for one question...I've never worked with this kind of phenolic tubing before. As I was working last night I realized that the spirals in the tubing are pretty large. Do I need to fill them with wood filler or bondo or can I get by with a high fill primer at the end? Thanks.