MCriscione
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- Jul 9, 2015
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Last week I found time to actually attach fins. Finally. It's been what, 4 or 5 months? More? It's finally starting to look rocket-like. Anyway, here's a quick summary.
Sometime in December I created fin alignment jigs out of foam board and templates printed from payloadbay.com I made three, so that I'd have one spare to support the other end of the tube. This let me pick the two that fit best to use on the fin itself when actually gluing them in. I tried to keep them at a minimal size (thus the 6 sides instead of only 3). The weak points at the tips of the fin cut outs I reenforced with some painters tape. Any tape would have been fine, I just happened to have that sitting next to me that day.
Using a method I've seen described somewhere else on this site, I lightly beveled the fiberglass fins. These fins are only 1/16th thick, so I didn't take much. I tried to only remove about 1/3 of the thickness from each side. I used a 'jig' essentially comprised of a flat board with two more pieces of wood (approx. 1/2" square?) screwed to it. Sandpaper was screwed down to lay flat along the base board. The fixed width/height allowed me to sand a consistent angle on each fin. I had to reposition the sandpaper a few times, I think once for each fin, as the repeated sanding in one location wore it down pretty well along the straight line/corner where the fin made contact. The process generated a lot of fiberglass dust, and as expected, caused a fair amount of itching, even when I did this with latex gloves on. Wrists and arms still got hit a bit. Even for this minimal work, a respirator is essential. Lots of this dust ends up in the air and you don't want to breath it. In my opinion, a dust mask isn't going to cut it for this. It may be tough to see in the photo, but I've also sanded to root edge, tab, and fillet areas of the fin in preparation for epoxy.
I described a while back that I had drilled some extra holes along the fin slot. You can see in this picture how I've used a syringe to inject the epoxy (rocketpoxy / G5000, tinted black for visibility) into the space between the airframe and the motor mount. This process seemed to work very well. I'd mix and fill the syringe, insert into each hole and squeeze out some epoxy in each direction, then move to the next hole. I got great coverage, and as you can see in the photos below, the dams I installed previously on the motor mount worked perfectly. If I was to do it again, I'd have made the dams narrower and saved some epoxy, as they aren't adding any strength past probably 1/8 - 1/4" away from the fins, but I had been worried about alignment of the motor mount and fin slots. Looks like I shouldn't have been worried, everything worked out fine.
You can see here the jig in use. The nice thing about the payloadbay jig template is that it prints fin thickness too, so there's no guess work or sloppy wobble so long as you cut right on the lines. The final fin alignment was near perfect. I did make one little un-fixable error here though. The fin slots in this kit are cut LONGER than the fin tabs. That means there is about 1/8" play forward and back when the fin is in the slot. I had intended to place all the fins hard against the back edge (maximizing the Cp towards the rear). This worked fine on the first fin. On the second fin, I forgot to check that positioning after putting on the fin alignment guides. The friction from putting the rear one on slid the fin to the 'forward' position, and I didn't think to correct it. I noticed as soon as I took the guides off to prep for attaching the third fin, which I put on 'correctly' at the back edge. Thus one of my three fins is about 1/8" further forward than the other two. I don't expect this to make any significant difference in flight characteristics, so I'm going to give it a pass. I caught it long after the epoxy had hardened and was uncorrectable at that point anyway.
Next up is the fin fillets, possible even this week. Because the fin tabs are a tiny bit longer than the gap between the motor mount and the body tube, and I neglected to sand them sufficiently to bring them fully in contact, there is a very small gap, probably 1/64" or 1/32" between the bottom of each fin and the BT. I plan to fill it with some medium CA prior to filleting to stop any epoxy from squeezing through when I form the fillets. Anybody see an issue doing that? I know CA ain't great for bonding strength, but I'm just looking for gap filling. it'll all get encased in fillet epoxy, so I think I'm good. Thoughts?
Sometime in December I created fin alignment jigs out of foam board and templates printed from payloadbay.com I made three, so that I'd have one spare to support the other end of the tube. This let me pick the two that fit best to use on the fin itself when actually gluing them in. I tried to keep them at a minimal size (thus the 6 sides instead of only 3). The weak points at the tips of the fin cut outs I reenforced with some painters tape. Any tape would have been fine, I just happened to have that sitting next to me that day.
Using a method I've seen described somewhere else on this site, I lightly beveled the fiberglass fins. These fins are only 1/16th thick, so I didn't take much. I tried to only remove about 1/3 of the thickness from each side. I used a 'jig' essentially comprised of a flat board with two more pieces of wood (approx. 1/2" square?) screwed to it. Sandpaper was screwed down to lay flat along the base board. The fixed width/height allowed me to sand a consistent angle on each fin. I had to reposition the sandpaper a few times, I think once for each fin, as the repeated sanding in one location wore it down pretty well along the straight line/corner where the fin made contact. The process generated a lot of fiberglass dust, and as expected, caused a fair amount of itching, even when I did this with latex gloves on. Wrists and arms still got hit a bit. Even for this minimal work, a respirator is essential. Lots of this dust ends up in the air and you don't want to breath it. In my opinion, a dust mask isn't going to cut it for this. It may be tough to see in the photo, but I've also sanded to root edge, tab, and fillet areas of the fin in preparation for epoxy.
I described a while back that I had drilled some extra holes along the fin slot. You can see in this picture how I've used a syringe to inject the epoxy (rocketpoxy / G5000, tinted black for visibility) into the space between the airframe and the motor mount. This process seemed to work very well. I'd mix and fill the syringe, insert into each hole and squeeze out some epoxy in each direction, then move to the next hole. I got great coverage, and as you can see in the photos below, the dams I installed previously on the motor mount worked perfectly. If I was to do it again, I'd have made the dams narrower and saved some epoxy, as they aren't adding any strength past probably 1/8 - 1/4" away from the fins, but I had been worried about alignment of the motor mount and fin slots. Looks like I shouldn't have been worried, everything worked out fine.
You can see here the jig in use. The nice thing about the payloadbay jig template is that it prints fin thickness too, so there's no guess work or sloppy wobble so long as you cut right on the lines. The final fin alignment was near perfect. I did make one little un-fixable error here though. The fin slots in this kit are cut LONGER than the fin tabs. That means there is about 1/8" play forward and back when the fin is in the slot. I had intended to place all the fins hard against the back edge (maximizing the Cp towards the rear). This worked fine on the first fin. On the second fin, I forgot to check that positioning after putting on the fin alignment guides. The friction from putting the rear one on slid the fin to the 'forward' position, and I didn't think to correct it. I noticed as soon as I took the guides off to prep for attaching the third fin, which I put on 'correctly' at the back edge. Thus one of my three fins is about 1/8" further forward than the other two. I don't expect this to make any significant difference in flight characteristics, so I'm going to give it a pass. I caught it long after the epoxy had hardened and was uncorrectable at that point anyway.
Next up is the fin fillets, possible even this week. Because the fin tabs are a tiny bit longer than the gap between the motor mount and the body tube, and I neglected to sand them sufficiently to bring them fully in contact, there is a very small gap, probably 1/64" or 1/32" between the bottom of each fin and the BT. I plan to fill it with some medium CA prior to filleting to stop any epoxy from squeezing through when I form the fillets. Anybody see an issue doing that? I know CA ain't great for bonding strength, but I'm just looking for gap filling. it'll all get encased in fillet epoxy, so I think I'm good. Thoughts?