Super Big Bertha -- Part 2: The fins
The Thrifty Rocketeer continues...
Well, I couldn't resist, so I started the build of the Super Big Bertha that Santa left me on my front porch.
One of the things I love about kits like this are the Through-the-Wall construction, as well as the sandwich fins. The first time I encountered this was in my Mega Mosquito kit, and I loved it and learned a couple of things. Apparently, not well enough too...
The first step after slicing open the package with the instructions in it, was photocopying the second page so that i wouldn't have to slice up my instructions. But the next step was to lay-out the balsa. I discovered two different sets of balsa: One set of four fins was about 1/8" thick, and one set of 8 fins was 3/16" thick and a solid sheet. That is, these were the exterior fin sheets. The thicker sheet had cut outs in the shape of triangles, that I removed and saved. (You never know when you're going to need this spare, scrap balsa.) In fact, I saved all the spare, scrap balsa "frame" once the fins were removed.
I use an Exacto knife to slice through the few connecting tabs that remain after the laser cutter has completed the trace of the fins. The technique of cutting the fins out is so exact now, that the fin basically falls out of the frame once the four remaining tabs are sliced. And I like it that way. Except for removing those few remaining tab stubs with a very light sanding, the fins are ready to go, right out of the frame.
One of the things the instructions state is to align everything in a dry fit before gluing. If I had read the instructions a bit more closely, I would have caught that the instructions specify CA glue to bond the three layers of the fin together. I overlooked this, as the instructions are printed in three languages, and the only designation is the CA on the glue bottle icon, as opposed to the wood glue icon which has no letters on the bottle.
So I goofed. I used Elmer's wood glue and spread it over the interior "struts" of the fin interior slab. That worked well, and I carefully positioned first one fin "cover" over this glued middle slab, and immediately weighted it down under a heavy book, as described to prevent warping. I'm glad I checked before leaving this to dry, as the top "cover" skated slightly and slid to one side. I would have had a mess trying to make a coherent fin if it had dried this way. I re-positioned the fin and more carefully laid the book on it.
I did this four times, being very careful not to allow any further skating of the compressed fins.
By the time I got the fourth fin set and compressed, it was time to check the first, and I found most of the wood glue had absorbed and was bonding. I flipped the fin over and proceeded to attach the outer fin cover to the other side. Again, this was done four times, and weighed down to prevent warping, which was very evident when not weighed. It worked.
Now, since I didn't catch this was supposed to be done with CA, I don't know how much different this was going to turn out than intended. I suspect the CA would have penetrated the balsa fin more and strengthen it at the cellular level. But, I am very pleased with how my wood glue sandwich fins turned out. Only one was slightly off position, and not enough to bother with.
After the fins dried overnight under the heavy books, I was ready to sand the leading edge round with a orbital palm sander. I choose 80 grit, which went incredibly fast, trimming the dust off like melted butter. It was easy to go too far, too fast, if you weren't checking constantly.
In a matter of minutes, I had all eight edges feathered down to almost a sharp leading edge of the fins.
I ran a brief hand-sand of 220 grit over them to true up any gouge and set them aside.
My next choice was to mix and apply a slurry of Elmer's wood glue over the exposed balsa surfaces. My choice is two drops to a 2-liter bottle's cap full of wood glue for the first coat. After drying and sanding, the second coat is ONE drop of water to a cap-ful of wood glue. I found that with a little help from an overhead fan, the fins were drying within a half hour and by two hours, I had sealed the fins. (NOTE: I do NOT seal the tabs that stick through-the-wall into the body tube. I leave that exposed raw balsa, so to accept and absorb wood glue later.)
I'm going to stop here and let the fins all dry as I turn my attention to the motor mount tube assembly for next installment. Once again, I'm doing things out of order to speed up the build, and it will cause me to make another mistake. Next time.
Until then, keep your glue capped and save your excess balsa trip pieces. And don't forget to rinse out your paint brush between applications of your wood sealer!
Well, I couldn't resist, so I started the build of the Super Big Bertha that Santa left me on my front porch.
One of the things I love about kits like this are the Through-the-Wall construction, as well as the sandwich fins. The first time I encountered this was in my Mega Mosquito kit, and I loved it and learned a couple of things. Apparently, not well enough too...
The first step after slicing open the package with the instructions in it, was photocopying the second page so that i wouldn't have to slice up my instructions. But the next step was to lay-out the balsa. I discovered two different sets of balsa: One set of four fins was about 1/8" thick, and one set of 8 fins was 3/16" thick and a solid sheet. That is, these were the exterior fin sheets. The thicker sheet had cut outs in the shape of triangles, that I removed and saved. (You never know when you're going to need this spare, scrap balsa.) In fact, I saved all the spare, scrap balsa "frame" once the fins were removed.
I use an Exacto knife to slice through the few connecting tabs that remain after the laser cutter has completed the trace of the fins. The technique of cutting the fins out is so exact now, that the fin basically falls out of the frame once the four remaining tabs are sliced. And I like it that way. Except for removing those few remaining tab stubs with a very light sanding, the fins are ready to go, right out of the frame.
One of the things the instructions state is to align everything in a dry fit before gluing. If I had read the instructions a bit more closely, I would have caught that the instructions specify CA glue to bond the three layers of the fin together. I overlooked this, as the instructions are printed in three languages, and the only designation is the CA on the glue bottle icon, as opposed to the wood glue icon which has no letters on the bottle.
So I goofed. I used Elmer's wood glue and spread it over the interior "struts" of the fin interior slab. That worked well, and I carefully positioned first one fin "cover" over this glued middle slab, and immediately weighted it down under a heavy book, as described to prevent warping. I'm glad I checked before leaving this to dry, as the top "cover" skated slightly and slid to one side. I would have had a mess trying to make a coherent fin if it had dried this way. I re-positioned the fin and more carefully laid the book on it.
I did this four times, being very careful not to allow any further skating of the compressed fins.
By the time I got the fourth fin set and compressed, it was time to check the first, and I found most of the wood glue had absorbed and was bonding. I flipped the fin over and proceeded to attach the outer fin cover to the other side. Again, this was done four times, and weighed down to prevent warping, which was very evident when not weighed. It worked.
Now, since I didn't catch this was supposed to be done with CA, I don't know how much different this was going to turn out than intended. I suspect the CA would have penetrated the balsa fin more and strengthen it at the cellular level. But, I am very pleased with how my wood glue sandwich fins turned out. Only one was slightly off position, and not enough to bother with.
After the fins dried overnight under the heavy books, I was ready to sand the leading edge round with a orbital palm sander. I choose 80 grit, which went incredibly fast, trimming the dust off like melted butter. It was easy to go too far, too fast, if you weren't checking constantly.
In a matter of minutes, I had all eight edges feathered down to almost a sharp leading edge of the fins.
I ran a brief hand-sand of 220 grit over them to true up any gouge and set them aside.
My next choice was to mix and apply a slurry of Elmer's wood glue over the exposed balsa surfaces. My choice is two drops to a 2-liter bottle's cap full of wood glue for the first coat. After drying and sanding, the second coat is ONE drop of water to a cap-ful of wood glue. I found that with a little help from an overhead fan, the fins were drying within a half hour and by two hours, I had sealed the fins. (NOTE: I do NOT seal the tabs that stick through-the-wall into the body tube. I leave that exposed raw balsa, so to accept and absorb wood glue later.)
I'm going to stop here and let the fins all dry as I turn my attention to the motor mount tube assembly for next installment. Once again, I'm doing things out of order to speed up the build, and it will cause me to make another mistake. Next time.
Until then, keep your glue capped and save your excess balsa trip pieces. And don't forget to rinse out your paint brush between applications of your wood sealer!
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