FlisKits D-Nelson Tomahawk build summary/tips

[Grant_Edwards]

Just finished building (but not painting) a FlisKits D-Nelson Tomahawk. I should have taken photos and done a proper build thread, but the table where I was doing the build was so cluttered the pictures would have been useless.

There's still a bit of sanding to be done and fillets to be touched-up, but here's what it looks like after all the gluing is done.

I'll follow up with some comments on the kit and a couple build tips.

 

[Grant_Edwards]

Overview

The Tomahawk has a 12" BT-20 lower body tube that transitions to a 8.5" BT-55 upper body tube with a 5" balsa nosecone. The base of each 45° forward-swept fin is a BT-5 Tube cut at angles that match the forward sweep of the fins.

There's a standard 18mm engine block with 1/4" of engine and 1/4" of rear BT-20 exposed so you can do the traditional masking-tape wrap for engine retention:



Fins and launch lug standoffs patterns are provided on cardstock and you cut them from 1/8 balsa stock after tracing the patterns onto the balsa. I find a razor saw works best for straight cuts.

When I ordered the kit, I assumed that there was a balsa transition and the upper BT-55 was a cargo section. It's not. The BT-20 is mounted in the BT-55 with two fiber centering rings (1" apart), and the transition is an old-school cardstock cone. I hadn't done one of those since the Honest John I built in 1972.

BT-20 to BT-55 Transition Jig

When gluing two different diameter body tubes together coaxially, the only way I've ever been able to get decent results is to have a "cradle" that holds them in alignment. I've never been able to eyeball those sorts of joints. All you really need is a chunk of angle-aluminum and a couple wooden spacers that you've sanded to the proper thickness:





Start with them a bit too thick, and sand them down, checking the size by placing them under the the dry-fitted tubes immediately adjacent to the joint. At first, they'll hold the large body tube up out of the angle. Sand them down until the larger tube just lays flush in the angle. Always sand them together so they're the same thickness. [The piece of plywood with the V block on it is left-over from some woodworking project or other, and has proved useful a number of times since.]

Once they're the right size, you move them to the far end of the smaller body tube, and it has no choice but to lay properly aligned:

 

[Grant_Edwards]

BT-20 to BT-55 Transition Mod

The directions say to glue the two centering rings on to the B-20 tube 1" apart, then insert them into the BT-55 tube so that part of the second (aft) ring is exposed (for the paper cone to be glued to). See figure A below. If you can do that, you're a better modeller than I, Gunga Din. After fiddling with various ways to do the dry fit, I could not get the BT-55 tube to stay in position. Either there was no ring exposed and the BT-55 to cone joint would be problematic, or the BT-55 tube would "fall off" the ring on one side or the other the next time a car drove by.

I solved that problem by gluing four centering "fins" to the BT-20 tube between the two centering rings (the "fins" were cut from scrap 1/8 balsa stock). The easy way to do it is to cut them so they're a bit trapezoidal: shorter than the center ring height at the front and higher at the back. See figure B below.

Then you sand them down flush with the centering rings.

Now it's trivial to glue the BT-20 assembly into the BT-55 so that it just touches the aft centering ring, while leaving that aft centering ring exposed enough to provide a "porch" for the paper transition cone to be glued to. See figure C below.

[The yellow line is kevlar string,]

 

[GlenP]

You could get a balsa coupler and cut the top tube in the middle to make a payload section, maybe vent it for an altimeter if you wanted. Nice build, and fin/tube design. Will be fun painting the inside of those small dia tube fins.

 

[Grant_Edwards]

Diagonal Cuts on BT-5 Tubes

Most of you probably already know these tips, but I'll post them anyway.

There is a cardstock marking guide for the 45° diagonal cuts for the 3 BT-5 fin base tubes. After marking the tubes, there are a couple ways to do the angled cuts.

Hobby Knife

1. Insert a 13mm engine in the tube to support both sides of the cut.
   
   
2. Cut, leaving the line, slowly and carefully with a razor blade or hobby knife.
   
   
3. Wick some thin CA into the cut ends, and let it harden/dry completely. Try not to glue too many of the tubes to fingers, (though that's better than gluing your fingers to your bench).
   
   
4. Sand the cut ends down to the line with fine sandpaper (320-400). Paper tubes saturated with CA are surprisingly hard, so it's slow going. Be gentle and patient and you'll get better results.

Razor Saw

The BT-5 tube cuts are 45° so if you've got a razor saw and miniature miter box, that also works well. Again, leave the lines, and do the CA/sanding as described above.

It will help if you pre-cut a 13mm engine casing at a 45° angle and insert it into the tube so that you're sawing down along cut-off end of the engine casing.

 

[Grant_Edwards]

You could get a balsa coupler and cut the top tube in the middle to make a payload section, maybe vent it for an altimeter if you wanted.

Yep, I was thinking about maybe doing that and trying to mount my Estes Astrocam in the upper section looking down at an angle through a window cut in the side.

Nice build, and fin/tube design. Will be fun painting the inside of those small dia tube fins.

I think the BT-5 interiors will be painted by hand with a small brush. I've got a little glass bottle of Testors red somewhere...

 

[Grant_Edwards]

Thinned Wood Filler vs. Cardstock

I tried using my usual thinned Elmers wood filler on the cardstock glue joint and on the joint between the cardstock transition cone and the BT-55 tube. That didn't work so great. The cardstock absorbs moisture from the filler and gets soft and expands. The joints themselves turned out OK, but areas of the cone into which I attempted to feather the filler ended up slightly wavy. Maybe after drying for a day or two they'll be better. It doesn't really look bad now, but I'm sure will look worse when painted — might have to hide that with some sort of roll pattern.

 

[Grant_Edwards]

Here's What I Screwed Up

After the two body tubes and transition cone were all glued together and dried, I picked up the result and happened to glance in the upper, open, end of the BT-55 tube where the nosecone is supposed to go. That's odd, there's a ring of something on the inside of the tube that's going to prevent the nosecone from seating completely into the tube. After an embarrassingly long time, I realize that I had applied a bead of wood glue about 1/2-3/4 inch inside the BT-55 tube, set the BT-55 tube down while applying glue to the BT-20's centering fins and aft centering ring, then picked up the BT-55 tube and inserted the BT-20 assembly in the wrong end of the BT-55 tube.

That means

1. I now have to carefully scrape, sand, slice the bead of hardened wood glue from the forward end of the BT-55 tube. I was able to remove enough of the wood glue away to get the nosecone to fit. I did scrape up the inside of the BT-55 tube a little, so I coated the inside of the upper end with some thin CA to reinforce it. The fallback plan was to just cut 1" off the top of the tube and end up with a slightly shorter rocket.
   
   
2. The joint between the forward centering ring and BT-55 tube is not glued at all. After making sure the kevlar string is safely out of they way (hanging out the back of the BT-20 tube) I dripped some wood glue down onto the forward centering ring, and slowly rotated the airframe to spread the glue around around forming a fillet between the forward centering ring and the BT-55 tube. It's a good thing that Titebond-III stays runny for a while.

 

[GlenP]

You can stiffen up cardstock transitions with a light coat of white glue on the inside surface, before you install it. So that won’t help you much now.

for engine mounts or baffles that need an extra fillet at the bottom of long tube, just out of reach, I drip some glue in there and use a long dowel to spread it around a bit. The runny glue rotation method works great for that too.

 

[Grant_Edwards]

You can stiffen up cardstock transitions with a light coat of white glue on the inside surface, before you install it.

I'll have to try that next time. I've got a pile of cardstock left over from making CD case inserts many years ago, and I've seen applications that will generate transition patterns to print. I should have made a few practice ones and done some experimentation. I also had a hard time getting the double-thickness part of the cone to curve smoothly.