Polecat Aerospace 4" Black Brant III

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Flyfalcons

Well-Known Member
Back home and will hopefully make some progress in the next few days. I checked the hardware - the U-bolts and nuts are 1/4-20. The holes drilled in the wood (at least of the remaining bulkheads I have) are 1/4", and the metal backplates are drilled to 5/16".

Flyfalcons

Well-Known Member
Last night I added the lower fillet on the upper centering ring and tied the nylon harness on. I just did a test fit to make sure the fins fit properly with the mount in place and all looks good. I am setting up the motor tube to be flush at the bottom of the body tube because I purchased a flanged retainer. If you get a standard retainer, you'll want to figure out how much the motor tube needs to overhang before gluing in the centering rings.

I had to give the fins slots a pass with my sanding block for a perfect fit, which I'd rather do than have loose-fitting fins from the factory.

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Flyfalcons

Well-Known Member
The nose cone bulkhead needs some sanding to get the best fit inside the nose cone, where the fiberglass layers add high spots here and there. I used an alignment mark to make sure I was fitting the bulkhead in the same position every time I sanded it.

Flyfalcons

Well-Known Member
A giant epoxy fillet seems to hold the nose bulkhead in well enough.

Flyfalcons

Well-Known Member
I beveled the leading edges of the fins a bit. They don't go to a point; there is about a 3/32" thick, square leading edge on the fins to help in the durability department during ground handling. I also lightly broke all the other squared-off edges to help prevent paint cracking.

Flyfalcons

Well-Known Member
I'm attaching the fins as per the instructions, which follow the time-honored method of 1) Glue fin in place. 2) Spend the next 30 minutes second-guessing yourself and tweaking it by .008 degrees at a time until the glue sets and you can't move it anymore. 3) Once the glue is set, never sight down the fin again for fear of noticing it being off by .003 degrees.

Flyfalcons

Well-Known Member
I did use the "double butter" method of gluing the fin, which results in a decent internal fillet. The instructions say an external fillet from the fin to body tube is unnecessary, but I'll still be doing those as well.

Flyfalcons

Well-Known Member
I had to take a picture of my 40% scale BBIII next to my 43% scale Edge 540.

Flyfalcons

Well-Known Member
Laying some Bondo down while the first set of fillets are curing. Looks like the Bondo will do a great job of filling the fabric weave while not being too difficult to sand down. So thanks to Jeff for the suggestion on that! Fillets are plain BSI 30-minute epoxy.