Upscale 3" Big Bertha

Scratch built upscale 3" Bertha. Tube, CRs, and nose from balsamachining.com. Fins made scratch in spirit of mega der red max (1/8 plywood core with voids for weight reduction with balsa skins).

Started with the fins ... printed upscale from original pattern and traced on a 12x24 sheet of 1/8 in ply. Cut out with jigsaw. Stole dowel idea from qquake for stack sanding and drilling. TTW to MMT roots.

I use 12x12" granite floor tiles and a weight (in the case a chop saw) to do the fin layup.

Cut fin slots in the balsamachining.com 3" tube. Taped off front and back of fin tab and used aluminum angle to mark and cut. Dry fit the fins (no pictures of fin shaping, sorry!). Final shot is dry fit next to my MDRM.

Long motor tube to reduce pressurization requirement. Put a 29mm engine block just below the upper CR to help strengthen where the recovery is looped around the MMT.

Glued 1/2 ply to top and bottom CR for rail buttons.

Keeping aft CR dry for internal fillets.

All 4 fins installed for initial tack (double glue joint to MMT). Tomorrow will be structural fillets inside and out. In cardboard and wood I use only titebond to keep things light but strong. This will be going up on 29mm F and G motors with a baby H if I'm feeling lucky!

First set of structural fillets (interior and exterior) down. Given the heavy bead of Titebond 2, wont get to another set until tomorrow. But should be super strong and relatively light weight when done!

Good build! That'll take more than a baby H no problem

Nytrunner said:

Good build! That'll take more than a baby H no problem

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Thanks! Yeah I'll probably toss an H128 in there at some point. This is the last of 9 Bertha models from BT5 through 3". My goal is to fly every motor class from 1/4A to G on only Berthas at a single launch

Clean and thoughtful build...

I have the same love for the Berthas. I like the idea of 1/4 A through G at a launch. I'm inspired!

Anybody have a scan of the Super Big Bertha Decals? I have a 3" in the build pile as well.

dmgrime said:

Cut fin slots in the balsamachining.com 3" tube. Taped off front and back of fin tab and used aluminum angle to mark and cut. Dry fit the fins (no pictures of fin shaping, sorry!). Final shot is dry fit next to my MDRM.View attachment 366474View attachment 366475View attachment 366476View attachment 366478

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That is such a crazy shot beside the chair! They look stock until you see the chair behind them! So cool!

Very nice!!!

Very cool!
Question: How in the world did you cut such straight lines with a jigsaw freehand?

Tobor said:

Very cool!
Question: How in the world did you cut such straight lines with a jigsaw freehand?

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I didn't [emoji2] Well kind of ... the jigsaw cuts were OK ... but then I doweled the 4 fins together and took a belt sander to them to square up. Works pretty good!

dmgrime said:

First set of structural fillets (interior and exterior) down. Given the heavy bead of Titebond 2, wont get to another set until tomorrow. But should be super strong and relatively light weight when done!
View attachment 366535

Click to expand...

Finally got all 4 sets of structural fillets done. They are quite thick so I may just finish with CWF instead of TBMT (aka Quick & Thick).

Slow going due to work, life, and cold temps!