The  Vertical Fin and Rudder are made from 6 to 8 lb. balsa sheet.

The finished target weight for this part is 17 grams, so in order to have any chance at all at achieving that weight, we need to be very selective in the wood that we use.

I started by cutting the various long pieces to shape and pinning them to my building board.

Almost all the balsa pieces are 3/8" thick, but you can see that the trailing edge is only 1/8 inch thick.

Starting at the bottom, I started gluing the various horizontal pieces into place.

In the interest of speed, I only roughly shaped the pieces before gluing them in place...  The final shape will be easier to achieve after all the parts are glued.

Add in the various triangular bracing pieces...

I continued up the rudder, shaping and adding in piece after piece.

Note that the plans call for "medium hard" balsa sheet for the 3 ribs that radiate from the rudder hinge point.

The rib thickness transitions from 1/4" to 3/16" to 1/8" balsa sheet the higher we go.

Work carefully to achieve 100% wood contact for the strongest joint possible.

With all the ribs in place, I just needed to finish off the trailing edge of the rudder.

In order to attach the trailing edge, I v-notched the trailing edge slowly and very, very carefully - test fitting repeatedly until all joints were just right.

I then place 1/8" scraps of balsa under the trailing edge to lift it to the middle of the rudder ribs.

I then glued the trailing edge in place.

Final assembly finished - complete with triangular bracing pieces at the top and bottom of the rudder.

From this angle we can see how the 1/8" shims lift the trailing edge to the middle of the ribs.

For the rudder hinge I took some Kevlar and cut on a 45 degree bias.

I'm actually making more than one Bubble Dancer simultaneously, so I'm using enough material for several hinges...

Next, I stretched the Kevlar as far as I could - trying to achieve a roughly 20 degree bias.

Remembering that the finished target weight is 17 grams, I'm checking how I'm doing - no too badly, since I will be removing a lot of wood during the airfoil sanding process.

I set my sanding block on a 2:1 angle and beveled the fin-to-rudder hinge joint to the specified 25 degree angle.

A closer view of the hinge bevels.

I had spread laminating epoxy on the Kevlar - leaving it sandwiched between two pieces of Plexiglass until it cured.

Now I needed to cut a hinge from my hinge stock.

Hinge is cut.

Weighing everything...

I will admit that this is more that double what this hinge should weigh for the simple reason that the plans call for .8 oz. Kevlar - which I've never bothered to purchase... so this hinge is made from the 1.7 oz. Kevlar that I have in stock.

I cut the hinge slot in the middle of both the rudder and the fin and then inserted the hinge.

I glued the hinge in place with CA. 

Using CA can be problematic if it is allowed to stiffen the Kevlar fibers across the hinge-line.  So I tested the idea out first on some scrap pieces and discovered that as long as I was careful to get most of the CA into the wood, and flexed the hinge while the CA was curing, the joint turned out absolutely fine.

In fact the final joint is quite "floppy" - excellent!

Hinge inserted - I sanded the outline shape of the assembly and re-weighed.

To shape the fin, I followed the documentation available on the CRRC website.

 

I drew out extended lines from each edge of the fin to converge at the edge of my workbench.

 

I then prepared my long sanding bar with 80-grit sandpaper on one half of the bar.

I taped across the fin at the location where the tapering ends to protect the wood from inadvertent sanding.

 

Next, I placed a half-thickness balsa shim at the bench edge on which the smooth portion of the sanding bar would rest. 

Then I began sanding across the fin until the end of the bar began lightly scratching on the blue tape.

Once the first side was done, I removed the shim, flipped the fin over and sanded the other side in the same fashion.

 

In this way an even taper is achieved on both sides.

 

I then repeated the process to taper the bottom portion of the fin.

To create the airfoil in the fin, I marked of lines a 5%, 10% and 25% of the fin chord.

Then using the slope values given on the plans, I set the fin on a block of wood and began sanding up to the lines.

Here I am sanding up to the 5% chord line.

Again, using 80 grit sandpaper makes the process go quite fast.

Once the 5% line has been reached, set the fin back and repeat the process - now sanding up to the 10% chord lines.

You can see that a longer sanding block will be needed.

The back portion of the fin is shaped by turning the fin around and sanding up to the 25% chord line.

Repeat this process for both sides.

I cut out a drawing of the rudder horn and stuck it to some carbon fiber plate.  I had made this plate earlier from 3 layers of 5.7 oz. carbon pulled to 20 degree bias.

I then used a strong utility knife to cut the carbon to the approximate dimension.

I finalized the by wet-sanding.

Note:  Some sandpaper wrapped around a wooden dowel works well to shape the concave curve of the horn.

Once the horn was shaped I checked it against the plans and located and drilled the pushrod hole.

I then lightly sanded the part of the horn that will be inserted in the rudder, so that it will glue better.

Content in progress...Using a sanding block, I created a notch in the end of the tail boom that will hold the fin in place.

I then wrapped some sandpaper around a dowel and very carefully rounded out the inside of the fin where it will mount on the boom.

You want to ensure that this groove remains centered and straight.

Here we see how the fin mounts in place on the boom.

Glue the rudder horn in place with some epoxy.

Then secure the horn with some .75 oz. fiberglass.

Turn the fin over and apply a small .75 oz. patch of fiberglass to the back side of the rudder horn as well.

Blot up excess epoxy and then once the epoxy has cured, sand lightly to make a smooth transition.

On the first Bubble Dancer I made, I left the covering of the entire fin/rudder until after the fin was glued in place... that was rather difficult.  So this time, I covered the rudder portion first - the fin portion, unfortunately; can only be done once mounted.

At any rate, using thickened epoxy on the curved slot, mount the fin to the boom.  Obviously be ready to properly align the fin while it cures.

Then using .75 oz. fiberglass, apply patches to the front and back of the fin.  Once it cures, trim and sand as needed.