Estes Aries SST, #1927, Build, Part 4, Smaller Fin Fitting

The root edge of the Rudder fins is slightly longer that the outside edge.

I marked the root edges with pencil.

The hand cut engine ribs were a bit higher than the fold height on the engine shrouds.

I sanded the top down until it fit just inside the dashed lines.

Double check the length of the engine ribs when set next to the 3/8" spacing from the root edge. Sand the rear end if needed until flush.

Estes Aries SST, #1927, Build, Part 3, Larger Wing Fins

The straight cuts were made with my break off blade knife.

Save your curved lines for an X-Acto style #11 blade.

That rear curved cut can be sanded by wrapping some 400 grit around an engine casing.

The Estes instructions suggest rounding the trailing edges of the wing. Pay attention to the single rounded leading edge of the engine rib.

I only rounded the flat side of the trailing edge near the root edge. Rounding an inside curve is difficult and unnecessary.

Estes Mini Arcas, #2441, Part 7, Fin Fill

When applying the CWF, clear that small notch above the root edge round cut out.

After sanding the dry CWF, round out the root edge cut out with a small rat tail file.

Here's the before and after CWF sand.

The small notch above the cut out aligns with the boat tail / body tube seam.

The fit of the fin root edge was very good.

The original ARCAS fins had a diamond taper. This smaller version just has a rounded leading edge. It would be very difficult to sand a diamond taper in 1/16" thick balsa.

Estes Monarch Build, #7214, Part 6, Rounding The Leading Edges

From the new Optima kit instructions:

The leading edges and outside edges are rounded.

The trailing edges remain square.

The short leading edge of the forward, smaller fins was tricky.

I used my sanding block to knock off the edges then a piece of 400 grit around my fingers to get the final round.

Here's the three forward fins,

note the squared trailing edges.

 

Here's the larger rear fins showing the rounded forward and outside edges.

Quest Intruder Kit Bash, Part 9, Joining Two Rounded Leading Edges

Joining two rounded leading edges can be tough.

How do you sand a rounded edge in a tight corner?

Round the smaller (forward) fairing piece leading edge first. Leave the leading edge of the wing fin square for now.

Then glue is to the (still square) larger wing fin.

My sanding block is at a 45 degree angle.

Round the wing leading edge while feathering the inside corner.

Don't sand hard into the corner.

Here's the finished round leading edge with (hopefully) a smooth transition between the two pieces.

Aerotech Mega Initiator #89541, Part 7, Prepping More Parts!

I know there is a LOT of parts prepping - 

Ask any wood worker. The secret to a great finish is good surface preparation.

   

Here's that inside corner on the leading edge of the fins.

On the top picture - 

With the primer shot, you can see the mismatch of the rounded leading edge at the center of the angle. Some corrective sanding was done.

Here's the same corner after a directed shot of filler/primer. More careful sanding should get things in line.

The fin after the second coat of filler/primer - 

There were some small lines that needed some CWF and more sanding.

It seems every slot was a hair too narrow.

The root edge notches and the fin slots in the body tubes were widened for a better fit.

I could slide my sanding block into the slots using 220 grit.

The fins were test fitted. 

The joint of the body tube and root edge was very good now, tight enough with no distorion of the tube..

Aerotech Mega Initiator #89541, Part 6, Fin Filling

One of the fins had a knot hole fill.

It won't be noticeable with an easy CWF fill and sanding.

This fin will go on the flying build.

Here's that inside angle trouble spot I mentioned earlier.

When sprayed with filler/primer you can better see what needs to be cleaned up with better rounding.

The root edge and fillet areas were taped off for the filler/primer spraying

The fin in back is before sanding,

The fin in front is after sanding with 400 grit.

Here's a close up of the sanded fin surface. 

It's easy to see the gray filler left in the shallow grain.

Aerotech Mega Initiator #89541, Part 5, Fin Shaping

The fins are 1/4" thick, five ply plywood. The plywood is very high quality.

The instructions don't mention it, but I'm going to round the edges on the presentation model.

I drew a pencil line for reference all the way around the exposed outside edges.

I started by knocking down the edge at a 45 degree angle with some 220 grit on a block. Sanding with the block helps keep the width consistent.

The tricky part is where the concave edge meets at the break in the leading edge corner. This took the most work.

You won't be able to judge how well the rounded edges meet until you spray it with some filler/primer later on.

On the left,

is the first rough knock down of the right side trailing edge.

On the right is a good rounded edge on a different fin.

I wouldn't sand the blackened laser cut root edge until you check the fit before gluing the fins thru the wall and against the motor mount tube.

Estes Constellation Build Part 9, Fin Pod Gluing

Each fin will be individually fitted.
Draw a line on the side of the pod side that will be glued to the fin.
Mark the outside edge of the fin with a "1" and the matching pod "1".

Match the edges of the pod and the outside curved edge of the fin. Lightly trace around the side of the pod.
(While not shown well in this picture, the tip of the pod should be even with the leading edge of the fin.)

Carefully cut off the pod recess.

After a bit of shaping checking against a bright light, the pods fits pretty well.

Glue fillets filled in the seams.
The fillets go from narrow at the tip to wide in the rear, the same shape as the teardrop shaped pod.

Estes Constellation, Part 8, Airfoiling The Fins

I used the rounded side of my old sanding block with 400 grit to sand the recess at the rear trailing edge.
This seemed to be the best way to get a round concave shape.

You could also use some 220 grit sandpaper wrapped around a 24mm engine casing for the round contour.

With the trailing edge facing up - 

Here's how that sanded recess looked after sanding.
You can see how the trailing root edge tip remains thick and less likely to break off. I still have to sand a "knife edge" to the remaining trailing edge.

With the fin held upside down,
On the left is looking directly at the root edge. The tip was rounded off.

On the right is the tapered trailing edge viewed from the rear of the fin.

Estes Constellation, Part 7, Shaping The Fins

The reissue Estes kit has you round just the leading edge of the fins. 

I'm showing an earlier Semroc build that follows the original Estes instructions where the fins are airfoiled. This is difficult because of the sharp tip at the rear of the root edge. 

This post shows one way to airfoil the fin and still keep that extended tip strong.

   

The old Estes instructions tell you to airfoil the fins. That rounded tip on the rear root edge could be a problem. Sand it down to a sharp taper and you run the risk of it easily breaking off.

The fin drawing in the instructions show a shading that suggests the shape penciled in at the right. This leaves the tip at the 3/32" thickness.

The end of the outside edge pod will be replaced so it was cut off now.

It's hard to see the trailing edge taper on the light balsa wood.
The leading and trailing edges were marked with an old, half dried out Sharpie pen.
You don't want a heavy black ink to soak into the balsa! In the end the black ink will be sanded off. Any remaining ink might show through the white paint.
When marking an edge like this, draw the line quickly allowing very little ink to color the balsa.

Semroc Wac Corporal Build, Part 5, Fin Shaping C

After doing the taper from the root edge - 

Start the leading and trailing edge tapers.

I've again marked the first high point with pencil.

Round off the sides to an rounded taper. The center area is the high point.

On the left is the root edge view, 

on the right is the leading and outside edge view.

The black ink remains for now. 

Each fin took 35 minutes to shape up to this point.

Here's all three, the shaping done.

The fin on the left has had the remaining ink sanded down with 400 grit.  

The edges are lightly rounded with 400 grit. Keep the root edge square.

Semroc Wac Corporal Build, Part 4, Fin Shaping B

Start by sanding the taper from the wide root edge to the thin outside edge. Concentrate on getting the outside edge thin - you don't have to taper the full face yet.

On the right you can see the taper cutting into the black inked edge.

Continue until you get the thin outside edge to about .05" thick.

It's a compromise. You want the outside edge thin while still keeping it strong enough to survive a hard landing.

To show where we're at now, I pencil shaded the fin face down the high point sand line.

Now sand down that face joining the two different widths - a smooth flat face from the root edge to the thinner outside edge.

Semroc Wac Corporal Build, Part 3, Fin Shaping A

The kit includes a cardstock marking guide to mark the centerline of the fins. It is supposed to be half the thickness of the 3/16" thick balsa.

Set the fin into the cardstock piece and trace around the top edge of the fin. On the right, the pencil line wasn't perfectly centered.

The instructions say you could use a marker to draw this centerline - NEVER use ink when making model rockets! The ink can bleed through the finish paint.

Instead of a hard to see simple centerline, I mark a high contrast edge. 

You can use ink - IF the balsa wood is sealed and be sanded off later.

Sealing the wood stops the ink from soaking in.

I seal the edges with a wipe of super glue. This also strengthens a thin fin edge.

Drip the CA glue onto a Q-tip and wipe the glue onto the side edges - but NOT the root edge. The root edge is left  unsealed as it is glued to the body tube.

After the CA glue seal, 

Mark the sides of the fin with a wide point marker.

Don't apply ink to the root edge.

Semroc Wac Corporal Build, Part 1, Fin Shape

Here's how Estes shaped the WAC Corporal fins in the 1960s.

The tapered sides are incorrect and the tips of the fins have sharp corners.

I doubt there were many available pictures and scale data available then.

This picture implies the fin taper shown above.

The Peter Always drawing shows the taper from the wide root edge to the thinned outside edge.

Note the fin corners are rounded off.

From the Semroc instructions, 

The fin taper matches the front view above.

The root edge profile shows the correct shape of the fin, the widest point at the center.