A Method To Make Overbuilt and Excessively Time Consuming Compound Beveled Fins

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Oct 25, 2016
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These are for low powered (and possibly mid powered) models, if anyone's interested. Posting will be of individual steps and sporadic (when I have the time and energy), but I'm aiming for completion by the end of the week.

Feedback is welcome.

Disclaimer: I'm not responsible for any damage it causes.
An hour and a half since posting... It's already excessively time consuming! :D
Incongruent has the attention span of a 14 y.o. :wink: But I've seen a sample of his finished work recently and it is very, very, very good ! Worth the wait I think.
Sorry about the wait. Anyways...

The first step is to get the scaled measurements and draw out the fin on a piece of stiff paper. I use matte board since it's available from art at school but cereal boxes, etc. will work as well.

On the last fin, I had a TTW tab which prevented the bottom from being squared up well. This time I'm trying a tab into the fin which will expose the bottom until the fin bottom is finished up.

This fin is based off of the measurements of the Patriot missile listed here:


Next, the template gets cut out and the edges are reinforced with CA, then sanded smooth. You can see how the fin and tab should fit together in the second picture.

It's possibly important to note that the tab is there to create an internal mounting point for necessary strength (and thus eliminate the need for an external fillet), and is in all likelihood not suitable for higher speeds.



The pattern is traced onto 1/32 ply and cut out. The sides are very slightly sanded. (To remove any splinters sticking out.) Too much sanding can make the fin undersized and is unnecessary at this step, though it doesn't hurt to sand now so long as is not overdone. (1/64 ply and other thicknesses will work as well, and as shown later the 1/32 ply is slightly flexible at the thin area, enough for me to make a stupid mistake. :facepalm:) The line where the leading and trailing bevels meet is marked for a 1/64 ply that will be added next.

The spars are cut by creating a strip of 1/64 inch plywood with a width slightly over the length of the bevel intersection (the line where the leading and trailing bevels meet) and from that, cutting trapezoids with the two bases a bit over the necessary lengths as defined by [h=(sD-T)/2] where sD is the scaled distance and T is the plywood thickness. If the fin has a compound bevel, as many scale fins do, then the trapezoid will narrow from root to tip. Make sure that the spars are flat, because they will be sandwiched between two peices of balsa and it really eases the job if you don't have to sand the balsa into a curve. It's also more true to scale (in most cases).

The quality of the cut doesn't matter as long as the side to be attached to the fin is roughly flat. The other sides will be sanded down and the spar will be reinforced from either side with balsa.

Next, the spars are held roughly perpendicular to the fin and along the previously drawn line on one end by a spring, with the each side overhanging the fin. For that, open the spring around the spar and let it pinch down, then place the spar as close as easily possible to the line and nudge it to place with a small, thin scrap of paper or wood. (alternatives work too) This is the usually most frustrating part.

Now drip a small amount of thin CA onto the spar, letting it wick itself into the joint, without letting the tip of the bottle touch the spar. The CA alone doesn't change the position, but if the nozzle touches, it can. I haven't seen the spring get glued yet, placing the drop about a centimeter (~1/2 inch) away from the spring. After that cures (not very long), the spring can be removed and if desired, the joint reinforced with a little extra CA, only in the joints with the excess wiped off, so that sanding the spar to shape is easier. Wood glue can get in the way of the balsa added later and doesn't go into the joint as well.

Now the fin and spar can be sanded to shape. A little extra plywood is good on the leading and trailing edges for the less careful. (Me) Verify the measurements, especially of the spar.





Balsa is cut oversize with the grain parallel to the spar. For this, place the fin onto a sheet of balsa thick enough to create the bevels and hold is to the spar and the edge of the wood are touching. Trace around (pencil works) this with as much extra room as you trust yourself not to cut into for each bevel section. Cut accordingly and contemplate on either how you cut well and wasted much balsa or gave yourself too little leeway and messed up a piece. Get over it and sand the side to be attached to the spar smooth and flat. (The side to be glued can also be sanded flat, but either way works.) Spread a thin film of glue onto the plywood surfaces in one quadrant, squeegee off the excess, and glue a corresponding balsa piece flush with the plywood. Repeat for the other piece on the same side, then after sufficient drying, cut the balsa to size (maybe a bit over), covering the slot for the TTW tab. Do the same for the other side.

My cuts are sloppy,
don't try and copy.
Time to change knife blade.

Sand the edges square. If little enough wood glue was used, the slot for the fin tab should be usable.
The tab in the pictures is a scrap used for demonstration. I do not condone crooked cuts that are supposed to be straight!

Yet oddly typical.





Maybe not as complex, but in the spirit of making an LPR build overly complex. You've definitely gone way above and beyond what I'm doing here :)

I'm doing something similarly (and needlessly) complex for my BT-70 and BT-80 Bertha upscales. For my BT-70 I'm using 1/8" basswood cores drilled out to reduce weight, skinned with 1/64" finnish ply skins (3ply). For the BT-80 I'm using 1/8" lite ply cores, also hollowed out, and 1/32" finnish ply skins. They've been fun to build and I've been toying with the idea of a stained finish given how good the ply looks! Don't have a build thread going but will try to grab some pics of what I've got so far (BT-70 is mounted in fillet stage, BT-80 fins are done but not mounted).
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Now for the sanding. :shock:

First, the taper is sanded into the fin, using the spars as a how-much-more guide and measuring every few strokes to verify. If you end up slightly thinner on one side than the other on the front and back ends of the fin, it's okay — you only need the center to be correct, since the sides will be sanded down into tapers anyways. I used 220 grit.

Once the tapers are sanded to satisfaction, masking tape is laid with one edge along the plywood spar as a sanding mask. The uncovered side is then sanded into a bevel reaching the center ply of the plywood, or more or less depending on preferance. (once again with 220 grit)

There was an issue that arose while sanding, however, and it was that the fin became thin enough that it flexed (this was accentuated by the lack of plywood in that area, the tab should probably be further back in similarly shaped fins) and too much balsa was taken off, which created a concave shape instead of a flat one. Our troubleshooting team is working on the issue and will be back later with a solution. Fortunately, despite the poor sanding, the fin slot slot was still intact.

Anyways, after the bevels are sanded, the masking tape is removed and another piece is applied to the other side and the process repeats. Chances are that if you're anything like me, the sanding is lopsided and you're not satisfied. So next, the fin is shaped further until satisfaction.

The edges get squared up if needed and the fin can be sealed, filled, and finished to preferance. After that, the tab gets glued into its slot.

That's all, folks!

Well, for the bulk of this post anyways. More pictures and stuff later.





While this technique has been around a long time, this is one of the first times I have seen someone take the time to actual make a tutorial. One of the keys to it working properly is using the harder wood as a guide so the softer wood is not sanded down too far.
Nice work so far Incongruent.
That turned out pretty well I'd say. I think you could get by with a thicker plywood core, like 1/16", to stiffen it more and reduce the flex you encountered. You could even drill lightening holes in the core to shave some weight if it's a concern.
Cool! You can also skin using cardstock. Similar technique with spars an stuff, but skinned with cardstock.
Rich is absolutely right, pictures are worth thousands of words. Hand shaping small pieces of balsa can be fun, nostalgic, therapeutic, and personally satisfying. Nice job !
I find it time consuming. :p I want them to look perfect and I want them done NOW! :D

But yea every now and then, I feel ya samb.
If you're using 1/32" thick ply, you could lengthen cut the tab nearly full length of the root edge. That might help prevent some of the flex you're getting.
If you're using 1/32" thick ply, you could lengthen cut the tab nearly full length of the root edge. That might help prevent some of the flex you're getting.

The issue with having a tab at all is that you restrict the bottom from being easily sanded square. I'm trying to see if having a slot that a tab will insert into will work, which is probably where most of the flex came from: the much narrower plywood near the tab slot. I was afraid to put the slot at the base of the spars, where the thickest balsa will be, but I'm trying that now. Hopefully the slot still works well and flexing is reduced.
Oh No!
I forgot to post pictures of the fin after finishing with CA.
The process it as usual: apply thin CA, sand, apply more thin, sand, etc. until satisfaction.

Here they are: