·:[ Darryl's Coot Building Journal ]:·
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[7 hrs] Here are some ribs for the vertical fin I built over the last couple of weeks.

The two ribs with lightening holes are flanged. These ribs were built out of some old paper-coated 2024-T3 aluminium 0.02″ stock that I had laying around. The material underneath is a brand new sheet of 12′ x 4′ x 0.02″ 2024-T3 for more parts. I also got a new sheet of 12′ x 4′ x 0.016″ 2024-T3 to use in the vertical fin and other tail components.

This evening, I rough-cut out the vertical fin skins from the 0.02″ 2024.

The vertical fin ribs require three sizes of lightening holes. And the holes must be flanged. I built these three flanging dies out of 2024-T3 aluminum bar stock.

A bolt goes through the center hole and a nut is tightened to crush the halfs together.

[4 hrs] After taking a detour to build a brake, I’ve actually created what appears to be an airworthy part. This is the 314-1-6 rib that is at the bottom of the vertical fin. It starts with 0.02″ 2024 T3 aluminium (coated with paper), with the pattern transfered onto it:

After drilling corner holes, cutting, bending and drilling lightening holes, the resulting rib looks like this:

Notice that there is also a 314-1-18 rib below the -6 rib. This is the top rib in the v. fin assembly, and I made it before making the brake. The rather mediocre result of making this rib without a brake prompted me to find a better solution. I’ll remake the -18 rib, since this is part of the structure on which a critical pulley for the elevator is mounted.

The last thing needed for the -6 rib is to flange the lightening holes. That will require making some flanging dies.

I have an el cheep-o sheet metal brake designed for doing duct work. It doesn’t allow for an adequate bend radius for doing aluminum in aircraft. I modified it a bit so that I can move the clamp back and put in a radius die. Several problems remain; like this brake, mine has tension adjusters. Unfortunately, the surfaces are warped in the wrong direction—the tension adjustment makes the warp worse! (Clearly, I need to pre-stress the metal in the opposite direction, but that is a pain). The other two problems are (1) it is not a finger brake, and (2) it is only 48″ wide.

What Coot builders need is a brake that allows bends from ~2″ wide to 62″ wide for the vertical fin spars. But, with a few exceptions, the sheet aluminum parts are pretty thin. For instance, most of the vertical fin parts require 0.02″ 2024 T3 aluminum. Ideally, we would all own light duty, extraordinarily wide finger brakes (or 2 brakes: an extra wide brake and a smaller finger brake), that allow changes in the bend radius. Good luck with that one–you are talking $1000 worth of equipment.

Commercial brakes tend to be way, way too heavy duty (and expensive) for Coot builders. So, here is my proposed solution: a ~$25 brake made of two 4×4s and 2×4s. Two 72″ 4×4s are connected together with 2 heavy duty hinges. I routed out the end of the 4×4s to the depth of the hinges:

I turned some scrap 1.06 OD pipe down to 1.0″ on the lathe just because I had a 1″ forstner drill bit for putting handle holes in the moving surface. The final surfaces were hand-shaved with a draw knife to remove slight height differences along the length of the 4×4s.

The whole thing is fastened to a bench using angle brackets. A 2×4 or 4×4 acts as the top surface, and is plained to a greater than 90 degree angle in a jointer, and hand-shaped for the proper bend radius. The tricky part is to clamp a 62″ wide piece of 2×4 to keep the bend radius uniform along its entire length. The solution is pretty simple, though. I have two “jobber” work benchs in which I can drill holes. (They are made out of retired concrete forms that were later turned into a 2′ high drum stage by adding 4×4 legs. Eventually, they were retired and donated to me. I stacked ‘em two high and bolted them together). A few pieces of all-thread and scrap wood makes clamping easy and flexible.

Here is a test run of the brake. I used some scrap 0.062″ 2024-t3 and cut a trapezoidal-shapped “rib” (i.e. wider on one end than the other, as many tail pieces are), drilled some corner holes and cut out the corners:

Now it is clamped in place:

Here is the result of the first bend:

And the second bend is in process:

Here is how the end pieces are done. First clamp for a 90 degree bend:

…bend to 90 degrees, and then add a wedge to exceed 90 degrees:

And here is the finished piece:

Not bad! This brake should allow me to make light aluminum parts for the tail and other assemblies. It’ll not be able to bent parts for the engine pylon, which uses 0.125″ 2024 T3. That is probably a job for a real brake.

Update: Oops…after writing this, I realized that the main spar for the vertical fin is actually 0.04″ 2024 T3—thicker than the test piece. I guess I’ll be testing the capacity of the brake with an expensive sheet of long and thick aluminum soon….