AC 43.13

It’s the bible for aircraft builders and mechanics. Everyone should have a copy of it. At least download it and have it near by on an iPad or something. You can find a copy of it here. Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair

I had to drag my copy of the book out today. Greg and I spent a lot of time today working on the aft baffle. We started by making the most accurate template we could, which took quite a bit of time really. But we ended up with a great fit.

Then we got to thinking about how we would transfer the pattern to a sheet of aluminum stock. The foam core board is representing a thinner piece of material. Making our template more cumbersome, the forward surface of the vertical section was the plane we wanted to use, but the top of the horizontal section. Additionally, there are attach tabs that must be bent in a few different directions. All of this has to be accounted for before cutting the aluminum. This takes a bit of planning.

So we opted to transfer the foam core pattern to a paper template we could use to mark and cut the aluminum. But it’s not quite as easy as just tracing our pattern. You have to add material in strategic places to account for the material that a bend uses. This is called bend allowance, and AC 43.13 tells us how to calculate that.

My baffles are made of .025″ 2024T3. The AC tells us that 1/16″ is the minimum bend radius for this material. I don’t have a bending brake, so I will probably make my bends using 2×4 lumber, and the radius on the corners is 1/4″ so I chose that as my bend radius. I’ll save you the math because it is simple and explained in the AC, but I have to add .413″ of material to compensate for the bend.

Not only do you have to calculate the bend allowance, but you also need to calculate the set back and the sight line. The sight line is the mark you line up in a bending brake so the bend comes out in the proper place. AC 43.13, again, walks you through this. So I’ll save the math, but here is how it looks on my template. We had to do this for the main bend as well as all the tabs on the baffle. There will be a total of 3 bends.

Note that I just realized my template is marked wrong. The bend allowance is marked correctly, it is the .413 high area between the two darker lines. The sight line is marked wrong. It is actually the middle line. It is slightly offset from center of the bend allowance, and is dependent on which side you will clamp in the bending brake (or between 2x4s). In my setup, the sight line is the distance of the radius, 1/4″, from the side being clamped.

Next we will cut the template out and then trace and cut it out of aluminum. Then we will see if all the math and templates did their trick. If not, I’ll resort to a more rudimentary method.

In between steps of the baffle art project, Greg worked on making a new oil cooler mounting bracket. It is pretty much done, we just need to cut off some material to make it smaller and lighter. We had to make a new bracket to move the oil cooler slightly. We wanted to have room for a few other components and will have better airflow through the cooler mounting it as low in the cowl as possible.


Lastly, Sean stopped by. He did some final fitting of the seats, both front and rear. He says he will be able to finish them up this week and on Sunday we should have a finished product. I can’t wait.

Not too bad for a single day in the Garaggio. Night.

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