To begin with, I am creating this in the Building Tips/Techniques forum. I tried to do that with my previous two posts, but somehow they were created in the general discussions forum by some mystical force.
Anyway, I was looking for advice on how to make the F-325 rear fuselage bottom skin for my old RV3 kit. Getting no "constructive" response, I thought I would share the method I used to make the part.
F-325 is the bottom skin that runs from the first bulkhead behind the pilot seat, back to the second bulkhead forward from the rudder hinge.
This skin is tapered in the sense that the rear of the tail section is narrower than the front, but to complicate matters, the bottom corners of the bulkheads it covers have bend radii that are smaller at the rear than at the front. This is a conical bend, and If there is a tool for doing that, I've never heard of it.
Fortunately, a friend of mine inherited his father in law's shop tools. His father in law was a well known aircraft mechanic and fabricator in our area. Along with the tools, he gained a great deal of knowledge of aircraft repair and building techniques from him.
We used a 10' sheet metal brake (although an 8' one would have worked for the RV3's 7' skin lenght). The brake has interchangable bend radii, and we used a 1/8" bend radius for the job.
We bent some sample material to try to establish a method that would bend the skin as desired. We shimmed the brake's clamp jaw apart with .125 material, and marked about 20 lines, 1/4" apart across a 4" wide sample. The .025 skin fit into the brake without being clamped. We "bent" the sample at each line, moving the brake about 14 degrees up. over the 5" width of the bend area, we achieved a smooth round bend of 80 degrees or so.
Next, we marked 20 lines 1/8" apart across a sample and repeated the process, bending a 2-1/2 inch area, at the same 14 degrees of brake action per bend. We found that the total bend was much less than the 80 degree bend we got with the first (wider) bend.
We reasoned that the .125 opening in the brake's jaws was the problem, so we repeated the test of bending the 20 markes at 1/8 spacing but using .0625 shims in the brake's jaws. This resulted in a bend more in the 80-90 degree range, again moving the brake about 12-14 degrees on each bend.
Finally, we put two shims in the brake, holding one end .125" apart, and 7' away, .0625 apart.
I marked the final part with 1/8" spacing across the desired bend area for the aft bulkhead, and 1/4" spacing across the desired bend area for the forward bulkhead.
Then we placed the part in the brake, with the aft end near the .0625 shim, and the forward end near the .125 shim.
We used a 12 degree brake action on each of the 20 marked lines, starting from the middlemost line, and working outward, so that the curve would be on the outside of the brake, rather than trapped inside the brake.
The result was not perfect, but was close enough that I could hand tweak the part so that it fit nicely over the ribs, with enough of an open bend that clamping it down over the bulkheads resulted in a good fit.
Hope this helps someone.
Jim Frisby
Anyway, I was looking for advice on how to make the F-325 rear fuselage bottom skin for my old RV3 kit. Getting no "constructive" response, I thought I would share the method I used to make the part.
F-325 is the bottom skin that runs from the first bulkhead behind the pilot seat, back to the second bulkhead forward from the rudder hinge.
This skin is tapered in the sense that the rear of the tail section is narrower than the front, but to complicate matters, the bottom corners of the bulkheads it covers have bend radii that are smaller at the rear than at the front. This is a conical bend, and If there is a tool for doing that, I've never heard of it.
Fortunately, a friend of mine inherited his father in law's shop tools. His father in law was a well known aircraft mechanic and fabricator in our area. Along with the tools, he gained a great deal of knowledge of aircraft repair and building techniques from him.
We used a 10' sheet metal brake (although an 8' one would have worked for the RV3's 7' skin lenght). The brake has interchangable bend radii, and we used a 1/8" bend radius for the job.
We bent some sample material to try to establish a method that would bend the skin as desired. We shimmed the brake's clamp jaw apart with .125 material, and marked about 20 lines, 1/4" apart across a 4" wide sample. The .025 skin fit into the brake without being clamped. We "bent" the sample at each line, moving the brake about 14 degrees up. over the 5" width of the bend area, we achieved a smooth round bend of 80 degrees or so.
Next, we marked 20 lines 1/8" apart across a sample and repeated the process, bending a 2-1/2 inch area, at the same 14 degrees of brake action per bend. We found that the total bend was much less than the 80 degree bend we got with the first (wider) bend.
We reasoned that the .125 opening in the brake's jaws was the problem, so we repeated the test of bending the 20 markes at 1/8 spacing but using .0625 shims in the brake's jaws. This resulted in a bend more in the 80-90 degree range, again moving the brake about 12-14 degrees on each bend.
Finally, we put two shims in the brake, holding one end .125" apart, and 7' away, .0625 apart.
I marked the final part with 1/8" spacing across the desired bend area for the aft bulkhead, and 1/4" spacing across the desired bend area for the forward bulkhead.
Then we placed the part in the brake, with the aft end near the .0625 shim, and the forward end near the .125 shim.
We used a 12 degree brake action on each of the 20 marked lines, starting from the middlemost line, and working outward, so that the curve would be on the outside of the brake, rather than trapped inside the brake.
The result was not perfect, but was close enough that I could hand tweak the part so that it fit nicely over the ribs, with enough of an open bend that clamping it down over the bulkheads resulted in a good fit.
Hope this helps someone.
Jim Frisby
