Jim Markle's bending Pietenpol horn bends
Like many builders, I really had no clue how I was going to make the formed control horns. Fortunately over the years I had gleaned enough information from other builders to be ready to give them a try.
My first attempt was to simply make two bends. In the following pictures you can see the bends making facets not a smooth curve.
However, someone said they though I could do better. Well, who could turn down such a challenge.
My next attempts match the plans much better and wasn't that difficult. Like every thing on this plane it just takes time. Here is how I do it.
These are the tools I used they consist of two bending blocks, a 1/2-inch round bar, pen, bronze bar and hammer (not pictured) to form the metal. I use the bronze bar to form the metal because wood splinters to easy and bronze is softer then steel so it doesn't leave a mark when you pound on it.
After cutting out the horn I first drill the holes on the ends and make the cuts for the flanges that need to bend out. As you can see I didn't do so well at following my line on one corner. I fixed it by drilling the hole a little larger.
Line up the bend line on the horn with the start of the radius on the block. In the picture you can see that I am above the line on the one end to help taper at the end. I use a brass bar to form my bends because it will last longer than hardwood and is softer then the steel so it will not leave marks. Try not to hit it with just the hammer
Time to form the end radius
On the plans you will see there is a brace that extends off the back of the horn. The only way I could figure out how to get the metal strip into the horn was to form a jog. I formed this jog over a piece of steel 1/2 the thickness of the brace.
Time to weld
I used a lot of clamps to hold the two halves together. Because the metal is so thin I used a 1/16-inch welding rod sparingly. On the trailing edge I simply fusion welded, no added filler rod, because the two halves are almost parallel to each other.
Finally, finish off the mounting flanges and bend the brace to fit your plane.