Trim tab - Another right mess

Sun 9th Dec 3hrs 10Min

Made bending break. Masked and sanded the site of the trim tab foam ribs. Trimmed the trim tab horns and match-drilled. Completed the Trim Tab trailing edge skin bend. Made blocks and bent end tabs. Match-drilled ends and skin to spar holes.

The replacement HS-902 and HS-907 arrived during the week along with the angle needed to make replacement R-915 A&B stiffeners and the angle to make a third(!) set of HS-908 attach angles. I am looking forward to preparing these pieces and eliminating all my past sins. However, I scarcely have the replacement pieces in my hands when I mess up another piece. This time it is the trim tab skin, but more of that later.

I made the bending break from two of my shelves as they were the only pieces of timber I had of about the right side. They in turn had been made from scraps of half-inch ply that I had left over from building the workshop and I had glued two thicknesses together to get the stiffness I needed for the shelves. The bend was not as smooth as I expected and it seemed to vary depending on where I had leaned on the break. However, I got it straight with a bit of trial and error. When I inserted the spar, I found that it was a little over-bent so I put it over the edge of another piece of half-inch ply and pushed gently down on the trailing edge. This spread the bend out a little until the spar fitted perfectly in position at the 'open' leading edge.

The manual shows pictures of the set-up for bending the tabs on the skin to form the ends of the trim tab. This is done using interlocking blocks, one of which is cut to go inside the skin and the other goes on top. The whole assembly is positioned on the edge of a bench and clamped tightly. Another picture shows the tab being bent with a mushroom set in a rivet gun. I also watched the Orndorff DVD again but George just refers to having bent the tabs using his flanging tool. I decided to go with the Vans procedure so I made the interlocking blocks from three-quarter inch MDF that I had lying around, clamped it all up and went at it initially with a timber block and a hammer and then finished off with the rivet gun.

To say the least of it, the results are not good. There were two problems. Firstly, despite heavy clamping, the blocks moved slightly, so the first bend did not end up on the line where it was supposed to be. Secondly, the rivet gun hammered the hell out of the skin leaving it over-bent and all dinged up. Maybe the part is okay from a functional point of view and the problems are only really visible at the edges but I don't think I can live with it.

Afterwards, I searched the forums for other people's ideas. The Orndorff video simply refers to using the flanging tool to make the bends but does not show it. I found that lots of builders have problems with this task, so much so that the replacement skin is with the accessories in the main on-line store rather in "the list" with virtually all of the other kit parts. Those who have done the job successfully are using timber blocks that are much thicker than mine - up to 2 inches. Some have also suggested using double-sided scotch tape between the skin and the blocks to prevent movement. Too bloody late now of course.