Horizontal Stabilizer – Dimpling

The next task on the horizontal stabilizer is dimpling the rivet holes that are used to attach the skin to the substructure. A dimpled hole is bent inward with a precision die that causes the hole to exactly match a flush rivet head. When riveted, the surface of the skin is smooth with no rivet bumps. This is one of the secrets to the RV-7’s speed. The smooth surface creates less drag than a surface with raised or blind rivets.

Both the substructure (ribs and spars) and the skin must be dimpled to accomodate the flush rivets. This picture shows one of the spars with some of the holes dimpled inward.

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I used a pneumatic squeezer to dimple the substructure. There are four holes in the very front of the outer rib that can’t be reached with the squeezer. Fortunately, I have a “pull-rivet” dimpler for these hard-to-reach spots. This is a dimple die through which you can insert a nail which can be pulled using a rivet puller. This places enough squeeze on the metal to shape the dimple.

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To dimple the skins, I’m using a C-frame dimpler from Cleaveland Aircraft Tools.  I’ve built a table for the dimpler that supports the skin during the process:

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When I used this tool on my Van’s practice airfoil, everything worked perfectly. The real skins on the horizontal stabilizer are considerably thicker and were a bit trickier to work with. Here are some dimpled holes that didn’t come out very well:

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Notice the wide indentation around the dimpled holes. The best way to gauge the quality of the dimples is to look at reflected light and see how much distortion there is. I found that I needed to stiffen my table a bit and raise the surface with respect to the lower, fixed die. I re-struck the same holes and got a much better result:

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Now that all of the components of the horizontal stabilizer have been completed and are ready for riveting, I am going to set them aside until I get my priming rig set up. The interior parts need to be primed to protect against corrosion before they can be riveted together. Here’s what the horizontal stabilizer looks like right now:

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