A Shim for the Win

It’s an imperfect world in which we live. For people like myself, that can add countless hours to an airplane build. I encountered one such imperfection while mating the Onex forward fuselage to the aft fuselage.

This drawing depicts what the joint looks like in the plans, viewed from below. You can see the gap under the middle screw (colored red). On the drawing, the skin flows smoothly. In real life, it does not.

I’ve removed the middle screw (you can see the reflection in the skin is no longer distorted, indicating the skin is no longer stressed) and made a 0.025-inch-thick shim to slide behind the skin to help bridge the gap caused by the overlapping 0.032-inch skins.

Two 0.032-inch-thick skins overlap at the joint. There are seven screws in the joint. Six of the screws do a fine job securing the skins tightly to the underlying structure. One screw is tasked with forcing a skin to fill a gap caused by the overlapping skins. When I tightened that screw, the reflection in my polished skins became distorted. It was an affront to my aesthetic. But it wasn’t just that the reflection was distorted (so I tell myself), it was also that the skin was clearly stressed. “Well,” I thought, “not on my airplane.”

The photos show the problem and my solution.

To make the shim, I drilled the fastener hole in a scrap and then cut the scrap to the needed width. I left the shim long so I could push it in place. Once in place and fastened, it was an easy matter to cut the shim off near the surface and file/sand it flush.

The screw is back in place with the shim behind. The distortion, therefore the stress, is much reduced. (You may have noticed the heads of the screws are aligned. This is another area where I lose hours of my life to the details.)

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