Horizontal Stabilizer – Rear Spar

The first major tail assembly is the horizontal stabilizer, and the first task in building the horizontal stabilizer is to assemble the rear spar. The rear spar is the strong internal structural component at the rear of the stabilizer that spans its entire width. It is built from two aluminum spar channels and reinforced with a pair (upper and lower) of thick aluminum reinforcement bars. The rear spar also has five hinge points for the attachment of the elevators, the moving control surfaces that control the pitch of the aircraft in flight.

In this picture, you can see the spar channels with the upper reinforcement bar in place:

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The reinforcement bars come with squared ends and edges that must be smoothed and rounded. The key to structural integrity with Aluminum (preventing cracks) is to avoid any sharp angles or sharp edges and corners that cause material stress. This picture shows the rounded ends of the reinforcement bars.

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You may notice that there are holes drilled into the table top. That is actually a sacrificial piece of MDF (medium density fiberboard) to protect the table top. In the first picture, you can see two clecos (temporary rivet-hole fasteners) that are inserted through the holes in the reinforcement bars and the spar channel. These go right into the MDF and hold everything in place. Each hole needs to be drilled to full size (1/8″ for -4 rivets). As I drill holes, I place clecos into every other hole to keep everything firmly aligned: drill and cleco, drill and cleco, …

Next, the elevator hinge brackets are clecoed into place and match drilled:

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The center hinge point is a bearing that is mounted between two steel brackets. In this picture, I am in the process of match-drilling the holes to hold the assembly together. One bracket is supplied with six rivet holes and the other bracket has none. This allows the builder to establish the perfect fit onto the spar before match-drilling the holes. You can see that the assembly (left and right brackets and the bearing) have been clecoed to the spar to ensure that the fit is correct during the drilling process.

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This is what the finished center hinge looks like once it’s been riveted together. I did coat the flange of the bearing with self-etching primer to prevent corrosion where the surfaces make contact. This is called a “faying” surface, where two pieces of metal contact each other.

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As is always the case when building an aluminum airplane, the builder assembles and disassembles things frequently. The rear spar has been assembled using cleco fasteners, then match drilled to ensure all the holes line up perfectly. Everything is then marked (so I can remember where it all goes) and then disassembled so the holes can be cleaned out and deburred. There must not be any aluminum chips or sharp edges on the holes when the rivets are set. If there are, the rivet strength can be compromised or the parts may be susceptible to cracking later on.

Next: The Horizontal Stabilizer Front Spar

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