H-stab fitting: the beginning

OK, so this is really a post for the entire weekend, sort of. I didn’t really have anything like a dedicated work session, but I did still get some stuff done.

In short, I moved on to the beginning of fitting the tail feathers, starting with the horizontal stab. The first step to doing this is to get the fuselage dead level, both front-to-back and side-to-side. This meant taking it down off the rotisserie setup and getting the forward portion sitting on the sawhorse I built for this eons ago. Then I ended up making stacks of shims out of thin plywood until I got everything nice and level.

With that done, it’s time to set the h-stab in place on the aft deck. But it doesn’t sit directly on the deck; under the forward spar splice angle there are a couple spacers (at this point, bolts will go through the bottom of the angle and through the deck). The aft spar, meanwhile, sits flush against the stab attach bars, which are in turn riveted to a bulkhead and project vertically through the aft deck. At this mount point, a 3/16” spacer is placed under the spar to properly position the stab.

To start with, the stab is centered on the fuselage, using the joint between the rear spar channels as a reference and centering that joint between the attach bars. Then the stab is loosely clamped into place at the rear spar. Next the stab needs to be leveled side-to-side. To do this, you make a couple of spacer blocks, which are placed approximately mid-span on the stab, and a long level laid across them. I used this level as a base for my small smart level, to get a more accurate reading. I ended up with the left side slightly low, so I made a thin shim from scrap stock to place on top of the left-hand spacer under the front spar, and that got my lateral level set.

At this point, the clamps on the rear spar are tightened to set that joint in place. Next, the stab needs to be squared to the fuselage centerline. This is done by measuring diagonal lines from each tip to a reference location on the fuselage side. The stab then gets tweaked until those measurements are equal (or, by the book, within 1/4”…I got closer than that). At this point, the front angle is clamped down tightly.

This ends up being the point at which I stopped work. The next step in the procedure is to use prepunched holes in the stab attach bars and drill through the rear spar, creating pilot holes for the attach bolts. I did not feel at all ready to commit to that without maybe doing some reading on the procedure. Somehow it all felt a little too easy to me. I suppose there is one more thing to be done before I commit to the holes, which is to check that the incidence is properly set. To do this, I’ll need to use a line between the tip rib tooling holes as a reference, and make sure that reference line is level just like the fuselage.

I also need to think about how I’m going to go about planning the attach holes a the forward spar. This is a spot that builders frequently mess up – getting proper edge distance on the bolt hole through all the various pieces can apparently be challenging, and it’s extremely important to get things right here. Overall, the entire empennage assembly is attached to the fuselage by just eight bolts. I definitely don’t want anything compromised back here.

So we’ll see about picking back up this week, and maybe I’ll work up the gumption to actually take a drill to this stuff.

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