Saturday, December 23, 2006

How fare the fairings?

Quite well, thanks. With the rough fitting finished yesterday, today's goal was to get the leg fairings mounted. The overall plan is to get the leg fairings aligned and mounted, then go back to working on the upper and lower intersections later. This allows me to defer the fitting problem I noticed yesterday with the lower fairings, but that's not the actual reason to back burner them for now. The gear leg fairings have to be aligned with the in-flight airflow so they don't act as forward rudders and impart unwanted yaw input in flight. Since it is the alignment of the leg fairings that is aerodynamically critical, it needs to be done before fitting the uppper and lower intersection fairings. The responsibility for any required give & take (positionally speaking) will be borne solely by them.

The starting point for today was with both leg fairings clamped to the legs:

The final alignment will require the plane to be up on jacks, but for now I had a nearer term task: the leg fairings are prevented from swivelling around after being aligned by means of a hose clamp that goes around a tab created by a couple of notchs cut in the top of the leg fairing. In order to cut the notchs in the right place, the fairings had to be pretty closely positioned to their final alignment.

I started by running some twine from each leg fairing back to the rear of the plane, where I duct taped the loose ends to a pair of jack stands:

The strings were held in place with pieces of tape:

It was the inside string on each side that I primarily interested in, and I wanted them each to run parallel to the centerline of the airplane. The centerline was determined by dropping a plumb bob from the center at both the front and rear of the plane, and connecting the two spots on the hangar floor with a chalk line. Each inner string was then measured for equal distance from the centerline at the front and back:

With the strings correctly aligned, it was easy to then align the leg fairings. I marked the lines I will later cut to make the tab that the hose clamp will hold. I've read that these tabs can be problematic as stress and vibration can break them, so I brought the leg fairings home along with my two-part epoxy and some fiberglass cloth. It's too cold in the hangar for the epoxy to set up. Once the chemicals get up to room temperature later tonight, I'll lay up some reinforcing flberglass in the areas that will become tabs.

I also spent some time planning the next steps. I'm planning on installing some rivnuts in the side of the fuselage and cowls and use screws to hold the fairings in place. Rivnuts are pretty cool - just as with a blind rivet, you don't have to have access to the back side of the surface to install them. You just drill a hole and push the rivnut in. It has a flange to hold it against the surface. You then screw a rivet puller mandrel into the rivnut and squeeze the handles. That pulls the mandrel, which causes the walls of the rivnut to compress against the inside of the surface.

Well, I wanted to practice installing some rivnuts in a lower consequence environment than the actual airplane. As you'll see, that was a fantastic decision! I tried both a size 8-32 and a size 6-32. The 8-32 went ok (it's on the left in the picture below) but required quite a bit of squeezing. I started on the 6-32 and was just noting how much easier it was to squeeze than the 8-32 when POP! went the mandrel. It just broke in half, leaving the remainder of its original length deep inside the rivnut. Had this happened on the airplane, I would have been (please forgive the vernacular) screwed. At this point, I do believe I will be using the 8-32s.

Here's another little job that I'm going to be facing. The blue fitting with the slant-cut face is the fuel tank vent, and as currently configured, it would be covered by the upper fairing. This I do not want. The slant-cut part is just a simple AN fitting that has been cut/ground down, so what I'll do is order an unmolested fitting of the same size and a matching female fitting. I'll get a short length of aluminum tube and borrow a flanging tool to make a short extension by attaching the female fitting to the end of the tube. I'll slant cut the end of the tube where it emerges from the fairing to preserve whatever the intended effect was of the slant-cut of the original fitting. I'll have to replace the male fitting too, and without even looking at the plans I fearlessly predict that getting to the other side of the fitting to remove the fuel vent tube is going to be an absolute bugger.

Oh, I'll see what I can do about touching-up the rusty spots on the leg mounts.

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