Saturday, January 06, 2007

Back to the grind(ing)

I arranged to borrow a second wing jack from the FBO at Bolton (the most helpful group of professionals that you could ever wish for!) and was able to borrow a tail stand from the builder of the other TZR-based RV-6. That left only one open issue: how was I going to lift that danged tail up into the stand? Sure, it only weighs 80 pounds for so, but it's an awkward lift that never fails to damage me physically to the degree that I swear never to do it again. And then along comes a job like this one that requires the tail to be elevated and I develop some sort of transitive amnesia about my previous experiences and heft it up.

The reason the tail needed to be lifted today was that I needed to do the final alignment of the leg fairings so I could trim and fit the upper intersection fairings. There's apparently a noticeable difference in the alignment of the gear legs in flight versus their position with the weight of the airplane compressing them, so the weight of the airplane has to be borne temporarily by wing jacks. There could also be a difference in alignment with the tail on the ground, so to further simulate an in-flight attitude the tail needs to be lifted and the airplane leveled.

Being as I only have one jack of my own, I have never lifted the entire airplane. I wasn't sure if it would be better to jack the wings prior to lifting the tail or vice-versa, but I decided lifting the wings first would be a bit safer as it is more unlikely that it would move (or fall off of) those heavy steel jacks than it was that the relatively frail wooden tail stand would slide or fall. I jacked the left wing until the wheel was just light enough to turn, then started on the right side. As I pumped the jack, I fixated on the left side and the right side tire, waiting for the same scuffing roll movement of the right side wheel. Eventually it seemed that I had been pumping much longer than expected, even considering a prior lack of experience with this, the borrowed jack. I eventually broke my fixation on the tire long enough to notice, with no small measure of shock and dismay, that the tail had, of its own accord, lifted itself way up in the air, and as a direct result the nose was close to assuming the position of a dog rooting out a field mouse. Huh, the CG is forward of the jack points; who knew?

While this unexpected intervention from the friendly gods of physics solved the brute force facet of lifting the tail, it presented a new, unexpected issue. You see, I had borrowed a tail stand, a tool that is only effective in one direction, e.g. holding the tail up. It is entirely useless in the function of holding the tail down. The simple expedient of lowering the wing jacks proved to be the answer - I was able to get the plane leveled with only a little bit of weight on the wheels. Given the inherent inaccuracies in this entire alignment process, I was ok with that.

I pulled the leg fairings off to do some sanding on their trailing edges, and touched up the paint on the plane where the old fairings had rubbed it away. I also got some paint on the bare areas of the gear legs while they were exposed. Those areas will all be covered by the new fairings, but it seemed a good idea from the corrosion prevention point of view to not leave any bare spots.

I plumb-bobbed a chalk line down the center of the fuselage again, unfortunately discovering some sort of defect in my chalk string dispenser in the process. Unwound to nearly the full length of the plane, it became irreparably jammed. No string out, no string in, no matter the extremity of the torque applied to the crank. I had to take it apart to untangle the internal disarray. I'm here to tell you, if you haven't yet had the pleasure and experience of disassembling a nearly full chalk line dispenser, don't be in any great hurry to do it. It's like opening a vacuum cleaner bag and rooting around through the mess of ingested dust mice and carpet fibers, except for being a much more colorful blue.

So, with the airplane more or less leveled and the fairings aligned to the best of my meager abilty (the far more discerning and infinitely more accurate judge of my alignment efforts will be the 185 mph airflow across the fairings in flight; I have no hope of achieving that precision of measurement with string), I could start trimming the upper intersection fairings.

As we're nearing the finish of the project, I thought it might be instructive to provide a 'before' picture:

The upper intersection fairings need to be trimmed fairly extensively around their perimeter to round out the unattractive rectangular shape they're molded in. I just sketched some curves with a Sharpie pen by eye, looking for a conservative cut line. I've learned to treat trimming glass in the same way I've learned to tell my clan of females what time we need leave the house for any given appointment. For exmple, I'll tell them that we absolutely must depart at say, 9:00, knowing full well that we really don't need to leave until 9:30. And, just as we usually manage to get out of the house by 9:45, I always manage to find a way to cut away too much material when I trim. So I was especially conservative today, making particularly sure that there would be a large enough 'flat' area where the fairing is flush with the fuselage skin to support a #8 screws and a tinnerman washers that would hold it in place.

I rough cut around the Sharpie line with a cutting disk in my die grinder, then used a sanding drum to grind a nice radius curve all the way around the fairing. Ack, the dust! But it all came out well and the fairings for the most part fit pretty well against the plane. The right side will need some adjustment, though.

I also had to cut a relief hole on the inside side of each to provide room for the screw on the hose clamp. I'm not thrilled with that aspect of the design, less because the opening is going to cause a little drag than because it's going to be a catch-all for the gunk thrown up by the landing gear when using grass runways. Of course, if I was concerned about drag, I would have done a better job of positioning the hole on the right side fairing than I did. The precision of my marking of the hole location was only marginally better than what would have been expected to result from a completely blind guess, so the ensuing drag resulting from the elargement of said hole is entirely my fault. So yeah, it's certainly not about drag at all. That's my story and I'm sticking to it, as they say.

The former fairings utlized a hole in the lower corner of the bottom cowl for a screw and nut to hold them in place and I'm going to re-use those holes to host #8 rivnuts. That way, I won't have to mess with the little nut way down there in the corner of the bottom cowl every time I have to remove the cowls. The pre-existing holes were also handy for getting a Cleco in place to hold the fairings:

There will be three more rivnut/screw locations drilled for each fairing, and I will also have to tap a hole for one of the screws on each fairing since the hole location is an area backed by an internal doubler. There's not enough grip length in the rivnuts to get a good solid fit through metal that thick.

You can start to see how it's going to look when it's done:

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