Saturday, January 1, 2011

Progress Stalled

Progress on the airplane has been stalled for a couple of days due to the nagging lower back injury sustained earlier this week. I've been to the chiropractor a couple of times and he, normally a veritable miracle worker, has only been able to move the pain point around to new locations. It started in my lower back, he moved it to a shooting pain down the length of my right leg, and finally got it cornered in an area I can live with. Not to put too fine a line on the final location, but let's just say that I had to cancel family plans to go to a two and a half hour movie because I didn't think I'd be able to sit through it. Emphasis on "sit."

That's all just fine with me since I can now stand and walk around almost as if nothing had happened to my back. The timing was perfect for getting back to work on the plane as we have been the appreciative recipients of a wonderful warm air mass sent up to us from the deep south. Can you imagine a temperature of 67F on New Years Eve? Well, of course you can; you live in Australia. But to us winter sufferers in Central Ohio this is a heat wave! I'm never one to waste good weather, so I ended up spending most of the day and evening out at the hangar.

I was working on the stall warning switch, so in effect my stalled progress became progress on the stall warning. What nice linguistic symmetry, albeit somewhat forced.

It started easily enough. Nearly all of the parts were located in a single brown bag.

The stall warning switch is essentially a micro switch sandwiched between two mounting plates. A metal vane is installed - this vane will protrude out through a slot cut into the leading edge of the left wing. When the angle of attack on that wing gets high enough to indicate that the wing will soon stall, the airflow that normally presses down on the vane will instead force it upward. This will cause the arm of the vane to activate the stall warning switch, thus closing an electrical circuit that will sound a warning horn in the cockpit.

The two mounting plates need to be final drilled to the sizes required for the two screws that will hold them together and for the screw that the vane will pivot around.

I just noticed that my Harbor Freight vise is "Made in Ching." I always knew that tool sizes at Harbor Freight were only approximations; now I know that spelling is too! (Cruel Karmic irony - I'm in here fixing the spelling of "vane" - I had used "vain" instead.)

This was still fairly early in the morning. We had yet to attain our eventual high of 67F, so a Thermos of coffee was still welcome.

Plus, I needed a little caffeine fortification before tackling the job of sorting out the washers that would be used as spacers when building up the switch mechanism. There were five different sizes all tossed into the same bag. There were three actual screw diameter sizes, with two of those three sets each having two separate widths. I separated them into piles by using visual inspection.

Then I ran into a problem with the plans. I know it might seem as if I'm being churlish when I call Van's to task on these minor problems with the normally industry-leading, excellent plans, but it is inarguably the case that these plans need to be as close to perfect as they can be made. The reason for that is the way the kit is certified in the eyes of the FAA. As an E-LSA, I am actually building this airplane on behalf of Van's. When it's done, I will certify that I followed the plans to the letter, and Van's will then inherit the liability for the safety and certification compliance of the completed aircraft. With that in mind, one would think that Van's would want to ensure that the thing is being built 100% correctly.

So here's why I bring this up. Do you see the washers labeled NAS1149FN432P, NAS1149FN416P, and NAS1149FN832P on the drawing? Well, you won't find any washers labeled that way in the kit inventory. What you will find instead are washers labeled AN960-4, AN960-4L, and AN960-8. Through a little intuition and detective work I was able to cross-identify those specs, but it would seem that Van's could correct that mismatch pretty easily and avoid any potential problems. On the other hand, the inventory had notes that pointed out the the AN960-4 is .032" thick, while the AN460-4L is .016" thick. My intuitive leap was that the #4 size rivet that was .032" think was probably the NAS1149FN432P. And so on. So the information is there, you just have to work for it.

Once I had that straightened out, it was a simple job to build the stack of parts into the final assembly.

With that done, a drilling template gets prepared. This template will be used to locate the holes that will be drilled in a rectangular piece of plastic. That piece of plastic will get mounted in the wing root where it will mate with a similar piece of plastic embedded in the side of the fuselage. The point of this exercise is to create a set of electrical terminals that will make contact when the wing is installed onto the airplane. This will allow the electrical current to flow out to the stall warning switch. The reason for this relatively complex way of making the electrical connection is that the wings are intended to be removable. You wouldn't want to have to connect and disconnect wires every time you wanted to remove or replace the wings. There would be too much risk of forgetting to do it, and there would be a lot of recurring stress on the connectors that would eventually lead to their failure.

There are two halves to the drilling template, so I marked the cut line with a Sharpie and separated the halves with the band saw.

The template gets clamped to the piece of plastic, the holes are drilled with #40 pilot holes, and are then final drilled to #19 or #30, depending on location.

The plastic terminal block then gets riveted to the mounting bracket that will hold it in place on the wing. A washer gets put on the shop head of the rivets, presumably to spread the pulling load of the rivet to keep it from simply pulling through the soft plastic.

Then it's time to start on the actual wiring. The first piece of wire is a short connector that will attach to the wing rib and provide a ground connection to the switch.

After fabricating that wire, the switch assembly gets mounted to the wing. The plans described what was going to happen. One screw would go through the hole at the bottom of the switch assembly and provide both a post for the ground wire and a pivot point for the switch assembly. The top screw would go into a slot; this would allow the switch assembly to pivot to so it could be precisely adjusted to just the right position. This triggered a recollection of a similar arrangement on the airsickness detectors (air vents) on the fuselage. Those too work by means of a screw running through a slot, and I remembered that the slot needed quite a bit of filing to make it work smoothly. I tested the switch assembly with a screw and sure enough, it needed some work with the file.

Quite proud of myself for proactively preventing the need to remove the switch after the hassle of installing it, I confidently proceeded with the installation.

Inexplicably forgetting that pride invariably goeth before the falleth, I blithely struggled with fitting the wing skin in place to ensure that the vane moved freely in the slot, assured in my knowledge that all would work well and there would be no reason to undo all of this effort. What could go wrong? After all, I had surmounted the difficulty in breaking the washer identification code, I had detected the slot trap and rectified the misfit problem, and really, it's just a simple switch, right?

And verily came the inevitable falleth. The vane was hard against the edge of the slot and would not move at all.

I removed the top clecoes on the wing skin and was able to remove the switch without having to completely remove the skin. Visual inspection of the switch assembly showed two things: the screw that was acting as a pivot for the vane was slanted, and the vane could do with moving inboard by one washer width. Both problems were addressed by loosening the two main screws, removing the pivot screw, changing the order of the washers from "two washers - vane - one washer" to "one washer - vane - two washers," and making sure the pivot screw didn't pick up any slant while re-tightening the main screws. In truth, I probably would have gotten away with just taking the slant out of the vane pivot screw.

Screwed back in place, the short ground wire is attached and a second wire, this one 68" long, is attached to the switch and run back to the wing root through the bushings in the ribs.

At the wing root terminal block, the 68" wire is bolted into place and another short ground wire is fabricated and attached to the terminal block.

The terminal block and second ground wire are mounted to the wing. Now we know why we needed nutplates on that rib.

The stall warning switch was the last thing from keeping me from installing the wing skins. It was getting dark and I wouldn't be able to work much longer, but there were a few folks hanging around the airport enjoying the temperate clime and I was going to need help flipping the wing in order to start skinning the bottom. I grabbed the help while it was available and couldn't resist the urge to get the first piece of skin clecoed on.

Being as it was New Years Eve, an impromptu party broke out in one of the neighboring hangars. There was champagne, munchie/snacks, and eventually two large pizzas acquired from the local biker's bar. As it got chilly, "Tool Junky" Al (his words!) pulled out a heater that delivers something like 12,500,000 BTUs and we all sat around trading friendly insults. Even Puppy Cabot joined in!

One thing you simply can't get away from on New Years Eve is the declaration of resolutions. My experience has taught me that the easiest resolutions to keep are the self-destructive ones, so my resolution for 2011 is to further my goal of becoming an International Man of Mystery and Intrigue by acquiring a taste for single malt Scotch whiskey. Dr. Doctor D, himself owning a gorgeous and impeccably maintained Piper Warrior in a hangar across from mine, was able to provide some fine practice materials.

I'm not sure that I will be able to keep my resolution (to me, Scotch still tastes like something I should be using to wipe on my arm before getting a shot), but I am 100% sure that I have never had a more enjoyable or more satisfying New Years Eve.


Joe Blow said...

It seems that someone of great intelligence, such as yourself, wouldn't go to a voodoo chiropractor to "heal" vertebral subluxations. ;-)

Rant off:
Enjoying your great blog!

DaveG said...

The chiropractor may not actually heal anything per se, but alleviating the symptoms of nagging lower back pain is good enough for me.

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