Thursday, January 21, 2010

Have you ever had that feeling?

That feeling that you've forgotten something? It can happen at the oddest times. You'll be walking towards your departure gate at the airport to board a jet for a vacation and you just can't shake the feeling that something is wrong.

Did I lock the door when we left the house?

Yes, I'm sure I did.

You run through a thousand possible things that could have slipped your mind. Then suddenly it hits you - you aren't carrying the laptop computer that is your ubiquitous travel companion on your normal business related travels.

You know the feeling, right? Well, keep it in mind for a few minutes.

After a couple of days out of the shop, I revisited the drawing that had confused me so much the last time I worked. Naturally, it made perfect sense almost immediately after having been away from it for awhile:

I think it was actually the text that caused my befuddlement:

But even the text isn't all that confusing, really. I must have just been fatigued; the cost of being a so-called knowledge worker is that there are days when the brain is every bit as fatigued as any other muscle can be after a long day of work. All it's saying is file those plastic bushings to a width of 3/16" on the F-1287C and 1/8" on the trim motor tray. Given the width of those parts are pretty close to 3/16" and 1/8" respectively already, the net result is that the bushings get filed away to the degree that they are just a little wider than flush with the surface of the parts. Easy, that. Makes you wonder what all the fuss was about.

I had also skipped over the stripping of the wires coming out of the trim motor and the crimping on of connectors because the crimper and strippers were out at the hangar. Having retrieved them, that step was soon accomplished. Interestingly, the plans call for one male and four female connectors on the trim motor wires and one female plus four male connectors on the wire that will run to the control panel. Further, the male connector on the trim motor is specifically assigned to one of the two all white wires.

Why? Why not five females on the trim motor and five females on the other wires? Hint: it has nothing to do with procreation.

I found the answer in the directions that come with the trim motor. They state that when 12 volts DC is applied to the white wires, the motor will run. If the polarity of the power applied to the wires is reversed, the motor will run in the opposite direction. Because you would never, ever want to have the trim control wired up backwards such that down becomes up and up becomes down, the connectors are installed such that the  white wires will always be connected in the same way. There was a jet (maybe a turboprop - I don't remember exactly) that crashed near here a year or two ago when on a maintenance test flight. The trim control had been reversed.

Neatness counts too, so an adel clamp is installed to keep the wires herded together:

You can see the nicely filed plastic bushings too.

The trim motor has to be attached to the trim tab, oddly enough, and that is done with a pivoting pushrod assembly. A threaded clevis and a stabilizing arm get bolted to a couple of aluminum frames:

The cotter pins that lock the castle nuts in place will get secured a little later. I just wanted to make sure that the parts weren't going to need to be disassembled again. Cotter pins are supposed to be a one-time use item.

The actual pushrod gets clecoed into place using two pre-drilled holes on each side and four more holes on each side are match drilled:

Drilling through the second side is a little tricky since the holes aren't offset from each other; the drill bit will hit the cleco on the other side if you aren't careful. The drilling leaves some messy holes, so the clecos are removed and the holes in the pushrod are deburred:

Once cleaned up, the rod gets riveted in using a new type of rivet, the AD4H. The directions state that you should put pressure on the clevis plates (the aluminum sides) as the rivets are being pulled. I wasn't sure what that was all about until I realized that just like the drill bit when drilling the second side of the pushrod would hit the cleco, the second rivet would hit the rivet across from it. I didn't figure that out until I got to the second rivet, though, so I concentrated extra hard on keeping pressure on the first rivet as I pulled it:

Looks great, doesn't it? I thought so too, but I just couldn't shake the nagging feeling that I had forgotten something.

What could it be? The rivet is nice and flush, and the shop head is nice and bulgy.

Hey, wait a second. Why can I see the shop head??

Oh. That would be because I forgot to put the pushrod back in!!

Brilliant! A purely decorative rivet!

Now, in case you were wondering what distinguishes an AD4H rivet from the normal LP4-3 rivets we've used thousands times before, I'm here to tell you. It is that they are inordinately hard to drill out! The 'AD' that I at first thought stood for Attention Disorder actually indicates Ain't Drillable. I tried and tried to drill down through the center of that rivet, but the mandrel is clearly made if Impervium. As in "completely impervious to drilling." I finally just ground the head off with the Scotchbrite wheel and pulled the rivet through the other way:

How fun!

Pressing on (so to speak), I quickly discovered why pressure is required when pulling the next rivets:

Once I had finished using 13 of the 15 provided rivets (and how I hope that I don't soon come across a step requiring three AD4H rivets!!) to rivet 12 holes, it all got bolted together, surprisingly easily. A threaded receptacle had to be inserted into the end of the pushrod and match drilled and riveted in. A bearing that will attach to the trim tab is then screwed in and secured with a jam nut. I couldn't get the threaded receptacle to go into the pushrod at first; the two starter holes drilled by Van's hadn't been deburred and the rough edges were blocking my progress. A 'T' size drill bit used as a reamer quickly resolved any confusion on the part of the pushrod - "you will accept this little gizmo, or else!"

At this point, there really is nothing much left to do until the fuselage kit arrives. And you know, I might just be okay with that.

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