Friday, December 19, 2008

Trammeled Not Trampled

I've now reached one of those highly important celebratory steps - trammeling the wings. This is again something that would take an accomplished A&P fifteen minutes to do, and has taken me three days. The first part is to draw a reference line on the top of the spar that marks the middle of the compression strut, and a crossing line at the halfway point. The easy way to do this is to take a square (which is actually a triangle-shaped piece of steel used for making things square), and lining up the top and bottom bolt holding the compression strut in, follow the line up and mark it.. You'll use these lines to measure whether or not your bays - the distance between compression struts - are square, and tighten or loosen the drag & anti-drag wires in order to pull the spar into a square shape.

Except, well, the holes aren't always - or even usually - straight up and down. There's a very slight angle that puts the line well off to one side. To make matters worse, the ribs and brackets get in the way and don't allow the square to easily lie there or line up. Also, your eyes are not in the center of your head - so if you look at something when standing directly over it with your left eye closed, your line will be way off to one side, or way off to the other if your right eye is closed.

Finally my IA disclosed some of the great truths in life:

1. Use a sharp pencil for a fine line.
2. You don't have to be absolutely precise or absolutely accurate - but you must be consistent. If the reference line is off by 1/4 inch, he said, that's fine - as long as it's off by 1/4 on every strut. So mark it as well as you can, and do it the same way, every time.
3. Loosen all your wires before you start, so the wing can be easily adjusted. Then start from the wing root and trammel out to the tip.
4. A piece of inner tube rubber cleaned with brake-kleen is slightly sticky and great for holding the drag wire.
5. If you have a 1/2 inch to square, only tighten the drag wire halfway (for 1/4 inch in this case) - the tension on the spars & other drag wire will pull it the rest of the way. It's geometry, Watson.

It went a lot faster after that!

But wait, there's more - after getting it all square, I have to go back, tighten all the bolts, and re-trammel after all the hardware is tight. This is slightly stymied by the two AN5-16 bolts called for actually need to be shorter than AN5-11. I don't have shorter bolts than that - parts run tomorrow!

I count today as four steps forward with only one small step back - measurable progress!

In other news, Lost Prophet's "Rooftops" is a good song, and I'd really like to hear it live, as it sounds like it'd be a lot better in person than car stereo.

2 comments:

Tony B. said...

I need to stop reading your blog. You're making me want to buy and "old" plane and start fixing it up.

On a Wing and a Whim said...

If you do, I advise strongly: find a good A&P / IA to work with, because it's a mentoring relationship. Then find the project.

Read away - after all, if the idea interests you now, you'll have your eyes open should the opportunity ever drop into your lap - and you'll be able and ready to reach out and grab that dream before it slips away. Maybe it'll be an antique, maybe an experimental, maybe restoring an old motorcycle, who knows?

After all, what is luck but melding the awareness of looking for opportunities and the knowledge and skills to use them with the opportunities that come?