Tuesday, November 21, 2006

AV - The Skin Game


Working out back of the shop under the shed roof. I'm cold and in a lot of pain. Khaki's, flannel shirt & a shop apron was warm enough when I started, afternoon sun heating up the tin roof. Not too bad at all. Except for the pain, which I pretend not to notice. Then the sun did it's trick and the breeze picked up. Eight to ten miles per hour according to the homemade anemometer spinning busily above the shop. But it's been known to lie.

California cold isn't really. Nothing at all like Duluth cold or Fairbanks cold. But I've been bent over the bench about four hours now and the pain is becoming The Pain. Kidney stone; something new for me. (If you haven't been there, you won't even come close :-)

A jacket helps. As does a cuppa coffee. But when I go back out to the shop the wind has picked up and spates of rain are coming in from the coast, invisible now, all light gone from a sky filled with dark scudding clouds. But no pills. Not yet. Not until I finish deburring the holes I've spent all afternoon laying-out and drilling.

Wing skins. Not big; two by eight feet, plus some trim. Outboard wing panels, upper surface. Only nine ribs. No stringers. But a lotta holes.

The holes for the spars reside in a couple of pieces of eighth-inch by one inch drug-store aluminum, the holes for the ribs are in a yard stick hijacked as a template. I used the computer to lay-out the holes, pricked the bullets, center punched the prick marks, drilled them on the drill press with the bit spinning 3100 rpm and the swarf winding away in a spiral as bright as a diamond. And then deburred.

The skins, left and right, stuck together belly to belly with a few squirts of spray glue, are laying on an old door pieced out with some scrap. The trim allows a couple of free holes to accept the cleco's that fasten the skins to the table.

Layout is school-boy geometry. Stretched black thread for my X, carpenter's square for my Y, corners trammeled with a twelve-foot piece of extruded angle, the intersections carefully marked with a Sharpie. I can live with plus or minus sixty but smile when the marks cross each other with an error of perhaps fifteen thou in 98.95 inches. Close enough.

The ribs have a riveting flange five-eighths of an inch wide with the Safe Zone being the middle third. For a safe structure the AN3 flush-heads should end up somewhere within a square about three-eighths inch on a side. But closer is better which is why I use templates for laying them out. Centered on the line then squared, the holes fall almost perfectly within a square only an eighth of an inch on a side. This is good. This is why I've spent four hours bent over the bench with the wind blowing up my ass. (Should of put on the jacket sooner. Should have bought Xerox in 1957... )

Once located relative to the line of holes for the spar cap rivets, the rib template is cleco'd to the panel and the twenty-six holes are drilled. The swarf is brushed away, the template moves to the next rib, the location is verified (BIG red arrows, hard to miss... but not impossible) and you do it all over again. Not a big deal. But the culmination of prior work spanning months.

After drilling comes deburring. Then flipping the panels over and deburring again. Dog-leg deburring tool, finger as a gauge. The skins are sixteen thou. Gotta be careful with the deburring, keep the skins supported. They'll be dimpled later and it's tough to get a burr out of the bottom of a dimple.

The only secret to building airplanes is to do something every day. Doesn't matter what you do or how long you do it, what matters is the work-habit. Do something every single day and you'll be surprised how quickly you run out of things to do. When that happens the only thing left is to climb in and go for a ride.

Today it was some holes in the wing skins. Tomorrow it will be something else but today is past and the holes are done and I get to come in the house and gobble a handful of pills and feel The Pain crawl back in its cave.

It's raining now but the coffee is hot and the pills have kicked in. Just another day in the solitary art of Flying Machine construction.


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