Tuesday, May 22, 2012

Stabilizer Spars

 With the leading edge formed the, 1" diameter, spar ends need to be flattened to fit to the, 1/2" diameter, leading edge tube.  These tubes are .035" thick so they're relatively easy to flatten.  I could have hammered the ends flat, as I did for the elevators, .065" thick.  I thought it could be done better with a vise with angled jaws.  Some c-clamps and blocks of 3/4" plywood worked fine.
 You need a clamp at the wide end of the taper to hold the blocks from opening.  Three clamps are needed at the flattened end, one on the tube and one on each side of it.  The clamps on each side keep the 2 blocks parallel.  As each clamp is tightened the other 2 get loose so you have to make 1/4 turns on each as you work the tube flat.  There is some spring back so you have to squeeze well beyond 1/2".
 I started with the front spar because I had a little extra length in the tube, in case I screwed up.  Also I needed to figure out how much the tube widened out forming it.  It was about 1-3/8" wide once it fit the 1/2" tube.
 On the front spar the widening can be even on both sides, but on the rear spar the aft edge must end up in a straight line.  By knowing how wide the tube was flattened, I was able to use the conduit bender to offset the ends of the rear spar about 3/16" to allow for the widening.
 Once flattened the rear edge is straight.  The fit came out very good.

 OK, the first end is easy.  The second end must be parallel to the first end or it won't fit the leading edge properly.  Because the spars are the same diameter I use them to find the top center.  First mark the top with a permanent marker.  Then I use a piece of aluminum angle to rub along the tubes leaving a nice center line mark without scratching the tubes.

 For the rear spar I need a line 90 degrees from the top to use when bending the end forward 3/16".  First wrap a piece of paper around the tube and mark the overlap point. Divide that length into quarters, in this case every 25/32". Then just mark the tube at the 1/4 mark.

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