From: Charles.K.Scott@dartmouth.edu (Charles K. Scott)
Newsgroups: rec.aviation.homebuilt
Subject: And Two Steps Back, Article 15, Building a Christavia
Date: 16 Dec 92 16:57:43 GMT
Organization: Dartmouth College, Hanover, NH
X-Posted-From: InterNews1.0a5@newshost.dartmouth.edu

And Two Steps Back, Article 15, Building a Christavia

Work on the Christavia has been slow of late for many reasons, the
cold, my family, my wife, money or lack of it etc. etc. :-).  Here's
where I am at the moment, I've two wings assembled with all ribs
constructed and mounted on the wing.  The ribs were dipped in varnish
before installing so that I could be assured of complete coverage of
varnish.  I had to dip each one about 4 or 5 times to get what I
considered proper coverage.  Naturally, the spar openings had to be
taped over so that there was no varnish where glue would have to
adhere.  I've fabricated the fuel bay diagonal brace, The Drag
Anti-drag wires, the the compression struts and most of the brackets. 
The aileron brackets are the subject of this update.

I've mentioned in previous postings the discrepancies I've discovered
in the plans, little things mostly, like dimensions that don't add up
etc.  Well I've discovered another little problem.  The flap and
aileron hinges are made of two pieces.  The part that bolts to the wing
spar is rather intricate and requires a piece of .080" metal to be bent
into a U shape and welded to a backing piece that is drilled.  The
backing plate is what is bolted to the spar.  The U part is also
drilled so that you can insert a bolt through it and the companion
piece that is bolted to the flap spar allowing the flap or aileron to
pivot on the bolt.  The plans call for the opening of the U shaped
piece to be 3/16".  The companion piece that is bolted to the flap or
aileron is made from .125" stock (1/8").  The plans also call for you
to weld a 1/16" washer on either side of the hole drilled in the flap
piece to act as a spacer and make the hinge nice and tight, with no
slop.  I didn't notice until I started jigging up the flap I built to
the wing to see just how everything lined up that I had a problem.  If
you add a !/16" to each side of a piece that's already .125", you end
up with a piece that's .250" thick, or 1/4".  For those who still don't
see why this is a problem, the openings in the already built hinges
were built to the spec. called out in the plans which was for the
opening to be 3/16".  So they don't fit.  But that's not all.  When I
attempted to simply hold the hinge pieces in place to at least get a
feel for how they are supposed to work, I realized that I had welded
them improperly.  They were never going to work, I screwed up and the
plans were incorrect.  But it's an ill wind that blows no one any good.
 I never liked the job I did welding the pieces in the first place, and
this is the perfect opportunity to do them over and get the job right. 
I already have the new backing pieces cut out, the problem during the
first attempt at fabricating them was that I couldn't successfully bend
the U pieces, they looked horrible after pounding them in a vice.  So
instead, I cut out two pieces and MIG welded them to the backing plate.
 That worked but the results were a bit sloppy because it was very
difficult to jig the two pieces in the proper place and then weld them
using the MIG welder of a friend. That was about 8 months ago.  I'm a
little wiser now and I've discovered a different way to bend the pieces
which makes the U piece much more precise and aligned.  Note: if I
could find some .080" mild sheet steel I might make my life a little
easier as it would likely be easier to bend.  But I'm using .080" 4130
sheet steel and that's undoubtedly complicating things.

Anyway, as I said I discovered a different way to bend the pieces.  I
fabricated a die (basically a slot) and found a slab of 1/4" steel that
can act as a mandrill, and by laying the piece I want to bend across
the slot and using the mandrill to push the piece into the slot (using
a press), the metal bends nicely and doesn't have all those hammer
marks from my first attempts.

That's where I am now.  I'm bending all the stuff I need and when all
the pieces are done, I have to shape them and file radius's on the
edges.  Then I have to figure out some sort way to jig everything in
place (U piece on backing plate) in the proper configuration this time
:-) and weld it.  I'm also looking around for a TIG welder because I'm
unhappy with what happens to everything when welding with Oxy
Acetylene.  If everything is clamped to a metal jig, it can make it
difficult to bring the work up to proper temperature because the jig
will wick the heat away.  There are several TIG welders here at the
College and a machinist friend has one.  I'm pricing them at the moment
but I'm not optimistic.  TIG appears to be the most precise way to weld
and possibly also the most versatile.  I'd love to have one in the shop
when I start welding up the fuselage.  The only problem is that this
kind of welding concentrates the heat so much that if you don't
normalize the weld when done, you risk developing cracks just outside
the welded area.  So you have to weld with the TIG, then heat the
welded cluster with the torch.

So that's where I am at the moment.  It appears to be a step backwards
but I'm not discouraged because as I said, I wasn't happy with the way
they turned out the first time, here's another opportunity to do the
job correctly, I know I'll sleep better knowing I've done the job the
right way.

Corky Scott.