Peter Cowan/Lexi Cameron wrote:

Would any of you who are experts on this please give me your comments?
I'm concerned about distortion issues when I re-weld my fuselage. The

front half of the fuselage is bolted to the concrete basement floor. About
half way back I have cut through all tubes except the lower longerons. I
have removed enough material from the top longerons and diagonals  to allow
me to raise the tail end about 3 inches. (Didn't like the lines of it the
other way plus the stab needed more negative incidence.) I may tack it
myself but I am bringing in an expert with either his tig or mig welder.
(His tig welds are gorgeous on thin wall tube but the machine weighs about
300lbs and I may not be able to move it into the basement for him.)

At this point the tail is not held rigidly except in its' distance off

the floor. Is there likely to be a lot of distortion as it is welded?

Thanks for any tips you might have.

What hapens is that the welded areas are at essentially zero stress at the
just below molten stage and then  contract from there as it cools.  This
causes various types of distortion, totally depending on the location & type
of weld.  If you are just rejoining longerons, I would expect a small but
measurable shortening of the longerons.  Depends a lot on the extent of the
welded area.  I assume you are following AC43.13-1B weld repair practice.
If not, PLEASE get a copy and design your weld repair to this FAA std.   It
might save your butt.

The worst distortions that I have personally seen are when welding diagonals
into a cluster at very small angles.   This makes a very long weld and causes
the most distortion.  If you do the same things on both sides, the
distortion should
be symmetric and not too serious. The most serious difficulty is in items like
exhaust systems and engine mounts which must bolt onto fixed mounting points
after welding.  A slightly distorted fuselage frame should be very unimportant.

Bill
Long EZ builder & pilot
mechanical engineer

 
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