There have been numerous discussions about lengthening the period of a
Lehman beyond the approximately 10 sec period and it becomes
"unstable."  

The question is - unstable in what way?  The period changes, the
centering fails, sensitivity goes haywire?  In particular, if the answer
is, "The centering fails," does the center just move around, or does the
beam "flop" to one side or the other?  

I'm doing some thought experiments during my long commute, and I'm
thinking about the sources of error.  In particular it's been mentioned
before that the center of rotation changes on the flexible hinge
designs.  That's intuitive, but how much and in what direction - toward
stability or instability? Has anybody figured out the actual path of the
center of rotation for a typical Lehman?  Crossed-X flex hinges were
mentioned, but in this application with side force, I don't think they
would be any less susceptible to the same de-centering.  

Another question is the upright's rigidity.  Several pounds of weight at
the end of a boom is a fair torque moment which is resisted by the
spring constant of the 'pipe' (in the pipe constructed versions)
vertical.  How much does this constant vary with temperature in standard
steel?  

Anyway, if there is an answer I should have looked up in the past posts,
just flame me, but please include an approximate date and subject header
I can go looking for!

Thanks,
Charles R. Patton
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