The use of simple geometry and statics assumes the
tower is perfectly rigid. It isn't. With the lifting
point at 25% of height and 50 pounds cantilevered
at the end, the tower will bend somewhat at
lift off.
Simple statics only holds when the strain (bending)
is less than 3%. When the strain is greater than
that, it becomes a "geometric nonlinearity" and
cannot be solved analytically. You have to use
finite element analysis to model it numerically.
So what will happen is that when the estimated
740 pounds is applied (as estimated by Jim L. and
Dave H.), some of that energy will be diverted
into bending the tower instead of lifting it.
It will take a greater pull to overcome that before
liftoff occurs.
Simple statics is fine for back of the envelope
guestimating but you should use a large fudge
factor. Perhaps size everything with
the assumption that it could take 1200 pounds
of lifting.
Besides all that, a general rule of thumb is
the lifting point should be no lower than one
third of the tower height. If you could raise
that guy anchor up to say the 15 foot level,
that would reduce stress on both the lifting
cable and the tower itself.
73 Jim K6OK
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