W3GM used bottom mount rotors on all of his 100' 25g towers.
The actual mast and the 'torque tube' were of different material.
He used thin-wall irrigation tubing, 100' of which weighed maybe
20 lbs. No idea what % of that was carried by the rotor, and what %
might have been hanging from the thrust bearing. The load was
light, in any case.
The effect of this was to allow the arrays.. 3 el fullsized 40, for
to rotate in the wind somewhat, taking peak loads off the rotor. I think he
turned the 40m array either with a tailtwister or a ham-4. In the time
I operated from there, maybe 4 seasons, I never remember a major rotor
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