Sidearm Physics

[00tlzivney at leo.bsuvc.bsu.edu]

I was thinking about antenna projects for this summer.  In particular,
I was looking up articles on sidearms.   I have a serious concern not
addressed explicitly by any of the references I have uncovered.

In particular, the moment arm is pretty large, 24-30".  With a moderate
sized antenna, say 10 sq ft and 50 lb/ft2 wind loading, that implies
10 x 50 x 24 = 6,000 in-lb of torque on the rotor trying to turn the
side arm.  This is way above most ham-type rotor capabilities, and is
interesting given that some of the articles show Ham-M or Tailtwister
rotors.  

I am worried that the brake could give way on the rotor, causing the
boom to bang into the side of the tower.  This would 1) be very noisy
to the neighbors, 2) do a number to the antenna, and maybe even 3) cause
structural damage to the tower itself.  Does anyone have any thoughts/
experience with this????  At the very least, it appears that the
antenna needs to be pinned to the mast on the sidearm & the sidearm
needs to be pinned to the rotor to eliminate any play.

What say????

Terry Zivney, N4TZ
00tlzivney at vsuvc.bsu.edu