----- Original Message -----
From: "Gary Schafer" <firstname.lastname@example.org>
> Hi Chuck,
> Yes I understand how the force is spread over time with the absorber. My
> question is more concerned with how it will affect the forces when
> oscillations are present.
> We know that energy can not be created or destroyed it can only be stored.
> So most of the energy that is absorbed by the donut will be given up in
> opposite direction to cause the antenna to rotate backwards from its
> direction from when a wind gust moves it.
> As it moves backwards the energy that was absorbed by the donut and the
> energy absorbed by the mast and antenna itself will all add together to
> the antenna in the opposite direction of the original rotation. If another
> wind gust in the opposite direction, or on another vector from the first,
> happens while things are unwinding the stored energy will add to the wind
> gust energy and speed up the antenna rotation more than the wind alone
> and cause greater stress on the rotator than if the donut were not there.
> The antenna would have a chance to move further with the donut than
> it. The antenna has a chance to pick up more momentum with the donut than
> without it.
> I know, energy can not be destroyed or created but it can be stored and
> released at different times and if that release time coincides with a wind
> gust then peak loads may be increased. I am not saying the donut is not a
> good thing just some things to think about.
> Another thought on the subject: It may be more convenient to mount the
> absorber under the rotor between the tower and rotor.
> Gary K4FMX
We've got one of these things installed at W6UE on some
especially heavy antennas (KLM KT34XA and 40M4 both
with 3" O.D. 1/4" wall "ICE" booms - total antenna weight
~300lbs). This stack has a huge mass moment of inertia. If
you set the OR-2800 rotator to its fast speed with a quick
stop (no speed ramp down), the stack will oscillate pretty
violently (i.e. high pucker factor). I'll have to video tape it
sometime and put the video up on the internet. I'd also like
to wach it in heavy Santa Ana winds, but I just haven't been
over there at the station during one of those occasions.
If figure its like any control system, its got dominate poles
and if they get too close to the right half plane, the response
of the system can become highly underdamped. The mass
moment of inertia of the system and the "Q" of the rubber
disc probably effect the position of the poles greatly. Get
the wrong combination of parameters and just the right
stimulus and you may have a Tacoma Narrows Bridge on
73, Mike W4EF....................................
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