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Re: Wind load calculations ?



Hi Jens,
    Yes, I didn't mention "galoping" or vibrations at 90 degrees to the
stream flow. There are a lot of things I didn't mention but the information
I gave you is quite conservative on many ocasions and so should be quite
safe.

    Resonance in wind is worst when the mechanical resonant frequency is
about 0.3 Hz. The best way to avoid resonance problems is to increase the
mechanical damping in your structure or have it so stiff that the mechanical
resonant frequency is quite high. Metal structures like antennas have quite
low amounts of mechanical damping and so these "High Q" structures will only
responed to the frequencies they are mechanically "tuned" to.

    A piece of relative stiff cord like the cord used for venetian blinds
tied between elements will often be sufficent to damp out vibrations. For an
element to vibrate it has to be able to move in both directions from its
rest position. a string preventing it moving one way will prevent vibration
in that plane in a way similar to the way a diode can be used to limit the
amplitude of oscillations in an oscillator circut. If it  can vibrate in two
planes then you need two strings at about right angles to each other.  I
have also used pieces of PVC pipe/conduit with holes the size and spacing of
the elements, inserted onto the elements of a 10/15m yagi to damp wind
induced vibrations. There a number of these type of approaches to wind
induced vibrations. The most important thing is that whatever you put on a
yagi away from the centre has to be non-conductive.

    This is a thought I haven't tried yet but should work quite well. For a
2 m yagi, try ping pong balls half filled with water, sealed and stuck on
the ends of the elements with silicon rubber. The water sloshing arround in
the balls should damp out the vibrations just as the liquids in a raw egg
makes it hard to spin on the bench top whereas you can spin a hard boiled
egg easily.

Regards,
Murray Peterson
VK2KGM

----- Original Message -----
From: "Jens Schmidt" <j.schmidt@paradise.net.nz>
To: "Murray Peterson VK2KGM" <vk2kgm@ihug.com.au>
Cc: "AMSAT-BB" <amsat-bb@AMSAT.Org>
Sent: Wednesday, January 31, 2001 11:29 PM
Subject: Re: [amsat-bb] Wind load calculations ?


> Hi Murray,
> Thanks a lot.
> That was just what I was after.
> Yes, it will be just as easy to do by hand (calculator) as on the PC.
> The drag coefficient was the thing I didn't know about.
> I would only have applied 35 m/s, since I don't count on living another
> 100 years, HI HI, but then this could be year 99.
> Interesting about the 'lift', I had one antenna break, (resonance
fatigue),
> I came home one evening, dark and raining, as usual looked up at the
> mast, and saw one half of a 2m beam pointing about 30 degrees up.
> Metric is fine. Now I will be able to specify much better, the specs
> needed for a rotator gearbox
>
> 73 Jens    ZL2TJT



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