In a message dated 97-11-13 03:16:11 EST, you write:
> IF, that is indeed the general case, then I believe it behoves one to
> try to park a beam 90° to the prevailing winds to minimize all the
> stresses as much as possible.
Hi, Roger --
This seemingly simple question about beam parking is surprisingly
complex. Making a statment like the above means you've made some
decisions/assumptions that may or may not be correct or significant.
Discussions by very knolwledgeable engineering types (W6NL, K5IU, etc.) of
physical and mechanical concepts such as cylinder flow, vectors, section
modulus, rotational torque, etc. have shown that even they can't agree on the
answer to the question.
> >I think that the parking direction varies from antenna to antenna.
Sometimes the >boom has the
> larger windload and >for other antennas the elements constitute the
> majority of the windload.
>
> OK! Of course you're right. But... and here's the real point.... If you
> know which direction minimizes the load on one's particular antenna....
> then doesn't it make sense, as long as one has to park an antenna
> somewhere.... to park it with the minimum wind loaded side to the
> prevailing wind... and of course to build the entire system taking the
> maximum figures into account? This is the intuitive part as I see it.
>
Well, then your decision is: do I park it with the boom pointing INTO the
wind so that any wind damage will result in losing elements that are a pain
but cheap to replace or do I park it so that the boom is ACROSS the wind
since the boom is stronger than the elements and more likely to come through
a wind with no damage (no lost elements)? This hasn't got anything to do with
rotational torque or its effect on the rotator/tower system.
> > Regardless of the math and/or physics introduced, there still doesn't
> >appear to be a consensus about which way to point the beam relative to
> the
> >wind.
>
> Well, I'm not sure why one wouldn't aim the beam in whatever direction
> results in the minimum wind load on the structure....IF THAT OPTION IS
> AVAILABLE. "Unless I'm losin' it," I hope that was the main point I
> tried to make in my wandering way. I DO understand all the exceptions.
>
IMO the result is that it doesn't really matter in WHICH direction it's
pointing. It might be more worthwhile spending time to make the antenna wind
load symmetrical on the boom/mast than worrying about which way it's
pointing.
> And I think this discussion has been most enlightening for getting
> people thinking about all the factors involved in stressing their
> system and how useful your program would be to them.
>
The MARC Program just calculates approximate antenna windload area and
doesn't get involved with any additional analysis.
73, Steve K7LXC
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|