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[TowerTalk] windforces on antennas

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Subject: [TowerTalk] windforces on antennas
From: baycock@HIWAAY.NET (Bill Aycock)
Date: Sun, 10 Jun 2001 14:13:19 -0500
I am a retired "Aerospace" Engineer, who started his professional career
called an "aeronautical" Engineer. At that time, we dealt mostly with
low-speed aero characteristics, which is what we are concerned with, with

The arguments over which area to use are all right, every one, and all
wrong, every one, depending on how you apply the numbers. Calculating drag
(wind force) is like Zen archery- first you shoot your arrow, then find it,
then call that spot the target. Simple.

In calculating drag, you need the effect of four factors- the
characteristics of the air - the reference area of the object we worry
about - the shape factor for that object ( called, usually, drag
coefficient) and the velocity of the air passing the object. Note that I
said "reference" area, not surface or projected area. It really doesnt
matter, so long as the area you use matches the one used to calculate the
form factor, or drag coefficient from measurements. If one is a little off,
the other, since it is derived from data, compensates. 

In practice, what is done is to take a shape- calculate the area projected
in some repeatable direction, place it in a wind tunnel, and measure the
force for various wind speeds. The relationship gives the form factor.
after that- you can add shapes together, make them larger or smaller, etc,
and, so long as you use the same method, you can calculate the drag for
other , similar shapes. 

THat is what the antenna manufacturers have done. They have standardized on
a form factor for antennas from measurements, and they give you a reference
area that, with the proper application of the wind speed, will give you the
proper force.

I know that Steve, our professional erector, has the proper specs and
numbers that are used according to the "Code". I cant remember the number
of that spec, but he has it.

Bill Aycock   ---   Persimmon Hill 
 Woodville, Alabama, US 35776
 (in the N.E. corner of the State)
      W4BSG   --   Grid EM64vr

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