[TowerTalk] Antenna & Tower Wind Load Ratings
Roger (K8RI) on TT
K8RI-on-TowerTalk at tm.net
Thu Jun 15 05:56:57 EDT 2017
Do you want a real, worst case scenario which gives you what to expect
for a real wind load and should be sufficient for almost any situation /
installation, or do you want a calculation that will give you the real
load at any speed for maximum wind load, remembering that an
instantaneous gust can cause element, boom, or tower failure? Having
flown for thousands of hours, I can attests that there are wind gusts
far beyond typical or forecast, followed by almost still air which is
like having the rug pulled out from under you. This can allow the
element or boom to rebound, with inertia.
Modeling on the face of itis not terribly difficult. However, including
"ALL" of the parameters and formulas can get very complicated
If you can deflect an element any number of degrees and calculate the
wind load the math exists to do that. If you want to do it over a wide
range in real time the math exists to do that as well. It's called
calculus. Unfortunately it takes Calc II (Derivatives and integrals of
trig functions) to do these calculations which is well beyond most
hams. I haven't used and Calc since I graduated in Dec of 1990 so it's
beyond me as well without a lot of reviewing.
OTOH I seriously doubt we even need be concerned about the finer
mathematical points as perpendicular air flow to the elements and to the
boom are "worst case scenarios". Anything else (with one basic
exception) causes less wind load.
IF the "flat plate" area is used, then you are using a worst case
scenario, with a built in safety factor. Where it gets difficult is
wanting accurate, real time calculations that give the actual wind load
for any speed and angle.
The one element (no pun intended) is wind induced oscillation, a normal
phenomena as the wind is rarely steady. This oscillation is rarely in
only one plane and can, at times, exceed the typical worst case wind
load for very short ti. The happens when the element rebounds in gusty
winds
73, Roger (K8RI)
On 6/15/2017 Thursday 1:33 AM, Kurt Andress wrote:
> I am just trying to catch up with you folks on some of this
> traffic......we are out of control here making things and planning
> projects and trips........
> Here is how I have seen it for the past few decades, and still how I
> see it today.....I am trying to really simplify this so it can be
> followed by most readers that really need to understand this......
>
> There are basically two types of "Antenna Area" values to be
> considered.....
>
> 1) The most basic and easiest one to determine is its "Projected Area"
> which is simply the sum of the lengths and widths of its combined
> members. This produces a profile of all members that are exposed to
> the wind, when the wind is normal (perpendicular) to the antenna
> members center axes. This number is always the same for every antenna,
> and every Yagi antenna has two, and only two Areas to be considered,
> one is when the wind is normal to the sum of its exposed elements, and
> the other is when the wind is normal to the boom members. Whichever
> one is the greatest is the maximum one each antenna has. This was
> clarified by Dick Weber, K5IU, P.E., in
> Communications Quarterly, 1993. It informed us
> that what everyone was doing to determine antenna loads on
> towers was being done wrong, according to existing conventional
> knowledge about how airflow over things in the air stream
> actually behave! We learned that everything we were doing
> was wrong, and that the wind flow over any and every kind
> of member in the wind stream produces resultant load vectors
> that are always only normal (perpendicular) to the center line axes of
> those members in the air stream at all the points on them, as they
> become deflected (bent) by the wind loads. This becomes a very
> difficult non-linear problem to solve with any kind of mathematics
> with any kind of the most expensive software available today! I have
> discussed this with some really qualified colleagues that run the
> really expensive software.......they just say "we need another level
> of software we don't have" to be able to do that......think about
> this......as each inch of an antenna element is deflected down wind,
> the actual forces on it are reduced because it becomes inclined to the
> wind......therefore the forces normal to it (the ones that are trying
> to bend it) are reduced.
>
> So, since we can't really do that, we go back to the EIA/TIA spec and
> find what they tell us to do with the linear engineering mathematics
> they know we can do!
>
> 2) There are established "Drag Coefficients" for the shapes of things
> in the air stream. They are defined in the EIA standards for us to
> use. As Jim Lux points out, they are dependent upon the Reynolds
> numbers for the sizes & shapes of things at various wind speeds! I
> went and studied the EIA 222-G criteria for this, and came to the
> conclusion that everything we might be doing with Amateur Antennas
> will fall into sub-critical regimes and that we will be ok using the
> standard Drag Coefficients of 1.2 for cylinders, and 2.0 for flat
> plates or rectangular things. These become the "Effective" or
> Effective Projected Areas" as the nomenclature has evolved, its all
> the same thing "Projected Area X Drag Coefficient!"
>
> As You & Jim Varney presented, there are so many other things to be
> considered, but this is Towertalk....where most of the readers have
> not spent several decades trying to figure out what is really going
> on.......
>
> So, the intent of my original post was not to dig this deep into this
> stuff, but to simply remind everyone that tower designers and antenna
> designers do not share the same skill sets and are not at all on the
> same page, so their ratings can not be correlated by anyone that is
> not a real engineer that spends the time to unravel it, and come up
> with his own understanding of how it really is.....
>
> I don't want to bash my friends/clients at some of the antenna
> companies, because I understand exactly how hard it is for them to
> even remain in business to deal with this market! It is a thankless
> endeavour!
>
> I happen to think that it is rather unfortunate, for an established
> industry, like this, to remain so blatantly disfunctional after so
> many decades.......I will continue to do what I do for my clients to
> get their things as right as possible.......which consumes almost all
> of my available bandwidth, which explains why I rarely show up here on
> TT....
>
> YMMV, as they say, and so on......
>
> 73, Kurt, K7NV
>
> Message: 5
>
> Date: Wed, 14 Jun 2017 07:01:04 -0700
> From: jimlux<jimlux at earthlink.net>
> To:towertalk at contesting.com
> Subject: Re: [TowerTalk] (no subject)
> Message-ID:<587f8125-a530-995c-fad2-e680a9515641 at earthlink.net>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> On 6/13/17 11:23 PM, Kurt Andress wrote:
>
>> I am going to make an attempt to present some information and history
>> that might make this conundrum more understandable for those that follow
>> this Reflector......
>> Don't shoot the messenger, I'm rooting for everyone that has a stake in
>> these matters! If I didn't, I would not have said a word!
>>
> So, in summary, sort of:
>
> Tower mfrs cite a "X square feet" as opposed to a "X lbs load"
>
> The X could be either the actual projected area (length x diameter) or
> some "effective area" (a number that you could plug into F = rho*V^2*A)
>
> Antenna manufacturers gave either actual projected area, or some
> "effective area" as well.
>
> So nobody really knows whether the tower or antenna is calculating for
> the coefficient of drag.
>
> And, of course, the Cd approximations in 222C are wrong (not only are
> the actual numbers off, but they don't allow for change in Cd with
> respect to Reynolds number regimes)
>
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--
73
Roger (K8RI)
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