Thanks for the info. I don't know as much about this as I'd like, but I'm
learning. From your comments I assume that 20-26 lbs would apply to round
tower legs (Rohn 65G here) and typical yagi booms and elements. Since I've
been using 36 lbs for my rough calculations, I should be sufficiently
over-designed :) I live in a 90 mph area and don't want any mistakes.
Would a 2" diameter tower leg have more or less drag than a 1/2" element? I
remember reading some info on this and seem to recall that the smaller
elements actually have more drag than the larger elements.
BTW, I'm sorry about the double post. I realized that I sent the original
question using the wrong email account, so I resent it this morning from the
correct account figuring the other one would bounce.
73,
Clay W7CE
----- Original Message -----
From: "Chris Wendling" <cpwendling@yahoo.com>
To: <towertalk@contesting.com>
Sent: Wednesday, January 23, 2008 7:09 AM
Subject: Re: [TowerTalk] Windload at 90 mph (was: Plumbing a tower)
> Jim,
>
> Yes, I figured you used a Cd of 1.0, which is a reasonable starting
> place.
> Nice to know there is another "aero" guy on the towertalk list.
>
> Below is a link to some very good information on communication
> structures and wind loads under various conditions of icing, etc. I
> think you'll find it very interesting.
>
> 73,
> CW-AI4MI
>
>
>
> <http://books.google.com/books?id=m-Uhn_O7O08C&pg=PA144&lpg=PA144&dq=coefficient+of+drag+for+wire+cable&source=web&ots=em_0WoRPLK&sig=ZeJ8s2lWzdEklgFL3Xu1JnQ2Af8#PPA133,M1>
>
> Jim Lux <jimlux@earthlink.net> wrote: Chris Wendling wrote:
>> Clay,
>>
>> Yes, you're in the ballpark.
>>
>> The formula for drag is 1/2*
>> rho*velocity-squared*frontal-area*coefficient-of-drag.
>>
>> I calculate about 26 lbs force for 90 mph for 1 sq-ft frontal
>> projection area.
>>
>> This assumes a coefficient of drag of 1.3 - typical for tubing or
>> wire profiles.
>> Also, the density of the atmosphere was assumed to be at STP
>> (standard temp and pressure at sea level.)
>>
>> The largest error contributor to these calculations is most oftem the
>> estimate of the drag coefficient- Surface roughness, Reynolds number,
>> interconnections on the tower, etc., may yeild different results.
>
>
> Indeed.. I just used 1.0 for Cd for rough and ready order of magnitude.
> I was thinking in terms of a tubular free standing tower (as opposed to,
> say, a lattice HDBX style) 1.3 might be a reasonable number of a
> smallish tube in that sort of wind, it would be near the peak in the
> Cd/Reynolds number curve. Somewhere around I have a spreadsheet that
> figures this stuff out, with handy data taken from Blevins, Fluid
> Dynamics Handbook.
>
>
>
> _______________________________________________
>
>
>
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