Clay,
Great question! Difficult answer. I'm assuming that the projected areas
are the same for both the 1/2" and 2" tubing in your question.
It may be counter-intuitive, but many times the smaller diameter tube, wire/
or cable will have the higher net drag with equivalent frontal projection
areas. This is due to the earlier onset of turbulent flow (as opposed to
laminar flow), something described by the Reynolds number. (Another
dimensionless quantity.) Don't want to get too far afield on the towertalk
list, so I'll stop here. But there are many excellent resources (try
Wikipedia for starters) that explain such topics as supercritical flow,
viscosity, Reynolds number, etc. if you have an academic interest in the
theory.
Realistically, you need not concern yourself with such details (analysis to
paralysis?) for a standard Ham tower project. Just use anywhere from 25 to 35
lbs force per square foot projected area at 90mph wind speed. Then use an
appropriate Engineering Safety Factor depending on location, icing, property
value, liability factors, etc.
73,
CW-AI4MI
W7CE <w7ce@curtiss.net> wrote: 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"
To:
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
>
>
>
>
>
> Jim Lux 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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