[TowerTalk] Angle of maximum wind force on Yagis
Guy Olinger, K2AV
k2av@contesting.com
Mon, 21 May 2001 12:50:16 -0400
With both dipoles in the wind, equally sharing, the VISIBLE cross-section offered the wind is at 1.4. HOWEVER, note that the wind does NOT flow across the elements at right angles, but at a 45 degree angle. If you draw a 45 degree section of a cylinder, it is NOT a circle, but an elongated ellipsoid, which has substantially less drag. Also since the element is at 45 degrees to the wind flow there is a shedding effect which allows wind flow behind the element in the direction toward the downwind tip. This further reduces the pull on the element since there is less of a vacuum behind the element than with the same flow cross section at right angles to the wind.
IN ADDITION, it is known that for a given wind velocity, there is an elongation of the circular shape at which the turbulence behind the element disappears and the flow becomes smooth. At this point there would be an almost light-switch reduction in drag. This may or may not come into play at wind velocities we would be concerned with.
The assumption you seem to be holding is that drag is in linear relationship to the visible cross section presented to the wind.
73
>
> From: "EUGENE SMAR" <SPELUNK.SUENO@prodigy.net>
> Date: 2001/05/21 Mon AM 11:52:10 EDT
> To: "Kurt Andress" <K7NV@contesting.com>, <towertalk@contesting.com>
> Subject: Re: [TowerTalk] Angle of maximum wind force on Yagis
>
> Hi, Kurt:
>
> If what you're asserting here is accurate (which I haven't satisfied
> myself that it is yet), then what you're saying is that my calculations show
> exactly the OPPOSITE of what I think they show. This is a serious
> discrepancy.
>
> I don't have the 19th Antenna Book (the 18th is my latest), but there
> are a couple of local Hamfests coming up. I'll pick up a copy and read
> through the section you cited.
>
> In the meantime, how about this? Take a hypothetical crossed dipole or
> turnstile antenna, made with the same amount of aluminum on either element.
> Say it's large enough to worry about wind loading. Orient it so the two
> dipoles are NS and EW.
>
> If the wind is from the east or west, you get some force, F, applied to
> the mast. The force is proportional to exposed area, L X W, of the element.
> If the wind is from the north or south, you get the same force, F, applied
> to the mast 90 degrees from the first example. I'm ignoring the end-on wind
> force. Also, I'm not using shape factor as it cancels in my analysis,
> below.
>
> Now let's say the wind is from the northeast. Both dipole elements
> experience the same force, whatever it is, right? The force on one dipole
> is proportional to effective area exposed to the wind. In my equation, that
> area is L X W X sin theta or, for 45 degree angle, L X W X 0.707. Total
> force at 45 degrees is twice this (two dipoles) or L X W X 1.41. This force
> is 1.41 times the force experienced by the single dipole broadside to the
> wind (merely L X W). I further assert that this is the MAXIMUM force that
> the crossed-dipole array would experience.
>
> If there is an error in my equation (and I'm not convinced there is
> yet), then it's in the cos theta term. I'll let you'all know.
>
>
> 73 de
> Gene Smar AD3F
> -----Original Message-----
> From: Kurt Andress <K7NV@contesting.com>
> To: EUGENE SMAR <SPELUNK.SUENO@prodigy.net>; towertalk@contesting.com
> <towertalk@contesting.com>
> Date: Monday, May 21, 2001 11:14 AM
> Subject: Re: [TowerTalk] Angle of maximum wind force on Yagis
>
>
> >Hi Gene,
> >The direction of maximum exposed antenna area, hence maximum force
> >developed by the antenna on the support structure, is either with the
> >elements or the boom broadside to the wind, whichever direction produces
> >the largest sum of projected antenna member areas is it. It's not somewhere
> >in between, as was thought a decade ago, somewhere in between produces the
> >lowest antenna loads on the structure.
> >
> >You can read about it in the K5IU articles cited in the post N8SM referred
> >to, or the latest ARRL Antenna Book, 19th Edition, Pg 22-18. Or, you can
> >get some software that does it.
> >
> >--
> >73, Kurt, K7NV
> >
> >http://yagistress.freeyellow.com
>
>
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>
>
List Sponsor: Champion Radio Products - We'll be at the Dayton Hamvention
with all of our safety equipment and other products. Stop by booth #559 and
say hi.
<A HREF="http://www.championradio.com">www.ChampionRadio.com</A>
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