For the boom, it would. But for the elements of something like a C31, which
flexes a good bit, particularly the 20 meter elements, the computation would be
unnecessarily harsh. The figures would come out like Rohn load figures,
substantially over-engineered to prevent lawsuits.
Nothing against over-engineering, mind you, but some situations, like single
tower, can be unnecessarily diminished by using such figures to limit the
installation.
73
>
> From: "EUGENE SMAR" <SPELUNK.SUENO@prodigy.net>
To: <towertalk@contesting.com>
> Date: 2001/05/21 Mon AM 11:00:05 EDT
> To: "Guy Olinger, K2AV" <k2av@contesting.com>,
> "TowerTalk" <towertalk@contesting.com>
> Subject: Re: [TowerTalk] Angle of maximum wind force on Yagis
>
> Guy:
>
> Thanks for the comments. I agree that flexible elements (Skyhawk) are
> likely to shed wind by bending a little,thereby reducing the actual force on
> the element/mast/tower. Would it be safe to say, though, that the equation
> I posed (first equation) is the MAXIMUM force likely to be encountered,
> given the assumptions I laid out in my posting? I'm satisfied that it is.
>
> 73 de
> Gene Smar AD3F
> -----Original Message-----
> From: Guy Olinger, K2AV <k2av@contesting.com>
> To: EUGENE SMAR <SPELUNK.SUENO@prodigy.net>; TowerTalk
> <towertalk@contesting.com>
To: <towertalk@contesting.com>
> Date: Monday, May 21, 2001 2:22 AM
> Subject: Re: [TowerTalk] Angle of maximum wind force on Yagis
>
>
> >I'm not at all sure that Cos Theta is a proper formula for the
> >diminishment of drag as the shapes are turned away from full head on. It
> >does describe the diminishment of viewed area from a point distant, but
> >an angled element presents the shape of an ellipsoid to the angled wind
> >flow. One could make a case that the drag diminishes faster than the
> >cosine factor. Further, elements built with flex exhibit a "shedding"
> >effect that reduces their drag in the higher winds.
> >
> >The complete formula for the drag of a typical yagi, as a function of
> >its turn to the wind is a right nasty equation, probably better measured
> >in a wind tunnel and "curved" than computed.
> >
> >73
> >-----------------
> >
> >Guy Olinger
> >Apex, NC, USA
> >
> >----- Original Message -----
> >From: "EUGENE SMAR" <SPELUNK.SUENO@prodigy.net>
> >To: <towertalk@contesting.com>
> >Sent: Monday, May 21, 2001 12:17 AM
> >Subject: [TowerTalk] Angle of maximum wind force on Yagis
> >
> >
> >> TT:
> >>
> >> I just finished putting together an equation to calculate the
> >angle of
> >> max wind force on a Yagi. (I couldn't find it in my own TT archives,
> >and
> >> wanted to shake the cobwebs out of my forty-something brain. This
> >isn't
> >> really that tough, though.)
> >>
> >> You'll need to know/calculate the effective area (diameter X
> >length X
> >> shape factor of 0.67) of the boom plus the boom-to-mast bracket (no
> >shape
> >> factor multiplier) as one term, and the total of all elements X shape
> >factor
> >> as another term, in the equation below. (You need the Yagi's assembly
> >> manual to get these dimensions. All dimensions must be in feet, so
> >you'll
> >> have to convert diameters to feet.)
> >>
> >> Assuming that zero degrees is where the Yagi is pointed, then
> >the
> >> following applies:
> >>
> >> Theta(max) = arctan [(Boom plus bracket area )/(element area)].
> >Theta(max)
> >> is the angle of maximum wind force on the antenna.
> >>
> >> I think we can all (most) agree that there are three other
> >> angles/directions that also satisfy this equation: one in each of the
> >other
> >> three quadrants around the antenna, theta(max) degrees from the boom.
> >>
> >> Once you know Theta (the angle), you can calculate the actual
> >force as
> >> follows:
> >>
> >> F(max) = W[(boom plus bracket area)(sin theta) + (element area)(cos
> >theta)],
> >>
> >> where F(max) is maximum force in pounds exerted by the wind and W is
> >the
> >> wind force in pounds per square foot. The remaining terms were
> >defined
> >> above.
> >>
> >> As an example, say you have a 21.5-foot boom 2M Yagi with 13
> >elements
> >> (this is my new/used KLM 144-148-13-LBX.) The effective boom-plus
> >bracket
> >> area is 1.91 sqft and the effective element area is 0.464 sqft.
> >Plugging in
> >> to the first equation yields Theta(max) as 76.4 degrees. The force at
> >this
> >> angle (second equation) from a 70 mph wind is (20.6
> >psf)(1.91sin(76.4) +
> >> 0.464cos(76.4)) = 40.5 pounds.
> >>
> >> If the example Yagi is facing north, this maximum wind force is
> >exerted
> >> on the antenna when the wind comes from 76.4 degrees, that is, from
> >slightly
> >> north of east. That's because the wind is almost perpendicular to the
> >> relatively large boom/bracket area, but it still catches the tiny
> >elements
> >> to exert some force on them, too. This max force is not found at
> >ninety
> >> degrees (out of the east) because an easterly wind would not _see_ the
> >area
> >> of the small element ends (OK - it's there but negligible).
> >>
> >> As we agreed above, the three other angles of max force are
> >slightly
> >> south of east, slightly north of west and slightly south of west.
> >(You can
> >> do the math to determine the exact azimuths.)
> >>
> >> As another example, let's say you have a 4-element 15M Yagi on a
> >short,
> >> heavy boom. (For this example exact lengths are not needed.) The
> >ratio
> >> [boom-plus-bracket area/element area] might be close to unity (one).
> >From
> >> the first equation, arctan 1 equals 45 degrees. In this example the
> >max
> >> force occurs at 45 degrees (also at 135 deg, 225 deg and 315 deg.)
> >>
> >> The equations assume a few things:
> >> 1. The wind sees all elements (the wide reflector doesn't hide the
> >shorter
> >> DE and Dirs.)
> >> 2. There is no allowance for turbulence in the vicinity of any
> >antenna
> >> component (although the shape factor, I've learned from TT, takes much
> >of
> >> this airflow into account. You can ignore shape factor completely if
> >your
> >> antenna uses a small boom-to-mast bracket. You decide what small
> >means.)
> >> 3. If you use tip-to-tip dimensions for each element, like for
> >> through-boom VHF elements, the equation ignores the fact that some of
> >each
> >> element is hidden inside the boom and not subjected to wind force. If
> >you
> >> use the lengths of split elements from the center support to the tip,
> >there
> >> is no hidden element length. It's all exposed to the wind force.
> >> 4. No, I didn't account for the force on hose clamps, taped or
> >otherwise.
> >> (Sheesh!)
> >>
> >> I want to calculate the force on each of my planned Yagis this
> >way to
> >> satisfy myself that I actually have more breathing room in tower
> >loading
> >> specs. But that's another discussion entirely.
> >>
> >>
> >> 73 de
> >> Gene Smar AD3F
> >>
> >>
> >> 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>
> >>
> >> -----
> >> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> >> Submissions: towertalk@contesting.com
> >> Administrative requests: towertalk-REQUEST@contesting.com
> >> Problems: owner-towertalk@contesting.com
> >>
> >>
> >>
> >
> >
> >
> >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>
> >
> >-----
> >FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> >Submissions: towertalk@contesting.com
> >Administrative requests: towertalk-REQUEST@contesting.com
> >Problems: owner-towertalk@contesting.com
> >
>
>
>
>
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>
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
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