[TowerTalk] wind load vs Rohn specs

[Kurt Andress]

Bill Coleman AA4LR wrote:

> On 9/3/00 1:26 AM, Kurt Andress at K7NV@contesting.com wrote:
>
> >Take one example:
> >
> >>40'
> >>Round 18.4x1.2=22.08
> >>Flat 11.0x2.0=22.0
> >>
> >
> >This is correct!
> >It says that it takes 18.4 SqFt of Flat Projected Area (FPA of your antenna
> >made from cylindrical members) to create 22.08 SqFt of Effective Projected
> >Area. This is where the drag coefficient is applied.
>
> This discussion of projected area has always made by head spin. Seems to
> me it doesn't account for antenna members that could add significantly to
> the drag of the installation.
>
> If we take a small tribander and optically project a shadow of the
> antenna vertically on the ground, then measure the area of that shadow,
> we fail to take into account say, the boom to mast plate. Since the plate
> sits vertically, it casts a small shadow, yet it presents a big area to
> the horizontal wind.
>
> Similarly, if we optically project a shadow horizontally against a wall,
> and measure the area of that shadow, we'll take into account the boom to
> mast plate, provided we have the antenna oriented correctly. However,
> we're likely to miss either the boom or one or more of the elements,
> whose shadows overlap one another.
>
> So, which is right? The vertical projection or the horizontal projection?
> Neither seems to be well correlated to the actual drag of the antenna.

The vertical projection technique would be useful if you plan to mount your
antenna with the elements vertically aligned or if all you get for wind events
are microbursts directly over the tower. In these exposures the wind will see
both boom and element areas simultaneously.

For normal installations, the horizontal exposure is the appropriate one to
chose. But trying to do it with a some-kinda projection against a wall, while
providing at least a six-pack of entertainment, is useless for determining how
the antenna will develop loads on a tower.

Antenna area measurements should be made with a calculator. Simply summing up
the Length x diameter of the members is more accurate.

The peak area (FPA or EPA) and hence load applied to a tower by a horizontally
polarized yagi type antenna will occur at either 0 deg or 90 deg orientation to
the wind. In between these two orientations the area (hence, load) will be less.

There is no longer (never was) a single peak peak load orientation caused by the
combined element and boom areas as used to be common thought. That notion was
effectively challenged in 1993 when K5IU dragged some old long standing
aerodynamic principles, kicking and screaming into the communications arena. I
think there have been several extensive posts regarding this on this reflector
and other places.

Your concern over the mast/boom plate is unwarranted because you are thinking
about it in the old obsolete reference frame.
For a TH7, the proper inclusion of the mast attachment or not makes a 2.2%
change in the effective area with the boom broadside to the wind, but who cares
when the area of the antenna pointed into the wind is 2.8 times greater than
broadside.


>
> Seems to me a better measure was the old "flat plate equivalent" -- which
> is the drag of the antenna expressed as equivalent to a flat plate of a
> given area. This FPA and EPA for round and square antenna members just
> leads to a lot of confusion.
>
> Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
>

Well, while intrigued by your proposal, it leaves me wanting to know more about
how it would actually work in practice, and how it would make anything better.

Things to address:

There are more than one specification used to design towers. They are all
configured to accept Flat Projected Area as an input. Inside each method the
drag coefficients, exposures, and windload formulas are applied.

They use different drag coefficients, different exposure factors, different wind
speed formulas. At the end of the day, they do not produce dramatically
different results, but the current notion of mindless mixing of methods is a
recipe for disaster.

How will the users (and the P.E's we hire) of the new "Flat plate equivalent"
value, generated in a similar unknown fashion, know how to work their way back
to the fundamental projected area to then commence evaluating their tower
according to its required design specification?

There are no universally accepted antenna design specifications. Nor are the
ones used for any particular antenna clearly defined. Hence, confusion reigns!

Seems likely that we would have the same confusion over a different number.

We need to first understand what we have done to apparently confuse established
science and it's child (analytical methodology) and sort that out before we go
ask for a new standard that we are likely to continue to misunderstand for the
same reasons.

I think we'll find that the current M.O. works just fine when the proper values
and methods are used.

-
73, Kurt, K7NV



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