[TowerTalk] wind load vs Rohn specs

[Kurt Andress]

Bill Coleman AA4LR wrote:

>
> >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.
>
> Now you are toying with me. The odd thing is, the way that these
> projected area values are calculated appears to be by using the vertical
> projection.

Maybe, but that is not how it should be done.

>
>
> >For normal installations, the horizontal exposure is the appropriate one to
> >chose.
>
> Right! Which is why the horizontal projection would appear to be more
> appropriate -- except that it isn't the one used in the "projected" area.
>
> > 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.
>
> My point exactly.

Great!

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

For clarification:
In my mind, the statement means that we take element #1 and sum up the exposed
areas of all its sections, then we do the same for every other element. Then we
add all the individual element areas together to get a total area for the
elements. The boom is done just like one of the elements.
I've never ever meant to imply that the leading elements shield the others from
the wind. I don't believe that is possible for the normal yagi element spacings.

>
>
> If you do this sum, then you have computed the area of the vertical
> projection of the antenna. (Assuming a standard yagi-type antenna) Just a
> few paragraphs before, you decried this projection as being inaccurate.
> Now you say it is the way to calculate them. Again, I don't understand.
>
> >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.
>
> Doesn't this depend on the configuration of the antenna members?

I'm sure I don't understand the question, I don't want to create any confusion by
trying to answer it.

>
>
> I know enough about aerodynamics to understand that a tubing member
> receiving clear, undisturbed area is going to have a different amount of
> drag than one receiving the turbulent air that has already passed over
> another antenna member. The effects are very non-linear and difficult to
> predict with simple algebra. The drag effects will vary with the speed of
> the wind, and also with the density (altitude, pressure, humidity) of the
> air.
>
> But I also know that you can't just ignore the area of the additional
> elements, even in turbulent air. The horizontal projection would appear
> to ignore that area completely, since the shadows of the elements (and
> possibly the boom) overlap one another.
>
> >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.
>
> Isn't there? I can think of two extreme cases. One is a very long
> wavelength antenna on a very short boom. Most of the drag will be
> contributed by the antenna elements. Virtually none by the boom. An
> alternative is a long boom antenna with very short elements (say a 70
> foot 1296 MHz beam). The elements contribute little drag to the antenna,
> virtually all by the boom. (Indeed, due to the turbulent effects of the
> elements, the antenna may offer less drag than the boom by itself)
>
> >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.
>
> Could you dig up some references?

Communications Quarterly, Spring, 1993, Determination of Yagi Wind Loads Using the
"Cross-Flow Principle", by Dick Weber, K5IU

The essence of the article is that the forces acting on the antenna members act
normal to their major axis. This means all element loads act parallel to the boom,
the boom loads act parallel to the elements.
You can go prove this to yourself by holding a piece of tubing out the window of
your car and take off down the road. When the tube is aligned in a vertical
position, your hand is pressed straight back, at a 90 degree angle to the major
axis of the tube. Now, tilt the top of the tube forward to a 45 degree angle and
you'll feel your hand being pushed at a the same 90 degree angle to the tube,
That's not straight back anymore it's up and back.

Since, the loads on an antenna are developed this way, there just is no way for
them to generate a peak load somewhere in between 0 & 90 degress azimuth.
Actually, somewhere in between 0 & 90 is where the minimum occurs. Its exact angle
is determined by the relative distribution of areas between the boom and elements.
If boom and element areas are equal, the minimum occurs at 45 degrees.

You'll find similar commentary a few times in the TT archives.

>
> >Your concern over the mast/boom plate is unwarranted because you are thinking
> >about it in the old obsolete reference frame.
>
> If someone puts a big mongo mast / boom plate on a very small antenna, it
> certainly would be a big contributor to the total drag on an antenna.
>
> >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.
>
> Since we can't control the wind direction, the appropriate computation
> would be for the "peak" drag on the antenna. If the structure can
> withstand the wind loads at the peak drag, then the loads at lessor drag
> orientations seem immaterial.
>
> But computing peak drag seems dependant on the specific configuration of
> the antenna. A low aspect-ratio yagi (1296 long boom) has peak drag 90
> degress to a high aspect-ratio yagi.
>
> So far we've talked about yagi's. What about other configurations,  like
> quads? How do we compute their "projected area"?
>

I'd be doing it the same way. Sum of all wires and spreaders in the 0 direction,
boom the same as above.

>
> I also know that many old biplanes got most of their drag, not from the
> whetted area of the wings or fuselage, but from round flying wires, which
> generate a ton of drag from the turbulent airflow they produce.
> Streamlined flying wires were introduced as far back as 1915, and were
> later eliminated entirely with cantilevered and monocoque construction.
>
> >> 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.
> >>
> >
> >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.
>
> You'd have one number, normalised to a given unit. You wouldn't have to
> deal with the wildly varying units and computations for this "projected
> area".
>
> >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.
>
> I have yet to see a concise definition of this Flat Projected Area. If I
> have a given antenna design -- how do I compute this FPA? You laughed at
> my vertical and horizontal projections. This leaves me more confused than
> before.
>
> Let's demystify this whole process by providing proper definitions.
>
> >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?
>
> I would assume they wouldn't use projected area at all, but use flat
> plate equivalent instead.
>
> >There are no universally accepted antenna design specifications. Nor are the
> >ones used for any particular antenna clearly defined. Hence, confusion
> >reigns!
>
> So is it OK to confuse people further?

Well, I guess you have missed the 20 or 30 posts I've made in the last 2-3 years
on this matter. Never once did I ever say it was ok for the antenna spec's to be
confusing! I've been trying to get them (antenna manufacturers) to fix the
problem, because that is where the problem lies!
I think it is stupid, misleading and unnecessary, when the right information is
just as easy to obtain.
Clear enough?


Please explain your new method of dealing with the problem, exactly how would I
take my 1" diameter x 72" long piece of tubing and arrive at its "Flat Plate
Equivalent Area."
Then I can think about what I'd do with that number.

Thanks!


--
73, Kurt, K7NV



--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com