[TowerTalk] Tower Questions

[Kurt Andress]

Bill Coleman AA4LR wrote:

Hi Bill,
Guess you missed the previous discussions about this stuff.

> 
> Shouldn't it be simple? What units does Rohn rate its towers in? I
> thought it was flat plate equivalent. That's very different from Flat
> Projected area.
> 

The Rohn cataloge designs are asking for flat projected area, They
provide two allowable area figures one for things made out of tubing,
the other for things made with flat or rectangular sections.

> 
> Isn't there a difference in flat projected area for a round member versus
> a square one? How does one convert from flat projected area to flat plate
> equivalent?
>

The generally accepted drag coefficient for flat plates is 2.0. The
value for long cylinders is 1.2.
Some design specs use these values some do not, so how one converts from
one to the other depends on which method is being followed.
 

> I know enough about aerodynamics to know that the shape of the element
> has a lot to do with its drag qualities. Round elements are moderately
> draggy, but oval shapes would be less so. (the bluff front edge isn't the
> problem - all "aerodynamic" car designs to the contrary -- it's the
> untapered trailing edge that causes all the drag) And movement of the
> element can change its drag profile - an element that can vibrate back
> and forth in a flutter will have a lot less drag than one that stays
> rigidly still.
> 
> So, you can't just compute wind drag by pretending the elements are
> perfectly flat blockers of wind.

You are correct here. The drag coefficients are accounted for in the
tower design process, since the different spec's that guide the process
are different, we need to leave this part to them. 
If we introduce a drag coefficient adjustment into the antenna area
before it goes thru the tower design process, we get erroneous results.

Here's an example:

The "Antennas R Us" antenna company specifies an "effective" area based
on the old EIA-222-C spec which uses a drag coefficient of .67 for
cylinders.

So, we take that and design a tower using EIA-222-F which uses the 1.2
drag coefficient. So we get a tower load based on a net drag coefficient
of .80 (1.2 x .67).

But the 222-F spec requires us to use a 1.2 drag coefficient. What we
get by using the "effective" antenna area is a tower load that is 2/3 of
what the tower spec would have used. So, we think all is fine but the
actual tower load is 1.5 times higher than we think.

There are other ways to mess this antenna area deal up, but that's how
this part of it should work. 
We should just have all antenna areas as flat projected values and then
let whichever design spec is used make the drag adjustment and figure
out the tower loads. 

-- 
73, Kurt, K7NV

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