[TowerTalk] wind load vs Rohn specs

[Bill Coleman AA4LR]

On 9/12/00 2:58 PM, Pete Smith at n4zr@contesting.com wrote:

>You'd have to get quantitative to definitively answer this, but I have a
>hunch that for less than 100 mph windspeed the area rather than the drag
>coefficient is decisive in determining the amount of drag (isn't there a
>V^2 in the equation someplace?)

You're correct that the effects of parasitic drag are proportional to the 
square of the airspeed. Parasitic drag has mostly to do with the surface 
friction of the air over the exposed body of the object.

The tricky part, however, is the turbulent airflow over round antenna 
elements and booms. Turbulent fluid flows are quite non-linear and 
difficult to predict. Some turbulent flows result in more drag, others 
result in less.

Turbulent flows exist at most slow wind speeds, too, although the affects 
are less important. Since we design antennas and structures for wind 
speeds of 70 mph and up, the drag from both parasitic and turbulent 
sources is significant.

--

I'm still trying to comprehend how a 40m rotatable dipole can have the 
same drag as a 3 element 40m beam. After all, they have the same 
projected area, right?



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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