[TowerTalk] wind load vs Rohn specs

[Bill Coleman AA4LR]

On 9/12/00 9:38 AM, Tower2sell@aol.com at Tower2sell@aol.com wrote:

>The answer is -- Use The Projected Area in the direction of the wind. If 
>the wind is blowing horizontal - use the horizontal projection. You may 
>have to rotate the antenna to find the maximum horizontal projected area.

You see, this confuses me.

Consider. We have a two element antenna on a 10 foot boom. Each element 
is 50 feet of 1 inch tubing.

I'll conceed that the drag broadside to the array is going to be less 
than the drag head on, so we'll deal with the drag head on.

>From your note, you imply that the drag of the antenna is equivalent to 
the drag area of a single element -- since the second element is 
completely hidden in our horizontal projection.

I don't see how this can be right. Naturally, the downwind element will 
not have as great a drag as the upwind one, since it is in disturbed air. 
But it won't have zero drag, either! And its drag will vary depending on 
a number of factors, including it's spacing from the first element.

Further, if we contemplate antennas having more elements, and longer boom 
lengths (less than 50 or so feet), their horizontal projections will 
remain the same, but the drag will continue to increase, right up to near 
the point where the antenna becomes a solid raft of tubes from one end of 
the boom to another, at which point we'll see a dramatic DECREASE in drag.

I just can't understand how a horizontal projection predicts the drag of 
the antenna to any degree. It can't pass the thought-problem test at all.



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


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