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[TowerTalk] Wind Loading redux

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Subject: [TowerTalk] Wind Loading redux
Date: Wed, 6 Jun 2001 23:19:23 -0400

     I won't feel offended if you hit "delete" at this point.

     Following up on my recent postings regarding wind loads presented by
Yagi antennas, I now know that I made the same error that numerous hams and
ham publications have been making for a couple of decades.  That is, I
assumed (logically) that the wind force on a Yagi tubular element was in
line with the wind itself, and that one compensated for the angle of wind on
the element (angle of attack) by using trigonometry  to reduce the effective
area presented to the oncoming wind stream.

     However, I have since read three of Dick K5IU's various papers
(available through ARRL's QEX/Communications Quarterly) in which he cites
several references on the subject of wind flow over tubular structures.  One
author, Hoerner, explained that the net force of wind blowing across a tube
is normal to the tube, i.e., at right angles to the axis of the tube.  This
is NOT logical but I'll accept it as true.  As I wrote to Dick, this is the
"and then a miracle occurs" part of the explanation.  I'd need to find and
read Hoerner's book to see a presentation of the physics involved here.

     Once you accept the principle of wind force as described (normal to the
tube's axis), then all the trig calculations are easy to understand.  The
net result is that, indeed, the maximum wind force on a Yagi is either
normal to the boom or to the elements, depending on which represents a
greater area, and NOT at some intermediate angle as I had calculated.

     Another point:  The nonsense of multiplying the area of a Yagi element
by 0.67 or 0.6 or anything less than unity (1) to account for its shape is
WRONG!  Dick's article saved me from making that mistake in my tower
calculations.  The correct number (which Dick calls drag coefficient) is 1.2
for long, thin tubes used in Yagi elements.  The drag coefficient for a flat
plate is 2.  Divide these two coefficients and you get 0.6, which might
explain why some folks used this number as a shape factor.

     Another point:  Armed with this new knowledge, I set about to calculate
the wind area of my still-in-the-box Bencher Skyhawk.  I took out the
assembly manual and calculated the exposed area (L X W, no shape factor) for
the boom and each tube of each element.  Guess what?  I calculated nearly
50% greater area FOR THE ELEMENTS ALONE than Bencher advertises for the
entire Antenna.  My figures show 12.2 sqft of element area and 4 sqft of
boom, whereas Bencher specs the Skyhawk at 8.5 sqft.

     I sent Bencher an e-mail last week asking them to verify my figures.
No reply as of this evening.

     A final point:  If you blindly point your mast to due north (please
don't start!) and attach all your long-boom Yagis from the same side of the
tower, you're unbalancing the torque on your tower system.  Dick wrote a
couple of papers on aerodynamic balancing of antennas to minimize tower
torque.  I'm now going to mount my four antennas (Skyhawk, 2M Yagi (23 foot
boom), D40 rotatable dipole, and triband V/UHF vertical) on the mast on
alternating sides, depending on how the math shakes out.  No point in
deliberately putting unnecessary stress in your tower system, if it can be
avoided simply.

73 de
Gene Smar  AD3F
P.S.  I'd advise you actually to calculate the area of your antennas, and
NOT rely on manufacturers' specs (Unless you know how they came up with the
numbers.  And I apologize for starting another re-thread.)  If you're
contemplating the purchase of an antenna, maybe the manufacturer will
sell/give you a manual for this purpose.  (But then again, maybe not.)

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