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## [TowerTalk] Yagi windload, yagi torque balancing,and why airplanes fly

 To: [TowerTalk] Yagi windload, yagi torque balancing,and why airplanes fly "Jim Jarvis" jimjarvis@ieee.org Sat, 15 Jan 2005 15:39:04 -0500
 ```With thanks to Dick Weber, who was kind enough to forward me copies of his paper on yagi windloading (communications quarterly, Spring 1993). Brosnahan is right. I stand corrected. Those of you who fly may recognize the "cross-wind force" as lift. The windload on a yagi does NOT peak at 45 degrees, as I had assumed, but rather DIPS. The reason for this is that the SIGN of the crosswind components of windload from the elements and boom are OPPOSITE. That is, they subtract. Once you draw the vectors, it's obvious. They ARE 90 degrees from each other, after all! Two key points from the paper: * Every US antenna manufacturer (at the time of Weber's writing) was incorrectly calculating the windload, and overstating it substantially. * Commonly accepted figures for wind force vs. windspeed included fudge factors, resulting in them being overstated. If you're in need of windload calculations...I commend Weber's paper to your attention. It partly answers why people get away with "overloading" towers... by using excessively conservative, incorrect, ratings to begin with. It also suggests that a freewheeling yagi should tend to turn itself to a position which minimizes force on the tower...at an angle determined by the relative areas of the boom and elements. That is...wind induced torque is not intuitive. As for my KT34XA torque balancing approach...it worked empirically. If I were doing it again, I think I'd try suggest using a counterweight on the director end, to bring the center of mass to the same point as the geometric center of the antenna. THEN to apply a trimtab to balance out the windload differences. Weber's paper is a must-read, if you're serious about any of this. n2ea jimjarvis@ieee.org _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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