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

[Jim Jarvis]

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 at ieee.org