Date: Wed, 13 Apr 2016 08:17:04 -0400
From: "Joe Subich, W4TV" <lists@subich.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] W6NL 40m Moxon
On 4/13/2016 7:54 AM, Jim Thomson wrote:
> ## if say the yagi pointing due east..and the wind is coming from the
> SW....heading to the NE, all the forces off all the els will be due
> east. All the forces off the boom will be to the due north.
With quartering winds (winds not perpendicular to either the boom or
mans), you have forces on the mast/tower to the north due to the boom
and forces on the mast/tower to the east due to the elements. The net
force on the tower/mast will be the *combination* of those forces to
the northeast (vector addition). That's no different that the vector
addition of forces on the tower due to tension in the guy wires.
I'm no mechanical or structural engineer but you simply can not treat
each load as if the others do not exist. At the very least there is a
superposition of forces - assuming that they are not dynamic and add in
some exponential manner.
73,
... Joe, W4TV
## This was posted to tower talk by kurt andress K7NV, back in aug 98.
New Methods:
In the Spring 1993 issue of Communications Quarterly, Dick Weber, K5IU,
(mech enginneer) published a paper describing wind flow over cylinders at
various wind attack angles.
The methods described resulted in very different values from what many of us
were getting. Leeson and myself independently made some test antennas and
separately
arrived at the conclusion that the Weber method was correct. I know that Roger
Cox at Hygain, and Tom Schiller at Force 12 also picked up on it. I have no
direct
knowledge about the others.
Leeson changed his spreadsheets, but couldn't change his book. I made the
changes for YS 2.0
Here are the changes that come out of the new method, it's termed "The Cross
Flow Principle" by Weber, or the "Sin^2 behavior of Cylinders in Yaw," by
Leeson:
The wind flow over the cylinders results only in loads that are perpendicular to
the axis of the cylinder. This means that all element loads result in forces
along the boom axis.
Asymmetric element placement along the boom DOES NOT result in a wind torque
imbalance. This makes the Leeson dummy element torque compensator unnecessary
and
ineffective.
The Max Projected Area of a Yagi is the largest value determined for the boom OR
the elements. If the boom area is larger than the total for the elements, the
boom area is the max area.
The MINIMUM is somewhere in between 0-90 deg azimuth. The min area angle is
determined by the ratio of the elements to boom area. If the boom and elements
areas are equal the MINIMUM area occurs at 45 deg.## ok, so a 45 deg angle
will not increase the effective wind load. on the contrary,it will result in
the min windload ( providing boom area + ele area are the same). This is all
old news.
http://lists.contesting.com/archives//html/Towertalk/1998-08/msg00499.htmlfor
the original complete posting by kurt. Jim VE7RF
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