Hi Guys,
Ok, here's the main difference between what you are thinking and what K5IU
pointed out is really happening. The resultant force on a cylinder inclined
to the wind is ALWAYS normal to the axis of the cylinder, NOT ALWAYS in
line with the wind.
Go try this, it's fun. Grab a 2'-3' long piece of tubing, jump in the car
and go someplace where nobody is watching, we don't want any of the good
folks around here placed under observation.
Hang the tube out the window with it in a vertical position. The wind
pressure on the tube will push your hand straight back. Normal to it's
major axis.
Now, incline the tube so the top end of the tube is forward and the bottom
end is back.
The wind on the tube will push your hand up and back, normal to whatever
angle you are holding the tube at. If it worked the way you are thinking it
would still be pushing your hand straight back.
The component of the resultant force that generates the load on the tower
is the one that goes down wind. That is Area x Pressure x SIN^3(Theta),
where Theta is the angle between the wind and the major axis of the tube.
The Resultant force, normal to the tube is AxPxSIN^2(Theta). Anyway, the
diagrams, formulas, and charts in the Antenna Book are easier to follow
than this.
If you really want to have fun, make a grossly imbalanced antenna and put
it up and watch it do things that are contrary to what you are expecting.
I made a 2" dia x 4' long boom, with one 3' long 1" diameter element
mounted at one end. I mounted the antenna on a 20' mast so that it was free
to rotate about a bolt right thru the center of the boom (and center of
boom tube).
Now, the old way of thinking would expect the end with the element to
immediately, and always go down wind and come to rest boom aligned to the
wind, just life a good old wind vane.
I left it up for a couple of months and drank lots of beer. Sometines it
came to rest with the element aligned with the wind, some times boom
aligned with the wind, sometimes with the element at the upwind end.
It would always find a stable position at 0, 90, 180, or 270 degrees to the
wind. What caused it to change positions was not wind loading, but
instability of the mast which would cause the mass imbalance to push it off
a stable place and on to the next.
This behavior is consistent with the resultant forces on the cylinders
being normal to the cylinder axes.
Now, if we chart the down wind components of a real antenna, we see two
peaks, one at 0 degrees, and another at 90 degrees. Whichever area (boom or
elements) is greatest is the azimuth of max load on the tower. At some
angle, defined by the relationship of the two areas, will be found the
azimuth of minimum load.
Yes, what I'm suggesting is backwards to what you thought. It's not my
fault, Father Physics & Mother Nature and people way smarter than I figured
it out. I'm just a messenger.
73, Kurt
"Guy Olinger, K2AV" wrote:
>
> With both dipoles in the wind, equally sharing, the VISIBLE cross-section
> offered the wind is at 1.4. HOWEVER, note that the wind does NOT flow across
> the elements at right angles, but at a 45 degree angle. If you draw a 45
> degree section of a cylinder, it is NOT a circle, but an elongated ellipsoid,
> which has substantially less drag. Also since the element is at 45 degrees to
> the wind flow there is a shedding effect which allows wind flow behind the
> element in the direction toward the downwind tip. This further reduces the
> pull on the element since there is less of a vacuum behind the element than
> with the same flow cross section at right angles to the wind.
>
> IN ADDITION, it is known that for a given wind velocity, there is an
> elongation of the circular shape at which the turbulence behind the element
> disappears and the flow becomes smooth. At this point there would be an
> almost light-switch reduction in drag. This may or may not come into play at
> wind velocities we would be concerned with.
>
> The assumption you seem to be holding is that drag is in linear relationship
> to the visible cross section presented to the wind.
>
> 73
>
> >
> > From: "EUGENE SMAR" <SPELUNK.SUENO@prodigy.net>
To: <towertalk@contesting.com>
> > Date: 2001/05/21 Mon AM 11:52:10 EDT
> > To: "Kurt Andress" <K7NV@contesting.com>, <towertalk@contesting.com>
> > Subject: Re: [TowerTalk] Angle of maximum wind force on Yagis
> >
> > Hi, Kurt:
> >
> > If what you're asserting here is accurate (which I haven't satisfied
> > myself that it is yet), then what you're saying is that my calculations show
> > exactly the OPPOSITE of what I think they show. This is a serious
> > discrepancy.
> >
> > I don't have the 19th Antenna Book (the 18th is my latest), but there
> > are a couple of local Hamfests coming up. I'll pick up a copy and read
> > through the section you cited.
> >
> > In the meantime, how about this? Take a hypothetical crossed dipole or
> > turnstile antenna, made with the same amount of aluminum on either element.
> > Say it's large enough to worry about wind loading. Orient it so the two
> > dipoles are NS and EW.
> >
> > If the wind is from the east or west, you get some force, F, applied to
> > the mast. The force is proportional to exposed area, L X W, of the element.
> > If the wind is from the north or south, you get the same force, F, applied
> > to the mast 90 degrees from the first example. I'm ignoring the end-on wind
> > force. Also, I'm not using shape factor as it cancels in my analysis,
> > below.
> >
> > Now let's say the wind is from the northeast. Both dipole elements
> > experience the same force, whatever it is, right? The force on one dipole
> > is proportional to effective area exposed to the wind. In my equation, that
> > area is L X W X sin theta or, for 45 degree angle, L X W X 0.707. Total
> > force at 45 degrees is twice this (two dipoles) or L X W X 1.41. This force
> > is 1.41 times the force experienced by the single dipole broadside to the
> > wind (merely L X W). I further assert that this is the MAXIMUM force that
> > the crossed-dipole array would experience.
> >
> > If there is an error in my equation (and I'm not convinced there is
> > yet), then it's in the cos theta term. I'll let you'all know.
> >
> >
> > 73 de
> > Gene Smar AD3F
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