The articles being discussed (Aerodynamic Balancing Parts 1 and 2 and
Cross-flow Principle) are worth the trouble to obtain. I learned quite a
bit from them when I read them in 2001. They set my thinking straight
regarding air flow over tubular antenna elements, and the resultant effect
on wind loading of towers.
I got to thinking about developing a demonstration of the principle of
cross-flow. The easiest to do (and easy is relative) is simply to fasten a
few short, narrow streamers (described further on) to a Yagi's boom and
elements. Then observe how they flutter behind the aluminum when exposed to
wind. The streamers I'm thinking of are the kind that kids attach to their
tricycles' handles, or the tape that has the word CAUTION printed on it.
You might even use streamer material that some model rockets use in lieu of
parachutes, available from hobby sources. The lighter the material, the
lower the wind speed needed to see the effect.
If I understand the cross-flow principle correctly, you will see the
streamer ALWAYS flutter a right angles to the boom or element, irrespective
of the wind direction (as long as the streamer is short enough to stay in
the air slipstream behind the element and not dangle several feet below it.
Kind of like a tell-tail on a boat sail for you mariners out there.) The
net amount of wind force can be seen in how horizontal the streamer is; the
closer to horizontal, the greater the effective wind force on the streamer.
Gene Smar AD3F
P.S. The articles also got me thinking that I ought not point my mast
northward and attach all Yagis from the same side of the tower. Attaching
all booms to the east side of the mast for example will increase the torque
on the rotator and tower unless the Yagis have each been designed to produce
zero torque, or unless you have designed all your Yagis, etc to work
together to produce a net zero torque or close to it. A minor consideration
for Small Guns but not so for long-boom stacked monobanders, perhaps.
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.
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