[TowerTalk] wind load vs Rohn specs

[Jeff Stai WK6I]

If your goal is to streamline the boom, you can make it symmetrical - sort
of an "eye" shape, kinda of like flattening the tube and pinching it into
a symmetrical (non-lifting!) airfoil shape.

The drag from a round tube occurs when the flow separates from the surface
as the air passes over perpendicular to the tube. I recall seeing techniques
where the flow separation occurs "later", by slightly disturbing the flow at the
leading edge. I imagine someone on this list more directly involved with
such things can elaborate further... it has been way too long since I studied
such things. If I recall correctly, the idea is to nudge the flow away from
the leading edge, changing its direction and causing it to wrap around the
trailing edge - the net improvement at the trailing edge is worth the loss at
the leading edge.

It could be as simple as running a narrow piece of tape over the top and bottom 
of an element or boom to nudge the flow away.

But the net improvement in our case may not be enough to be worth the effort....?

73 - js WK6I

Ken Hirschberg wrote:
> 
> Mike - You're not kidding!  Look at the pictures of aircraft antennas.
> It works, and people have been doing it for years, mostly for VHF / UHF
> antennas and for the masts for HF antennas.  The nice thing about a
> plane, of course is that under normal circumstances, the wind direction
> is known.  That would be do-able to some extent for big HF beams with a
> wind sensor and automated rotator.
> 
> While were at it, I've seen no mention about interference drag at the
> point of connection between boom and elements.  That drag could be
> reduced either with lightweight fairings or with a boom to element
> attachment that was designed for low drag.
> 
> Streamlined tubing is available in both steel and aluminum, but there's
> just no market for telescoping sizes.  I have used small lengths of it
> for some of aircraft antennas designs that I have produced.  What is
> available is WAY too heavy for elements, but may be practical for booms,
> again if you can arrange to "park" the antenna with its boom
> perpendicular to the wind.
> 73, Ken
> 
> michael watts wrote:
> 
> > How about using streamlined elements?  Just like the
> > steamlining cowls that we put around wires and tires
> > on airplanes.  Anybody know how to make such a thing
> > is telescoping sizes?
> >
> > I'm not sure if I'm kidding or not.
> >
> > Mike wy6k

-- 
jeff stai
WK6I DM13
NAR #21059 TRA #3356 L2
jstai@home.com / wk6i@arrl.net
ROC web page: http://www.rocstock.org/

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