[TowerTalk] HF beam against the wind.

Roger L. Elowitz K2JAS@worldnet.att.net
Mon, 10 Nov 1997 16:21:16 -0500

Hi Steve, K1PEK,  and the multitudes,

Congrats on your first input to TowerTalk.  The "rush" one feels when
one hits the "send" button knowing that thousands all around the world
are reading [and judging] your every word... is a thrill indeed... at
least it is for me. Glad you enjoy the experience too.

It seems that this subject of where to park a beam in a blow is easily
becoming confused so I'll try once more to clarify by restating my

Is it preferable to turn or park one's beam into the wind [elements
taking the brunt of the blow] or 90 to it [boom taking the brunt of
the blow]?

We agree that the maximum wind load is calculated with the elements
taking the brunt of the blow and it is that number that will determine
the proper selection of tower, it's height and guying (or no guying)
and rotor to be used as well as the wind area of the country and
maximum likely winds to be experienced. [I'm sure I've probably left
out a few other things too].

Now assuming everything is properly designed and selected.... my second

Wouldn't it be prudent to turn the beam perpendicular to the prevailing
wind to minimize the stress on both the beam and the tower... in the
event that the blow should reach a level that would exceed the maximum
specs... and that by turning the beam thusly... more surely improve the
survivability of all parts of the system?
At 07:47 AM 11/10/97 +0000, Steve, K1PEK wrote:
>Hi Roger, as my first input to TT, I hope this answer doesn't sound
short or
>presumptuous.  Do beams have more than one wind  load specification?
>however the industry assumes that the max. load spec is what is most
>important, which I agree with.  Yes, you're right that the spec is
>to be the one measured with elements broadside to wind direction.  

Yes Steve, we are in full agreement on all the above points.

>Yes, there's an empirical answer to this question which , in part, is
based on one
>of Einstein's theories correlated with vector analysis and basic wind

I take it you are kidding about Einstein who worked in areas of physics
Newton rarely dreamed of. Newtonian physics coupled with vector
analysis are all that are needed here.... methinks.

>Rather than trying to convey formula symbols that may not
>resolve through E mail, suffice it to say that when one wades into the
>surf of the ocean, many of us turn sideways to the wave force, unless
we are
>wearing hi impact polymer groin protection.   Pictures, images in our
>and excrutiating pain often substitute for scientific equaltions.
>Regards,  Steve, K1PEK

I love your bather-in-the-surf analogy!  It's hard to argue with and
even harder to test first-hand!

Some other posters have mentioned the "weather-cocking" tendency of
"freewheeling" beams as evidence that a weather-cocked beam experiences
the "least wind resistance" and therefore the least wind load.  That
concept... I fear, is in grave error... simply because it is in this
position that the beam is measured to derive it's MAXIMUM WIND LOAD

And therein lies the confusion!  

Because the beam weathercocks I believe, it is NOT turning into a
position of it's minimum wind loading resistance... but rather, it's
minimum ROTATING moment of resistance.  [There are probably better
words to explain this phenomena.] This is especially so considering a
long boom beam with asymmetrically mounted elements... ie. more
elements on one side of the mast to boom bracket than on the other.

Consequently, the beam streamlines into the wind by balancing the
rotational forces. This result most likely puts the least pressure on
the rotor braking system- granted. That is likely a VERY IMPORTANT
POINT... that should not be forgotten.  However, if one assumes that
the brake is large [strong] enough to take the maximum rotating force
of a beam parked crosswise to the wind... then wouldn't it make more
sense to do so given the reduced wind load on the tower?

[At this point in my reasoning I think I have not properly taken into
consideration the effect of the destructive twisting forces that would
be transferred to the tower structure assuming the rotor brake does an
adequate job.] 

Other posters have suggested that purists extend their booms on the
appropriate side or add compensating wind catching plates that would
tend to balance the beam in a crosswise direction to the
wind....[equalizing the tendency to rotate] while at the same time
having it expose itself with the LEAST wind load.

This sounds like an idea that not only validates my thinking but also
seems to make a great deal of cautionary sense. 

Well, that's my best thinking on the subject... and I have little doubt
more can certainly be said. All replies are of course welcome...both on
and off-line.

With best 73 to all,

Roger, K2JAS
Morganville, NJ
(...in all seriousness folks!)


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