[TowerTalk] Torque balancing

[Red <RedHaines@centurytel.net>]

Hello to John, Tod, and all TTs:

I am very interested in obtaining copies of the articles mentioned by 
John and Tod.  I hope I am not too old to learn from them and thus 
correct the errors of my thinking.

Do you agree with the following simple experiment to demonstrate the 
correctness of your ad absurdum example?  If so, let us see if the 
combination of peer review and multiple independent confirmations by the 
testing will support or reject the theory.

Build a model of a boom,  simply a tube of handily available material 
and dimensions, pivoted at its geometric center.  I don't believe it is 
necessary to achieve a 40 ft by 3 inch diameter simulation; use what you 
have handy.  Mount it where it is exposed to wind and observe that it 
has little or no tendency to weather vane.

Add a simulated antenna element to one end of the simulated boom.  Mount 
it, pivoted at the center of the boom as before, and observe its 
behavior in wind.

If the results are unclear, continue by putting two experiments in the 
wind at the same time: one simulating a balanced boom with no element 
and one simulating the same boom but with a simulated element at one end.

If the result remains ambiguous, add a weather vane or wind sock in the 
same wind to clearly indicate the wind direction and a subjective 
measure of gustiness.  Add a torque measuring element to the simulated 
booms for a quantitative assessment, with due regard for the 
inaccuracies, of the torques.  That might be base on weights, springs, 
or other principles that you may prefer and find convenient to assemble.

I welcome the improvement of this proposal by others and I particularly 
welcome seeing reports of those who complete an experiment.

As a picture is to deductive discourse, so are experimental results 
proportional to analyses. 

I look forward to seeing the referenced documents by Mr. Weber as well 
as the results of measurements by all who attempt them.

73 de WOØW

W0UN -- John Brosnahan wrote:

>  My apologies to anyone who
> is getting bored with all of this.  But it IS an important topic since
> the analysis has been done incorrectly by the ham antenna
> manufacturers for so many years.  I hope my word picture--
> without any mathematics (or drawings!)--makes it all a bit easier
> to follow.
>
> --John
>
> Hi, Jim--
>
> I am not sure if we are in agreement or not, but let me show you
> the ad absurdum case to make sure we are both talking apples.
>
> Take a 40 ft boom (arbitrary) of 3 inch tubing and attach it to
> the mast in the exact center.  Now attach, ala the old Telrex
> method, a single element (oh, say a 20M reflector) at one END
> of that boom.  (Telrex style element used to greatly reduce any quibble
> about mounting brackets, since Telrex ran their elements through
> the boom.)
>
> OK, so far?  Only ONE element and it is at the very end of the
> boom, 20 ft from the mast.  It LOOKS like a classic weather vane
> doesn't it, because of total asymmetry.  BUT it needs NO
> torque balancing!
>
> Wind hitting one half of the element (no matter the angle) will produce
> a vector force that can be defined as the sum of two vectors -- one 
> perpendicular
> to the element and one parallel to the element.  The parallel vector
> has NO effect on the element--wind has no "stiction".  The perpendicular
> vector on one half of the element gets transferred as a force trying to
> rotate the element in a counterclockwise direction.  BUT the same wind
> on the other half of the element also gets broken down into the same two
> vectors and the parallel vector once again can be ignored.  The 
> perpendicular
> vector also gets transferred to the boom, but this time in a clockwise
> direction.
>
> Or just take your hand and push on one side of the element and see how
> the force tries to push on the boom.  Doing the same thing on the other
> side with your hand will also push on the boom but in the opposite
> direction.  The two forces balance.
>
> In reality it is MUCH more complex but it is not what has typically been
> modeled.  For instance, take a 45 degree wind to the element.  The upwind
> side will WANT to bend in such a way as to become MORE broadside to
> the wind.  And the downwind side of the element will want to bend in such
> a way as to become less broadside to the wind -- just due to the fact 
> that
> they are attached on opposite ends.  One half is attached to the boom on
> the downwind end and one half is attached to the boom on the upwind
> end.
>
> But this effect is zero for a completely straight (stiff) element, and 
> only plays
> a role when the elements are bending a lot on the wind.  And if they bend
> more in the wind it is probably because they are thinner -- and have 
> less wind
> area to start with.
>
> So, to first order, assuming the elements stay straight under the 
> force of the
> wind, then there is NO need to compensate for any elements -- even if 
> there
> is only ONE element at one end of the center-mounted boom.
>
> Element flex adds a small extra thing to think about as does any element
> projection that has any surface area parallel to the boom, such as the 
> end
> cap on traps.  The bodies of the two traps in an element cancel each 
> other
> out since they are parallel to the element.  But both upwind ends of any
> set of trap end caps will add to the load on the boom.   But once 
> again this
> is a pretty small area and the resulting forces are very low.
>
> So, to first order at least, you can ignore the elements and only 
> compensate
> the boom if it is off center mounted to the mast.  Or better still, 
> mount the boom
> in the center and add some weight to one half to balance the boom.
>
> OK?    ;-)
>
> 73--John

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