[TowerTalk] Thrust Bearing, etc: more answers from UST calcs

[Jim Lux]

On 2/8/13 7:54 PM, SPWoo wrote:
> Regarding failure points: I've seen photos of two fallen US Tower
> retractable towers and both of them failed at the very bottom.  The
> problem with designing a retractable tower such that it will fail at
> the top is that of cost.  The bottom section would have to be super
> massive and I wonder if the tower will be price competitive.
>
> Regarding tower wind loading: US Tower specifies wind loading in
> terms of round members although they don't always make that clear.

And I'm not sure that specifying "round members" is valid. Members in 
the 1-4" range at 70mi/hr are in a flow regime where the Cd changes 
rapidly with the Reynolds number.  A 1" tube at 70 mi hr has Re=50k, 4" 
is 200k.  The corresponding Cd are 1.01 and 0.54...

So the drag of a 4" tube is 1/2 that of the same length 1" tube, not 4x. 
  I guess that makes it "safe".. bigger tubes have less drag than small 
tubes on a cross sectional area basis.

But the fact that it changes seems a bit tricky, especially because what 
they are really doing is giving you a load (in pounds) translated back 
into some assumed projected area.


> On my documents they didn't make it clear.  OTOH antenna
> manufacturers like to expresstheir wind loading numbers in terms of
> flat members.  Therefore you need to divide the antenna wind loading
> number by 0.6 and compare that number to the published tower wind
> loading figure.

Where's the 0.6 come from?   For 1-2" tubing at typical wind speeds, Cd 
for a cylinder and a flat plate are about the same.

  Also, the tower wind loading includes all
> accessories such as mast, rotor, coax cable, etc.  A mast of 2" OD
> and 15' length has a wind loading of 2.5 sq. ft.  This is often
> overlooked by hams.
>