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.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|