>> So the drag of a 4" tube is 1/2 that of the same length 1" tube, not 4x.
The drag coefficient may be 1/2, but when multiplied by 400% more face area
is the drag not 2x?
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Lux
Sent: Saturday, February 09, 2013 8:29 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Thrust Bearing, etc: more answers from UST calcs
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.
>
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