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

[Michael Tope]

Jim,

With regard to your comments below, are you assuming laminar or 
turbulent flow? I just grabbed my copy of Leeson's "Physical Design of 
Yagi Antennas" and he discusses this same issue of a rapid change in 
drag coefficient for wind speeds and tubing diameters of practical 
interest to antenna builders for the case of turbulent flow. He then 
states "conservative design, however, dictates a less aggressive 
choice", referring to the choice between assuming turbulent flow or 
laminar flow when doing these sorts of design calculations (for laminar 
flow this transition from ~constant drag coefficient to rapidly changing 
drag coefficient occurs at much higher wind speeds). UBC and EIA-222 (at 
least the versions that were current when his book was published) both 
appear to assume laminar flow.

Leeson presents calculations from both UBC and EIA-222 formulas both of 
which show an ~0.6 ratio between cylindrical member and flat-plate 
member drag coefficients.

73, Mike W4EF........................

On 2/9/2013 6:28 AM, Jim Lux wrote:

> 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.
>