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.
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