[TowerTalk] Where did 0.6 come from?

[Jim Lux]

On 2/10/13 8:30 AM, SPWoo wrote:
> I'm not an engineer by profession; so I'm not saying that this is correct.  In various software programs and Excel spreadsheets I obtained from ham-related sources use a Cd of 1.2 for round members and 2.0 for flat members.  A specific example of this is the AntennaMast Calculator that came with the ARRL Antenna Handbook.
>
>

And Kurt's page gives similar numbers..

What's odd is that Cd of 1.2 for transverse flow over a cylinder is for 
a VERY low Reynolds number (like 400  =>  0.5" cylinder at 1 mi/hr) and 
laminar flow.

Of course, Cd is lower than 1.2 for anything with higher Re, so maybe 
it's a safe assumption.  But it also doesn't mean that the drag force on 
a 2" round pipe is 0.6 of the drag force on a 2" flat strap.

The curves of Cd vs Re are very different for a cylinder and a flat 
plate and a square member.   So what the "standard" does, probably, is 
take a worst case.. Cd is never bigger than 1.2 for a cylinder.  Cd is 
never bigger than 2.0 for a flat plate.  Both might be less, but then, 
the standard overestimates the loads, and the resulting structure will 
be safe.

I think one thing the Kurt's page makes clear is that the tower 
standards and the "wind ratings" that derive from them are not 
necessarily based on actual aerodynamic analysis.  They're a standard 
where you plug in the parameters to see if your proposed design meets 
the standard.

For instance, it doesn't really matter what the wind speed actually is: 
you use the "wind zone" (older versions) or the "reference wind speed by 
county" (newer versions).  What matters is that when you calculate the 
"wind load" by  multiply the frontal area of your tower by the 
pounds/square foot number for your area.

That is, the standard gives a common and uniform way to determine what 
the design loads for your structure should be (whether or not those 
happen to be the real loads.. hopefully, the design load is always 
GREATER than the actual loads you expect to see)

   After that it's straightforward structural analysis.