[TowerTalk] Wind loading comment misdirected? W7VP

[Jim Lux]

At 02:44 PM 3/26/2006, NPAlex at aol.com wrote:

>===========================================
>I don't think I said a thing that contridicted your comments and Rohn's
>requirements.  I believe it is IO (intuitively Obvious) that a Flat surface
>drag/profile factor would be larger then a round surface.
>============================

While intuitively obvious, it's also wrong.

Oddly, there are lots of situations where the Cd of a round member is >1.0 
(i.e. the drag area is larger than the projected flat plate area)..By the 
way, a flat plate usually has a higher Cd than a long body with a flat 
front of the same cross sectional area; think about the flow behind the 
plate. It all depends on the Reynolds number and all those things about 
laminar and turbulent flow.

You can play with the numbers at:
http://www.fluidmech.net/jscalc/cdre01.htm

Try putting in different diameters of the cylinder like 0.1, 0.01, and 
0.001 ft
with airflows of 10, 50, 100 ft/sec (88 ft/sec = 60 mi/hr)

0.1 ft = 1.2 inches..

The classic area where this comes up is in biplanes, where the bracing 
wires, although small in cross section, have a drag area that's larger than 
that of a substantially larger streamlined strut.