[TowerTalk] Wind Surface Area estimates

[Jim Lux]

On 3/9/12 6:50 AM, Gary Slagel wrote:
> Hi guys... more questions!
>
> I'm thinking looking at putting an A4S and and some kind of 40M yagi on the same mast.  I plan to rotate the 40M yagi 90 deg from the A4S to minimize interaction.  I'm thinking somewhere in the future I may even add another A4S somewhere down the tower a ways.
>
> The A4S has an advertised windload of 5.5 sq ft.  The 40M yagis around the same 5.5 sq ft.  But, I'm thinking the 40M yagi is 5.5 sq ft looking at it from the front.  Looking at the side of the 40M yagi (2" boom and 22.6 ft) I'd guess more like 2.5 sq ft.  Same with the A4S from the side.... a 2" 18' boom should be around 2 sq ft.  So... total windload with the yagi's arranged 90 deg from each other should be 5.5 + 2 = 7.5 sq ft from one side and 5.5 + 2.5 = 8 sq ft from the other side.
>

A tube looking end on has a drag much more than the cross sectional area 
of the tube.

Also consider what happens when the wind is at a 45 degree angle..

But.. let's for a minute assume that the drag end on is zero, and cross 
ways to the tube it's 1. And when the wind comes at 45 degrees, the drag 
is 0.707 (the projected cross sectional area)

You have two tubes at right angles, one (Tube A) is oriented N-S and the 
other (Tube B) is E-W.

When the wind is from the north,  Tube A has no drag and Tube B has a 
drag of 1, for a total load of 1.

When the wind is from the east, Tube A has a drag of 1 and Tube B has a 
drag of 0, for a total load of 1

But, when the wind is from the north east, tube A has a drag of 0.7 and 
tube B has a drag of 0.7, for a total drag of 1.4.


So, your calculation is off by a bit (without looking at the end on drag 
of a tube).


(I don't recall Cd for flow along a tube.. let's say it's Cd=0.01, since 
this is skin friction, the area is the wetted surface area 
(diameter*pi*length).  A 2" diameter tube that is 20 feet long will be 
about 10 square feet.  That works out to 0.1 square feet "equivalent 
flat plate area".  however, the Cd could easily be 0.05 or 0.1 so you 
could be as big as 1 square foot)

I have a handbook that gives Cd for tubes at various angles and Reynolds 
numbers, but I can't seem to lay my hands on it right now)


There's also the practical detail that a lot of the drag will come from 
"interference drag" where the crossing members are.

Maybe there's some charts showing the drag polar diagram of an antenna 
out there. (the Yagi design book might have some?)