FW: [amsat-bb] Wind Pressure: Mesh vs Solid

["Bill Greene VE7WFG" <bgreene@xxxxxxxxx>]

HI Dave,

I was wondering who those guys were driving down the freeway with the
dishes sticking out of the window of the car ! What a sight !

Just kidding Dave ....... Thanks for the science! I actually was
wondering about wind loading as I want to mount a small dish on the
cross boom with the Yaesu 5600B's.

Hope you and yours enjoy a most health and happy holiday !

Best of 73 de VE7WFG

Bill
Armstrong BC
DO00jk
Amsat 29777 
Amsat Area Coordinator 
www.billgreene.ca




-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org] On
Behalf Of Dave Guimont
Sent: December 19, 2003 8:54 PM
To: amsat-bb@AMSAT.Org
Subject: [amsat-bb] Wind Pressure: Mesh vs Solid


Some time back, on this reflector, a conversation developed with part of

the thrust saying that at higher wind speeds, forces on a mesh approach 
those of a solid dish.  Now, I am certain that is true at much higher 
speeds, but at over 100 mph that concern is of no consequence to us.

I very much doubted that statement, and for those interested I've done
some 
empirical testing....I am not an engineer, but 27 years of flying Navy 
fighters on both sides of Mach 1, I have a pretty good grounding in
laminar 
flow, burble, lift, drag, back side of the curve, flat plate area and a 
host of other factors that I have long since forgotten, as I retired 37 
years ago...

THE EQUIPMENT:  a 30" x 40" solid Primestar dish, 1' X 1' piece of solid

metal and a 1' x 1' piece of 1/4" galvanized hardware cloth, properly 
tethered to hook into a spring scale graduated in ounces and a 1.2 meter

dish built from a kit produced by TEKSHARP in San Jose, CA.  The dish
from 
TEKSHARP is in kit form and 1/4" galvanized hardware cloth is used to
cover 
the framework.
The kit is an outstanding example of good construction.  It is extremely

rugged, stainless hardware, precision cut parts, and goes together like
an 
Erector Set.  It is about 1/2 the weight of the Primestar and several db

better on sun noise comparisons.  I have nothing to do with the company 
other than being a VERY satisfied user....

THE TESTS:  Hanging out my car window, with my grandson driving, we did 
measurements on the mesh and the solid plate at speeds from 40 to 70
miles 
an hour.

TEST RESULTS: The following figures are based on a "wind tunnel speed"
of 
60 mph, the other speeds are a progression that one would expect at
these 
speeds.

At 60 mph the solid plate indicated a force of 7.2#.  The mesh square 
indicated a force of 3.2#.  The solid dish (flat plate area of ~12.7
sq') 
is ~ 91# total pressure.

The mesh covered dish (flat plate area of ~ 1 sq' for ribs and
supporting 
structure) represents ~7# of force and the remaining mesh area of 11.7
sq' 
with ~37# results in a total force of 38# on the mesh dish when
positioned 
into the wind on a vertical plane.

I elevated the solid dish to provide a minimum face to a wind from any 
direction.  My mesh dish will present about the same face to the wind at

maximum elevation, but the forces are so little more for a mesh dish in
ANY 
position I no longer have to be concerned about wind from any direction
or 
velocity....For those not aware, we rarely get 45 mph gusts, once every 
several years, but I've lived in about every area of the US, and these 
figures maybe of interest to residents in those other areas...






            73, Dave wb6llo@amsat.org
                    Disagree: I learn.... 

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