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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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