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Dish orientation and wind loading



Hi Greg,
                Wind load is calculated by multiplying the stagnation
presure by the coefficent of drag (and lift if the shape is likely to
produce lift) and by the crossectional aera presented to the wind.

    The stagnation pressure is the presure due to the unit kinetic energy of
the fluid flow. (A fluid is something that flows; it could be gas or liquid)

    I don't know what unit system you prefer but the calculations are much
more simple in the metric system so I will give you a metric example (you
can translate it by multiplying by the appropriate conversion factors -
otherwise ask me).

    By the way, for Sydney, NSW, Australia (it is probably typical of a lot
of places) the design wind speed (wind speed expected to occurr once in
every 100 years) is 47 metres per second (170 km/h or 105 MPH) so 50 MPH (23
m/s) could be a bit low.

The stagnation pressure is half the density of the fluid (air) times the
square of the velocity.
ie 0.5 x 1.2 kg/m3 x 23m/s x 23m/s = 317 Pa  (A Pascal is one Newtow per
square metre)

For the screened grill which is 30 inches x 36 inches that is 0.75 metres x
0.9 metres which is 0.675 m2. If this was a solid sheet normal to the wind
direction you would use a coefficent of drag of 1.0.

For the open screened grill which is basically a collection of "rods" as far
as the wind is concerned, then a coefficent of drag of 1.0 would be quite
adequate. The dish area used in the calculation would be the area which
resists the wind which is probably about two thirds of the area of a solid
plate.

For the screened grill in a 50 MPH wind:-

Aera        =        0.675 x 2/3 = 0.45 m2
Drag Co  =        1.0
Stagnation Presure    =    317 Pa

Wind Load                    =    0.45 x 1.0 x 317 = 143 N  (14.6Kg force or
32 lbs.)

There are complicated ways of reducing the design wind speed but if you use
the basic design wind speed your design will be safe.

However, wind is a dynamic load, not just a static load and so wind induced
vibration should also be avoided.

    The most hazardous mechanical resonant frequency to avoid is 0.3 Hz
(dynamic wind loads have most of there energy at this frequency - ie a gust
every 3 seconds) but if in doubt, is is better to have all mechanical
resonant frequencies above 10 Hz as there is usually almost no significant
component of wind loading above that frequency.

    The problem arrises particularly with metal structures because metal
structures behave very elastically (low energy loss - mechanically) and so
at resonance, they often have damping factors of 1% or less. The electrical
equivilent is a resonant circuit with a Q of 50 or more. (ie television IF
band pass filter would have that sort of Q) That means a cylclic force which
is applied to a structure of this sort at
the structure's resonant frequency will have 50 times the affect that the
same force would staticly!

    There is an old black and white film of a bridge in the USA (Tocoma
Narrows - or a name like that) which shown a bridge which was otherwise well
designed and constructed, oscillating and eventually
collapsing in a moderate breeze. This film graphically illustrates this
effect.

    As for direction to park the array; facing into the wind is probably the
best as it places lest moments on the structure which could damage your
rotators. The dynamic loading effects are pribably the ones which can cause
the most damage and so making sure there is nothing to "flap in the breeze"
is probably more important.

Regards,
Murray Peterson
VK2KGM

----- Original Message -----
From: "Greg D." <ko6th_greg@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, September 04, 2001 12:19 PM
Subject: [amsat-bb] Dish orientation and wind loading


> Hi folks,
>
> Ok, so Winter's coming, and believe it or not, there *are* seasons
> in California.  While I'm not worried about snow collecting in my
> canned helix, I am worried about the wind.
>
> The normal park position for my rotor is due South.  That's where
> the club's repeater is, and perhaps more importantly, that's where
> most storms come from around here.  When all I had were a couple of
> beams, this presented the minimal wind load.  Now I have this 30x36"
> screen covered grill up there to add to the wind problem.
>
> I believe that having the grill point straight up would present the
> minimum wind loading for it, but that would put the two beams at
> probably the worst orientation - high and broadside to any wind.
>
> I know that the window screen I used on the grill isn't as bad as
> putting a solid dish up there, but it's still going to be pretty bad.
> I read something years ago that mesh C-band dish was as good as solid
> as far as the wind was concerned.
>
> So, two questions:
>
> 1) how much force will a 30x36" screened grill put on the tower in,
> say, a 50 mph wind?
>
> 2) given that I have to pick a compromise direction to park the rotor,
> what is best?  The other antennas are a 8 element 2 meter beam
> (vertically polarized), and a 2x8 element 70cm beam.  You can see
> what I've got at:  http://www.jps.net/gregd/hamsite.htm
>
> I'm going to guess that the beams are going to contribute less than
> the dish, so parking the rotor either due east or west (putting the
> dish in profile, but the beams broadside) will be best.
>
> Comments?
>
> Thanks,
>
> Greg  KO6TH
>
>
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