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Re: Wind load calculations ?



Hi Jens,
    I don't have a URL with anything on it but I can tell you about wind
loading. (I am a Structural Engineer).

    There are genrally two types of forces which act on objects within a
fluid flow (gas or liquid):-
1) Drag which arises due to the differences in presure between the leading
surface and the trailing surface of the object. This is a force in the
direction of the stream flow.
2) Lift which arises due to the differences in pressure between surfaces
which are parallel to the average stream flow lines. This is a force which
acts at 90 degrees to the average stream flow. (ie the direction of stream
lines if there is no object in their path).
 For rods and other similar sections of which a yagi type antenna is made,
the lift forces are usually less significant than the drag forces.

Both these forces are proportional to the stagnation pressure of the flow
(the pressure a pieto tube type annamometer measures - they use these to
measure air speed in aeroplanes or water speed in boats etc.) when the fluid
flow arround the object dosen't  form compression waves (the fluid can move
out of the way - ie for air this means that the velocity of the flow with
respect to the object is sub-sonic - Note at super-sonic velocities
compression waves form where the air is compressed and due to adibatic
compression - the amount of heat energy in a mass of air remains constant
and the volume decreases the temperature rises - ie half the volume the
absolute temp doubles - air at 23 C or 296 K will rise to 592 K or 319 C -
objects entering the atmosphere are heated because of adiabatic compression
of the air - not "friction" as you hear so many people say)

The stagnation pressure is half the density of the fluid times the velocity
squared. For example, in Sydney the wind velocity used for design (100 year
return wind) is 47 m/s.

therefore:
    velocity    47 m/s
    density    1.2 kg/m3
    pressure   1.325 kPa

The actual force applied to the object in the wind is dependant on the shape
of the object and how much crossection it presents to the wind flow. The
effect of the shape and orientation of the object is quantified by
coefficents. There is a drag coefficent and a lift coefficent. For example
an aeroplane's wing is designed to have a low drag coefficent and a high
lift coefficent which acts in a vertically upwards direction.

    Antenna elements, on the other hand are not designed for their
aerodynamic properties and actually usually have a larger drag coefficent
and a smaller lift coefficent. The easy and conservative approach to this is
to assume a drag coefficent of 1.0 and that it can act in any direction as
wind and the turbulance it casuses can act in almost any direction.

    The only other thing to do is to calculate the crossection of the
element at 90 degrees to the wind. For example, the driven element in a 20 m
yagi is round, 10m long and 30 mm diameter. That means that the crossection
presented to the wind is 10 x 0.03 which is 0.3 square metre. Unsing a
coefficent of 1.0 and the stagnation pressure of 1.325 kPa we get  398 N or
40 kg of force distributed eavenly along the element. Simply calculate the
drag on each element, the boom, the rotor, the mast etc and ad them up to
get the total force on your antenna system.

    Note. The 1.0 drag coefficent is conservative for almost all antenna
elements except a parabolic dish. For a parabolic disk facing the wind use a
drag coefficent of 1.2.

I hope this is a general enough run down for you to get started. You can ask
me any more specific questions you have. I did these calcs in metric but I
could explain them in imperial measurements if you need me to.

Regards,
Murray Peterson B.E.
VK2KGM
Sydney NSW,
Australia

----- Original Message -----
From: "Jens Schmidt" <j.schmidt@paradise.net.nz>
To: "AMSAT-BB" <amsat-bb@AMSAT.Org>
Sent: Wednesday, January 31, 2001 5:52 PM
Subject: [amsat-bb] Wind load calculations ?


> Hi all,
> I would appreciate some download sources for
> software to calculate the loading on antenna systems
> from wind.
> Generic is preferred of course.
> Thank you,
> 73 Jens    ZL2TJT

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