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Re: Wind load calculations ?



Hi Murray,
Thanks a lot.
That was just what I was after.
Yes, it will be just as easy to do by hand (calculator) as on the PC.
The drag coefficient was the thing I didn't know about.
I would only have applied 35 m/s, since I don't count on living another
100 years, HI HI, but then this could be year 99.
Interesting about the 'lift', I had one antenna break, (resonance fatigue),
I came home one evening, dark and raining, as usual looked up at the
mast, and saw one half of a 2m beam pointing about 30 degrees up.
Metric is fine. Now I will be able to specify much better, the specs
needed for a rotator gearbox

73 Jens    ZL2TJT


Murray Peterson VK2KGM wrote:

> 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

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