How do windsocks work?

[packman]

A question being kicked aroud the crewroom at the moment that even the oldest and boldest can not provide a reasonable answer for. How does a windsock work?

[Pegasus77]

They work from 9 till 5, and then the towercontroller comes out to give them the evening off.

But seriously... I don't really get your question? How does the wind blow up the thing?

Or do you mean the fact that they are mostly calibrated at 15kts to stand fully blown still in the wind. If the wind blows harder, they start moving like a flag (dunno the english word for that, in Dutch we would say: wapperen), and if the wind blows less hard than 15 kts, they are bent down a bit.

[packman]

To clarify, a windsock is a convergent duct. If wind blows through it velocity increases and pressure reduces so how does it not collapse in on itself?

[Pegasus77]

Good question... Thanks for making my brains starting to work again during the last days of my holiday!

[phd]

It may have something to do with the rigid ring which usually keeps the opening of the windsock open. I have a small non-rigid windsock made of ripstop nylon which has no stiffening ring around the upwind opening and this rarely stays properly inflated - it often shows a tendency to collapse in on itself.

It may also be that the initial assumptions about the airflow speed increasing are incorrect or over estimate this effect. I agree in a venturi with a constrained airflow both up and downstream of the narrower section there will be an increased flow rate and reduced pressure in the venturi. However the windsock is different - the air both up and downstream of the windsock is not constrained. At the upwind opening of the sock there may be local slowing of the airflow due to turbulence around the mouth of the sock, and some air may spill around the outside of the sock in eddies. There could then be a net increase of pressure inside the sock which is sufficient to keep it inflated. I bet if you placed the nozzle from the end of an air-start hose inside the windsock and blasted a jet of air through it to re-energise the flow inside the sock, it would suck it closed tighter than the sphincter of the Air Transat pilot who lost both engines of his A330 in mid-atlantic.

Has anyone out there ever measured the pressure inside and outside of a windsock? If so that is very sad and you need to find yourself a good psychiatrist as you need help.. .-----------------------------------------------. .Pobody's nerfect

[Pegasus77]

How about a visit to the psych for people who have their own windsock :) :)

[weirdy]

I think Fishbed has hit the nail on the head. A windsock may look like a venturi tube, but the air in it doesn't act like it was in a venturi. I think that it is a consticted pipe. Put your finger over a hosepipe and let a little of the water out, what happens is an increased outlet pressure and an inflated hosepipe, same as a windsock.

[ 26 February 2002: Message edited by: weirdy ]</p>

[chiglet]

P77,. .At EGCC [at least] they work 24/7 the windsocks, [all FOUR of them] ARE illuminated at night <img src="wink.gif" border="0"> . .As to how they work:- <img src="confused.gif" border="0"> . .Wind blows through a [calibrated] fabric tube.. .So,you are not "really" interested...0-5kts . .Not a "lot" happens.. .But at 5-10kts, the "tube" starts to 'stiffen',so a "little" bit more interest.. .11-15kts, a "lazy" lump <img src="wink.gif" border="0"> , and it is "starting" to get interesting. .16-25kts, a "real" stiffy :) . .we aim to please it keeps the cleaners happy. .Above that, I really daren't say <img src="rolleyes.gif" border="0">

[ 26 February 2002: Message edited by: chiglet ]</p>

[whatbolt]

Bit off the thread-but do you know why a weather cock is called a weather cock-because if it was a weather fanny the wind would blow through it-bit like a weather sock really ???

[Dan Winterland]

Most windsocks have a gauze panel at the end that allows the presssure differential to equalize and prevent the thing from collapsing.

God, this is a boring thread!

[ft]

Consider the air going around the "intake" as well. It has to have accelerate laterally to follow the narrowing cone. No acceleration without a pressure gradient so there will be a lower than ambient pressure on the outside of the cone.

I don't think it is unreasonable to assume near ambient pressure not long after the outlet from the cone. Assuming constant mass flow throughout the cone, this will mean a higher pressure in the inlet created by ram effect in the oncoming wind which falls off towards the outlet.

Low pressure on the outside, higher pressure on the inside -&gt; cone stays inflated.

Please find any errors in thinking or assumptions. <img src="smile.gif" border="0">

Cheers,. . /ft

[englishal]

...probably the same reason a parachute doesn't collapse on itself.....

By the way, here is the way to make your very own Windsock from a length of PVC pipe...http://www.geocities.com/CapeCanaveral/Lab/3929/windsock.html

Isn't the internet great <img src="wink.gif" border="0">

EA

[Wings]

Has anyone noticed if or where on Jepp Charts or Aerad charts they have a decode of the windsock at the airport ?. .What I mean is, two entries above differ on the wind strength required to make a windsock fly stiff horizontally. One guy says 15 Kts, one says 25 kts. Has anyone seen it written anywhere that (say) Heathrow windsocks stand stiff and horizontal at 15 Kts, 25 Kts or something else ?

p.s. I know the Tower Controllers give us the wind velocity prior to Takeoff and Landing, but I make a point of looking at the windsock anyway.

[Cyclic Hotline]

This design works well!. .<a href="http://www.angelfire.com/yt2/dc3windtee/" target="_blank">Wind Tee</a> <img src="tongue.gif" border="0">

[pigboat]

At CYYT, they have a twenty foot length of schooner chain attached to a pole. When the chain is standing out straight, they allow there is a fair breeze blowing. <img border="0" title="" alt="[Smile]" src="smile.gif" />