Flag Wrapping.....

[MoonRocker]

Ok can someone explain the science here. A club I go to has a flag pole and when a member dies we put the flag at half mast. Our flag pole is on the side of the building high up at an angle of approx 45 degrees. Now, inevitably, when a wind gets up it gets wrapped around the pole. What I don't get is, why does it never unwrap itself, the wind blows in all directions but the flag will only wrap in the same direction as its first wrap, getting more and more wrapped around the pole. I have never seen it unwrap, ever. I'd have though that if the wind changed direction it would unwrapaAny ideas why not? thanks

[Calibax]

When unfurled, the flag presents a large surface area to the wind so little force is needed to move it enough to encircle the pole. When wrapped around the pole it presents a small surface area so a much greater force of wind is required to unwrap it.

[MoonRocker]

surely when it's wrapped once there's the same area left both sides but it always wraps further, it never unwraps.

[Calibax]

True, but the wind probably prevails mostly from a single direction even when it's blowing about a bit. Also there may be some friction due to the layers adhering together which may further prevent unwrapping.

[wildwood]

Just having a stab here. Could it be the aerodynamic properties of the round pole, decreasing the air pressure on top so the banner lifts up easier?

[Wak]

Also having a stab at it! Is it possible that the wind rubbing against the cloth induces a charge of static electricity in the cloth which causes it to stick to itself and therefore becomes more difficult to unwrap due to this adhesion????
What material is the flag made from???
I'm probably talking rubbish but you never know!

[Sipowicz]

Clingfilm?

[Wak]

L.O.L or maybe sellotape ????

[Khandro]

is it always wound clockwise ar anticlockwise or sometimes either?

[ChuckFickens]

You can stop it happening at all by hanging the flag from a couple of rotating collars similar to the rings you'd hang shower curtains from.

[MoonRocker]

wak: it's a new heavy cotton flag
chuck:yes there are ways to stop it, last one we had sewn ball bearings in the hem to minimise it.
Khandro: it can wrap both ways but it only goes one way once that way is decided.

Forget the higgs boson, solve this!

[Khandro]

As the wrap can be in both directions, it has to be related to the wind direction on that particular day.
Chuck; The pole is at 45 degrees, so unless some device is also fitted, the uppermost ring would descend down the pole. Also flagpoles usually have a finial on the end which would hamper the sliding on of the rings.

[MoonRocker]

"As the wrap can be in both directions, it has to be related to the wind direction on that particular day."

yes but no matter how long we leave it, it never uwrapps, the wind blows in all directions over time.

[ChuckFickens]

"Chuck; The pole is at 45 degrees, so unless some device is also fitted, the uppermost ring would descend down the pole."

The kits for it come with a circular clamp that goes around the pole to stop just that happening. also the finial can normally be unscrewed on most poles, but if not the rings can probably be fitted by taking the pole out the mount and putting them on from the bottom.

http://www.usflags.co...s/thumbs/0001699.jpeg

[Wak]

I suppose one could always sew a length of suitable metal chain down each long side of the flag to weight it down and to stop it being blown over the flag pole???? Trial and error should sort it out.

[Khandro]

Wak; ^^ A non-starter; the flagpole is at an angle of 45 degrees, and would hang like a crumpled chapati.

[wildwood]

Just tie a light rope to the top corner of the flapping end and tie the other end to your neighbour's chimney.

[jomifl]

I''m not sure whether this explanation is correct but here goes:
Air flowing above a certain velocity past a pole at right angles, generates turbulence eddies which break away from the pole sides alternately. This is what makes taut strings vibrate in the wind. When the flag has been wrapped once the aerodynamics of the pole/flag combination are asymmetrical so the eddies break away on one side only, this happens to be the side which wraps the flag again the same way. The wind direction would not make any difference.

[heathfield]

The region with the lowest air pressure will be on the lee side where the flag is closest to the pole. The pole edge of the flag will try to move into this region. Once the process has started, it will continue. Even when the low pressure region is facing the wind, it will maintain the lowest air pressure, and the nearest part of the flag will keep trying to move into it. If the wind is strong enough the drag on the rest of the flag will keep it flying straight. But if the wind is gusty, each time the wind drops a bit, and drag is reduced enough, the flag will again try to move into that lowest pressure region, and end up being wrapped round the pole.

[Khandro]

Working on an a priori basis that jomifl and/or Heathfield are correct, (I can't contradict either of them!) you are still stuck with the problem, and only chuck has given a satisfactory way out of it. I would further suggest that you re-position the flagpole vertically, which is after all the proper, correct historical orientation for flags, - 45 degree poles are namby-pamby.

[chakka35]

Calibax explained why it happens in the very first answer.