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Why flags flutter

Why flags flutter

thread1-180705: Why do flags flutter?


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The old thread has been closed, so I've opened a new one to continue the discussion.

I watched the formula 1 grand prix on TV at the weekend, and at the start of the grid they had flags for "formula 1" and for "the FIA". These flags were fluttering.

The ripples moved in the direction 'flag pole bottom' to 'flag tail top'.

Each ripple had a direction approximately parallel to a line from 'flag pole top' to 'flag tail bottom'.

In the previous discussions I'd implied the ripples moved downwind and mentally I'd pictured them as vertical. But clearly vertical ripples travelling downwind provide no means of supporting the flag. Also a straight horizontal flag also has no means of supporting itself.

Watching the flag, it occurred to me that the ripples are the means by which the flag supports itself- well, more accurately, the means by which the wind supports the flag.

I believe the tail of the flag tries to drop and as it does so, it pendulums down and round towards the pole, crumpling as it does so.

If you look at a flag pole so that the pole is on the left and the flag is on the right, these crumple zones run in this direction \. The wind hitting them provides lift and causes them to run up the flag as well as downwind.
/| These three lines represent an arrow; the direction the ripples moved.

RE: Why flags flutter

Hi All,

Just a thought,if the pole is of the usual configuration, i.e. tapered from bottom to top, the vortices spilling off of the bottom of the pole would be stronger than those at the top, no?

If so, this would account for their greater influence on the lower part of the flag and the difference in their passage across it. A dropping tail would fall into an area of greater influence and lift as previously described?

Just my 2d worth, please comment, but be kind to me winky smile.


Pete Rihoy,


RE: Why flags flutter

OK - so why did the flags on the moon appear to flutter...

RE: Why flags flutter

Because there was a slight breeze in the Mojave desert on the day they filmed that sequence.............! Sorry, different thread winky smile

Pete Rihoy,

I know the voices aren't real, but they have some great ideas winky smile

RE: Why flags flutter

That's exactly what I was thinking. shadeshappy



RE: Why flags flutter

O.K. Folks,

The reason for my interest in this topic is because I am trying to promote this action.

Whilst I accept the fact that a burgee flying from a halyard (no pole) does flutter, the general consensus seems to be that vortices created by a pole enhances the effect.

It occurs to me that flag flown off centre from the pole may have greater movement due to an imbalance in the vortices travelling down each side of it. Does anyone have any evidence for or against this?  I will, of course, be experimenting, but would welcome input to arrive at a start point, to save replicating work already done. smile

Pete Rihoy,

I know the voices aren't real, but they have some great ideas winky smile

RE: Why flags flutter

Good point. Hopefully I can get a bit of free time to set up an experiment to validate. It should be very simple to observe a difference if there exists one.



RE: Why flags flutter

Flag practically has no bending stiffness, torsional stiffness or compression stiffness. Also, how do you define it's natural mode of vibration. Can you say flag has flutter in the classical sense? I don't know...

RE: Why flags flutter

Could you hold the flag @ the leading edge corners with your fingertips and would it still flutter?

The leading edge shape in in the post is the pole.   If the flag were held only @ the corners, the leading edge would now be the thickness of the flag material.  

I think that since both sides are equal, the wind is creating the same pressure on both sides, and the flag being light weight, is 'sandwiched' between these two areas of a slightly higher pressure.  Also, one possiblility, is the friction of the wind maybe grabbing parts of the flag and pulling it in the direction of airflow.

One thing I thought of while writing this, is how would the density of thread come into account.  Does are pass through the small 'pores' in between the thread?  Would a really thing of a non porous material (maybe a really thin sheet of mylar) act in the same fashion?


RE: Why flags flutter

Any minute variation in the wind pressure would send the flag into oscillations. Try it with a piece of paper.



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