Drage Forces on Flags 3

[timh]

Does anyone know of any published data on the drag force cooefficients for different types and sizes of flags. The Australian wind code does not give any guidance.

 

[PJD]

I haven't seen anything published but I've seen several designs using a 50% flag size with the normal force method. Contact flagpole mnfs. located in the SWEETS catalog for more info.

 

[timh]

I think I may have an answer to my own question. <br>
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One of my engineers has found a reference book with some published data on the drag resistance of flags. It comes from &quot;Fluid Dynamic Drag&quot; by Hoerner 1965 pp 3-25. This refernece quotes research done by Faithorne &quot;Drag of Flags&quot;, ARC RM 1345 (1931). I would like to know if anyone has done any further research on it.<br>
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The reference gives two graphs, one where drag is expressed as a function of the material weight (tested on an aspect ratio=1.0 and wind speed of 30ft/sec) and the second shows drag expressed as a function of aspect ratio (tested for a height of 4ft and wind speed of 80ft/sec). As an example of the second graph, an aspect ratio of 1.0 gives a Cd=0.06 wheras an aspect ratio of 4.0 gives Cd=0.2.<br>
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If anyone would like to read the full test of this article (1 page) I will galdly email a scanned image.<br>
<br>

 

[DonBoudreaux]

I have some information that may be of value to you. Send me a FAX number and I will send you 4 pages that will help you. You can contact me at: donboudreaux@hotmail.com

 

[nvy]

in the british code there is a formula for <br>getting an approximate pressure coefficient<br>on a flag.<br>this can be used to design the flagpole supporting<br>it.

 

[Polecat]

One formula for this was developed by the U.S. Navy and appears in the <u>American Civil Engineer's Handbook</u> (Merriman & Wiggin edition).&nbsp;&nbsp;It is strictly an empirical formula and was arrived at through some wind tunnel testing at the Naval Yard in Washington, DC.<br><br>Force exerted by flag: F = 0.0003*A*V^1.9<br>F = pounds<br>A = flag area in ft2<br>V = wind velocity in mph.<br><br>Another formula developed by Sighard Hoerner in a book called <u>Fluid Dynamic Drag</u> is this:<br>F = 0.000218*A*V^2 --- Where the same definitions apply, except that it was tested for flags having an L/W ratio of 1.5.<br><br>Compared to some other formulas available, these seem to give comparable results for smaller flags (say, 4x6), but become very conservative for larger ones or high wind velocity.<br><br>Hope this will help.&nbsp;&nbsp;Don't forget to add the wind on the flagpole itself.&nbsp;&nbsp;By the way, if a flag is mounted on a very tall building (or a pole exceeding 30 ft), then one must add a height and gust factor to the equation.&nbsp;&nbsp;See ASCE 7-95 for more on this.<br><br><br><br> <p>wyf (The Polecat)<br><a href=mailto: > </a><br><a href=

http://www.spiraleng.com>SESI</a><br>

 

[waz]

The polecat when you say it is conservative for large flags and winds do you mean that the actual wind force is less than that calculated. I am looking at a 13ft deep x 3ft wide flag in winds between 30 - 70 mph.

 

[oldrunner]

I'm surprised that no one has mentioned the Guide Specifications for Design of Metal Flagpoles, ANSI/NAAMM FP 1001-97.

You didn't state the flag material, but the code provides values for nylon and cotton flags, and for polyester flags.

For nylon and cotton: Wf = .0010 x V^2 x Af^.5 x Ch
For polyester: Wf = .0014 x V^2 x Af^.5 x Ch

where V = design wind velocity in mph.
Af = flag area in ft^2
Ch = 2.01 x (z/900)^(2/9.5) where z = height of the
pole in feet.

It's probably a good idea to obtain this code, or if you have the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminares and Traffic Signals, 1994, there are mostly the same procedures except the AASHTO Standard doesn't have the flag load formulas.

 

[RHK]

If you go to Standards.com.au and look at the next draft for the Australian Wind Code (1170 Part2), I think you will find the drag of flags in one of the Appendices. If not, Hoerner is a very good reference for drag of odd shapes.

The best bet with flags and banners is to specify the size of the halyard to be used on the flag. When the drag exceeds the capacity of the rope, it breaks and the flag blows away (hopefully long before the structure falls down).

It appears to me that the general public is quite forgiving when structures blow over. They talk of freak wind gusts and rogue waves, not appreciating that we think we know what we are doing. So loosing the flag will not reflect on your expertise.




Russell Keays