A wind question that doesn't involve tubas

[poomshanka]

Howdy folks...

We're having a banner made for an outdoor stage backdrop. It's 20' wide X 13' high, and will be affixed to a truss system that looks like this:

http://img.en.china.cn/0/0,0,396,17968, ... 79679c.jpg" target="_blank

The banner is printed on a breathable mesh that allows wind to pass through it (and not cause the truss system to come crashing down if a stiff breeze kicks up). Problem is the porous material allows light to show through from the other side and make the artwork hard to see under certain conditions.

We had an idea to put a second backdrop up to block out the backlight. It'd be flooded black, made of a much more open mesh, and have slits cut into it. The two would be separated by the depth of the truss, say around a foot or so.

Question: Would this present any "additive wind resistance" issues that would make the two backdrops more prone to wind influence than just one by itself? It sounds like they'd actually act as wind breaks for each other, especially the back one with the more porous structure. I dunno, though. I'm sure there's a force-loading equation out there, but I slept through that chapter in Physics.

Any thoughts from you aerodynamic specialists out there? Thanx...

...Dave

[DonShirer]

Yes, any added screen as you describe would add resistance, but probably only a fraction of the original amount. (Try testing a mockup with a big fan.) Can you attach diagonal bracing lines without a danger of decapitating a cast or crew member?

[Tom]

Is labor time/cost an issue for rigging?

If so, I'd just go with the single backdrop as I think the benefit of having a second, black backdrop behind it for the sole purpose of making it easier to read really will will be minimal, especially if it is particullary sunny. Remember, if the sun isn't on you sitting on stage, it is on your audience, and they'll not be especially interested in looking straight into the setting sun.

As for wind, any backdrop or banner, perforated or not, will catch the wind and will flap around. Expect this and don't be surprised when the flapping backdrop is slapping against the truss or making tons of noise. I think having two backdrops setup one behind the other would actualy magnify the problem.

At the end of the day, be sure to consult with your stage techs and check the load rating on the tuss (especially if this truss has any lights hung on it). Screens and backdrops can be surprisingly heavy and wind can really add a lot of stress. Make sure you have appropriate chains and motors for the rigging and that there are safety ties also. If a truss or light grid comes down in heavy wind, people will be seriously injured or killed. It is not something to be taken lightly.