Hey all,

I'm working on an illusion in which an assistant will be hunched down on the stage, covered with a cloth. I need to have them positioned directly over some sort of air "blast" type thing, so that on cue, the cloth can be blown off in a quick burst. I have experimented so far with my air compressor, at about 150 PSI, and while it will launch a 36" silk off my hand and embed it in the rafters of my workshop, it isn't nearly strong enough to do this on a human sized scale. Any ideas? Thanks!

Phil

If you attach the cloth to objects in tubes, the cloth will be lifted without loss of the air.
Like a confetti launcher.

The problem is not pressure but volume. You need to get a large volume of air out in a short amount of time. The exhaust end of a shop vac may be suffieient, but more likely you will need to make an air cannon. An air cannon is an air receiver (tank) with a solenoid valve attached (1 inch or larger). The bigger the valve and associated piping the higher the volume (blast) of air. Just fill the tank to the required pressure and energize the solenoid valve. The entire contents of the tank should vent immediately through the valve, thus creating a sufficient blast of air for your purpose. Obviously the size of the tank will affect the size of the blast.

Check out a google search for halloween props- especially pneumatic ones. You should easily be able to find air cannons there. Alternatively, you may want to attach the cloth to some fishing line and just pull it off. MulTiple attachment points will yield different results.

Why not

Use a string and have a hidden assistant "YANK" the cloth away?

(unless they're south o the Mason Dixon line, then they'd be a confederate... not an assistant).

Tim is really right about the volume of air. It really boils down to a question of practicality. If you had unlimited resources, a dedicated stage and everything else to make the effect work you would need large pneumatic butterfly valves attached to massive accumulator tanks in an array under the stage, but the bigger problem is if you got enough air power to get the cloth to pop properly it could be dangerous for other elements on the stage, including the performer.

We've had very good luck blowing cloths around with commercial wind machines. They do have plenty of power, but because they have to ramp up to speed they don't have the same pop you're probably looking for. Yes, it's possible to start the wind machines under the stage and have a louver arrangement with a diverter under the stage. The vents would be closed, the fans would get up to speed diverting the wind another direction, then, that outlet closed and the louvers opened sending the air immediately up to the stage. Again, these are very difficult mechanical prospects that we would consider if we were installing this and a theater where we were doing five shows a day for a 10 year run.

We tried a reveal similar to what you're describing using a large metal grating under the performer. In this particular case, the stage was made out of steel deck platforms so we simply had two high-speed when machines directly under the grate but pointing upstage. The fans were put up to speed and when we were ready we simply had two stagehands tilt the fans straight up blowing the fabric up. It did not have the exact pop you would get from an air cannon type of device, but it did have plenty of control and plenty of lift.

From a practical standpoint, if at all possible. I agree that you should just use line and jerk it up in the air. In the old days of special effects. We used what we call an elasticized explosion when we had to create a "bomb" right next to the talent. There would be a very small black powder flash pot inside the box, but the sides of the box were attached to monofilament lines running off stage different directions where they were attached to elastic bungee cords. The terminations of these monofilament lines were inside the box and join together with a fusible link that would pop as soon as the flash pot went off. We fired the flash pot creating the puff of smoke, the flash burned through the loop releasing the lines that jerked all the parts of the box apart traveling 20 feet across the stage. It appeared that the pressure of the explosion blew the parts of the box away but you could stand right beside it with no fear. You could have a similar technique with trick line running through small holes in the cloth then attached to buttons or some other catch and then a final loop that attaches to the prop, or the stage in a mousetrap release. The other end of course runs into the wings through a pulley and then through an appropriate sized bungee or elastic loop. When you fire the release, the cloth flies into the air on cue. You do have to contend with lighting, but it seems pretty practical. For quick and dirty shows we didn't have time to rig the bungee, we simply attached the line to the end of a 4 to 6 foot wooden stick. The stagehands would just pop stick yanking the cloth up.

Ultimately weere really talking about various ways of puppeteering a cloth with either threads or air. Good luck and let us know what you end up trying!

I am old school. When in doubt, pull the line.

Nice post Ray for those that don't realize this part of the business.

Richard

OR

A bunch of helium balloons between two light-weight cloths... no strike that.

lol... Richard, I'm with you! If it was for my show, personally, I'd stick to a hard asset like pulling a string because I know that will work every time in most every circumstance. I end up having to design a lot of weird alternative methods for certain long run themepark shows where they love to push the envelope with technology. It doesn't necessarily make it better, but it is really fun to play with these massive budgets where you're designing a theater from scratch for show. Unless you're designing your own long run show for a Vegas showroom, you're just not going to get these consistent circumstances that will make these things practical... But again, they're fun to play with!

As always, my theory is that if the method is more interesting than the effect... you're doing something wrong!

Most air cannons that I am familiar with are extremely loud, although someone else may be able to correct me on that. The big thing I see here involves the simple physics behind this. The volume of air required on a sudden blast isn't just what's needed to push the cloth up, but what is needed to override the air pressure from above. Remember that old science experiment where you lay a yardstick over the edge of a table, and smack the free end? It goes flying across the room. Now, do the same but cover it with a single sheet of newspaper. The air pressure from above holds the paper AND yardstick in place. This time when you smack the yardstick, it will break.

The air pressure around the cloth would have to equalize first (gradually) before a spread out cloth would begin to lift. The wind machines under the stage (think Marilyn Monroe's uplifting dress) provides that. As Ray mentioned, an air cannon with the necessary force would probably result in trauma due to a concussion blast.

I would build a window fan into the base, along with some colored up lighting underneath the fan. The assistant could flip the switch, then the air pressure would build under the cloth and blow this up with dramatic affect.

If the assistant stands up with the blast this should look good, give them a foot switch to kill the fan. I would be worried about the cloth blowing straight up or off to the side, we would have to adjust the square size and weight of the cloth to build a pillar of air under the center.

The whole thing could be a 2 foot square maybe 1 foot high base. Move it into position and plug it in as needed.

I would allow a LOT of time and money for R&D. A typical window fan won't give you that much output but ultimately it's a matter of trying something, then adjusting as you learn. The weight of cloth will play a huge roll. We usually try china silk as it looks great in the air and is very light but it's also more delicate depending on your application.

Good luck!

OK, an air cannon is probably not practical for many reasons including noise, but they are fun to play with! I have seen air guns in several museum shops that are about 10" in diameter and 12" long. If I remember correctly, they have a diaphragm of rubber stretched across them which is pulled back then rapidly released causing a column of air that can be felt across the room. If you can have the person on a small pedestal, perhaps a version of this, in conjunction with the string pull idea, would give the desired effect.

I like the fan idea too, or perhaps the exhaust end of an industrial dust collector. These would need to have moving louvers or a moving slat arrangement to give a blast of air. If the fan has to ramp up to speed the result won't be a blast