Lockdown Challenge: Dancing with snakes?

Lockdown Challenge #34 – Waving Air-Snakes  

A Caribbean carnival artist called Peter Minshall came up with amazing waving giant figures for the 1996 Atlanta Olympics. Here we take a look at the effect he used, how a long flexible tube can be made to spring up and then fall down sideways in a kind of waving action when fed by a hairdryer.

The easiest way to get started on making Waving Air-Snakes is to tape a length of lightweight tubing of some kind onto a hair drier with a cool setting. You can use strips of dustbin liner folded and taped along one side, or plastic packaging film that comes in tube form, or even lightweight airtight cloth. Aim for a tube 6-10cm in diameter. Now set the hair drier to cool and point it upwards and hold it there. With luck, the tube will fill up in jerks with air, until it’s pointing straight up. Then, with a whoosh, the air goes out, and the tube bends over back to the ground, ready to start again.

What is happening? When you start, the tube is horizontal and blocks off the fan. Then, the air comes from the fan and fills up the first piece of tube.  But the rest is still lying down and a ‘crimp’ in the tube stops the air getting out. So as the air carries on coming from the fan, it fills up more tube and moves the crimp upwards.  After a second or two the crimp reaches the top and the air whooshes out, depressurising the whole tube in a fraction of a second so that it lies down again ready to begin again.

It is more random, but the air-snake is like an RC oscillator, that favourite device found all over the place in physics and electronics. In an RC (resistor-capacitor) oscillator, electric current piles in from the battery through the R, filling up the C, just like air filling up the air-snake tube.  It’s the voltage on the C that rises (in an upside-down exponential curve) rather than the height of a tube.  When the voltage gets too high, a neon tube, a DIAC, or another semiconductor like the famous ‘555’ IC short-circuits the C suddenly, just like the crimp getting to the top of the air-snake tube lets the air whoosh out. Then the voltage drops down to zero for the cycle to begin again.

It’s useful to make a stand for the hair drier. But make sure the air inlet can’t be blocked, and don’t leave it alone while it’s running. (But why would you? It’s too much fun to watch!) Always switch off the hair drier if the air isn’t flowing through it. If the snake tends to crimp and stop the air right at the hair drier too much, you can double the tube thickness near to the hair dryer.

The air-snake doesn’t have to be vertical! Try it at an angle. You can add lightweight features at the top of the tube. A snake-head, or a paper hand, maybe? A narrowing in the tube will make the tube fill to higher pressure and go straighter in the lower section. And you can add one or more small air-snakes to a large air-snake, or make a dancing figure with the body and two waving arms.

Why not try long-distance waving? 200 years ago, semaphore towers with huge waving arms sent military messages for miles. At night, you can light up a translucent air-snake from the inside using a torch or LEDs. Or what about making two Air-Snakes with hands (‘Air-Arms’?). They can shake hands – or have a boxing match.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org

There is also the back catalogue of Lockdown Challenges from 2020 to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.