Travelling on air
Hovercraft (air-cushioned vehicles or ACVs) are a British invention.
Christopher Cockerell built the first one in 1956, using a coffee tin, an industrial air-blower and some bathroom scales. This was a brilliant opportunity for our GCSE applied science students to see successful British science in action.
AVCs work on land or sea and are the fastest ships in the world (60mph).
One crossed the Channel in 22 minutes. But a big hovercraft uses about 1 tonne of aviation fuel per trip, so they're not very environmentally friendly.
GCSE students explore questions like: Why do things move? Speed up? Slow down? Any example where friction doesn't complicate our understanding is very handy.
Hovercrafts have not been too successful as a means of transport, but they have been brilliant for military uses, search and rescue, surveying and lawn mowing. They can get to places nothing else can reach: how about being stuck in the middle of a mud flat during a force 9 gale - what else is going to be able to come and get you? Some scientists are suggesting using them for exploring other planets.
Powerful engines turn fans which blow air under the craft (lift air) and provide power to some sort of propeller which pushes the craft along (thrust air). A "skirt" traps the air under the craft. The air flow into the cushion under the hovercraft has to be higher than the air loss under the skirt (or the craft sinks). If the air flow is too great the craft tilts over. Air molecules in the cushion bump into the bottom of the craft, providing enough force to lift it off the ground.
For the SRN4 Mk III hovercraft, built in Britain, this involves lifting 300 tonnes, 418 passengers and 60 cars. Not bad for a bunch of tiny particles.
* www.st-anselms.org.uk www.sciencesite.tk
* Flying fish Hovercraft: www.flyingfishhovercraft.co.uk
John-Paul Riordan Science teacher, St Anselm's Catholic School, Canterbury