Grindstone cowboys

This week the teams decide to follow their leaders. With the help of their Zimbabwean-born expert Giles Pearson, the Techno Teachers plan to construct a rather unorthodox vertical-axis machine with only two blades, called a Darrieus rotor (see Scrapheap science for more information). This will be competing against a rather more traditional design if all goes well for the orange-clad Manic Mechanics. Their expert, Jim Barr, has a familiar farm-type horizontal-axis design in mind, with many blades.

Of course, being BMW mechanics, the Manics' first inclination is to mount a car axle with its differential as a bearing between the shaft and the sails. However, a differential reverses the rotation when used like this, and the coffee grinder that must be powered only works one way. This confusion, along with the exact direction the blades will turn, comes back to haunt the Mechanics throughout the day.

Back to basics
Both teams scavenge a large amount of scaffolding bars and angle iron. It looks as if this will be a 'back to basics', welding-and-structure-building day, similar to the Beach Boys' effort in the 'Demolition' challenge. Then Tony of the Manic Mechanics finds a ready-made motorway lighting gantry with wheels. Expert Jim quickly realises that this could form the basis of the windmill with a little modification.

Tony then gets the idea that they should be looking for all sorts of other prefabricated components, rather than custom-making parts for themselves. He presents an off-the-shelf hub casing to Jim, but Jim can't see how this plan can possibly work. Eventually, Tony is persuaded that some hard work will be required to compete successfully in Scrapheap Challenge - it isn't simply an Easter egg hunt. The Manics settle on a wheel hub (what else?) on which to mount their blades, but soon get a bit snippy with expert Jim as he seems to keep changing his mind.

The feeling in the Robert and Cathy camp is that both teams appear to be building 'wibbly wobbly windmills'. This is borne out as the Manics yet again change the plan for the spokes on which will rest their sails: they dismiss thin sheet in favour of strong 2.5cm (1in) tube. All this donkey work takes up much of the day.

Bad omen?
The Teachers, on the other side of the wall, have their own troubles. They are deep in a measurement crisis as Tim has cut the poles for their structure too short. However, unlike at the barber's, where more can be taken off but none put back on, super-welder Juliet gets to work rectifying his mistake. With three hours to go, the Technos seem to have sorted out their measurement troubles. But by now the weather has turned atrocious. Whether or not this is a bad omen for the test day remains to be seen.

The Teachers must incorporate some kind of initialising mechanism in their design as their windmill will not be self-starting. So they devise a system where they can wrap a rope around the base of the windmill and then all pull together to give it a jump start - like a giant spinning top. This design works omni-directionally, and the grinder, as mentioned before, only goes one way. Their windmill's real operational strength, however, is that, once in motion, it can make use of wind coming from any direction. The Manics, on the other hand, will have to point theirs so that it always faces the wind, thus leaving it open to potential weakness if the wind changes direction.

Up effluent inlet
With one hour to go, the Manics have almost all of their blades on, but the Technos are back to their old measuring problems: their blades are half a metre too short! As there are only two of them, and their combined surface area is all the Teachers have for catching the wind, this is a critical mistake. But as time is getting very short, they will have to live with it. Meanwhile, the Manics have just realised that the coffee grinder works only in one direction. Their rallying cry - 'Come on!' - is heard for miles around and they set to work trying to avoid disaster.

The car axle they have chosen as their bearing works extremely well as it has very low friction, but they have put the blades on back to front! The Manics are not the only ones up effluent inlet, however - the Teachers have just realised that, with ten minutes to go, their whole assembly has locked solid. A last-second fix will have to be augmented by some tinkering at Beachy Head in the morning. All this and the judge proclaims that he doesn't fancy either team's chances!

Bodged-up car scrap versus high-tech flimsiness
The next day the teams arrive on the south coast to news of a gale warning. Despite this, the Technos - in a boom-or-bust frame of mind - decide to add to their short aerofoil sails. The Manics heed the meteorological advice and cut their sails down a bit, though. Their worry is that, if the wind is too strong, there will be nothing to stop their machine shaking itself to bits - it has no in-built device to add artificial friction or to tilt the head away from fatally strong gusts.

And then it begins to rain.

Sacks of coffee beans and collection bowls at the ready, the teams start the grind that could lead them to the next round. The Teachers wind up their machine and pull the rope with all their combined might. The rotor does a passable impression of a helicopter and all but takes off. The team quickly apply their grinder and things get off to a roaring start.

The Manics, too, have a good run to begin with, but soon the wind drops and their emasculated sails find it hard to cope with this eventuality. Although they do not come to a grinding halt, the tempo slows for the embattled garage guys. The Met Office warning seems to have been for naught as, with 30 minutes to go, the wind not only drops further still, but changes direction. This does not affect the Teachers, who soldier on regardless, but the Mechanics must swivel their entire machine if it is to catch the draught.

Ten minutes to go and the wind gets up again, which allows the Manics to catch up. Apparently oblivious to their pupils' reactions when the programme is aired, Teachers Andy and Tim sing and dance like lunatics as they go about the business of winding up their rotor and grinding more coffee.

At the very end, it seems to be neck and neck as the wind at last shows some strength. Only when the two very damp teams hang their collected coffee on the official scales does it transpire that the Techno Teachers have breezed through to the next round.

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