Magnificent men (and women) and their flying machines

NOW we know the reason for the Mona Lisa's enigmatic smile in Leonard da Vinci's famous painting. She was trying stifle a laugh at the artist's belief that man could fly.

Viewers may have had the same amused reaction when the narrator of Leonardo's Dream Machines announced that teams would build of two of his creations, an 80ft crossbow that fired cannonballs and a flying machine. Annoyingly, the end of the first episode left both matters unresolved. We'll have to wait until next week to discover if today's team of experts can make da Vinci's plans work.

He left behind a vast archive of thousands of scientific and engineering drawings - blueprints for extraordinary machines including helicopters, tanks, submarines and massive war engines.

The aim of the exercise was to see if da Vinci was an inspired engineer or an artist with a technological vision.

If he was right, man could have flown centuries earlier. The clues are in his work. Constructing the machines in three months caused real headaches for the two teams.

The giant crossbow, in da Vinci's plans, was 80ft wide and 50ft long. Despite objections from Paulo Galluzzi, director of the Science Museum in Florence, it was decided to make the weapon 30ft shorter. As team leader David Hepworth pointed out, that would still make it bigger than any crossbow ever built.

It was reckoned that the weapon would be more effective than a cannon, which made a habit of blowing up unexpectedly, killing both friend and foe, and couldn't be relied on for range and accuracy because of recoil. A crossbow the size of a house would hit the target more frequently and efficiently, if only these modern builders could construct it using materials available in the 15th Century.

You really didn't expect such intelligent men to make the simple mistake they did. Design engineer Ivan Williams's plans were in feet and inches, yet some of the components were ordered by the metre. The inevitable result was that they didn't fit.

Bonding wood together caused problems too. The laminate opened up when the wood was bent. "It's nothing short of a disaster," said Hepworth. "If it wasn't for the fact that I was a Yorkshireman, I would crying at this point."

The first tests were disastrous and the team went back to the drawing board. We've yet to see the da Vinci-inspired glider built by aircraft restorers Steve Roberts and Martin Kimm get off the ground.

Their design takes the tail from one of his drawings and marries it to his most common wing shape. They're not deviating from the 500-year-old design, but aren't sure whether it will fly.

No wonder pilot Julie Leden, three times world hang-gliding champion, is worried. We left her arguing with the plane-makers over exactly how she was going to control the craft.

The model of the glider nose dived to the ground rather than stay up in the air - "the thought of which concerns me," she said, not unreasonably.