Another twist

Royal Navy helicopters that have been used to back-up amphibious assault missions for the past 30 years have been given a new lease of life supporting ground troops in Afghanistan.

The Sea King HC Mk 4 helicopters have been fitted with new composite main rotor blades that generate an increase of up to 2,000lb (908kg) in maximum hover mass. The blades, from US firm

, have also raised maximum forward speed by 15 per cent at 10,000ft above sea level.

Previously the Sea King has only been able to operate at low levels. But the new blades enable it to fly at high altitude, making it suitable for service in the rugged terrain of Afghanistan.

Over the last year, the blades have undergone rigorous performance testing and flight evaluation by the MoD's Aircraft Test and Evaluation Centre using a Sea King test aircraft owned by defence company Qinetiq.

Nigel Peterson,

business group technical manager of rotor evaluation, said Carson blades first came to the attention of the MoD and Qinetiq about five years ago.

At the time, the MoD was investigating ways to extend the life of its aircraft, and Peterson said Carson's claim that the blades could add 2,000lb extra thrust to old helicopters was encouraging.

Testing on the blades began on the Sea King a little over a year ago when the MoD placed an urgent request for extra helicopter force in Afghanistan.

Technical limitations of the original blades have prevented the aircraft being used at high altitude. 'Helicopters are performance-limited at high altitude,' said Peterson. 'As the air gets thinner the blades work harder and then the engine must work harder.'

One of the design problems with the Sea King's original blades was their tips. As a helicopter flies forward, one blade cuts into the airflow and one goes backward. The tip of the blade cutting into the airflow moves faster than the tip of the blade that is going backward.

As the aircraft's blades approached the speed of sound, shockwaves occured at the tips, and as the blades increased rotational speed, the shockwaves travelled down the blades. This caused significant vibration through the aircraft.

The end of the Carson blades are paddle-shaped, which helps reduce shockwaves as the tips of the blades approach the speed of sound.

'It gives you more lift and you're able to fly forward faster,' said Peterson.

He added that the blades also have a noticeable twist in them to increase efficiency. 'The old blades were just straight, almost like a wing,' he said.

For the Sea King's traditional missions this wasn't a problem. 'The aircraft was happy operating from the back of ships, and doing maritime Royal Marine commando work at low level and sea level,' said Peterson.

Over the years the helicopters have seen action in, among other places, the Falklands, Bosnia and Iraq. 'It was never envisaged two or three years ago that they'd be able to fly over the hills and mountains of Afghanistan,' he said.

Sea King helicopters can hold up to 16 personnel, and tasks in Afghanistan include transporting troops — work previously carried out by Chinook helicopters.

While the new blades have given fresh life to the old Sea King, the chances are there will not be any further major structural improvements made to the aircraft in the future. The helicopters are outdated compared to others in operation, such as the EH101 Merlins.

As a result, Afghanistan could be one of the Sea King's final major missions as they are due to go out of service in 2017.