Eurocopter's "neighbourly helicopter" programme has given rise to the EC120 and EC135, which have noise levels 6dB lower than the International Civil Aviation Organisation standard, making them the quietest in the business.

This year, the work will progress with a demonstration of the EC155 main rotor, fenestron and inlet, which will reduce noise to 8dB below the limit. "Our strategy is to be the best in the market," says Galland. The main rotor is optimised aerodynamically for shape and operating regime, with particular attention paid to blade-vortex interaction, which accounts for most of the characteristic "slapping" noise of a helicopter on approach. The results of the efforts to refine the blade shape, particularly at the extremities, and to reduce tip speed from 41,340ft/min (210m/s) to 35,430ft/min, have demonstrated that a helicopter flying at 500ft (150m) is virtually inaudible until it has reached an angle of 45¼ to the listener, he says. "We were extremely surprised."

Adapting the rotor to the prevailing conditions is the key, he says, so that, for example, rotor speed is reduced when the helicopter is operating near the ground, increasing as it moves away from the noise-sensitive area. This requires some sort of automatic speed control, obtained through coupling the engine and rotor control systems. The EC155 has a preliminary automatic speed control system, allowing a 2-3% rotor speed variation. "We're looking at 10%," says Galland.

Source: Flight International