Just a few years ago Eurocopter scored a major coup at an HAI Heli-Expo show, amazing the crowds by introducing a new helicopter as a completely certified aircraft instead of a promise-laden prototype encumbered with the usual waits for first flight, inflated claims of launch customers and delays in FAA approval. Instead, Eurocopter presented its EC 130 as a done deal.

In much the same way, Eurocopter is quietly working on a new main rotor blade intended initially for use aboard the Franco- German manufacturer’s EC 155B1 light twin. Dubbed the “neighborly helicopter,” the new machine is hoped to be as much as seven to nine decibels quieter than the proposed new ICAO rotorcraft noise regulations. In the EC 155’s case (for a helicopter weighing as much as 6,000 pounds), the regulation would limit noise to no more than 93 decibels.

While substantial, even this amount of noise reduction might not be enough. Support for bringing the noise restrictions for rotorcraft more into line with those for fixed-wing aircraft was voiced at last December’s meeting of ICAO’s environmental protection committee, with the prevailing scuttlebutt that the final requirement will be a maximum effective perceived noise level (EPNL) in the neighborhood of 85 decibels.

In sound, decibels are defined in terms of power per unit surface area on a scale beginning at the threshold of human hearing–zero decibels–upward toward the threshold of pain, about 120 to 140 decibels. For perspective, the sound level in the average residential home is about 40 decibels, average conversation is about 60 decibels, typical home music listening levels are about 85 decibels, a loud rock band about 110 decibels and a jet engine at point blank about 150 decibels.

Eurocopter engineers hope to achieve their desired noise reductions with an all-composite main rotor blade with a twisted and swept tip and an active servo flap on the trailing edge. Servo flaps have long been used on certain helicopters, most notably on the revolutionary–not to mention quiet and easy-to-fly–intermeshing rotor helicopters created by Charles Kaman, founder of Kaman Aerospace.

Also key to the proposed design is a 4-percent reduction in main-rotor rpm during cruise flight, controllable via the helicopter’s FADEC. Under investigation is the use of active rotor hub struts for better high-frequency noise dampening. Such active struts would function in much the same way noise-canceling headphones do, by using sound–in this case, vibration–developed in reverse phase to effectively cancel out unwanted noise. Tests of such struts have already been conducted aboard a
BK 117 testbed helicopter. A full-up demonstrator fitted with the entire quiet system is slated to fly in the second quarter of 2005.