Vecchio A., Valent L., Janssens K., Berthe A.

A common trend is observed in the helicopter industry that aims at positioning helicopter not only as a special transportation system for risky missions related to military operations but also as a common transportation mean. In particular, helicopters may offer a valid alternative to regional mobility that helps reduce air traffic congestion while providing a door-to-door service. In this perspective, the helicopter industry is currently striving to improve NVH performances such as to achieve passenger comfort levels that can be compared with the ones of regional transportation aircraft. For a full understanding of helicopter vibroacoustic behaviour, it becomes essential developing NVH testing techniques that yield reliable results also in operational conditions. NVH testing in helicopters during flight is a difficult matter. Noise levels are severely affected by the strength and vicinity of sources. Jet engines, gearbox and rotors can be considered as separated sources that excite the cabin simultaneously. Those sources are strongly tonal, rpm dependent and highly correlated. The high reflectivity of acoustic fields limits dramatically the applicability of standard acoustic test techniques such as Acoustic Intensity or Near Field Acoustic Holography. The use of test data acquired in the helicopter cabin during flight conditions to locate and separate sources is therefore a major challenge. This paper presents an alternative testing technique that proves able to deal with harsh environmental testing conditions while improving measurement reliability. A novel acoustic probe is used that couples a miniaturized microphone to a particle velocity sensor and provides accurate in-situ measurements of particle velocity synchronously with sound pressure level. This allows computing acoustic intensity maps and acoustic impedance, and paves the way for a better understanding of vibroacoustic comfort in helicopter cabins. Noise maps of the helicopter cabin in operating conditions are derived that point out critical areas susceptible to design modifications.