.Real-life enclosures, such as aircraft fuselage, car interiors and helicopter cabin, are very often characterized by a strong fluid-structure coupling. This behaviour heavily affects the helicopters design procedures aimed at reducing noise and vibration levels on which, in turn, fatigue and human comfort strongly depend. However, identifying the critical spectrum areas where a possible overlapping between the helicopter response and the operating force spectrum may occur, is quite a tougher target to get. This is mainly due to i) a not so clear a-priori knowledge of the structural components concerned with vibro-acoustical coupling to the closure, and ii) a continuous changing of the in-flight boundary conditions which move the frequency response spectrum of the helicopter from the one obtained in laboratory. Through an intensive testing campaign on a EC-135 helicopter cabin, new techniques aimed to the proper modal parameter extraction are presented in this paper. Four modal analyses have been performed on the cabin, a pure acoustical modal analysis of the enclosure, an experimental modal analysis of the internal and external surface and an operational modal analysis of the external surface. The common approach is to consider just the cabin internal surface coupled to the cavity, and to neglect the possibility to consider an acoustical reference in the OMA analysis. In this paper it is highlighted the worth of the proper excitation method for an EMA as well as the reference choice for an OMA, showing the right guidelines to be pursued from a designer. In particular it is here pointed out that an acoustical loading must be preferred also for the cabin external surface modal parameter extraction, in order to directly excite the acoustics of the cavity in an experimental modal analysis, while in operational conditions it would be more efficient an acoustical response as reference.
On the vibro-acoustic operational modal analysis of a helicopter cabin
MUCCHI, Emiliano;
2009
Abstract
.Real-life enclosures, such as aircraft fuselage, car interiors and helicopter cabin, are very often characterized by a strong fluid-structure coupling. This behaviour heavily affects the helicopters design procedures aimed at reducing noise and vibration levels on which, in turn, fatigue and human comfort strongly depend. However, identifying the critical spectrum areas where a possible overlapping between the helicopter response and the operating force spectrum may occur, is quite a tougher target to get. This is mainly due to i) a not so clear a-priori knowledge of the structural components concerned with vibro-acoustical coupling to the closure, and ii) a continuous changing of the in-flight boundary conditions which move the frequency response spectrum of the helicopter from the one obtained in laboratory. Through an intensive testing campaign on a EC-135 helicopter cabin, new techniques aimed to the proper modal parameter extraction are presented in this paper. Four modal analyses have been performed on the cabin, a pure acoustical modal analysis of the enclosure, an experimental modal analysis of the internal and external surface and an operational modal analysis of the external surface. The common approach is to consider just the cabin internal surface coupled to the cavity, and to neglect the possibility to consider an acoustical reference in the OMA analysis. In this paper it is highlighted the worth of the proper excitation method for an EMA as well as the reference choice for an OMA, showing the right guidelines to be pursued from a designer. In particular it is here pointed out that an acoustical loading must be preferred also for the cabin external surface modal parameter extraction, in order to directly excite the acoustics of the cavity in an experimental modal analysis, while in operational conditions it would be more efficient an acoustical response as reference.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
