Acoustical Methods Towards Accurate Prediction of Rotorcraft Fuselage Scattering

Abstract

Main and tail rotors are major sources of noise for helicopters and contribute significantly to ground noise footprint. The main research effort in the past was concentrated on the noise generation mechanism and its reduction. Even though the scattering of noise generated by helicopter rotors has been recognized to deeply affect noise spectrum and directivity of the isolated rotary-wing system, there has not been an extensive research effort towards the comprehension of the phenomenon. This is particularly important in dealing with tail rotor noise, for which the wavelength of the harmonics is comparable or smaller than the characteristic dimension of the fuselage. In order to boost research activities on noise propagation in presence of the fuselage, a specific Action Group (AG24) has been constituted in the Helicopter Responsables framework of the Group for Aeronautical Research and Technology in EURope (GARTEUR). The G forocuups oisf on the development and validation of numerical prediction methods, addressed within the WP1 of the AG. The experimental activities carried out in the second WP of the action are described in a separate paper presented at this forum. Accurately predicting the effective helicopter external noise under the influence of the fuselage requires the use of advanced Analysis tools overcoming limitations of classical acoustic analogy methods. The evaluation of the scattered acoustic field allows the interior noise prediction and the evaluation of fuselage vibro-acoustical feauters. One of the aim of the WP1 is to expand the limits of current noise prediction tools, enabling the development of new vehicles exploiting shielding effects and controlled surface impedance thus reducing the environmental impact of helicopters and increasing public acceptance. From a scientific point of view, this objective is addressed through a validation of the different prediction design Tools developed within the participants’ institutions. Most of the codes solve the Helmholtz equation, using a Boundary Element Method for addressing the discretized Green’s function integral formulation of the solution.