This paper investigates different methodologies for the evaluation of the acoustic disturbance emitted by helicopter’s main rotors during unsteady maneuvers. Nowadays, the simulation of noise emitted by helicopters is of great interest to designers, both for the assessment of the acoustic impact of helicopter flight on communities and for the identification of optimal-noise trajectories. Typically, the numerical predictions consist of the atmospheric propagation of a near-field noise model, extracted from an appropriate database determined through steady-state flight simulations/measurements (quasi-steady approach). In this work, three techniques for maneuvering helicopter noise predictions are compared: one considers a fully unsteady solution process, whereas the others are based on quasi-steady approaches. These methods are based on a three-step solution procedure: first, the main rotor aeroelastic response is evaluated by a nonlinear beam-like rotor blade model coupled with a boundary element method for potential flow aerodynamics; then, the aeroacoustic near field is evaluated through the 1A Farassat formulation; finally, the noise is propagated to the ground by a ray tracing model. Only the main rotor component is examined, although tail rotor contribution might be included as well. The numerical investigation examines the differences among the noise predictions provided by the three techniques, focusing on the assessment of the reliability of the results obtained through the two quasi-steady approaches as compared with those from the fully unsteady aeroacoustic solver.

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This paper presents one of the main objective of WP1 of Clean Sky GRC5 MANOEUVRES project, which consists in the correlation of ground noise data measured during flight tests, with numerical predictions obtained by a numerical process aimed at the analysis of the acoustic field emitted by helicopter rotors in arbitrary unsteady manoeuvring flight. Two of the helicopter trajectories analysed by the dedicated GRC5 flight test campaign are considered. Noise measurements obtained by microphones located on the ground at several positions along and aside the ground projection of the vehicle fly-over trajectory are used for correlation. The numerical simulation starts with the aeromechanic identification of the flown trajectory, followed by the corresponding prediction of aerodynamic loads, rotor noise radiation and far field atmospheric propagation.@en

One of the main factors that limit public acceptance of rotorcraft is the noise emitted over densely populated areas, which prevents a wider diffusion of these vehicles capable of performing flight operations otherwise unattainable. It is the result of complex phenomena generated aerodynamically, through the main and tail rotors, as well as mechanically, by the engine and transmission system. Significant research effort is currently made for reducing rotorcraft acoustic impact, both by helicopter manufacturers and by research institutes and academia.@en

This paper investigates different techniques for the evaluation of the acoustic disturbance emitted by helicopters in unsteady maneuvers. Nowadays, the simulation of noise emitted by helicopters is of great interest to designers, both for assessing the acoustic impact of helicopter flight on communities and for identifying optimal-noise trajectories. Typically, these simulations consists in the atmospheric propagation of a near-field noise model, extracted from an appropriate database determined through steady-state flight simulations/measurements. In this work, two steady-flight/unsteady-flight noise equivalence criteria are examined, in order to assess their suitability by comparison with noise predictions directly evaluated through a fully-unsteady aeroacoustic solver. They differ in the flight parameters used to select the near-field noise model within the database to be associated to a given unsteady-flight condition.@en

This paper presents a sound synthesis approach that allows investigating the noise annoyance of rotorcraft in relation to the source components and flight path. The sound synthesis is based on a source-transfer-receiver model, describing the noise emission and directivity of the sources and their propagation towards the receiver point on the ground. Several noise propagation effects are considered such as the Doppler shift, distance effect, atmospheric absorption, ground reflections and the effects of head and body. A case study is presented, showing the complete modeling and synthesis procedure for a four-bladed twin-engine helicopter. The source data was obtained from numerical simulations. Flyover sounds were synthesized for a baseline trajectory and a noise-optimized flight path.@en