Numerical characterization of helicopter noise hemispheres

Abstract

Numerical tools aiming at the evaluation of ground acoustic impact of helicopters typically rely on databases given in terms of acoustic disturbance over hemispheres surrounding the helicopter (noise hemispheres). These are evaluated for a discrete number of steady flights falling within the flight envelope. The objective of the present work is the identification of flight parameters to be considered for the characterization of noise hemispheres, particularly when related to unsteady maneuvers. To this purpose, different approaches based on steady flight aeroelastic/aerodynamic/aeroacoustic predictions are examined for assessing their capability of simulating the acoustic impact of helicopters in arbitrary unsteady flight. The numerical investigation demonstrates that at least three parameters, including disk loading, are required to adequately characterize noise hemispheres. Conversely, the similarity of kinematic parameters alone may yield steady flight acoustic predictions poorly correlated with those obtained for unsteady maneuvers

Helicopter noise footprint prediction in unsteady maneuvers

“…In approach A, similarity of advance ratio, , and flight-path angle, , is assumed to guarantee the similarity of the emitted noise, whereas in approach B, , rotor thrust coefficient, C T , and rotor disk orientation with respect to relative wind, TPP , are considered as the parameters that characterize noise equivalence between different flight maneuvers. This means that, for a given unsteady flight condition, in approach A the noise source is determined by extracting the noise hemisphere corresponding to the same values of and from the steady-flight database, whereas in approach B the noise source is determined as the one associated to the same values of , C T , and TPP (see also Gennaretti et al 16,17 ). In order to assess the quality of the acoustic predictions obtained by these approaches, their simulations are compared with those determined by the fully unsteady solution (approach C) based on the general aeroacoustic formulation described in ''Near-field noise prediction'' section.…”

“…The flight dynamics tool applied in steady flight trimming and unsteady maneuver identification applies a low-fidelity main rotor model suited for this kind of problems. 17 This fact combined with the observation that the prediction of rotor acoustic disturbance requires accurate evaluation of blade dynamics and aerodynamics (especially when BVI phenomena occur) motivates the introduction of high-fidelity aeroelastic and aerodynamic solvers in the third step of the presented analysis.…”

Integrated methodology for the prediction of helicopter rotor noise at mission level

“…However, co-ordinate transformations between source-observer are evaluated only once per source point, thus significant computational savings occur compared to the traditional observer-time dominant (or retarded-time) numerical algorithm [36]. followed in [10] to properly apply the inverse square law for spherical geometric spreading.…”

“…The integrated methodology was used for the purposes of noise abatement procedure design, although it was applied to simplified hypothetical flyovers, rather than complete helicopter mission trajectories. Recently, Gennaretti et al [10], [11] introduced a hemisphere-based approach to predict the acoustic footprint of helicopter unsteady maneuvers. A database of noise spheres was developed, relied on steady flight aeroelastic, aerodynamic and aeroacoustic predictions.…”

2.3 - Developing a Novel Contactless Sensor for Helicopter Rotor State Measurements

“…This algorithm retrieves interpolated SPL (Sound Pressure Level) distributions within a database of pre-calculated steady-state acoustic predictions. The input for this process is given by the current values of three parameters, to which noise emission is strongly related: the helicopter advance ratio, the main rotor thrust coefficient, and the main rotor tippath-plane (TPP) angle of attack, or TPP-AOA [5]. The on-board measurement of these parameters poses some difficulties.…”