Among the several sources of acoustic annoyance produced by rotorcraft in operating conditions, blade-vortex interactions (BVIs) capture the interest of much of the current research. This paper deals with the reduction of BVI noise from helicopter main rotors by application of the active twist rotor concept (ATR), exploiting smart materials for twisting blades through higher-harmonic torque loads. An optimal, multi-cyclic, control approach is applied to identify the control law driving the ATR actuation during the occurrence of severe BVI events. Numerical predictions are obtained through a computational tool that is able to predict the aeroelastic response of the rotor blades and the emitted noise in arbitrary steady flight conditions. The approach for the control law identification is described and numerical results concerning aeroelastic and aeroacoustic performance of the controlled rotor are presented to assess the proposed methodology.
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 nearfield 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 quasisteady approaches as compared with those from the fully unsteady aeroacoustic solver.
An optimal procedure for the design of rotor blade that generates low vibratory hub loads in nonaxial flow conditions is presented and applied to a helicopter rotor in forward flight, a condition where vibrations and noise become severe. Blade shape and structural properties are the design parameters to be identified within a binary genetic optimization algorithm under aeroelastic stability constraint. The process exploits an aeroelastic solver that is based on a nonlinear, beam-like model, suited for the analysis of arbitrary curved-elastic-axis blades, with the introduction of a surrogate wake inflow model for the analysis of sectional aerodynamic loads. Numerical results are presented to demonstrate the capability of the proposed approach to identify low vibratory hub loads rotor blades as well as to assess the robustness of solution at off-design operating conditions. Further, the aeroacoustic assessment of the rotor configurations determined is carried out in order to examine the impact of low-vibration blade design on the emitted noise field.
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