Jacopo Serafini

Assessment of Computational Models for the Effect of Aeroelasticity on BVI Noise Prediction

This paper deals with the computational analysis of acoustic fields generated by helicopter rotors when Blade-Vortex Interactions (BVI) occur. The prediction procedure starts from the determination of the steady periodic blade deformations. Then, the BVI-affected, unsteady aerodynamics solution is obtained by a potential-flow boundary integral formulation suited for aeronautical configurations experiencing blade-wake impingements. It is applicable to blades with arbitrary shape and motion and evaluates both wake distortion and blade pressure field. Finally, the noise field radiated by the rotor is computed through an aeroacoustic tool based on the Ffowcs Williams and Hawkings equation. The numerical investigation examines the sensitivity of BVI noise prediction on the aeroelastic model applied for the calculation of blade deformations, and assesses the accuracy of the results through correlation with experimental data concerning a helicopter main rotor in descent flight. Noise predicted is examined in terms of both acoustic pressure signatures and noise radiation characteristics.

Effects of Biodynamic Feedthrough in Rotorcraft/Pilot Coupling: Collective Bounce Case

This paper discusses the aeroelastic interaction between the helicopter and the pilot called collective bounce. The problem is mostly studied in the time domain, using the multibody system dynamics approach to model the dynamics of the vehicle and the aeroelasticity of the main rotor and a linear or quasilinear transfer function approach for the voluntary and involuntary dynamics of the pilot. Different models are considered for the aerodynamic forces acting on the rotor, ranging from blade-element/momentum theory to a boundary-element method used independently and in cosimulation with the multibody model. The problem is analyzed in hover and forward flight, highlighting modeling requirements and the sensitivity of the stability results to a variety of parameters of the problem.

Aeroelastic Modeling Effect in Rotor BVI Noise Prediction

The acoustic field generated by a helicopter main rotor experiencing blade-vortex interaction (BVI) during a descent flight path is examined. The prediction procedure starts from the determination of the aeroelastic steady periodic solution. Then, a boundary integral formulation for the velocity potential suited for configurations where stong wake/blade impingement occurs is applied. It is fully three-dimensional, can be applied to blades with arbitrary shape and motion and performs the calculation of both wake shape and blade pressure field. Finally, the noise field generated by the helicopter rotor is evaluated through an aeroacoustic tool based on the Ffowcs Williams and Hawkings equation. The numerical investigation discusses the sensitivity of BVI noise prediction on the aeroelastic models applied for the calculation of blade steady periodic deformations. The eects of the dierent blade deformations given by the aeroelastic solvers considered are examined both in terms of local acoustic signatures and in terms of noise radiation characteristics.

Optimal Design and Acoustic Assessment of Low-Vibration Rotor Blades

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.

Rotor Dynamic Wake Inflow Finite-State Modeling

Wake inflow modelling is a crucial issue in the development of efficient and reliable computational tools for flight mechanic and aeroelastic analysis of rotorcraft. The aim of this work is the development of a finite-state, dynamic wake inflow modelling for rotors in steady flight conditions, based on the simulations provided by aerodynamic solvers of arbitrary accuracy. It is related to the well-known Pitt-Peters model, which relates the coefficients of an approximated linear distribution of the wake inflow over the rotor disc to thrust and roll and pitch moments. A three-step identification procedure is proposed. It consists in: (i) evaluation of wake inflow due to harmonic perturbations of rotor kinematics, (ii) determination of the corresponding inflow coefficient transfer functions, and (iii) their rational approximation. Wake inflow models related to both flight dynamics statespace variables and rotor loads are presented and discussed. Wake inflow models predicted through aerodynamic solutions provided by a boundary element method for potential flows are compared with the Pitt-Peters one, and validated by correlation with the inflow directly calculated by the aerodynamic solver, for a rotor subject to arbitrary perturbations.

Photogrammetric Detection Technique for Rotor Blades Structural Characterization

This paper describes an innovative use of photogrammetric detection techniques to experimentally estimate structural/inertial properties of helicopter rotor blades. The identification algorithms for the evaluation of mass and flexural stiffness distributions are an extension of the ones proposed by Larsen, whereas the procedure for torsional properties determination (stiffness and shear center position) is based on the Euler-Prandtl beam theory. These algorithms rely on measurements performed through photogrammetric detection, which requires the collection of digital photos allowing the identification of 3D coordinates of labeled points (markers) on the structure through the correlation of 2D pictures. The displacements are evaluated by comparing the positions of markers in loaded and reference configuration. Being the applied loads known, the structural characteristics can be directly obtained from the measured displacements. The accuracy of the proposed identification algorithms has been firstly verified by comparison with numerical and experimental data, and then applied to the structural characterization of two main rotor blades, designed for ultra-light helicopter applications.

Assessment of Helicopter Pilot-in-the-Loop Models

The aim of this paper is the evaluation of several pilot models found in the literature, suited for helicopter pilot-assisted and pilot-induced oscillations analyses. Three main topics are discussed: (i) sensitivity of rotorcraft-pilot couplings simulations on the application of the different pilot models available in the literature; (ii) effect of vehicle modeling on active pilot modeling; (iii) effects of interactions between active and passive pilot models. The focus is on hovering flight, where a specific adverse rotorcraft-pilot coupling phenomenon, the vertical bounce, may occur. Pilot models are coupled with a comprehensive aeroservoelastic model of a mid-weight helicopter. The numerical investigations are performed in frequency domain, in terms of eigenanalysis and frequency response analysis.

Helicopter noise footprint prediction in unsteady maneuvers

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.

Structural characterization of rotor blades through photogrammetry

This paper deals with the use of photogrammetry for the experimental identification of structural and inertial properties of helicopter rotor blades4. The identification procedure is based upon theoretical/numerical algorithms for the evaluation of mass and flexural stiffness distributions which are an extension of those proposed in the past by Larsen, whereas the torsional properties (stiffness and shear center position) are determined through the Euler–Bernoulli beam theory. The identification algorithms require the knowledge of the blade displacement field produced by known steady loads. These data are experimentally obtained through photogrammetric detection technique, which allows the identification of 3D coordinates of labeled points (markers) on the structure through the correlation of 2D digital photos. Indeed, the displacement field is simply evaluated by comparing the markers positions on the loaded configuration with those on the reference one. The proposed identification procedure, numerically and experimentally validated in the past by the authors, has been here applied to the structural characterization of two main rotor blades, designed for ultra-light helicopters. Strain gauges measurements have been used to assess the accuracy of the identified properties through natural frequencies comparison as well as to evaluate the blades damping characteristics.

Helicopter Vibratory Loads Alleviation through Combined Action of Trailing-Edge Flap and Variable-Stiffness Devices

The aim of this paper is the assessment of the capability of controllers based on the combined actuation of flaps and variable-stiffness devices to alleviate helicopter main rotor vibratory hub loads. Trailing-edge flaps are positioned at the rotor blade tip region, whereas variable-stiffness devices are located at the pitch link and at the blade root. Control laws are derived by an optimal control procedure based on the best trade-off between control effectiveness and control effort, under the constraint of satisfaction of the equations governing rotor blade aeroelastic response. The numerical investigation concerns the analysis of performance and robustness of the control techniques developed, through application to a four-bladed helicopter rotor in level flight. The identification of the most efficient control configuration is also attempted.