Helicopters operating in high-speed forward flight conditions frequently generate High-Speed Impulsive (HSI) noise, posing significant challenges for effective noise control and limiting their operational flexibility. To address this critical issue, this study proposes a novel active noise reduction approach that dynamically adjusts rotor diameter, aiming to alleviate the generation and intensity of HSI noise. Utilizing the Chinese Laboratory of Rotorcraft Navier-Stokes (CLORNS) solver combined with the Ffowcs Williams-Hawkings (FW-H) acoustic analogy, aerodynamic and acoustic characteristics of the AH-1G helicopter rotor undergoing diameter adjustments were comprehensively simulated. This study methodically investigated key factors influencing the acoustic field, including the scale of rotor diameter retraction and the speed of retraction. The results clearly demonstrate the effectiveness of the proposed approach, achieving significant noise reductions ranging from 4 dB to 7 dB at crucial operational points while ensuring aerodynamic stability. Parametric analyses further reveal that the scale of rotor diameter retraction is a decisive factor in noise mitigation, and slower retraction speeds are beneficial, contributing to smoother flow transitions and diminished sound pressure fluctuations. Moreover, to overcome potential noise amplification induced by rotor control actions, this research introduces a compensation method incorporating a dynamic parameter adjustment mechanism. The developed control strategy is particularly effective in eliminating the noise growth observed in regions characterized by abrupt noise increases. Overall, this study provides an innovative and practical solution for helicopter noise management, substantially enhancing operational flexibility and acoustic comfort in high-speed flight conditions.

Coaxial counter-rotating propellers exhibit non-negligible aerodynamic and acoustic challenges due to the close spacing between the two rotors and significant mutual aerodynamic interference. This study establishes a Bayesian optimization-based aerodynamic shape optimization framework for high-speed coaxial contra-rotating propeller blades, aiming to enhance aerodynamic efficiency under cruise conditions. Within this framework, the multiple reference frame (MRF) method coupled with Reynolds-averaged Navier-Stokes (RANS) equations was adopted to accurately evaluate the aerodynamic characteristics, including the blade-to-blade aerodynamic interaction between the blades, air compressibility effects at high rotational speeds, and the influence of complex blade geometry. A Kriging surrogate model was developed to map the relationships between design parameters and aerodynamic responses, with a composite infill criterion implemented to accelerate convergence. Aerodynamic-structural integrated parameterization was implemented, independently defining chord length, twist distribution, and sweep configuration for both forward and aft propellers. The optimized configuration demonstrates a comprehensive aerodynamic efficiency of 0.8413 when accounting for spinner and rotating shaft effects, representing a 1.53% improvement over the baseline design. Load distribution analysis reveals that the peak loading positionsvare located at 0.75R for the forward propeller and 0.7R for the aft propeller. The optimized design scheme obtained in this study elucidates the geometric characteristics of high-efficiency and high-speed coaxial counter-rotating propeller blades.

It is difficult to simulate the strong interference and serious flow separation of Fenestron by the CFD method based on the widely used RANS equation, and the detailed experimental data, which could be used to validate the aerodynamic and noise numerical methods, is unavailable. The experimental investigation on the aerodynamic and noise characteristics of Fenestron is carried out. In view of the complex internal flow field in duct and the relative motion between the stationary duct and the rotating rotor, a comprehensive aerodynamics and pressure measurement scheme is designed based on the bottom support rig. In this measurement scheme, the thrust generated by the rotating rotor can be measured by the rotating shaft balance, the thrusts from Fenestron are measured by the external balance and the pressures on duct inner wall are monitored by a pressure measuring system. To fully capture the noise directionality of Fenestron, a series of noise observers located at an arc array are arranged. In terms of the Fenestron test models, the baseline model, the performance improvement model based on the high-performance tail rotor and the noise reduction model based on the non-uniform blade distribution are designed respectively. By the designed measurement scheme, aerodynamic forces and pressure distributions and noise were measured for the three different Fenestron models. The results show that the aerodynamic thrusts of the tail rotor and duct increase greatly and the noise increases slightly for the performance improvement model because of the larger aerodynamics. The rotor aerodynamic performance of the noise reduction model is reduced, but the modulation effect of the tail rotor improves the forces of the duct. The noise radiated by the noise reduction model is reduced and a good noise reduction result is obtained in frequency domain.