Noise analysis of variable-speed rotor technology on the coaxial lift-offset helicopter

The impact of variable-speed rotor technology on the aerodynamic and aeroacoustic characteristics of a coaxial lift-offset helicopter was investigated using a numerical analysis approach. The free wake vortex lattice method was utilized to predict aerodynamic performance, while the Ffowcs-Williams Hawkings acoustic analogy was employed to evaluate the noise impact. The Harrington coaxial rotor was used as the baseline configuration. Results show that reducing the rotational speed, in combination with lift-offset, leads to a decrease in overall power required during forward flight by lowering both collective and cyclic pitch. This is primarily due to a reduction in profile power across the speed range. However, excessive rotor deceleration can lead to an expanded reverse flow region, increasing induced power at high forward speeds. From an acoustics perspective, the reduced rotor speed mitigated loading noise radiated toward the ground, especially in the region 40-60 degrees below the rotor plane, achieving a maximum 2.6 dB reduction at the evaluated observer location. This study indicates that the technology can offer advantages for both performance and noise in forward flight conditions.