Aerodynamic and Aeroacoustic Effects of Variable Rotor Speed in a Coaxial Rotor System

Coaxial helicopters offer performance advantages for high-speed flight, but their tightly coupled rotors generate complex aerodynamic interactions and intense noise. This study investigates the potential of Variable Rotor Speed (VRS) to improve the aerodynamic efficiency and reduce the noise of a coaxial rotor system, using the Sikorsky XH-59A as a reference. A free-wake vortex lattice method, validated against test data, was coupled with an acoustic prediction model to evaluate rotor performance and noise across forward speeds from 80 to 160 kts and rotor speeds from 85 to 100% of nominal. The analysis reveals that moderate RPM reductions (to about 90–95% of nominal) can enhance aerodynamic performance, yielding net rotor power savings by significantly lowering profile drag, while only slightly increasing induced power to maintain thrust. However, an excessive RPM drop ( ≈ ≈ 85% at high speed) expands the reverse-flow region and raises induced power. VRS is also shown to mitigate aeroacoustic noise: at 80 kts, reducing RPM by 15% decreases blade–vortex interaction noise by approximately 3 dB, and at 150 kts, it still achieves a 1–2 dB noise reduction by suppressing unsteady blade loading during blade-crossover events. Furthermore, increasing the inter-rotor spacing provides additional noise abatement (by weakening wake–blade interactions) with minimal impact on power. The findings demonstrate that a judicious VRS strategy, especially in combination with optimized rotor spacing, can significantly improve coaxial rotorcraft efficiency and acoustic signature.