An Acoustic Investigation of a Coaxial Helicopter in High-Speed Flight

The desire for a vertical takeoff and landing (VTOL) aircraft capable of high forward flight speeds is very strong. Compound lift-offset coaxial helicopter designs have been proposed and have demonstrated the ability to fulfill this desire but, with high forward speeds, noise is an important concern that needs to be addressed. The study in this paper utilizes a Rotorcraft Comprehensive Analysis System (RCAS) model of the XH-59 aircraft, in conjunction with a noise prediction code, PSU-WOPWOP, to computationally explore the acoustics of a lift-offset coaxial helicopter. Specifically, unique characteristics of the XH-59 coaxial helicopter noise are identified; and design features and trim settings specific to a compound coaxial helicopter are considered for noise reduction. At some observer locations, there is constructive interference of the coaxial acoustic pressure pulses, such that the two signals add completely. The locations of these constructive interferences can be altered though, by modifying the upper-lower rotor blade phasing, providing an overall acoustic benefit. Significant noise reduction (and power reduction) is possible by reducing rotor tip speeds – an option available because the coaxial rotor in a compound configuration does not need to provide all the propulsive force. A dual-swept tip blade also enables noise reduction at an in-plane, forward, target observer. Compete CFD analyses, coupled with PSU-WOPWOP, should be explored in the future for a comprehensive acoustic evaluation of lift-offset coaxial helicopter noise.