Performance and Acoustics of a Stacked Rotor with Differential Collective Pitch

Performance and acoustics of a 1.108 m radius stacked rotor in hover with an axial spacing between the upper and lower rotor of 0.73 chords were measured. The differential collective pitch of the rotor system, defined as half the difference between the lower rotor pitch and upper rotor pitch was varied along with the azimuthal spacing between the upper and lower blade sets. Coupled comprehensive analysis and acoustics simulations were conducted using the Rotorcraft Comprehensive Analysis System (RCAS), the acoustic solver PSU-WOPWOP, and the broadband noise module from The University of Maryland Acoustic Code (ACUM). Increased sensitivity to azimuthal spacing was observed for large, negative differential collectives, resulting in 76.5% change in thrust and 46% change in power over 11.25o change in azimuthal spacing. The trends in individual and total system thrust were predicted with good accuracy, but the total power tended to be overpredicted. Overall sound pressure level decreased by 3.5 dB at -2o differential collective and 90o azimuthal spacing due to reduced broadband levels, indicating a possible quiet mode of operation for this type of rotor. Acoustic predictions captured thickness and loading noise trends, but significantly underpredicted broadband levels.