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 rotors of 0.73 chords were measured. The differential collective pitch of the rotor system, de ned as half the difference between the upper rotor pitch and lower 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, the acoustic solver PSU-WOPWOP, and the Brooks, Pope, and Marcolini method implemented in ANOPP2. Increased sensitivity to azimuthal spacing was measured for large, negative differential collectives, resulting in a 76.5% change in total rotor thrust and a 46% change in total rotor power over 11.25 ° change in azimuthal spacing. The trends in individual and total system thrust were predicted with good accuracy, but the total power was overpredicted. Overall sound pressure level decreased by 3.5 dB at – 2° differential collective and 90° azimuthal spacing due to reduced broadband levels, indicating a possible quiet mode of operation for this type of rotor. By assessing the highest performance and lowest noise, an optimum rotor con guration was observed at an azimuthal spacing of – 45° and differential collective of 2°. Acoustic predictions captured tonal and broadband noise trends in most conditions. Conditions where broadband noise was underpredicted could be a result of vortex interactions not modeled by the broadband simulations.