The Effect of Phase Offset Angle for Coaxial Co-Rotating Rotors

The aerodynamic performance and noise generation of stacked co-rotating rotor systems are investigated in this paper. A test apparatus is employed which allows the rotor blade pitch and the separation distance between the two bladed rotors to be varied. Instrumented motors and a novel phase control system are used to vary the azimithal phase offset between the upper and lower rotors. This configuration also allows the upper and lower rotor thrust and torque to be measured independently. The phase offset is shown to have a strong effect on tonal noise generation at all separation distances, with the lowest noise achieved when the blades rotate out of phase with one another. The broadband noise also varies with phase offset; at close separation distances, the lowest noise broadband noise levels occur when the lower rotor leads the upper rotor. However, the phase offset for minimum broadband noise levels shifts linearly toward the upper rotor leading as the separation distance is increased. At a separation distance of 0.65 rotor radii, the phase offset is no longer effective in controlling broadband noise due to an apparent break down of the upper rotor wake. In general, the phase offset for lowest broadband noise is associated with the offset for the highest power loading for any separation distance. The power loading of the rotor system does not change in low speed edgewise flight, but both tonal and broadband noise increase and phase control is no longer effective in reducing broadband noise. Stacked rotor configurations with several different blade pitch settings are also compared to co-planar (conventional) two and four bladed rotors operating at the same total rotor system thrust. The stacked configurations all demonstrate a higher aerodynamic efficiency than the co-planar configurations, with the most efficient stacked configuration have the lower rotor blade pitch increased to balance the thrust across the upper and lower rotors. With these pitch settings, the broadband noise no longer varies significantly with phase offset angle. While this configuration also has the lowest noise levels of any stacked rotor configuration, the broadband noise is not significantly different than a four bladed co-planar design and the tonal noise levels are slightly higher.