Performance and Acoustic Measurements of a Stacked Rotor with Unequal Radii

The loads and acoustics of a coaxial, corotating, or stacked rotor with unequal upper and lower rotor radii in hover are experimentally investigated. First, the loads and noise of an isolated two-bladed 1-m radius rotor are presented. Then, the effect of blade tip vortex interactions on the noise of a stacked rotor with equal upper and lower rotor radii are examined. It was previously observed that the miss distance, defined as the distance between the lower rotor blade and upper rotor tip vortex, correlates with the broadband noise levels. These results indicated that shortening of the lower rotor radius to the location where the upper rotor tip vortex crosses the lower rotor plane could reduce broadband noise. The loads and acoustics of a stacked rotor configuration (axial spacing of 0.73 rotor blade chords and azimuthal spacing of 90°) were measured, where one rotor had 90% of the other's radius, at a constant thrust of 150 lbf and a tip Mach number of 0.42 (476 ft/s). Compared to the equal-radii stacked rotor, the shortened upper rotor radius had negligible changes in noise and power, and the shortened lower rotor radius resulted in a 4.5 dB decrease in noise with a 2% increase in power. The noise reduction occurred over broadband-dominant frequencies and was due to removing vortex interaction, which generates turbulent boundary layer noise. Compared to a conventional four-bladed rotor at the same thrust, the shortened lower rotor radius, stacked configuration generated similar total noise with a lower dominant blade passage frequency, and required 6.8% less power.