An Experimental Study of Stacked Rotor Tip Vortex Interaction in Hover

Measurements were made using flow visualization to track the blade tip vortices of a 1.108-m radius stacked rotor in hover. Vortex measurements of an isolated, two-bladed rotor were also made and correlated with the Landgrebe wake model. When a second rotor was introduced to the system, the interaction between the upper and lower rotor vortices resulted in deviations from the Landgrebe wake model. Changing the azimuthal spacing revealed large changes in the trajectory of the upper and lower rotor vortices, particularly at small, negative azimuthal spacings. The miss distance, or distance between a rotor blade and vortex, was measured. Positive miss distances where the lower rotor blade passed below the upper rotor tip vortex were associated with greater noise between 1 and 5 kHz, possibly from turbulent boundary layer noise or blade- vortex interaction noise. The greatest performance and lowest broadband noise corresponded with large miss distance and minimal vortex-vortex interaction, which occurred at an azimuthal spacing of - 45 degrees . The tip vortex trajectory, and thus miss distance, could be modified by utilizing differential collective. This resulted in a 4.5-dB reduction in broadband noise at negative differential collectives by allowing the lower rotor blade to pass above the upper rotor vortex and achieving negative miss distances. At azimuthal spacings where the miss distance was always negative, the airfoil self-noise dictated the broadband noise level.