Experimental Investigation of the Effect of Azimuthal Spacing on the Performance of a Stacked Rotor in Hover and Forward Flight

The unconventional configuration of a 2 × two-bladed stacked rotor with a diameter of 0.82 m is studied experimentally throughout this paper. With the rotational speed kept constant at 2453 RPM and the dimensionless axial spacing fixed at 0.06, the main objective is to assess the effect of azimuthal spacing across multiple configurations in forward flight, varying the shaft angle and freestream velocity. First, an analysis of the baseline rotor in forward flight is presented, featuring four blades evenly spaced in a single plane. This is followed by results for the stacked rotor in hover flight, revealing a consistent trend with the literature: a low-performance region offering lower blade loading values for smaller azimuthal spacings, when both rotors closely overlap, and a region of increased performance for larger azimuthal spacings in both positive and negative directions. Most azimuthal spacings exhibit higher performance relative to the baseline rotor, with a maximum positive change of 7.82%. Finally, a performance comparison between the baseline rotor and the stacked rotor in forward flight reveals a similar trend to hover flight regarding the low-performance region for small azimuthal spacings, but with a less significant region where the stacked rotor outperforms the baseline rotor. Moreover, the vibratory loads of the stacked rotor depend on the azimuthal spacing, varying from a two-per-revolution harmonic at an azimuthal spacing of 0° to a four-per-revolution harmonic at 90°, resulting in higher vibratory loads transmitted to the rotor hub when the blades of both rotors are unevenly spaced.