Experimental Study Of Axial Spacing Effects On Stacked Rotor Performance In Hover And Forward Flight

Abstract

The dual co-rotating coaxial rotor, referred to as a stacked rotor, is an unconventional configuration featuring blades arranged in multiple planes with unequal azimuthal spacing. This study presents an experimental investigation of a 0.82 m diameter stacked rotor operating at a constant rotational speed of 2468 RPM. The aim is to assess the effects of azimuthal spacing and axial spacing on system performance in terms of blade and power loading. Azimuthal spacing was varied across the full range, with finer resolution near blade overlap (0? ), and two dimensionless axial spacings were tested: a smaller value of 0.06 and a larger value of 0.115. In hover, the larger axial spacing consistently outperformed the smaller one in blade loading and power loading, while showing reduced sensitivity to azimuthal spacing due to weaker aerodynamic interference between rotors when azimuthal spacing is varied. Compared to the baseline single rotor, the larger spacing exceeded its performance across all azimuthal spacings, whereas the smaller spacing exhibited a low-performance region near blade overlap. In forward flight, the percentage change in blade loading relative to the baseline generally decreased compared to hover, yet remained notably positive near an azimuthal spacing of ±90? . However, this configuration typically generated greater drag than the baseline in that region, while the aerodynamic balance shifted in favor of the stacked rotor, producing less drag and higher power loading, when azimuthal spacing approached 0?. Although the rotor was not trimmed for lift or propulsive force, the results indicate that in forward flight, larger axial spacing can match baseline performance when blades are closely aligned in azimuth, whereas smaller spacings perform better at larger azimuthal separations.