Lichten Award Paper: Experimental Measurements and Low-Order Modeling of Stacked Rotor Performance in Hover

Abstract

Coaxial, co-rotating ('stacked') rotors have been shown in the past to increase performance and decrease noise. Due to renewed interest in Urban Air Mobility, research of stacked rotors has increased. This paper reports an experimental study of 1.108 m-radii fixed-pitch stacked rotor with variable axial and azimuthal spacing performed to quantify effects of rotor geometry on total and individual rotor performance. Compared to a conventional, four-bladed rotor, figure of merit was found to increase by 6.4% at small axial spacings. A new rotor concept using azimuthal variation for thrust control was validated. It was observed that total thrust can be varied up to 17% through an azimuthal spacing change of 22.5°. Additionally, a low-order model code, Blade Interaction Prediction (BLIP), was developed to efficiently predict the thrust of closely-spaced rotor blades. BLIP couples vortex element method and blade element momentum theory to combine the effects of chord-wise circulation and rotor inflow. Excellent correlation was observed with experimental results and it was found that small azimuthal angles are most effective to vary total thrust.