Measurements and Modeling of Stacked Rotor Wake and Performance

Abstract:
Stacked co-rotating rotors offer a mechanically simple alternative to conventional coaxial counter-rotating systems, but their aerodynamic performance is strongly dependent on both axial and azimuthal blade spacing. This study experimentally and numerically investigates the effects of rotor spacing on the performance and wake structure of model-scale stacked rotors in hover. A dedicated test platform was developed to measure thrust, power, and phase-resolved 2D-3C particle image velocimetry flow fields for two-bladed stacked rotors over axial spacings of Δz/c=0.75 to 5 and azimuthal spacings of ϕ = 0° and 90°. Relative to isolated two- and four-bladed baseline rotors, the stacked configurations exhibited measurable variations in total hub loading and induced flow structure as a function of spacing. The flow field results show that changes in axial spacing alter the relative position of the lower rotor within the convected wake of the upper rotor, producing corresponding changes in inflow, effective angle of attack, and total thrust. Azimuthal spacing further modifies these trends by shifting the phase relationship between upper- and lower-rotor blade passages. To interpret these effects, a coupled blade element momentum theory and Blade Interaction Prediction model was developed. The model captures the primary trends in measured thrust and sectional loading, demonstrating that both wake convection and chordwise blade interaction are required to predict the aerodynamic behavior of closely spaced stacked rotors.

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