Vortex Particle Analysis of Side-by-side Overlapping Rotors in Forward Flight

Abstract

A Vortex Particle method was implemented to examine the wake structure and rotor performance of a pair of fixed pitch variable RPM side-by-side overlapping rotors in hover and forward flight conditions up to 50 kts. The side-by-side configuration is geometrically differentiated from traditional tandem rotors by the freestream wind direction. Due to the mutual rotor aerodynamic interaction, a momentum theory or blade element method would not accurately capture the rotor performance and therefore an unsteady wake method is necessary, however, the results shown here can inform these lower fidelity methods used for simpler sizing or performance analyses. The analysis varied the rotor shaft lateral separation distance, vertical shaft separation distance, twist and taper profile, rotor speed, and forward airspeed. Decreasing the lateral distance between the two rotors increases the induced velocity in localized regions on the blades of the lower rotor, reducing angle of attack and introducing an N/rev component to the instantaneous rotor thrust. Increasing the rotor overlap reduced mean rotor thrust by a maximum of 4.2% at 50 kts forward airspeed. Decreasing the vertical rotor separation caused a similar trend with higher induced velocity at the lower rotor plane. Similar trends occurred for both propeller-like and rectangular blades.