Distributed electric propulsion and vertical takeoff and landing has recently opened a new design space for urban air mobility. However, the use of multiple rotors operating in close proximity introduces complicated aerodynamic interactions that are not well understood, are not captured through conventional design tools, and need to be addressed in the conceptual design stage. This study investigates the accuracy of the viscous vortex particle method (VPM) in modeling rotor-on-rotor aerodynamic interactions in a sideby-side configuration as encountered in tilt-rotor, quadrotor, and distributed propulsion aircraft. The VPM approach has the potential to enable the use of mid/high fidelity models capturing multirotor interactions during conceptual design. Validation of the individual rotor is presented in both hovering and forward-flight configurations at both low and high Reynolds numbers. Validation of the hovering multirotor is then presented, followed by a detailed parametric study of rotor-to-rotor interactions during hover and forward flight, constructing the response surface of thrust, torque, and propulsive efficiency as a function of operational parameters.

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Recent developments in electric aircraft technology have enabled the use of distributed propulsion for the next generation of vertical lift vehicles. However, the ability to rapidly assess the performance of these design concepts, with sufficient fidelity, is a current weakness of this nascent industry. This paper explores the capacity of the viscous Vortex Particle Method (VPM) to model wake interactions found in distributed propulsion. The elements of the vortex particle method are summarized, and a new approach for the calculation of vortex stretching through the complex-step derivative approximation is presented. Preliminary validation is performed on vortex ring cases resembling the fundamental dynamics encountered in propeller wakes. Unsteady wake dynamics of individual propellers are successfully modeled, replicating the instabilities that lead to vortex breakdown as observed experimentally. Comparing the method with results from momentum theory, it is shown that VPM is consistent with theoretical values of near and far field induced velocities, and a notable feature is its ability to model near/far field transition. Furthermore, VPM is able to fully characterize induced velocities across the entire wake, from the stable region where momentum theory operates, through instability transition and eventual vortex breakdown. The simulation of a multirotor configuration of two tip-to-tip propellers is shown, displaying the capacity of VPM to model wake mixing. The results presented here are intermediate steps in the development of a mid-fidelity modeling tool for the early design stages of distributed-propulsion electric aircraft.

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