Enhancement and Validation of VPM-Derived State-Space Inflow Models for Multi-Rotor Simulation

Abstract

State-space inflow models have long been the standard for rotor wake modeling for flight dynamics and control simulation. As rotorcraft design continues to trend toward Future Vertical Lift (FVL) multi-rotor configurations in order to overcome the limitations of traditional design, the use of state space inflow models must similarly evolve to capture the complex aerodynamic interactions inherent to these new rotorcraft configurations. There is a demand for a state-space inflow model that accurately captures the significant aerodynamic interactions that occur between multiple rotors, ducted fans, wings, and complex airframes that preexisting inflow formulation fails to address. This paper discusses the ongoing effort to establish a robust methodology for deriving a state-space inflow model suitable for FVL applications from first-principle based viscous Vortex Particle Method (VPM) using the CIFERR system identification tool. The paper focuses on areas of enhancement that expand the identified inflow model's accuracy and usefulness as a tool for flight dynamics simulation of multi-rotor configurations. First, the effects of wake distortion due to rotor tip-path plane (TPP) rotation are thoroughly investigated for a co-axial configuration across a number of airspeeds and implemented into the inflow model formulation for improved simulation accuracy. Next, the rotor interference on the aerodynamic surfaces and the fuselage is explored, with attention placed on addressing this interference as a unified formulation. For the rotor interference on fuselage (3-D body) estimation, investigation of improvement through multi-point sampling is performed. The paper also examines the control design application through constructing the linear time invariant aircraft model with integrated state-space inflow model and verifying the accuracy of gain and phase margins and crossover frequencies. For each of these areas of investigation, the impacts on precision flight dynamics simulation are assessed through frequency and time domain response analysis and comparison to the simulation with first principle based VPM. The validation results of the predicted response data show excellent agreement with VPM simulation and further justify this method of inflow model identification as an effective tool for multi-rotor and FVL applications.