Finite State Coaxial Rotor Inflow Model Improvements via System Identification

For real-time rotor inflow calculations, the finite state inflow models are commonly used due to the fact that they are more computationally efficient compared to CFD models. For coaxial rotor configuration, the finite state Active-Receiving Rotor Inflow Model (ARRIM) developed from potential flow theory, was shown to correctly predict power variations with rotor thrust in hover. However, potential flow theory cannot capture the complex aerodynamic interactions between upper and lower rotors in forward flight. In order to address this problem, a system identification approach is used to quantify both rotors' mutual interference effects using results from GT-Hybrid, a high-fidelity free-wake model. There are significant differences in wake skew angles predicted by GT-Hybrid and those calculated using momentum theory. Corrections needed in ARRIM influence coefficient matrix (L-matrix) in capturing wake distortions due to rotor-to-rotor flow interactions are identified. The identified results show a linear correlation between the required corrections to the wake skew angle and advance ratio. ARRIM with the identified wake skew angle and L-matrix corrections is implemented in FLIGHTLAB R and is seen to improve its inflow predictions.