A Free Wake Linear Inflow Model Extraction Procedure for Rotorcraft Analysis

Linearized inflow models have been used to represent dynamic wake effects for control law development and flight dynamics simulation of conventional main rotor/tail rotor helicopters. For advanced rotorcraft configurations based on compound-coaxial and multi-rotor distributed propulsion, rotor-on-rotor and rotor-on-wing interactions lead to a breakdown of classical dynamic inflow theory. An approach for extracting low-order inflow models from comprehensive aerodynamic analyses has been investigated as part of continuing research and development toward an automated procedure for inflow/wake model identification. This paper describes the initial results for inflow model extraction from a full-span free wake analysis in hover, forward flight, and maneuvering flight conditions. Emphasis has been placed on the model structure formulation to yield identified inflow models that capture the critical dynamics associated with the rotor wake and induced velocity without being over-parameterized. Model structures previously reported in the literature do not capture all wake dynamics observed in a free wake model, some of which impact the coupled rotor-body response characteristics. Wake distortion effects due to tip path plane angular rate, off-rotor interference, and coaxial rotor interactions are examined herein.