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Linear Inflow Model Synthesis for Advanced Rotorcraft Configurations


Current linear rotorcraft flight dynamics models are dependent on finite-state inflow theory based on potential flow modeling at the rotor plane [1]. These inflow models have few parameters and are readily available in linear state-space form, making them easy to implement in flight dynamic models for stability assessment and control system design studies. These types of models have been developed for [2] and extensively used in [3] modeling single main rotor helicopters. The future Army rotorcraft fleet will include configurations beyond the traditional single main rotor/trail rotor helicopters (e.g., tiltrotors and compounds) and advances in simulation modeling are required. Physics-based inflow models (e.g. free-vortex wake models, CFD, VPM, etc.) provide a more accurate and wholly generic representation of the rotor wake and can capture interference effects between the rotor and other rotors [4], other lifting surfaces, the fuselage, and the ground [5]. Rotor interference effects become critical when the wake of one rotor system is immersed in another, such as in coaxial helicopters. The physics-based models are nonlinear and so are limited in their use in flight dynamics and control studies. State-space inflow models of coaxial configurations have begun to be developed, but validation data is limited and needs to be improved in forward and maneuvering flight [6, 7]. A method to numerically determine a real-time linear state-space model directly from physics-based models that captures the key interference effects would provide a pathway for accurate model development and improved stability analysis and control law design of advanced configurations. The goal of the effort will be to develop and validate a methodology to determine linear time-invariant (LTI) parametric inflow models from physics-based models for any rotorcraft configuration. The parametric model should include interference effects of other aircraft components. The inflow model should be easily included in existing rotorcraft simulation tools. The methodology should be applicable to not only single main rotor helicopters, for which linear inflow models exist, but be generic and easily extendible to coaxial, compound, or other rotorcraft configurations. The validation should consist of wake and full aircraft dynamics comparisons (e.g. time histories, frequency responses, trim analysis, etc.) with higher-order models, the non-linear simulation from which the model was derived, and experimental/flight data, if available. The methodology developed would reduce the iterative nature of control law design and lead to cost savings and increased efficiency in the development process of aircraft flight control systems. The feasibility for determining accurate linear inflow models for advanced rotorcraft configurations and an initial model structure will be established in Phase I. If the ability is confirmed, Phase II will fully develop the capability. The primary interests of this study are (1) linearization capability, (2) accuracy, and (3) scalability of parameter representations. There are broad dual-use applications for a linear representation of advanced rotorcraft inflow dynamics. Major military rotorcraft manufactures all have multi-rotor aircraft and other advanced configurations in use or development. The inflow modeling of these configurations has proven to be problematic and a key barrier to proper predictive capability of the aircraft flight dynamics characteristics. PHASE I: Determine the feasibility to extract a real-time parametric linear inflow model for advanced rotorcraft configurations from a physics-based model that includes interference effects of other aircraft components. Propose an initial model structure and compare to other inflow model types and document the advantages and disadvantages of each method. Describe the methodology that will be used to obtain the model and all simplifications or assumptions needed to produce the linear model. Describe the process that will be used to obtain time history and frequency response data for validation. PHASE II: Develop and validate a software tool that implements a numerical method to generate parameterized LTI state-space inflow models for use in rotorcraft flight dynamics analysis. The tool should require generic inputs, including aircraft states and control inputs, and be able to correctly predict the inflow at the rotor(s) in both hover and forward flight. The model should be applicable to a range of physics-based models. Demonstrate the derived model in a flight dynamics simulation of a representative test aircraft. Compare results of this model with prior work using finite-state and higher-order models, including predictions of transient response with flight test data. Model predictive capability will be evaluated by comparing time histories and frequency responses of the high-order and linear models. Time histories should provide an error of 25% or less, and the frequency responses should have costs of less than 100 [8]. Provide documentation for incorporation of this model with existing flight dynamics modeling tools. PHASE III: Transition the tool for commercial use with military and industry customers. Since the developed tool is generic, it will be compatible with each customer’s existing physics-based inflow models. The government will use the tool to develop accurate flight dynamics models to support handling qualities (through piloted simulation) and control law development for advanced rotorcraft configurations. Industry customers could use the tool for similar purposes.
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