Hybrid Unsteady Simulation of Helicopters: HUSH Shreyas Ananthan * and James D. Baeder † University of Maryland, College Park, MD Jayanarayanan Sitaraman ‡ National Institute of Aerospace, Hampton, VA Seonghyeon Hahn § and Gianluca Iaccarino ¶ Stanford University, CA HUSH (Hybrid Unsteady Simulation of Helicopters) is a computational framework developed to predict and analyze the rotor-wake aerodynamics, the blade structural dynamics, the rotor performance, and the re- sulting aeroacoustics of rotorcrafts operating over a range of flight conditions. The key to this approach is the coupled simulation of a high-fidelity computational fluid dynamics model, e.g., an unsteady RANS solver, with a comprehensive blade structural dynamics model to accurately simulate the aerodynamic environment in which the rotor blades operate. The coupled numerical model is solved iteratively along with an appropriate vehicle trim algorithm until a converged solution for the desired flight condition is obtained. The data exchange between the domain-specific solvers is facilitated using a Python-based library which provides the flexibility of coupling different computational codes using a standard set of interfaces with minimal modifications to the participating codes. The framework has been used to successfully analyze various rotor configurations, includ- ing traditional articulated and hingeless rotors (UH-60A, HART-II, model DNW rotor, etc.). More recently, the framework was used in the analysis of more exotic configurations such as bearingless rotors (MDART) and rotors with trailing edge flaps (SMART). The results from the simulations have been extensively validated with measurements from the UH-60A Airloads Program, the HART II test program, and the full-scale wind tunnel experiments of the MDART rotor. I. Introduction Reliable prediction of the highly complex aerodynamic operating enviroment of rotorcraft is difficult even for steady forward flight conditions. The difficulty arises from the necessity of capturing a combination of physical phe- nomena that include transonic and compressibility effects on the advancing blade, dynamic stall on the retreating blades and the interaction of the rotor blades with the returning vortex wake. Moreover the problem is highly aeroe- lastic with strong fluid structure coupling and demands a true multi-physics simulation. Despite various advances in individual disciplines, confident prediction of the rotorcraft aeromechanics and acoustics still remains a significant challenge to the rotorcraft community, and hinders the development of efficient, quieter rotors. Traditional aerodynamic models used in comprehensive rotorcraft simulations are commonly based on lifting line theory and incorporate simple empirical models to simulate the blade unsteady effects, and to account for the influence of the rotor wake. These models are strictly valid only in the flight regimes where the experimental data used to derive the empirical models were obtained. Thus, these models are severely limited in their capability to predict the wide range of aerodynamic effects encountered by the rotor blade. Designing and conducting reliable experiments for a wide range of flight conditions, especially maneuvers, is a very costly enterprise and may even be impossible. Numerical simulations provide a more cost-effective way of analyzing rotor aerodynamics. However, the development and analysis of a comprehensive rotor structural and aerodynamic model is not without its challenges. The primary challenge arises from the strong fluid-structure interaction that needs to be adequately modeled in the simulations. A full continuum dynamics treatment of the rotor structural and aerodynamic interaction is impractical. 1 A modular approach where partitioned domains interact via a common interface is preferable. This allows the individual solvers to use the most efficient, domain-specific solution technique in solving the governing equations. * Assistant Research Scientist, Dept. of Aerospace Engineering, University of Maryland, shreyas@umd.edu, AIAA Member † Associate Professor, Dept. of Aerospace Engineering, University of Maryland, College Park, MD, baeder@umd.edu, AIAA Member ‡ Research Scientist, NIA, 100 Exploration Way, Hampton VA, Jayanarayana.Sitaraman-1@nasa.gov, AIAA Member § Postdoctoral Researcher, Stanford University CA, hahn@stanford.edu ¶ Assitant Professor, Stanford University, CA, jops@stanford.edu 1 of 29 American Institute of Aeronautics and Astronautics 26th AIAA Applied Aerodynamics Conference 18 - 21 August 2008, Honolulu, Hawaii AIAA 2008-7339 Copyright © 2008 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.