Fully coupled flight dynamics/CFD simulations of a helicopter with actuator disk models for the main rotor and tail rotor

The purpose of this study is to develop a coupled flight dynamics/CFD simulation tool of a helicopter capable of modelling the interactional aerodynamic effects between the tail rotor, main rotor, airframe, and surrounding environment. The code couples a multi-degree-of-freedom dynamic model of the helicopter with a Navier-Stokes CFD solver. A focus of this study is to develop an efficient and low-cost design tool for ship-based autonomous Vertical Take-off Unmanned Air Vehicle (VTUAV) systems. To improve efficiency, the method uses actuator disk coupling of the rotors and avoids full resolution of moving boundary conditions within the flow field. A generic UAV model (with properties similar to the MQ-8C unmanned helicopter) was developed and integrated with an autonomous flight controller. The controller flies the UAV along prescribed trajectories or holds it at fixed positions relative to a ship or other external objects. Actuator Disk Models (ADM) were used to embed the main rotor and tail rotor inflow aerodynamics in the CFD calculation. The CFD solver models the main rotor/tail rotor inflow, ground effect, impingement of rotor wakes on fuselage and empennage, and the effect of unsteady airwake from external objects. Fully coupled simulations were performed of a helicopter hovering in an open domain, of trimmed flight at prescribed wind speeds (to trim characteristics in forward flight), and of a time-accurate acceleration/deceleration maneuver in a 25-knot headwind. Finally, simulations of an approach/landing trajectory to a Navy frigate were performed, and the response of the rotorcraft and attitudes and controls were analyzed. The ADM predicted reasonable physical behavior in trim and complex flow environments, but there are some concerns about the accuracy and stability of using an ADM of tail rotor where the computational grid is coarse. The addition of the tail rotor inflow in the CFD calculations captured main rotor/tail rotor interactions, and predicted a reduced directional control margin compared to cases without ADM coupling of the tail rotor.