Descending Flight Simulation of Tiltrotor Aircraft at Different Descent Rates

Helicopters and tiltrotor aircrafts are known to fall into an unstable state called vortex ring state when they descend rapidly. This paper presents a six degrees of freedom descending flight simulation of a tiltrotor aircraft represented by the V-22 Osprey, considering the interaction between fluid and a rigid body. That is, an aircraft affects the surrounding flow field by rotating the rotors, and flies with the generated force as thrust. Similarly, an orientation of the airframe is controlled by aerodynamic force which is generated by manipulating the shape. This numerical analysis is a complicated moving boundary problem involving motion of an air-frame or rotation of rotors. As a numerical approach, the Moving Computational Domain (MCD) method in combination with the multi-axis sliding mesh approach is adopted. In the MCD method, the whole computational domain moves with objects in the domain. At this time, fluid flow around the objects is generated by the movement of the boundaries. In addition, this method removes computational space restrictions, allowing an aircraft to move freely within the computational space regardless of a size of a computational grid. The multi-axis sliding mesh approach allows rotating bodies to be placed in a computational grid. Using the above approach, the flight simulation at two different descent rates is performed to reveal a behavior of a tiltrotor aircraft and a state of the surrounding flow field.