Autorotation design and simulation for a small-scale helicopter

Safety of manned and unmanned aircraft is strictly related to the capability of managing emergency situations. In the case of helicopter engine failure, the autorotation manoeuvre represents a valid possibility for a safe emergency landing. Such manoeuvre is made of two different main phases: 1) the steady descent, where the rotor angular rate is kept in a proper range and the helicopter descends with constant velocity, and 2) the final flare, where the kinetic energy stored in the main rotor is used to generate a braking force, so to reduce the vertical velocity to a minimum value. For a manned piloted helicopter, experience and piloting skills of the pilot are mandatory for performing a safe emergency landing. For unmanned rotorcraft, since the remote pilot does not have the direct perception of linear accelerations and attitude motion, the remote autorotation is indeed an extremely hazardous task. For this reason, there is a significant interest in the design of a control algorithms allowing a remotely piloted helicopter to automatically perform the autorotation manoeuvre, which is a crucial feature for all missions over populated areas and/or for all aircraft carrying expensive payloads. In this paper, a preliminary investigation on steady descent conditions in autorotation, and a first design of a complete autorotation maneuver has been made through a model-based design approach. Also, a closed loop control system has been developed, to perform the two main phases of autorotation. Simulations results show the suitability of the proposed approach for a wide range of initial conditions (altitude and advancing velocity).