This paper discusses the development of flight dynamic model and control law to enable autonomous autorotative landing for small unmanned helicopters. For this, a novel modeling and system identification approach based on acceleration prediction model is proposed for accurate simulation of the plant dynamics during the unpowered descent flight of the unmanned helicopter. The flight dynamics model identified using the flight test data obtained through manual autorotation flights of the unmanned helicopter is implemented on a real-time computer to create a hardware-in-the-loop simulation (HILS) setup to design and tune a PID controller in the HILS environment to facilitate safe landing of the helicopter in the event of unpowered flight. This controller is then validated on a small unmanned helicopter using open source PixHawk 2.0 flight controller board to demonstrate autonomous autorotative landing starting from different height and velocity conditions. For vertical autorotation experiments performed, the controller is observed to maintain steady autorotative descent through application of suitable negative collective to maintain rotor RPM and then arrests the rate of descent by flaring at the appropriate height, which is defined as a function of descent rate, to ensure landing at near zero vertical speed.
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