A fault-tolerant controller is presented that is able to recover a quadrotor from an upset condition in the case of a single rotor failure. With only 3 rotors the quadrotor is not fully controllable anymore, however by using precession the quadrotor can be made controllable again in roll and pitch. A step-by-step recovery strategy is proposed in which controllability is actively recovered before recovery of attitude and altitude. Control allocation is based on the Incremental Non-Linear Dynamic Inversion (INDI) principle and takes rotor saturation into account by solving a constrained quadratic optimisation problem. The controller is validated in a real life test environment where the quadrotor is thrown into the air with only 3 propellers from which it has to recover using the techniques presented in this paper.

Mountain ranges all over Earth have long been tourist attractions for their monumental size, beautiful nature, clean air and possible leisure activities. Though attractive, mountain activities may form a threat to human safety. Search and rescue (SAR) teams are constantly stand-by and often have multiple rescue sorties per day. These missions are slow and dangerous for the involved personnel. In extremely rare cases rescue missions use helicopters for search by pilot eyesight. These mission types on average cost e3, 300 per hour, are dangerous for personnel, have a small endurance and have a large downtime. There is a clear need to improve effectiveness and safety of these rescue missions...