This video illustrates our recent work on the algebraic conditions to obtain static hovering (i.e., with zero linear/angular moments) and on our method to control any generic multi-rotor with static hovering capability, even when propellers are tilted and the force/moment actuation is coupled.
We applied our method to rotor-failure flight of an hexarotor, which, in its standard form, is typically (and mistakenly) considered a fail-safe platform by definition.
Differently from other failsafe methods in the literature, which force the vehicle to follow dynamic hovering limit cycles with the possibility to collide with the surrounding environment, our method allows to keep the platform in static hovering, thus in a totally safe condition.
Details on the method are given in 
- , “Control of Statically Hoverable Multi-Rotor Aerial Vehicles and Application to Rotor-Failure Robustness for Hexarotors”, in 2017 IEEE Int. Conf. on Robotics and Automation, Singapore, 2017.