A collapsible flying device , comprising: a drive motor ; a drive shaft rotatingly driven by the drive motor ; a rotor hub extending from the drive shaft to be rotated therewith by the drive motor ; and at least two rotor blades coupled to the rotor hub enabling pivoting of each of the at least two rotor blades through a pivot range between a collapsed position in which the rotor blade is oriented towards the rotor hub , and an expanded position in which the rotor blade is oriented away from the rotor hub ; and at least two biasing elements , each biasing element being coupled to one of the at least two rotor blades , the biasing elements biasing the at least two rotor blades toward the collapsed position when the at least two rotor blades are in a first section (PR1) of the pivot range (PR) adjacent to the collapsed position and biasing the at least two rotor blades toward the expanded position when the at least two rotor blades are in a second section (PR2) of the pivot range (PR) adjacent to the expanded position, wherein each of the at least two rotor blades is stable in the collapsed position and the expanded position when the drive shaft is rotated at a constant rotational speed; characterised in that wherein each of the biasing elements is an elastic member connected to a biasing point on one of the at least two rotor blades and a corresponding base point on the rotor hub , wherein a distance between the corresponding base point and the biasing point has a local maximum when the one of the at least two rotor blades is between the first section (PR1) and the second section (PR2) of the pivot range (PR). a drive motor ; a drive shaft rotatingly driven by the drive motor ; a rotor hub extending from the drive shaft to be rotated therewith by the drive motor ; and at least two rotor blades coupled to the rotor hub enabling pivoting of each of the at least two rotor blades through a pivot range between a collapsed position in which the rotor blade is oriented towards the rotor hub , and an expanded position in which the rotor blade is oriented away from the rotor hub ; and at least two biasing elements , each biasing element being coupled to one of the at least two rotor blades , the biasing elements biasing the at least two rotor blades toward the collapsed position when the at least two rotor blades are in a first section (PR1) of the pivot range (PR) adjacent to the collapsed position and biasing the at least two rotor blades toward the expanded position when the at least two rotor blades are in a second section (PR2) of the pivot range (PR) adjacent to the expanded position, wherein each of the at least two rotor blades is stable in the collapsed position and the expanded position when the drive shaft is rotated at a constant rotational speed; characterised in that wherein each of the biasing elements is an elastic member connected to a biasing point on one of the at least two rotor blades and a corresponding base point on the rotor hub , wherein a distance between the corresponding base point and the biasing point has a local maximum when the one of the at least two rotor blades is between the first section (PR1) and the second section (PR2) of the pivot range (PR). <EOS>