Abstract: A displacement mechanism for effecting both rotary movement of a movable member (for example a fin) with respect to a fixed member (for example a flight vehicle body) from a first, inoperative position to a second, operative position about a longitudinal axis and axial movement of the movable member along that axis to effect subsequent locking in the second position, comprises a fluid operated vane motor to effect rotation and a fluid operated piston/cylinder arrangement to effect the axial movement. Preferably the vane motor and the piston/cylinder arrangement are closely integrated one with another.