The objective of this paper is to generate a desired flight path to be followed by an autonomous quad-rotor helicopter. The novel approach consists in the explicit formulation of both under-actuation constraints, as there is 6 degrees of freedom and only 4 inputs. A simple but efficient method of trajectory generation is proposed. In addition of classical trim trajectories, we propose first and second order maneuvers. Then, we study both possibilities of moving at a constant or a linearly varying velocity.