PRINCIPLES OF HELICOPTER FLIGHT UNDER VARIOUS CONDITIONS ARE REVIEWED, GIVING SPECIAL ATTENTION TO THE OPERATION OF THE MAIN ROTOR. A BRIEF HISTORY OF HELICOPTER DEVELOPMENT IS PRESENTED, TOGETHER WITH A SUMMARY OF THE MAIN COMPONENTS OF A HELICOPTER AND A CLASSIFICATION OF THE VARIOUS TYPES OF HELICOPTERS. THE CHARACTERISTICS OF THE MAIN ROTOR AND ITS OPERATION DURING AUTOROTATION AND DURING AXIAL AND OBLIQUE FLOW ARE CONSIDERED. ALSO CONSIDERED ARE VERTICAL AND HORIZONTAL FLIGHT, ALTITUDE GAIN AND DESCENT, TAKEOFF AND LANDING, EQUILIBRIUM, STABILITY, AND CONTROLLABILITY, TAKING INTO ACCOUNT THE AERODYNAMIC FORCES ACTING ON THE HELICOPTER DURING THE VARIOUS MANEUVERS.