Hub Mast Rotor blades Uses Rotor System Due to the unique operating characteristics of the helicopter— its ability to take off and land vertically, to hover for extended  periods of time, and the aircraft’s handling properties under  low airspeed conditions—it has been chosen to conduct tasks  that were previously not possible with other aircraft or were  too time- or work-intensive to accomplish on the ground.  Today, helicopters are used for transportation, construction,  firefighting, search and rescue, and a variety of other jobs  that require its special capabilities.  [Figure 1-3] The helicopter rotor system is the rotating part of a  helicopter that generates lift. A rotor system may be mounted  horizontally, as main rotors are, providing lift vertically; and  it may be mounted vertically, such as a tail rotor, to provide  lift horizontally as thrust to counteract torque effect. In the  case of tilt rotors, the rotor is mounted on a nacelle that  rotates at the edge of the wing to transition the rotor from a  horizontal mounted position, providing lift horizontally as  thrust, to a vertical mounted position providing lift exactly  as a helicopter. The rotor consists of a mast, hub, and rotor blades.  [Figure 1-4] The mast is a hollow cylindrical metal shaft which extends  upwards from and is driven by the transmission. At the top  of the mast is the attachment point for the rotor blades called  the hub. The rotor blades are then attached to the hub by  several different methods. Main rotor systems are classified  according to how the main rotor blades are attached and  move relative to the main rotor hub. There are three basic  classifications: semirigid, rigid, or fully articulated, although  some modern rotor systems use an engineered combination  of these types. All three rotor systems are discussed with  greater detail in Chapter 4, Helicopter Components, Sections,  and Systems.  With a single main rotor helicopter, a torque effect is created  as the engine turns the rotor. This torque causes the body of  the helicopter to turn in the opposite direction of the rotor  (Newton’s Third Law: Every action has an equal and opposite  reaction, as explained in Chapter 2, Aerodynamics of Flight).  To eliminate this effect, some sort of antitorque control must  be used with a sufficient margin of power available to allow  the helicopter to maintain its heading and prevent the aircraft  from moving unsteadily. The three most common controls  used today are the traditional tail rotor, Fenestron (also called  a fantail), and the NOTAR$^{®}$. All three antitorque designs will  be discussed in Chapter 4, Helicopter Components, Sections,  and Systems.  Figure 1-3.  The many uses for a helicopter include search and rescue  (top), firefighting (middle), and construction (bottom). Figure 1-4.  Basic components of the rotor system.  1-3