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## Uses 

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] 

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## Rotor System 

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. 

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