Turbine Compressor Accessory Gearbox Transmission System Combustion Chamber The two-stage turbine section consists of a series of turbine  wheels that are used to drive the compressor section and  other components attached to the accessory gearbox. Both  stages may consist of one or more turbine wheels. The first  stage is usually referred to as the gas producer (N1 or NG)  while the second stage is commonly called the power turbine  (N2 or NP). (The letter N is used to denote rotational speed.) If the first and second stage turbines are mechanically  coupled to each other, the system is said to be a fixed turbine  (turboshaft). These engines share a common shaft, which  means the first and second stage turbines, and thus the  compressor and output shaft, are connected. The compressor may consist of an axial compressor, a  centrifugal compressor, or combination of the two. An axial compressor consists of two main elements: the rotor  and the stator. The rotor consists of a number of blades fixed  on a rotating spindle and resembles a fan. As the rotor turns,  air is drawn inward. Stator vanes are arranged in fixed rows  between the rotor blades and act as a diffuser at each stage  to decrease air velocity and increase air pressure. There may  be a number of rows of rotor blades and stator vanes. Each  row constitutes a pressure stage, and the number of stages  depends on the amount of air and pressure rise required for  the particular engine. A centrifugal compressor consists of an impeller, diffuser,  and a manifold. The impeller, which is a forged disc with  integral blades, rotates at a high speed to draw air in and  expel it at an accelerated rate. The air then passes through  the diffuser, which slows the air down. When the velocity  of the air is slowed, static pressure increases, resulting  in compressed, high pressure air. The high-pressure air  then passes through the compressor manifold where it is  distributed to the combustion chamber via discharge tubes. On most turbine assemblies used in helicopters, the first  stage and second stage turbines are not mechanically  connected to each other. Rather, they are mounted on  independent shafts, one inside the other, and can turn  freely with respect to each other. This is referred to as a  “free turbine.” When a free turbine engine is running, the  combustion gases pass through the first stage turbine (N1)  to drive the compressor and other components, and then past  the independent second stage turbine (N2), which turns the  power and accessory gearbox to drive the output shaft, as  well as other miscellaneous components. If the airflow through the compressor is disturbed, a  condition called surge, or compressor stall, may take effect.  This phenomenon is a periodic stalling of the compressor  blades. When this occurs, the pressure at the compressor  is reduced and the combustion pressure may cause reverse  flow into the compressor output. As the airflow through  the compressor is reduced, the air pressure then increases  temporarily correcting the condition until it occurs again.  This is felt throughout the airframe as vibrations and is  accompanied by power loss and an increase in TOT as the  fuel control adds fuel in an attempt to maintain power. This  condition may be corrected by activating the bleed air system  which vents excess pressure to the atmosphere and allows  a larger volume of air to enter the compressor to unstall the  compressor blades. The accessory gearbox of the engine houses all of the  necessary gears to drive the numerous components of the  helicopter. Power is provided to the accessory gearbox  through the independent shafts connected to the N1 and  N2 turbine wheels. The N1 stage drives the components  necessary to complete the turbine cycle, making the engine  self-sustaining. Common components driven by the N1  stage are the compressor, oil pump, fuel pump, and starter/ generator. The N2 stage is dedicated to driving the main  rotor and tail rotor drive systems and other accessories such  as generators, alternators, and air conditioning.  The transmission system transfers power from the engine to  the main rotor, tail rotor, and other accessories during normal  flight conditions. The main components of the transmission  system are the main rotor transmission, tail rotor drive  system, clutch, and freewheeling unit. The freewheeling unit  or autorotative clutch allows the main rotor transmission to  drive the tail rotor drive shaft during autorotation. In some  helicopter designs, such as the Bell BH-206, the freewheeling  unit is located in the accessory gearbox. Because it is part  of the transmission system, the transmission lubricates it to  Unlike a piston engine, the combustion in a turbine engine is  continuous. An igniter plug serves only to ignite the fuel/air  mixture when starting the engine. Once the fuel/air mixture  is ignited, it continues to burn as long as the fuel/air mixture  continues to be present. If there is an interruption of fuel, air,  or both, combustion ceases. This is known as a “flameout,”  and the engine must be restarted or re-lit. Some helicopters  are equipped with auto-relight, which automatically activates  the igniters to start combustion if the engine flames out. 4-10