•  Improper valve timing Propulsive Efficiency construction features of any gas turbine engine is the type of  compressor or compressors for which the engine is designed.  A propeller is used with an engine to provide thrust. The  engine supplies bhp through a rotating shaft, and the  propeller absorbs the bhp and converts it into thrust hp. In this  conversion, some power is wasted. Since the efficiency of any  machine is the ratio of useful power output to the power input,  propulsive efficiency (in this case, propeller efficiency) is the  ratio of thrust hp to bhp. On the average, thrust hp constitutes  approximately 80 percent of the bhp. The other 20 percent  is lost in friction and slippage. Controlling the blade angle  of the propeller is the best method of obtaining maximum  propulsive efficiency for all conditions encountered in flight. Four types of gas turbine engines are used to propel and  power aircraft. They are the turbofan, turboprop, turboshaft,  and turbojet. The term “turbojet” was used to describe any  gas turbine engine used in aircraft. As gas turbine technology  evolved, these other engine types were developed to take the  place of the pure turbojet engine. The turbojet engine has  problems with noise and fuel consumption in the speed range  that airliners fly (.8 Mach). Due to these problems, use of  pure turbojet engines is very limited. So, almost all airliner- type aircraft use a turbofan engine. It was developed to turn a  large fan or set of fans at the front of the engine and produces  about 80 percent of the thrust from the engine. This engine  was quieter and has better fuel consumption in this speed  range. Turbofan engines have more than one shaft in the  engine; many are two-shaft engines. This means that there are  two sets of compressors and turbines that drive them. These  two shafted engines use two spools (a spool is a compressor  and a shaft and turbines that driven that compressor). In a  two-spool engine, there is a high-pressure spool and a low- pressure spool. The low-pressure spool generally contains  the fan(s) and the turbine stages it takes to drive them. The  high-pressure spool is the high-pressure compressor, shaft,  and turbines. This spool makes up the core of the engine, and  this is where the combustion section is located. During takeoff, when the aircraft is moving at low speeds  and when maximum power and thrust are required, a low  propeller blade angle gives maximum thrust. For high- speed flying or diving, the blade angle is increased to obtain  maximum thrust and efficiency. The constant-speed propeller  is used to give required thrust at maximum efficiency for all  flight conditions. In a reciprocating engine, the functions of intake, compression,  combustion, and exhaust all take place in the same combustion  chamber. Consequently, each must have exclusive occupancy  of the chamber during its respective part of the combustion  cycle. A significant feature of the gas turbine engine is  that separate sections are devoted to each function, and all  functions are performed simultaneously without interruption. Turbofan engines can be low bypass or high bypass. The  amount of air that is bypassed around the core of the engine  determines the bypass ratio. As can be seen in  Figure 1-43 ,  the air generally driven by the fan does not pass through the  internal working core of the engine. The amount of air flow  in lb/sec from the fan bypass to the core flow of the engine  is the bypass ratio. Bypass ratio =  100 lb/sec flow fan  = 5:1 bypass ratio  20 lb/sec flowcore Some low-bypass turbofan engines are used in speed  ranges above .8 Mach (military aircraft). These engines use  augmenters or afterburners to increase thrust. By adding  more fuel nozzles and a flame holder in the exhaust system  extra fuel can be sprayed and burned which can give large  increases in thrust for short amounts of time.  The turboprop engine is a gas turbine engine that turns a  propeller through a speed reduction gear box. This type of  engine is most efficient in the 300 to 400 mph speed range and  can use shorter runways that other aircraft. Approximately  80 to 85 percent of the energy developed by the gas turbine  engine is used to drive the propeller. The rest of the available  energy exits the exhaust as thrust. By adding the horsepower  The major components of all gas turbine engines are basically  the same; however, the nomenclature of the component parts  of various engines currently in use varies slightly due to  the difference in each manufacturer’s terminology. These  differences are reflected in the applicable maintenance  manuals. One of the greatest single factors influencing the  A typical gas turbine engine consists of: Gas Turbine Engines  Types and Construction 1-37 1.  An air inlet, 2.  Compressor section, 3.  Combustion section, 4.  Turbine section, 5.  Exhaust section, 6.  Accessory section, and 7. The systems necessary for starting, lubrication, fuel  supply, and auxiliary purposes, such as anti-icing,  cooling, and pressurization.