15 MODULE : GAS TURBINE ENGINES 15 MODULE GAS TURBINE ENGINE AS OUTLINED IN PART-66, APPENDIX 1. NOTES SUB-MODULE 17 - TURBO-SHAFT ENGINES Arrangements, drive systems, reduction gearing, couplings, control systems. 15 Sub Module : 17 Turbo Shaft Engines Page 1 15 MODULE : GAS TURBINE ENGINES TURBOSHAFT ENGINES 1. The turbine engines used in helicopters come in various sizes from a range of 300 to 3000 or more horsepower. 2. They are referred to as shaft turbine or turbo shafts. This is because the power extracted from the engine is used to turn a shaft. The shaft turns the transmission for the main rotor and tail rotor of the helicopter. 3. The engine drives the main rotor through a transmission and belt or centrifugal clutch system. The anti-torque rotor is driven from the transmission. 4. The shaft turbine may produce some thrust but it is primarily designed to produce shaft horse power. Accomplished by using the same basic components found in a turbojet with the addition of turbine wheel (wheels) to absorb the power of the escaping gases of combustion. 5. Turbo shaft engine may also contain a gear reduction to reduce the speed of the turbine, which exceeds 30,000 RPM on some engines. 6. CLASSIFICATION : A) Direct shaft turbine B) Free shaft turbine 7. Both types have been successful, however the free turbine is the most popular. 8. Turbine engines are ideally suited for powering helicopters, because they operate most efficiently at the constant RPM required by a helicopter. 9. Most turbo-shaft engines drive their output shaft with multistage free turbine that extracts as must energy as possible from the exhaust gases. 15 Sub Module : 17 Turbo Shaft Engines Page 2 15 MODULE : GAS TURBINE ENGINES Fig, Turbo shaft engine design Typical turboshaft with Axial flow compressor and free turbine COMPONENTS 1. Basic components : compressor, diffuser, combustor, turbine wheel or wheels, and exhaust. 2. The free turbine derives its power from a separate turbine not connected to the compressor (figs.) 3. The direct drive turbo shaft engine must run at a constant speed, with the compressor and the power output attached to the same shaft. 4. The free turbine, however, can vary the speed of the compressor as required to maintain power turbine output. 1. Engine inlet and particle separator head 2. Air Starter and accessory box 3. Oil cooler fan 4. TV3-117 VM engine Fig. Engine fitted on Mi-8 Russian Helicopter 15 Sub Module : 17 Turbo Shaft Engines Page 3 15 MODULE : GAS TURBINE ENGINES 5. VR – 14 Main Transmission of Main Gear Box 6. Tail rotor drive shaft ENGINE OPERATION COMPRESSOR 1. The airflow through the turbo shaft engine begins at the inlet where the air enters the compressor . 2. The most popular shaft turbine today is a combination of radial and centrifugal compressors. 3. Usually first stages of compression will be axial while the last stage will be centrifugal. 4. COMBINATION OF AXIAL and CENTRIFUGAL COMPRESSOR REASONS a) By reducing the number stages of axial flow compressor, the size of the engine may be reduced in length without an appreciable change in diameter and if the engine were built with a centrifugal compressor, only the diameter of the engine would increase. b) The likely hood of foreign object damage is greatly reduced by eliminating the smaller stages of axial compression. c) The most significant reason, the speed of the airflow through the engine is increased by using this combination of axial and centrifugal compressor. DIFFUSER 1. The air from the combined compressor is then directed to the diffuser section where it is straightened before entering the combustion area. 2. Some of this air is used to support combustion and the rest used for cooling purpose. COMBUSTOR 1. Although different types of combustors may be used, the most popular for the turbo shaft is annular combustor using reverse flow. (Figure) 2. At this point fuel is added to the air and is ignited. 3. This heats the air which rapidly expands and passes through a nozzle assembly and is directed to the turbine wheel. 4. This wheel is rotated by the gases passing over it and turns the compressor, which generates more air to repeat the process. TURBINE 1. It is at this point the two turbo shafts vary in operation. 2. In the free turbine another turbine wheel is placed behind the compressor turbine. This turbine drives a shaft to power the helicopter. 3. The direct drive turbine may also have another turbine wheel but this is fixed to the compressor and also drives a power shaft used to power the helicopter. 4. The way the shaft and turbine are used for power varies from engine to engine. Fig. Cold end drive turbo shaft 15 Sub Module : 17 Turbo Shaft Engines Fig. Hot end drive turbo shaft Page 4 15 MODULE : GAS TURBINE ENGINES 5. Some engines drive from the compressor end(cold end) while others drive from the combustor end (hot end). Both have advantages and disadvantages. 6. The main advantage of driving form the hot end is that less shafting is required. However, the gear reduction is exposed to more heat in this area. POWER TO WEIGHT RATIO 1. With the reciprocating engine, a weight ratio of one pound per horsepower was considered ideal and today it is not unusual to produce a turbine with ratio excess of four horsepower per pound. 2. This increases the payload of the helicopter, provides extra power for emergency condition and under certain atmospheric condition that require increased power. 3. SHAFT HORSE POWER (SHP) : The power the engine was specifically built to produce. 4. EQUIVALENT SHAFT HORSE POWER : Another term sometimes used to indicate the power of a turbo shaft engine. ESHP takes into consideration both the shaft horsepower delivered to a) rotor shaft through gear box, and b) the thrust developed at the engine exhaust and is calculated as additional power. FLAT RATED 1. It is known that both the engine performance and the helicopter performance will deteriorate with changes in atmospheric conditions. 2. A turbo-shaft engine whose allowable output power is less than the engine is physically capable of producing. TORQUE LIMITED 1. The term is not used in regards to the engine but, power the engine may deliver to the transmission. 2. It is the transmission that is torque limited or capable of withstanding only so much of the engine’s potential power output. 3. This can however, be of some benefit during operations when engine power deteriorates. 4. If all the power is not used, additional power may be available because of the torque limitations. This often occurs with the single engine operation of a twin engine helicopter. PARTICLE SEPARATORS 1. The turbo shaft engine require a greater airflow operation, and one of the most critical requirements of this air is that it be clear of foreign objects. 2. This can be difficult in helicopter operations where landings are often conducted in unimproved areas, with dust and sand being blown into the air by downwash of the main rotors. 3. This often requires the use of a particle separator on the inlet of the engine. 15 Sub Module : 17 Turbo Shaft Engines Page 5 15 MODULE : GAS TURBINE ENGINES LYCOMING T 53 SERIES TURBOSHAFT ENGINE 1. 2. 3. 4. Is a turbo shaft engine with a two stage free-power turbine and a two-stage gas-producer turbine. It has a combination axial-centrifugal compressor and an annular atomizing type combustor. It is torque limited in horsepower to 1250 SHP for 5 minutes and 1100 SHP for max. cont. operation. The first stage turbine turns counterclockwise. The second stage turbine and the out put shaft turn clockwise. 5. The major sections : a) Air inlet b) Compressor rotor c) Diffuser d) combustion e) Exhaust BASIC OPERATION 1. The air is compressed by a five –stage axial compressor and single-stage centrifugal compressor. 2. The air then passes through the diffuser where the high velocity air is converted to high pressure and the air flow is returned to a radial flow. 3. Provisions are made on the diffuser for bleeding air from the compressor 4. As the air leave the diffuser, it enters the combustion area where the pressure is reduced, the velocity is decreased, and the direction is changed. 5. The fuel is introduced into the chamber by 22 atomizers at the aft end of the combustor. 6. As the combustion takes place, the temperatures will reach 3,500oF. 7. Flowing out of the combustor, the gases again reverse direction and flow across the two-stage gas producer turbine and two-stage power turbine. 8. The power turbines drive a shaft passing through the compressor turning the gear reduction, the engine output shaft, and the N2 gear box. 9. The gas producer turbines turn the compressor which extracts approximately 2/3 of the energy to produce more air for combustion. 15 Sub Module : 17 Turbo Shaft Engines Page 6 15 MODULE : GAS TURBINE ENGINES 10. In addition to driving the compressor it also turns the N1 accessory gearbox on the bottom of the engine. BOEING CH-47 CHINOOK HELICOPTER 1. Tandem rotors are two horizontal main rotor assemblies mounted one behind the other. 2. However, it requires the expense of two large rotors rather than the more common one large main rotor and a much smaller tail rotor. 3. The Boeing CH-47 Chinook is the most common tandem rotor helicopter today. BOEING CH-47 R : The Drive Shafting 1. The drive system includes: five transmissions (forward, aft, combining, number one and two engine nose box), and aft vertical shaft, nine synchronizing shafts (drive shafts), and two engine drive shafts(cross shafts). 2. Each transmission has an independent oil lubrication system that is air cooled through a heat exchanger. 3. Each transmission’s oil pressure and temperature 15 Sub Module : 17 Turbo Shaft Engines Page 7 15 MODULE : GAS TURBINE ENGINES is monitored. 4. The maintenance panel has indicators for chip detectors and debris screens that latch when metal particles are detected in the oil system. A tripped latched would indicate possible transmission failure. 1. The drive system provides power to turn the rotors. 2. Engine output is connected to the engine transmission, then to the combining transmission. 3. From the combining transmission the power is directed, through the synchronizing shafts, to the forward and aft transmission. 4. The reduction in gear ratio between the engine and the rotor wing blades is 66.96 :1. 5. All synchronizing shafts are free floating at their inputs and secured to the transmission at their outputs. TYPICAL TAILROTOR TRAMSMISSION DRIVE 15 Sub Module : 17 Turbo Shaft Engines Page 8