Turboshaft Engines (15.17) PDF

Turboshaft Engines (15.17) Learning Objectives 15.17.1 Describe arrangements drive systems, reduction gearing, couplings and control systems (Level 2). 2024-07-23 B1-15b Gas Turbine Engine Page 243 of 290 CASA...

Turboshaft Engines (15.17) Learning Objectives 15.17.1 Describe arrangements drive systems, reduction gearing, couplings and control systems (Level 2). 2024-07-23 B1-15b Gas Turbine Engine Page 243 of 290 CASA Part 66 - Training Materials Only  Turboshaft Engines Purpose of Turboshaft Engines A turboshaft engine is similar to a turboprop. It has a shaft driven by extra turbine stages or dedicated free turbine stages to extract as much energy as possible from the exhaust gases. The shaft is connected to a reduction gearbox which can be used as an Auxiliary Power Unit (APU) to drive a helicopter rotor or power industrial applications, e.g. a vehicle driveshaft, electrical generators, or a ship’s propeller shaft. Turboshaft engines drive something other than a propeller by delivering power to a shaft. These engines do not produce exhaust thrust. They are common on helicopters. Turboshafts are free turbine or xed. As APUs have been discussed elsewhere, helicopter applications will be covered here. Turbo-shaft engine con gurations 2024-07-23 B1-15b Gas Turbine Engine Page 244 of 290 CASA Part 66 - Training Materials Only  Helicopter Turboshaft Engine Purpose of Helicopter Turboshaft Engines In a helicopter, power from the engine is transferred through a drive system to the main rotors. A typical drive system is termed the ‘power train’ and consists of the following components: Drive shafts Couplings Transmission Clutch Freewheeling unit. Free turbine turboshaft engines commonly have two gearbox sections: An accessory gearbox driven by the gas generator An output, or power, gearbox driven by the free power turbine. The accessory gearbox drives the components essential to engine operation, such as: Gas generator (N1 or Ng) governor Starter/generator Fuel control unit Lubrication unit Ng tachometer. The power gearbox drives the following: Main rotor transmission output shaft Power turbine (N2 or Np) governor Np tachometer Hydraulic pumps. 2024-07-23 B1-15b Gas Turbine Engine Page 245 of 290 CASA Part 66 - Training Materials Only  Turbo-shaft gearbox arrangement 2024-07-23 B1-15b Gas Turbine Engine Page 246 of 290 CASA Part 66 - Training Materials Only  Drive Shafts Both xed-shaft and free turbine engine output shafts are coupled to the helicopter’s main rotor transmission by a drive shaft. The purpose of the drive shaft is to: Transfer power from the engine to the transmission Compensate for expansion and contraction of the airframe. Turbo-shaft main rotor gearbox turbine coupling 2024-07-23 B1-15b Gas Turbine Engine Page 247 of 290 CASA Part 66 - Training Materials Only  Couplings To enable drives to be used via shafts, the movement between the engine and the shaft must be compensated for, done by couplings of varying design. These take up thermal expansion, misalignment and airframe to engine movement. Coupling example On helicopters, it is common to have two engines supplying one drive shaft (rotor and tail rotor). When xed-wing or helicopter aircraft have more than one engine contributing to a single output shaft, the input drives are summed together in a combining gearbox. Unlike the accessory or reduction gearboxes, combining gearboxes include oil pressure and temperature warning systems. Modern combining gearboxes electronically sense if one engine has failed and allow the other engine to exceed its normal operating parameters. Free wheel units allow the gearing to continue turning if one engine input has seized. If one engine fails or seizes, a sprag clutch disconnects that engine. 2024-07-23 B1-15b Gas Turbine Engine Page 248 of 290 CASA Part 66 - Training Materials Only  Multi-engine combining gearbox 2024-07-23 B1-15b Gas Turbine Engine Page 249 of 290 CASA Part 66 - Training Materials Only  Transmission Because gas turbine engines rotate at very high speeds (35 000 rpm) and the main rotor of a helicopter needs to rotate at very low speeds (300–400 rpm), some form of reduction gearing must be incorporated. The purpose of the reduction gearbox is to reduce engine rotational speed and increase torque. Depending on engine con guration, it can be part of the engine or part of the transmission in the helicopter airframe. Fixed-shaft engines are equipped with a reduction gearbox similar to a turboprop engine. In other aircraft types, the main rotor transmission is a high-ratio reduction gearbox. Transmission reduction gearboxes can be a combination of spur gear (to change direction and speed) and planetary gear types. Inside a main rotor transmission 2024-07-23 B1-15b Gas Turbine Engine Page 250 of 290 CASA Part 66 - Training Materials Only  Main rotor transmission 2024-07-23 B1-15b Gas Turbine Engine Page 251 of 290 CASA Part 66 - Training Materials Only  Clutch/Freewheeling Unit Since rotating aerofoils provide lift in a helicopter, the rotor system must be free to rotate if the engine fails. The freewheeling unit automatically disengages the engine from the main rotor in the event the engine stops providing power to the main rotor. This allows the main rotor to continue turning at normal in- ight speeds. As a safety feature, a freewheeling unit (overrunning clutch) is located between the engine and the main rotor transmission. The freewheeling unit ensures that the rotor cannot back drive the engine. The most common freewheeling unit assembly consists of a one-way roller or sprag clutch located between the engine and main rotor transmission. When the engine is driving the rotor, inclined surfaces force rollers against an outer drum. If the engine fails, the rollers move inwards, allowing the inner portion to exceed the speed of the outer drum. In this condition, engine speed is less than that of the drive system and the helicopter is in an autorotative state. Sprag clutch 2024-07-23 B1-15b Gas Turbine Engine Page 252 of 290 CASA Part 66 - Training Materials Only  Sprag clutch engaged and disengaged Transmissions and reduction gearing can be used for many applications, such as the helicopter’s main and tail rotors. On helicopters, it is common to have two engines supplying one drive shaft (rotor and tail rotor). When xed-wing or helicopter aircraft have more than one engine contributing to a single output shaft, the input drives are summed together in a combining gearbox. The combining gearbox either has a dedicated oil supply or uses the main engine oil system. Unlike the accessory or reduction gearboxes, combining gearboxes include oil pressure and temperature warning systems. Combining gearboxes need a way to sense if engine power is not equal. They do this via torque equalisers, which mechanically sense engine power and signal the engine fuel controls to re-equalise if they are out of tolerance. The torque equalisers also signal one engine to compensate for failure of the other engine. Modern combining gearboxes electronically sense if one engine has failed and allow the other engine to exceed its normal operating parameters. Free wheel units allow the gearing to continue turning if one engine input has seized. If one engine fails or seizes, a sprag clutch disconnects that engine. 2024-07-23 B1-15b Gas Turbine Engine Page 253 of 290 CASA Part 66 - Training Materials Only  Turboshaft Engine Control Systems Automatic-Control Constant-Speed Engines Some turboshaft engines, such as APUs, are designed to operate at a constant speed, and fuel ow is adjusted depending on demand. The engine fuel system is fully automatic and does not require external control. Modern systems are electronic. Earlier systems use the mechanical control system shown below. APU automatic hydro-pneumatic fuel control Fuel Control The fuel control unit consists of the governor, acceleration limiter, fuel pump and fuel lter, all housed in one assembly. The pneumatic thermostat valve is connected to the acceleration limiter by the control line, which contains compressor discharge pressure (CDP). During start, and whenever a high Exhaust Gas Temperature (EGT) is sensed, the normally closed pneumatic thermostat valve opens and relieves CDP from the acceleration limiter. This results in reduced fuel ow to the fuel spray nozzle and therefore lowers the EGT. 2024-07-23 B1-15b Gas Turbine Engine Page 254 of 290 CASA Part 66 - Training Materials Only  Over-Temperature Protection System To prevent high Turbine Gas Temperature (TGT) during the start cycle and during operation, an over- temperature protection system is installed. The control is a temperature-actuated air valve, normally closed, mounted in the exhaust duct and connected to the acceleration limiter through a normally open thermostat control valve (three-way valve). At a preset temperature, the temperature-actuated valve cracks. This bleeds off some pressurised air from the acceleration limiter, with the result that the fuel pressure and EGT decrease. The temperature difference between the valve cracking point and the full open position is about 11 °C. If bleed air is selected, the thermostat control valve is energised and connects the control pneumatic thermostat valve to the bleed air load control valve to control the EGT by controlling the bleed air load. Automatic Control of Helicopter Engines On a simpli ed typical turboshaft engine installed in a helicopter, the engine control consists of the following: Gas producer (N1) controls Power turbine (N2) controls. Control of the gas producer is operated by the pilot’s twist grip on the collective stick. Power turbine controls (N ) are operated from the collective system. Raising the collective stick gives the following 2 results. Rotor blade angle increases Torque increases Rotor speed decreases. Gas Producer (N1) The gas producer is controlled by the hydromechanical fuel control. The controls consist of a exible control cable which extends from the throttle twist grip on the collective stick to the fuel control input lever. The hydromechanical fuel control receives the pilot’s signal for a given level of power and adjusts the engine fuel ow to provide the desired power within the rpm and EGT limitations of the engine. 2024-07-23 B1-15b Gas Turbine Engine Page 255 of 290 CASA Part 66 - Training Materials Only  Power Turbine Controls (N2) The power turbine controls consists of a mechanical linkage between the rotor collective system and the power turbine governor input lever. Movement of the collective stick repositions the governor shaft. As the pilot increases collective pitch, more power is needed to drive the rotor. The N2 governor senses the drop in rpm and schedules the hydromechanical fuel control to increase fuel ow to the N1 (gas producer). The N1 increases the gas ow to the N2, which increases power, therefore increasing rotor rpm. This action prevents rpm variations as power changes are made. Power turbine control system In twin-engine helicopters, the Torque Control Unit matches torque output between engines. In single-engine helicopters, rotor torque is monitored on the cockpit torque indicator. The pilot maintains torque output within the limitations of the power train and transmission. 2024-07-23 B1-15b Gas Turbine Engine Page 256 of 290 CASA Part 66 - Training Materials Only  Torque indicator On some turbine engine helicopters, the twist grip has been eliminated in favour of a power lever for free turbine engines. The N1 usually has three positions: ground idle, ight idle and full N1. The N1 system speeds up and slows down as a function of N2 so a steady rotor rpm can be maintained during all ight conditions. Modern turboshaft helicopter engines have full FADEC-controlled engines. 2024-07-23 B1-15b Gas Turbine Engine Page 257 of 290 CASA Part 66 - Training Materials Only