Turbo-Propeller Engines PDF

Turbo-Propeller Engines (15.16) Learning Objectives 15.16.1 Describe gas coupled, free turbine and gear coupled turbines on turbo-prop engines (Level 2). 15.16.2 Describe reduction gearboxes tted to turbo-prop engines (Level 2). 15.16.3 Describe integrated engine and propeller control...

Turbo-Propeller Engines (15.16) Learning Objectives 15.16.1 Describe gas coupled, free turbine and gear coupled turbines on turbo-prop engines (Level 2). 15.16.2 Describe reduction gearboxes tted to turbo-prop engines (Level 2). 15.16.3 Describe integrated engine and propeller controls on turbo-prop engines (Level 2). 15.16.4 Describe overspeed safety devices on turbo-prop engines (Level 2). 2024-07-23 B1-15b Gas Turbine Engine Page 233 of 290 CASA Part 66 - Training Materials Only  Turbo-Prop Engines Purpose of Turbo-Prop Engines A turboprop engine is a gas turbine engine that drives a propeller to produce thrust. Turboprop engines are similar in design to turbojet engines except that the power produced by a turboprop engine is delivered to a reduction gear system that spins a propeller. Reduction gearing is necessary in turboprop engines because optimum propeller performance is achieved at much slower speeds than the engine’s operating rpm. Turboprops, like all gas turbine engines, have a compressor section, combustion section, turbine section and exhaust section. These sections carry out the same functions as if they were installed in a turbojet engine. However, a turboprop engine is designed with a few differences. The turbines of a turboprop engine extract 80% to 90% of the energy from the exhaust gases after driving the compressors to drive the propeller. To do this, most turboprop engines use multiple-stage turbines. In addition, the turbine blades in a turboprop engine are designed to extract more energy from the exhaust gases than the blades found in a turbojet engine. Turboprop engines are used extensively in business and commuter type aircraft because the combination of jet power and propeller ef ciency provides good performance characteristics at speeds between 300 and 450 mph. In addition, most turboprop engines provide the best speci c fuel consumption of any gas turbine engine. All turboprop engines utilise a propeller reduction gearbox. There are two basic turbine con gurations for driving the reduction gearbox: Gas-coupled – referred to as a ‘free turbine’ Gear-coupled – referred to as a ‘ xed shaft’. 2024-07-23 B1-15b Gas Turbine Engine Page 234 of 290 CASA Part 66 - Training Materials Only  Gas-Coupled Turboprop The free turbine is connected only to the gearbox and propeller shaft. This arrangement allows it to seek its optimum design speed while compressor speed is set at its design point (point of best compression). The propeller on a free turbine can go to the feather position on shutdown because the starter rotates only the gas generator turbine and is not loaded by the propeller and power turbine during engine start. The turbine that drives the propeller is turned only by the hot exhaust gases leaving the compressor turbine. The ratio between the propeller rpm and the compressor turbine rpm is not constant. Advantages: The propeller can be held at very low rpm during taxiing, with low noise and low blade erosion. The engine is easier to start, especially in cold weather. The propeller and its gearbox do not directly transmit vibrations into the gas generator. A rotor brake can be used to stop propeller movement during aircraft loading when engine shutdown is not desired. Disadvantage: The engine does not have the instantaneous power of reciprocating engines. Gas-coupled, free turbine turbo-prop con guration 2024-07-23 B1-15b Gas Turbine Engine Page 235 of 290 CASA Part 66 - Training Materials Only  Gear-Coupled Turboprop A second method of transferring the exhaust gas energy to the propeller is through a xed shaft. In this case, the main turbine typically has an additional turbine wheel that extracts the energy needed to drive a propeller. With these xed-shaft engines, the main power shaft goes directly into a reduction gearbox to convert the high-speed, low-torque turbine output into low-speed, high-torque energy to drive a propeller. The ratio between propeller rpm and turbine rpm is constant. Gear-coupled, two-stage centrifugal ow turbo-prop A xed shaft turbo-propeller engine is shut down with the propeller in ne pitch to minimise the load on the starter when the engine is being started. This provides the minimum resistance to turning and thus prevents an excessive exhaust gas temperature occurring during the starting cycle. Some propellers have a special ne-pitch setting. 2024-07-23 B1-15b Gas Turbine Engine Page 236 of 290 CASA Part 66 - Training Materials Only  Propeller Reduction Gearbox Reduction Gearboxes Turboprop engines operate at high rotational speeds; all turboprop designs must incorporate a reduction gear assembly that can convert the engine’s high-speed, low-torque rotational speed to a more useable low speed and high torque. Although reduction gear assemblies are used on some reciprocating engines, a turboprop reduction gear system must perform under more extreme operational conditions. For example, it is not uncommon for a turboprop engine to operate at rotational speeds in excess of 40 000 rpm. These speeds far exceed the 2200 rpm that most turboprop propellers operate, and therefore all turboprop engines must incorporate a reduction gear assembly. The main engine oil system normally lubricates the reduction gearbox. Often chip detectors are incorporated in the reduction gearbox. There are two main types of reduction gears: Parallel spur Epicyclic. 2024-07-23 B1-15b Gas Turbine Engine Page 237 of 290 CASA Part 66 - Training Materials Only  Parallel Spur Gears Parallel spur gears have the advantage of being mechanically simple and relatively cheap to manufacture. They can be either straight cut or helical cut and do not give the same reduction ratio as an epicyclic gear system. Parallel spur gears Relevant Youtube link: PW100 engine reduction gearbox with 3D animation 2024-07-23 B1-15b Gas Turbine Engine Page 238 of 290 CASA Part 66 - Training Materials Only  Epicyclic Reduction Gears Epicyclic reduction gear assemblies incorporate a central sun gear wheel driving a number of planet gears running inside a ring gear. This layout is very compact and can have a huge variety of reduction ratios, depending on gear teeth ratios or which of the three components is locked. It is generally used with two segments of epicyclic gears. An advantage it offers over a spur gear set is keeping the propeller on the same axis as the engine output shaft. Reduction gearbox assembly View Interactive Flash Animation: Planetary Gears (Interactive Animation) 2024-07-23 B1-15b Gas Turbine Engine Page 239 of 290 CASA Part 66 - Training Materials Only  Control and Safety Devices Engine Control Most turboprop engines operate in either Alpha mode or Beta mode. In Alpha, or ight operation mode, the propeller governor is adjusted by the condition lever to set the propeller rpm. When the condition lever setting is xed, the power lever operates the fuel control unit to control the amount of fuel delivered to the engine. Increasing fuel ow by moving the power lever forward increases engine power. With the governor attempting to maintain a constant rpm, an increase in engine power causes the propeller governor to increase the propeller blade angle. On the other hand, a decrease in engine power causes the propeller governor to decrease the propeller blade angle. In Beta or ground mode, which is below ‘Flight idle’, the propeller blade angle and engine rpm are controlled by the power lever position. This allows for minimum thrust during ground operations and allows reverse thrust to be selected. One reason this is necessary is because, unlike a reciprocating engine, a turboprop engine takes more time to react to fuel ow and power changes. With this delayed reaction time, turboprop aircraft cannot use varying engine rpm to effectively control the aircraft on the ground. Therefore, to facilitate ground-handling characteristics, the gas generator speed is held relatively constant while propeller pitch is varied as necessary to produce the desired amount of thrust (Beta range). For engines with full FADEC, the power lever controls both engine and propeller through all conditions. Such engines do not have Alpha or Beta modes. 2024-07-23 B1-15b Gas Turbine Engine Page 240 of 290 CASA Part 66 - Training Materials Only  Mechanical and FADEC turbo-prop control 2024-07-23 B1-15b Gas Turbine Engine Page 241 of 290 CASA Part 66 - Training Materials Only  Overspeed Safety Devices Overspeed is a condition in which the engine or propeller speed (depending on con guration) is higher than the speed selected by the pilot. An overspeed governor is tted as a backup if the propeller governor fails to control engine/propeller rpm. It is located on the side of the reduction gearbox, cannot be adjusted in ight and is independent of pilot control. In an overspeed condition, the overspeed governor bleeds off propeller oil pressure, causing the blade angle to increase. This increase in pitch places more load on the engine and slows the propeller down. Overspeed safety device con guration 2024-07-23 B1-15b Gas Turbine Engine Page 242 of 290 CASA Part 66 - Training Materials Only