TURBOPROP ENGINE TYPES

1. fixed shaft/ constant speed
2. split shaft/ free turbine engine

FIXED SHAFT

One type of turboprop engine is the fixed shaft constant speed type such as the Garrett TPE331. [Figure 14-2] In this type engine, ambient air is directed to the compressor section through the engine inlet. An acceleration/diffusion process in the two- stage compressor increases air pressure and directs it rearward to a combustor. The combustor is made up of a combustion chamber, a transition liner, and a turbine plenum. Atomized fuel is added to the air in the combustion chamber. Air also surrounds the combustion chamber to provide for cooling and insulation of the combustor.

Figure 14-2. Fixed shaft turboprop engine.

The gas mixture is initially ignited by high-energy igniter plugs, and the expanding combustion gases flow to the turbine. The energy of the hot, high velocity gases is converted to torque on the main shaft by the turbine rotors. The reduction gear converts the high r.p.m.—low torque of the main shaft to low r.p.m.—high torque to drive the accessories and the propeller. The spent gases leaving the turbine are directed to the atmosphere by the exhaust pipe.

Only about 10 percent of the air which passes through the engine is actually used in the combustion process. Up to approximately 20 percent of the compressed air may be bled off for the purpose of heating, cooling, cabin pressurization, and pneumatic systems. Over half the engine power is devoted to driving the compressor, and it is the compressor which can potentially produce very high drag in the case of a failed, windmilling engine.

In the fixed shaft constant-speed engine, the engine r.p.m. may be varied within a narrow range of 96 percent to 100 percent. During ground operation, the r.p.m. may be reduced to 70 percent. In flight, the engine operates at a constant speed, which is maintained by the governing section of the propeller. Power changes are made by increasing fuel flow and propeller blade angle rather than engine speed. An increase in fuel flow causes an increase in temperature and a corresponding increase in energy available to the turbine. The turbine absorbs more energy and transmits it to the propeller in the form of torque. The increased torque forces the propeller blade angle to be increased to maintain the constant speed. Turbine temperature is a very important factor to be considered in power production. It is directly related to fuel flow and thus to the power produced. It must be limited because of strength and durability of the material in the combustion and turbine section. The control system schedules fuel flow to produce specific temperatures and to limit those temperatures so that the temperature tolerances of the combustion and turbine sections are not exceeded. The engine is designed to operate for its entire life at 100 percent. All of its components, such as compressors and turbines, are most efficient when operated at or near the r.p.m. design point.

Figure 14-3. Powerplant controls—fixed shaft turboprop engine.

Powerplant (engine and propeller) control is achieved by means of a power lever and a condition lever for each engine. [Figure 14-3] There is no mixture control and/or r.p.m. lever as found on piston engine airplanes. On the fixed shaft constant-speed turboprop engine, the power lever is advanced or retarded to increase or decrease forward thrust. The power lever is also used to provide reverse thrust. The condition lever sets the desired engine r.p.m. within a narrow range between that appropriate for ground operations and flight.

Powerplant instrumentation in a fixed shaft turboprop engine typically consists of the following basic indicator. [Figure 14-4]

• • Torque or horsepower.
• • ITT – interturbine temperature.
• • Fuel flow.
• • RPM.
• Torque developed by the turbine section is measured

Figure 14-4. Powerplant instrumentation-fixed shaft turboprop engine.

by a torque sensor. The torque is then reflected on a cockpit horsepower gauge calibrated in horsepower times 100. Interturbine temperature (ITT) is a measurement of the combustion gas temperature between the first and second stages of the turbine section. The gauge is calibrated in degrees Celsius. Propeller r.p.m. is reflected on a cockpit tachometer as a percentage of maximum r.p.m. Normally, a vernier indicator on the gauge dial indicates r.p.m. in 1 percent graduations as well. The fuel flow indicator indicates fuel flow rate in pounds per hour.

Propeller feathering in a fixed shaft constant-speed turboprop engine is normally accomplished with the condition lever. An engine failure in this type engine, however, will result in a serious drag condition due to the large power requirements of the compressor being absorbed by the propeller. This could create a serious airplane control problem in twin-engine airplanes unless the failure is recognized immediately and the affected propeller feathered. For this reason, the fixed shaft turboprop engine is equipped with negative torque sensing (NTS).

Negative torque sensing is a condition wherein propeller torque drives the engine and the propeller is automatically driven to high pitch to reduce drag. The function of the negative torque sensing system is to limit the torque the engine can extract from the propeller during windmilling and thereby prevent large drag forces on the airplane. The NTS system causes a movement of the propeller blades automatically toward their feathered position should the engine suddenly lose power while in flight. The NTS system is an emergency backup system in the event of sudden engine failure. It is not a substitution for the feathering device controlled by the condition lever.