Turbine Aircraft Systems - Engine

Question 1

Describe the major components of a gas turbine engine.

Answer 1

A typical gas turbine engine consists of:

1. An air inlet
2. Compressor section
3. Combustion section
4. Turbine section
5. Exhaust section
6. Accessory section
7. The systems necessary for starting, lubrication, fuel supply, and auxiliary purposes, such as anti-icing, cooling, and pressurization

Question 2

Explain the basic operation of a gas turbine engine.

Answer 2

The basic gas turbine engine contains four sections: intake, compression, combustion, and exhaust.

1. To start the engine, the compressor section is rotated by an electrical starter on small engines or an air-driven starter starter on large engines.
2. As compressor RPM accelerates, air is brought in through the inlet duct, compressed to a high pressure, and delivered to the combustion chambers.
3. Fuel is then injected by a fuel controller through spray nozzles and ignited by igniter plugs.
4. The fuel/air mixture in the combustion chamber is then burned in a continuous combustion process and produces a very high temperature (4,000ºF.), which heats the entire air mass to 1,600-2,400ºF.
5. The mixture of hot air and gases expands and is directed to the turbine blades, forcing the turbine section to rotate, which in turn drives the compressor by means of a direct shaft.
6. After powering the turbine section, the high velocity excess exhaust exits the tail pipe or exhaust section. Once the turbine section is powered by gases from the burner section, the starter is disengaged, and the igniters are turned off.
7. Combustion continues until the engine is shut down by turning off the fuel supply.

Question 3

This compressor has an impeller surrounded by a ring of diffuser vanes. The impeller is driven at high speed by a turbine. Air is drawn into the air inlet and directed to the center of the impeller. The air is then forced outward centrifugally into a diffuser, where the pressure of the air is increased. The pressurized air is then supplied to the combustion section.

Answer 3

Centrifugal-flow compressor

Question 4

This consists of two main elements, a rotor and a stator. The rotor, turning at high speeds, has blades fixed on a spindle that takes in air at the compressor inlet and impels it rearward through a series of stages paralleling the longitudinal axis of the engine. The action of the rotor increases the compression of the air at each stage, accelerating it rearward through several stages. With this increased velocity, energy is transferred from the compressor to the air in the form of velocity energy. The stator blades act as diffusers at each stage, partially converting high velocity to pressure. Each consecutive pair of rotor and stator blades constitutes a pressure stage; the greater the number of stages, the higher the compression ratio. Most present-day engines utilize from 10 to 16 stages.

Answer 4

Axial-flow compressor

Question 5

the most fundamental gas turbine engine. The relatively small frontal area results in a small mass of air accelerated through the core to a high velocity; no fan is utilized. Air is compressed, ignited in the combustion section, and expelled at the rear of the engine at high velocity to drive a turbine, which in turn drives a compressor. Used on aircraft that fly at high airspeeds and altitudes. Inefficient and loud at low altitudes.

Answer 5

Turbojet

Question 6

a gas turbine engine with a duct-enclosed axial flow fan at the front of the engine, driven by the engine’s turbine section. A portion of the incoming air entering the engine is compressed and enters a combustion chamber; the other portion is also compressed and bypasses the combustion section altogether. Both are joined together downstream to produce thrust. In high bypass engines, the fan/bypass air produces most of the thrust, approximately 80%. The two types of turbofan engines, low bypass and high bypass, refers to the amount of air bypassing the core. Turbofan engines are a compromise between turbojet and turboprop engines, resulting in better high-altitude performance than the turboprop, and better low-altitude performance than the turbojet.

Answer 6

Turbofan

Question 7

basically a turbojet engine that utilizes exhaust gases to turn a propeller. Two methods are used: 1) high-velocity exhaust gases turn a propeller directly via a compressor shaft and reduction gear box; or 2) exhaust gases turn a power turbine, which is connected to the propeller via a shaft and reduction gear box. A turboprop engine moves a large mass of air at a low velocity and is the ideal application for low-altitude and lower speed aircraft.

Answer 7

Turboprop

Question 8

much like a turboprop engine; high velocity exhaust gases turn a power turbine connected to a rotor via a shaft and reduction gearbox. Normally used in helicopters where a turboshaft turns both the main and tail rotors.

Answer 8

Turboshaft

Question 9

Define the term “bypass ratio.”

Answer 9

The bypass ratio is the ratio of the mass airflow in pounds per second through the fan section of a turbofan engine to the mass airflow that passes through the gas generator portion of the engine.

Question 10

What is the function of the stator vanes on an axial flow compressor?

Answer 10

Stator vanes receiver air from the air inlet duct or from each preceding stage and increase the pressure of the air and deliver it to the next stage at the correct velocity and pressure. They also control the direction of air to each rotor stage to obtain the maximum possible compressor blade efficiency.

Question 11

What are the basic components of a turboprop engine?

Answer 11

The typical turboprop engine can be broken down into assemblies as follows:

1. The power section assembly with the major components of gas turbine engines (compressor, combustion chamber, turbine, and exhaust sections);
2. The reduction gear or gearbox assembly—those sections peculiar to turboprop configurations;
3. The torquemeter assembly, which transmits the torque from the engine to the gearbox of the reduction section; and
4. The accessory drive housing assembly.

Question 12

What do N1, N2, and N3 indicate?

Answer 12

Spool speed expressed in percent RPM.

N1: on a turbo prop is the gas producer speed. On a turbofan or turbojet engine is the fan speed or low pressure spool speed.

N2: the high pressure spool speed on engines with 2 spools and the medium pressure spool on engines with 3 spools

N3: the high pressure spool on engines with 3 spools

Question 13

Explain the function of a torquemeter.

Answer 13

Turboprop/turboshaft engines are designed to produce torque for driving a propeller and power output is measured by a torquemeter. The torquemeter measures power applied to the shaft and is calibrated in percentage units, foot-pounds, or psi.

Question 14

the divergent section of the engine after the compressor and before the combustion section. It has the all-important function of reducing high-velocity compressor discharge air to a slower velocity at increased pressure. This prepares the air for entry into the flame burning area of the combustion section at a lower velocity so that the flame of combustion can burn continuously.

Answer 14

The main function of the diffuser in a turbine engine.

Question 15

Define the term “EGT”

Answer 15

Exhaust gas temperature (EGT) - the temperature of the exhaust gases as they enter the tail pipe, after passing through the turbine.

Question 16

Define the term “TIT”

Answer 16

Turbine inlet temperature (TIT) - the temperature of the gases from the combustion section of the engine as they enter the first stage of the turbine. The TIT is the highest temperature inside a gas turbine engine and is one of the limiting factors of the amount of power the engine can produce.

Question 17

Define the term “ITT”

Answer 17

Interstage turbine temperature (ITT) - the temperature of the gases between the high-pressure and low-pressure turbine wheels.

Question 18

Define the term “TOT”

Answer 18

Turbine outlet temperature (TOT) - like EGT, turbine outlet temperature is taken aft of the turbine wheel(s).

Question 19

What is the definition of a “hot start”

Answer 19

this is when normal engine rotation occurs, but with exhaust temperature exceeding prescribed limits; usually caused by an excessively rich mixture in the combustor. Fuel to the engine must be terminated immediately to prevent engine damage.

Question 20

Define the term “hung start”

Answer 20

a condition in which normal “light-off” occurs yet engine RPM remains at a low value rather than increasing to the normal idle speed; often the result of insufficient power to the engine from the starter. The engine should be shut down immediately.

Question 21

In gas turbine engines, this is a condition in an axial-flow compressor in which one or more stages of rotor blades fail to pass air smoothly to the succeeding stages. A stall condition is caused by a pressure ratio that is incompatible with the engine RPM.

Answer 21

Compressor stall

Question 22

Descibe a compressor stall indication

Answer 22

a rise in exhaust tmperature or RPM fluctuation, and if allowed to continue, may result in fame-out and physical damage to the engine

Question 23

the ratio of turbine discharge to compressor inlet pressure. Pressure measurements are recorded by probes installed in the engine inlet and at the exhaust. Once collected, the data is sent to a differential pressure transducer, which is indicated on a flight deck EPR gauge. An EPR gauge is used to indicate the power output of a turbojet/turbofan engine.

Answer 23

Engine pressure ratio (EPR)

Question 24

What are igniters?

Answer 24

turbine discharge to compressor inlet pressure. Pressure measurements are recorded by probes installed in the engine inlet and at the exhaust. Once collected, the data is sent to a differential pressure transducer, which is indicated on a flight deck EPR gauge. An EPR gauge is used to indicate the power output of a turbojet/turbofan engine.

Question 25

What are thrust reversers?

Answer 25

In turboprop aircraft, the reversible pitch propeller acts as a thrust reverser by re-directing thrust forward. In turbojet/turbofan engines, a thrust reverser is a device installed in the exhaust gas stream, usually somewhat to the rear of the nozzle. The engine exhaust gases are mechanically blocked and diverted in the reverse direction by an inverted cone, half sphere, or other means of obstruction, which is placed in position to reverse the flow of exhaust gases. They significantly reduce landing distances, as well as brake fade, and contribute to extended tire life. There are two types, mechanical blockage and aerodynamic, and they are sometimes referred to as clamshell, cascade, or petal-door type thrust reversers.

Question 26

What are the two basic types of turbine engine lubrication systems?

Answer 26

Both wet- and dry-sump lubrication systems are used in gas turbine engines. Most turbojet engines are of the axial-flow configuration, and use a dry-sump lubrication system. However, some turbine engines are equipped with a combination dry and wet type of lubrication system.

Question 27

What is the main difference between wet- and dry-sump lubrication systems?

Answer 27

Wet-sump engines store the lubricating oil in the engine proper, while dry-sump engines use an external tank that is usually mounted on the engine (or somewhere in the aircraft structure near the engine).

Question 28

How is the oil cooled in a turbine engine?

Answer 28

To ensure proper temperature, oil is routed through either an air-cooled or a fuel-cooled oil cooler. With an oil-to-fuel heat exchanger, not only is heat removed from the oil but the fuel is kept at proper temperature, thus eliminating the potential for ice formation within the fuel.