Powerplant

Question 1

Describe the major components of a gas turbine engine.

Answer 1

An air inlet, compressor section, combustion section, turbine section, exhaust section, accessory section, the systems necessary for starting, lubricating, fuel supply, and auxiliary purposes, such as anti icing, cooling, and pressurization

Question 2

Turbine engines are classified according to the type of compressors they use. What are the three types if compressors found in turbine engines?

Answer 2

Centrifugal flow, axial flow, and centrifugal-axial flow

Question 3

Describe a centrifugal-flow compressor

Answer 3

The 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 centrifugally outward into a diffuser, where the pressure of the air is increased. The pressurized air is then supplied to the combustion section

Question 4

What is the main function of the diffuser section of a turbine engine?

Answer 4

The diffuser is 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 so that the flame combustion can burn continuously

Question 5

Describe an axial-flow compressor

Answer 5

The axial flow compressor 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 of to 16 stages.

Question 6

Explain the function of stator vanes.

Answer 6

The function of the stator vanes is to receive 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 7

Explain the operation of a centrifugal-axial flow compressor.

Answer 7

Utilizes both kinds of compressors to achieve the desired compression. A typical free power turbine engine has two independent counter-rotating turbines. One turbine drives the compressor, while the other drives the propeller through a reduction gearbox. The compressor stages combined with a single centrifugal compressor stage. The axial and centrifugal stages are assembled on the same shaft and operate as a single unit.

Question 8

What are the four types of gas turbine engines?

Answer 8

1. Turbojet
2. Turbofan
3. Turboprop
4. Turboshaft

Question 9

Explain a Turbojet.

Answer 9

Consists of a compressor, combustion chamber, turbine section, and exhaust section. The compressor section passes inlet air at a high rate of speed to the combustion chamber, which contains the fuel inlet and igniter for combustion. The expanding air drives a turbine, which is connected by a shaft to the compressor, sustaining engine operation. The accelerated exhaust gases from the engine provide thrust. Turbojet engines are limited in range and endurance. They are slow to respond to throttle applications at slow compressor speeds.

Question 10

Explain a Turbofan.

Answer 10

Developed to combine the best features of the turbojet and turboprop. Turbofan engines create additional thrust by diverting secondary airflow around the combustion chamber. The turbofan bypass air generates increased thrust, cooks the engine, and aids in exhaust noise suppression and provides, turbojet type cruse speed and lower fuel consumption. The inlet air that passes through a turbofan engine is usually divided into two separate streams of air. One stream passes through the engine core, while the second bypasses the engine core. A turbofans bypass ratio refers to the ratio of the mass airflow that passes through the fan divided by the mass airflow that passes through the engine core.

Question 11

Explain a Turboprop.

Answer 11

A turbine engine that drives a propeller through a reduction gear. Exhaust gases drive a power turbine connected by a shaft that drives the reduction gear assembly. Reduction gearing is necessary in turboprops because optimum propeller performance is achieved at much slower speeds than the engines operating RPM. Turboprop engines are most efficient at speeds between 250 and 400 mph and altitudes between 18,000 and 30,000 feet. They also perform well at the slow airspeeds required for takeoff and landing are fuel efficient. The minimum specific fuel consumption of the turboprop engine is normally available in the altitude range of 25,000 feet to the tropopause.

Question 12

Explain a Turboshaft.

Answer 12

Delivers power to a shaft that drives something other than a propeller. The biggest difference between a turbojet and turboshaft engine is that on a turboshaft engine, most of the energy produced by the expanding gases is used to a drive a turbine rather than produce thrust. Many helicopters use a turboshaft gas turbine engine. In addition, turboshaft engines are widely used as auxiliary power units on large aircraft.

Question 13

Explain the term engine pressure ratio (EPR).

Answer 13

EPR is 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 date is seen 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 14

Define EGT.

Answer 14

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

Question 15

Define TIT.

Answer 15

Turbine inlet temperature - The temperature of the gases from the combustion section of the engine as they enter the first stage of the turbine.

Question 16

Define ITT.

Answer 16

Interstate turbine temperature - The temperature of the gases between the high pressure and low pressure turbine wheels

Question 17

Define TOT.

Answer 17

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

Question 18

What is the N1 indicator providing?

Answer 18

Represents the rotational speed of the low pressure compressor and is presented on the indicator as a percentage of design RPM. After start, the speed of the low pressure compressor is governed by the N1 turbine wheel. The N1 turbine wheel is connected to the low pressure compressor through a concentric shaft.

Question 19

What is the N2 indicator providing?

Answer 19

Represents the rotational speed of the high pressure compressor and is presented on the indicator as a percentage of design RPM.

Question 20

Why is there sometimes an N3 indicator in turbofan engines?

Answer 20

A triple spool engine would have an N3 indicator. This is a turbofan engine design where the fan is the N1 compressor followed but the N2 intermediate compressor and then the N3 high pressure compressor, all of which rotate on separate shafts at different speeds.

Question 21

Where are the highest temperature located in a turbine engine?

Answer 21

Occurs at the turbine inlet. TIT is therefore usually the limiting factor in turbine engine operation.

Question 22

Explain the factors that would affect the thrust output of a turbine engine.

Answer 22

1. Air density
2. Increasing temperature
3. Humidity

Question 23

How does an air turbine starter accomplish turning the high pressure compressor?

Answer 23

An axial flow turbine that turns a drive coupling through a reduction gear train and a starter clutch mechanism. The air to operate the air turbine starter is supplied from either a ground operated air cart, the APU, or a cross bleed start from an engine already operating.

Question 24

What are igniters?

Answer 24

A typical gas turbine engine is equipped with igniters that provide a high heat intensity spark used to ignite the fuel air mixture. A typical ignition system includes two exciter units, two transformers, two intermediate ignition leads, and two high tension leads. As a safety factor, the ignition system is actually a dual system, designed to fire to igniter plugs. This type of ignition system provides a high degree of reliability under widely varying conditions of altitude, atmospheric pressure, temperature, fuel vaporization, and input voltage.

Question 25

What is the function of the accessory section on a gas turbine engine?

Answer 25

To provide space for mounting of accessories necessary for operation and control of the engine. Generally, it also includes accessories concerned with the aircraft, such as electric generators and hydraulic pumps. Secondary functions include acting as an oil reservoir and/or oil sump and housing the accessory drive gears and reduction gears.

Question 26

What are the basic components of a turboprop engine?

Answer 26

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

Question 27

What information is provided by a torquemeter in a turboprop aircraft?

Answer 27

Turboprop and shaft engines are designed to produced torque for driving a propeller and power output is measured by a torquemeter. This measure the power applied to the shaft and is calibrated in percentage units, foot-pound, or psi.

Question 28

Explain the operation of a jet engine thrust reverser.

Answer 28

A thrust reverser is a device fitted in the engine exhaust system that effectively reverses the flow of the exhaust gases. The flow does not reverse through 180; however, the final path of the exhaust gases is about 45 from straight ahead. This, together with the losses in the reverse flow paths, results in a net efficiency of about 50 percent. It produces even less reverse thrust if the engine RPM is less than maximum in reverse.

Question 29

Explain a Target reverser.

Answer 29

Simple clamshell doors that swivel from the stowed position at the engine tailpipe to block all of the outflow and redirect some component of the thrust forward.

Question 30

Explain a Cascade reverser.

Answer 30

Normally found on turbofan engines and are often designed to revere only the fan air portion. Blocking doors in the shroud obstructs forward fan thrust and redirects is through cascade vanes for some reverse component. Cascades are generally less effective than target, particularly those that reverse only fan air, because they do not affect the engine core, which continues to produce forward thrust.

Question 31

Is reverse thrust application more effective at high or low speeds?

Answer 31

High speeds for two reasons, the net amount of reverse thrust increases with speed; and the power produced is higher at higher speeds because of the increased rate of doing work. Kinetic energy of the airplane is being destroyed at a higher rate of speed, to get maximum efficiency it should be used as soon as the aircraft touches down.

Question 32

How is inadvertent deployment of thrust reversers prevented while in flight?

Answer 32

The systems normally contain several lock systems; one to keep reversers from operating in the air, another to prevent operation with the thrust levers out of idle detention, and/or an auto-stow circuit to command reversers stowage at any time thrust reversers deployment would be inappropriate, such as during takeoff or while airborne.

Question 33

Explain the start sequence of a gas turbine engine.

Answer 33

1. The compressor section is rotated by an electrical starter on small engines or an air driven 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 chamber
3. Fuel in 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 temp (4,000F), which heats the entire air mass to 1600-2400F
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