Explanations: Turbine Engines

Question 1

The compressor outlet is the same as the what?

Answer 1

the burner inlet.

Question 2

In what direction does the nozzle diaphragm in a turbine engine deflect the gases?

Answer 2

In the direction of turbine wheel rotation.

Question 3

What is the purpose of profiling a compressor blade?

Answer 3

Profiles prevent serious damage to the blade or the housing if the blade should contact the compressor housing.

Question 4

What is the Brayton cycle?

Answer 4

It is the energy exchange cycle used in a turbojet engine. The pressure of the air remains relatively constant as the energy from the burning fuel is added to it. Since the pressure is constant as the volume of the gas increases, its velocity increases.

Question 5

What is the difference between a turboprop and turbojet engine?

Answer 5

In summary, turboprop engines are more efficient at lower speeds and are suitable for shorter-range flights, while turbojet engines are optimized for high-speed flight and are used in high-performance aircraft requiring supersonic capabilities.

Question 6

What is a two-spool (split-spool) compressor?

Answer 6

The compressor and high-pressure turbine are both split into two segments. Each compressor segment is driven by its corresponding turbine using two separate drive shafts, with one inside the other.

The high pressure rotor is governed for speed, but the low-pressure rotor is free to operate at its own best speed.

As the density decreases at altitude, the compressor load decreases and the low-pressure rotor increases its speed.

Question 7

What sort of duct is the exhaust cone aft of the turbine jet engine?

Answer 7

It is a divergent duct, increasing pressure and lowering velocity.

Question 8

What are the stator vanes at the end of the discharge of an axial-flow compressor called?

Answer 8

straightening vanes.

Question 9

What are the n1 and n2 compressors in a dual axial flow or twin spool compressor system.

Answer 9

The n1 compressor is the low-pressure compressor and

the n2 compressors is the high-pressure compressor.

Question 10

What names does the stator element of a turbine assembly have?

Answer 10

turbine inlet nozzle vanes, turbine inlet guide vanes, and nozzle diaphragm vanes are the three most commonly used.

They are located directly aft of the combustion chambers and immediately forward of the turbine wheel. This is the highest or hottest temperature that comes in contact with metal components in the engine.

Question 11

What is a hung start?

Answer 11

When an engine is unable to accelerate to a speed high enough to keep running without help from the starter.

Question 12

What are the two basic elements of a gas turbine engine?

Answer 12

Stator and rotor.

Question 13

What do the impeller and diffuser do in a centrifugal compressor?

Answer 13

The impeller adds energy to the air flowing through the engine and speeds it up. The diffuser slows the air down and increases its pressure.

Question 14

What does trimming a gas turbine engine consist of?

Answer 14

adjusting the fuel control to give the engine the correct idle and maximum RPM.

Question 15

What will happen is a bearing is allowed to retain it’s magnetism?

Answer 15

they will be damaged by foreign ferrous materials they attract.

Question 16

What is Tt7 and what is Pt12

Answer 16

Tt7 is turbine discharge total temperature

Pt12 is compressor inlet total pressure.

Question 17

What is TBO?

Answer 17

The recommended operating time between overhauls.

Question 18

Why must the materials used to mark the components in the hot section be carefully chosen?

Answer 18

so that they do not contaminate the structure of the metal when it is exposed to high temperatures.

Question 19

What is a compression ratio in turbine engines?

Answer 19

The ratio of the pressure of the air at the discharge of a turbine engine compressor to the pressure of the air at its inlet.

Question 20

What are impulse, reaction, and impulse-reaction turbine blades?

Answer 20

Impulse - The airflow strikes the blades, transferring its momentum to the blades

Reaction - the air flows energy is converted to rotational motion as it flows over the turbines. The pressure difference between the inlet and outlet of the turbine cause the rotation.

Impulse-reaction- combines both elements of the two design.

Question 21

What is the difference between the thrust produced by a turbojet or turbofan engine and that produced by a turboprop engine?

Answer 21

An engine driven propeller imparts a relatively small amount of acceleration to a large mass of air, and a turbojet/fan imparts a greater amount of acceleration to a smaller mass of air.

Almost all of the useful heat energy in a turboprop engine is used to drive the compressor and the propeller, and very little is used to provide exhaust thrust.

Question 22

What is a can type combustion chamber?

Answer 22

It consists of an outer case, and inside is a perforated stainless steel combustion chamber or liner. Both are removed as a unit for routine maintenance.

Each can has its own fuel injector, igniter, liner, and casing.

Question 23

How does turbine engines deal with airborne particulates?

Answer 23

There is normally efficient dust and particle separators in the induction sections.

Question 24

Why are turbine outlet temperatures monitored?

Answer 24

Because it is impractical to measure TIT, so TOT must be monitored. If TOT is within range, it is reasonable to assume TIT is also.

Question 25

Why must turbine blades be reinstalled in the same slot from which they were removed?

Answer 25

turbine blades are individually weighed and coded for installation in the disks in such a way that they best distribute the weight evenly around the disk.

Question 26

What is the difference between single and double entry centrifugal compressors?

Answer 26

single entry has scrolls on only one side, while double entry has scrolls on both sides.

A double entry is much like two single entrys back to back.

Question 27

How does stress rupture failure appear?

Answer 27

minute hairline cracks on or across the leading edge of the blade.

Question 28

What does a hot inspection of a gas turbine engine consist of?

Answer 28

a visual and dimensional inspection of the combustion section, turbine inlet guide vanes, turbine wheels, and all other components that operate in the high temp gas path.

They are conducted on a time or cycle basis, and when there has been a deterioration of certain of the engine parameters, such as EGT, EPR, or fuel flow.

Question 29

What problem can occur with excessively high compressor RPM?

Answer 29

if the compressor RPM is high, relative to the amount of air flowing though the engine, the angle of attack of the compressor blades will become excessive and a compressor surge or stall can develop.

To prevent a compressor stalls, the inlet guide vanes are turned to the correct angle, and the bleed-air valves are automatically opened by actuators controlled by the fuel control. The bleed air valves reduce the back pressure and allow more air to flow through the compressor to reduce the angle of attack of the compressor blades.

Question 30

How do bleed valves prevent compressor stall?

Answer 30

By allowing some of the air to be vented overboard to prevent the air from “piling up” in the higher pressure stages of the compressor.

Question 31

How are impulse reaction blades structured?

Answer 31

There is an impulse section at its root and a reaction section at its tip.

The exit pressure of an impulse reaction turbine blade is relatively constant across its length.

Question 32

What do most automatic fuel control units sense?

Answer 32

Inlet air temperature
compressor RPM
burner pressure (compressor discharge pressure)
position of the power lever (throttle)

Question 33

What two factors affect the angle of attack of axial flow compressor blades?

Answer 33

the velocity of the air through the engine and the RPM of the compressor.

If the airflow is restricted, reducing velocity, the AOA of the compressor blades will increase to such a point that compressor stalls can occur.

Question 34

What are annular and can-annular combustion chambers?

Answer 34

Annular have a ring-shaped configuration with combustion happening around the circumference.

can-annular are hybrid, typically featuring several individual cans arranged in a circular pattern.