Basic Gas Turbines

Question 1

What are the four types of reaction engines?

Answer 1

Rocket,
Ram Jet,
Pulse Jet,
Gas Turbine

Question 2

Describe a rocket engine

Answer 2

Self contained air and fuel in pressurised canisters

Question 3

Describe a Ram Jet engine

Answer 3

No moving parts, Divergent entry duct and convergent exhaust duct. Must have a large forward airspeed to operate (600kt)

Question 4

Describe a pulse jet

Answer 4

No moving parts, similar to a Ram Jet, Divergent entry duct and convergent exhaust duct. Works on an intermittent cycle.

Question 5

Describe a Gas Turbine

Answer 5

A Gas Turbine has a turbine driven compressor. It does not require forward airspeed to compress the air entering the engine.

Question 6

What factor limits the maximum thrust from a gas turbine?

Answer 6

Maximum Turbine Temperature determines maximum thrust.

A large increase in gas temperature from the air intake to the turbine means a large increase in the volume of gas, a large acceleration of the gas, and as a result thrust is produced.

Question 7

How does density affect the amount of thrust produced?

Answer 7

Cold intake air allows the engine to produce more thrust because cold air has high density. High mass air flow through the engine produces more thrust.
As Density reduces, mass airflow and thrust decreases.

Thrust output is a product of Mass Airflow and acceleration

Question 8

Thrust output is a product of:

Answer 8

Mass Airflow and Acceleration

THRUST = Mass Airflow x Vj - V.
Vj is the jet exhaust velocity and V is forward speed.

Question 9

A Spool refers to

Answer 9

A compressor at the front of an engine and it’s accompanying set of turbine wheels at the rear.

Question 10

EPR - Engine Pressure Ratio refers to?

Answer 10

The ratio of exhaust gas pressure compared to inlet air pressure. Indicates thrust output of a turbojet.

Question 11

What would a blocked EPR Probe cause?

Answer 11

An over-read of the EPR gauge.

Question 12

In a High Bypass (Turbofan) engine, describe the N1 Fan

Answer 12

A Large Fan added at the front of the compressor, driven by an extra turbine at the rear of the engine. Produces thrust by giving a relatively small acceleration to a large mass of air.

This bypasses the combustion chamber via a duct around the outside of the engine core.

Question 13

What does by-pass ratio mean?

Answer 13

The ratio of air which bypasses the engine core compared to the air which passes through the core.

It’s usually around 5:1 or higher. Approximately 75% of thrust is produced by by-pass fan air.

Question 14

What is the equation for by-pass ratio?

Answer 14

By-pass mass flow
—————————
Engine core mass flow

Question 15

How are spools numbered?

Answer 15

Front to back:
N1
N2
N3
Etc.

Question 16

Describe a free power turbine turboprop

Answer 16

The most common design of turboprop
Extra turbine stage called the “Free power turbine”.
It is on a separate shaft (Spool) not connected to the compressor or any other shaft or spool. The power from the turbine is fed via the shaft to a reduction gearbox and from there to an output shaft connected to the propeller.

Lower starting torque
Variable prop speed
High torque at low RPM

Question 17

Describe a direct drive turboprop

Answer 17

The output shaft to the reduction gearbox comes off the main compressor/turbine spool instead of having it’s own independent turbine.

Question 18

Describe a compound turboprop

Answer 18

Variation on direct drive design.

Two spools, propeller gear box driven from the low pressure compressor spool.

Question 19

Why is the reduction drive necessary on a turboprop?

Answer 19

Needs to reduce the very high RPM of the turbine to a manageable speed to use by the propeller. Small turboprop engines can rotate at 50000rpm, whereas propellers rotate closer to 2000rpm.

Question 20

Why do turboprops have low exhaust velocity?

Answer 20

The additional turbines absorb extra energy from the gas stream. A small amount (10%) of thrust is produced by the exhaust.

Question 21

Describe the Brayton cycle compared to the Otto Cycle?

Answer 21

Continuous cycle:

• Intake
• Compression
• Combustion
• Exhaust

One constant cycle - as opposed to single stages as found in the Otto Cycle

Constant pressure during combustion

Question 22

What is the role of a convergent duct?

Answer 22

Increases velocity

Decreases pressure and temperature

Question 23

What is the role of a divergent duct?

Answer 23

Decreases velocity

Increases pressure and temperature.

Question 24

Describe a Centrifugal Flow Compressor.

Answer 24

Consists of an impeller to accelerate the mass airflow and impart a pressure rise. Has a diffuser (Divergent duct) to further increase the pressure.

Air flows into the centre of the impeller and centrifugal force causes it to accelerate and flow outwards, a pressure rise also occurs through the impeller due to the addition of kinetic energy by the rotation of the impeller.