The Compressor

Question 1

How does the compressor increase the mass flow through the engine

Answer 1

By compressing and raising the pressure of the air

Question 2

Pressure ratio

Answer 2

Ratio of compressor output to compressor inlet pressure

Question 3

Advantages and disadv of a centrifugal compressor

Answer 3

Simple
Robust
Short length
Not prone to stall surge

Large frontal area 
Poor acceleration 
Low pressure ratio 
Low mass flow - very limited by mass flow 
High SFC

Question 4

The axial flow compressor

Answer 4

Rotors and stators form divergent ducts which convert kinetic energy to pressure energy

Question 5

Pressure ratio of one stage in the axial flow compressor

Answer 5

1.2 : 1 or 1.1: 1

Question 6

Stator ensures that

Answer 6

The air is presented to the next rotor at the correct angle

Question 7

What happens to velocity, pressure and temp of over the rotor or the stator

Answer 7

Rotor - velocity, pressure and temp increase

Stator - velocity reduces, pressure and temp increases

Question 8

Axial flow compressor adv and disadv

Answer 8

Adv 
Small frontal area 
High pressure ratio 
High mass flow 
Low SFC 

Disadv
Expensive and complex
Prone to stall and surge
Easily damaged

Question 9

Axi CF compressor

Answer 9

Combination of axial and centrifugal

Found in APUS

Question 10

Relationship between SFC and pressure ratios

Answer 10

Greater the pressure ratio, the greater the SFC

Question 11

Stall and surge of the rotor blades

Answer 11

Operation outside the limits of RPM, pressure ratio and mass flow will result in a breakdown of flow around the compressor blades

If one stage stalls - turbulent air from one stage might hit the others causing a surge

Question 12

Where is the surge margin reduced and what can be done to increase the margin

Answer 12

Low air mass flow and low pressure ratio

Increased margin by the effect of VIGVs and stability bleed valves

Question 13

Causes of stall and surge

Answer 13

Anything that disrupts the airflow

Unusual a/c attitude

Damage of ice accretion on intake blades

Rapid changes in fuel flow

Question 14

Symptoms and effects of rotor blade stall

Answer 14

Very loud bang/rumbling
Pressure fluctuations - vibrations - could lead to blade failure
Flame out - reduction in mass flow - increase in gas temp - overheating of turbine and combustion sections

Question 15

Pilot actions when encountering rotor blade stall

Answer 15

Slowly close the throttle to restore the correct RPM/axial velocity

If it persists - shut down the engine

Question 16

Compressor stability bleed valves operation and purpose

Answer 16

High operating speeds - force conditions beyond the stability line - rear stage could become overloaded which could breakdown the airflow in the compressor

Allows a surplus of air to be dumped into the bypass duct

Question 17

Problem with compressor stability bleed valves

Answer 17

Below 75% RPM runs the risk of the valves being opened inadvertently - could lose the engine to flame out

Question 18

Variable inlet guide vanes purpose

Answer 18

To guide the air before the HP compressor so the first stator has optimum AoA

Between LP and HP compressor

Can be fixed or variable

Question 19

Multi spool designs allows

Answer 19

Each compressor to run at its optimum speed

Reduces the need for complex air control systems

Question 20

Engine bleed air effects on the engine

Answer 20

Reduces mass flow and thrust

SFC increases, EGT increases

Taken from different compressor stages

Question 21

What can sense engine overheat

Answer 21

A thermo switch that senses the temp of the air at the cooling outlet

Once it reaches a limit = warning on the f/d - eng needs to be shut down

Question 22

Accessory cooling for low RPM/starting the engine

Answer 22

Air from the compressor is thrown through a Venturi - sucks air in

A valve controls the flow of air from the compressor to ensure the injector system only operates during the ground running

Question 23

Severe compressor blade stall can occur

Answer 23

When compressor output falls below combustion input

Reduction of of mass flow at a constant throttle setting - causes an increase in gas temp (reduction of cooling air) - overheating of the combustion and turbine sections