Basic Gas Turbines Flashcards
Maximum thrust from a gas turbine is limited by the…
maximum turbine temperature
higher temp means greater volume which means more mass flow
EPR is a measure of
Static pressure at the exhaust outlet to the inlet air pressure.
A blocked EPR probe will cause the gauge to over or under-read?
Over-read (high amount of static pressure)
The main difference between a turbojet and a turbofan engine is
The addition of a large fan (N1) to the compressor which produces most of the thrust, compared to a turbojet which gets all the thrust from the exhaust (mass flow of air)
The benefit of a high bypass turbofan in comparison to turbojets is
Better efficiency and quieter
Turboprops have low exhaust velocity because…
Most of the thrust (exhaust) produced is absorbed by the turbine and delivered to the propeller via a reduction gearbox.
The Gas Turbine working cycle is known as the brayton cycle and is characterised by…
constant pressure during combustion, and simultaneous occurence of all events (inductions, compression, ignition, exhaust).
What do convergent ducts do to static pressure and temperature, and velocity
Decrease pressure and temperature, increase velocity
What do divergent ducts do to static pressure and temperature, and velocity
Increase pressure and temperature, decrease velocity
What is different about a centrifugal flow compressor to an axial flow compressor
A centrifugal flow compressor makes use of an impeller to accelerate mass air flow and impart a pressure rise, and a diffuser (divergent duct) to further increase the pressure.
An axial flow compressor makes use of a series of rotor blades (impellers) seperated by stator vanes (diffusers - divergent ducts) which increase pressure at each stage.
Stator vanes are shaped to
direct air from one set of rotors to the other at the correct angle of attack
An advantage of an axial flow compressor to a centrifugal flow compressor is…
an axial flow compressure gives a bigger pressure rise for the same frontal area (pretty much a bunch of centrifugal compressors lined up)
A limitation of stator vanes is that they…
only provide air to the rotors at the correct angle of attack during design RPM (92%-98%)
The purpose of an Variable stator Vane airflow control system is to
maintain the correct angle of the stator vanes to deliver air to the rotors at the correct angle of attack during operation away from design RPM
A stall occurs when…
The compressor blade angle of attack exceeds the critical angle of attack
A stall is LIKELY to occur during….
rapid RPM changes
A surge (flow reversal) is likely to occur when
the pressure between the compressor and combustion chamber begins to neutralise
Symptoms of a surge are
dropping RPM, increased fuel flow, and an immediate rise in EGT
Compared to a centrifugal flow compressor, axial flow compressors…
ahieve greater efficiency and power, and are simpler and cheaper, however centrifugal compressors trade power for reliability (less likely to stall or surge)
What do bleed air valves do?
Used on axial flow compressors, a valve is opened during off-design RPM periods to reduce the compressor outlet static pressure
When are a rich flameout and lean flameout likely to occur
Rich: low speed high power climb
Lean: High speed low power descent
Remember power is fuel flow, too much power with not enough air is too much fuel, and vice versa
Of all the types of combustion chambers (single, multiple, tubo-annular, and annular), which is superior and why?
Annular in terms of combustion efficiency, cost and weight
Less bullshit to get through, less energy to lose
Nozzle Guide Vanes are
Stationary Convergent Ducts located at the front of a TURBINE which increase velocity to help rotate the turbine
What are the two types of rotor blade types?
Impact: Absorbs energy from air flow like a water wheel
Impact/Reaction: Lots of little aerofoils. More efficient than just impact.
The purpose of a shroud is to…
Reduce gas leakage, reduce vibration, and increase strength of the blades.
What is blade creep and what are the 3 types?
Blade creep is a gradual increase in length over time which can cause damage as the blades hit the casing or brake. Primary blade creep occurs during manufacturing, and secondary blade creep occurs during service, and tertiary creep occurs at the moment of failure.
What type of electrical circuits are temperature probes set up in?
Parallel, provides an average reading.
The purpose of the support strut is to
keep the exhaust flow streamline (maintain efficiency)
The purpose of the jet pipe is to
transfer exhaust gases from the turbine to the propelling nozzle
Is the propelling nozzle divergent or convergent, and why?
Convergent, gives the airflow a last little velocity boost.
What is the main way in which noise is supressed in a jet engine
Mixing cool air (low velocity)
What is thrust augmentation and the two main types?
A method used to increase thrust short term.
Afterburner: Extra fuel burnt in jet pipe to increase velocity
Water/methanol injection: Added at compressor stage to increase density (water), and burn in the compressor (methanol)
At what speeds is reverse thrust NOT used
below 60-80kts
Cold stream reversers make use of what type of reverse path
via ‘blocker doors’
Hot stream thrust reverses make use of what type of reverse path?
Clamshell doors
What is the purpose of an interlock
Prevents accidental activation of reverse thrust during high power operation
Why does a gas turbine engine use LESS oil than a piston engine?
No oil is burnt during the combustion phase
What is the ‘beta range’?
Used on turboprop aircraft, blade angle is no longer governed by prop governor, but by power lever and pil pressure control system….ultimately allows less power output on the ground.
What is a Negative Torque System?
provides a signal to increase propeller blade angle (increase prop torque) to stop the propeller driving the engine.
When is an NTS activated?
During fuel interruptions, gust loads on the propeller, high compressor air bleed conditions at low power, and normal shutdowns (prop still spinning but not engine)
What does the safety coupling do in a turboprop?
disengages the gearbox in the case of a propeller driving the engine (overspeed)
Does the pilot need to manually change the fuel mixture as altitude is changed?
No, the Fuel Control Unit is automatic, no mixture control.
What is the purpose of an atomising fuel spray nozzle
Mix fuel and air to give good atomisation for efficient combustion
What is the purpose of ‘continuous ignition’?
instant relight in the case of a flameout. The igniter plugs energy output is lowered (less joules) to preserve the unit.
What is an igniter plug?
Like a spark plug with a high energy (joule), hot, high voltage spark.
Where would you expect to find the highest gas velocity in a turbojet engine?
At the end of the exhaust nozzle
The purpose of a diffuser in a gas turbine engine is to
increase pressure and reduce velocity
As air temperature increases, what happens to thrust?
Decreases
Where would you expect to find the highest gas pressure in a turbojet engine?
Diffuser
Where would you expect to find the highest gas temperature in a turbojet engine
In the combustion chamber
The engine core in a turbofan refers to
The compressor, combustion chambers, turbines and exhaust
The maximum thrust output of a jet engine will increase or decrease with a decrease in temperature?
increase
The effect an increase in altitude alone will have on the low pressure compressor of a gas turbine engine if constant thrust is maintained is
rpm will increase
A divergent duct causes subsonic pressure and velocity to
increase and decrease respectively
What effect does the RAM effect have on the thrust output as TAS increases
Thrust increases
What happens to the velocity, gas pressure, and temeperature as air flows through the turbine section of a turbujet
Velocity INCREASES, gas pressure reduces, and temperature reduces.
Tertiary creep occurs…
At the point of failure
Unstable airflow through the compressor is likely to result in
compressor stall
Where is maximum gas pressure in a turbine engine found?
At the compressor exit
The purpose of variable inlet guide vanes is to
prevent compressor blade stall during operation off design rpm
Why does turbine aircraft range increase with altitude?
Higher alt allows operation at design RPM which further allows the aircraft wings to maintain an efficient wing angle of attack
When bleed air is use, what happes to gas temp, thrust, and SFC
increase in gas temp, reduced thrust, increased SFC
Maximum gas pressure in a gas turbine engine is found
at the compressor exit/combustion chamber inlet
The main reason for a limit on the maximum gas temperature in a gas turbine is to
protect the turbines from overheating
Burning fuel in the combustion chambers is necessary to
increase the volume and velocity of the gases
When leaving the compressor, does air pass through a divergent or convergent duct?
Divergent, aim is to increase pressure further
In a free power turboprop, the propeller is driven by the…
rear low pressure turbine (fastest RPM)
