AFT aero and systems notes Flashcards
What are the 4 types of reaction engines?
Rocket
Ram Jet
Pulse Jet
Gas Turbine
What is a rocket?
Self contained air and fuel in pressurised container
What is a ram jet? What entry and exhaust duct type does it have?
No moving parts, requires large forward speed to operate
Divergent entry duct
Convergent exhaust duct
What is a pulse jet?
Similar to Ram jet but on intermittent cycle
What is a gas turbine?
Turbine driven compressor. Does not require forward airspeed to compress the air entering the engine
What is maximum thrust of a gas turbine limited by?
The maximum turbine temperature
If there was a large increase in gas temperature from the air intake to the turbine, what happens to the volume of gas, acceleration of the air and thrust produced?
Volume of gas increases
Acceleration of air increases
Thrust produced increases
What happens to thrust as air temperature increases?
Thrust decreases
What happens to thrust as altitude increases?
Thrust decreases due to less dense air
What happens to thrust if the mass air flow decreases?
Thrust decreases
What is the thrust formula?
Thrust = (Mass airflow X Jet exhaust velocity) - Aircraft forward speed
Describe a basic pure turbo jet.
One or more compressors driven by set of turbine wheels.
What is a spool?
Compressor at the front of an engine and its accompanying set of turbine wheels at the rear
How does a pure turbo jet deliver its thrust?
Reaction to the rearwards acceleration of exhaust gases after it has passed through the turbine sections of the engine
What happens to the static pressure at the exhaust outlet of pure turbo jet compared to the intake?
Roughly double
What is the EPR?
Engine pressure ratio.
Static pressure at the exhaust outlet compared to that at the intake
What indication does the EPR give to pilots?
Thrust output
What would a blocked EPR cause to the gauge?
Cause it to overread
Describe the basic high by-pass jet (turbo fan)
Not all air passes through the engine core and combustion chamber. Large fan (N1) is at the front of the compressor and driven by extra turbine at rear of engine
What is the by-pass ratio?
Ratio of air which bypasses the engine core compared to the air which passes through the core
What is the formula for the by-pass ratio?
By-pass ratio = (By-pass mass flow) / (Engine core mass flow)
What would an engine sound like, efficiency be with a high by-pass ratio?
Quieter and more efficient
Where is the majority of thrust produced in a high by-pass ratio engine?
From the by-pass fan air
Where is the majority of thrust produced in a low by-pass ratio engine?
From the jet exhaust
What are the advantages of the free power turbine turbo prop?
Lower starting torque (since starter doesn’t drive the prop)
Variable prop speed
High torque at low speed
What is the main difference between gas turbine and piston cycle?
Gas turbine is continuous
What is the role of convergent ducts?
Increase velocity
Decrease pressure
Decrease temperature
What is the role of divergent ducts?
Decrease velocity
Increase pressure
Increase temperature
In the compressor, what happens in the centrifugal flow?
Impeller accelerates mass air flow and imparts a pressure rise and a diffuser (divergent duct) to increase pressure
What does an axial flow compressor consist of?
Rotor blades attached to main rotor assembly
Stator vanes between each rotor blade (do not rotate)
What is the role of the rotor blades and stator vanes in axial flow compressor?
Rotor blades increase velocity of airflow
Stator vanes are located between each rotor blade, do not rotate. They increase pressure by forming divergent duct
How are rotor blades shaped in axial flow compressor?
Aerofoil shaped and therefore must be presented at correct angle of attack
How are stator vanes shaped in axial flow compressor?
Shaped to direct air from one set of rotors to the next at correct angle of attack
Where is the largest air load in the axial flow compressor?
Rear most blades of compressor
Compared to a centrifugal compressor of the same frontal size, what does an axial flow compress do to air and pressure?
Consumes more air and gives bigger pressure rise
What types of airflow control are there?
Variable inlet guide vanes (VIGV)
Variable stator vanes
Bleed air valve (surge valve)
What are variable inlet guide vanes? Where are they located?
Change angle at which relative airflow meets the first stage compressor, preventing surge and stall.
Located at front of compressor
What are variable stator vanes?
Change angle of each row of stator vanes to maintain correct angle of attack when engine operating ‘off design RPM’
What are bleed air valve (surge valve)? Where is it used? What sort of compressor is it used in?
Used with axial flow compressors, valve towards rear of compressor.
Valve opens when operating off design RPM to reduce compressor outlet pressure and increase mass airflow across early stages of compressor
What percentage of total mass airflow enters the combustion chamber through the primary zone? What is it for?
20%
Flame stabilisation and combustion
What percentage of air enters the primary zone through holes in the wall of the flame tube?
20%
How much of the total mass airflow is used for cooling and dilution further down the chamber?
60%
What percentage of air is used for combustion? What is it called?
12%
Primary air
Where does primary air enter the combustion chamber?
Through the swirl vanes at the front of the combustion chamber
What is the purpose of the swirl vane?
Reduce axial flow
Keep flame stable in the primary zone of combustion
Where does the highest gas temperature occur during combustion?
In the primary zone of combustion
When is a rich flameout possible?
Mixture is too rich
low speed and high powered climb
When is a lean/weak flameout possible?
Mixture is too weak
High speed and low power descent
With a multiple combustion chamber, how is pressure equalised? What else does this device allow?
With interconnectors.
Allows flame propagation between chambers
What is the most effective combustion chamber in terms of efficiency, cost and weight?
Annular
What are NGV?
Nozzle guide vanes.
Positioned in front of each turbine wheel.
Stationary aerofoil blade shaped as convergent ducts
What is the purpose of the NGV?
To increase velocity of gas stream onto next set of turbine blades at correct angle of attack
What are turbine blades and what are their purpose?
Blades attached to turbine disc.
Extract energy from gas stream to drive compressor, fan or gearbox
What are shrouds?
Caps on the tips of some blades to reduce gas leakage, reduce vibration and increase strength
What is blade creep? What causes it?
Increasing in length of turbine blades
Caused by excessive exposure to high temps, centrifugal forces and stress
What are the different types of blade creep and when do they occur?
Primary creep (during manufacture) Secondary creep (during service life) Teriary creep (failure)
What is the most critical temperature in the engine?
Turbine temperature
In a turbo jet engine, how much energy of the total energy of the gas stream is used to drive compressor?
75%
In a turbo jet engine, how much energy is used for thrust?
25%
What are support struts?
Used to keep exhaust flow streamlined
What does the jet pipe do?
Carries exhaust gases from turbine to propelling nozzle
What is the propelling nozzle?
Convergent to Increases the velocity of exhaust to give final increase in thrust
What is the main source of noise from a turbo jet?
Exhaust noise
What is the main method of noise suppression for turbo jets?
Increase frequency (Hz) of exhaust noise from low to high. Done by mixing cool air with hot exhaust gases
Which is quieter, turbo prop or turbo fan?
Turbo fan
What is the main source of noise from a turbo fan?
The fan and the turbine
What is thrust reversal?
Angling the thrust to go forwards
What does thrust reversal do to landing distance?
Decreases landing distance
When should thrust reversers be used?
Early stages of landing roll when speed is high.
Why should we not used thrust reverse at low speeds?
Hot turbulent air can enter the engine along with foreign objects
What reverse thrust is used on turbofans?
Cold by-pass only (coldstream reversers such as blocker doors)
How do blocker doors work?
Blocker doors in the by pass duct to redirect coldstream air out through cascade vanes in the engine cowl
What are interlocks?
Prevent thrust lever from being moved to reverse position unless engine is running at low power setting
What are turbine oils like?
Medium to low viscosity
Stable under high operating temperatures
What two things act together to maintain a constant RPM in a turbo prop?
Fuel control unit
Propeller governor
What happens in turbo prop as power levers are advanced?
FCU sends extra fuel to combustion chamber which increases engine output and tries to drive turbine faster.
Prop governor detects this and coarsens blades to absorb extra energy and provide more thrust at constant RPM
What is the beta range?
In a turbo prop, the ground handling range, where the prop blade angle isn’t controlled by the prop governor but via the power lever position via oil pressure control system
What is the NTS?
Negative torque system.
Provides a signal which increases prop blade angle to limit negative shaft torque
What conditions is NTS likely to operate?
Temporary fuel interruptions
Air gusts loads on prop
Normal descents with lean fuel scheduling
High Compressor air bleed conditions at low power
Normal shutdowns
What is TSS?
Thrust sensitive system (autofeather)
What does TSS do?
Automatically feathers prop if there is a power loss during take-off.
Automatically increases blade angle and causes prop to feather
What is the safety coupling?
Safety feature of the turbo prop.
Disengages the reduction gear from engine if it’s operating about a preset negative torque value. (much higher value then required for NTS)
After being disengaged, when will the reduction gear be re-engaged?
Automatically during feather or engine shutdown
What is a prop brake?
Rapidly slow prop after engine shut down.
Prevents windmilling on ground
In a turbojet fuel system, what happens to fuel flow as altitude increases?
Fuel flow decreases
What are impellers used for in the fuel system?
Boost pumps in tanks to supply high pressure
What problems could there be if using AVGAS instead of kerosene in a jet engine?
Decrease range
Poor altitude performance
Over temperature
Over speed
What is the specific gravity and flash point of AVTUR compared to AVGAS?
AVTUR SG = 0.8, flash point = 46 degrees
AVGAS SG = 0.72, flashpoint = -40 degrees
How is fuel measured?
Capacitance type system with probes (capacitors) in fuel system
What is a standpipe?
Prevents accidental dumping of entire fuel load if the dumps not shut off manually at desired quantity
What are baffles?
Dividing partitions in fuel tank used to limit movement of fuel
Hot bleed air from the compressor, hot oil or electrical system is used to prevent ice on what devices?
Wing leading edge and props on turbo props
What negative effect does hot air anti ice have?
Decrease in thrust when air is taken from compressor
What is the normal procedure for starting the turbine engine?
Starter and ignition on
When N2 = approx 10/15% add fuel and watch for light off
Check for rapid TOT increase to confirm light off
What speed is self sustaining?
35-50%
When can a hot start occur?
High fuel flow
Low RPM
Tailwind
What are the indicators of a hot start?
After light off, TOT increases beyond maximum
What are the indicators of a wet start
Added fuel at 15% but no light up occurs
What is a hung start?
Not enough starter power to reach self sustaining speed
What may a hung start lead to?
A hot start
What should be done in the event of a hot, wet or hung start?
Shut fuel off
Keep starter on
What is the formula for specific fuel consumption?
Fuel flow / thrust produced
Is a high or low specific fuel consumption (SFC) preferred?
Low
What will cause an increase in specific fuel consumption?
Increase in ambient temperature
Decrease in compressor pressure
Operation outside design RPM
What are the advantages of turboprop?
Higher thrust at low forward speeds
Lower thrust specific fuel consumption
Quieter than turbofan or pure jet
More suited to short sectors
What are the disadvantages of turboprops?
Engines are heavier
Not suited to high altitude
Not suited to long sectors
What is the APU used for?
Provide source of electrical and pneumatic power independent of ground support or aircraft engines
In the event of an engine failure, what can be done to assist with electrical load?
Turn on the APU
What happens to engine bleed air that is not required by the aircraft of APU?
Dumped overboard downstream of turbine section
What will activate the APU automatic shutdown?
Fire detection Automatically activated fire extinguisher Low oil pressure High oil temp High exhaust gas temp Overspeed Duct overheat
What happens in a convergent duct if the flow is supersonic?
Velocity decreases
Pressure rises
Density rises
What happens in divergent duct if the flow is supersonic?
Velocity increases
Pressure decreases
Density decreases
What is subsonic flow?
Flow velocity less than speed of sound
What is sonic flow?
Flow velocity at speed of sound
What is supersonic?
Flow velocity greater than speed of sound
What is Mfs?
Mach Free Stream
Mach number of aircraft through the air
What is the local mach number?
Mach number of the flow at a particular location on an aircraft/aerofoil
What is Mcrit?
Critical mach number.
Mach number which first produces evidence of local sonic flow.
Start of transonic range
Above what speed does shockwaves form on the wing?
Mcrit
What is Mcdr?
Critical drag rise mach number.
Significant shockwave related problems as shockwave grows and moves rear
What is higher, Mcrit or Mcdr?
Mcdr
What is Mdet?
Detachment mach number.
Aircraft mach number at which bow waves attaches to leading edge or nose.
What speed range does Mdet mark? And what is the flow type?
Marks end of transonic speed range
All flow is supersonic above Mdet
What is Mmo?
Maximum operating mach number.
Highest mach number at which aircraft can be intentionally flown
What happens if you fly above Mmo?
Shock wave formation
Control problems
High airframe stresses
What happens to velocity, mach number, static pressure, density, temp and total pressure in a normal shockwave?
Decrease in velocity and mach number (subsonic)
Increase in static pressure, density and temperature
Less total pressure
Where are oblique shockwaves likely to form?
Around compressive corners in supersonic flow
What happens to airflow in oblique shockwave?
Change in direction
Reduction in velocity and mach number
Rise in density and static pressure
What happens to static pressure, velocity and temp in expansion waves?
Decrease in static pressure and temp
Increase in velocity
What is the speed range for subsonic?
0-0.75M
What is the speed range for transonic?
0.75 - 1.20M
What is the speed range for supersonic?
1.2-5-0M
What is the pitching tendency as shockwave forms?
Pitch down
What is mach tuck?
CP moving rearwards generating a nose pitch down
What does the Mach trimmer do?
Automatically applies back elevator trim to compensate mach tuck as speed increases
What are the advantages of swept back wings?
Delays compressibility effects
Increase Mcrit and Mcdr
Increase force divergent number
Reduced CL changes with both speed and AoA
Improved lateral and directional stability
What are the disadvantages of swept back wings?
Tendency to stall tip first
Reduced max CL and higher stall angle
Reduced trailing edge device effectiveness
Dutch roll tendency
Structural complexity
High induced drag at high angle of attack
What is super-critical wing section?
Flattened top surface to reduce top surface acceleration and delay compressibility effects
What does super-critical wing section do?
Helps lift at low speed and high angle of attack
What happens to lift, drag, pitch and centre of pressure as aircraft passes Mcrit?
Increase in drag
Decrease in lift
Pitch down
CP move rearwards
What do vortex generators do?
Delay shock induced separation by reenergising boundary layer.
What can be done to assist with dutch roll tendency?
Yaw dampers
What do yaw dampers do?
Automatically applying a cancelling rudder input when yaw is sensed
What are the two types of yaw dampers?
Series and parallel
What does the parallel type yaw damper do? When can it be switched on/off?
Moves rudder pedals
Must be off for take off/landing
What does the series type yaw damper do? When can it be on/off?
Does not move rudder pedals
Can be on at all stages of flight
What provides the signals to the yaw damper?
Rate gyro
What does a swept wing do to stall angle of attack, maximum CL and drag?
Increases stall angle of attack
Reduces max CL
Increases drag
What is the lift and drag like of a swept wing compared to a straight wing?
Less lift more drag
As a swept wing approaches the stall, what happens to centre of pressure?
CP moves forwards and inwards
In a swept wing plane, what is tendency of pitch at stall?
Pitch up
What does increased gross weight do to stall speed?
Increases the stall speed
What do use of flaps do to stall speed and CL?
Decrease stall speed
Increase max CL
What does a forward CoG do to stall speed?
Increases stall speed
At higher altitude, what is the stall speed IAS and EAS?
IAS and EAS stall speed increases.
IAS greater than EAS
What is Va?
Manoeuvring speed
What happens if full control deflection occurs below Va?
G stall before load limit is reached
What happens if full control deflection occurs above Va?
Load limits being exceeded before stall occurs
What is the Va for a heavier aircraft compared to a lighter one?
Faster for heavier gross weight
In equilibrium what do all the forces equal?
Weight = lift drag = thrust
Where does lift act through?
Centre of pressure
What is the lift formula?
CL x 1/2 x p Vsquared x S Wherer CL = Coefficient of lift p = air density V= velocity or TAS of air flowing around wing S = Surface area of the wing
What is the formula for power?
Thrust x TAS
What is Vimd?
The speed where total drag is minimum
Indicated minimum drag
What is Vimd also known as?
Minimum drag speed Best lift/drag ratio Minimum fuel flow Best endurance Maximum excess thrust Max angle of climb
Where does max still air range occur?
1.32 Vimd
In a headwind, what do you need to do to achieve max range?
Fly faster then 1.32 Vimd
What speed do you need to fly at for max endurance?
Minimum fuel flow
Vimd
What speed are you said to be flying at if you are in the region of reverse command?
Less that Vimd
In a steady climb, what is thrust, drag, lift and weight?
Thrust > Drag
Weight > Lift
Climbing using maximum excess thrust would be climbing at what?
Max angle of climb (Vx)
Vimd
Climbing using max excess power would be climbing at what?
Max rate of climb (Vy)
What happens when flaps are extended?
Increase in lift
Increase in drag
Lower Vimd compared to clean
What is the TAS and IAS for max rate of climb at a higher altitude compared to lower altitude?
TAS = Higher IAS = slightly lower
What happens to the minimum control speed as altitude increases?
Decreases
What does a forward CoG do to minimum control speed?
Lowers it
What does flap extension do to minimum control speed?
Increases it
What does gear extension do to minimum control speed?
Increases it