Aircraft - Powerplant Flashcards
Boyles Law
Pressure and volume inversely proportional.
P x V = constant
Assuming constant temperature
Charles Law
Volume and temperature are proportional.
V / T = constant
Assuming constant pressure
Combined Gas Law
P x V / T = constant
Bournoulli’s equation
Static pressure + dynamic pressure = total pressure (constant)
Newton’s laws
1) A body will continue in rest or in uniform motion in straight line, unless acted upon by a force
2) Rate of change of momentum is proportional to (and in direction) of applied force
3) Action and reaction are opposite and equal
Work
Force x distance
Power formula
Force x distance / time
[the rate of doing work]
Force equation
Mass x acceleration
Thrust
A force based on acceleration and mass of air pushed backwards:
F = M x (V(jet) - V(flight))
Where V(jet) = speed of air existing turbine or propeller
V(flight) = speed of air encountered
Horsepower measurements
- Indicated
- Friction
- Brake
- Thrust
Indicated HP - Theoretical reciprocating engine power, based on pressure developed in cylinders, ignoring work done in the engine.
Friction HP - Work done within the engine
Brake HP - Power delivered to the shaft
Thrust HP - Power converted to thrust (brake HP with propeller efficiency)
Relationship between BHP, IHP and FHP
BHP = IHP - FHP
Piston engine efficiency
- overall
- thermal
- volumetric
Overall: 80%
Thermal: 33% (heat lost through exhaust)
Volumetric: 80%
Piston energy type chain
Chemical energy ->
Heat energy ->
Pressure energy ->
Mechanical energy
Piston strokes
Induction
Compression
Power
Exhaust
Connecting rod
- description
- small and big ends
Joins piston to crankshaft.
“Small end” at piston
“Big end” at crankshaft
Top dead centre & bottom dead centre
Based on piston and combustion chamber at the top and crankshaft & connecting rod at the bottom.
So TDC is small cylinder volume, BDC is max cylinder volume.
Piston engine
- Mean Effective Pressure (MEP)
The average pressure exerted on the piston during the power stroke.
This determines the theoretical power of the engine.
3 issues with theoretical 4 stroke engine
Air has momentum.
At BDC and TDC linear movement of piston is very small despite rotational movement of crankshaft being constant (“area of ineffective crank angle”).
Combustion of fuel and increase in pressure is not instantaneous.
Changes in practical 4 stroke engine based on the issues with theoretical
Valve timing
Ignition timing
4 stroke valve timing
Both valves lead and lag.
This maximises the amount of mixture that can enter and reduces required valve speeds.
Some loss of power due to exhaust opening before power stroke is complete, but this is minimal due to limited piston movement at BDC.
Valve overlap
Lead and lag of the inlet and exhaust valves leads to a period when both are open.
This allows the momentum of exiting exhaust gas to “pull in” new mixture, called scavenging.
Valve timing - fixed or variable?
Valve timing is optimised based on the shape of the cam, which is fixed, so valve timing is fixed.
It doesn’t vary with RPM.
[Note: determined in terms of angles, not speed, obviously higher RPM leads to faster physical valve speeds, but same point in the cycle]
4 stroke ignition timing
- General
- Variable?
Ignition is “advanced”, so happens before TDC, to ensure that power peaks as piston leaves period of ineffective crank angle.
Timing DOES change based on RPM (unlike valve timing) as combustion delay is fixed, so as the engine gets faster the spark needs to happen earlier to achieve the right timing.
Radial engines
- description
- # cylinders
Pistons mounted radially around the crank shaft. Need to have an odd number of cylinders to allow timing (e.g. 1, 3, 5, 2, 4).
Creates a big area with a lot of drag, and upside down cylinders have issues with oil pooling.
Crank throw
This is half of the stroke distance.
It is the range of movement of the connecting rod connection at the crankshaft, between central point (centre of crankshaft) and its TDC or BDC point.
Alternatively could be described as the amplitude of the vertical movement of the crankshaft/connecting rod connection.
Crank throw - considerations around weight
Shorter crank throw reduces weight as components can all be smaller.
Thus aircraft engines tend to be “short stroked”.
Camshaft
Controls valve timing. Driven via gears by the crankshaft and rotates at half the speed of the crankshaft, so valves each open and close once per cycle.
Piston engine
- Crankcase
Main structural element of engine.
Mounting points to mount to airframe, for accessory gearbox and for cylinders.
Internal passages allow oil to flow and a breather allows air to vent.
Piston Engine
- Crank Assembly
Crank shaft
Connecting Rods
Pistons
NOT Crank Case
Piston engine
- Accessory gearbox
Drives oil, ignition, hydraulic, pneumatic systems and sometimes engine starter motor.
Exhaust valve cooling
Exhaust valve is hollowed and partly filled with sodium. This melts at high temperature and transfers heat from the valve head to the stem.
Valve bounce
Caused when spring closing valve vibrates at its resonant frequency. This prevents the valve from staying gas tight.
Resolved by having two counter wound helical springs (one within the other) holding the valve closed, which have different resonant frequencies.
Valve gear - tappet
Fitted between the camshaft and push rod, creates the right clearance to ensure valve movement range is correct.
Has some clearance for thermal expansion so it can fully close when hot, but too much clearance means the valve won’t fully open.
Limitation on normally aspirated engines
Engine power limited by the amount of air that enters the cylinders. The greater the mass of air in the cylinder, the more combustive power can be created.
Naturally this is less at higher altitude where outside air pressure is lower.
Manifold Absolute Pressure (MAP)
Mixture pressure at inlet manifold just before going into cylinder. Can be measured by a MAP gauge and is a good indicator of MEP and thus engine power.
Controlled by the throttle.
What does MAP gauge show when:
- Engine off
- Engine idle
- Engine full throttle
Engine off - shows outside pressure, so 29.92 in Hg at MSL in ISA.
Engine idle - zero
Engine full throttle - Just less than 29.92
Calculating power of piston engine
- MEP method
Power = Force x distance / time
Use PLANE
Pressure (MEP) & Area (of piston crown)
Length (Stroke length)
Number of cylinders
Engine (RPM / 2 - only one power stroke per 2 rotations)
Calculating power of piston engine
- Torque method
Power = Force x distance / time
Distance x force x rpm
= torque x rpm
Torque measured at the gearbox between engine and propeller
Fixed vs variable pitch propeller engine power monitoring
Fixed pitch generally just look at rpm.
Variable pitch power is a function of rpm and manifold pressure, so have a MAP gauge as well as rpm.
Piston engine
- Clearance volume
- Total volume
- Swept volume
- Compression ratio
Clearance volume = Volume @ TDC
Total volume = Volume @ BDC
Swept volume = the difference
Compression ratio =
Total volume : Clearance volume
High piston compression ratio effects
High compression ratio improves thermal efficiency, but increases likelihood of detonation.
Squared or square cage engine
Piston engine where piston diameter = stroke.
Volumetric efficiency
- definition
Ratio of mixture induced in cylinder to mass of air (at standard temp/pressure) that would fill the swept volume.
In normally aspirated engine this is affected by the ease of inflow and ease of exhaust flow, along with density of the air.
Effect on volumetric efficiency of:
- Altitude
- RPM
Assuming constant MAP (big assumption), volumetric efficiency increases with altitude due to easier exhaust flow.
Also increases with RPM due to increased momentum of gases.
Power at altitude for piston engines
Consider variable pitch propeller where MAP and rpm can be controlled
At low altitude throttle is less than full for given MAP, so as you climb volumetric efficiency increases and throttle can be opened to maintain MAP.
When throttle is fully open you are at peak power and above that altitude power will decrease as MAP falls.
Note: MAX power is always decreasing as throttle always full and MAP decreasing with altitude.
Engine oil designations
Number indicates viscosity (higher number - higher viscosity).
Letters in front indicate additives.
[New engines run in with zero additive oil to reduce lead in time]
Dry sump oil system
Used oil falls into a sump at the bottom of the engine. A scavenge pump (stronger than the main oil pressure pump to prevent pooling in the sump) pumps it through the cooler into the main oil tank (onto a de-aerator plate).
Hot well
This is a perforated metal cylinder over the oil tank outlet. When engine started it lets the warmer oil through more easily than the more viscous colder oil. This allows an initial good flow of warm oil before the entire tank is warm.
Oil system filters
2 coarse filters fitted before pumps (suction filter before pressure pump, scavenge filter before scavenger pump) to protect the pumps from damage.
A fine pressure filter fitted after pressure pump to protect components, has a bypass valve.
Magnetic plugs
AKA chip detectors.
Magnetic parts on oil return lines (from sump pump via scavenger pump to coolers and main oil tank) to provide WARNING of bits of metal.
Pressure regulation in oil system (piston)
Pumps are engine driven and higher pressure than required to ensure they work. Have pressure relief valves fitted in parallel that return oil from outlet to inlet side of pump if pressure is too high.
Controlling oil cooling
Might have shutters to allow level of oil cooling to be controlled.
An anti-surge valve allows coolers to be bypassed in case of blockage or excessive cooling causing “oil coring” (congealing in the cooling matrix).
Location of oil cooler in dry and wet sump systems
In dry sump cooler is between scavenger pump and the oil tank (in the return line).
In wet pump it is fitted in the supply to the engine.
Impact of oil temperature on oil pressure
Cold oil is more viscous and gives higher oil pressure
Impact of low oil quantity on oil pressure
Low amount of oil has to do more work so temperature increases. This temperature increase results in reduced oil pressure.
Wet sump system
Similar to dry sump but no oil tank, all oil is stored in the sump at bottom of the engine.
Uses “splash lubrication” where crankshaft moving through oil flings it around the engine.
Wet sump disadvantages
Sump affected by inverted flight so wet sump can have oil starvation (dry sump has a tank with ready supply).
Splash lubrication at high rpm can cause excessive oiling, plus the splash lubricating oil isn’t filtered or cooled.
Usually need more oil for wet sump.
Checking oil level dry vs wet sump
Oil level in dry sump needs to be checked immediately after flight as some will drain back to sump.
Wet sump wait until cooled.
Hydraulicing
When oil pools in inverted cylinders (inverted or radial engines) and causes damage when engine starts as it is not compressible.
Need to remove spark plugs to drain and start with a manual cranking to check.
Cylinder Head Temperature (CHT)
Hottest part of the engine so useful to measure. Usually measure the hottest cylinder (last one, with worst air cooling).
Functions of engine oil
Lubrication
Cooling
Cleaning
Protection
& HYDRAULIC operations
Magneto system description
Engine drives a rotating magnet within a U-shaped stator, with primary (few, thick) and secondary (many, thin) windings around it. This generates an AC current.
The primary winding low tension (LT) circuit is regularly broken, which induces a pulse in the secondary winding high tension (HT) circuit.
Large number of secondary windings means high voltage, which is distributed to a spark plug by the distributor.
NOTE: Tension = Voltage
Prevention of arcing in the magneto system
Arcing can occur at the breaker points in the low tension circuit when the circuit is broken due to high induced current.
A capacitor is installed parallel with the contact breaker to prevent this (called a condenser) which also assists in breaking of the primary circuit.
Capacitor in magneto circuit PREVENTS ARCING.
Ignition system & magnetos
Works opposite to normal system. In normal operation the ignition switches are open, so the LT breaker controls the primary windings. When a magneto ignition switch is grounded, the primary coil is connected to earth so the breaker does nothing and the magneto fails.
As a result of this a break in the ignition wiring will result in permanent ignition firing, not a failure of ignition.
So NOT fail safe on the ground.
Reference to ignition switching system. What does ignition switch activated mean?
This is what is activated when you turn the key to magneto 1 or magneto 2.
So its action is to connect the primary system to earth.
In this sense in the “dual” setting the ignition switch system is considered disactivated, not activated.
Pressurised magnetos
Magnetos use insulating property of air to isolate components. So at high altitudes may need to be pressurised to prevent arcing.
Spark plug augmentation (3)
Need sufficient voltage even at low rpm. Gearbox can do this at low rpm (idle) but need extra help at startup:
i) Impulse magneto has a spring-loaded clutch that stores energy and releases it (twice per engine revolution)
ii) HT Booster coil supplies a high voltage directly to the distributor, by powering primary coil with battery and collapsing it with a trembler interrupter.
iii) LT Booster coil uses aircraft battery directly to power the primary coil.
Spark plug retardation
The 3 augmentation methods (impulse magneto, HT booster, LT booster) all have methods for retarding ignition during starting to prevent sparking too far ahead of TDC which would damage engine.
Detonation
AKA knocking
Rapid spontaneous combustion of mixture after burning has begun (i.e. after ignition). Flame rate about 1000ft per second instead of 60 to 80ft.
Energy produced is given up as heat and shock rather than piston movement.
Occurs AFTER normal ignition.
Pre-ignition
- description
- effect of rpm
When mixture ignites before the spark.
Can cause engine to “run on”, which is continuing to run after ignition is switched off.
Unlike detonation, this gets WORSE with higher rpm.
Octane rating
Characteristic of fuel indicated anti-knock value. Above 100 it is a “performance rating”.
Based on the anti-knock qualities of a mix of X% iso-octane and (100-X)% heptane.
It is ok to temporarily run with fuel of a higher rating, but NEVER with a lower rating.
Factors influencing detonation
Increased temperature or pressure of mixture increases risk of detonation.
- Carb heat on with higher power;
- High power & low rpm
- Overheated cylinders
Impact of carb heat on mixture
Air less dense so rich mixture.
Stoichiometric mixture
Chemically correct mixture which achieves complete combustion, 15:1 by mass.
Typical range of mixture
8:1 - rich
20:1 - lean
Appropriate mixture settings
At 15:1 (stoichiometric) high risk of detonation due to excess latent heat.
10% rich will absorb this heat, but use 15% in practice to ensure all cylinders are ok.
At high power settings use 20% rich.
Take-off use 30% rich.
Power chart against mixture settings
Effect of lean mixture on temperatures
Less fuel means the gas burns cooler, however it also burns slower so more heat is transferred to the engine.
- Low gas burn temp
- High cylinder head temp
- Low exhaust gas temp
Other issues with mixture settings
- Popping back
- Idling
Lean mixture can lead to “popping back” when mixture is burning when the inlet valve opens.
At low (idling) rpm exhaust gas momentum is reduced so retention of some exhaust gas dilutes the mixture. So need rich mixture at idle.
However, prolonged idling with rich mixture (e.g. on the ground) causes spark plug fouling.
Fuel volatility
This is readiness to vaporise.
We need fuel to vaporise in the mixture to enable combustion, but not in the fuel tanks or lines.
Gravity feed ceiling
More likely in jet craft, this is a ceiling for use of gravity feed of fuel.
Climbing without the positive pressure from a fuel pump can lead to “foaming”, the vaporising of fuel in lines due to the decrease in air pressure.
Vane type fuel pump
Two double ended vanes split circular housing into 4 segments. They rotate and also slide across each other so segments change in size. Segments large at inlet and small at outlet so pressure changes create the flow.
Basic float carburettor
Float in the carburettor operates a needle valve that cuts of fuel supply at a certain level (fuel delivered under pressure from fuel tanks).
Exits via a u-bend through a “jet” narrowing that controls flow, then further through an exit at the choke/throat venturi part of carburettor, before the throttle.
Venturi effect as air flows past fuel outlet causes fuel to be sucked out.
Carb
- Throttle effect
Throttle controls the amount of air (air meaning air & fuel mixture) allowed through by the butterfly valve.
The higher the flow of air, the greater the venturi effect and so the more fuel is extracted to maintain the right ratio.
Carb
- Pressure balance duct
At high air speed pressure increases in carb throat due to ram air, thus reducing amount of fuel drawn out. Conversely at high altitude lower density of air would en-richen the mixture.
Therefore a pressure balance duct joins the fuel float chamber to the carb intake to ensure correct mix ratio across AIRSPEEDS and FLIGHT CONDITIONS.
Carb
- Diffuser
This is a perforated piece sitting after the jet in the tube from the carb fuel chamber to the nozzle. It has a pressure connection after it to the fuel chamber which adjusts rate of fuel flow to ensure correct mixture ratio across ENGINE SPEEDS.
Required as fuel flow from nozzle isn’t linear with engine power otherwise.
ALSO helps atomise fuel
Mixture control - functionality and purpose
Can function in a variety of ways.
Purpose is to adjust mixture to take account of air density and also to reduce fuel consumption.
Carb
- Slow running jet
This is an outlet positioned by the throttle butterfly valve when almost closed to deliver a rich supply at idle. It has an idle-cut out that closes it to allow the engine to shut down.
Power enrichment
Provides additional fuel for rich mixture required at high power settings (to provide cooling at high power settings).
Can be provided via a variable size main jet, or a separate power jet controlled via a cam connected to the throttle controls.
Carb
- Accelerator pump
Connected to the throttle, provides an injection of fuel into the venturi when throttle is opened, to prevent weak or lean cut.
Piston
- Direct vs indirect fuel injection
Direct injection delivers fuel directly to cylinder heads. Highly fuel efficient but requires complex timing.
Indirect delivers fuel on low pressure, continuous flow basis to the air intake of each cylinder.
Indirect fuel injection system
Have both engine driven pump and electric pump (for redundancy and starting) delivering fuel to “fuel control unit”. Deliver at higher pressure than needed with PRV in parallel and reverse to control pressure.
Fuel control unit first metered by mixture control, then throttle (which is linked to the butterfly valve mechanically).
Then delivered to a manifold which splits it to the multiple injectors. Injectors help vaporise and also enable engine cut off.
Exhaust Gas Temperature (EGT)
- How it is used
- hottest temp
This gauge is used to adjust mixture. 15:1 creates highest temperature, richer than this and extra fuel aids cooling, leaner and there is less fuel to burn.
Setting power cruise and economy cruise (mixture)
Power cruise: Find peak EGT then enrichen until it falls a bit, should be at about 12:1.
Economy cruise: Find peak EGT then lean until it falls a bit, should be at about 17:1.
NOTE: Only works in the cruise, doing this at higher power settings can cause detonation.
Ground boosted vs altitude boosted superchargers
Ground boosted supercharges increase horsepower on the ground. Engine needs to be stronger to withstand extra pressure.
Altitude boosted aim to maintain sea level power as the aircraft climbs.
Power vs altitude for ground boosted supercharger
Power vs altitude for altitude boosted supercharger
2 unexpected features of power vs altitude relationship for altitude boosted supercharger
1) Take off power can be boosted significantly above cruise level for 5 mins
2) Power climbs initially up to the rated altitude (unlike normally aspirated or ground boosted which always reduce with altitude). This is due to exhaust back pressure reducing with altitude, improving scavenging.
Altitude boosted engine
- Full throttle height chart
Full throttle heights
For altitude boosted superchargers.
Rated altitude is the point at which full throttle is reached for maximum cruise MAP, after this power starts to fall.
If MAP is lower (i.e. lower rpm) the entire power curve is lower, but full throttle not reached until a greater height.
Components of a centrifugal impeller/compressor
How centrifugal impellers/compressors work
Fast spinning (40,000rpm) impeller gives velocity, pressure and temperature to the air. Centrifugal force pushes it to the outsides to the channels of the diffuser, which trade some of the airs kinetic energy for pressure energy (velocity down, pressure & temp up).
Pressurised air then ducted to an inlet or another impeller.
The impeller and diffuser each contribute about half the rise in pressure.
Turbocharger
- description
- restrictions vs superchargers
Hot exhaust gases used to drive a turbine, which drives the compressor.
The compressor can only be used to compress pure air, unlike supercharger which can compress fuel/air mixture.
This is due to high exhaust gas temperatures, which could ignite the mixture.
Turbocharger
- Advantages & disadvantages
+ No power taken from the engine
+ Less complex and lighter than supercharger
+ As engine rpm increases, exhaust gases increase and so does the boost
- Turbo lag
Controlling turbocharger output
- Simple system
Simple systems just have a PRV and “over-boost” warning light when activated.
Controlling turbocharger output
- Single controller, waste gate
“Waste gate” (which redirects exhaust gas to waste when open) is operated by a “waste gate actuator” (hydraulic - engine oil).
Waste gate is NORMALLY OPEN. The actuator closes it due to pressure differential to allow exhaust gas into turbocharger.
Critical altitude (turbocharged engine)
Attribute of turbocharged engine.
This is the altitude at which throttle is fully open and engine is producing maximum amount of power.
Beyond this point exhaust back pressure will reduce power available (due to all exhaust being directed to the turbine).
Turbo & waste gate during phases of flight
Startup: Waste gate held open by spring (natural position)
Idle: Low manifold pressure causes waste gate to partially close, but little exhaust gas so less than full turbo power
Takeoff: Partially open
Climb: Waste gate gradually closes, turbine speed increases
Critical altitude: Waste gate fully closed, turbine at max speed.
Beyond critical altitude: Waste gate fully closed, compressor outlet pressure can’t be maintained, MAP falls.
Dual Pressure Control Unit System
An extra turbocharger control system designed to balance pressure on each side of the throttle butterfly valve. Without this, the standard turbo control system gives excess output at low MAP settings (and “bootstrapping”).
Pressure feeds from each side of the throttle affect a diaphragm, which controls a bleed valve that is linked to the waste gate actuator.
Intercooler
Heat exchanger at the inlet manifold which exchanges high temperature heat of turbo compressed air with colder ram air.
Reduces risk of detonation due to high temperature compressed air being used in the engine.
FADEC
Full Authority Digital Engine Control
A computer controls the various power settings (throttle, mixture, cowl flaps, ignition) through a single lever.
FADEC components
- Electronic Engine Controller (EEC)
- Fuel Metering Unit (FMU)
FADEC includes all related auxilliary components, not just computers
FADEC safety
Critical system so needs dual electronic control units/channels (redundancy) and its own independent backup power system (permanent magnet alternator or battery with longer endurance than the aircraft).
Described as SINGLE FAULT TOLERANT, one failed data input is acceptable.
FADEC power
Has its own 3 phase magnetic alternator as a primary power supply, driven by the engine.
FADEC controls/buttons
NOT throttle control
Power lever
Crank switch (to start, piston only?)
Possibly mode selector
External vs internal supercharger
Internal - supercharger, engine driven
External - turbocharger, exhaust driven
Diesel vs petrol engine differences
No spark plugs (compression ignition).
So need higher compression ratios (14:1 to 28:1 compared to 9:1 petrol).
This requires a strong (and thus heavy) engine.
Need direct fuel injection, no carburettor, NO MIXTURE!
Hotter, so need liquid cooling usually.
Diesel vs petrol fuel
Diesel (kerosene) is heavier (SG 0.8 to 0.9) and has more controlled detonation at high compression.
Less volatile and higher flashpoint so safer and more stable.
Poor lubricant however.
Diesel vs petrol OTTO cycle differences
Induction introduces only air to the system.
Fuel is not introduced at the same time so compression compresses only the air.
Fuel introduced at TDC as the big pressure and temperature increase would cause it to ignite too early otherwise.
Diesel engines
- Direct fuel injection
Similar to petrol fuel injection.
Engine and electric pumps with PRV deliver pressurised fuel to a control unit.
The control unit knows throttle setting and rpm and delivers pulses of fuel to a manifold, which distributes it to individual injectors (simple mechanical injectors that deliver fuel based on pressure received).
[Older system]
Diesel engines
- Common rail injection
Supply pump provides pressurised fuel to a common rail. The common rail feeds fuel to individual solenoid injectors, which themselves control when and how much fuel is injected.
An electronic control unit controls both the pressure in the common rail and the individual solenoid injectors.
Diesel engine fuel injection high or low pressure?
High pressure
Variable pitch propellers
RPM or MAP first when:
- increasing power
- decreasing power
RPM then MAP to increase power (to prevent detonation)
MAP then RPM to decrease power
Variable pitch propeller effect of increase in TAS
Increase in TAS reduces the effective AoA of blade, which the CSU is trying to keep constant.
In practice, the TAS increase reduces the force on the blades, which results in an increased RPM. The CSU responds to the increased RPM by coarsening blades.
Gas turbine cycle
- Compression
- Combustion
- Expansion
- Exhaust
Called OTTO if steps happen separately, Brayton cycle if they happen continuously.
Turbine engine
- Constant or variable pressure & volume?
CONSTANT pressure
NOT constant volume
[Piston = constant volume]
Pressure thrust
An additional component of jet thrust based on increase in pressure ONLY when propelling nozzle is CHOKED (i.e. at exit velocity limit).
Pressure thrust = A x (rho - P0)
A: nozzle area
rho: nozzle static pressure
P0: atmospheric pressure
Total jet thrust
Thrust = M x (V(jet) - V(flight)) +
A x (rho - P0)
or velocity thrust + pressure thrust
Thrust vs power
Power = force x speed, or rate of doing work
Power at start of take off roll is zero and increases with speed.
Thrust can be considered to remain constant across the whole of the speed range.
Flat rated engine
Jet engines produce more power when temperatures are low and safety limits might be breached in cold air.
Flat rated engines have fuel flow limitations for temperatures below a given level to stay within safety limits.
Engine is described as “temperature limited” above the flat rated temperature, thrust reduces as temp rises.
Jet engine thermal efficiency
Conversion of thermal energy to kinetic energy
30%
Single Spool Axial Flow Turbojet
- nature and where they are used
Single spool means single shaft connecting turbine blades to compressor blades (multiple of each).
Axial flow means flow straight through the jet engine.
This is slim, low air mass and high velocity. Low drag (good for fast flight) but not used by airliners as it is noisy and inefficient at cruise speed.
Twin Spool Bypass Turbojet
Two shafts, connecting LP (low pressure) compressor (at front) to LP turbine (at back), and HP compressor & turbine (in-between). LP compressor feeds some air to a bypass outside the combustion/turbine zone. This is more thermally efficient (no combustion) and better at low speeds. It insulates some of the noise of the core.
Core can be smaller so less weight.
i = Bypass air mass:core air mass [or cold air:hot air]
Produces MORE pressure than single spool.
Triple Spool High Bypass Ratio Turbo Fan
Have a third intermediate compressor & turbine in the middle on a third shaft, inside the core along with the HP compressor/turbine and LP turbine.
High bypass ratios with most power delivered by the large LP fan at the front.
Bypass ratio
Bypass air mass / HP compressor air mass
[Need to deduct if given total air mass]
Turbo-prop engines
Use high efficiency of continuous combustion of gas turbine engines to drive propeller shaft (propellers more efficient at mid altitudes and <300kt).
Need a gearbox as turbine shaft speeds are faster than desired propeller rpm.
Most exhaust temperature used to rotate turbine, but some jet thrust used.
Equivalent shaft horse power = Shaft HP + Jet thrust
Free Turbine Engines
Twin-spool turbo prop engines which have one standard compressor/shaft/turbine driving a disconnected turbine (the “free turbine”), which connects to the prop shaft.
Thus prop rpm can be independent from the compressor rpm.
Subsonic turbine engine inlet
Called “pitot” intake. Lip to prevent turbulence of air entering compressor, even in crosswinds. Convergent throat then divergent diffuser to increase pressure into compressor.
Smooth profile to minimise turbulence.
In strong crosswinds (startup, takeoff run, sev. turbulence) can get surge or flameout (or hot/hung start).
Supersonic turbine engine inlet
Pitot intake creates high speed air which can cause issues occasionally with subsonic, but will certainly cause shockwave in supersonic. Engines can’t cope with this.
Use a “multi-shock” inlet with small shockwaves, or variable throat, to slow down airflow without increasing drag.
Compressor pressure ratio
Absolute output compressor delivery (output) pressure / absolute compressor inlet pressure.
This is the measure of performance of a compressor.
Centrifugal vs axial flow compressor
Can have double sided centrifugal compressor, but difficult to have multiple stages due to energy loss of passing air to the next stage, so axial more popular.
One disadvantage of axial however is that the rotors are aerofoils and can stall, whilst centrifugal aren’t aerofoils, just directing air outwards to the diffuser.
Compression ratio around 4.5:1 for centrifugal, 35:1 for axial.
Centrifugal can be shorter though, so good for APU, although need big inlet.
Axial flow compressor
- functionality
Each stage has rotor blades and a set of stator blades.
Rotor: Increase velocity, pressure and temperature
Stator: Convert kinetic energy to pressure, so decrease velocity, increase pressure and temperature
Stators also fix airflow for next stage.
Pressure increases around 10-20% each stage.
Axial flow compressor
- shape of compressor section
Need to oppose the force of high pressure further back pushing towards low pressure at front of engine.
Use a convergent duct shape called “air annulus”. The outer case size tapers whilst the shaft size increases so rotor size squeezed from both directions.
Compressor
- stagger angle
This is the “twist” in the rotor blades of a compressor to manage the difference in velocity of air at the tip of the blades compared to the root.
Axial compressor safety margin
Stalling of the axial compressor rotors can lead to a flow breakdown (surge) or reversal of airflow (deep surge).
Chart of pressure ratio to air mass flow shows a “working line” which is separated by a safety margin from a “stability line”, beyond which is the “unstable area”.
“Acceleration line” may sit between the two.
Note that the safety margin is much smaller with low air mass flow.
Axial compressor safety margin chart
Axial compressor
- Variable inlet guide vanes & variable stator vanes
Variable inlet guide vanes adjust flow to the first stage to ensure it hits rotors at an appropriate angle of attack - essential during startup.
Variable stator vanes can do the same thing for later stages.
Axial compressor
Inter-stage bleed
- When it is used
- Change in flow over early and late stages
They open during start up to reduce angles of attack in early stages and maintain axial flow through all stages.
They INCREASE flow over early stages and REDUCE flow over later stages.
Compressor stall symptoms
- EGT - high/low
- EPR - high/low
- TGT - high/low
- Thrust
- Fuel Flow
- RPM
- Other
- High EGT
- Low EPR
- High TGT
- Reduced thrust
- Rumbling or banging from compressor
- Fluctuating rpm & fuel flow
Compressor surge
- Description
- Deep surge
- Cause
This is when pressure in combustion chamber exceeds compressor pressure and hot gases move forwards.
In deep surge combustion gases flow all the way into compressor, which can be caused by over-fuelling, but less common now with better engine controls.
Primarily caused by EXCESS FUEL, not compressor stall.
When is compressor stall most likely? (2)
Low RPM
Acceleration/deceleration
Centrifugal compressor stall/surge
Centrifugal compressor doesn’t experience aerodynamic stall, but severe intake icing, impeller damage or over-fuelling can still cause surge and loss of power.
Axial flow blade construction
- connection
- purpose of stubber/clapper
Blades are loosely fitted with a “fir tree root” which allows them to adjust based on balance of forces on them (e.g. centrifugal). Causes a tinkling sound when windmilling on the ground.
Blades can have a “snubber” or “clapper” in the middle to PREVENT AERODYNAMIC INSTABILITY, but wide chord blades don’t need this.
Purpose of loose fit compressor/turbine blades
Allows for thermal expansion and prevents damage due to vibration.
Typical combustion chamber efficiency
Up to 98%.
Turbine Combustor
- Functionality up to primary zone
Consists of inner “flame tube” and outer “air casing”.
150m/s air from last compressor stage is straightened by stators then diffuser converts more kinetic to pressure to reach 110m/s.
20% of air is “primary air” directed to flame tube, rest is “secondary air” to the outside.
110m/s still too fast for 3m/s kerosene flame rate so gets swirled in the primary zone of flame tube to slow down.
Turbine Combustor
- Combustion in primary zone to exhaust
Fuel injector sprays fuel which atomises in turbulent air. Igniter provides initial spark but stops once engine started.
No detonation as pressure is constant so use 15:1 stoichiometric ratio (based on primary air) which produces very hot 2100C.
Secondary air added in the secondary zone and dilution zone to reduce temperatures to 1000 to 1500C which turbines can withstand.
Combustor designs
- tubo annular
- annular
- can type
Tubo-annular: Several flame tubes in a common air casing - lighter, more compact, easier maintenance.
Annular: One flame tube, which is shorter, stiffer, lighter and more EVEN THERMAL OUTPUT to turbine. However manufacturing and maintenance can be tricky.
Can-type: Several tubes in one outer casing. 2 igniters connect all flame tubes.
Combustion chamber drain valves
All have them to remove unburnt fuel
Reverse flow annular combustor
Air flow from the compressor is directed back around the combustion chamber and enters it from the rear. Some loss of efficiency, but it allows the compressor and combustion chamber to be closer together, so smaller.
Good for helicopters & small turbo-prop.
[Has a VAPORISER facing opposite way to air flow]
Turbine Fuel spray nozzle types
- pressure jet injector
- air spray injector
1) Pressure Jet Injector
Either simplex which delivers single spray of high pressure fuel (not good at low rpm) or duplex with two orifices (to cover high & low rpm).
2) Air spray injector
Compressor air enters inner and outer swirl chambers and fuel injected in-between. Finely atomised spray with improved combustion.
Fuel vaporiser
Alternative to fuel spray nozzles.
U-shaped tube with holes in it that distributes fuel to combustion chamber. Not used often.
Continuous ignition
Can be used on take-off from wet runway or flight through heavy rain.
Ignition continues (at low power setting) even though flame already present, to prevent flame out.
Turbine ground ignition process
Starter
Rotation
Ignition
Fuel [AFTER ignition]
Turbine blade operation
Nozzle Guide Vanes (NGVs) (stators) direct airflow from combustion chamber on to rotor blades. 2 types of forces used:
1) Impulse - NGVs are convergent, accelerate the gas (-VE pres., +VE vel.) and U shaped rotor blades are driven
2) Reaction - NGVs don’t need to be convergent but direct the gas to reaction blades in a different direction, which creates a reaction force.
NOTE: No aerodynamic force, no stall.
Pressure at stator and rotor with impulse and reaction blades
Blade type Stator Rotor
Impulse DOWN constant
Reaction DOWN DOWN
“In Due Course the Reaction Dies Down”
- Impulse Down Constant
- Reaction Down Down
Why is turbine life limited?
Extreme heat causes them to increase in size over lifetime, which gives them a life limit. Called “blade creep”.
Turbine blade cooling methods (3)
Convective: Void inside the blades allows heat to pass out via convection through vents.
Impingement: Jets of cool air blown onto internal blade surfaces and exit through holes in the trailing edge.
Film: Cool air blown inside the blades which exits through holes on the surface, creating a cooling “film” over the blade surface.
Active Clearance Control
This is controlling the turbine blade tip clearance by controlling bleed air onto the turbine casing. The internal circumference can be finely controlled this way.
Effect of taking bleed air from compressor (e.g. for anti-icing).
- mass flow
- thrust
- turbine temp
Lower mass flow
Reduced thrust
INCREASED turbine temperature
[NOTE: Bleed air taken from compressor, NOT fan!]
Jet exhaust design
- exhaust cone
Gas enters from turbine at 370m/s, which would cause a lot of energy loss to friction. So an “exhaust cone” in centre creates a divergent duct, converting velocity to pressure energy and keeping hot air from hanging around last turbine stage.
Rear struts holding the cone straighten the flow.
Jet exhaust design
- propelling nozzle
Gas held at about 290 m/s (mach 0.5 relative to exhaust system) until the propelling nozzle, which is a convergent duct and accelerates air to mach 1.0.
At mach 1.0 the nozzle is “choked” and thrust is capped, but increase in static pressure at exhaust relative to atmospheric pressure creates an additional pressure thrust.
Jet exhaust design
- convergent divergent nozzle
Attempts to increase thrust by increasing pressure differential at the exhaust (divergence) rather than just using convergence to reach maximum momentum thrust.
Low bypass vs high bypass exhaust
Low bypass usually mix the two air streams in the same exhaust.
High bypass will have separate co-axial nozzles.
Turbine exhaust noise suppression
Medium bypass can have mixer chutes which combine the high and low pressure cases to limit noise.
High bypass allow the outer low pressure air to sheath the noisier high velocity central air.
Turbo jet engines can have lengthened jet pipes called “exhaust hush kits”.
Reverse thrust
Clamshell doors and bucket systems both redirect the core turbine exhaust air forwards for reverse thrust.
Cold stream reversal systems redirect the bypass air forwards. Less powerful as low pressure air can’t be turned through as great an angle.
Can cause surging and structural damage if used at the wrong time, so MULTIPLE interlocks are essential (e.g. on the ground, idle thrust).
Auxiliary gearbox (turbine)
Connects jet engine to powered systems and starter motor.
Starter motor must be connected to HP spool.
Non-essential components connected via “quill shaft” or “shear neck”, weak point that breaks if component seizes to prevent excess load on gearbox & drive system.
Fuel and oil pumps are essential so no quill shaft.
What accessories does the auxiliary gearbox power?
AC generator (through CSD)
Oil pumps
Hydraulic pumps
High pressure fuel pumps
Main bearing housing (labyrinth seals)
Bearings (i.e. ball bearings) are the major jet moving parts, so important for oil. Temperatures are hot so rubber seals no good, use “labyrinth” seals instead which use high pressure air and a groove & thread system to prevent oil escaping. Pressure controlled with venting.
Results in highly aerated oil which must be separated centrifugally.
Gas Turbine Oil pressure relief
Older system had a typical pressure relief valve arrangement to limit oil pressure.
Modern engines need higher pressure though, especially for bearings mounted at high level. Thus use “full flow” oil system instead, where higher engine rpm results in higher oil pressure.
Gas Turbine Engine Oil types
Need low viscosity oil.
Temp ranges are high (-40 to +250C) because rotational speeds are high, but no reciprocating parts. At high altitude temperatures can fall so need low viscosity to maintain flow.
Main uses of internal bleed air
- Engine anti-icing
- Internal engine & accessory cooling
- Bearing chamber sealing (labyrinth seals)
- Keeping hot air out of turbine blade cavities
[Internal means uses for within the engine itself, external is for other parts of the aircraft]
Fuel Oil Heat Exchangers (FOHE) or
Fuel Cooled Oil Coolers (FCOC)
Exchange oil heat with cool fuel to prevent fuel waxing and help cool oil (before main ram air cooling of oil).
Oil pressure usually higher than fuel pressure as oil in fuel is less of a problem than fuel get into hot parts of the engine where it shouldn’t be!
Jet engine starting procedure
Starter motor drives HP spool (N2) initially. At 15% rpm fuel HP shut-off valve is opened and fuel enters to start combustion.
Starter continues to help until self-supporting speed (30% rpm), at which point starter and igniter drop out.
Engine accelerates up to idle speed of 60% N2 (25% N1).
2 types of jet starter motor
Electric (DC!) - Need TWO power sources (ground power unit & air cart)
Pneumatic (often as a backup, using air from a running engine or APU)
Windmilling jet start
This is an in-flight start, windmilling of compressor blades due to air flow is used instead of starter motor.
Jet engine malfunctions
- dry start
- hung start
- hot start
- wet start
Nil rotation
Dry start - Low EGT, no fuel flow
Hung start - (Caused by WEAK starter motor e.g. low pneumatic pressure) - Low rpm (20%), low fuel flow, high EGT
Hot start - (Caused by overfuel, maybe previous failed start) - V. high EGT
Wet start - (Failed ignition) - low rpm, low EGT, some fuel flow
Dry running turbine engine
After a wet start, need to clear fuel from the engine. Close fuel HP shut-off valve, switch off igniters, run engines with starters to blow fuel out.
Without this get flames bursting out of back of engines!
Maximum re-light altitude
May be an altitude limit for re-lighting engines to ensure sufficient air mass flowing through engines.
Exhaust Pressure Ratio
Alternative indication to N1 for engine thrust.
= Exhaust pressure / compressor inlet pressure
Can give over-read if inlet is blocked, very dangerous, check N1.
APU
- required to function
- power produced
- altitude
Need air flow so either a permanent source or doors that open if it is required.
Run at a constant speed so 3-phase AC can be powered without a CSDU.
Might have altitude restriction for starting (25000ft) but can operate to higher altitude (40000ft).
APU - how it is started
Needs AC power to start
Either ground power or battery
Temperature, Static Pressure and Velocity in Divergent and Convergent Ducts
Divergent: + Static Pressure, + Temp, - Velocity
Convergent: - Static Pressure, - Temp, + Velocity
Jet fuel flow adjusted in relation to which factors?
Compressor inlet pressure (NOT compressor outlet pressure).
So forward speed & ambient air pressure.
Jet thrust and fuel consumption with increasing altitude
Thrust decreases as air density is lower
Fuel consumption decreases a little as a result of decreasing temperature [could be stay constant]
Engine vibration indications
- unwanted frequencies
Filtered to remove unwanted frequencies
Electric circuit connections of multiple thermocouples
Connected in parallel
Pusher vs tractor propellers
Tractor pull the aircraft (e.g. normal single propeller plane), pusher look backwards and push the aircraft forwards.
Propeller reduction gearboxes
Necessary for turboprop engines which produce 10,000 to 15,000 rpm. Use “epicyclic” gears to transmit torque whilst reducing rpm.
Optimal angle of attack for prop blades
4 degrees
Forces on propeller
Total reaction acts perpendicular to the “thrust face” of the blade.
Broken into 2 components:
- Torque: i.e. prop torque, which opposes engine torque in the plane of rotation
- Thrust: at 90 degrees to the plane of rotation of the blade
Blade twist and thickness
Need bigger angle of attack at the root, and also thicker blade - for additional aerodynamic assistance and also to account for larger forces at the root.
Variable pitch prop positions
Variable pitch prop alpha and beta ranges
Divided by the flight fine pitch stop.
Alpha is the coarse side of this, i.e. flight range and feathered.
Beta is the fine side of this, i.e. taxy and reverse.
Divide at FFP is important as fining beyond this (into beta range) during flight can cause loss of control.
Forces acting to turn prop blades
Aerodynamic Twisting Moment (ATM) on the blade isn’t in line with the axis, it acts to COARSEN the pitch.
Centrifugal Twisting Moment (CTM) opposes this (FINING the pitch) and is stronger.
Thus it is more difficult to coarsen than fine the blade.
Windmilling (ATM & CTM)
Reaction force on blades when windmilling becomes drag (reverses direction) so ATM acts with CTM to fine the blades.
This is ok on single prop as it acts to maintain rotation and aid restart, but creates asymmetric drag on multi-engine so need a feathering mechanism.
Pitch Control Unit (PCU)
Combination of:
- Pitch Control Mechanism (PCM)
- Constant Speed Unit (CSU)
Pitch Control Mechanisms (PCM)
- Double acting
- Single acting
2 types.
Double acting have a hydraulic (engine oil or hydraulic system) cylinder with controls in both directions.
Single acting use a counterweight to coarsen and hydraulic control to fine. Counterweight uses centrifugal force so no effect when prop is stationary.
Single acting PCM in hydraulic or engine failure
- single engine
Counterweight acts to coarsen and weak spring & hydraulics act to fine.
So failure of hydraulics leads to coarse blades, but engine failure weakens counterweight and prop fines.
This allows a restart.
Single acting PCM in hydraulic or engine failure
- multi engine
In multi engine the spring acts to coarsen (along with counterweight) so hydraulic or engine failure lead to coarse blades.
Thus require a “centrifugal latch” to lock blades at fine when rpm decreases, to allow restart.
HOWEVER may need to act to prevent this as feathering is best for failed engine.
Constant Speed Unit (CSU)
- response to low and high rpm
Acts to maintain the rpm selected by pilot by coarsening if rpm rises and fining if rpm falls.
CSU functionality
A hydraulic spool (or “landed”) valve direct engine driven hydraulic force to either a coarsen, fine or neutral position.
It is connected to a spring which is opposed by flyweights that are subject to centrifugal force with rpm.
High rpm (overspeed) leads to the flyweights exerting force over the spring and moving the landed valve to coarsen position, which feeds the PCM.
Pilot rpm selection affects the spring compression.
Feathering
- engine
- rpm lever direction
Unless you have a free power turbine turbo-prop, need to stop the engine before feathering to prevent excess forces.
Pull back rpm lever through coarsen to feather.
Unfeathering
Unfeathering allows engine restart in the air. However oil pressure is lost as engine is off!
Can have an accumulator to store hydraulic power, or a solenoid valve to deliver high pressure oil to fining side of spool valve.
Auto feathering
Only used during take off and landing (armed by power lever position). Detects low rpm (indicating engine failure) and feathers to reduce drag from the failed engine.
Propeller control systems
- two lever
- single lever
Two lever - Power and rpm levers. For beta range set rpm lever to fully fine, then pulling power back through detents selects ground and reverse ranges.
Single lever - Power lever AND condition lever! Condition lever selects air start, run, ground stop and feather but NOT REVERSE. Power lever then controls power. Governor targets constant rpm.
Multi prop synchronising and snychrophasing
Synchronising matches rpms by designating one engine master and showing others as rpm differences. Unsynchronised props get “beat” frequency causing noise and vibration.
Synchrophasing achieves specific phase differences (specific settings by aircraft) to reduce noise further.
Need to achieve rpm synchronisation before synchrophasing can be done.
Prop lever direction
Linked to rpm.
So pushing forwards is high rpm (takeoff & landing), pull back in the cruise for lower rpm.
Prop torque measurement (2)
Helical oil pressure system
AC phase on inner and outer shaft
Specific Fuel Consumption (SFC)
Fuel per unit of power.
= Fuel per hour / thrust
Which part of a turbine engine is most temperature limited?
Turbine. Issues with blade creep are the likely effect of high temp.
NOT combustion chamber which is designed for high temperatures.
What is trending in engines?
Comparing performance parameters of an engine to the rest of the fleet, in order to diagnose potential problems.
Pressurisation and dump valve (turbine)
Pressurisation prevents fuel flow unless pressure is sufficient for atomisation.
Dump valve returns fuel to tank once engine stops.
Flight idle vs ground idle
Flight idle is at higher power level to allow for better preparation for go around.
Expansion or compression in convergent duct
EXPANSION!
This is because the convergence reduces static pressure.
