Powerplants Flashcards
1
Q
What is Boyles law
A
If mass if constancy
P1V1=P2V2
2
Q
What is Charles law
A
If pressure constant
V1/T1=V2/T2
3
Q
What is the general gas equation
A
P1V1/T1=P2V2/T2
4
Q
What is the equation of continuity
A
AVp
Constant mass flow
5
Q
What is Bernoulli’s equation
A
Static pressure + dynamic = constant
6
Q
What is convection
A
Hot air / fluid rises
7
Q
What is conduction
A
Heat energy passes directly molecule to molecule
8
Q
What is radiation
A
Heat transfer with electromagnetic waves
9
Q
What is Newton’s first law
A
Body will continue unless acted upon by external force
10
Q
What is Newton’s second law
A
Rate of change of momentum proportional to force applied
F=ma
11
Q
What is Newton’s third law
A
Every action
12
Q
Momentum formula
A
Mass x velocity
13
Q
Thrust formula
A
Mass x acceleration
14
Q
Work done formula
A
Force x distance
15
Q
Power is…
A
Rate of doing work
16
Q
Torque is..
A
Twisting force or moment
Depends on:
Force applied
Length of lever arm
Included angle
17
Q
What is efficiency
A
Measure of output compared to input
18
Q
4 stages of the 4 stroke cycle “Otto cycle”
A
Suck = induction Squeeze = compression Bang = combustion Blow = exhaust
19
Q
Advantages / disadvantages of inverted 4 stroke in line engine
A
Oil could collect at top of piston (hydraulicing)
Cylinders at front more cooled
Better forward view
20
Q
Advantages and disadvantages of radial engines
A
All cylinders cooled evenly
Bottom cylinders still have oil collected
Large frontal area = lots of drag
21
Q
Advantages / disadvantages of horizontally opposed engines
A
Slight temperature differential
Better view over top of engine
Short crank shaft = less “whip”
No hydraulicing
22
Q
What is the crankshaft made it of
A
Hardened steel
23
Q
What does the crankshaft do
A
Convert linear motion into rotational
24
Q
What is a crankcase made out of
A
Iron
25
How is oil transferred from the main journals to the big end journals
Via internal oilways
26
What is the stroke distance
Distance from top to bottom of stroke (top dead centre to bottom dead centre)
27
What is throw distance
Distance between centre of crankshaft to top / bottom of stroke
2 x throws = 1 stroke
28
What are connecting rods
Connects pistons to crankshaft
29
What is a gudgeon pin
In piston head
Allows a bit of movement
30
What do piston rings do
Allow a air tight seal
31
What are the inlet / outlet valves called
Popper valves
32
How do the poppet valves open
Cam shaft pushes up on the rocker arm (which piviots round the rocker shaft) opening and closing poppet valves
Rotates at half the speed of crankshaft
33
What is the exhaust poppet filled with
Sodium to absorb the heat
34
What is the swept volume
Area at top of piston x Stoke
35
What is compression ratio
Swept volume + clearance volume (total volume) / clearance volume
36
What is the firing order of a 4 cylinder engine
1,3,4,2
37
What is break horse power
Torque x RPM
38
What is indicated horse power
PLANE / 33000
= theoretical before friction
39
What does PLANE stand for
```
P = indicates mean effective pressure
L = length of stroke
A = area of piston crown
N = number of power stokes per min
E = number of cylinders
```
40
What does BHP equal
IHP - FHP
41
What factors affect power output
Density : pressure / temp / humidity
Mixture ratio : higher alt = before richer
Exhaust back pressure = static pressure down = pressure acting on open exhaust pipe reduces
42
What is thermal efficiency
Work out (BHP) / work (energy) container in fuel
43
How much of the fuel energy gets used in a piston
30% gets used usefully
44
Mechanical efficiency is...
Ratio of BHP to IHP
% power in engine available to turn propeller
80-85% typically
45
What is Volumetric efficiency
Ratio of volume of mixture into cylinder on induction stoke compared to swept volume of cylinder
Indicated how well engine is breathing
Typically 85%
46
What is the ineffective crank angle
10° before and after top dead centre
47
What is valve lead
20° before TDC
48
What is valve lag
50° after BDC
49
What is specific fuel consumption
Ib (kg) / BHP / hour
Mass of fuel / horsepower produce / unit time
50
What is oil used for
Minimise friction
Cools = absorbs heat
Cleaning = removes deposits
Protection = protects from corrosion
Hydraulic operations
Seals / cushions impact loads of piston
51
What 4 types of oil are there
Straight mineral oil
Detergent oil
Ash less dispersant
Synthetic oil
52
What are the characteristics of straight mineral oil
No additives
Evaporates (oxidises) at high temperatures
Sludge can form which can block filter
Typically used for first 50 hours of engine operation
53
What is detergent oil (flushing oil)
High cleaning effect = blocks filters
Not approved for aero engines
54
What is ashless dispersant oil
No carbon forming limitation of straight oil and does not form ash deposits like detergent oil
Approved for all aero piston engines
55
What is synthetic oil
Very expensive = not usually used
Better than other oils
56
What are oils graded according to
Viscosity
57
What is multi grade oil
Thinner in winter
Thicker in summer
58
Advantages and disadvantages of a wet sump oil system
+ simple
- can’t do aerobatics
- oil not cooled after being in engine
59
When is a good time to check oil in a wet sump system
Before start
60
What is a dry sump oil system
Oil cooled after been in engine
Can do aerobatics as positive flow all way round
Makes shallower engine
61
When is a good time to check oil in a dry sump system
ASAP after shutdown
62
What is a hot well in a dry sump oil system
Helps promote the heating up of oil
63
What is a spur gear pump
Provides flow of oil
One gear driven by engine which drives a second gear
64
What is a pressure relief valve
If too much pressure then a sample of the pressure will push against a spring which will release the pressure
65
How does a oil cooler work
When cold thermovalve is open so the oil can flow round outside = not cooled
When hot = thermovalve closed = forces oil through cooling system
66
Types of oil filter
Cartridge type
Canister type
Ridged to increase filter area
If filter blocked = bypass valve allows oil to still flow
67
What do cowl flaps do
Can be moved to regulate the amount of hot air leaving the air cooling
68
What do engine baffles do
Attempt to evenly cool the engine
69
How to measure cylinder head temperature
Temp of hottest (usually rear most) cylinder
Measure by thermocouple
Voltage proportional to temp
Does not require aircraft power
70
What is liquid cooling for engine
Coolant mixture of water / anti freeze
If cold = bypasses coolant heat exchange
Controlled by thermostatic valve
71
What is the advantages of a liquid cooled engine
More consistent temperature across cylinders
Good for cabin heating / window de-misting
72
Disadvantages of liquid cooled engines
Weighs more
Leak = engine overheats quickly
Greater maintenance
Greater complexity
73
What is mixture ratio
Mass of air compared to mass of fuel
74
What is the mixture ratio to burn all fuel and all oxygen
15:1
Chemically correct (stoichiometric)
75
What does a fully rich mixture do
Helps cool inside of engine (9:1)
76
What is best power mixture
12:1
77
What is detonation
Shock wave propagating infront of shockwave = occurs after ignition
78
What is pre-ignition
High temps cause sharp edges (spark plugs etc) to become hot (like coal)
On subsequent compression strokes these hot points may ignite the mixture before normal ignition
79
What is normal heptane
Easily detonated and has a zero reference number
Fuel
80
What is iso octane
Fuel
Does not readily detonate and has a reference number of 100
81
What does a octane rating of 80 mean
20% heptane 80% octane
82
What is a higher octane number
Greater resistance to detonation
“Anti knock” number
83
What is tetra ethyl lead like when used in fuel
Reduces combustion temperature but corrosive to valves and spark plugs
84
What is ethylene di-bromide like when added to fuels
Not corrosive
But cumulative position
Affects mental capacity
85
Can you use a higher grade fuel
Yes but not a lower
86
What is fuel ice
Cooking effect on water droplets from the evaporation of fuel
87
What is throttle ice
Freeing of water vapour in the air at or near the throttle
88
What is impact ice
Supercooled water droplets in atmosphere coming into contact with intake
89
What is carb heat
Hot unfiltered air into carburettor
Makes mixture richer
90
How many volts is required to jump a 1mm gap at sea level pressure
3000v
91
How many volts is required to jump a 1mm gap in a aircraft ignition system
20,000v
92
What is a magneto
A generator
A transformer
A distributing system
93
In a magneto what does a condenser do
Reduces arcing at the contact breaker and assists the rapid collapse of the magnetic field
94
How to switch off the magnetos
Ignition switch grounds (earths) the PRIMARY CIRCUIT
95
What is a auxiliary starting system for
At low rpm during starter motor engagement the magneto will not produce a high enough voltage
96
Why is started ignition delayed
Engine rotating slowly
To prevent kick back
Spark is delayed so it still achieves peak pressure 10° after top dead centre
97
What is impulse coupling
Fitted between engine and magneto
Provides retarded fat spark for starting
Engine shaft winds up a spring
Spring overcomes cams = drives hub and magneto generator at high speed
98
What is a HT booster coil
For starting bigger piston engines
Uses a battery red into a trembler circuit
Created pulsed DC
99
How quickly does avgas burn
60-80 feet per second
100
How quickly does avtur (diesel) burn
2-3 feet per second
101
What is the approx compression ratio of a diesel
22:1
102
What is the approx thermal efficiency or a diesel
45%
103
What is a glow plug for in a diesel
To heat combustion chamber to improve fuel vaporisation when started from cold
104
What is the induction stroke of a Diesel engine
Only air drawn into cylinder
105
What is the compression stroke of a Diesel engine
Air compressed and fuel is injected into cylinder where is ignites under pressure
106
What is the power stroke of a Diesel engine
Power output is controlled by the amount of fuel injected into cylinder
107
What is the exhaust stroke of a Diesel engine
As per petrol
108
What is the most common fuel injection system for modern diesels
Common rail systems
109
What is the benefits of fuel injection
Accurate metering of fuel
Fuel is delivered direct to each cylinder
Only air travels through inlet
No Venturi in inlet = no icing
110
What is a electric diesel fuel pump for
Vapour suppression when operating at high ambient temperatures
Back up in the event of a failure of the engine driven pump
Engine priming during start
Excess fuel is returned to inlet side of fuel pump
Min fuel return occurs at full throttle
111
What is the engine control unit (ECU)
Controls injection parameters
```
Receives inputs from;
Throttle position
Mixture lever
Engine rpm
Temperature
```
112
What is a common rail injection system
Generation of the injection pressure is separate from the injection itself
High on easier pump generates in an accumulator a pressure of upto 1600 bar
Independent of engine speed
These high pressures improve vaporisation of fuel
113
How are Diesel engines cooled
Liquid cooling
Mixture of water and anti freeze
114
How does supercharging work
compressor rises temperature and pressure
```
Manifold absolute pressure (MAP)
= increases
= more mass of air in engine
=More fuel can be burned
= more power
```
115
What is a altitude boosted system
A supercharged designed to maintain sea level MAP with increasing altitude
116
What is a ground boosted system
Designed to increase MAP above sea level standard conditions
117
What is a waste gate
Let’s exhaust gasses escape without going across compressor turbine = controlling amount of pressure
118
Difference between supercharger and turbocharger
Supercharger is connected directly to crankshaft
119
What is a supercharger
Connected directly to crankshaft
As alt increases power increases (to an extent) exhaust back pressure reduces
120
To increase power what order should things be done to mixture rpm and MAP
1 richen mixture
2 increase rpm
3 increase MAP
121
To decrease power what order should things be done to mixture rpm and MAP
1 decrease MAP
2 decrease rpm
3 adjust mixture
122
What is rated power
Specified manifold absolute pressure (MAP) and rpm that is permissible for safe continuous operations
123
How can you measure rpm
Gauge;
Mechanically driven by a flexible drive shaft from the engine turning a magnet inside a drag cup
Tacho-generator, 3 phase generator driven by engine = synchronous motor turning at same speed as generator - motor turns magnet in drag cup
124
Manifold absolute pressure
Power developed - mass of fuel burned determined by mass of air it mixes with
Measures between throttle valve and inlet valve
2 capsules, one sealed
The other open to manifold pressure
Normally aspirated engines at sea Level (max power)
Pressure slightly less than ambient due to induction system losses
125
What is a oil pressure gauge
Bourbon tube
126
How is oil temperature measured
Wheatstone bridge
127
How is cylinder temperature measured
Thermal couple
128
How is exhaust gas temp measured
Thermocouple
Used to make adjustments to mixture ratio
Mixture adjustment carried out less than 75% power
129
How is fuel flow / pressure measured
Pressure drop across a fixed orifice is directly proportional to flow through it
Bourbon tube
130
What is blade angle
Angle between plane of rotation and chord line
Measured 70/75% of the distance from hub to tip
131
What is flight fine pitch
The finest pitch available in flight
132
What is the alpha blade pitch range
Range of blade angles within which Constant speed is maintained
133
What is the beta range for blade pitch angle
Range of blade angles which are scheduled (selected manually)
134
What is propeller efficiency
Typically 80%
Power output = product of thrust and TAS
power input = product of torque and RPM
135
Where is a fixed pitch prop most efficient
Cruise
136
What is ATM (propellers)
Aerodynamic turning moment
Increases blade angle
137
What is CTM (propellers)
Centrifugal turning moment
Decreases blade angle
138
What is a reduction gear box
Reduces prop speed compared to High speed engine
139
What is positive torque
Engine driving the propeller
140
What is negative torque
Propeller is driving the engine
141
what is pitch control unit
Uses operating pins (or links) to swivel blades in the hub
142
What is a single acting pitch control unit
In a twin = if oil pressure drops = feathers naturally
In a single = oil pressure drops = fine blade pitch to try restart
A spring (assisted by compressed nitrogen) acts on the piston to increase blade angle
Engine oil fed to the other side of piston to reduce blade angle
143
How is aerodynamic turning motion (ATM) and centrifugal turning motion (CTM) balanced
Use of counterweights on blade roots (reduces CTM)
144
What is a centrifugal larch
Holds blade at ground fine position on shut down
Only works when blade already in ground fine position so it won’t work in flight
145
What is a constant speed unit
Controls how much oil that is allowed into pitch control unit
146
What are governor fly weights for
Maintains blade angle for constant rpm
If prop over speeds slightly = governor flyweights = reduces oil supply to PCU
= coarser
147
How to unfeather a single acting prop
Uses a accumulator to provide oil pressure to unfeather prop
148
What is a double acting PCU
Uses oil on both sides instead of spring on one side
Greater control of blade angle
To unfeather = uses 3 phase motor to drive pump
149
What is a mechanical pitch lock
Loss of hydraulic pressure = lock blade angle
150
When does a automatic feathering system work
Only at high throttle (take off)
151
What does a pitch stop do
Prevents blades becoming to fine in the air
152
What are the 3 conditions in a condition leaver
Run
Stop
Feather
153
What is synchronising of propellers
Synchronise to same engine rpm
More comfort
Reduce airframe fatigue
Reference to a master rpm
154
What is synchrophasing propellers
Blades interlinked instead of exact same phase
Increases comfort
Reduces cabin fatigue
155
What is negative torque signal
Prevent propeller driving engine when low power setting and sufficient airspeed
156
Which of Newton’s laws related to gas turbine engines
3rd law
Every action
Solely reaction of mass of air exiting the engine
No ‘pushing effect’
157
What cycle refers to gas turbine engines
Brayton cycle
158
Where is the highest pressure in a gas turbine engine
Entry to combustion zone
159
What is the compression in a gas turbine engine
Air pressure increases
| Entire unit contained in a convergent duct to maintain constant axial velocity whilst pressure increases
160
What is a diffuser in a gas turbine engine
Divergent duct between compressor and combustion section
Highest pressure occurs here
161
What is the combustion section of a gas turbine engine
Continuous combustion
Theoretically constant pressure
Highest gas temp occurs here
162
What is the exit of a gas turbine engine like
Convergent duct
Gas velocity increases as it exits the combustion chamber and enters nozzle guide vanes
163
What does net thrust equal
Net thrust = mass (Vj-Va)
```
Vj= jet velocity
Va = aircraft velocity
```
164
When is max thrust for a aircraft
When static
165
What happens to thrust above tropopause
Decreases more rapidly
166
How does bleed air affect thrust
Reduces mass flow
| Which reduces thrust
167
What does intake heating do to thrust
Reduces air density
Which reduces mass of air
Which reduces thrust
168
What is flat rated thrust
Cooler / denser air means full power is produced before throttle reaches max detent
= rpm limited
169
What is specific fuel consumption
Mass of fuel / unit of thrust / time
170
What is propulsive efficiency
Efficiency with which kinetic energy is translated into propulsive force
At low speeds the high velocity exhaust wastes more energy than the propeller
Prop more efficient at low speeds
Some engines have efficiently of 80/90%
171
Characteristics of single spool axial flow turbojet
Only efficient at high speed
Small frontal area
Inefficient and noisy
172
Most common type of reduction gear box
Sun and planet gear train
173
Purposes of gas turbine air inlet
Provide supply of turbulence free air, evenly distributed to the face of the compression
Present air at Mach 0.5
Maximise ram effect
174
What happens to pressure velocity and temperature at the gas turbine air inlet
Pressure increases
Velocity decreases
Temperature increases
175
Advantages of centrifugal compressor
Simple
Robust
Short length
Not prone to stall and surge
176
Disadvantages of centrifugal compressor
```
Broad frontal area
Low mass flow
Low pressure ratios
Poor acceleration
High SFC
```
177
Axial flow compressor pressure ratio and temperature rise over one stage
1.1:1 to 1.2:1
25°c
178
What happens to velocity pressure and temperature across a compressor rotor
All increase
179
What happens to temperature pressure and velocity across a compressor stator
Velocity reduces
| Temperature pressure increases
180
Advantages of a axial flow compressor
Small frontal area
High mass flow
High pressure ratios
Low SFC
181
Disadvantages of axial flow compressor
Expensive
Complex
Prone to stall / surge
Easily damaged
182
What is a axi-cf compressor
Half centrifugal half axial
Makes for shorter engine
183
Variable inlet guide vanes (VIGV)
Present incoming air at the optimum angle for subsequent rotor stages
184
What is internal engine air
Airflow that does not contribute directly to engine thrust
Bearing chamber sealing
Compressor and turbine blade tip clearance
Internal cooling
185
Where is the sensor for engine over heat (turbine over heat)
On some engines the thermo switch is in the cooling air outlet
186
What is accessory cooling
Air through a nozzle from the compressor sucks in atmospheric air to cool the engine while running on the ground
187
Multiple can combustion system
Easy servicing
Doesn’t present gas stream to all areas of turbine
188
Tubo annular combustion chamber (can-annular)
Servicing more difficult
Presents gas stream to a greater area of turbine compares to multi can
189
Annular combustion chamber
Presents gas stream to all areas of turbine
190
What % of air in the combustion section does what
20% in direct support of combustion
80% round outside
Of that 80%
- 20% shapes flame
-40% cooling
20% dilution
191
Fuel to air ratio in primary zone of combustion chamber
15:1
192
Fuel to air ratio overall
60:1
193
Reverse flow vaporising nozzle
Helps vaporise fuel to be sprayed
194
Duplex nozzle
Like a finger over a hose
Helps maintain high fuel pressure during low engine running
195
On start how many spark and at what energy does the High energy ignition unit supply
60-100 parks per min
| 8-10 joules
196
When engine on continuous ignition what energy are the sparks
4-6 joules
197
Most common turbine blade attachment method
Fir tree route
198
What is blade creep
Elongation of metal beyond its elastic limit
199
What are turbine blades made out of
Nickel based alloys
With additional elements such as tantalum, aluminium, and chromium
Made with single crystal technology
200
What is active clearance control
Uses bleed air around the turbine casing causing it to expand
201
Purpose of the exhaust system
Straightens
Accelerated
And directs exhaust gasses
202
What is the angle of reverse for reverse thrust
135/150°
203
What is no rev thrust indication
Fully stowed and locks are fully engaged
204
What is a Amber reverse thrust indication
Locks are disengaged (in transit)
205
Green reverse thrust indication
Thrust reverser fully deployed
206
Where are gas turbine temperature sensors located
Exhaust section or turbine
Ideal place would be turbine inlet
207
What is a optical pyrometers
Non contacting device that intercepts and measures thermal radiation
208
What seal is used on the main bearing housings for a gas turbine
Labyrinth seal
209
What is the most common oil system on gas turbines
Full flow recirculating system
Used oil fed here
210
What is a fuel cooled oil cooler (FCOC)
Aka (FOHE) fuel oil heat exchanger
Pump oil round fuel pipes to cool oil and heat fuel = before fuel filter to stop ice forming
Low pressure fcoc acts as fuel heater
High pressure fcoc acts as oil cooler
211
Centrifugal breather
Spins round to separate oil and air
212
Primary oil filtration
Provided by large capacity filter in combined scavenger line
Remove fine particles
213
Secondary filtration
After pressure pump
Much coarser
Ensures coolers and oil jets not at risk of blockage when scavenger filter bypassed
214
Fuel control unit senses
```
Thrust lever position
Air temp and pressure
Engine speed
EGT
Compressor delivery pressure
```
215
What does a acceleration control unit ACU do
Limits fuel flow to prevent over fuelling
216
FADEC
Full authority digital engine control
Closed loop
```
Computed functions of:
Throttle lever angle
TAT
altitude
Mach number
```
217
If both FADEC channels fail
Engine in safe condition
Fuel flow to min
VSV to fully open
Oil cooler to wide open
Active clearance control ACC shut off
218
What is “self sustaining speed”
Speed at which the engine will accelerate without the aid of starter motor
219
wet start
Faulty ignition unit
Important to let it drain
No indication of rise to EGT
Stagnation of RPM
220
Hot start
EGT exceeds max
Causes
Low air pressure to air started
Failure to completely drain after wet start
Strong tail wind in jet pipe
Early opening of fuel valve
221
Hung start
Rpm stagnated with high EGT
Causes
Fuel control malfunction
Premature starter disengagement
Compressor stall
222
How long can take off thrust be used
5 mins
223
How long can max contingency thrust be used for
2.5 mins
When baulked Landing one engine out
224
How long can intermediate contingency thrust be used for
Used 1 engine failed
Unlimited duration
225
How long can max continuous thrust be used for
Unlimited duration
| All engines
