AT 103 Third Exam Flashcards
!! What are the advantages of a fuel injected system !!
Freedom from vaporization icing
more uniform delivery of air fuel mixture to each cylinder
improved control of fuel/air ratio
instant acceleration after idling with no tendency to stall
increased engine fuel efficiency and power
what is bendix
a servo
Difference between fuel injected and carburated
carburator - mixer
Fuel injected - meterer
fuel air mixture in fuel injected does not mix in the Venturi
carb vs injected similarity
both have an air inlet tube that goes through a Venturi
throttle regulates air through system
What does the idle valve do
connected to throttle linkage so that at idle, another part will open to let a little bit of fuel in in order to not over lean the mixture
What are the two functions of the fuel divider
ensures equal distribution of metered fuel to the nozzles
provides isolation of each nozzle for clean engine shutdown
What operates the fuel divider
fuel flow pressure
where does the air - fuel mixture meet?
right at the point before entering the cylinder
What are the two ways to produce thrust for turbine engines
aerodynamic action
jet reaction
How does aerodynamic action work
uses propellers
lift is produced along the horizontal plane
large mass of air through small change in velocity
how does jet reaction work
mass of air is heated inside the engine and discharged at high velocity through a shaped nozzle
Mass of air and amount determines thrust
(Small mass of air through large velocity change)
Who demonstrated the first jet propulsion (and when)
Heron of Alexandria
Egypt
first century AD
Quick turbine development history!
1232 Chinese weapon rockets
1872 Dr F Stolze first true gas turbine engine - never ran under own power
1939 - Ernst Heinkel Aircraft HE 178
1941 - Sir Frank Whittle designed Gloster Meteor
1942 - Messerschmitt Me 262 world’s first operational jet fighter
What is the Otto cycle
intake
compression
power
exhaust
What is the Brayton cycle
Otto cycle but with events occurring at the same time, however in different locations within engine
What happens in a convergent duct (supersonic)
air compresses
velocity decreases
pressure increases
density increases
What happens in a diverging duct (supersonic)
air expands
velocity increases
pressure decreases
density decreases
How does air flow through a chocked nozzle
velocity increases when converging, exhaust accelerates to speed of sound and forms shockwave creating pressure differential across nozzle that contributes to thrust
How does a non-air-breathing engine work
carries needed oxygen in fuel
generate heat by chemical reaction between fuel and oxidizer
how does turbojet work
takes in air
compresses
mixes compressed air with fuel
air fuel mixture ignited/burned
hot gasses pass through turbine
air forced out specially shaped exhaust nozzle
Benefits of the turbofan engine
increased propulsive efficiency
lower noise
lower fuel consumption
What does a bypass engine with a 4:1 ratio mean
(High bypass engine)
for every 4 molecules that enter, only one actually goes through the engine, the other three go into the bypass for cooling
What is thrust measured in
pounds
What is thrust determined by
change in momentum experienced by air flowing through the engine
momentum of the fuel
force caused by difference in pressure across the exhaust nozzle multiplied by the area of the nozzle
!! What is net thrust !!
thrust produced by the engine in flight
!! what is gross thrust !!
produced by the engine while engine is static and not moving ((test stand))
Thrust total =
thrust (core engine) + thrust (fan)
thrust indication for a small jet
% RPM
large jet engine indication
engine pressure ratio
(ratio of the turbine discharge total pressure to the compressor inlet total pressure)
What is the inlet air duct
part responsible for supplying the constant, undisturbed flow of subsonic air to compressor
(usually considered part of the airframe)
What are variable inlet ducts for
accommodate different air speeds by lowering/raising a wedge or moving a taper plug in or out of a duct
Centrifugal compressors advantages
rugged
light weight
ease of construction
high pressure ratio for each stage
what happens if a centrifugal compressor is too big
tip speed increases and efficiency decreases
difficult to streamline engine
pros/cons of axial-flow compressor
heavy
costly
higher compression ratio
easier to streamline
single spool axial flow compressor characteristics
limited stages
rearmost stages become inefficient/front stages become overloaded
airflow can become restricted and can lead to compressor surge
In a dual spool axial flow compressor, which compressor starts the engine and why
N2 due to lighter weight
compressors are made up of
rotor blades and fixed stator vanes which are shaped and positioned to form a series of divergent ducts
Why do blades have a gap at idle
centrifugal force creates the seal as the engine speeds up
where is the highest pressure point in the engine
diffuser section
what is the diffuser section
diverging area for air to decrease velocity and increase pressure
Advantages / Disadvantages of multi-can
A: individual cans can be removed
D: uneven temperatures which can cause turbine failure due to the extreme difference
can-annular advantages
can remove cans
shorter cans with lower pressure drop
uniform temperatures even with a clogged fuel nozzle
Pros / Cons of annular combusts
most efficient
minimum cooling
even temperature air
Cannot be replaced without removing engine
What is the turbine section
power producing component of the engine
Where is the hottest gas inside the turbine engine
turbine inlet guide vanes
What is a reaction blade
produces force by aerodynamic action (airfoil)
what is an impulse blade
produces turning force by the energy required to change the direction of airflow
what is creep
deformation of metal that is continually under high centrifugal loads and temperatures
what is metal fatigue
weakening of metal subjected to repeated cycles
what is corrosion
electrolytic action that occurs when rallying agents combine with elements in the air to form salts
Lubrication primary characteristics of turbine oil
Low Viscosity
High Film Strength (does not break under high pressure)
Low volatility (does not evaporate at low air pressures of high altitude)
secondary characteristics of turbine oil
high flash point
low pour point
good anti-foaming
low carbon deposit
what is the wet sump system
pressurized oil lubricates rotor coupling and bearings
accessory drive gears are splash lubricated
dry sump system
after lubricating/cooling, oil is returned to tank by scavenger pumps
Pressure subsystem
supplies correct amount of clean lubrication oil at the proper pressure and temperature
scavenge subsystem
picks up oil after it has performed functions and returns oil to oil tank
Vent subsystem
provides slight pressure on the oil in the oil tank to allow positive flow of oil
where is oil usually filtered in a turbine engine
right after the pressure pump and again before being sprayed
What is the chip detector
system that detects ferrous metal flakes or chips in the oil system
(canary in a coal mine)
What is the oil pressure system reading
difference between pressure produced at the oil pump and the pressure of air in the vent system
What is the difference between Jet A, A-1, B
Jet A: freezes at -40F
Jet A-1: freezes at -58F
Jet B: freezes at -60F
Kerosene vs Avgas
K: 6.7 pounds per gallon
Av: “6” pounds per gallon
Kerosene has a lower heat energy per pound but higher per gallon
What is the issue with turbine fuel viscosity
contaminants and water are suspended in jet fuel and take a long time to get picked up by the tank pumps
water can also freeze
what do fuel nozzles do
vaporize fuel to mix with air
if fuel flow is increased too quickly…
over rich mixture may result
excessive TIT or produces compressor surge
fire may go out
if fuel flow is reduced too quickly
engine may experience lean die-out
where is the TIT read
at the turbine inlet guide vanes
What does FADEC do
controls all engine functions and eliminates need for backup hydro-mechanical control
controls engine power through direct control of EPR
advantages of FADEC
no engine trimming required
improved engine starts
constant idle speed
saves fuel by improved engine bleed air management
fully modulates active tip clearance
automatically limits critical engine pressures and speeds, reducing wear and possible damage
What happens if both channels fail
actuator motors are spring loaded to fail safe positions
fuel flow goes to minimum
how does the turbine ignition system work
continuous combustion process so ignition is only needed during start
what are the three different types of instrument gauges
temperature
pressure
mechanical movement
