AT103 Review Through Exam 3 Flashcards
Early Gas Turbine Engine Inventors/Users
1232 - Chinese began to use rockets as weapons.
1872 - Dr. F. Stolze designed the first true gas turbine engine. This engine never ran under its own power.
1939 - The aircraft company Ernst Heinkel Aircraft flew the first flight of a gas turbine jet, the HE178.
1941 - Sir Frank Whittle designed the first successful turbojet airplane, the Gloster Meteor. In 1942 he shipped an engine prototype to General Electric in the United States. America’s first jet plane was built the following year.
1942 - Messerschmitt Me 262, the world’s first operational jet fighter.
Brayton Cycle vs. Otto Cycle
Brayton is the same as Otto except events occur at the same time and events occur at different locations within the engine
Convergent vs. Divergent
C: Velocity decreases, Pressure and Temperature increase
D: Velocity increases, Pressure and Temperature decrease
Non-Air vs. Air Breathing
NA: Carries all the needed oxygen within their fuel
AB: Takes air
Types of Air Breathing Turbine Engines (4)
Turbojet
Turboprop
Turboshaft
Turbofan
Turbojet Engines
Takes air in through the air inlet
Compresses the air
Mix compressed air with fuel
Air and fuel mixture ignited and burned
Hot gases passes through turbine
Air forced out of a specially shaped exhaust nozzle forming a high-speed jet
Turboprop Engines
Drives a propeller
A derivative of the turbojet engine
Two methods to drive propeller
Connecting the propeller to the compressor
Connecting the propeller to a turbine separate from the core engine (Free-Turbine Engine)
Turboshaft Engines
Commonly used for helicopters
More efficient at constant RPM required by helicopters
Most drives output shaft with multistage free turbine
Reduction gearbox is remote from the engine
Turbofan Engines
Increased propulsive efficiency
Lower Noise
Lower fuel consumption
Lengthened blades in early stage(s) of low-pressure compressor
Air from the fan section flows around the outside of the core engine
Fan can produce 30-75% of the total thrust
Bypass Ratio
Ratio of the mass or air moved by the fan to the mass of air moved by the core engine
Types of Bypass Ratios (3)
Low-Bypass (Less than 2 : 1)
High-Bypass (Ratio of 4 : 1 or greater)
Ultra-High-Bypass (Exceeds 30 : 1)
What is thrust measured in?
pounds (lbs)
What is thrust determined by? (3)
Change in momentum experienced by air flowing through the engine
The momentum of the fuel
The force caused by the difference in pressure across the exhaust nozzle multiplied by the area of the nozzle
Types of thrust (2)
Net
Gross
Net thrust
Thrust produced by the engine while in flight
Gross thrust
Thrust produced by the engine while engine is static and not moving
Thrust indication for smaller jet engines
Compressor speed in % RPM
RPM is approximately proportional to the trust being produced by the engine
Thrust indication for larger jet engines
Engine Pressure Ratio (EPR)
Ratio of the turbine discharge total pressure to the compressor inlet total pressure
Methods of increasing thrust (3)
Water Injection
Duct Heater
Afterburners
Thrust Horsepower
Can be computed when engine thrust (lbs) and aircraft speed (mph) are known
What are turboprops rated in?
ESHP
What do turboprops take into account for? (2)
Shaft horsepower to the propeller
Thrust developed at the exhaust
Engine Station Designations (11)
AM or 0 - Ahead of the Engine
1 - Entrance to the aircraft air inlet duct
2 - Fan (N1 compressor) inlet
3 - N2 compressor inlet
4 - Burner inlet
5 - N2 Turbine inlet
6 - N1 Turbine inlet
7 - Diffuser
8 - Afterburner combustion chamber
9 - Afterburner duct
10 - Exhaust nozzle discharge
Gas Turbine Engine Sections (2)
Cold
Hot
Parts of the Cold Section (3)
Inlet air duct
Compressor
Diffuser
Parts of the Hot Section (3)
Combustion
Turbine
Exhaust system
Inlet Air Duct
Usually considered a part of the airframe
Responsible for supplying a constant undisturbed flow of subsonic air to the compressor
Designed specifically to match the engine being used
Types of inlet air ducts (2)
Subsonic
Supersonic
Subsonic inlet ducts
Usually a divergent type
Air flowing into a divergent duct… Velocity decreases Pressure increases
Turboprop Engines
Propeller reduction gearbox interferes with air flow
Ways to deliver air in turboprop engines (3)
Ducted Spinner Inlet
Conical Spinner Inlet
Under-scoop Inlet
Pratt & Whitney PT6 Engine
Reverse flow
Foreign Object Damage (FOD)
Common practice to cover engines when not in use
Warm compressor air is routed to prevent…
inlet ice formation (bleed air)
Supersonic inlet ducts
Air must be below the speed of sound entering the compressor
A convergent-divergent (CD) inlet duct is used
Supersonic air entering slows down until…
the narrowest part of the duct
What happens when supersonic air enters the narrowest part of the duct?
Air is reduced to speed of sound and a normal shockwave forms
Air becomes subsonic after flowing through the normal shockwave
Air is further slowed through divergent duct
Variable inlet ducts are used to accommodate…
different air speeds
How do variable inlet ducts accommodate different air speeds? (2)
Lowering or raising a wedge
Moving a taper plug in or out of the duct
Types of Compressors (2)
Centrifugal
Axial-Flow
Advantages of Centrifugal Compressors (4)
Rugged
Light Weight
Ease of Construction
High Pressure Ratio for each stage of compression
Centrifugal Compressor components (3)
Impeller
Diffuser
Manifold
Centrifugal compression ratio
6 : 1 to 7 : 1
Centrifugal Compressors
Volume of air moved depends on the diameter of the compressor
Disadvantages of diameters too large (2)
Tip speed increases and efficiency decreases
Difficult to streamline the engine
Types of Centrifugal Compressors (2)
Double-Entry
Multi Stage
Double-Entry Centrifugal Compressor
Difficult to design inlet ducts for front and rear air supply
Multi Stage Centrifugal Compressor
High pressure rise per stage
Experiences pressure loss between stages
Axial-Flow Compressors
Air passes straight through the compressor
Made up of stages of rotor blades between stages of fixed stator vanes
Rotor blades and stator vanes are airfoil shaped and positioned to form a series of divergent ducts
Axial-Flow Compressor disadvantages (2)
Heavier than centrifugal compressors
Costly to manufacture
Axial-Flow Compressor advantages (2)
Higher overall compression ratio
Easier to streamline
Types of Axial-Flow Compressors (2)
Single-Spool
Dual-Spool
Single-Spool Axial-Flow Compressors
Stages of compressor is limited
What happens when there are too many stages in a Single-Spool Axial-Flow Compressor? (3)
Rearmost stages become inefficient
Front stages become overloaded
Airflow through compressor become restricted and can lead to compressor surge
Dual-Spool Axial-Flow Compressors
Rearmost compressor = High-pressure compressor (N2)
Driven by forward stage turbine (High-pressure turbine)
N2 compressor is governed by the fuel control and used by the starter to start the engine due to its lighter weight
Three-Spool Axial-Flow Compressors
Fan = Low-pressure (LP) compressor (N1)
Intermediate-pressure (IP) compressor (N2)
High-pressure (HP) Compressor (N3)
All driven by separate turbines (three shafts)
Path through Three-Spool Axial-Flow Compressor
- N1, N2, N3 Compressors
- Diffuser
- Combustion
- Turbine
- Exhaust system
Hybrid Compressor Engine
Uses both axial-flow and centrifugal compressors
Blade Attachment
Blades are loosely attached to the compressor drum or disk
Centrifugal forces from a running engine will hold the blades in their intended position
Commonly attaching by the dovetail or fir tree method
Blade Design
Blades have twists designed to produce the correct pressure gradient along the length as velocity changes
Much like small propellers
To maximize efficiency (seal):
Abradable strips are mounted in the compressor housing and/or the blade tip
What counteracts Bernoulli’s Principle
Blade twists
Guide Vanes and Stator Vanes
Usually fixed (Variable controlled hydraulically with fuel from fuel control)
Turns air so it flows correctly for the rotor blades
Stator Vanes have airfoils positioned to create divergent ducts (decrease velocity and increase pressure)
Diffuser Section
At the rear or the compressor
Diverging area for air to decrease velocity and increase pressure
Highest pressure point in the engine
Causes of Surges and Stalls (5)
Excessive rotor blade angle of attack
Obstruction to inlet
Excessive pressure in the burner section
Abrupt flight maneuvers
High crosswind during takeoff and low airspeed
Combustion Section Requirements (7)
Minimum pressure loss in gases
High combustion efficiency (low smoke emission)
Low risk of flame blowout
Combustion occurring entirely within the combustor
Uniform temperature distribution throughout the gases
Low enough temperature of gases leaving combustor to prevent turbine damage
Combustor design provides easy starting
What are Combustors made out of?
Thin sheets of corrosion resistant metal
Most Common Types of Combustors (3)
Multiple-Can
Can-Annular
Annular
Multiple-Can Combustors
Usually between 8-10 cans are used
Igniters installed in only 2 cans
Crossover tubes connects the cans
Advantage of Multiple-Can
Individual cans can be removed
Disadvantages of Multiple-Can (2)
Uneven temperatures
Can cause turbine failure when cans fail due to extreme temperature differences
Can-Annular Combustors
Consist of individual cans mounted on an annular duct
Hot gases are collected, then directed into the turbine
Advantages of Can-Annular (3)
Individual cans can be removed
Shorter cans (lower pressure drop)
Uniform temperatures even with a clogged fuel nozzle
Annular Combustors
Makes the most efficient use of space
Efficient mixing of fuel with air
Requires minimum amount of cooling air
Provides even temperature air
Cannot be replaced without removing the engine from the aircraft
Turbine Section
The power-producing component
¾ of the gases from combustion section is converted to SHP to drive compressor and fan
Types of Turbines
Axial Turbine
Radial-Inflow Turbine
Turbine Inlet Guide Vanes (TIGV)
Mounts between the combustion section and first stage turbine
Forms a series of convergent ducts to increase velocity and decrease pressure
Directs air to turbine blades at the right angles
Highest temperature point in the engine
Highest pressure and temperature points in the engine
Pressure: Diffuser Section
Temperature: Turbine Inlet Guide Vanes (TIGV)
Turbine design
Normally made of heavy nickel alloy forging
Blades are attached with a fir-tree method (accounting for thermal expansion)
Turbine disk and blades are sometime machined out of a single piece of material and are called “blisk”
Turbine Blade Designs (3)
Reaction Blade
Impulse Blade
Reaction-Impulse Blade
Reaction Blade
Produce turning force by aerodynamic action (airfoil)
Impulse Blade
Produce turning force by the energy required to change the direction of airflow (bucket)
Reaction-Impulse Blade
Combo of Reaction and Impulse Blades
Turbine Cooling
Amount of power a gas turbine engine can produce is limited by the maximum temperature the turbine inlet can tolerate
Turbine Inlet Temperature (TIT)
Turbine cooling improves efficiency
Flowing compressor bleed air through hollow guide vanes and rotor blades
Causes of Turbine Failures (3)
Creep
Metal Fatigue
Corrosion
Creep
Deformation of metal that is continually under high centrifugal loads and temperatures
Metal Fatigue
Weakening of metal subjected to repeated cycles
Corrosion
Electrolytic action that occurs when alloying agents combine with elements in the air to form salts
Accelerated by exposure to extremely high temperatures
Turbine Engine Exhaust
Energy remaining after exhaust gases passes through the turbine are utilized to produce thrust
Exhaust system must straighten and accelerate the exhaust gases
The end of the tail pipe is the exhaust nozzle or jet nozzle
The outlet area determines the velocity of gases leaving the engine
Most turbojet and low-bypass turbofan have an outlet area in a choked condition
Gases are accelerated to the speed of sound at the end of the tail pipe and can…
no longer accelerate
For turbine engines equipped with afterburners
Variable-Area Nozzle
Variable-Area Nozzle
Opens or closes automatically based on the fuel flow of the engine
Earlier afterburner engines have only two positions
Modern afterburner engines control their nozzles with the electronic engine control (EEC)
For efficiency
The divergent portion of the nozzle must vary automatically as airflow changes
Some nozzles use a wall of air to form the divergent portion boundary of the nozzle
Noise Suppressors
Noise is one of the primary complaints
The amount of noise relates to the velocity of the exhaust gases
The distance the noise travels relates to the frequency of the sound
The amount of noise cannot be decreased without sacrificing thrust
First sounds to be heard are always the low-frequency
Increasing the frequency will shorten the distance of the noise
Thrust Reversers
Used to assist the brake system in slowing down the aircraft after landing
Same function as propellers with reverse- pitch capability
Turbojet and turbofan engines reverses some of its thrust to aid deceleration
Thrust reversers diverts 40-50% of the engine’s rated forward thrust rearward
Used on the ground to decrease landing roll
Some aircrafts use thrust reversers to allow descent at a steep angle without building up excessive speed (Very limited)
Operates by deflecting part of the exhaust gases or fan discharge air forward
Actuated by the pilot using the control mounted on the engine throttle
Throttle is retarded to the idle position, then the reverse thrust control in moved
Afterburners
The air flowing out of the exhaust is hot and contains large amounts of oxygen
First used on a civilian aircraft by the Concorde
Although afterburners use more fuel, it also helps aircraft reach cruising altitude quicker
Also known as a “reheat” system in the United Kingdom
When operating the afterburner, back pressure on the turbine would increase if exhaust nozzle stays the same
Variable exhaust nozzles are used to control turbine back pressure
Vector Thrust
Either through changing the direction of the exhaust nozzle or the turbine engine entirely
British Aerospace Sea Harrier is one of the first to use vector thrust for vertical takeoff capabilities
To increase the thrust produced by vector-thrust engines, plenum-chamber burning (PCB) is used
Similar to afterburners, fuel is injected into the fan-discharge air and burned
Increase thrust by as much as 50%
Lubrication System in Gas Turbine Engines
Absorb large amount of heat
Typically 5 – 8 gallons of low viscosity synthetic oil
Oil circulates through the engine at a high flow rate
Normally about a pint an hour is consumed
Turbine shaft bearings
Absorb large amount of heat
Similarities to reciprocating engines (5)
Lubrication reduce friction in the gears and bearings
Absorb heat from the bearings
Protect metal parts against corrosion
Pick up and carry contaminants into filters
Routed through an oil-to-fuel heat exchanger
Lubrication System Subsystems (3)
Pressure Subsystem
Scavenge Subsystem
Vent Subsystem
Lubrication System Components (3)
Oil Filters
Oil Coolers
Chip Detector
Oil Filters
Wire-mesh screen can remove contaminants up to 40 microns
Pleated fiber filter up to 15 microns
Oil Coolers
Modern oil coolers are of the oil-to-fuel type
Serves dual purpose
Removes heat from the oil
Warms the fuel
Chip Detector
Magnetic chip detectors are installed in the scavenger system
Picks up ferrous metal flakes or chips
They are easily removed for routine inspection
Some are connected into an electrical circuit that indicates metal particles by illuminating a warning light on the flight deck
Turbine Engine Fuels (2)
Jet A / A-1
Jet B “Wide-Cut Fuel”
Jet A Fuel
Similar to commercial kerosene
Similar to the military JP-8
Jet A starts to freeze at -40°F
Jet A-1 starts to freeze at -58°F
Jet B Fuel
Blend between gasoline and kerosene
Similar to the military JP-4
Jet B starts to freeze at -60°F
Tetraethyl Lead (TEL)
Causes deposits to form on the turbine blades
Lubrication Properties of Avgas (2)
Avgas does not lubricate as well as kerosene (lubricity)
Can cause excessive wear on fuel control system
Microbial Growth in Jet Fuel Tanks
Microscopic organisms live and multiply at the interface between water and fuel forming scum
The scum holds the water that is in contact with the tank structure causing corrosion
Scum Prevention Methods
Additives (Ethylene Glycol Mono-methyl Ether, PFA 55MB, Prist)
Mixed into the fuel at the refinery or
Injected into the fuel as it is pumped into the tanks
Turbine engines use a…
continuous combustion process (not timed)
Ignition is only needed normally during the…
start sequence
Unlike aviation gasoline, turbine engine fuel requires a…
much hotter spark for ignition
Ignition system is normally turned on during (4)
Takeoff
Landing
Flight into Turbulent Conditions
When Engine Anti-Ice System is Actuated
In case of a turbine engine flameout
2 Ignition Exciters
2 High-Tension Leads
2 Igniters
Cooling-down period is called a…
“duty cycle”
Two types of ignition systems
Intermittent-Duty System
Continuous-Duty System
To start a gas turbine engine
The compressor is first rotated with a starter
Ignition system is energized
When the engines reaches a specified RPM, fuel is sprayed into the combustors and the engine starts
Once engine accelerates to a self-sustaining idle speed, the starter and ignition systems are turned off
Start sequence is usually programmed so the pilot only needs to initiate the action (press and hold a button)
For dual-spool axial-flow engines, the starter rotates the N2 compressor
Commonly Used Started Types (2)
Air Starters (Large Engines)
Electrical Starters (Small Engines)
Air Starters
Light weight with good amount of torque produced
Requires large volumes of low pressure compressed air
APU, GPU, and cross-flow of compressor bleed air
Air starters have their own lubrication system in the starter housing
Magnetic chip detector is built into the drain plug
Electrical Starters
Series-wound electric starters
Connected to the engine through a ratcheting starter jaw similar to reciprocating engines
Starter jaw engages a mating jaw on the engine starter drive
Once engine is self-sustaining, the starter jaw moves away from the jaw in the engine
Electrical power is then stopped to the starter
What connects to the crankshaft?
Connecting rod
The top limit of the stroke is called
Top Dead Center (TDC)
Parts of a spark plug (3)
Electrodes
Ceramic Insulator
Metal Shell
Fine Wire Spark Plug
Similar construction to the massive electrode type
Electrodes are made of platinum and iridium
Platinum and iridium ensure maximum conductivity and minimum wear
Most superior spark plug
Too high volatility will form bubbles in the fuel lines causing…
“vapor locks”
