Subjects 1 - 3 Flashcards
- Fundamentals - Engine performance - Engine inlet systems
How does a turbojet produce thrust?
By accelerating air through the core
What is the advantages to a Piston Engine
- Lower manufacturing cost than Jet Engine
What is the advantages to a Jet Engine
- Fewer moving parts
- More time in service (less time between overhaul)
- Higher Power:Weight ratio
- Can produce thrust by either propeller or jet action
What is Boyles law?
If temperature is constant:
- Pressure and Volume are inversely proportional
What is Charles Law?
If pressure is constant:
- Volume and temp are directly proportional.
What is the Brayton Cycle?
Pressure across the COMBUSTION phase remains constant
- Gas volume and velocity increase
Can be used in open or closed cycles.
What is Bernoulli’s theorem?
Total Kinetic and Potential energy = Constant
- Conservation of energy
- As velocity increases, pressure decreases
What happens to Velocity, Pressure and Temp in a subsonic convergent duct?
Velocity: Increase
Pressure: Decrease
Temp: Decrease
What happens to Velocity, Pressure and Temp in a subsonic divergent duct?
Velocity: Decrease
Pressure: Increase
Temp: Increase
What happens to Velocity, Pressure and Temp in a supersonic convergent duct?
Velocity: Decrease
Pressure: Increase
Temp: Increase
What happens to Velocity, Pressure and Temp in a supersonic divergent duct?
Velocity: Increase
Pressure: Decrease
Temp: Decrease
Is density changed when air passes a supersonic duct?
Yes
Is density changed when air passes a subsonic duct?
No
In a normal shockwave, how is the Velocity, Density, Energy in airstream, Static Pressure and direction affected?
Velocity: Slows to subsonic
Density: Greatly increases
Energy in airstream: Reduced
Static Pressure: Increases
Direction: Does not change
In a oblique shockwave, how is the Velocity, Density, Energy in airstream, Static Pressure and direction affected?
Velocity: Slows down but remains supersonic
Density: Increases
Energy: Some energy wasted (heat)
Static Pressure: Static pressure increases
Direction: Flow follows surface contour
In a Expansion wave, how is the Velocity, Density, Energy in airstream, Static Pressure and direction affected?
Velocity: Increased supersonic flow
Density: Decreases
Energy: No loss in energy
Static Pressure: Decreases
Direction: Follows surface contour
What is Newtons 3rd Law?
For every action there is an equal and opposite reaction
How does aircraft produce thrust?
Changing the momentum of the mass of air
What is the two methods of creating thrust?
- Jet action
- Aerodynamic action
What is another name for aerodynamic action and jet action?
Aerodynamic - Cold jet thrust
Jet Reaction - Hot jet thrust
How does aerodynamic action produce thrust?
By accelerating a large mass of air through a small change in velocity
How does jet action produce thrust?
By accelerating a small mass of air through a large change in velocity
Of the two methods to produce thrust, which one is more efficient and why?
Cold jet thrust
- Due to Ek=1/2 m x v^2
Thrust is divided into 2 categories, what are they?
- Gross thrust
- Net Thrust
Thrust produced is determined by what 2 things?
- Change in airflow/fuel momentum
- Force caused by difference in pressure across the nozzle multiplied by the area of the nozzle
What is net thrust equal to?
Change in airflow momentum (fuel+air) + Chocked nozzle thrust
What is chocked nozzle thrust?
Pressure drop across nozzle
Describe the stages of the engine in terms of forward or rearward thrust
Inlet - No thrust
Compressor - Fwd thrust (energy taken from turbine)
Combustion - Fwd thrust (created by fuels energy release)
Turbine - Rearward thrust (to power compressor)
Exhaust - Rearward thrust (gas expands through the nozzle)
What is the equation for resultant thrust for the forward/rearward thrust?
Resultant thrust = Rearward thrust - Fwd thrust
At what nozzle condition does aircraft operate in for high speed operation?
Just below chocked nozzle
- The velocity of the gases remain unchanged, EGT only varied with the speed of sound
Does EGT increase or decrease with an increase in the speed of sound?
Increase
What is the chocked nozzle condition?
- When airflow in the nozzle reaches Mach 1, extra energy is converted to pressure
- A portion of the thrust is created by the difference between exhaust and ambient pressure.
Does thrust increase or decrease with an increase in temperature?
Thrust decreases with an increase in temp
How does an increase in altitude affect thrust?
DOUBLE EFFECT
- The temperature drop slightly increases thrust
- The decrease in pressure reduces thrust
OVERALL:
- Increase in altitude = Decrease in thrust
- The decrease in thrust is more than the increase due to temp drop
What is the thrust lapse rate?
Rate of thrust reduction with increase in altitude
What is the air temp lapse rate
1.9 degrees per 1000ft
When is the thrust maximum?
When aircraft is stationary
How does increasing airspeed decrease thrust produced when aircraft begins to move?
Thrust is slightly decreased when it is operating at the chocked nozzle thrust.
What is Ram effect?
When aircraft moves forward air is rammed into the inlet duct
What is Ram Recovery?
When the ram air pressure exceeds the atmospheric pressure.
- When the thrust increase from the ram effect exceeds the thrust decrease from airspeed, this is RAM RECOVERY.
- Increase in pressure reduces load on compressor, increasing mass and net thrust.
Overall, does increase in airspeed increase or decreases thrust?
Increases thrust with increase in airspeed due to ram recovery.
What does a centrifugal compressor use to indicate thrust?
RPM
What does a dual spool centrifugal compressor use to indicate thrust?
EPR (Engine pressure ratio)
What is Thrust Horsepower?
Measure of work done per time (POWER)
What is the equation for Thrust HP?
(Net thrust x Airspeed) / 375 mile-pound per hour
What is 1HP equal to in mile-lb per hour?
375 mile-lb per hour
For turboprops, what is the equation for Thrust HP?
Brake HP x Efficiency (n)
- Turboprops have a standard efficiency of 80%
What is the equivalent HP and where is it measured?
Sum of shaft HP and Net Thrust of exhaust gas
- Measured at the output shaft
What is the equation for ESHP when STATIONARY?
SHP + (Net Thrust / 2.5)
(1 SHP = 2.5lb thrust)
What is the equation for ESHP in FLIGHT?
ESHP = SHP + (Net Thrust x Airspeed / 375 x n)
- Consider Airspeed
What are the 4 types of Jet Reaction Engines?
- Rocket
- Ram Jet (Athodyd): Subsonic = Divergent, Supersonic = Convergent
- Pulse Jet: Flapper valve
- Gas turbine
What are the two types of Gas Turbines?
- Thrust producing
- Torque producing
How do turbojets work?
- Accelerates small mass of air to large velocities
Advantages/Disadvantage for turbojets.
Advantages:
- Fast
Disadvantages:
- Noisy
- Poor fuel economy
- Inefficient thrust
How do turbofans work?
Bypassing air - Large mass of air accelerated by small velocity
Describe high bypass and low bypass turbofans
- High bypass turbofans (4:1) produces 90% of thrust from fan.
- Low bypass engines (2:1) can mix gases in exhaust and afterburn it
Advantages/Disadvantage for turbofans compared to turbojet
Advantages:
- Efficient thrust
- Economical / Quiet
- Fast below subsonic
- More air available for afterburn
Disadvantages:
- Limited to subsonic (unless low Bypass ratio or afterburn is used)
Describe turboprops
Extra turbines to extract remaining exhaust energy to drive a propeller on a splined shaft.
- Less than 10% of exhaust gas is included in thrust.
Advantages/Disadvantage for turboprop
Advantages:
- Good Power:Weight ratio
- Efficient/Economical thrust
- Good acceleration at medium speeds.
Disadvantages:
- Can be noisy
- Relatively slow
What is a turboshaft?
- Similar to turboprop, but the shaft is connected to a gearbox and torque is multiplied.
Advantages/Disadvantage for turboshaft?
Advantages:
- High shaft HP
- Fast start (can get max power quickly)
- High Power:Weight ratio
- Burns low grade fuel
Disadvantages:
- Not as economical as diesels in big applications
How are ultra high bypass geared turbofans more efficient?
Multiplied torque from gearing down
- less load on turbine so turbine spins faster RPM
What is the benefit for Propfans (Contra-Rotating)
- Combines the efficiency of turboprops and speed of turbofans
What applications are turbojets best for?
High speed supersonic
- Poor efficiency at low speeds
- Very noisy
What applications are turboprops used for?
For high efficiency low subsonic speeds and when a good Power:Weight ratio is needed
- Complex transmission
- Noisy
- Limited top speed
What are the applications for a turbofan?
Good efficiency at medium speeds and quieter
- More complex than turbojet
- Suspectable to FOD
What are the applications for Propfans?
Higher fuel efficiency than turbofan
- Noisy
- Complex
- Under development
Whats the applications for a Geared Fan?
Higher efficiency and quieter than turbofan
- Complex
- Hugh maintenance due to gearbox
What is a single entry centrifugal compressor
When the impeller disc has vanes only on 1 side
What is the advantage and disadvantage to having a double entry centrifugal compressor?
Advantage:
- Allows narrower engine diameter and higher mass airflow
Disadvantage:
- Rear facing side does not fully benefit from ram effect because air has to turn inwards from the plenum chamber
How many components make up a dual stage centrifugal compressor?
2 impellers
How many components make up a dual stage axial flow compressor?
2 rotors, 2 stators
Single spool, dual spool and triple spool are components that make up what type of compressor?
Axial flow
What components make up an Axial-Centrifugal compressor?
1 stage of each:
- 1 rotor, 1 stator
- 1 impeller, 1 diffuser
In a triple spool, what components make up N1, N2 and N3
N1= Fan
N2= Intermediate compressor
N3= High pressure compressor
How does a free power turbine work?
A separate turbine drives the propeller instead of a compressor
What are the advantages/disadvantages to a Free Power Turbine?
Advantages:
- Can seek the optimum design speed by itself
- Easier to start (especially in cold weather)
- Rotor does not transmit vibrations to the gas generator
- Rotor brake can be used to stop rotor from turning while engine runs
- Rotor can be held at a very low RPM during taxi (prevents noise)
Disadvantages:
- Lack of instant acceleration
The blades in the aft fan are an extension of what?
Extension of the turbine blades
Describe airflow in a axial and centrifugal compressor
Axial:
- Straight through, no change in direction
- Parallel to the compressor
Centrifugal:
- An impeller receives air at the center and flings it out.
In a turboprop, where is the Fixed turbine connected to?
Compressor and gearbox
In a turboprop, where is the Free turbine connected to?
Directly to the rotor gearbox.
Are the tip speeds on a propfan subsonic or supersonic?
Supersonic tip speeds
What are the main applications for a turboshaft?
Helicopters, Powerplants, Ships
What are the 2 main purposes of an engine inlet?
- Provide a constant flow of subsonic air to the compressor with little drag
- Increase air pressure
What are Pitot/Straight inlets and where are they used on?
- Used in nearly all subsonic planes
- Air goes straight through
What are divided inlets and where are they used on?
Two or more inlet ducts feed into one engine
- Used on fighter jets
What are internal/external compression inlets and where are they used on?
They are supersonic inlets that controls the formation of shockwaves to slow airflow to subsonic. The inlet cone goes in/out
- Used on the SR-71 blackbird
What is the variable throat inlet?
Controls the formation of shockwaves and volume of air entering
What are Engine mounted inlets and what is an example of a plane they are used in?
Shortest duct, minimal duct losses
- 787, 777
What are wing mounted inlets and what is an example of a plane they are used in?
Low drag profile, minimal duct losses
- Comet
-Engines have become too big to be wing mounted
What are Belmouth inlets and what is an example of a plane they are used in?
No duct losses, but high drag profile
- Helicopters
What are single entrance inlets and what is an example of a plane they are used in?
Aerodynamically clean but inefficient due to length of duct
- F11 underscoop
What are Fuselage mounted inlets and what is an example of a plane they are used in?
Low drag profile and duct losses
- F22, F35
- Sudden flight maneuvers cause imbalance of ram pressure.
What is the purpose of a subsonic inlet diffuser?
To increase the pressure by slowing down subsonic air
What is the most efficient inlet for a turboprop? What are disadvantages of this inlet?
Ducted Spinner Inlet
- Disadvantage: - Complex to maintain and difficult to provide anti-icing
What is an effective alternative to a Ducted Spinner Inlet?
Conical Spinner Inlet
What inlet is used on a turboprop when the reduction gears are offset to the propeller?
Underscoop Inlet
What type of duct do all supersonic aircraft use?
Convergent-Divergent Duct
- C-D Duct
Describe airflow in a Supersonic inlet
- Supersonic air is pinched to create a normal shockwave and slowing the airflow down to subsonic. Pressure and density increase
- The subsonic air passes the divergent section where it slows down further, increasing the density and pressure.
Describe airflow in a supersonic exhaust
- The subsonic flow of gasses aft of the turbine is pinched, increasing it to Mach 1. It is unable to flow any faster (Chocked nozzle) so pressure and density increase as it axially compresses.
- Once the supersonic air passes the divergent duct, it rapidly expands and loses its pressure/density, accelerating the gas to supersonic.
What are the two types of Variable Geometry Inlets?
- Inlet Spike (Movable Plug)
- Movable Wedge (A wedge in the airstream creates C-D Duct)
What is an Inlet vortex dissipator?
Compressed bleed air ducted from inlet to prevent vortex from forming and pulling in FOD
Why are inlet screens rarely used on fixed wing?
Can be blocked with ice and metal fatigue can ingest metal into engine
- Used on rotorcraft and test stands due to proximity from ground
What do Inlet Particle Seperators do?
Use centrifugal force or inertia to separate heavier particles from entering engine and maintains constant airflow to compressor.
What are Blow-In Doors on the inlet?
Provides additional air to engines during ground operations when enough air isn’t applied.
What are the main problems with inlet ice?
- Ice reduces efficiency and airflow
- Chunks of ice can detach and damage compressors
What are the 3 Anti-ice systems?
- Bleed air
- Electrical
- Hot engine oil
How does Bleed Air Anti-Ice work?
Bleeds air from first few stator stages in compressor to provide surface heating of the inlet
- Slight EGT rise when anti-ice is selected
- PT2 drain has two purposes; drains water away and acts as ice suppressor.
What anti-ice method is most common on turboprops and turboshaft?
Electrical anti-ice
Describe Electrical Anti-Ice
Uses heat strips made of high resistance material wrapped in neoprene.
Should electrical anti-ice be used when the engine is not running?
No, should only be used when the engine is running to prevent overheating.
When is the Hot Oil Anti-ice used?
To supplement electrical and bleed air anti-ice.
How does the Hot Oil Anti-ice work?
Hot oil from the engine is pumped around the inlet duct and struts before returning to an oil cooler and then to the oil tank.
