Total Pressure=Static Pressure+Dynamic Pressure

• Static pressure - the potential energy of fluid molecules at rest (pressure).
• Dynamic pressure - the kinetic energy of fluid molecules in motion.  It is a measure of the force of the fluid molecules as they move through the system.(Velocity)

• a subsonic nozzle is convergent and as velocity increases, pressure decreases.
• a subsonic diffuser is divergent and as velocity decreases, pressure increases.

• at supersonic speeds, the airflow has an opposite effect when ecountering convergent and divergent openings.
• a supersonic nozzle is divergent and as velocity increases, pressure decreases.
• a supersonic diffuser is convergent and as velocity decreases, pressure increases.

Compressor

Combustion chamber

Turbine

suck/squeeze/bang/blow

This operating cycle consists of 4 events which occur simultaneously: intake, compression, combustion and exhaust.

• Intake - air enters
• compressor - air is compressed
• combustion chamber - air/fuel mixture is ignited
• turbine - ignited gases accelerate through the turbine which then turns the compressor to continue the Brayton cycle
• Exhaust - after leaving the turbine, there is still enough pressure to force the hot gases through the exhaust duct and jet nozzle at a very high speed

Pressure:  increases from inlet to the burner and decreases after(highest at beginning of burner section)

Temperature:  increases until turbine (hottest at end of burner section)and then decreases

Velocity:  decreases from inlet to compressor, slightly increases in compressor, decreases in diffuser, increases in burner and turbine and increases/levels out at a very high velocity in exhaust section

• gross thrust - a measurement of thrust due solely from the velocity of the exhaust gases. Produced by a stationary engine; perhaps while mounted on a test stand, or an aircraft completing a ground run up.
• net thrust - thrust that corrects for the effect of inlet airflow velocity: 

Net Thrust = m [(V_final - V_initial)/t]

• As temperature increases, density decreases, and thrust decreases.
• As temperature decreases, density increases and thrust increases.

• As pressure increases, density increases, and thrust increases.
• As pressure decreases, density decreases and thrust decreases.

• As an aircraft climbs, pressure and temperature will normally drop.
• With an increase in altitude, the rate of thrust decreases because a pressure drop is greater than the thrust increase resulting from a temperature drop.
• An engine will produce less thrust as it increases in altitude.

• As the inlet veocity (V initial) approaches the magnitude of the exhaust velocity (V final) thrust is reduced.
• If the mass of air and fuel is held constant, thrust will decrease as airspeed increases.

• The difference btwn inlet and exhaust velocities decreases as the acft increases speed.  However, more and more air is being rammed into the inlet, increasing the mass and pressure of inlet air.
• This offsets the decrease in acceleration and results in a neutral effect or slight increase in thrust at subsonic speeds.
• As airflow becomes compressible(supersonic), mass due to ram effect increases at an increasing rate.
• Ram effect is especially important to high performance acft due to high mass airflow at supersonic speeds.

• Engine pressure ratio (EPR) gauges - for acft that rely on the propulsive power of the exhaust gases of a gas turbine engine, such as turbojets and turbofans.
• Torque meter - propeller or rotor driven acft use a torque meter gauge to indicate power available, such as a turboprop and turbo shaft.
• Tachometer - provides crew with an indication of engine speed.  Calibrated in % RPM.

• Single entrance - simplest/most effective design. Directly in front of engine.  Positioned to collect generally undistrubed air.
• Double entrance - 2 inlets (AV-8), allows pilot to sit lower in fuselage/reduces friction losses due to length.

Problems: located on side so boundary layer air and skin friction may distort incoming air.

cannot be made very large without increasing drag 

• subsonic: divergent in shape b//c of relative incompressibility of subsonic airflow;shape will increase airflow pressure while reducing its velocity
• supersonic: convergent/divergent shape; will initially converge at supersonic speeds;the highly compressible supersonic airflow is slowed to a value less than sonic velocity and pressure will be increased; at the outlet of the convergent shape the airflow is relatively incompressible and the shape must be divergent to continue the conversion of velocity into high pressure airflow for use in the compressor.

• Primary function - to supply enough air to satisfy the requirements of the combustion section.  The compressor increases the pressure of the airflow from the air inlet duct and directs it to the burners in the quantity and at the pressure required.
• Secondary function - to supply compressor bleed air to operate various components throughout the engine and acft.

• Comprised of stators and rotors
• The turbine section drives the compressor and the accessories.  It also designed to increase airflow velocity.

• Turbine section is the most highly stressed part of the engine
• The higher the temp the turbine section can bear, the higher the thrust that can be produced.  For this reason, titanium and ceramics are used in the construction

• blade elongation
• cumulative process and excessive temperatures over long periods may result in permanent blade deformation

• Turbine blades are not welded to the rotor shaft. They are attached by a method called "fir tree"
• This attachment method prevents the thin metal blades fro cracking at the attachment points by allowing them to expand when heated
• this does not eliminate thermal stress.  it does improve the turbine blades ability to handle high temperatures and repeated heating and cooling.

• the turbine section is comprised of stators and rotors.  Stators come before rotors.
• Stators prepare the airflow from the combustion chamber for the harnessing of power by the turbine rotor.  The stators deflect the gases at a specific angle in the direction of turbine wheel rotation.
• Rotors convert the heat energy (potential and kinetic) from the burner chamber into mechanical energy, approximately 75%.  The other 25% is used for thrust.

• Exhaust section must direct the flow of hot gases rearward to cause a high exit velocity to the gases while preventing turbulence

• convergent type
• takes slow subsonic gases from the turbine section and gradually accelerates them through the convergent section
• gases cannot exceed the speed of sound
• as the gas velocity increases, the ability of the pressure to push the molecules from behind decreases.

• it is a variable geometry convergent divergent nozzle
• controls expansion and velocity of exhaust gases.
• exhaust gases are accelerated to high sonic speeds in the convergent section.  At this point the gases are highly compressible.  To allow the airflow to continue its velocity increase, the volume outward and rearward must increase through the divergent section.

4 main parts: spray bars, flame holder, screech liner and variable exhaust nozzle.

• Spray bars - introduce fuel to the afterburner; located in the forward section of duct.
• Flame holder - form of flame stabilization;located downstream of fuel spray bars;provides region in which airflow velocity is reduced and turbulent eddies are formed; allows for the proper mixing of fuel and air for combustion.
• Screech Liner - inner sleeves to control a phenomenon known as screech (violent pressure flunctuations caused by cyclic vibrations that greatly reduce efficiency).
• Variable Exhaust Nozzle - commonly called turkey feathers; can close for basic engine subsonic operation or open to allow the gases to expand at the proper rate when the afterburner is being used; this prevents the gases from backing up and causing a back pressure, which can stall an engine.

• If AOA to the rotors is too low, the comression ratio will be low & the compressor will be inefficient.
• If AOA to the rotors is too high, a compressor stall is possible
• Anything that decreases the inlet airflow or increases compressor RPM will increase the AOA and therefore increase the possibility of a compressor stall

with constant PCL position:

• RPM decay
• ITT rise (interstage turbine temp)
• change in engine noise/loud noise
• flucuations in torque
• fuel flow
• T-34(RPM decay/ITT rise/noise/torque changes/compressor speed(N1)/fuel flow/flames&smoke)

1. airflow distortion (most common cause)
2. mechanical malfunction

1. Variable Inlet Guide Vanes (IGV) and stator Vanes:  failure to change the AOA will cause too much or too little airflow at low engine speed.

2.  Fuel Control Unit (FCU):

a.  determines proper amt of fuel to be introduced into the combustion chamber

b.  An over rich mixture causes excessive burner pressure and a back flow of air into the compressor that leads to a compressor stall

c.  too lean a fuel mixture may cause the engine to flame out which can be just as hazardous

3.  Foreign Object Damage (FOD)

when an object damages the delicate blades of the compressor; this results in deformation of the blades which change its aerodynamic properties

4.  Variable Exhaust Nozzles

If the nozzle fails to open, an excessive back pressure will be produced which could lead to a compressor stall

• avoid erratic or abrupt power control lever (PCL) movements, especially at low airspeeds or high AOA
• pilot should maintain at least the prescribed minimum airspeed and avoid abrupt changes in aircraft attitude to allow the proper amount of smooth air to enter the inlets.
• avoid flight through severe weather and turbulence

1.  Variable Inlet Guide Vanes: AOA can be changed at low engine speed;this maintains the velocity of the air(and the angle at which it strikes the blades)within acceptable limits for low airflow conditions.  It also permits high airflow with a minimum of restrictions.

2.  Dual/Twin Split Spool Axial Flow Compressors: allows the front rotor to turn at a slower RPM than the rear rotor. this allows the front rotor to turn without being chocked by the low airflow.

3.  Bleed Valves:  installed near the middle or rear of the compressor to "bleed" (vent to the atmosphere) air and increase airflow in the front of the compressor at low engine RPMs.

4.  Variable Exhaust Nozzle: used to unload the pressure during afterburner operation.

• 1st reaction is to reduce the attitude of the acft which will reduce the inlet's AOA.  This allows turbulent free air to enter the inlet at the proper velocity.
• The PCL should then be retarded to just below stall threshold to allow the engine to catch up with the inlet airflow.
• Once engine indications return to normal, the PCL may be slowly advanced to the desired setting.

Inlet Ducts

Compressor Section

Combustion/Burner Section

Turbine Section

Exhaust Section

• The amount of fuel required to produce one pound of thrust
• Propulsive force behind the turbojet is dependant upon the amount of fuel added to the air mass.  This is a proportional relationship: more air requires more fuel.

• turbojet derives its thrust by highly accelerating a small mass of air through the engine.
• all the air entering the inlet traverses through the gas generator
• the turbine section of the gas generator extracts only the necessary power from the hot gas stream (75% of the total energy) to drive the compressor and accessories
• the remaining evergy from the airflow is used for thrust by accelerating gases out the exhaust section.

• the propulsive efficiency of an engine is determined by the efficient conversion of KE to propulsive force by its propelling mechanism
• at low acft speeds the turboprop is more efficient than the turbofan or turbojet. at higher speeds the turbojet is more efficient than the 2
• the turbofan is a cross btwn the turbojet and turboprop. the turbofan's propulsive efficiency is btwn the turbojet and turboprop

Advantages: Lightest specific weight

higher and faster

Disadvantages: low propulsive efficiency at low forward speeds

high TSFC at low altitude/low airspeeds

long takeoff roll

Advantages:  higher thrust at low airspeeds

lower TFSC

shorter takeoff distance

noise reduction/10 to 20 % over turbojet

Disadvantages:  Higher specific weight

larger frontal area

inefficient at higher altitudes

Centrifugal Flow Compressor

Axial Flow Compressor

Axial Centrifugal Compressor

• 3 main components: impeller(rotor inducer), diffuser and manifold
• may be utilized in: single stage, multiple stage or dual faced
• greatest application is on small engines
• compression ratios btwn 6:1 & 7:1

Advantages: rugged, low cost, good power output over a wide range of rpm, high pressure increases per stage

Disadvantages: large frontal area required, impractical for multiple stages

• axial(strait line) flow of air through the compressor section of the engine; 2 main elements: rotor blades and stator vanes
• use of multiple stages can produce very high overall compression ratios
• current axial flow compressors have efficiencies near 90% and compression ratios approaching 15:1

Advantages:  high peak efficiencies, small frontal area reduces drag, high ram efficiency,better combustion efficiency, improved high-altitude performance, good starting flexibility

Disadvantages: at low inlet spd, airflow decreases(compressor stall), reduced air supple to reat of compressor for high-speed acft(ram effect), good efficiencies only possible over narrow speed, high cost, difficult to manufacture, high starting power requirements 

• the propeller, which provides the majority of the thrust, imparts a small amount of acceleration to a large mass of air
• the majority of thrust, approximatly 90%, is a result of the large mass being accelerated by the propeller

• blade
• hub
• pitch change/dome assembly

• blades are installed into the hub and the hub is attached to the propeller shaft
• the pitch change/dome assembly is the mechanism that changes the blade angle of the propeller

• purpose - to prevent the propeller blades from reaching supersonic speeds
• operation - converts high rpm/low torque to low rpm/high torque
• T-34 has a 2 stage reduction gear system that reduces power turbine rpm at 15:1 ratio

• Torque shaft - (inner shaft) coupled to the compressor by the compressor extention shaft.  the other end of the torque shaft is connected to the reduction gear box.  This shaft carries the load from the propeller and produces the torsional deflection
• Reference shaft - rigidly connected to the torque shaft at the compressor extension shaft.  The other end is not rigidly connected.  The reference shaft does not twist and therefore provides the reference to the twisting torque shaft.

• the propeller assembly, the reduction gear box, along with the torquemeter assembly may be connected to the gas generator in 2 possible configurations:

1. attached to the front of the compressor drive shaft
2. attached to the free/power turbine

• it accelerates a very large mass of air with its propeller to a moderate speed.

Advantages:  very high thrust at low airspeeds, excellent take off, slow speed and low altitude characteristics, superior for lifting heavy loads off short and medium length runways

Disadvantages:  heavier and more complicated acft, limited speeds (400-450 kts)

• Alpha - PCL from flight idel to full power
• Beta (ground operations) - PCL from flight idel to max reverse

2 basic sections:

• gas generator
• free/power turbine section

When the compressor is divided into 2 completely independent rotor spools, each driven by its own turbine and drive shaft.  One spool is the low pressure compressor and the other is the high pressure compressor.

• Low pressure compressor:at front of compressor;provides initial pressure increase to airflow from inlet; must spin slower
• high pressure compressor: located after low pressure compressor, further increases pressure;turned at higher speeds by high pressure turbine; is smaller and lighter

• high peak efficiencies
• small frontal area reduces drag
• straight through flow=high ram efficiency
• better combustion efficiency than cent.
• with dual spool: starting flexibility is greater & improved high altitude performance

• decreased airflow in compressor at low inlet speeds(could lead to a compressor stall)
• good efficiencies over narrow rotational speed
• difficult to manufacture
• high starting pwr requirements

• Can - used on older centrifugal compressor engines;airflow ducted to individual combustion cans arranged around the circumference of burner section
• Annular - the liner of the annular combustion chamber consists of a continuous, circular, inner and outer shroud around the outside of the compressor drive shaft.
• Can-Annular - used on large, high performance engines;combination of can and annular

Advantages:

• strength/durability/ease of maintenance
• individual units inspected/replaced without distrubing rest of engine

Disadvantages:

• poor use of space
• greater pressure loss
• uneven heat distribution to turbine
• malfunction of 1 can may lead to turbine damage.....

Advantages:

• uniform heat distribution across face of turbine
• better mixing of fuel/air
• better use of space

Disadvantages:

• cannot be removed easily
• found on smaller engines
• structural problems arise due to large diameter

Advantages:

• even temp distribution at turbine inlet
• greater stability and lower pressure loss than can type
• efficient

Disadvantage:  Expensive

• it is mechanically independent from the gas generator
• exhaust gases from the gas generator turbine drive the power turbine.  this power turbine is connected to the main transmission (reduction gear box) through a coaxial main drive shaft.  The main drive shaft can be located on the rear or front of the engine.

• the propulsive energy from the exhaust is negligible; all remaining energy is extracted by the free or power turbine to drive the rotor assembly
• so in the turboshaft engine, virtually all of the pressure energy is converted into shaft horsepower

• Pressure applied to a confined liquid is transmitted equally in all directions without the loss of pressure and acts with equal force on equal surfaces.
• the shape of the container has no effect on the pressure or force relationship

• Force - a push or pull (expressed in pounds)
• Pressure - the amount of force per unit area: P = F/A
• Area - in a hydraulic system the unit area is a square inch

Pressure is the force acting upon 1 square inch of area:

P = F/A

• the main purpose of a hydraulic system is to multiply force
• provide extra power and mechanical advantage in various acft components
• hydraulics operate the flight controls and utility systems such as the landing gear, wing fold, wheel brakes and other such units.

• it is similar to the workings of the power steering or pwer brake system in an automobile
• the force that is inputted by the aviator is multiplied and then applied
• a complete acft hdraulic system consists of a power system and any number of actuating subsystems
• hydraulic systems for military acft operate near 3000 PSI

• AC - alternating current - electricity that reverses direction
• DC - direct current - electricity that flows in one direction

• AC power requires less current and the use of smaller acft wiring and is lighter in weight
• AC components are light weight, simple and reliable

• provides various electrical components with their power requirements through several buses.
• designed so that the equipment attached to a particular bus has similar power requirements and impact on flight safety

1.  Essential bus:  equipment required for flight safety
2. Primary bus:  mission
3. Secondary (or Monitor) bus:  power to convenience circuits (cabin lighting)
4. Stater bus:  power to start engines

• Volatility - a measurement of a liquid's ability to convert to a vaporous state
• Flashpoint - the lowest temp of a combustible substance (fuel) that would ignite with a momentarily application of a flame.

They are inversely related:  a fuel that is highly volatile will have a low flashpoint and a fuel that is not very volatile will have a high flashpoint

• JP4 - highly volatile, low flashpoint of -35F
• JP5 - low volatility, high flashpoint of 140F, used onboard acft carriers b/c its the only 1 safe for shipboard use
• JP8 - low volatility, flashpoint of 100F

• Normal rated thrust -no time limitation (cruising speed)
• military rated thrust - only 30 minutes (takeoff)
• combat rated thrust - no time limitation b/c afterburner is being used

• lower tendency to leave coking deposits
• stronger chemical stability at high pressures
• very corrosive
• limited shelf life of 6 months

property of a fluid that resists the force tending to cause the fluid to flow.

inversely related to temp: as temp increases viscosity decreases (hot = thin = low viscosity)

oil viscosity is the measure of its ability to flow at a specific temp

to reduce friction caused by metal to metal contact

essential to prevent wear in mechanical devices where surfaces rub together

• provide adequate supply of clean oil at proper pressure and temp
• remove heat fro engine
• remove contaminants from system

2 categories:  wet sump and dry sump

1. pressure subsystem:  supplies lubricating oil from tank to main engine bearings/accessory drives
2. scavenge subsystem:  removes oil from bearings/accessory drives through oil coolers and returns it to tank (greater pumping capacity than pressure pumps
3. breather pressurizing subsystems:  connects individual bearing compartments and oil tank with breather pressurizing valve to help minimize oil leakage

Bleed air

mechanically driven

• compressor discharge air at high pressure and temperature is bled from the engine through ports or valves at intervals along the compressor case and at the end of the diffuser
• it is ducted as a xource of power for operating air conditioning units, cockpit pressurization and engine anti ice
• dual axial compressor engines usually have 3 separate bleed air systems: high pressure, low pressure and interstage bleed air
• the high and low pressure systems are used to drive acft/engine components and interstage bleed valves are required to ensure compressor stability

• taken directly from the main shaft connecting the turbine to the compressor
• used for tachometers, hydraulic pumps, generators,alternator and other accessories mounted near or connected directly to the engine

ensures compressor stability

ducted overboard to prevent compressor stall during low thrust operations

not avaliable at high thrust settings

lacks steady volume or pressure

• started accelerates compressor to establish airflow through engine
• ignition is activated
• fuel is added
• starter continues to accelerate engine

• Hot start - exceeding max temp for turbine
• Hung start - temp in turbine continues to rise and compressor RPM stabilizes below normal
• False start - compressor RPM stabilizes below normal/temp is within limits
• Wet start - mixture does not light off initially (most dangerous type of abnormal start)

• most common type on small engines
• mechanically connected/mounted on either engine accessory gear box or front gram of engine

• most common type on large turbine engines
• the air turbine(attached to engine) accelerates the compressor (air supplied by ground cart/APU)
• on multi engine acft after 1 engine is online, bleed air from that engine is used to start remaining engines

• Annular Gap - protrudes into the combustion chamber
• Constrained Gap - stays outside combustion chamber and operates at a cooler temp than the annular gap plug