Engines Flashcards
Describe the behavior of subsonic airflow in a nozzle and diffuser
Airflow at subsonic speeds is considered incompressible.
At subsonic speeds, airflow in a nozzle will increase velocity with a decrease in pressure. The nozzle will be a convergent shape
Subsonic airflow in a diffuser will decrease velocity with an increase in pressure. The shape of the diffuser will be divergent.
Describe the behavior of supersonic airflow in a nozzle and diffuser
As airflow approaches supersonic speeds it becomes more compressible.
A Nozzle will always increase velocity and decrease pressure. Its shape will be divergent at supersonic speeds
A diffuser will always decrease velocity and increase pressure. Its shape will be convergent at supersonic speeds
what are the three main sections of a gas generator
the three main sections of a gas generator are
compressor
combustion chamber
turbine
Describe the Brayton cycle
The four components of the operating cycle (intake, compression, combustion, exhaust) happen SIMULTANEOUSLY in the Brayton Cycle.
Describe the Otto cycle
The four events of the operating cycle occur SEQUENTIALLY in the Otto Cycle
(same 4 events- suck, squeeze, bang, blow)
Describe the basic components of the gas generator
The basic components of the gas generator
Inlet
Compressor
Diffuser
Combustion
Turbine
Exhaust
Describe:
Gross Thrust
Net Thrust
Thrust = mass x acceleration
Gross Thrust: measured on stationary engines or on aircraft groun run-ups. Based on standard day conditions (29.92, 15C). Gross thrust ignores the velocity of the air at the inlet- measures exhaust gas velocity only
Net Thrust: thrust corrected for the effects of inlet airflow. More realistic. Net thrust = mass * (Vfinal - Vinitial)/t
How does atmospheric temperature effect thrust in a gas turbine engine
As air increases temperature, its density decreases. Density is related to the mass of air
T= m x a
as temperature increases, density (mass) decreases, thrust decreases
* above 36,000 ft temperature stabilizes and will no longer offset pressure changes
How does altitude effect thrust?
Higher altitudes have decreased air density (mass)
T = m x a
as altitude increases, density (mass) decreases, thrust decreases
altitude has a greater impact upon overall thrust than temperature
What is the effect of engine RPM on thrust?
Increasing RPM will increase thrust.
HOWEVER, at lower RPM settings there is very little increase in thrust when the throttle is increased.
At higher RPM settings a relatively small increase in throttle will produce a large increase in thrust
Describe the effect of airspeed on thrust
airspeed
as the inlet velocity approaches the exhaust velocity thrust is reduced
T = m x a
acceleration is between the inlet and the exhaust, as these get closer to equal, acceleration decreases leading to reduced thrust production
what is the effect of ram effect on thrust
Ram effect: at higher speeds more air molecules are rammed into the inlet = increased air mass
The ram effect greatly increases thust at SUPERSONIC speeds
what are the cockpit instruments that measure thrust?
The cockpit instruments the measure thrust are
Engine Pressure Ratio (epr)
Torquemeter
Tachometer (Monitor Only)
what is a nozzle?
diffuser?
Nozzle: increase velocity, decrease pressure
Diffuser: increase pressure, decrease velocity
What is the function of the Inlet?
The Inlet will increase air pressure and decrease velocity.
What is the function of the Compressor:
The compressor
increases velocity, temperature, Pressure, Total Pressure
What is the function of the Diffuser
The diffuser is part of the compressor. It will increase pressure and decrease velocity.
At SUBSONIC speeds the diffuser will be DIVERGENT
SUPERSONIC = CONVERGENT
The pressure is the highest at the diffuser
What is the function of COMBUSTION?
Air/Fuel Combusted, increase temperature, increase velocity.
Slight decrease in Pressure
What is the function of the turbine?
the TURBINE increases velocity, decrease Pressure.
Kinetic energy from the turbine is used to turn the compressor
What is the function of the exhaust?
The Exhaust has the final velocity increase and pressure decrease
What does the EPR indicate?
What type of engines have an EPR?
The EPR gauges indicate the pressure ratio between the inlet and exhaust airflow
used in most turbojet and turbofan aircraft
What is the torquemeter
thetorquemeter indicates
SHAFT HORSEPOWER AVAILABLE
What is the Tachometer?
The Tachometer provides an indication of engine rotation speed (RPM)
CALIBRATED IN % RPM
*does not measure thrust. provides for a quick assessment of the amount of energy being used by the engine
Describe inlet ducts:
inlet ducts provide the proper amount of high pressure, turbulent free air that is steady and uniform to the first stage of the compressor
act as diffusers. the subsonic inlet is divergent in shape (increase airflow pressure and decreases airflow velocity)
Supersonic inlet is CONVERGENT/DIVERGENT: it must first slow down the airflow to subsonic speeds, then it acts like a subsonic inlet duct
Describe compressors
The compressors main function is to supply enough high pressure air to satisfy conduction
driven by energy from teh turbine
airflow pressure and velocity increase
Describe the burner section of a gas turbine engine
what types of airflow happen in this section?
the combuston/burner section provides the proper mixing of the fuel and air to ensure proper combustion
air is divided into two types;
primary air (25%) which is mixed with fuel for combustion
secondary air (75%) flows around the chamber and is used for cooling the chamber walls, turbine, and controls the flame within the chamber itself. This air can also be used in the after burner section
Describe the turbine section of a gas turbine engine
transforms 75% of the heat energy of the expanding gases from the combustion chamber into mechanical energy to drive the compressor and accessory gear box
25% of energy used for thrust
describe the phenomenon of creep in a gas turbine engine
Creep is an abnormal elongation of the turbine blades due to overheating and it can result in permanent blade deformation
prevent it with “Fir Tree” attachments
Describe the exhaust section of a gas turbine engine
The exhaust section directs the flow of hot gasses rearward causing a high exit velocity of the gasses while preventing turbulence
2 kinds of exhaust nozzles
Convergent: direct gasses to a focal point and accelerates gasses from the turbine leading to increased velocity
Convergent/Divergent: SUPERSONIC aircraft. convergent section accelerates gasses to the speed of sound, divergent section increases the supersonic air even faster
describe the afterburner section of a gas turbine engine
increase the maximum thrust available from an engine for short periods of time
increase thrust by 50% at a cost of using 300% more fuel
Describe the components of a reciprocating engine
the basic components of a reciprocating engine are
crankcase
cylinders
pistons
connecting rods
valves
valve-operating mechanism
crankshaft
Single entrance ducts
location
disadvantages
single entrance ducts
location: directly in front of engine
disadvantages: short single ducts can cause stall at slow airspeed or high AOA
Divided entrance ducts
pros:
cons:
Divided entrance ducts
less friction loss
cons: more air turbulence, larger and shorter inlets
What is the variable geometry inlet duct?
The variable geometry inlet duct can change shape.
at subsonic speeds it is divergent. At supersonic speeds it becomes CONVERGENT/DIVERGENT
What are the three types of compressor
The three types of compressors are:
centrifugal
axial
axial- centrifugal
what are the components of a centrifugal flow compressor?
the three components of a centrifugal flow compressor are:
impeller- accelerates the airflow toward the diffuser.
diffuser-converts high velocity to high pressure airflow (total pressure remains the same) point of highest compression
manifold- directs airflow to the combustion chambers
*highest pressure increase per stage
**large, lots of drag, single stage
What are the elements of an axial-flow compressor
The 2 main elements of the axial-flow compressor are
rotor- increases air pressure
stator- reduces the airflow velocity produces an increase in airflow pressure
*one rotor blad and one stator form a SINGLE STAGE
GREATEST OVERALL COMPRESSOR RATIOS (they are multi-staged compressors)
Higher combustion efficiency due to multiple stages
Where is the point of highest compression?
The point of highest compression is the diffuser (just before the air goes into the combustion section)
divergent design
What are the three types of combustion chambers used?
The three types of combustion chambers are:
can- greater pressure loss, cold spots on turbine. Strong, easily maintained
annular- continuous inner and outer shroud that surrounds the compressor drive shaft- uniform heat distribution, smaller, maintenance is a bitch
can-annular- efficient, lower pressure loss but pricy
What are the parts of the turbine
The turbine consists of
stationary stator
rotor
What are the parts of the exhaust section
Outer Duct
Inner Cone
Radial struts
What are the components of the afterburner?
The afterburner consists of
Spray bars- introduce fuel
flameholders- help mix fuel and air, keep flame in the duct
screech liner- shock absorber
variable exhaust nozzle- allos for the acceleration of the subsonic gasses (its the iris)
