53 Questions
Where does the compressor discharge air enter?
Turbine plenum
What is the purpose of the turbine plenum?
To collect compressor discharge air
What is attached to the rear outer flange of the second stage compressor diffuser?
Turbine plenum
What is the function of the torus turbine assembly?
To generate power
How many paths of structural support are provided by the turbine plenum arrangement?
Two
What happens to the mass airflow in the turbine plenum?
A large portion of it passes through the combustor and turbine unit
What is the primary function of the turbine wheel in an APU?
To change the direction of the gas flow and produce torque
What is the purpose of the reduction gear train in the accessory drive section?
To reduce the speed of the compressor shaft and increase the torque
What is the function of the sun gear in the planetary reduction gears?
To connect the compressor shaft to the reduction gear train
What is the result of the high velocity gas flowing through the turbine nozzle?
A change in the direction of the gas flow, producing torque
What is the purpose of the torsion shaft in the accessory drive section?
To connect the compressor shaft to the reduction gear train
What is the function of the planetary reduction gears in the accessory drive section?
To reduce the speed of the compressor shaft and increase the torque
What type of impeller is used in the second stage of the compressor?
Single-sided single-entry
How many bearings support the compressor/turbine shaft?
A single race ball bearing and a bronze bush
Where is the single bronze bush located?
Forward of the turbine between the second stage compressor and turbine
How are the first and second stages of the compressor connected?
Through a series of crossover ducts
What is the function of the crossover ducts?
To act as diffusers and transfer air
Where is the turbine plenum chamber located?
After the second stage compressor
What sequences electrical power to the fuel shut-off valve during APU start?
Centrifugal speed switch
What is the primary function of the rate control valve in the fuel system?
To regulate fuel pressure during on-speed conditions
How is the starter motor de-energized during APU start?
When the centrifugal speed switch reaches 95%
What occurs when the centrifugal speed switch reaches 95% during APU start?
The starter motor is de-energized
What is the purpose of the fuel governor during APU start and on-speed conditions?
To govern fuel flow
What is the purpose of the secondary orifice in the fuel system?
To open at 75 PSIG fuel pressure when flow divider spring tension is exceeded
Which electrical system component is responsible for igniting the fuel/air mixture?
High energy ignition unit (HEIU)
What is the primary source of power for the APU electrical components during initial start and operation?
Aircraft's battery
At what fuel pressure does the secondary orifice open?
75 PSIG
What is the output capacity of the APU generator?
115 volt AC
What is the purpose of the APU electrical system during initial start and operation?
To provide a means of starting the APU, controlling the fuel supply, igniting the fuel/air mixture, sequencing APU acceleration, and monitoring APU operation
What happens at point A in the APU start-up sequence?
The oil pressure sequencing switch activates
What is the purpose of the 35% speed switch in the APU start-up sequence?
To de-energise the starter motor
What is the state of the APU at point D in the start-up sequence?
On speed with EGT stabilising
What happens to the FCU at point E in the APU start-up sequence?
It changes from acceleration limiting to fuel governing mode
What is the result of the APU reaching point E in the start-up sequence?
Electrical power from the APU generator and/or bleed air from the APU can now be utilised
What happens to the pneumatic thermostat at point E in the APU start-up sequence?
It changes from acceleration limiting to load control mode
What is the primary function of the Load Control Valve (LCV)?
To regulate the amount of air that is bled from the APU for aircraft services
What is the maximum percentage of compressor discharge air that the APU can provide without detriment to its performance?
8%
What is the purpose of the three-way solenoid valve in the Load Control System?
To connect the LCV to the pneumatic thermostat and the control air line to the FCU
What is the purpose of the pneumatic thermostat in the Load Control System?
To monitor the temperature of the turbine section
Where is the Load Control Valve (LCV) mounted?
On the APU plenum chamber
What is the type of valve that the Load Control Valve (LCV) is?
Pneumatically operated butterfly valve
What is the function of the three-way solenoid valve in the load control system?
To pneumatically disconnect the FCU and connect the LCV to the pneumatic thermostat
What is the purpose of the load control solenoid in the load control valve operation?
To open the ball valve (B) and close ball valve (C)
What happens to the diaphragm (D) when the load control valve is opened?
It moves against spring tension and air pressure in chamber (E) to slowly open the butterfly valve (F)
What is the purpose of the ball valve (C) in the load control valve operation?
To close the load control valve
What is the function of the spring tension in the load control actuator diaphragm chamber?
To keep the load control valve closed
What is the purpose of the lever pivoting at point (X) in the load control valve operation?
To open ball valve (B) and close ball valve (C)
What is the function of the three-way solenoid valve in the APU operating mode?
To connect the thermostat to either the FCU or LCV
What is the purpose of the acceleration limiter circuit in the FCU?
To connect the thermostat to the FCU when the solenoid is de-energized
What is the condition required for the load control circuit to be connected to the thermostat?
When the 95% speed switch is made and bleed air is selected to on
What is the purpose of the air pressure regulator valve in the APU system?
To receive air from the turbine plenum and reduce it to approximately 20 PSI
What happens when an over-temperature occurs in the pneumatic thermostat?
The high expansion material expands, unseating the ceramic ball valve and bleeding off the control air pressure
What is the purpose of the load control valve in the APU system?
To control the load of the APU under varying engine pressures and temperatures
Study Notes
APU Major Sections
- The APU consists of three major sections: compressor, accessory drive, and power section.
- The compressor section consists of a two-stage centrifugal impeller compressor, diffusers, and ducting.
- The accessory drive section consists of a reduction gear train and housing.
- The power section consists of turbine, combustion, and exhaust components.
Compressor Section
- The compressor section has two stages: a double-sided double-entry impeller and a single-sided single-entry impeller.
- Both stages are located on a single shaft with the turbine.
- Air is transferred between the first and second stages by a series of crossover ducts that act as diffusers.
- The compressor/turbine shaft is supported by a single race ball bearing and a single bronze bush.
Turbine Nozzle Assembly
- High-velocity gas from the turbine nozzle is directed against the turbine wheel vanes, causing a change in gas flow direction.
- The resultant force produces the torque to rotate the turbine wheel.
- The torque developed by the turbine is returned through the shaft to rotate the compressor.
Turbine Wheel Assembly
- The turbine wheel is a radial inflow turbine wheel.
- The turbine wheel vanes are subjected to high-velocity gas, producing torque to rotate the turbine wheel.
Accessory Drive Section
- The accessory drive section consists of a reduction gear train and housing.
- A torsion shaft splines between the compressor shaft and the sun gear of the planetary reduction gears.
Power Section
- The power section consists of turbine, combustion, and exhaust components.
- The turbine plenum collects compressor discharge air in preparation for bleed air extraction or combustion.
- The turbine plenum is attached to the rear outer flange of the second stage compressor diffuser and is sandwiched between the exhaust pipe flange assemblies.
APU Start-up and Operation
- The start-up events of the APU are sequenced by the oil pressure sequencing switch, ignition system, fuel shut-off valve, and exhaust gas temperature (EGT).
- The APU accelerates to 35% speed, then 95% speed, and finally reaches on-speed condition.
- The fuel governor controls fuel supply during start-up and on-speed conditions.
- The electrical power is generated and supplied to the aircraft's systems during APU operation.
Electrical System
- The electrical system provides a means of starting the APU, controlling the APU fuel supply, igniting the fuel/air mixture, sequencing APU acceleration, and monitoring APU operation.
- The main components of the electrical system are generator, starter motor, tachometer, generator start meter, centrifugal speed switch assembly, and high energy ignition unit (HEIU).
Load Control Valve (LCV) Operation
- The LCV is a pneumatically operated butterfly valve that regulates the amount of air bled from the APU for aircraft services.
- The LCV system components include three-way solenoid valve, air pressure regulator, pneumatic thermostat, and LCV.
- The load control valve operates in closed and open modes, controlled by the load control solenoid, air pressure regulator, and pneumatic thermostat.
Pneumatic Thermostat
- The pneumatic thermostat is located in the APU exhaust flange and exposed to exhaust gas.
- The thermostat operates in two modes: start acceleration (no bleed air) and load controlling.
- The thermostat regulates the air pressure holding the load control valve in the open position, and closes the load control valve in case of over-temperature.
Learn about the major components of a compressor power section in aerospace engineering, including turbines and exhaust systems. Identify the key parts of a compressor and their functions. Test your knowledge of aerospace engineering systems.
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