Assignment On Aircraft Power Systems
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In this assignment, I intend to provide a brief presentation about the principle of operation of the aircraft electrical power systems, specifically, Integrated Drive Generators (IDG) in a modern aircraft. I will compare and contrast the fluid and electrical methods of power generation and give a brief on the safety precautions to be followed when handling aircraft electrical power supplies and during a functional test.
There are various kinds of power generation on an aircraft, namely electrical (generators & batteries), fluid (hydraulics & pneumatics). Depending upon the application, the power sources may vary.
AIRCRAFT ELECTRICAL SYSTEM
The main function of any aircraft electrical system is to generate, regulate and distribute the electrical power in the aircraft. Modern aircrafts use more electrical power as they are equipped with more Electronic Flight Instrument System (EFIS). This is done so as to reduce pollution in the environment, for a clean and safe flight.
Electrical power is used to operate:
Aircraft Flight Instrument- Electronic Centralized Aircraft Monitoring (ECAM)
Essential Systems- Primary Flight Display (PFD), Navigation Display (ND)
Passenger Services- In-Flight Entertainment (IFE)
Most of the aircraft systems use a power standard of 115 [email protected] 400Hz; 28VDC.
There are various electrical power sources on the aircraft:
Engine Driven AC Generators (EDG)
Auxiliary Power Units (APU)
Ram Air Turbine (RAT)
External/Ground Supply (Spoor, 2012)
Components in an electrical system include:
AC Generator/Alternator produces AC power.
Constant Speed Drive Unit (CSDU) regulates the rotating speed (input) to deliver a constant output from the engine to the generator.
Integrated Drive Generator (IDG), Generator and CSDU connected either coaxially or side-by-side. Mostly coaxial connection.
Transformer Rectifier Unit (TRU) converts the AC power to DC power
Generator Control Unit (GCU) monitors and controls the generator output.
Each engine powers up one or more generators. Airbus A380 has four engines and four generators. Whereas, Boeing 777 has two engines and two generators. The power produced by these generators is used to power up the entire aircraft. (Boeing, 2012)
All the other power sources, except external supply, are used in case of emergencies only.
External power supply is used on ground when the engines and the APU are not running. It is supplied via Ground Power Unit (GPU). GPU can be portable or stationary and they provide AC power through the external plug on the nose of the aircraft. They can be used to power up an entire aircraft or a specific area in the aircraft. (Spoor, 2012)
Figure : Aircraft Electrical System (Classic Jet Sims, 2012)
AIRCRAFT HYDRAULIC SYSTEM
The principle of a hydraulic system is to convert fluid power into mechanical power and then use it for various applications on the aircraft, such as extension/retraction of landing gears, flaps, and speed and wheel brakes.
The power source here is the hydraulic fluid. The pressure for generating the hydraulic power is supplied by the Engine Driven Pump (EDP) and the Electric Motor Driven Pump (EMDP). They are both mounted on the engines.
Commonly used hydraulic fluids on the aircraft are:
MIL-H-5606 (Mineral Based Fluid)
MIL-H-83282 (Synthetic Hydrocarbon Based Fluid)
MIL-H-87257 (Same as MIL-H-83282, but it has been improved in its low temperature viscosity)
MIL-H-8446 (Phosphate-Ester Based Fluid) (Wikipedia, 2012)
Special care must be taken when handling the hydraulic fluid. Some of the hydraulic fluids are skin/eye irritant. Also it is extremely important for the fluid to be free from any contamination for flight safety.
Advantages of the hydraulic system as a power source includes:
Approximately 100% system efficiency, with only negligible loss due to friction. (EAC, 2012)
Figure : Basic Hydraulic System (Free Online Private Pilot Ground School, 2006)
COMPARISON OF AIRCRAFT ELECTRICAL AND HYDRAULIC SYSTEM
Power is generated by the engine, by rotation of turbine shaft.
Power is generated by incompressible hydraulic fluid. Pressure for hydraulic actuation is supplied by EMDP and EDP.
Similar to that of an electric circuit.
Components include: Generators/Alternators, Batteries, Switches, Bus bars, Fuses, Circuit Breakers, Voltage Regulators, Ammeters, and Wires. (Free Online Private Pilot Ground School, 2006)
Components include: Reservoir, Pump, Hydraulic Fluid, Accumulators, Actuators, Filters, Hydraulic Fuses, Tubing, Pipes, Valves, and Seals.
Lights, Radio Equipment, Indicators, Warning Systems, Heaters, Starting motor. (FlightLearnings, 2012)
Extension/retraction of landing gears, flaps, and speed and wheel brakes.
Principle of Operation
Generate, Regulate and Distribute electrical power generated.
Work is done by moving an incompressible fluid, Pascal's Law. (EAC, 2012)
EMERGENCY DEPLOYMENTS IN EVENT OF GENERATOR(S) FAILURE
In case of main engine driven generator (EDP) failure, there are various methods to ensure continued power supply for safe landing:
Auxiliary Power Unit (APU)
Ram Air Turbine (RAT)
When the main engine generators fail, APU or RAT is deployed. When the secondary methods fail, batteries are used as the last resort for a safe landing and evacuation.
AUXILIARY POWER UNIT (APU)
Auxiliary Power Units (APUs) is a small gas turbine engine which provides electrical, hydraulic and pneumatic power (depending on the design) for ground or in-flight operations. It is also used as a backup source in emergency situations. It is commonly found on large aircrafts. It is usually located in the tail end of the aircraft. When started, it normally runs on 100% speed. It produces 115VAC @ 400Hz. Some can also generate 28VDC.
The main purpose of the APU is to start the engines in the aircraft. This is because the engine requires high rotational speed for its self-sustaining operation. In this case, APU is started by battery or hydraulic accumulator and then used to generate the necessary rotational speed. It is also used to run the equipment when the engines are shutdown. This is used for air conditioning for the passengers prior to engine start. However, it can also be connected to a hydraulic pump to power up hydraulic equipment (flaps and flight controls). (Wikipedia 2012)
RAM AIR TURBINE (RAT)
Ram Air Turbine is a small airflow driven turbine that is connected to either a hydraulic pump or an emergency alternator or a main generator. In normal flight, it is stowed away in closed compartment in wing or fuselage. It is deployed only when there is total power loss due to engine failures. It provides enough power to run the critical systems, such as, flight controls. The power generated by the RAT depends on the airspeed. More the speed, more the power generated will be. Some aircrafts use RAT in normal operation. For example, in crop dusting RAT is used to power up the chemical sprayer. (Wikipedia, 2012)
Aircraft batteries are used to start the engines or the APU. But they are also used in emergency situations as a last resort for safe landing. There have been cases where it has been used to restart the engines even after engine flame-out. They also act as a buffer for the DC connections. It regulates the voltage to ensure the power quality for the equipment connected to it.
Two types of batteries are used in aircrafts today, Lead-acid and Nickel Cadmium (Ni-Cd). Lead-acid batteries are either valve regulated or vented. They are used in light and general aircrafts. Ni-Cd batteries are used in larger aircrafts. Lead-acid batteries are generally heavier and have less capacity than the Ni-Cd ones. (Dr. Thomas, 2010)
Figure : Lead-Acid Battery (Chief Aircraft, 2012) Figure 4: Nickel-Cadmium Battery (Surplustraders.net, 2012)
AIRCRAFT ELECTRICAL SYSTEM USING FREQUENCY WILD AC GENERATOR
From 1950s to early 1960s some turboprop aircrafts used 'frequency wild', now Variable Frequency (VF) system. Airbus A380 and Boeing 787 Dreamliner are some of the modern civil aviation aircrafts that have adopted the VF system again.
Figure : Simplified Frequency Wild System (Scribd, 2012)
As shown in Fig.4, the Variable Frequency Generator (VFG) is directly connected to the engine gear box. (Langlois, 2004) states: 'VFG is a conventional wound rotor synchronous alternator'. There is no CSD or IDG in this system. This is done so as to improve the reliability of the system and also to reduce the weight of the aircraft. Airbus A380 uses aluminium cables instead of copper for weight reduction. The system is also fully computerized. Contactors and Breakers have been replaced by solid-state devices for increased reliability and performance (Global Aircraft, 2012).
Frequency Wild AC power can only be supplied to the resistive circuits such as the electrical heater, engine de-icing heaters, windshield heating, etc. as shown in Fig. 4. The AC power generated is sent to the TRU where the voltage is stepped down from 360Hz-760Hz (Langlois, 2004) to 28V and then rectified to DC voltage. Then it is used by the DC components.
CONTROL & PROTECTION
A traditional Constant Frequency unit has only a constant frequency of 400Hz voltage supply. A constant speed drive unit drives the generator. It is also expensive and not easy to maintain. A VFG is flexible as it can handle a range of frequencies rather than a constant one. In this unit, there is no CSD or IDG. It is more advantageous over Constant Frequency Unit because:
It has low maintenance
Process temperature is controlled within VFG itself.
Equipment protection from excess torque damage
The variable frequency generator (VFG) is used on applications with a high percentage of resistive AC or DC loads and provides power to many of the next generation aircraft. Hamilton Sundstrand's variable frequency starter generator (VFSG) and motor controller system provides main engine start and control.
The voltage is controlled around the generator. This is done by controlling field strength of the AC generator by feedback signal from the voltage regulator, which keeps the voltage constant irrespective of the speed and load variations.
VFGs are preferred over DC systems because they were much less affected by poor brush performance at higher altitudes and they had better power to weight ratio.
AC/DC ELECTRIC MOTORS AND ACTUATORS PERFORMANCE
DC actuators are typically high-speed reversible series-wound motors. Their output is converted to driving torque using step-down gearbox. These motor actuators are self-contained units, as they combine both electrical and mechanical devices. They can exert linear thrust over a short distance.
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