Module 13,11
Module 13 – Aircraft aerodynamics, structures and systems
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Module 13 – Aircraft aerodynamics, structures and systems
• Main use of compressed air: pressurization and conditioning system, anti-ice protection, engine start up system.
Methods to provide compressed air:
• Engine bleeding
• Generation through the APU (Auxiliary Power Unit)
• Generation through some ground support equipment
Introduction
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Module 13 – Aircraft aerodynamics, structures and systems
• On aircraft, the air conditioning system has the function to maintain comfort environmental conditions (temperature, humidity and air composition) during all flight phases.
• The air conditioning system must be designed to extract and introduce heat in the cabin.
• Comfort conditions must guaranteed also in critical environment.
Air conditioning system
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Module 13 – Aircraft aerodynamics, structures and systems
• The pneumatic system takes hot air from the compressor.
• Mixing hot air, taken from the engine, with cold air passed through a refrigerating cycle, it is possible to obtain the air at correct
temperature and humidity for the maintenance of the desired environmental cabin conditions.
Air conditioning system
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Module 13 – Aircraft aerodynamics, structures and systems
• In order to guarantee the maximum comfort of passengers the air temperature, present in the cabin, must be comprised between 18° C (65° F) and 24° C (75° F)
• The relative air humidity must be about 20-30 %
• In normal conditions, the air conditioning system must guarantee in the cabin an air flow of about 1 lb per minute for each person. This value cannot be less than the half in case of failure of the system
Air conditioning system
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Module 13 – Aircraft aerodynamics, structures and systems
• There is not a real conditioning system, they are equipped with a ram air system. The air comes directly from outside the aircraft:
same pressure and temperature of the air in which the flight occurs.
• The air that comes from outside is filtered and heated before entering the aircraft cabin through a series of ducts.
• Main problem: how to heat the air.
• The air can be heated using an engine exhaust heat exchanger or a combustion heater
Small not-pressurized aircraft
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Module 13 – Aircraft aerodynamics, structures and systems
• Ram air coming from a forward facing air duct passes through a heat exchanger in which hot exhaust gasses of engine pass.
• After the air is heated, it enters a chamber in which cold air coming from another aircraft intake flows into. Some valves control the hot and cold air flows in order to reach the desired temperature.
• At this point the air mixture enters the cabin.
Exhaust heat exchanger system
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Module 13 – Aircraft aerodynamics, structures and systems
• Advantages: cheap and very efficient.
• Disadvantages: very dangerous in case of an internal leak. This damage can cause carbon monoxide poisoning. In addition this system doesn’t operate if the aircraft is stationary and there are no fans installed.
Exhaust heat exchanger system
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Module 13 – Aircraft aerodynamics, structures and systems
• They use the aircraft fuel to heat the air. The air is provided by a fan, while the fuel is directly taken by the aircraft fuel system.
• The air-fuel mixture is ignited by a spark plug; the exhaust gases travel through the exhaust outlet.
Combustion heater system
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Module 13 – Aircraft aerodynamics, structures and systems
• Air for the air conditioning system is bled from the engine.
• Hot air is directly taken from the compressor and it must be cooled to reach a suitable temperature.
• Hot air, taken from the engine, is mixed with cold air coming from a refrigerating cycle to obtain the desired temperature and humidity
Large pressurized aircraft
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Module 13 – Aircraft aerodynamics, structures and systems
The air conditioning systems also uses a percentage of recycled air from the cabin.
Recycled air is filtered and mixed with pure air.
Advantages:
The system must elaborate a smaller external air flow
It is possible to maintain the relative humidity around acceptable values, reducing the need to add humidity to the dry air that comes from the refrigerating cycle.
Large pressurized aircraft
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Module 13 – Aircraft aerodynamics, structures and systems
Air cooling is achieved using some “packs” that can include two cycles:
• Air cycle
• Vapor cycle.
The number of cooling packs depends on the size of the aircraft.
When it is necessary to install more than one pack, these packs work independently to provide air for each compartment they work for.
Air cycle and vapor cycle machines
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Module 13 – Aircraft aerodynamics, structures and systems
Pack = turbine that drives a compressor on a rotor shaft.
The turbine and the compressor wheels are similar: they consist of a cast wheel and some blades made of aluminum alloy.
The turbine wheel rotates within a nozzle ring, while the compressor wheel rotates within a diffuser ring
Air cycle and vapor cycle machines
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Module 13 – Aircraft aerodynamics, structures and systems
• Most common refrigerating cycles on aircraft are air cycles.
• Air cycles are thermodynamic cycles in which the air undergoes some transformations in order to reach the desired temperature and pressure conditions.
• The theoretical refrigeration cycle is a reverse Joule cycle.
1. The external air, flows into the compressor, in which it is subjected to a compression.
2. Then in the second phase an isobaric cooling happens
3. while in the next phase an expansion occurs through the turbine, until the pressure present in cabin is reached
4. The air is expelled from the fuselage by control valves
Air cycle machines – reverse joule cycle
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Module 13 – Aircraft aerodynamics, structures and systems
Reverse joule cycle
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Module 13 – Aircraft aerodynamics, structures and systems
• Bootstrap cycle = improved standard refrigeration cycle.
• The fluid is subjected to two compressions and passes through two heat exchangers before entering the turbine.
• Heat exchangers reduce the air temperature before entering the second compressor and the turbine
• Fan: connected to turbine and compressor, cools them when aircraft is ground. In flight cooling is provided by the airflow.
• Water separator: installed after the turbine and before the air flows into the cabin. It provides a whirling movement to the air, to
facilitate the formation of microscopic water drops. Then drops are drained to avoid condensation in cabin
Bootstrap cycle
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Module 13 – Aircraft aerodynamics, structures and systems
Bootstrap cycle
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Module 13 – Aircraft aerodynamics, structures and systems
In refrigerating vapor cycles the air cooling is achieved with a
refrigerant fluid. This fluid is able to absorb heat during the evaporation process.
Major components of a typical vapor cycle machine:
• Liquid receiver
• Thermostatic expansion valve
• Evaporator
• Turbo-compressor
• Condenser.
Vapor cycle machines
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Module 13 – Aircraft aerodynamics, structures and systems
• Refrigerant stored in the liquid receiver.
• From this reservoir the fluid passes in the thermostatic expansion valve and reaches the evaporator.
• In the evaporator the hot air coming from engines boils the
refrigerant and then enters the cabin at a much lower temperature
• The vaporized refrigerant fluid flows into the compressor (coupled with the turbine) and reaches a high temperature and pressure.
• Then the hot gas enters the condenser and is cooled by the ram air coming from outside.
• The refrigerant condenses and it is pumped back to the liquid receiver to start a new cycle.
Vapor cycle machines
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Module 13 – Aircraft aerodynamics, structures and systems
Vapor cycle machines
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Module 13 – Aircraft aerodynamics, structures and systems
• The pressure controller is connected to the outflow valve: it sends some signals to the valve in order to regulate its aperture
• When the aircraft is on ground, the valves are maintained in the complete open position, in order to ensure the air exchange even if the air conditioning system is activated. After the take-off the
valves moves towards the closed position, which is never reached in order to guarantee the air exchange during the flight.
• Only in the case of failure the outflow valve closes, preserving the cabin pressure, for a time interval sufficient to lead the aircraft at an altitude where the pressurization isn’t necessary.
Pressure controller and outflow valve
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Module 13 – Aircraft aerodynamics, structures and systems
• The safety valve operates when the difference between the cabin pressure and the external pressure is bigger than a specific value utilized for the fuselage design.
• Generally the safety valves are activated when the cabin pressure exceeds the limit value of about 0.25 psi.
Safety valve
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Module 13 – Aircraft aerodynamics, structures and systems
In the cockpit, some instruments, which permit pilot to controls the aircraft pressurization, are installed:
o • The cabin altimeter
o • The cabin variometer (expressed ft/min) o • The cabin differential pressure indicator
• The cabin altimeter indicates the cabin altitude. It remembers that the cabin altitude is defined as the atmospheric height at which the value of the pressure inside the fuselage corresponds.
• The cabin variometer controls the pressure rate inside the cabin, referring to the altitude variation per minutes [ft/min]
• The cabin differential pressure indicator indicates the pressure difference between the inside and the outside of the aircraft [psi]
Additional instruments
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