15.15 Power Augmentation Flashcards
What are power augmentation systems used for?
To restore or boost power for improved performance
Without these systems, payload, fuel load, take-off, and climb performance would be affected.
What is a potential drawback of fitting larger engines for increased performance?
Increased weight, frontal area, and specific fuel consumption
This would consequently reduce the distance the aircraft could fly.
What factors affect the maximum power output of a gas turbine engine?
Density or weight of the airflow, air pressure, air temperature
If air pressure decreases or temperature increases, mass flow through the engine decreases.
List three methods to compensate for power loss in gas turbine engines.
- Increasing the engine speed
- Increasing the fuel flow
- Increasing the air density
Why might increasing the engine speed not always be a viable option?
Due to mechanical limitations and over speed limits
Especially true for older engines.
How can air density be increased to compensate for power loss?
By cooling the air using water sprayed into the engine air intake
This cooling increases air density and decreases air temperature in the combustion chamber.
What is the effect of decreased combustor air inlet temperature?
More fuel can be burnt before reaching turbine inlet temperatures
This increases engine power and recovers power loss at low air densities.
Name a system used to augment engine power on turboprop and turboshaft engines.
Water methanol system
What is a common application of water injection systems?
Used on turbojets
What is the primary use of afterburners?
Used on military aircraft and was also used on Concorde
Which method is most suited for engines with centrifugal compressors?
Compressor Inlet Injection System
What is the disadvantage of using only water in the Compressor Inlet Injection System?
Water can freeze during flight at high altitudes
What is added to the water in the water/methanol mixture to prevent freezing?
Methanol
What is the typical mixture strength of water to methanol used?
Approximately 60% water to 40% methanol by volume
How does the water/methanol mixture affect compressor inlet air?
Reduces temperature, causing air density and thrust to increase
What does the Combustion Chamber Injection System primarily increase?
Mass flow through the turbine relative to the compressor
What is the result of reduced pressure and temperature drop across the turbine?
Greater pressure within the jet pipe, producing more thrust
What does the reduction in turbine gas temperature permit?
More heat to be added, resulting in a greater change of momentum to the gas flow
What does increased rotational speed in the combustion system provide?
Further additional thrust
What is the primary purpose of water/methanol injection systems?
To increase thrust for takeoff
What is the term used for the maximum allowable thrust for takeoff when using water injection?
Wet thrust
What conditions are required to obtain the wet thrust rating?
Predetermined turbine discharge pressure or engine pressure ratio for the prevailing ambient conditions
What limitations are associated with the wet thrust rating?
Time-limited and altitude limitation
Why are water injection systems not commonly used on turbine engines today?
Advancement in engine technology
How does water injection compensate for reduced thrust on warm days?
By lowering the air temperature and increasing air density
What component is actuated by the control shaft when the power lever is moved towards maximum power?
A microswitch in the fuel control
What does the water injection speed reset servo do during water injection?
Resets the speed adjustment to a higher value
What happens if the water injection system is not armed or if there is no water available?
Nothing happens when the water injection switch is switched on
What is the role of methanol in water-methanol injection systems?
Acts as an antifreeze and contributes to thrust production in the combustion chamber
Fill in the blank: Water injection systems are typically not used on _______ engines.
high bypass turbofan
True or False: Water injection systems can be used at any altitude without limitations.
False
What component increases the set speed during water injection?
Speeder spring
What is the function of the shuttle valve in the water injection system?
Acted upon by water pressure during water injection
What does the water injection speed reset servo displace to increase the force of the speeder spring?
A lever on the cam-operated lever linkage
What is afterburning?
A method of augmenting the thrust of an engine to improve takeoff, climb, and combat performance in military aircraft.
What are the advantages of afterburning over a larger engine?
It does not increase weight, frontal area, or overall fuel consumption.
What is the maximum temperature reached by the engine during afterburning?
More than 1700 °C.
How is fuel introduced in the afterburning process?
Fuel is introduced and burned between the engine turbine and exhaust nozzle using unburned oxygen in the exhaust gas.
What is the purpose of the afterburning jet pipe’s larger area?
To obtain a reduced velocity gas stream.
What type of nozzle is fitted to an afterburning jet pipe?
Either a two-position or a variable area propelling nozzle.
What happens to the nozzle during non-afterburning operation?
The nozzle is closed.
What prevents an increase in pressure in the jet pipe during afterburning?
The nozzle opens to provide an exit area suitable for the increase in the volume of the gas stream.
What is the role of the flame stabiliser in afterburner operation?
To provide a region for turbulent eddies to assist combustion and reduce gas velocity for flame stabilization.
What initiates combustion in the afterburner?
An igniter.
What is the construction of the burner section in an afterburner?
Several circular concentric fuel manifolds supported by struts inside the jet pipe.
What materials are used in the construction of the afterburning jet pipe?
Heat-resistant nickel alloy.
What is the purpose of insulation in the afterburning jet pipe?
To prevent the heat of combustion from being transferred to the aircraft structure.
Describe the construction of the propelling nozzle in an afterburner.
Similar material and construction as the jet pipe, often with two movable eyelids operated by actuators.
Fill in the blank: The afterburning process involves the introduction and burning of fuel between the _______ and exhaust nozzle.
[engine turbine]
