15.2 Engine Performance Flashcards
What principle does a gas turbine engine operate on?
Newton’s law of momentum
For every action, there is an equal and opposite reaction.
How does a turbofan generate thrust?
By accelerating a large quantity of air.
What is one disadvantage of turbojet engines compared to turbofans?
Turbojet engines accelerate a smaller quantity of air, leading to higher fuel consumption and noise.
What happens to air in the inlet diffuser of a gas turbine engine?
Changes to its temperature, pressure, and velocity occur due to the ram effect.
What role does the compressor play in a gas turbine engine?
It mechanically increases the pressure and temperature of the air.
What occurs in the burner section of a gas turbine engine?
The temperature of the air is increased by combustion of fuel.
What is the purpose of the turbine in a gas turbine engine?
To remove energy from the hot gas and drive the compressor.
What does the exhaust nozzle do in a gas turbine engine?
Discharges the exhaust gas at a high velocity to produce thrust.
What is the relationship between force, mass, and acceleration expressed in Newton’s second law?
Force is proportional to the product of mass and acceleration.
What is the gravitational constant used for acceleration?
32.2 feet per second squared (ft/sec²).
What is Gross Thrust?
Thrust developed when the engine is on the ground and stationary.
What is Net Thrust?
The actual thrust developed by the engine at any given moment.
How is Net Thrust calculated?
Net Thrust = Gross Thrust - Ram Drag.
What does Choked Nozzle Thrust refer to?
Thrust produced when the exhaust nozzle is choked and maximum efficiency is reached for subsonic flight.
What is the effect of a convergent duct in a gas turbine engine?
Increases velocity and decreases pressure.
What is the effect of a divergent duct in a gas turbine engine?
Decreases velocity and increases pressure.
How is thrust horsepower related to thrust and speed?
Power = Force x Speed.
What is the relationship between thrust in pounds and thrust horsepower at 375 mph?
1 lb of thrust develops 1 THP at a speed of 375 mph.
What is Equivalent Shaft Horsepower (ESHP)?
The power unit obtained by adding the shaft horsepower and residual exhaust gas thrust in turboprop engines.
What does Thrust-specific Fuel Consumption (TSFC) indicate?
The amount of fuel an engine needs to produce 1 kN of thrust for one hour.
What is the fuel requirement for a high bypass engine under the International Measurement System?
38 kg of fuel per hour for each kN of thrust.
What is the definition of thrust distribution?
The thrust forces resulting from changes in pressure and momentum of the gas stream.
What happens to loads in the forward part of the engine due to Newton’s second law?
Loads are in the forward direction.
What is the significance of drag in a gas turbine engine?
It is a significant force that occurs as gases pass through the exhaust section.
What happens to the thrust of a turbojet engine if the aircraft speed increases while inlet velocity remains constant?
Thrust decreases.
What does the term ‘mass flow’ refer to in the context of thrust calculation?
The mass of air flowing through the engine per second.
What is the formula for calculating jet thrust?
Thrust = (mass flow in lb/sec) x (jet velocity - inlet velocity) / 32.2.
What is the primary focus when discussing engine efficiencies?
How efficiently fuel energy is converted to jet velocity and how this velocity is used to push the aircraft forward.
What does the thickness of the arrows in the energy flow diagram represent?
The percentage of the energy flow.
What is the main use of the fuel energy in a gas turbine engine?
For compression.
What part of fuel energy is used in the engine intake?
To make the ram pressure.
What is a significant loss of energy in a gas turbine engine?
High temperature of the exhaust gas.
What is internal efficiency?
The ratio of kinetic energy to fuel energy.
What are the two parts of kinetic energy in internal efficiency?
- Propulsive energy
- Energy loss due to speed differences
What is external efficiency also known as?
Propulsive efficiency.
What does external efficiency indicate?
How much of the kinetic energy is converted into aircraft movement.
What factors primarily influence external efficiency?
- Aircraft speed
- Exhaust gas velocity
How is total efficiency defined?
The product of internal and external efficiency.
What does total efficiency show?
How much of the fuel energy is converted into aircraft movement.
Fill in the blank: Internal efficiency is the kinetic energy divided by the _______.
[fuel energy]
True or False: All energy from the working cycle can be converted into aircraft movement.
False.
What is the definition of bypass ratio?
The amount of airflow in lbs/sec from the fan bypass compared to the amount of air that flows through the core of the engine.
How are turbofan engines categorized?
As high-bypass or low-bypass in accordance with their bypass ratios.
What is the typical bypass ratio for most commercial aircraft?
8:1
What type of turbofan engines are used in speed ranges above 0.8 Mach?
Low-bypass turbofan engines.
What does the Engine Pressure Ratio (EPR) reflect?
How much power is being developed by an operating turbine engine.
How is EPR calculated?
By comparing the pressure at the inlet of the compressor to the pressure at the turbine exhaust area.
Where are the sensors for measuring EPR installed?
In the engine.
What is the purpose of the EPR gauge located on the flight deck?
To set the power for take-off on many types of aircraft.
What components make up the EPR indicating system?
- Inlet pressure sensing probe
- Exhaust pressure sensing probe
- Pressure ratio transmitter
- Pressure ratio indicator for each engine
What does the EPR indicating system show for a low-bypass ratio engine?
The engine’s power output and is used for setting thrust and monitoring performance.
Fill in the blank: The EPR indicating system consists of one inlet pressure sensing probe and an _______.
exhaust pressure sensing probe
True or False: The EPR transmitter converts pressures into a ratio.
True
What is the output signal provided by the pressure ratio transmitter?
Signifying EPR.
What is the typical velocity range of gas from the turbine entering the exhaust system?
750 to 1500 ft/sec
What causes the speed velocity of airflow to decrease in the exhaust system?
High friction and diffusion
What is the standard outlet gas velocity held at?
Approximately 950 ft/sec
What design feature is used to reduce rotating velocity losses in the gas stream?
Core support fairings
What is the effect of the propelling nozzle on gas velocity?
Forms a convergent duct, increasing gas velocity
What conditions cause the exit velocity of exhaust gases to be subsonic?
Low thrust conditions
What happens when the propelling nozzle becomes choked?
No further increase in velocity can be obtained unless the temperature is increased
What is pressure thrust?
The pressure difference across the propelling nozzle effective over the nozzle exit
What happens to static pressure of exiting gases as upstream total pressure increases above choked conditions?
Increases above atmospheric pressure
What is crucial for the design of the propelling nozzle?
Obtaining the correct balance of pressure, temperature, and thrust
What are the potential risks of using a small nozzle?
Possibility of engine surging
What is the consequence of using a large nozzle?
Values obtained are too low
Fill in the blank: The exhaust gases pass to the atmosphere through the _______.
propelling nozzle
What is engine rating?
Engine rating refers to the thrust performance guaranteed by the manufacturer for a new engine under specific operating conditions.
Define static thrust.
Static thrust is the thrust measured with the engine stationary, as when the aircraft is initiating the take-off roll.
What is ram effect?
Ram effect is a rise in pressure above atmospheric pressure at the engine inlet due to the forward velocity of an aircraft, increasing mass airflow and gas velocity.
How does temperature and altitude affect thrust?
Higher temperature and altitude typically decrease thrust performance due to reduced air density and less efficient combustion.
What does flat rated mean?
Flat rated means an engine of high horsepower can be restricted to a lower horsepower rating without changing output, allowing operation under demanding conditions.
What are the five major thrust ratings for turbine engines?
- Maximum take-off thrust
- Maximum go-around thrust
- Maximum continuous thrust
- Maximum climb thrust
- Maximum cruise thrust
What is maximum take-off thrust?
Maximum take-off thrust is the highest thrust an engine is permitted to supply for take-off, limited to a few minutes.
What is the maximum go-around thrust?
Maximum go-around thrust is the highest thrust an engine can supply for a go-around, identical to the maximum take-off thrust and also limited to a few minutes.
What is maximum continuous thrust?
Maximum continuous thrust is the highest thrust an engine can supply without time limitations, used only in emergencies.
What is the maximum climb thrust rating?
Maximum climb thrust rating is lower than take-off thrust, used only for climbing until cruising speed is reached.
What is maximum cruise thrust?
Maximum cruise thrust is the maximum thrust approved for normal cruise operation.
What happens to thrust as aircraft speed increases?
Thrust output temporarily decreases as speed increases from static but may increase again due to ram recovery.
What defines the flat rate temperature?
The flat rate temperature is the upper temperature limit at which an engine can provide maximum take-off thrust.
How does air density affect aircraft performance at high altitudes?
Air density decreases with altitude, requiring more runway for take-off and reducing rate of climb and approach speed.
How does temperature influence air density?
Warm air is less dense than cold air, resulting in fewer air molecules in a given volume.
What is the impact of humidity on engine power?
High humidity reduces the amount of air available for combustion, leading to an enriched mixture and reduced power.
True or False: High humidity is a major factor in computing density altitude.
False
What is a key danger at high-altitude airports during high temperatures?
Low-performance aircraft may struggle to lift off or maintain necessary climb rates due to reduced engine power and lift.
