15.2 Engine Performance Flashcards
What is gross thrust?
Gross thrust is static thrust, computed when the aircraft is at rest. It is the difference in velocity between the air entering and the air exiting the engine.
What is net thrust?
Output thrust of the engine in flight. (Less than an engine that is static) due to the velocity change between the air entering and leaving being less.
Can be calculated by measuring the difference between what the static thrust would be and the ram drag.
What is Bernoulli’s principle?
Total pressure, kinetic energy and potential energy in a system is constant
What is a choked exhaust nozzle also known as?
Jet nozzle
What is the core of a turbofan engine also known as?
The gas generator.
What is the purpose of a turboprop or turboshaft engine?
To transmit all useful power by a shaft that drives a fan or propeller.
What is choked nozzle thrust?
The increased pressure in the exhaust (cruise to takeoff power) where the pressure across the exhaust opening creates additional thrust as it’s above the ambient pressure.
How do you calculate the rated thrust of an engine?
Finding the sum of the forward forces and subtracting the sum of the rearward forces
What are the two types of forces within an engine?
Reactive & Pressure
When can you calculate using thrust horsepower?
When the aircraft is in motion.
What is the unit for thrust horsepower?
Watts
What type of engine is equivalent shaft horsepower used?
Turbo prop (propeller)
How do you calculate equivalent shaft horsepower?
By adding the shaft’s actual HP and the HP from the jet thrust of the engine.
What is the thrust specific fuel consumption?
A ratio of fuel consumption to produced engine thrust
What happens to the fuel flow when a flight is in cruise compared to when it is on the ground?
It increases as the air density is lower. The fuel flow must increase to maintain the air mass needed to be accelerated.
What is engine efficiency split into?
External efficiency
Internal efficiency
What is external engine efficiency also known as?
Propulsive efficiency
What is internal efficiency also known as?
Thermal efficiency
What is the engines internal (thermal) efficiency?
The percentage of total heat generated that is converted from fuel energy to useful kinetic energy.
What is the engine bypass ratio?
The ratio of air flow mass bypassing the core of the engine to the air flow mass entering the core
What is a geared turbo fan?
A gear is located between the fan and the turbine to create the optimum fan speed
What is the engine pressure ratio (EPR)?
A way of indicating the thrust level. It is relative to the atmospheric conditions.
How is EPR measured?
The pressure ratio between the core exhaust duct and the air inlet (PT50/PT20)
What happens to pressure, temperature and velocity in an inlet?
Pressure increases
Temperature increases
Velocity decreases
What happens to the velocity of air as it travels through the compressor stages?
Rotors speed up the air flow
Stators slow down the airflow
The air is kept at as constant air flow speed as possible.
What factors can affect the thrust produced by an engine?
Temperature
Altitude
Air speed
How does the temperature of the air affect the thrust produced?
The denser the air, the more thrust can be produced, meaning in colder temperatures more thrust can be produced as the air is much denser.
(But not at higher altitudes!)
What is the pressure of air at sea level?
14.7PSI or 1013HPa
How does altitude affect pressure?
Pressure decreases as altitude increases.
A drop in pressure will reduce the air density and therefore reduce the thrust that can be produced.
How does an increase in air speed affect an engine?
Decreases the amount of thrust produced by the engine due to the reduction in air mass acceleration; but increases the amount of thrust due to the ram effect (which increases air pressure by the forced air)
What happens to the compressor efficiency as the blades rotate faster?
It begins to drop when the blade tips reach the speed of sound.
What is a flat rating?
When an engine is tested and referred to an ISA day, it will deliver a certain amount of thrust at temperatures specified.
It’s referred as ‘flat’ due to the flat shape of the full power curve and the point on the temperature scale where the thrust or power drops below 100%
What is 1HP in ft.lb/s?
550 ft.lbs/s
What happens to fan speed at a higher altitude?
The rpm increases due to the air density decreasing
What happens to thrust and fuel consumption when water enters a jet engine?
The thrust increases, fuel consumption remains the same
What happens to thrust with high air humidity?
Reduces the thrust output
What is specific fuel consumption a ratio of?
Fuel consumption to Thrust
Where is the idling RPM higher; sea level or at altitude?
At altitude
What does the term Pb mean?
Burner pressure measured at the combustion chamber
How is thrust horsepower calculated?
Multiplying engine thrust by the airspeed
What happens to the density of air if it is more humid?
It is less dense
What is the main factor considered when designing a jet engine?
Maximum turbine temperature (as it’s the most highly stressed component in the engine)
