M15.7 Flashcards
What is the main job of the exhaust on a jet engine?
To accelerate the airflow coming from the turbine to efficiency make thrust
What is the main job of the exhaust on a turbo-prop?
To safely guide the exhaust gases overboard
Why does the exhaust system only make a small amount of thrust?
Because most of the energy has gone into powering the turbines
What is the job of the APU exhaust system?
It guides all gases overboard because all the energy has gone into the turbine
What does a muffler do in the exhaust system?
It reduces the noise
Why do some exhaust systems have a tube called the exhaust duct or tall pip?
It prevents excessive turbulence and prevents reverse flow
What is the exhaust called for the hot gas flow?
Primary nozzle
What is the exhaust called for the cold gas flow?
Secondary nozzle
What is a combined exhaust nozzle?
A larger exhaust that mixes the hot and cold exhaust gases which gives a higher propulsion efficiency
Why would a special mixer be used on the exhaust system?
To get a higher propulsion efficiency compared to the combined exhaust nozzle
To achieve optimum thrust from a given mass what must happen in the exhaust?
The gases must expand completely in the exhaust nozzle
How is the exhaust airflow caused?
By the pressure ratio between the turbine discharge and ambient pressure
When would the pressure ratio of the exhaust change?
When the engine speed changes (increase in speed gives an increase in ratio)
What is the highest pressure ratio a convergent exhaust can go up to?
2:1
What happens if the pressure ratio of the exhaust is too high?
The static pressure is greater than the ambient pressure which causes a burst in all direction which is very inefficient
What are used for pressure ratio exhausts higher than 2:1?
A convergent-divergent nozzle (also called C-D nozzles)
What type of aircraft use convergent-divergent nozzles?
Aircraft that fly faster than the speed of sound
Why do most modern turbofan engines have large exhausts?
To prevent choking
Why must high exhaust gas velocities be avoided?
Because they decrease propulsion efficiency, cause great turbulence, and therefore make a loud exhaust noise
What is the unit commonly used to express noise?
Effective perceived noise decibel (EPNdB)
What type of engine is the quietest?
High bypass ratio jets
What are the two methods used to reduce noise level of an engine?
- selection of a design engine
- selection of dampening methods
What are aircraft linings made from to reduce noise?
Porous surfaces supported by honeycomb structure
Why is honeycomb used to reduce noise?
It allows sound to waves to be reflected in the chambers which converts sound energy into heat energy
What is a hush kit?
It mixes ambient air to the exhaust gas before it enters the nozzle to reduce noise
Why cant thrust reversers be used at the ideal angle of 180 degrees?
It would cause major damage to the engine
What is the normal angle of thrust reversers?
45-60 degrees
Why can’t thrust reversers be used in flight?
Because it will cause very dangerous aerodynamics
Where are clam shell door type thrust reversers used?
On older jet engines with low bypass ratios
How are clamshell door thrust reversers operated?
By pneumatic bleed air from the high pressure compressor
Where are bucket door type thrust reversers used?
On older jets with a low bypass ratio
How are bucket door type reversers operated?
Hydraulically
What are reverse thrusters that deflect cold air called?
Fan reversers
Why are reverse thruster not used on the hot core of the engine?
Because it would need a lot of maintenance on them
How are thrust reverser sleeves operated?
Pneumatically by drive motors or hydraulically by actuators
How many doors does the pivoting door thrust reversers have?
4
How are pivoting door thrust reversers operated?
Hydraulically
Why would the engine compressor sometimes stall when thrust reversers are deployed?
Because the airflow through the engine will be disturbed
Why is operating the thrust reversers at low speeds bad?
Because it can ingest its own reversed airflow
What does ingestion from its own thrust reversers cause?
Severe compressor stall and high exhaust gas temperatures
How would reverse thrusters flow affect FOD?
It kicks up dirt and dust from the runway
On some aircraft reversed airflow causes air cushions, why is this bad?
It decreases the load on the main landing gear which decreases brake efficiency
What is special about the inward airflow from thrust reversers?
They don’t direct the airflow into the wheel wells or into the group to whirl up dirt
What is important to remember about a four engine aircraft thrust reverser system?
Don’t blow reversed air into the other engines
What must happen to a faulty thrust reverser?
It must be reactive properly so it doesn’t deploy in flight
What are the three main subsystems to a thrust reverser?
- control system
- actuation system
- airflow deflection system
What is the control system for thrust reversers?
To activate the thrust reversers and to increase the engine power for reverse thrust
What is the reverse actuation system?
It moves the airflow deflection mechanism by signals from the reverser control system
What is the airflow deflection system?
It directs the airflow in the optimum direction for safe reverse thrust
What is the main component of the control system?
The reverse lever in the cockpit
What is the reverse control system always protected by?
The air ground logic so it cant work in flight
What is the reverse interlock system used for?
It makes sure that the engine power can only be increased when the reverser are fully deployed
Why must the hydraulic reverser systems be synchronised?
So they all deploy at the same time to prevent uneven reverse thrust
Where are pneumatic reverser actuation systems only used on?
Translating sleeves
What is the indication in the cockpit that the thrust reversers aren’t in the stowed position?
An amber reverse unlock message
When would a green reverse light come up in the cockpit?
When it is fully deployed and the throttle interlock is released
