In a turbocharged or turbo-normalised engine, the fuel is added ___ the air is compressed.
Note: If fuel was added beforehand, the fuel & air mixture could not be put through the hot compressor.
What is the critical alt. for a turbocharger?
The altitude at which the turbo can't maintain the pressure required to produce the rated engine power. Above this altitude, engine performance will start decreasing.
What can cause a compressor stall?
- Air entering the inlet at too steep an angle, causing turbulence
- Dirt on the compressor blades (similar to frost on an airplane wing)
- Wear or damaged compressor blades reduces their critical angle
- If bleed or surge vales don't open correctly
What is Ng/N1?
Ng or N1 is the RPM or a %RPM of the gas generator (compressor turbine). Also referred to as the low pressure spool.
What is Np or N2?
The rotational rate of the propeller turbine or power turbine.
How do Turboprops, Turbofans and Turbojets accelerate the air they take in?
- Turboprop - very large mass of air, small acceleration
- Turbofan - large mass of air, medium acceleration (more efficient)
- Turbojet - large mass air, very high acceleration (high exhaust velocity = wasted energy).
What's the air to fuel ratio of a turbine?
What controls cabin pressure in a pressurized aircraft?
Regulating outflow air
A turbocharger works by increasing the ___ of air to the engine.
What is bypass ratio?
The ratio of air flowing through the bypass duct to air which flows through the combustion chamber.
What does the wastegate control in a turbocharger?
The wastegate controls the amount of exhaust gas that flows through the turbocharger. A fully open wastegate means very little or no exhaust gass will flow through the turbocharger, making is basically inoperative.
When are compressor stalls most likely to occur?
During accelerating or starting
What is Engine Pressure Ratio (EPR) and why is it important?
Ratio of turbine inlet air pressure to exhaust pressure. Indicator of engine health.
What stops the prop from reversing in flight?
Low pitch stops
What determines the level of boost in a turbocharged piston engine?
The wastegate which diverts exhaust gas through or around the turbocharger turbine.
Cabin pressure is regulated through a ___ inflow of bleed air and a ___ outflow of air through ___.
- constant inflow
- variable outflow
- via outflow valves
True/False - The majority of energy created by combustion in a turbojet is used to drive the compressors?
What % of the air passing through the combustion chamber of a gas turbine is combusted with the fuel?
Thrust reversers on a turbofan are generally called ___ and operate by reversing ___.
Cascades, bypass air.
Two purposes of the fuel additive "Prist"
- Fuel anti-freeze
- Prevents bacteria and fungal growth in the fuel tanks
Engine bleed valves vs. Engine bleed air?
Engine bleed valves - prevent surges or stalls
Engine bleed air - provide bleed air to systems
On a multi-spool turbojet, the starter motor normals turns the ___
What do hydraulic accumulators do?
Smooth out pressure variations in a hydraulic system.
3 limiting factors for turbojet engines?
- Inlet air temp
- Rotor speed
- Shaft torque
What is the job of the stator veins between the axial compressors?
- Reduce rotational component of airflow
- Increase air pressure
- Guide air to the next compressor at the correct angle
How do you determine power output on a turboprop?
Prop RPM and Torque
What does a diffuser tube do in a turbine engine?
Decrease airflow velocity and increase pressure
What do you get from turbofan over a turbojet?
- Acceleration of larger air mass
- More thrust
- Less noise
An increase in oil pressure to the prop hub creates a ___ pitch, while a decrease causes the prop to ___
How does the beta valve work when reverse is selected?
The beta valve opens allowing oil pressure to the prop hub.
The term used in a free turbine to describe how combustion exhaust gas drives the power turbine?
In what situations is asymmetric thrust greatest?
- high angles of attack
- high power settings
3 purposes of an APU (commonly in the tail)?
- Electrical power
- Pneumatic air
- Can start the aircraft's engines
What do fusible tire plugs do?
Produce a controlled deflation if tire becomes too hot.