Ch. 29 The Turbine Engine Flashcards
Turbine engines are either _____ or _____ types.
Free-turbine or fixed shaft
Turbine engine pros (8 total)
- Smoother (less noticeable vibration)
- Quieter
- Better power / weight ratio
- More reliable
- Overhauled at greater intervals (hours)
- Easier to start in the cold
- Require less maintenance between overhauls
- Consume less engine oil
Turbine engine cons (4 total)
- More expensive to purchase & overhaul
- Thirstier (consume more fuel)
- Less tolerant of abuse
- More surprising when they fail (being quieter, they give less warning of failure)
The gas turbine is made up of what 4 parts?
- Compressor
- Combustion chamber
- Turbine
- Accessory gearbox assembly
What is a compressor & what are the 2 basic types?
The compressor draws filtered air into the plenum chamber & compresses it.
Axial & centrifugal are the 2 types.
Plenum chambers hold air at a pressure slightly higher than that of the surrounding air. Plenum chambers stabilize the pressure of air before it enters the compressor
Air moves from the compressor to the _____, where fuel is added & burned.
Combustion chamber
Half of the air that enters an engine compressor is used for combustion and the other half is used for _____.
Cooling the metal around the hot flame
Temperatures at the turbines are around 700-900 degrees C
The very hot, fast moving air passes through the _____ and then through the _____
Compressor turbine; power turbine
If the airflow through the compressor is disturbed, a condition called _____, may take effect.
Compressor stall (aka surge)
The pressure at the compressor is reduced from lack of incoming air & the combustion pressure may cause reverse flow into the compressor output.
What’s the difference between a free turbine or fixed turbine engine?
Free: the 1st & 2nd stage of turbines are not mechanically connected to each other, only a gas path
Fixed: the 1st & 2nd stage of turbines are mechanically coupled to each other; the compressor & output shaft are connected
What are the functions of the turbines within a two-stage turbine engine?
The first one, or compressor turbine, drives the compressor section & referred to as gas producer (seen as N1 or NG).
The second one, or power turbine, drives the other components attached to the accessory gearbox (seen as N2 or NP).
What is TOT?
Turbine outlet temperature
The temperature of gas that’s expelled from the exhaust.
Depending on the manufacturer, TOT may be referred as ITT (inter-turbine temperature), exhaust gas temperature (EGT), measured gas temperature (MGT), T4, or T5.
What is a turbine engine cycle?
The number of times the engine has been started & used at takeoff power. Taking metal from a cool rest & heating it to 700 degrees C.
Jet engines have longer lives than helicopter engines because they cruise at high altitudes at one power setting vs helicopter engines that have huge changes in power requirements.
Why do transient limits of temperature exist?
Because it is difficult to accurately measure temperatures deep within an engine & how that heat is affecting the metal.
Particularly for startup & sometimes during operation: metal can’t stand high temperatures for prolonged periods, & the short period allowed for a high temperature during starting means this high temperature won’t exist long enough to affect the turbine blades.
The power available from a turbine engine depends upon _____ & _____.
Pressure altitude & temperature
The major reason turbine engines can develop more power with _____ air is that many are “temperature limited” at the turbine wheel.
Colder
On a cold day, the air arrives at the combustion chamber cooler than on a hot day. Thus, more fuel (energy & power) can be added to the air before the temperature limits are reached.
More fuel in means more power out.
The major reason turbine engines can develop more power with _____ air is that many are “temperature limited” at the turbine wheel.
Colder
On a cold day, the air arrives at the combustion chamber cooler than on a hot day. Thus, more fuel (energy & power) can be added to the air before the temperature limits are reached.
More fuel in means more power out.
Turbine engines are affected more by _____ than by the density of the air.
Air temperature
Whereas a piston engine is more affected by the density of the air rather than temperature.
There is less power available on hotter days, or the TOT is higher on a hot day than on a cold day for the same torque used.
Turbine engines are affected by high humidity only at _____ & _____.
Very high temperatures & very high humidity levels
Whereas piston engines are affected by humidity by reducing the amount of power available.
What is specific fuel consumption (SFC)?
It’s the weight of fuel required to produce one shaft horsepower for 1 hour.
It’s used to measure the relative fuel efficiency.
At low power settings, over 60% of the effort produced by the engine goes to drive the _____, and what is left over is useful power.
Compressor
When operating at high power, the useful power to drive the compressor is _____, so proportionally less is used by the compressor.
Greatest
It is more fuel efficient to run the engine closer to the limits rather than at idle power.
Turbine engines are specifically designed to perform better at higher power settings / close to their design limits.
What are the 3 main instruments for turbine engine helicopters?
- N1 speed (compressor)
- TOT (temperature of exhaust gases)
- Torquemeter
What are the 3 types of torquemeters?
Oil pressure torquemeter, strain gage torquemeter, & optical sensor torquemeter
_____ or _____ in oil pressure torquemeters are added to take out the immediate effects of pressure fluctuations.
Dampers or accumulators
The _____torquemeter uses a series of gears in the engine accessory gearbox to move back & forth depending upon the power demanded from the engine.
Oil pressure
This torquemeter will not read reliably if the oil temperature isn’t within normal operating range.
The _____ torquemeter is used to measure the amount of power being applied to the drive shaft.
Strain gage
The drive shaft between the engine & the transmission.
The _____ in a turbine engine maintains the free (or power) turbine N2 at a constant speed by varying the fuel flow & compressor speed.
Governor
The engine power is adjusted by the governor, the pilot controls the blade pitch.
The _____ in a turbine engine maintains the free (or power) turbine N2 at a constant speed by varying the fuel flow & compressor speed.
Governor
The engine power is adjusted by the governor, the pilot controls the blade pitch.
With a hydro-mechanical fuel control system, a _____ ensures the engine governor maintains a constant rotor speed with different power conditions.
Droop compensator
It is a mechanical linkage that auto-inputs commands to the governor, so the system doesn’t lag too much when you pull or reduce power.
If the governor doesn’t maintain the N2 (rotor speed) constant, it may be slowly _____, wandering or hunting.
Oscillating
A governor may have _____ if, following a power change, it doesn’t return to the same N2 (rotor speed) at the same power setting.
Hysteresis
_____ is the change in rotor speed that happens briefly following a power change.
Transient droop
What is an isochronous governor?
A governor that maintains the same speed in the mechanism controlled (engine) regardless of the load.
_____ are placed in fuel control lines to reduce the effect of rapid changes in pressure, & these further slow the response.
Accumulators
Rapid power increases are not handled well as the engine must accelerate but avoid compressor stalling. Decelerating is even harder to do quickly without flaming out the engine.
What is FADEC? What are it’s 5 functions?
Full Authority Digital Electronic Control
- Fuel flow regulation (during all phases of flight)
- Monitors engine parameters (N1, N2, engine pressure, TOT, exceed limits & signals warnings)
- Start sequence (opening of start valve, monitoring shaft speeds, controlling fuel flow & igniters)
- Thrust reversers
- Bleed valves (controls airflow during engine start, engine turbine cooling, prevent engine stall)
FADECs allow pilots to ignore engine parameters as they take care of them.
Simply put droop is _____.
Reduction in rpm
