30 Questions
What is the primary source of power for a gas turbine engine during flight?
Ram effect of the incoming air
What type of turbine wheel is used in the pure reaction turbojet engine?
Single-stage turbine wheel
What is the purpose of the combustion chamber in a gas turbine engine?
To burn the air/fuel mixture
What is the name of the British Royal Air College cadet who patented the first turbojet aircraft engine?
Frank Whittle
What is the working fluid used in an aircraft gas turbine engine?
Air
What is the primary function of the compressor in a gas turbine engine?
To compress the air
What is the primary function of the gas generator in a turboshaft engine?
To extract energy from the combustion process
What is the main difference between a fixed turbine and a free turbine turboshaft engine?
The power take-off mechanism
Which of the following is NOT a common application of turboshaft engines?
Commercial airliners
What is the primary purpose of an aircraft auxiliary power unit (APU)?
To drive generators and hydraulic pumps
How many types of gas turbine-powered jet engines are commonly used in today's aircraft?
4
What is the name of the turbine that drives the free turbine system in a turboshaft engine?
Power turbine
What is the primary purpose of the compressor in a turbojet engine?
To increase the air pressure
What is the result of the heat generated by the burning fuel in a turbojet engine?
The air expands and flows towards the rear of the engine
What is the ratio of airflow in a low-bypass engine?
1:1
What is the main advantage of the turbojet engine?
Enhanced efficiencies at high-speed, high-altitude operations
What is the term for the air that passes through the core engine in a low-bypass engine?
Hot gas stream
What is the advantage of a mixed exhaust design in a low-bypass engine?
Noise suppression
What is the primary difference between a turbojet engine and a turbofan engine?
The ability to use higher turbine temperatures without a corresponding increase in jet velocity
What is the result of the air exiting the engine at a higher velocity than the incoming air?
The engine produces more thrust
What is the fan ratio of a high-bypass engine?
4:1 to < 9:1
What is the current trend in commercial and business jets in terms of engine choice?
Turbofan engines are becoming more popular
What is required to accomplish the high-bypass ratio in a high-bypass engine?
A large-diameter fan
What is the term for the air that is discharged by the fan and ducted overboard from a short duct?
Cold gas stream
What percentage of the airflow accelerated by the fan rotor blades is ducted past the core engine?
Up to 90%
What is the purpose of the pressure within the fan duct in ducted fan engines?
To reduce airflow separation from the blades at speeds over Mach 1
Why do high-bypass engines and ducted fan engines produce more fan thrust than low-bypass engines?
Because they suffer less loss through skin friction with their short ducts
What is the purpose of the engine station reference system?
To identify the location of a component or fitting along the axis line of the engine
What is the term for the airstream that is ducted overboard, either directly behind the fan or at the rear of the engine?
Bypass airstream
What is an alternative arrangement of the fan, where it is powered by a shaft from the turbine or is an extension of the turbine blades?
Aft-fan arrangement
Study Notes
Bypass Ratios
- Low-bypass engine: airflow is divided approximately into two halves between the fan and compressor
- Medium-bypass engine: 2:1 or 3:1 ratio
- High-bypass engine: 4:1 to < 9:1 ratio
- Ultra-high-bypass engine: > 9:1 ratio
Low-Bypass Engine
- Air is divided into two halves: fan and compressor
- Fan air may be ducted overboard or pass through the core engine
- Core engine air is compressed, combusted, and discharged through the hot exhaust nozzle
- Turbofan engine can have non-mixed or mixed exhaust
- Mixed exhaust design offers noise suppression by diluting hot gases
High-Bypass Engine
- Fan ratio: 4:1 to < 9:1 (four to nine parts of air go through the fan for every part that goes to the core engine)
- Requires a large-diameter fan
- Example: diagram showing changes in pressure, velocity, and temperature during ground run-up
History of Propulsion
- Frank Whittle, British Royal Air College cadet, wrote a thesis advocating for the use of gas turbine engines in 1930
- Whittle patented the first turbojet aircraft engine, which used a compressor impeller driven by a turbine
- The engine developed was a pure reaction turbojet, where total thrust came from reaction to the hot gas stream
Gas Turbine Engine
- The aircraft gas turbine is a heat engine using air as a working fluid
- Consists of a compressor, combustion chamber, and turbine
- Four types of gas turbine engines: turboshaft, turbo-propeller, turbojet, and turbofan
Turboshaft Engine
- Delivers power through a shaft to operate something other than a propeller
- Can be a fixed turbine or free turbine turboshaft engine
- Free turbine turboshaft engine has two major sections: gas generator and free turbine
- Used in industrial applications, such as electrical power-generating plants and surface transportation systems
- Also used in aviation, particularly in helicopters and aircraft auxiliary power units (APUs)
Turbojet Engine
- Uses the acceleration of airflow throughout the engine to produce thrust
- Well suited for high-speed, high-altitude operations
- Basic operating principles: air enters through an inlet duct, is compressed, mixed with fuel, and ignited
- Expanding air spins turbine wheels, which drives the compressor, and exits the engine at a higher velocity
Turbofan Engine
- Consists of a multi-bladed ducted propeller driven by a gas turbine
- Up to 90% of airflow accelerated by the fan rotor blades is ducted past the core engine
- Fan tip speed may exceed Mach 1 to deliver the correct amount of air
- High-bypass engines and ducted fan engines produce more fan thrust than low-bypass engines
Engine Stations
- Reference system using engine stations to identify the position of a component or fitting
- Engine stations follow the gas path and refer to basic locations such as the compressor inlet, compressor outlet, and turbine inlet and outlet
- Number of designated stations varies according to engine complexity
This quiz covers the different types of bypass ratios in gas turbine engines, including low, medium, high, and ultra-high bypass ratios.
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