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Questions and Answers

What happens to thermal efficiency as the compression ratio increases?

Thermal efficiency increases. (correct)

Thermal efficiency decreases significantly.

Thermal efficiency remains constant.

Thermal efficiency becomes unpredictable.

What is specific fuel consumption (SFC) used to measure?

Fuel consumed per pound of thrust produced. (correct)

Engine cooling capacity.

Efficiency of fuel burning only.

Overall aerodynamic performance.

Which condition causes an increase in specific fuel consumption at higher aircraft speeds?

Reduction of thrust requirements.

Increased fuel efficiency.

Decreased mass airflow.

Ram effect causing increased mass airflow. (correct)

What is a common cause of degraded efficiency in compressors and turbines?

Wear, damage, or contamination. Signup and view all the answers

What does a lower specific fuel consumption (SFC) indicate?

Higher fuel efficiency. Signup and view all the answers

How does thrust vary with changes in altitude and temperature?

Thrust improves with decreasing temperature. Signup and view all the answers

What type of efficiency is achieved through adiabatic compression?

Maximum pressure with least temperature rise. Signup and view all the answers

What mechanism controls the maximum fuel flow to prevent engine over speeding?

Engine power limiter. Signup and view all the answers

What is the formula for calculating thrust in a gas turbine engine?

Thrust = Mass x (Vj - Va) Signup and view all the answers

What is momentum thrust in the context of gas turbine engines?

Thrust generated by the mass flow and velocity difference Signup and view all the answers

At what air speed does the nozzle reach Mach 1, thus forming a shock wave?

At exactly the speed of sound Signup and view all the answers

What happens when the pressure in the jet pipe exceeds 1.4:1 compared to ambient pressure?

Significant pressure thrust begins to be produced Signup and view all the answers

What type of nozzle design is typically required to fully exploit pressure thrust?

Convergent/divergent nozzle Signup and view all the answers

What is the unit of measurement for thrust in gas turbine engines?

Newtons or pounds-force (lbf) Signup and view all the answers

What distinguishes pressure thrust from momentum thrust in gas turbine engines?

Momentum thrust is based on mass flow rates Signup and view all the answers

Why is pressure thrust considered negligible for engines designed for commercial passenger aircraft?

These engines are designed to have exit nozzles at Mach 1 Signup and view all the answers

How does decreased temperature affect thrust?

It increases air density and thus increases thrust. Signup and view all the answers

What occurs to thrust as altitude increases while keeping airspeed and RPM constant?

Thrust decreases due to reduced air density. Signup and view all the answers

What is the effect of ram effect on thrust at higher airspeeds?

It causes thrust to recover and then increase. Signup and view all the answers

What happens to thrust if there is an increase in forward speed without ram effect?

Thrust may reduce due to increased momentum drag. Signup and view all the answers

What does flat rated thrust refer to in gas turbine engines?

Thrust that is consistent up to a certain temperature limit. Signup and view all the answers

How does pressure change with altitude compared to temperature?

Pressure decreases at a faster rate than temperature. Signup and view all the answers

What role does air density play in thrust generation?

Higher density contributes to greater thrust. Signup and view all the answers

What is the influence of ambient temperature on maximum thrust at a given throttle setting?

Thrust increases until it reaches a limit at high temperatures. Signup and view all the answers

What is the Kink Point in relation to turbine engines?

The temperature above which engines become temperature limited Signup and view all the answers

Which rating is used only during take-off operations for turbo engines with water injection?

Take-Off Wet Thrust/SHP Signup and view all the answers

Which thrust rating can be used by the pilot during emergencies without a time limit?

Maximum Continuous Thrust/SHP Signup and view all the answers

How are turbine engines primarily thrust rated?

Engine Pressure Ratio and Fan Speed Signup and view all the answers

What distinguishes Take-Off Dry Thrust/SHP from Take-Off Wet Thrust/SHP?

The inclusion of water injection Signup and view all the answers

What rating should be used for normal climb to cruising altitude?

Maximum Climb Thrust/SHP Signup and view all the answers

Which of the following thrust ratings is limited during take-off operations?

Take-Off Wet Thrust/SHP Signup and view all the answers

What is the purpose of engine power ratings in certification?

To ensure the engine performs at specified thrust levels responsibly Signup and view all the answers

Which of the following best describes the Turbojet engine?

An engine that operates on the principle of a constant pressure cycle Signup and view all the answers

What is a characteristic feature of multi-spool design in jet engines?

It allows for improved efficiency and thrust generation Signup and view all the answers

In the context of bypass engines, what is the purpose of bypass air?

To provide additional thrust without consuming extra fuel Signup and view all the answers

Which of the following concepts is integral to understanding the operation of a ram jet?

It relies on the forward motion of the vehicle to compress air Signup and view all the answers

What distinguishes turbo-shaft engines from other types of jet engines?

They convert thermal energy into mechanical power Signup and view all the answers

Which Newton's law of motion is primarily involved in propulsion systems like jet engines?

The action-reaction principle Signup and view all the answers

What type of engine arrangement is commonly used in high bypass turbo fan engines?

Parallel dual spool arrangement Signup and view all the answers

Which gas turbine engine type is most suited for propelling helicopters?

Turbo-prop Signup and view all the answers

Which of the following is NOT a characteristic of turbojet engines?

High efficiency at low speeds Signup and view all the answers

In gas turbine engines, what is the significance of having multiple spools in the design?

Allows each spool to operate at different speeds for optimal efficiency Signup and view all the answers

Study Notes

Thermal Efficiency and Compression Ratio

Increasing compression ratio leads to higher thermal efficiency.
This means the engine converts more fuel energy into useful work.

Specific Fuel Consumption (SFC)

SFC is a measure of how much fuel an engine consumes per unit of power produced.
Lower SFC indicates better fuel efficiency.

SFC and Aircraft Speed

Higher aircraft speeds can lead to increased SFC due to factors like drag and air density.

Compressor and Turbine Efficiency

Degradation in compressor and turbine efficiency can occur due to fouling, wear, or damage.

Thrust and Altitude/Temperature

Thrust decreases with increasing altitude due to lower air density.
Thrust also decreases with increasing temperature due to reduced air density.

Adiabatic Compression

Adiabatic compression is a process where no heat is exchanged with the surroundings.
Achieving high adiabatic compression efficiency contributes to higher engine efficiency.

Maximum Fuel Flow Control

The engine's fuel control system limits fuel flow to prevent over-speeding, protecting the engine.

Thrust Formula

Thrust = Mass flow rate of air * (Exit velocity - Inlet velocity) + (Exit pressure - Ambient pressure) * Nozzle area

Momentum Thrust

Momentum thrust is generated by the acceleration of air through the engine, increasing its velocity.

Shock Wave Formation

Nozzle reaches Mach 1, creating a shock wave, when the pressure in the jet pipe exceeds ambient pressure by a factor of 1.4:1.

Pressure Thrust

Pressure thrust is generated by the difference in pressure between the jet pipe and the ambient air.
A convergent-divergent nozzle is typically needed to fully utilize pressure thrust.

Thrust Units

Thrust is measured in units of force, commonly in pounds (lbs) or Newtons (N).

Pressure Thrust vs. Momentum Thrust

Pressure thrust is generated by the pressure difference, while momentum thrust is due to velocity change.
Passenger aircraft engines prioritize momentum thrust, making pressure thrust negligible.

Temperature and Thrust

Decreased air temperature can decrease thrust due to lower air density.

Altitude and Thrust

Thrust decreases with increased altitude, even at constant airspeed and RPM, due to lower air density.

Ram Effect and Thrust

At higher airspeeds, ram effect increases thrust by compressing the air entering the engine.

Forward Speed and Thrust

Increasing forward speed without ram effect can slightly decrease thrust due to increased drag.

Flat Rated Thrust

Flat rated thrust refers to a consistent thrust output across a range of operating conditions.

Altitude and Pressure/Temperature

Pressure decreases exponentially with altitude, while temperature decreases at a slower, linear rate.

Air Density and Thrust

Thrust directly correlates with air density. Higher density air provides more mass for acceleration.

Ambient Temperature and Thrust

Higher ambient temperatures reduce air density, limiting maximum thrust achievable at a given throttle setting.

Kink Point

Kink Point in a turbine engines' operating chart is a point where an increase in fuel flow results in a plateau in thrust and significant increase in turbine exhaust temperature.

Take-Off Thrust Rating

Take-Off Wet Thrust/SHP is used only during take-off operations utilizing water injection.

Emergency Thrust Rating

Maximum Continuous (MC) Thrust/SHP is limited by engine temperature and can be used by the pilot during emergencies with no time limit.

Primary Thrust Rating

Turbine engines are primarily thrust rated based on Take-Off Dry Thrust/SHP, representing maximum thrust available without water injection.

Take-Off Dry Thrust/SHP vs. Take-Off Wet Thrust/SHP

Take-Off Wet thrust includes thrust generated by water injection, leading to higher thrust potential than Take-Off Dry Thrust.

Normal Climb Thrust Rating

Maximum Continuous (MC) Thrust/SHP should be used for normal climb to cruising altitude.

Thrust Rating Limitations

Take-Off Dry Thrust/SHP is limited during take-off operations.

Engine Power Rating Purpose

Engine power ratings are used in certification and operational procedures to ensure safe and efficient performance.

Turbojet Engine Description

A turbojet engine compresses air, burns fuel, and expands hot exhaust gases to generate thrust.
It is characterized by direct exhaust of high velocity gases, resulting in a low bypass ratio.

Multi-Spool Design

Multi-spool design in jet engines allows for the independent control of different compressor and turbine sections, improving efficiency and performance.

Bypass Air

In bypass engines, a portion of air is bypassed around the core engine and mixed with the exhaust to improve efficiency.
This bypass air contributes to lower noise levels and higher efficiency compared to turbojet engines.

Ram Jet Basics

Ram jet engines rely on the compression of air by the high-velocity flight of the aircraft.
They are typically used at high speeds where the ram effect is sufficient for compression without a compressor.

Turbo-Shaft Engine

Turbo-shaft engines are similar to turboprops but do not have a propeller.
They use the turbine shaft to generate power for a gearbox, which drives a rotor or other mechanical device.

Newton's Law of Motion and Propulsion

Newton's Third Law of Motion (action-reaction) is fundamental to propulsion.
Explains how the ejection of a high-velocity stream of exhaust gas produces an equal and opposite force (thrust) that moves the aircraft forward.

Turbo Fan Engine Arrangement

High bypass turbo fan engines typically employ a dual-spool arrangement, with one spool for the fan and another for the core engine.

Gas Turbine Engine for Helicopters

Turbo-shaft engines are best suited for propelling helicopters.
They efficiently generate power to drive the main rotor, providing lift and control.

Turbojet Engine Characteristics

Turbojet engines are not known for their high bypass ratios and low noise levels as those features are characteristic of turbofan engines.

Multi-Spool Engines

Multiple spools in gas turbine engines allow for:
Independent optimization of compressor and turbine stages, minimizing engine weight and improving efficiency.
Better control over airflow through different stages, maximizing performance across different speeds and altitudes.

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