Gas Turbine Engines Fundamentals

Fundamentals of Gas Turbine Engines

• EASA Part-66 CAT A document
• Revision ID: 1SEP2014
• Author: For Training Purposes Only
• Release Date: September 19, 2014

Training Manual Information

• Copyright is held by Lufthansa Technical Training (LTT)
• Any use outside of training is prohibited (including copying and reproduction)
• Permission is required for reproduction from LTT.

Gas Turbine Engine Fundamentals

• Controlled flight over long distances is only possible with a suitable aircraft engine
• The engine creates necessary movement for the aircraft
• The engine also provides hydraulic power, electric power and bleed air for the pneumatic system.

Introduction to Aircraft Engines

• Two main engine types exist: Piston type engines and gas turbine engines
• The first airplanes were powered by piston engines that turned a propeller
• Piston engines are simpler in design and are more fuel efficient than gas turbine engines
• Piston engines are not as efficient at high altitudes.
• Piston engines are smaller than gas turbine engines.

Types of Gas Turbine Engines

• Turbojet engines were the first type of gas turbine engines used on aircraft.
• Turbojet engines are loud and need a lot of fuel due to high exhaust velocities.
• Turbofan engines were developed from turbojet engines to be quieter and more fuel efficient
• Turboprop engines are specially designed to produce shaft horsepower (used to drive a propeller)
• Auxiliary power units (APUs) are small gas turbines for electric and pneumatic power for aircraft

Newton's Laws of Motion

• All aircraft engines work on the same principle, to create force necessary to move the aircraft
• Jet propulsion is the propelling force created in the opposite direction of the airflow through a jet nozzle
• The reaction engine uses Newton's third law of motion (for every force there is an equal and opposite reaction)

Acceleration and Work

• Acceleration is created when a body of mass is accelerated
• The force formula relating mass and acceleration is: Force = Mass × Acceleration (F=m×a)
• There's an increase in the airflow pressure which results from the higher velocity of the air

Brayton Cycle

• Brayton Cycle is a series of changes in the state of gas in thermodynamic cycles commonly used for converting heat into mechanical work
• The Brayton Cycle is a thermodynamic cycle of a Turbojet engine.
• Curve (0-3) in the graph represents isentropic compression process
• Compression of airflow decreases the volume and increases the temperature
• Curve (3-4) represents isobaric heat addition
• Curve (4-0) is the isentropic expansion process through which gases are expanded to atmospheric pressure

Energy and Velocity

• Thrust is generated due to airflow acceleration, and not pushing against the atmosphere.
• The magnitude of thrust changes as a result of outlet velocity alterations in a jet nozzle.

Environmental Effects on Thrust

• The thrust is impacted by four main environmental factors : ambient air pressure, air temperature, altitude and aircraft speed.
• Changes in the air pressure and temperature impact air density which, in turn, affects the thrust.
• Increasing altitude leads to decreasing air pressure.
• Lower air density creates lower thrust because of less mass and vice-versa

Thrust of Turbojet, Turboprop and Turbofan Engines

• The various gas turbine types exert thrust by accelerating different amounts of air at different velocities, and different amounts of energy into torque
• Turbo-Jet engines are designed for high-velocity gases; The remainder of the gas energy goes towards thrust
• Turbo-prop engines change a majority of the energy into rotary motion via a propeller
• Turbo-fan engines combine the advantages of turbojet and turboprop, with a fan that drives a large quantity of air at a low velocity, and an additional core engine that accelerates air at a high velocity.

Constructional Arrangement

• Modern aircraft use modular construction compared to the non-modular engines from earlier years, which were comprised of many individual, and interdependent, parts.
• Modular engines are a set of separate modules that are easily removed and replaced
• The construction process for modular engines is better and has advantages to both the manufacturer and the operator

Engine Materials

• Typical materials used in modern jet engines include aluminum alloys, ceramics, composites, cobalt-base alloys, nickel-base alloys, corrosion-resistant steels, and titanium alloys.
• These materials are chosen for their strength, lightweight properties and cost-effectiveness