Section 5 EO C232.01 – IDENTIFY THE CHARACTERISTICS OF GAS TURBINE ENGINES

A jet engine is a reactive engine, which propels itself by ejecting material to create a force, as described by Newton’s third law of motion.

Newton’s third law states that for every action there is an equal and opposite reaction. All propulsion systems rely on this fact in some way. A jet engine propels itself in one direction by ejecting a fluid (hot gas) in the opposite direction.

The amount of thrust developed by ejecting hot gas depends on the mass and velocity of the material ejected. To develop a lot of thrust, a lot of material must be ejected or else it must be ejected at high velocity. Most of the mass ejected by a jet engine comes from the air, which is scooped up from the atmosphere that the jet is passing through. That scooped air is raised to a high velocity by burning fuel.

Since the jet engine can always get more air, its thrust duration is limited only by the amount of fuel that it has available.

150 BC – Hero. An Egyptian philosopher and mathematician, invented a toy (Aeolipile) that used steam to rotate on top of a boiling pot of water. The escaping steam caused a reaction that moved several nozzles arranged on a wheel.

1232 – Battle of Kai-Keng. Chinese soldiers used rockets as weapons to repel the Mongols at the Battle of Kai-Keng. Burning gunpowder and the reaction principle were used to propel the rockets. After Kai-Keng, the Mongols used rockets and it is believed that they brought the technology to Europe.

1500 – Leonardo da Vinci. He drew a sketch of a device, the chimney jack, which rotated due to the movement of smoke and hot gases flowing up a chimney. This device used hot air to rotate a shaft, which turned a spit. The hot air from the fire rose upward to pass through a series of fanlike blades that turned a shaft, which turned the roasting spit.

1629 – Giovanni Branca. He developed a stamping mill for bending metal. His stamping mill used jets of steam to spin a turbine, which rotated a shaft to operate the machinery.

1872 – Dr. F. Stolze. He designed the first true gas turbine engine. His engine used a multi-stage turbine section and a flow compressor. This engine never ran under its own power.

1930 – Sir Frank Whittle. He designed a gas turbine for jet propulsion in England. The first successful use of this engine was in April, 1937. His early work on the theory of gas propulsion was based on the contributions of most of the earlier pioneers of this field.

1939 – Heinkel Aircraft Company. This company flew the first gas turbine jet, the HE178.

1941 – Sir Frank Whittle. He designed the first successful turbojet airplane, the Gloster Meteor. Whittle improved his jet engine during World War II and in 1942, he shipped an engine prototype to General Electric in the United States. America’s first jet aircraft was built the following year.

1942 – Dr. Franz Anslem. He developed the axial-flow turbojet, which was used in the Messerschmitt Me 262, the world’s first operational jet fighter.

After World War II, the development of jet engines was directed by a number of commercial companies. Jet engines soon became the most popular method of powering high-performance aircraft.

Show the cadets a slide or distribute a handout of Figure C-1.

The earliest jet to fly was a ramjet, the simplest jet engine, which has no moving parts. The speed of the aircraft forces air into the small volume of the engine, increasing air pressure and density. Ramjet application is restricted by the fact that its air compression depends on forward speed. The ramjet develops no static (stationary) thrust and very little thrust in general when travelling below the speed of sound. As a consequence, a ramjet vehicle requires some form of assisted takeoff, such as another aircraft, and so it has been used primarily in guided-missile systems.

In 1930, Sir Frank Whittle’s ingenious idea of placing a turbine into the stream of hot exhaust gases allowed the operation of a compressor to solve the problem of running the engine at low speeds or static conditions. This is the secret of the turbojet engine and of all other refinements of the design, such as turboprops, turbofans and turboshafts.

Point out to the cadets the turbine shown in Figure C-2 and the absence of a turbine in Figure C-1.

Another benefit of turbines in jet engines is that they provide power for all sorts of ancillary flight instruments and other systems. In a modern airliner, turbine power provides everything from radio communications with the air traffic control tower to hot water for the passengers.

Show the cadets a slide or distribute a handout of Figure D-1.

The four basic parts of any gas turbine jet engine are the compressor, combustor, turbine, and nozzle, all of which process air, or core air, which travels through the engine. In the most common gas turbine aircraft engine, the turbofan, there is also the fan, which provides bypass air as well as core air, and a mixer, which combines the core airflow with the bypass airflow. The reduced engine noise levels and the excellent fuel efficiency of the turbofan engine have made it the engine design of choice for most modern commercial applications. Examples of varied turbofan applications are the CF-18’s two GE F404 low bypass turbofan engines and the A380 Airbus’s four Rolls-Royce Trent 900 high bypass turbofan engines; two dissimilar applications that both favour turbofan technology.

Fan. The fan is the first component in a turbofan. The fan pulls air into the engine. The air then splits it into two parts. One part continues through the “core” or centre of the engine, where it is acted upon by the other engine components. The second part “bypasses” the core of the engine, travelling through a duct to the back of the engine where it produces much of the force that propels the aircraft forward.

Compressor. The compressor is the first component in the engine core. The compressor squeezes the air into a smaller volume, increasing its pressure. The air is then forced into the combustor.

Combustor. In the combustor the air is mixed with fuel and ignited, producing high temperature, expanding gases.

Turbine. The high-energy airflow coming out of the combustor goes through the turbine, causing the turbine blades to rotate. The task of the turbine is to convert the linear gas motion into rotary mechanical work to drive the compressor, which then feeds the combustor with high-pressure air.

Nozzle. The nozzle is the engine’s exhaust outlet. The hot, high-pressure gases that have passed through the turbine, combined with the colder air that bypassed the engine core, produce a force when exiting the nozzle that acts to propel the engine, and therefore the aircraft, forward. The nozzle may be preceded by a mixer, which combines the high temperature air coming from the engine core with the lower temperature air that was bypassed in the fan. The mixer results in a quieter engine.

Afterburner. In addition to the basic components of a gas turbine jet engine, one other process is occasionally employed to increase the thrust of a given engine. Afterburning consists of the introduction and burning of raw fuel between the engine turbine and the jet nozzle, utilizing the unburned oxygen in the exhaust gas to support combustion. The increase in the temperature of the exhaust gases further increases their velocity as they leave the propelling nozzle, which thereby increases the engine thrust. This increased thrust could be obtained by the use of a larger engine, but this would increase the weight and overall fuel consumption.