LESSON 9 Jet Engines

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Flight Theory

Jet Engines

112-inch-fan PW4098


The most powerful commercial engine in the world – powers the 777-300, a stretched version that is the longest commercial aircraft.

Its diameter nearly as wide as the fuselage of a Boeing 737.

It uses hollow titanium, shroudless fan blades

The 4 primary components to all gas turbine engines

The Compressor

The Combustor

The Turbine

The Exhaust

The Compressor

The compressor is the leading part of the engine.

The function of the compressor is to raise the pressure and reduce the volume of the air as it pumps the air through the engine.

The air which enters is compressed by means of an axial type flow compressor or a centrifugal type design.

Compressor Types

Centrifugal Flow compressors

Centrifugal compressors were used mostly in early model engines.

Flow exits at a 90˚ angle to the shaft

A diffuser then turns the flow parallel to the shaft

The pressure rise is more per stage when compared to the axial type.

Pressure increases by an average factor of 4, axial flow about 1.2 per stage.


Axial Flow Compressors

Most all engines today are of the axial type flow compressor.

The compressor section is comprised of a multi stage rotating multi-blade rotor with stators in between

Each stage multiplies the pressure ratio.

Engine can be narrower than centrifugal flow compressors, resulting in less drag.


The Combustor

The combustor is the chamber in which the compressed air is mixed with fuel and ignited by use of kind of a spark plug.

Needless to say this is where highest temperatures are reached

As a pilot you need to be concerned with the temp on starting.

Only about 25% of the air is mixed with the fuel and ignited the rest is used to cool the sleeves of the combustor section liner and the hot gases before they enter the turbine.

Multiple Combustion Chamber

Used mainly on centrifugal compressor engines

Air from the compressor is ducted into the burner cans

The cans are arranged radially around the engine.


Can-annular Combustion Chamber

This type bridges the gap between the Multiple and the Annular types

Burners are arranged inside a common air casing

This makes it more compact and is easier to overhaul

Burner Can


Has 1 combustion chamber in an Annular shape

For the same power output, the length of the chamber is only 75% of the can-annular system of the same diameter.

There are no problems with separate combustion chambers.

The Turbine

The turbine is downstream of the combustor and extracts energy from the hot gases as they flow by.

The turbine can also have multiple stages with cross sectional area of each blade increasing as you get farther aft in the engine.

This is because of the reduced velocity of the exhaust gases toward the rear of the engine.

Kind of the reverse of the compressor at the other end.

The turbine then drives the compressor (usually the same shaft) and any accessories or a transmission to drive a propeller.

Compressor/Turbine Relationship

Turbine Blade With Cooling Holes

Hot Hot Hot…

Gases may enter the turbine at temps of 850˚ to 1700˚ C.

The higher the temp the higher the efficiency

So they blow cool air out holes in the blades of the turbine.

Nickel alloys are used in construction to stand up to the heat.


The Exhaust Section


The exhaust section rounds out the engine consisting of the exhaust duct (tail pipe), exhaust cone, and nozzle.

The primary job of the exhaust system is to increase the velocity of and direct the gases in such a way as to provide the necessary thrust.

This process converts energy available in the exhaust stream in the form of heat and pressure into the production of thrust.

Basic Exhaust

The Exhaust Section

The exhaust gases reach close to Mach 1 in relation to the gas temperature.

This is as fast as they can go unless an increase in temp. occurs.

The solution is to add a variable area nozzle usually found on high performance jets.

By increasing the exit area, higher amounts of fuel can be used (afterburner) the temperature rise which would normally prohibit this can be controlled by increasing the flow by expanding the nozzle.

The Exhaust Section

In an afterburning engine fuel is injected into the exhaust section through use of a spray bar.

Remember that 75% of the air is still unburned and is available for combustion in this area.

Types of Gas Turbine Engines




Turbofan or Fanjet

High Bypass Turbofan



Linear Aerospike


A turbojet is a turbine which no excess power (above that required by the compressor) is supplied by the turbine.

All the available energy in the exhaust gases is converted to kinetic energy of the jet which supplies the propulsion force.

The high velocity imparted to the exhaust gases provides the thrust for propulsion. In most turbojet engines approx. 65% of the energy developed is used to drive the compressor section while the remaining 35% is used as thrust.



A turboprop, is a gas turbine engine in which the turbine provides power in excess of that required to drive the compressor which is used to drive a prop.

Some thrust is expelled through the exhaust pipe however more blades or stages have been added to the hot section to extract more power from the exhaust gases to drive the prop.

The Cessna caravan on floats.



Sometimes a separate free wheeling turbine is used to extract power from the exhaust gases to drive rotor blades in a helicopter.

This type of engine is referred to as the Turboshaft engine.


A turbofan or fanjet is similar to a turbo prop except that the excess power of the turbine is used to drive a fan or low pressure compressor in an auxiliary duct usually around the primary duct.

This fan could be at the forward or aft position of the engine however most commercial airliner engines locate the fan in the front.

High-Bypass Turbofan

The High-Bypass turbofan engine combines the best of both the turbo prop and the turbojet engine in one.

Of course as speed decreases in a turbojet engine its efficiency decreases because of the wasted kinetic energy, the faster the engine goes through the air the less difference there is between exhaust velocity and air velocity thus more efficiency.

High-Bypass Turbofan

By the same token the turbo prop is very efficient at lower velocities because of the amount of air the prop can move but at higher velocities drag increases and efficiency decreases as the prop blade tips approach the speed of sound.

The high bypass turbofan relinquishes to the turbojet at speeds greater than Mach 1.

The high by-pass turbofan has an advantage in that it is much quieter than the turbojet.

The Ramjet

The only air breathing engine developed that is capable of speeds much above Mach 3 is the ramjet.

This engine compresses the air by the sheer ram effect as it enters the intake nozzle.

However, the airflow must be slowed to subsonic speeds before adding the fuel and ignition.

This makes the combustion process easier but also drives up the temperatures inside the engine.

The Scramjet

This engine is much like the ramjet however the airflow remains all supersonic. This keeps the temperatures down to a manageable level but igniting the fuel air mixture is described as trying to light a match in a tornado.

The scramjet also depends on extensively critically shaped inlet and exit nozzle arrangement. Neither the ramjet or the scramjet are capable of self starting from no or low airspeed. So there would still be a requirement to have a turbojet or fanjet somewhere to get the plane to fast enough speed for operation of the these two.


The Linear Aerospike engine

The linear aerospike engine is much like a rocket engine however, one of the major differences, and the most notable, is the absence of a bell-shaped nozzle.

The linear aerospike engine uses the atmosphere as the exterior “wall” of its nozzle, which provides a self-optimizing, altitude-compensating effect.

The resulting unconstrained flow of gases allows for maximum performance from launch to main engine cutoff. The engine runs off of liquid hydrogen and liquid oxygen.

Thrust augmentation for short field capabilities

Some engines are situated on the under side of the wing so as to augment short field take off capability.

The C-17 employs thrust from its high by-pass turbofan by diverting it across the wing and flaps for superior short field takeoff and landing.

Still others employ variable directional nozzles for deflecting thrust downward to help accelerate ROC


Thrust Reversers

The thrust reverser deflects the directs the thrust generated by the engine forward to decelerate the plane.

The typical reverser should be able to reverse at least 40% of the engine’s thrust for slowing the aircraft.

There are two common types of thrust reversers: Clamshell and cascade.

The throttle has a special position known as “reverser idle” which activates the reverse thrust.

The clamshell is located in the rear of the engine sometimes forming the rear section of the engine nacelle.

Thrust Reversers

The cascade reverser uses numerous turning vanes in the gas path to direct the gas flow outward and forward during operation.

Blocker doors cover the fan exit and a sleeve moves to expose the cascade vanes that direct the fan air forward. The cascades are rows of airfoils that turn the air forward.

Sometimes both are used on the same engine. Thrust reversers are most effective at higher speeds.

The higher speed prevents ingestion of debris and of hot exhaust gases into the engine.

Most are used only in the first part of the landing roll out, and as a general rule, are not used below a forward speed of 60 knots.

Variable geometry duct for supersonic aircraft

In supersonic aircraft, the inlet duct is designed to slow down the incoming air to subsonic speeds to be processed by the compressor.

The forward part of the duct slows velocity to Mach 1 and the divergent subsonic section further slows the air and increases the pressure.

Variable geometry duct for supersonic aircraft

Sometimes a moveable spike is used to vary the area of the intake and slow the velocity.

This method is used on the SR-71 engines.

Another way is to use a protuberance in the airstream to create a shock wave which causes a diffusion of the airflow which in turn slows the airflow.

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