Chapter: 1

Fundamentals of aircraft gas turbine engines

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1.1. Introduction:
Propulsion
• The Random House College Dictionary defines
propulsion as "the act of propelling, the state of
being propelled, a propelling force or impulse"
and defines the verb propel as "to drive, or cause
to move, forward or onward. ''
• Marrriem-Webster dictionary defines propulsion
as “ the force that moves something forward: the
force that propels something; the action or
process of propelling; something that propels”
• Further more the word propel means to push or
drive something forward or in a particular
direction.
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 Introduction …

• From these definitions, we can conclude that the

study of propulsion includes:
– the study of the propelling force,
– the motion caused, and
– the bodies involved.

• Propulsion involves an object to be propelled plus

one or more additional bodies, called propellant.
• The study of propulsion is concerned with vehicles
such as automobiles, trains, ships, aircraft, and
spacecraft.

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 Introduction…
• Methods devised to produce a thrust force for
the propulsion of a vehicle in flight are based on
the principle of jet propulsion (the momentum
change of a fluid by the propulsion system).
• The fluid may be:
– the gas used by the engine itself (e.g., turbojet),
– a fluid available in the surrounding environment
(e.g., air used by a propeller), or
– stored in the vehicle and carried by it during the
flight (e.g., rocket).

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 1.2. Role of engine in aircraft development
• Some milestones in the long journey spanning nearly one
century may be mentioned briefly.
• For several decades, piston engine coupled with
propellers provided the necessary power for early aircraft.
• The turbojet engines (the first jet engines) invented
independently by Sir Frank Whittle in Britain and Dr. von
Ohain in Germany powered aircraft from the early 1940s.
Such jet engines paved the way to the now, highly
sophisticated military and comfortable civil aircraft.
• During the middle of the twentieth century, airlines relied
upon low-speed subsonic aircraft whose flight speeds
were less than 400 km per hour powered by turbojet
and/or turboprop engines.

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 …..
• In the late 1960s and early 1970s, the wide-bodied aircraft
(Boeing 747, DC-10 and AirbusA300) powered by turbofan
engines flew at transonic speeds, that is, at speeds less than
950 km/hr. Even now civilian aircraft fly at the same
transonic speeds.
• On the other hand, military airplanes, fighter airplanes, for
example, fly at supersonic speeds, that is at speeds less than
2400 km/hr. Such fighter planes are fitted with turbofan
engines that have afterburners.
• X-planes, which are hypersonic, aircraft fitted with
scramjet/rocket engines, fly at speeds less than 9600 km/hr.
• Space shuttles, which also have rocket engines, fly at
hypersonic speeds of less than 28000 km/hr.

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 …..
• It is interesting to compare the flight time
between popular destinations such as Los
Angeles and Tokyo on different airplanes
- 9.6 hrs for Boeing 747,
- 6.2 hrs for Concorde, and
- only 2 hrs for hypersonic aircraft.
• It may be stated here that given humankind’s
endless ambitions, it is difficult to anticipate
the shape, speed, and the fuel of the flying
machines even for the next few decades.

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 …..
• It is a fact that evolution of aero-vehicles and aero-
propulsion are closely linked.
• Unlike the eternal question of the chicken and the
egg, there is no doubt as to which came first.
• The lightweight and powerful engine enabled
humans to design the appropriate vehicle structure
for both civil and military aircraft.
• The interdependence of the performance
characteristics of aero-vehicles (including aircraft,
missiles, airships, and balloons) and their aero-
propulsion system, the evolution of both aero-
propulsion and aero-vehicles are very interesting
indeed.
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 1.3 CLASSIFICATIONS OF AEROSPACE ENGINES
• Aerospace engines are classified into two broad
categories, namely, air-breathing or non–air-
breathing engines .
• Air-breathing engines (or airbreathers) are engines
that use the air itself, through which the vehicle is
flying, both as an oxidizer for the fuel in the
combustion chamber and as a working fluid for
generating thrust.
• Non–air-breathing engines are the rocket engines
wherein the propulsive gas originates onboard the
vehicle. Rocket engine is defined as a jet propulsion
device that produces thrust by ejecting stored
matter, called the propellant.
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….

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 1.3.1 CLASSIFICATION OF JET ENGINES
• Jet engines are further subdivided into
numerous categories. Jet engines are
subdivided into ramjet, pulsejet,
scramjet, gas turbines, turbo-ram, and
turbo-rocket engines (Figure 1).
• A brief description of each type is given
here.

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 a. RAMJET

• The ramjet engine is an aero-thermodynamic-

duct to give its full name.
• It is composed of three elements, namely, an
inlet, which is a divergent duct, a combustion
zone, and a nozzle, which is either convergent
or convergent–divergent.
• There are no rotating elements, and neither a
compressor nor a turbine. Layouts for
subsonic and supersonic ramjet are illustrated
in subsequent figures.

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…..

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 b. PULSEJET
• A pulse jet engine (or pulsejet) is a very simple form
of internal combustion engine.
• A pulse jet is similar to a ramjet, except that a series
of spring-loaded shutter-type valves (one-way
valves) is located ahead of the combustion section.
In a pulse jet, combustion is intermittent or pulsing
rather than continuous.
• Air is admitted through the valves, and combustion
is initiated, which increases the pressure, closing the
valves to prevent backflow through the inlet.
• The hot gases are expelled through the rear nozzle,
producing thrust and lowering the pressure to the
point that the valves may open and admit fresh air. 14
…..

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 c. SCRAMJET
• Scramjet is an acronym for Supersonic Combustion
Ramjet.
• The scramjet differs from the ramjet in that combustion
takes place at supersonic air velocities through the
engine.
• This allows the scramjet to achieve greater speeds than a
conventional ramjet, which slows the incoming air to
subsonic speeds before entering the combustion
chamber. Projections for the top speed of a scramjet
engine (without additional oxidizer input) vary between
Mach 12 and Mach 24 (orbital velocity).
• It is mechanically simple, but vastly more complex
aerodynamically than a jet engine. Hydrogen is normally
the fuel used. 16
…..

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 d. TURBORAMJET
• The turboramjet engine combines the turbojet engine for
speeds up to Mach 3 with the ramjet engine, which has
good performance at high Mach numbers.
• The engine is surrounded by a duct that has a variable
intake at the front and an afterburning jet pipe with a
variable nozzle at the rear.
• During takeoff and acceleration, the engine functions as a
conventional turbojet with the afterburner, while at other
flight conditions up to Mach 3, the afterburner is
inoperative. As the aircraft accelerates through Mach 3, the
turbojet is shut down and the intake air is diverted from the
compressor, by guide vanes, and ducted straight into the
afterburning jet pipe, which becomes a ramjet combustion
chamber. 18
 …..
•

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 e. TURBOROCKET
• The turborocket engine could be considered as an
alternative engine to the turbojet/ramjet.
• However, it has one major difference in that it
carries its own oxygen to provide combustion.
• The engine has a low-pressure compressor driven
by a multistage turbine; the power to drive the
turbine is derived from combustion of kerosene
and liquid oxygen in a rocket type combustion
chamber.

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…..

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 1.3.2.CLASSIFICATION OF GAS TURBINE ENGINES

• Gas turbine engines are the power plants for

all the flying aircraft/helicopters (and in this
case may be denoted as aero engines or aero
derivative gas turbines) and sources for
power in miscellaneous industrial
applications in automotive, tanks, marine
vessels, and electric power generation.
• In addition to the advantages of the jet
engines over the reciprocating engines,
vibration stresses are relieved as a result of
rotating rather than the reciprocating parts.
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 a. TURBOJET ENGINES

• Turbojet engines were the first type of jet engines

used to power aircraft as early as the1940s.
• The turbojet engine completely changed air
transportation. It greatly reduced the expense of air
travel and improved aircraft safety.
• The turbojet also allowed faster speeds, even
supersonic speeds. It had a much higher thrust per
unit weight ratio than the piston-driven engines,
which led directly to longer ranges, higher payloads,
and lower maintenance costs.
• Military fighters and fast business jets use turbojet
engines:
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 a. TURBOJET ENGINES

• Turbojet engines may be classified as single

spool or double spool.
• Each may be further subdivided into
centrifugal and axial compressor types.
• Turbojet single-spool axial compressor in
figure next slide represent compressor
employed in the single spool turbojet.
• The first turbojet engines of Frank Whittle
(W.1) and von Ohain (He S-1) incorporated
centrifugal compressors.

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a. TURBOJET ENGINES

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 a. TURBOJET ENGINES
• Turbojets also may be classified as afterburning or
nonafterburning.
• Afterburner is added to get an increased thrust.
• Afterburner is found in fighter aircraft, and is when only
absolutely necessary.
• If a pilot runs too long with the afterburner on, he or she
risks running low on fuel before the mission is completed.
• The only civil transport aircraft that is fitted with
afterburner is the Concorde.
• The turbojet engine was the most popular engine for
most high-speed aircraft, in spite of the higher fuel
consumption. When high speed and performance are
important, the cost of fuel is less important.

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a. TURBOJET ENGINES

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 b. TURBOPROP
• A turboprop engine differs from a turbojet engine in that
the design is optimized to produce rotating shaft power to
drive a propeller, instead of thrust from the exhaust gas.
• The turboprop uses a gas turbine to turn a large propeller.
• The shaft that connects the propeller to the turbine is also
linked to a gearbox that controls the propeller’s speed.
• The propeller is most efficient and quiet, when the tips
are spinning at just under supersonic speed. Moreover, no
propeller is capable of withstanding the forces generated
when it turns at the same speed of the turbine.

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 b. TURBOPROP

• Turboprop engines may be further classified

into two groups, depending on the turbine
driving the propeller.
• In the first group, the propeller is driven by
the same gas turbine driving the compressor.
• In the second group, additional turbine
(normally denoted as free power turbine)
turns the propeller. Next figure illustrates a
turboprop engine with a single turbine
driving the compressor and propeller.

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b. TURBOPROP

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 c. TURBOSHAFT

• Turboshaft engines are defined here as

engines used in powering helicopters.
• The general layout of a turboshaft is similar
to that of a turboprop, the main difference
being that the latter produces some residual
propulsion thrust to supplement that
produced by the shaft-driven propeller

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c. TURBOSHAFT

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 d. TURBOFAN ENGINES
• Turbofan engines were designed as a compromise
between the turboprop and turbojet engines.
• A turbofan engine includes a large internal propeller
(normally denoted a ducted fan) and two streams
of air flowing through the engine.
• The primary stream travels through all the
components like a turbojet engine, while the
secondary air passes through the fan and is either
ducted outside through a second nozzle identified
as the cold nozzle or may mix with the hot gases
leaving the turbine(s) and both are expelled from a
single nozzle .
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d. TURBOFAN ENGINES

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 d. TURBOFAN ENGINES
• Turbofan engines have a better performance,
greater fuel economy than turbojet at low
power setting, low speed, and low altitude.
• Turbofan engines may be classified as
– Forward fan or
– aft fan configuration
– Low or
– high BPR

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d. TURBOFAN ENGINES

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d. TURBOFAN ENGINES

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d. TURBOFAN ENGINES

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d. TURBOFAN ENGINES

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Evolution of aero engines

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Classification of aeroengines based on their speed

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