3, Aircraft engine.ppt

Aircraft engine

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Shaft engines

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In-line Engine

This type of engine has cylinders lined up in one row. It
typically has an even number of cylinders, but there are
instances of three- and five- cylinder engines. The biggest
advantage of an inline engine is that it allows the aircraft to
be designed with a narrow frontal area for low drag. If the
engine crankshaft is located above the cylinders, it is
called an inverted inline engine, which allows the propeller
to be mounted up high for ground clearance even with
short landing gear. The disadvantages of an inline engine
include a poor power-to-weight ratio, because the
crankcase and crankshaft are long and thus heavy. An in-
line engine may be either air cooled or liquid cooled, but
liquid-cooling is more common because it is difficult to get
enough air-flow to cool the rear cylinders directly.

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Power-to-weight ratio is a measurement of actual
performance of any engine or power sources. It is
also used a measure of performance of a vehicle
as a whole, with the engine's power output being
divided by the kerb weight of the car, to give an
idea of the vehicle's acceleration.
Curb (kerb) weight is the total weight of a vehicle
with standard equipment, all necessary operating
consumables (such as motor oil and coolant), a full
tank of fuel, and not loaded with either passengers
or cargo

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Rotary engine

Rotary engines have all the cylinders in a circle
around the crankcase like a radial engine, but
the difference is that the crankshaft is bolted to
the airframe, and the propeller is bolted to the
engine case. The entire engine rotates with the
propeller, providing plenty of airflow for cooling
regardless of the aircraft's forward speed. Some
of these engines were a two-stroke design,
giving them a high specific power and power-to-
weight ratio.

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V-Type Engine
Cylinders in this engine are arranged in two in-
line banks, tilted 30-60 degrees apart from each
other. The vast majority of V engines are water-
cooled. The V design provides a higher power-to-
weight ratio than an inline engine, while still
providing a small frontal area.

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Radial engine

This type of engine has a row of cylinders arranged in a
circle around a crankcase located in the middle. The
combination of cylinders must be an odd number in each
row and may contain more than one row. The odd
number of cylinders allows for every other cylinder to be
on a power stroke, allowing for smooth operation. A
radial engine has only one crank throw per row and a
relatively small crankcase, making the engine very light
for its power output. Because of the cylinder
arrangement, radial engines cool very evenly and run
very smoothly.

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In military aircraft designs, the large frontal area of the
engine also served as an extra layer of armor for the
pilot. This is also a radial's main disadvantage; the large
area makes the front of the aircraft very blunt and
aerodynamically inefficient. The upside-down cylinders
can also collect engine oil when the engine hasn't been
run in awhile, causing a condition known as hydraulic
lock.

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Hydrolock (short for either hydraulic lock or
hydrostatic lock) is a condition of an internal
combustion engine in which an incompressible liquid
has been introduced into its cylinder(s), resulting in the
immobilization of the engine's pistons. The liquid
causing this malfunction is often water, hence the prefix
"hydro-". Hydrolock occurs in a 4-stroke engine when
liquid is sucked into the engine's cylinder(s) during the
intake stroke and, due to the incompressibility of the
liquid, makes the compression stroke impossible. This,
in turn, prevents the entire engine from turning, and can
cause significant engine damage if one attempts to
forcibly turn over or start the engine. Typically,
connecting rods will be bent, making the engine
uneconomical to repair.

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Opposed Engine

An opposed-type engine has two banks of cylinders opposite each

other. The crankshaft is located in the center and is being driven

from both sides. The engine is either air cooled or liquid cooled,

but air cooled versions are more common, by far. Opposed

engines are mounted with the crankshaft horizontal in airplanes,

but are often mounted with the crankshaft vertical in helicopters.

The advantage of a horizontally-opposed engine over a radial is

that it allows better visibility and eliminates hydraulic lock. An

opposed engine also has a relative advantage in being mostly free

of vibration because the pistons are located left and right of the

crankshaft and act as balance weights for each other. Opposed,

air-cooled four or six cylinder piston engines are by far the most

common engines used in small general aviation aircraft requiring

up to 400 horsepower per engine. Aircraft which require more than

400 horsepower per engine tend to be powered by turbine

engines. 15

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Turboprop

While military fighters require very high speeds, many 17
civil airplanes do not. Yet, civil aircraft designers
wanted to benefit from the high power and low
maintenance that a gas turbine engine offered. Thus
was born the idea to mate a turbine engine to a
traditional propeller. Because gas turbines optimally
spin at high speed, a turboprop features a gearbox to
lower the speed of the shaft so that the propeller tips
don't reach supersonic speeds. Often the turbines
which drive the propeller are separate from the rest of
the rotating components so that they are free to rotate
at their own best speed (referred to as a free-turbine
engine). A turboprop is very efficient when operated
within the realm of cruise speeds it was designed for,
which is typically 200 to 400 MPH.

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Turboshaft

Turboshafts are used primarily for helicopters and
auxiliary power units. A turboshaft engine is very similar
to a turboprop, with a key difference: In a turboprop the
propeller is supported by the engine, and the engine is
bolted to the airframe. In a turboshaft, the engine does
not provide any direct physical support to the helicopter's
rotors. The rotor is connected to a transmission, which
itself is bolted to the airframe, and the turboshaft engine
simply feeds the transmission via a rotating shaft. The
distinction is seen by some as a slim one, as in some
cases aircraft companies make both turboprop and
turboshaft engines based on the same design.

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Jet engines

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Jet engines
The key part of a jet engine is the exhaust nozzle. This is the
part which produces thrust for the jet; the hot airflow from
the engine is accelerated when exiting the nozzle, creating
thrust, which, in conjunction with the pressures acting inside
the engine which are maintained and increased by the
constriction of the nozzle, pushes the aircraft forward

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Turbojet
A turbojet is a type of gas turbine engine that was
originally developed for military fighters during World
War II. A turbojet is the simplest of all aircraft gas
turbines. It features a compressor to draw air in and
compress it, a combustion section which adds fuel
and ignites it, one or more turbines that extract power
from the expanding exhaust gases to drive the
compressor, and an exhaust nozzle which accelerates
the exhaust out the back of the engine to create
thrust. When turbojets were introduced, the top speed
of fighter aircraft equipped with them was at least 100
miles per hour faster than competing piston-driven
aircraft.

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The relative simplicity of turbojet designs lent
themselves to massive wartime production, but the war
ended before any turbojets could be mass-produced. In
the years after the war, the drawbacks of the turbojet
gradually became apparent. Below about Mach 2,
turbojets are very fuel inefficient and create
tremendous amounts of noise. The early designs also
respond very slowly to power changes, a fact which
killed many experienced pilots when they attempted to
transition to jets. These drawbacks eventually led to the
downfall of the pure turbojet, and only a handful of
types are still in production.

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A gas turbine is a rotary machine, similar in principle to a
steam turbine. It consists of three main components - a
compressor, a combustion chamber and a turbine. The air
after being compressed into the compressor is heated
either by directly burning fuel in it or by burning fuel
externally in a heat exchanger. The heated air with or
without products of combustion is expanded in a turbine
resulting in work output, a substantial part, about two-thirds,
of which is used to drive the compressor. The rest, about
one-third, is available as useful work output.

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Turbofan
A turbofan engine is much the same as a turbojet, but
with an enlarged fan at the front which provides thrust in
much the same way as a propeller. A turbofan has extra
turbine stages to turn the fan. Thus, more power is
extracted from the exhaust gases before they leave the
engine. This operation is a more efficient way to provide
thrust than the jet nozzle alone, resulting in improved
fuel-efficiency. Turbofans were the first engines to use
multiple spools; concentric shafts which are free to
rotate at their own speed; in order to allow the engine to
react more quickly to changing power requirements.
Although the fan creates thrust like a propeller, the
surrounding duct frees it from many of the restrictions
which limit propeller performance.

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Turbofans are more efficient than propellers in the
trans-sonic range of aircraft speeds, and can operate in
the supersonic realm. Turbofans are coarsely split into
low-bypass and high-bypass categories. Bypass air
flows through the fan, but around the jet core, not
mixing with fuel and burning. The ratio of this air to the
amount of air flowing through the engine core is the
bypass ratio. Low-bypass engines are preferred for
military applications such as fighters due to high thrust-
to-weight ratio, while high-bypass engines are preferred
for civil use for good fuel efficiency and low noise. High-
bypass turbofans are usually most efficient when the
aircraft is traveling at 500 to 550 miles per hour (800 to
885 KM/h), the cruise speed of most large airliners. Low-
bypass turbofans can reach supersonic speeds, though
normally only when fitted with afterburners.

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The term supersonic is used to define a speed that is
over the speed of sound (Mach 1). At a typical temperature
like 21°C (70 °F), the threshold value required for an
object to be traveling at a supersonic speed is
approximately 344 m/s, (1,129 ft/s, 770 mph or 1,238
km/h). Speeds greater than 5 times the speed of sound
are often referred to as hypersonic. Speeds where only
some parts of the air around an object (such as the ends
of rotor blades) reach supersonic speeds are labeled
transonic (typically somewhere between Mach 0.8 and
Mach 1.2).

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An afterburner (or reheat) is an additional component added to
some jet engines, primarily those on military supersonic aircraft.
Its purpose is to provide a temporary increase in thrust, both for
supersonic flight and for takeoff (as the high wing loading
typical of supersonic aircraft designs means that take-off speed
is very high). On military aircraft the extra thrust is also useful
for combat situations. This is achieved by injecting additional
fuel into the jet pipe downstream of (i.e. after) the turbine. This
fuel is ignited by the hot exhaust gases and adds greatly to the
thrust of the engine. The advantage of afterburning is
significantly increased thrust; the disadvantage is its very high
fuel consumption and inefficiency, though this is often regarded
as acceptable for the short periods during which it is usually
used.

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TYPES & HOW JET ENGINE WORK

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Aircraft maintenance is the technology related to the
actions required to maintain (or improve) the airworthiness
and the designed-in reliability of an aircraft and its systems,
subsystems, and components throughout the life-cycle of
the aircraft.

Airworthiness is a term used to dictate whether an
aircraft is worthy of safe flight. It is illegal in most
countries to fly an aircraft without first obtaining an
airworthiness certificate from the responsible government
agency. The airworthiness usually must be maintained by
a program of inspections by an authorized Aircraft
Maintenance Technician, typically performed annually, or

after a fixed elapsed flight time, such as every 100 hours.

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Airworthiness is a term used to dictate whether an aircraft
is worthy of safe flight. It is illegal in most countries to fly an
aircraft without first obtaining an airworthiness certificate
from the responsible government agency. The airworthiness
usually must be maintained by a program of inspections by
an authorized Aircraft Maintenance Technician, typically
performed annually, or after a fixed elapsed flight time, such
as every 100 hours.

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Aircraft maintenance checks are periodic checks that
have to be done on all aircraft after a certain amount of
time or usage. Airlines casually refer to these checks as
one of the following: A check, B check, C check, or D
check. A and B checks are lighter checks, while C and D
are considered heavier checks.

A Check
This is performed approximately every month. This check
is usually done overnight at an airport gate. The actual
occurrence of this check varies by aircraft type, the cycle
count (takeoff and landing is considered an aircraft
"cycle"), or the number of hours flown since the last
check. The occurrence can be delayed by the airline if
certain predetermined conditions are met.

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B Check
This is performed approximately every 3 months. This
check is also usually done overnight at an airport gate. A
similar occurrence schedule applies to the B check as to
the A check.

C Check
This is performed approximately every 12-18 months. This
maintenance check puts the aircraft out of service and
requires plenty of space - usually at a hangar at a
maintenance base. The schedule of occurrence has many
factors and components as has been described, and thus
varies by aircraft category and type.

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D Check
This is the heaviest check for the airplane. This check
occurs approximately every 4-5 years. This is the check
that, more or less, takes the entire airplane apart for
inspection. This requires even more space and time than
all other checks, and must be performed at a maintenance
base. Often, older aircraft being phased out of a particular
airlines' fleet are stored or scrapped upon reaching their
next D Check, due to the considerable cost.

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