Unit 2: Classification of Aerospace Vehicles
According to :
◦ aerodynamic shape,
◦ generation of forces,
◦ application and usage.
1. Aircraft
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Aircraft is a type of flight vehicle that operates inside Earth’s
atmosphere.
As it operates inside Earth’s atmosphere, aircraft always operates
under the influence of Earth gravity field and Earth’s atmosphere
field, which causes the aircraft to experience Earth’s gravitational
pull (weight) and aerostatic/aerodynamic forces in their operation.
Thus, in order fly, an aircraft needs to generate lift force equal to
its weight (by using aerostatic forces, aerodynamic forces or
propulsion’s reaction force).
Aircraft is also usually streamline shaped in order to minimize
drag forces.
2. Spacecraft
 Spacecraft is a type of flight vehicle that operates outside Earth’s
atmosphere.
 Since it operates outside Earth, spacecraft no longer experience
aerostatic/aerodynamics force, hence, a spacecraft is not required to have
a streamlined shape (although some spacecraft is designed with
streamlined shape because some of their region of operation is inside
Earth’s atmosphere).
 In the outer space, a spacecraft is influenced by the gravity field of all space
objects, such as planets, satellite, and stars, although the effect may
become negligible after a certain distance.
 These gravity field is usually being used by the spacecraft to accelerate or
decelerate without its propulsion system.
 Aside from the space objects’ gravity field, a spacecraft is also influenced
by other environment such as electromagnetic field (from sun or other
stars), solar radiation etc.
1. Airplane (Aerodynamic Craft)
 Airplane (aerodynamic craft) is a type of aircraft that uses aerodynamics force
generated by moving airplane’s wing relative to the wind as its lifting force.
An airplane is usually characterized by a long span object called wings. These socalled wings are designed with a certain cross-section shape called airfoil, causing
wings to generate aerodynamic lift force if it moves relative to the air.
In general, airplane can be classified again based on the relative movement
between airplane’s wing and fuselage. The classification consists of fixed-wing
airplane, rotary-wing airplane and mixed-fixed-rotary-wing airplane.
a. Fixed Wing Airplane
 Fixed wing airplane is an airplane with one or more pairs of wings
commonly attached symmetrically to the left and right side of its fuselage
permanently, thus not allowing the wings any relative movement to the
fuselage.
In order to generate its lift, fixed wing airplane generate wing’s relative
movement to the air by moving the whole airplane forward.
As the consequence, a fixed wing airplane must keep moving forward with a
minimum velocity above the stall speed so that the airplane’s wing can
generate enough lift force to counteract aircraft’s weight and fly.
1. (a) Fixed Wing Airplane
b. Rotary Wing Airplane
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Rotary wing airplane is an airplane with a pair of wing and its wing axis
commonly attached to the upper section of its fuselage, thus allowing the
wing to rotate around its axes and move relative to the fuselage.
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These rotating wings are commonly called the rotors and the rotary wing
aircraft the helicopter (or Chopper in common language).
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By rotating its rotor, the helicopter is capable of generating wing’s relative
movement to the air without the need to move the whole airplane.
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Consequently, the helicopter is capable of vertical take-off, hovering in the
air and also flying in all directions .
c. Mixed Fixed-Rotary Airplane
Mixed fixed-rotary airplane is an airplane with certain configuration that
allows it to act as fixed wing airplane or rotary wing airplane at certain
condition.
b. Rotary Wing Airplane
Sarang (ALH) Helicopter
c. Mixed Fixed-Rotary Airplane
V-22 OSPREY VTOL
2. Airship (Aerostatic Craft)
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Airship (aerostatic craft) is a type of aircraft that uses buoyant force generated
from the air around it as its lifting force.
An airship is usually characterized by a big air container called envelope that
carries light-than-air air (such as hot gases or helium). These light-than-air air
produces a buoyant force that cause an airship to fly/float on the air.
In general, there are only two types of airships, which are hot air balloon and
zeppelin. The difference between hot air balloon and zeppelin lies on their
propulsion system. Hot air balloon does not have any propulsion system and
relies on wind to move, while zeppelin usually has piston powered propeller as
its propulsion. Zeppelin also has an empennage at its rear section to maintain
its stability.
3. Surface-Effect Craft
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Surface-effect craft or ground effect craft is a type of aircraft that use
aerodynamics force enhanced with surface effect as its lifting force.
Similar to airplane (aerodynamic craft), surface-effect craft is also
characterized by a long span object called wings (although its wing planform
differs from the one commonly used in the airplane).
Surface-effect craft’s wings are also designed with certain cross-section shape
(airfoils) that will generate aerodynamic lift force when it moves relative to the
air. (wing in ground effect or WIG)
However, surface-effect craft’s wings are placed very close to the ground/sea
surface so that it experience surface effect that enhances surface-effect craft’s
aerodynamics. Thus, surface-effect craft commonly flies very close to the
ground/sea surface.
4. Rocket (Propulsion-Lifted Craft)
 Rocket (propulsion-lifted craft) is a type of aircraft that uses reaction force
from its propulsion system as its lifting forces.
 A rocket is characterized by its long cylindrical shaped body with a
conical shape at its front end and a nozzle at its rear end that exerts very hot
fluids at extreme speed.
 As the reaction of the hot fluids exertion, the rockets experience forward
reaction force (as stated in the third law of Newton) which is used to lifting
force that counteracts rocket’s weight and flies the rocket itself.
A monoplane (top), biplane (middle), and tri-wing
aircraft (bottom).
1. Manned Aerial Vehicle
Manned aerial vehicle is a type of flight vehicle that is
operated by humans (pilots and/or crews) inside the
aerial vehicle itself.
2. Unmanned Aerial Vehicle (UAV)
Unmanned aerial vehicle is a type of flight vehicle that
is operated automatically by artificial ntelligences inside
the aerial vehicle or remotely by humans outside the
aerial vehicle.
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The airframe of a fixed-wing aircraft consists of the
following five major units:
 1. Fuselage
 2. Wings
 3. Stabilizers
 4. Flight control surfaces
 5. Landing gear
A rotary-wing aircraft consists of the following four
major units:
 1. Fuselage
 2. Landing gear
 3. Main rotor assembly
 4. Tail rotor assembly
Fuselage
 Holds the structure together and
accommodates passengers and/or
cargo.
Cockpit
 The command and control section of an
airplane.
Vital instruments for controlling the
airplane on the ground as well as when
flying.
Powerplant
(engines)
 Engines generate thrust and provide
hydraulic and electric power.
Wing
 Wings generate lift.
 Wing design is a crucial factor in aviation:
a wing is designed to reduce drag at the
leading edge and generate lift by its airfoil
section.
Slat
Flap
 Slats are fitted at the leading edges of the
wings, and deploying them increases the angle of
attack of the wings.
 Flaps adjust the camber of the wings, increasing
lift.
 Extending the flaps increase the camber of the
wings airfoil, thus increasing lift at lower speeds.
 Spoilers decrease lift.
 Spoilers are fitted on top of the wings, and are
used to reduce lift on a section of the wing without
Spoiler
increasing the airspeed of the airplane or without
increasing drag significantly.
 Ailerons increase or decrease lift asymmetrically,
in order to change roll and, thus, move the aircraft
left or right while flying.
Aileron  Ailerons work asymmetrically as a pair: as the
right aileron goes up, the left one comes down and
vice- versa, thus making the aircraft roll right or
left, respectively.
Horizontal
stabiliser
 The horizontal stabiliser helps maintain
an airplane's equilibrium and stability in
flight.
Elevator
 Elevators increase or decrease lift on
the horizontal stabiliser symmetrically in
order to control the pitch motion of an
airplane.
 They work symmetrically as a pair: when
the elevators are up, the aircraft
ascends; when the elevators are down,
the aircraft descends, and when the
elevators are horizontal, the aircraft flies
straight.
Vertical
stabiliser
 The vertical stabiliser prevents lateral
movements of the airplane.
 Without a vertical stabiliser, most aircraft
would lose lateral control, tend to slip,
increase drag and become uncontrollable.
Rudder
Undercarri
age
(landing
gear)
 The rudder controls the yaw motion of
an airplane. The rudder is a hinged
surface fitted to the vertical stabiliser.
 In the air, the rudder is primarily used
to coordinate left and right turns (the
turns themselves are done with the
ailerons) or to counter adverse yaw (e.g.
when crosswinds pushes the airplane
sideways).
The undercarriage, also known as landing
gear, provides a platform for the aircraft to
stand as well as plays an important obvious
role in landing and take-off.
Primary Group
 The ailerons control the rolling (or banking) motion of the aircraft.
This action is known as longitudinal control.
 The elevators control the climb or descent (pitching motion) of the
aircraft. This action is known as lateral control.
 The rudder determines the horizontal flight (turning or yawing
motion) of the aircraft. This action is known as directional control.
Secondary Group
 Trim tabs are small airfoils recessed into the trailing edges of the
primary control surface. Trim tabs let the pilot trim out an
unbalanced condition without exerting pressure on the primary
controls.
 Spring tabs are similar in appearance to trim tabs but serve an
entirely different purpose. Spring tabs are used for the same
purpose as hydraulic actuators. They aid the pilot in moving a larger
control surface, such as the ailerons and elevators.
Auxiliary Group
 WING FLAPS.—Wing flaps give the aircraft extra lift. Their
purpose is to reduce the landing speed. Reducing the landing speed
shortens the length of the landing rollout. The use of flaps during
takeoff serves to reduce the length of the takeoff run.
 SPOILERS.—Spoilers are used to decrease wing lift. In the
raised position, they greatly reduce wing lift by destroying the
smooth flow of air over the wing surface.
 SPEED BRAKES.—Speed brakes are movable control surfaces
used to keep the airspeed from building too high when the aircraft
dives. Speed brakes slow the aircraft's speed before it lands.
 SLATS.—Slats are movable control surfaces that attach to the
leading edge of the wing. When the slat is open (extended forward),
a slot is created between the slat and the wing leading edge. Highenergy air is introduced into the boundary layer over the top of the
wing. At low airspeeds, this action improves the lateral control
handling characteristics.
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The primary factors to consider in aircraft structures are strength,
weight, and reliability.
Many forces and structural stresses act on an aircraft when it is
flying and when it is static.
When it is static, the force of gravity produces weight, which is
supported by the landing gear. The landing gear absorbs the forces
imposed on the aircraft by takeoffs and landings.
During flight, any manoeuvre that causes acceleration or
deceleration increases the forces and stresses on the wings and
fuselage.
Stresses on the wings, fuselage and landing gear of aircraft are
tension, compression, shear, bending, and torsion. These
stresses are absorbed by each component of the wing structure and
transmitted to the fuselage structure.
The empennage (tail section) absorbs the same stresses and
transmits them to the fuselage.
These stresses are known as loads, and the study of loads is called
a stress analysis. All stresses are analyzed and considered when an
aircraft is designed.
TENSION : Tension is defined as pull.
 It is the stress of stretching an object or pulling at its ends. Tension is the
resistance to pulling apart or stretching produced by two forces pulling in
opposite directions along the same straight line.
 For example, an elevator control cable is in additional tension when the pilot
moves the control column.
 COMPRESSION : If forces acting on an aircraft move toward each other to
squeeze the material, the stress is called compression.
 Compression is the opposite of tension. Tension is pull and compression is
push.
 Compression is the resistance to crushing produced by two forces pushing
toward each other in the same straight line.
 For example, when an airplane is on the ground, the landing gear struts are
under a constant compression stress.
 SHEAR : Cutting a piece of paper with scissors is an example of a shearing
action.
 In an aircraft structure, shear is a stress exerted when two pieces of fastened
material tend to separate.
 Shear stress is the outcome of sliding one part over the other in
opposite directions.
 The rivets and bolts of an aircraft experience both shear and tension stresses.
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BENDING : Bending is a combination of tension and
compression.
 For example, when bending a piece of tubing, the upper
portion stretches (tension) and the lower portion crushes
together (compression).
 The wing spars of an aircraft in flight are subject to
bending stresses.
 TORSION: Torsional stress is encountered in engine
torque on turboprop aircraft.
 Engine torque tends to rotate the aircraft in the direction
opposite to the direction the propeller is turning.
 This force creates a torsional stress in the fuselage.
 Also, torsional stress on the fuselage is created by the
action of the ailerons when the aircraft is manoeuvred.
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VARYING STRESSES : All structural members of an aircraft are
subject to one or more stresses.
 Sometimes a structural member has alternate stresses; for example,
it is under compression one instant and under tension the next.
 The strength of aircraft materials must be great enough to withstand
maximum force of varying stresses.
 When an aircraft is on the ground, there is a bending force on the
fuselage. This force occurs because of the weight of the aircraft. This
bending action creates a tension stress on the lower skin of the
fuselage and a compression stress on the top skin. These stresses
are transmitted to the fuselage.
 When the aircraft is in flight, bending occurs because of the reaction
of the airflow against the wings and empennage. Lift forces act
upward against the wings, tending to bend them upward.
 The wings are prevented from folding over the fuselage by the
resisting strength of the wing structure. The bending action creates a
tension stress on the bottom of the wings and a compression stress
on the top of the wings.
An aircraft must be constructed of materials that are both light and strong.
Early aircraft were made of wood. Lightweight metal alloys with a strength
greater than wood were developed and used on later aircraft. Materials
currently used in aircraft construction are classified as either metallic materials
or nonmetallic materials.
 METALLIC MATERIALS : The most common metals used in aircraft
construction are aluminium, magnesium, titanium, steel, and their alloys.
 Alloys :
 composed of two or more metals.
 Base metal is the metal present in the largest amount.
 All other metals added to the base metal are called alloying elements.
 Adding the alloying elements result in a change in the properties of the base
metal. For example, pure aluminium is relatively soft and weak. However,
adding small amounts or copper, manganese and magnesium will increase
aluminium's strength many times.
 Heat treatment (tempering, annealing, carborising etc.) can increase or
decrease an alloy's strength and hardness.
 Alloys are important to the aircraft industry. They provide materials with
properties that pure metals do not possess.
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Aluminium :
 Aluminium alloys are valuable because they have a high strength-toweight ratio.
 Aluminium alloys are corrosion resistant and comparatively easy to
fabricate.
 The outstanding characteristic of aluminum is its lightweight.
 Magnesium :
 Magnesium is the world's lightest structural metal. It is a silvery-white
material that weighs two-thirds as much as aluminum.
 Magnesium is used to make engine gear box casings.
 Magnesium's low resistance to corrosion and high inflammability has
limited its use in conventional aircraft.
 Titanium :
 Titanium is a lightweight, strong, corrosion resistant metal.
 Titanium is ideal for applications where aluminum alloys are too weak
and stainless steel is too heavy.
 Titanium is unaffected by long exposure to seawater and marine
atmosphere.
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Steel Alloys :
 Alloy steels used in aircraft construction have great strength.
 These materials withstand the forces that occur on today's modern
aircraft.
 These steels contain small percentages of carbon, nickel,
chromium, vanadium, and molybdenum.
 High-tensile steels will stand stress of 50 to 150 tons per square
inch without failing. Such steels are made into tubes, rods, and
wires.
 Another type of steel used extensively is stainless steel. Stainless
steel resists corrosion and is particularly valuable for use in or near
water.
Transparent plastic, reinforced plastic, composite and carbon-fibre
materials.
 Transparent Plastic :
 Transparent plastic is used in canopies, windshields, and other transparent
enclosures.
 Reinforced Plastic :
 Reinforced plastic is used in the construction of radomes, wingtips,
stabilizer tips, antenna covers, and flight controls.
 Reinforced plastic has a high strength-to-weight ratio and is resistant to
mildew and rot. Because it is easy to fabricate, it is equally suitable for
other parts of the aircraft.
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Reinforced plastic is a sandwich-type material.
It is made up of two outer facings and a center
layer. The facings are made up of several layers
of glass cloth, bonded together with a liquid
resin. The core material (center layer) consists
of a honeycomb structure made of glass cloth.
Reinforced plastic is fabricated into a variety of
cell sizes.
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High-performance aircraft require an extra high strength-to-weight
ratio material. Fabrication of composite materials satisfies this
special requirement.
Composite materials are constructed by using several layers of
bonding materials (graphite epoxy or boron epoxy). These materials
are mechanically fastened to conventional substructures.
Another type of composite construction consists of thin graphite
epoxy skins bonded to an aluminum honeycomb core.
Carbon fibre is extremely strong, thin fibre made by heating
synthetic fibres, such as rayon, until charred, and then layering in
cross sections.
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The principal structural units of a fixed-wing aircraft are the fuselage,
wings, stabilizers, flight control surfaces and landing gear.
FUSELAGE
 The fuselage is the main structure, or body, of the aircraft. It provides space
for personnel, cargo, controls, and most of the accessories. The power
plant, wings, stabilizers and landing gear are attached to it.
 There are two general types of fuselage construction:
 Welded steel truss – used in smaller aircraft and some helicopters
 Monocoque designs - commonly used structure type
Monocoque Design :
 The monocoque structural design is divided into the following sub-types:
 Monocoque
 Semi-monocoque
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True monocoque construction uses formers, frame assemblies, and bulkheads
to give shape to the fuselage.
The monocoque design uses stressed skin to support almost all imposed
loads. This structure can be very strong but cannot tolerate dents or
deformation of the surface. It is similar to a coke aluminium can.
Since no bracing members are present, the skin must be strong enough to
keep the fuselage rigid. Thus, a significant problem involved in monocoque
construction is maintaining enough strength while keeping the weight within
allowable limits. Due to the limitations of the monocoque design, a semimonocoque structure is used on many of today’s aircraft.
Monocoque—A shell-like fuselage design
in which the stressed outer skin is used to
support the majority of imposed stresses.
Monocoque fuselage design may include
bulkheads but not stringers.
Semi monocoque design overcomes the strength-to-weight problem of
monocoque construction.
 In addition to having formers, frame assemblies, and bulkheads, the semi
monocoque construction has the skin reinforced by longitudinal members.
 The semi-monocoque system uses a substructure to which the
airplane’s skin is attached. The substructure, which consists of bulkheads
and/or formers of various sizes and stringers, reinforces the stressed skin
by taking some of the bending stress from the fuselage.
 The main section of the fuselage also includes wing attachment points and
a firewall (fireproof partition between the rear of the engine and the cockpit
or cabin to protect the pilot and passengers from accidental engine fires).
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Semi-Monocoque—A fuselage design that
includes a substructure of
bulkheads and/or formers, along with
stringers, to support flight loads
and stresses imposed on the fuselage.
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The semimonocoque fuselage is constructed primarily of aluminum alloy,
although steel and titanium are used in high-temperature areas.
Longerons take the primary bending loads .
The longerons are supplemented by other longitudinal members known as
stringers.
Stringers are more numerous and lightweight than longerons.
The vertical structural members are referred to as bulkheads, frames, and
formers. The heavier vertical members are located at intervals to allow for
concentrated loads. These members are also found at points where fittings
are used to attach other units, such as the wings and stabilizers.
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The welded steel truss was used in smaller aircraft, and it is still
being used in some helicopters. The truss type fuselage frame is
assembled with members forming a rigid frame e.g. beams, bars,
tubes etc. Primary members of the truss are four longerons plus
diagonal members.
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The wings are airfoils attached to each side of the fuselage and are the
main lifting surfaces that support the airplane in flight.
Airfoil—An airfoil is any surface, such as a wing, propeller, rudder, or even
a trim tab, which provides aerodynamic force when it interacts with a
moving stream of air.
The principal structural parts of the wing are spars, ribs, and stringers.
These are reinforced by trusses, I-beams, tubing, or other devices,
including the skin.
The wing ribs determine the shape and thickness of the wing (airfoil).
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In most modern airplanes, the fuel tanks either are an integral part
of the wing’s structure (wet wing), or consist of flexible containers
mounted inside of the wing.
Attached to the rear, or trailing, edges of the wings are two types
of control surfaces referred to as ailerons and flaps.
Ailerons extend from about the midpoint of each wing outward
toward the tip and move in opposite directions to create
aerodynamic forces that cause the airplane to roll.
Flaps extend outward from the fuselage to near the midpoint of
each wing. The flaps are normally flush with the wing’s surface
during cruising flight. When extended, the flaps move
simultaneously downward to increase the lifting force of the wing
for takeoffs and lift and drag during landings.
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The correct name for the tail section of an airplane is empennage. The
empennage includes the entire tail group, consisting of fixed surfaces such as
the vertical stabilizer and the horizontal stabilizer. The movable surfaces include
the rudder, the elevator, and one or more trim tabs.
The rudder is attached to the back of the vertical stabilizer. During flight, it is
used to move the airplane’s nose left and right. The rudder is used in
combination with the ailerons for turns during flight.
The elevator, which is attached to the back of the horizontal stabilizer, is used to
move the nose of the airplane up and down during flight.
Trim tabs are small, movable portions of the trailing edge of the control surface.
These movable trim tabs, which are controlled from the cockpit, reduce control
pressures. Trim tabs may be installed on the ailerons, the rudder, and/or the
elevator.
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The landing gear is the principle support of the airplane when parked, taxiing,
taking off, or when landing. The most common type of landing gear consists
of wheels, but airplanes can also be equipped with floats for water
operations, or skis for landing on snow.
The landing gear consists of three wheels—two main wheels and a third
wheel positioned either at the front or rear of the airplane. Landing gear
employing a rear-mounted wheel is referred to as tail wheel airplanes.
When the third wheel is located on the nose, it is called a nose wheel, and
the design is referred to as a tricycle gear. A steerable nose wheel or tail
wheel permits the airplane to be controlled throughout all operations while on
the ground.
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The powerplant usually includes both the engine and the propeller.
The primary function of the engine is to provide the power to turn
the propeller. It also generates electrical power, provides a vacuum
source for some flight instruments, and in most single-engine
airplanes, provides a source of heat for the pilot and passengers.
The engine is covered by a cowling, or in the case of some
airplanes, surrounded by a nacelle.
Nacelle—A streamlined enclosure on an aircraft in which an engine
is mounted.
The purpose of the cowling or nacelle is to streamline the flow of air
around the engine and to help cool the engine by ducting air around
the cylinders.
The propeller, mounted on the front of the engine, translates the
rotating force of the engine into a forward acting force called thrust
that helps move the airplane through the air.
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Most gas turbines operate on an open cycle in which air is taken
from the atmosphere, compressed in a centrifugal or axial-flow
compressor, and then fed into a combustion chamber. Here, fuel
is added and burned at an essentially constant pressure with a
portion of the air. Additional compressed air, which is bypassed
around the burning section and then mixed with the very hot
combustion gases, is required to keep the combustion chamber
exit (in effect, the turbine inlet) temperature low enough to allow
the turbine to operate continuously. If the unit is to produce
shaft power, the combustion products (mostly air) are expanded
in the turbine to atmospheric pressure. Most of the turbine
output is required to operate the compressor; only the remainder
is available to supply shaft work to a generator, pump, or other
device. In a jet engine the turbine is designed to provide just
enough output to drive the compressor and auxiliary devices.
The stream of gas then leaves the turbine at an intermediate
pressure (above local atmospheric pressure) and is fed through a
nozzle to produce thrust.
generation of forces