AIRCRAFT ENGINE ASSEMBLY AND TESTING

Content:

1. Introduction to Air craft engine

2. Aero engine description

3. Aero engine manufacture, Assembly, overhaul and testing.

4. Manufacturing strategy and processes.

5. Forging

6. Casting

7. Fabrication

8. Welding

9. Electro chemical machining

10.Electro discharge machining

11.Composite materials and sandwich casing

12.Special Manufacturing process, laser machining, robotics, Plasma

spray.

13.Cellular Manufacturing.

14.Quality assurance

15.Engine Assembly.

16. Engine Testing and dispatch

1. Introduction to Air craft engine 1.0 Jet engine development:

The development of the gas turbine engine as an aircraft power plant has been so rapid that it is difficult to appreciate that prior to the 1950s very few people had heard of this

method of aircraft propulsion.

Development of aircraft engine can not be credited to any individual. Sir whittle of

England, Hanse Van Ohoin and Max muller of Germany and Secondo Campini of Iatly

had got some success in their effort to develop the aircraft engine. In fact in 1930 Whittle

had tried to patent the engine, but did not succeeded in his effort. Finally he had

developed first working jet engine in 1937. The whittle engine formed the basis of the

modern gas turbine engine and from it many engines were developed such as Welland,

Derwent, Nene and Dart.

The Derwent and Nene turbo jet engines had world wide military application. The Dart

turbo propeller engine became world famous as the power plant for Vickers Viscount

aircraft.

Although other aircraft may be fitted with latter engines termed as twin spool, triple

spool, bypass, ducted fan, un ducted fan and profane, these are inevitable development of

Whittle early engine.

1.1 Jet engine theory:

Centuries ago in 100 A.D., Hero, a Greek philosopher

and mathematician, demonstrated jet power in a machine called an "aeolipile." A heated,

water filled steel ball with nozzles spun as steam escaped.

Over the course of the past half a century, jet-powered flight has vastly changed the way

we all live. However, the basic principle of jet propulsion is neither new nor complicated.

Centuries ago in 100 A.D., Hero, a Greek philosopher and mathematician, demonstrated

jet power in a machine called an "aeolipile." A heated, water filled steel ball with nozzles

spun as steam escaped. Why? The principle behind this phenomenon was not fully

understood until 1690 A.D. when Sir Isaac Newton in England formulated the principle

of Hero's jet propulsion "aeolipile" in scientific terms. His Third Law of Motion stated:

"Every action produces a reaction ... equal in force and opposite in direction."

The jet engine of today operates according to this same basic principle. Jet engines

contain three common components: the compressor, the combustor, and the turbine. To

this basic engine, other components may be added, including:

A nozzle to recover and direct the gas energy and possibly divert the thrust for

vertical takeoff and landing as well as changing direction of aircraft.

An afterburner or augmentor, a long "tailpipe" behind the turbine into which

additional fuel is sprayed and burned to provide additional thrust.

A thrust reverser, which blocks the gas rushing toward the rear of the engine,

thus forcing the gases forward to provide additional braking of aircraft.

A fan in front of the compressor to increase thrust and reduce fuel consumption.

An additional turbine that can be utilized to drive a propeller or helicopter

rotor.

1.2 Aircraft engine type:

Constant demand for the greater efficiency, economy and quieter engines has produced a

numbers of variations of basic jet engines. Some variations of the engine are described

below.

The turbofan engine:

A high bypass turbofan

engine.

A turbofan engine is

basically a turbojet to which a fan has been added. Large fans can be placed at either the

front or rear of the engine to create high bypass ratios for subsonic flight. In the case of a

front fan, the fan is driven by a second turbine, located behind the primary turbine that

drives the main compressor. The fan causes more air to flow around (bypass) the engine.

This produces greater thrust and reduces specific fuel consumption.

For supersonic flight, a low bypass fan is utilized, and reheat is added for additional

thrust.

An ultra high bypass jet engine.

A logical approach to improving fuel consumption is even higher bypass technology.

Mechanical arrangements can vary. During the 1980s, GE developed the Unducted Fan

UDF engine which eliminated the need for a gearbox to drive a large fan. The jet

exhaust drives two counter-rotating turbines that are directly coupled to the fan blades.

These large span fan blades, made of composite materials, have variable pitch to provide

the proper blade angle of attack to meet varying aircraft speed and power requirements.

Power plants such as the UDF engine are capable of reducing specific fuel consumption

20-30 percent below current subsonic turbofans.

The Turbojet Engine

Turbo jet engine.

The turbojet is the basic engine of the jet age. Air is drawn into the engine through the

front intake. The compressor squeezes the air too many times normal atmospheric

pressure and forces it into the combustor. Here, fuel is sprayed into the compressed air, is

ignited and burned continuously like a blowtorch. The burning gases expand rapidly

rearward and pass through the turbine. The turbine extracts energy from the expanding

gases to drive the compressor, which intakes more air. After leaving the turbine, the hot

gases exit at the rear of the engine, giving the aircraft its forward push ... action, reaction!

For additional thrust or power, an afterburner or augmentor can be added. Additional fuel

is introduced into the hot exhaust and burned with a resultant increase of up to 50 percent

in engine thrust by way of even higher velocity and more push.

The Turboprop/Turbo shaft Engine

A turboprop or turbo shaft engine.

A turboprop engine uses shaft power to turn a propeller and the thrust is produced by the

propeller. As in a turbojet, hot gases flowing through the engine rotate a turbine wheel

that drives the compressor. The gases then pass through another turbine, called a power

turbine. This power turbine is coupled to the shaft, which drives the propeller through

gear connections.

A turbo shaft is similar to a turboprop engine, differing primarily in the function of the

turbine shaft. Instead of driving a propeller, the turbine shaft is connected to a

transmission system that drives helicopter rotor blades; electrical generators, compressors

and pumps; and marine propulsion drives for naval vessels, cargo ships, high speed

passenger ships, hydrofoils and other vessels.

2.0 Aero engine description

First few engines used to power the initial aircrafts were piston engines. The principle of

working of these engines is similar to the automobile engines. These engines are latter on

replaced by Jet engines for most of the applications because of its limitations and high

vibrations. A piston engine is depicted in the below picture.

Typical piston engines

The application of gas turbine engine for the aircraft propulsion has overcome the

inherent weakness of the piston engines. Some of the Gas turbine engines and their

applications are depicted below

Turbo Shaft gas turbine engine

As the gas turbines are not self starter some kind of starter is required to start the main

engine. Specially designed an air turbine starter to start the main engine is depicted

below.

Typical gas turbine starter

The gas turbine starter engine stars the main gas turbine engine. Main engine provides

power for the propulsion of the main aircraft. The picture below depicts the single gas

turbine engine power aircraft.

Typical jet powered aircraft

Gas turbine engine is utilized to power UN manned aircrafts also. A typical engine for

this application and an unmanned aircraft is depicted in below pictures.

Typical turbojet engine

Un manned Aircraft

A gas turbine engine is essentially a heat engine using air as a working fluid to provide

thrust. Cross section view of a typical gas turbine engine is shown below.

Cross sectional view of a typical gas turbine engine

Typical Exploded view of Aero engine

Exploded view of modern engine shown in the above Fig. Description of the major

engine sub assemblies is given below.

Air Intake

The air intake reduces the velocity and increases the pressure of air entering to a

level suitable for operation of the compressor. The flow of air into the compressor should

be free of turbulence to obtain maximum operating efficiency. The air intake does this

function with minimum energy loses. The air intake is divergent in order to transform the

kinetic energy of the air into the pressure. Typical views of the air intakes are shown.

.

Compressor

The function of a compressor is to increase the pressure of incoming air so that

the combustion process could be done effectively. There are two methods by which the

compression is effected, basically classified with regards to the direction of flow of air.

The centrifugal compressor consists of an impeller and a diffuser. Air enters the

compressor at the centre of the impeller and is then compressed by the rotational motion

of the impeller. Thus the rotational velocity of air is increased. This increase in velocity is

converted to increase in pressure through the diffuser. In this type of compressor, air flow

takes a full 90 before entering the combustion chamber.

The axial flow compressor consists of a series of rotating rotor blades followed by a

stationary row of stator blades, whose combination is called a stage. An axial compressor

usually has many numbers of such stages to achieve the desired pressure increment.

Basically, the velocity of air is increased through rotor blades and pressure is increased

when the high velocity air pass through the stator blades. This happens subsequently so

that a high outlet pressure from the compressor is achieved. In this type of compressor,

air flows parallel to the axis of the engine. To obtain a higher operational flexibility, the

compressor may consist of two or more rotating assemblies each rotating at its optimum

speeds.

Centrifugal Impeller

Modern high performance engines invariably utilize the axial flow compressor,

and many small engines use the centrifugal flow compressor.

Combustion Chamber

The function of a combustion chamber or combustor is to burn the fuel air

mixture and to direct the products of combustion onto the turbine. Air enters the

combustion chamber at a very high pressure and high velocity. The velocity of air is

slightly slowed down before entering the combustion chamber so that a stable

combustion may take place. Otherwise the flame will be blown away by air. Usually, a

very small proportion air is actually burnt inside the combustion chamber; the remaining

air is used only for cooling purposes. Different types of combustion chamber are

designed. A typical combustion chamber is shown below.

Turbine

The hot gases from the combustion chamber flow through the turbine assembly.

The turbine extracts kinetic energy from these gases and converts into mechanical energy

to drive the compressor. Similar to the construction of an axial flow compressor, the

turbine consists of a series of stationary stator blades and rotating rotor blades. The

number of such stages to be employed depends on the amount of power to be extracted.

The turbines operate in a torturous environment and the materials from which they are

fabricated should be able to withstand high temperatures and severe stresses.

Section view of a typical gas turbine is shown below.

Pictures of some of the turbine parts shown below.

Typical Stator

Typical turbine rotors

Gas generator rotor

Nozzle guide vane of a gas turbine engine

Fuel system

The function of the fuel system are to provide the engine with fuel in a form suitable for combustion and control the flow to the required quantity necessary for easy

starting, acceleration and stable running at all engine operating conditions. To do this

one or more fuel pumps are used to deliver the fuel to the fuel spray nozzle, which

injects into the combustion chamber in form of an atomized spray. A view of typical

fuel system is shown below.

Fuel pump assy.

Oil SystemThe lubrication system is required to provide lubrication and cooling for all gears,

bearings and splines. A view of the lube system is shown below.

Exhaust System: The purpose of a nozzle is to expand the gases to the atmosphere and in

doing so give a final velocity impetus to the gases. The hot gases flowing from the

turbine are straightened in an exhaust pipe. It then expands to the atmosphere through the

nozzle. The nozzle may be shaped like a simple converging cone for low speed aircraft.

Its shape differs to a converging then diverging cone for high speed aircraft. This is done

for the proper expansion of gases. A typical view of exhaust system is shown below.

These are the major components of a gas turbine engine. Gas turbine engines are

comprised of several other important components. Any number of variations in or

arrangements of these components are possible. But the intention is always the same

to achieve the required thrust.

3. Aero engine manufacture, Assembly, overhaul &testing.During the design stage of the aircraft gas turbine engine, close liaison is required to

be maintained between design, manufacturing, development and product support to

ensure that the final design is a mach between the engineering specification and the

manufacturing process capability.

The functioning of jet engines with its high power to weight ratio, demand the

highest possible performance from the each components. Consistence with these

requirements, each components must be manufactured at lowest possible weight and

cost and also provide mechanical integrity through a long service life.

Consequently, the methods used during the manufacture are diverse and are usually

determined by the duties each component has to full fill.

No manufacturing technique or process that in any way offers an advantage is

ignored and most available engineering methods and processes are employed in the

manufacture of these engines. In some instances, the technique or process may appear

by some standards to be elaborate, time consuming and expensive, but is only

adopted after confirmation that it does produce maximized component lives

comparable with test rig achievements.

Engine components are produced from a variety of materials like Aluminium

alloy, Magnesium alloy, high tensile steel and high temperature nickel and cobalt

alloy. Forgings and castings are used to manufacture various engine components. A

proportion of components are cast using the investment casting process. Whilst

fabrications, which form an increasing content, are produced from materials such as

stainless steel, titanium and nickel alloys using modern joining techniques i.e.,

tungsten inert gas welding, resistance welding, electron beam welding and high

temperature brazing in vacuum furnaces.

The methods of machining engine components include grinding, turning,

drilling, boring and broaching whenever possible, with the more difficult materials

and configurations being machined by electro-discharge, electro-chemical, laser hole

drilling and chemical size reduction.

Structural components i.e., cold spoiler, location rings and by-pass ducts,

benefit by considerable weight saving when using composite materials.

In addition to the many manufacturing methods, chemical and thermal professes are used

on part finished and finished components. These include heat treatment, electro-plating,

chromate sealing, chemical treatments, anodizing to prevent corrosion, chemical

treatments, chemical cleaning and mechanical cleaning, wet and dry abrasive blasting,

polishing, plasma spraying, electrolytic etching and polishing to reveal metallurgical

defects. Also a variety of barreling techniques for removal of burrs and surface

improvement are used for the finishing of the components. Most processes are concerned

with surface changes, some give resistance to corrosion whilst others can be used to

release unwanted stress.

The main structure of an aero gas turbine engine is formed by a number of circular

casings, which are assembled and secured together by flanged joints and couplings

located with dowels and tenons. These engines use curvic and hurth couplings to enable

accurate concentricity of mating assemblies which in turn assist an airline operator when

maintenance is required.

4.0 Manufacturing strategy and processes.

Manufacturing technology is changing and will continue to change to meet the

increasing demands of aero-engine components for fuel efficiency, cost and weight

reductions and being able to process the materials required to meet these demands.

With the advent of micro-processors and computers, full automation of components

considered for in house manufacture are implemented and all other components being

procured from the world-wide supplier network.

This automation is already applied in the manufacture of cast turbine blades with

computer numerical controlled (C.N.C) grinding centers, laser hard facing and film

cooling hole drilling by electro-discharge machining (E.D.M). Families of turbine and

compressor discs are produced in flexible manufacturing cells, employing automated

guided vehicles delivering palletized components from computerized storage to

C.N.C machining cells that all use batch of one techniques, the smaller blades, with

very thin airfoil sections, are produced by integrated broaching and 360 degree

electro-chemical machine (E.C.M) while inspection and processing are being

automated using the computer.

Typical CNC shop

Tolerances between design and manufacturing are much closer when the design

specification is matched by the manufacturing proven capability.

Computer Aided Design (C.A.D) and Computer Aided Manufacturing (C.A.M)

provides an equivalent link when engine components designed by C.A.D can be used

for the preparation of manufacturing drawings, programmes for numerically

controlled machines, tool layouts, tool designs, operation sequence, estimating and

scheduling. Computer simulation allows potential cell and flow line manufacture to

be proven before physical machine purchase and operation, thus preventing

equipment not fulfilling their intended purpose.

Each casing is manufactured from the lightest material commensurate with the

stress and temperatures to which it is subjected to in service. For example,

magnesium alloy, composites and materials of sandwich construction are used for air

intake casings, since these are the coolest parts of the engine. Nickel based alloy

steels are used for the turbine and nozzle. For casings subjected to intermediate

temperature i.e., by-pass duct and combustion outer casings, aluminum alloys and

titanium alloys are used.

Major processes used for the manufacturing of the aero engine components are

explained in below paragraphs.

5. Forging:The engine drive shafts, compressor discs, turbine discs and gear trains are forged

to as near optimum shape as is practicable. Compressor blades with thin airfoil

sections with varying degrees of camber and twist are forged in a variety of alloys.

Nevertheless precision forging of these blades is a recognized practice and enables

one to be produced from a shaped die with the minimum of further work.

The high operating temperatures at which the turbine discs must operate

necessitates the use of nickel base alloys. The compressor discs at the front end are

produced from titanium. The higher strength of Titanium at the moderate operating

temperatures together with its lower weight provides considerable advantage over

steel.

Forging calls for a very close control of the temperature during the various

operations, an exceptionally high standard of furnace control equipment, careful

maintenance and cleanliness of the forging hammers, presses and dies.

Annular combustion rings can be cold forged to exacting tolerances and surfaces

which alleviates the need for further machining before welding together to produce

the combusting casing.

H.P. Compressor casings of the gas turbine engine are forged as rings or half rings

which, when assembled together, form the rigid structure of the engine. They are

produced in various materials, i.e., stainless steel, titanium and nickel alloys.

6. CASTING:

An increased percentage of the gas turbine engine is produced from cast components

using sand casting. Typical example of die and investment casting technique is shown

below.

Investment casting is becoming the most acceptable technique in use because of its

capability to produce components with surface that require no further machining. It is

essential that all the castings are defect free by discipline of cleanliness during

process of casting otherwise they could cause component failure.

All castings are inspected/tested for correct chemical composition and mechanical

properties and are subjected to radiological and microscopic examinations to make

sure that the castings are defects free.

The complexity of configurations together with tight tolerances in size and

surface finish is totally dependent upon close liaison with design, manufacturing,

metallurgist, chemist, die maker, furnace operator and final caster.

In the pursuit of ever increasing performance, turbine blades are produced from

high temperature nickel alloys that are cast by the investment casting or lost wax

technique. Directionally solidified and single crystal turbine blades are cast using this

technique in order to extend their cyclic lives.

Automatic casting is used in the production of equi-axed, directionally solidified

and single crystal turbine blades. The lost wax process is unparallel in its ability to

provide the highest standards of surface finish, repeatable accuracy and surface detail

in a cast component. The increasing demands of the engine have manifested itself in

the need to limit grain boundaries and provide complex internal passages. The moulds

used for directionally solidified and single crystal castings differ from conventional

moulds in that they are open at both ends; the base of a mould forms a socketed

bayonet fitting into which a chill plate is located during casting. Metal is introduced

from the central sprue into the mould cavities via a ceramic filter. These and

orientated seed crystals, if required, are assembled with the patterns prior to

investment. Extensive automation is possible to ensure the wax patterns are coated

with shell material consistently by using robots. The final casting can also have their

rises removed using elastic cut-off wheels driven from robot arms.

Investment casting

7. FabricationMajor components of the gas turbine engine i.e, bearing housings, combustion

and turbine casings, exhaust units, jet pipes, by-pass mixer units and low pressure

compressor casings can be produced as fabricated assemblies using sheet materials

such as stainless steel, titanium and varying types of nickel alloys.

Other fabrication techniques for the manufacture of the low pressure compressor

wide chord fan blade dies, hot twisted in a furnace and finally hot creep formed to

achieve the necessary configuration. Chemical milling is used to recess the centre of

each panel which sandwiches a honeycomb are finally joined together using

automated furnaces where an activated diffusion bonding process takes place.

Typical CNC Machine

8. WeldingWelding processes are used extensively in the fabrication of gas turbine engine

components i.e., resistance welding by spot and seam, tungsten inert gas and electron

beam are amongst the most widely used ones today. Care has to be taken to limit the

distortion and shrinkage associated with these techniques.

Tungsten inert gas (T.I.G) welding

The most common form of tungsten inert gas welding in use is the direct current

straight polarity i.e. electrode negative pole. This is widely used and the most

economical method of producing high quality welds for the range of high

strength/high temperature materials used in gas turbine engines. For this class of

work, high purity argon shielding gas is fed to both sides of the weld and the welding

torch nozzle is fitted with a gas lens to ensure maximum efficiency for shielding gas

coverage. A consumable four percent throated tungsten electrode, together with a

suitable non-contact method of arc starting is used and the weld current is reduced in

a controlled manner at the end of each weld to prevent the formation of finishing

cracks. All welds are visually and penetrates inspected and in addition, welds

associate with rotating parts i.e. compressor and/or turbine are radio logically

examined to quality acceptance standards. During welding operations and to aid in

the control of distortion and shrinkage the use of an expanding fixture is

recommended and whenever possible, mechanized welding is employed together with

the pulsed arc technique

A typical T.I.G welding operation is illustrated below.

Electron beam welding (E.B.W)

This system, which can use either low or high voltage, uses a high power density

beam of electrons to join a wide range of different materials and of varying thickness.

The welding machine comprises an electron gun, optical viewing system, work

chamber and handling equipment, vacuum pumping system, high or low voltage

power supply and operating controls. Many major rotating assemblies for gas turbine

engines are manufactured as single items in steel, titanium and nickel alloys and

joined together i.e., intermediate and high pressure compressor drums. This technique

allows design flexibility in that distortion and shrinkage are reduced and dissimilar

materials, to serve quite different functions, can be homogenously joined together.

For example, the H.P turbine stub shafts requiring a stable bearing steel welded to a

material which can expand with the mating turbine disc. Automation has been

enhanced by the application of computer numerical control to the work handling and

manipulation. Seam tracking ensures that the joint is accurately followed and close

loop under bead control guarantees that the full depth of material thickness is welded.

Electron beam welding machine

9. Electro-chemical machining (E.C.M)This type of machining employs both electrical and chemical effects in the removal of

metal. Chemical forming, electro-chemical drilling and electrolytic grinding are

techniques of electro-chemical machining employed in the production of gas turbine

engine components.

The principle is that when current flow between the electrodes immersed in a solution of

salts, chemical reactions occurs in which metallic ions are transported form one electrode

to another.

In chemical forming the tool electrode (the cathode) and the work piece (the anode) are

connected into a direct current circuit. Electrolytic solution passes, under pressure,

through the tool by electrolytic action. A hydraulic ram advances the tool electrodes into

the work piece to from the desired passage.

Electrolytic grinding employs a conductive wheel impregnated with abrasive particles.

The wheel is rotated close to the surface of the work piece, in such a way that the actual

metal removal is achieved by electro-chemical means. The by-products, which would

inhibit the process, are removed by the sharp particles embodied in the wheel.

Stem drilling and capillary drilling techniques are used principally in the drilling of small

holes, usually cooling holes, such as required when producing turbine blades.

Stem drilling

This process consist of tubes (cathode) produced from titanium and suitably insulated to

ensure a reaction at the tip. A twenty percent solution of nitric acid is fed under pressure

onto the blade prodding holes generally in therein of 0.026 in. diameter. The process is

more speedy in operation that electro-discharge machining and is capable of drilling

holes up to a depth two hundred times the diameter of the tube in use.

Capillary drilling

Similar in process to stem drilling but using tubes produced form glass incorporating a

core for platinum wire (cathode). A twenty percent nitric acid solution is passed

throughthe tube onto the workpiece and is capable of producing holes as small as 0.009

in. diameter. Depth of the hole is up to forty times greater than the tube in use and

therefore determined by tube diameter.

Automation has also been added to the process of electro-chemical machining with the

introduction of 360 degree E.C machining of small compressor blades. For some blades

of shorter length airfoil, this technique is cost effective than the forged finished shaped

airfoil. Blades produced by E.C.M employ integrated vertical broaching machines for

broaching the blade root feature, such as a fir-tree, and then by using this as the location,

electro chemical machining from both sides is done to produce the thin airfoil section in

one operation.

10. Electro-Discharge Machining (E.D.M)This type of machining removes metal from the workpiece by converting the kinetic

energy of electric sparks into heat as the sparks strike the workpiece.

An electric spark results when an electric potential between two conducting surfaces

reaches the point at which the accumulation of electrons has acquired sufficient energy to

bridge the gap between the two surfaces and complete the circuit. At this point, electrons

break through the dielectric medium between the conducting surfaces and moving from

negative (the tool electrode) to positive (the workpiece), strike the latter surface with

great energy.

When the sparks strike the workpiece the heat is so intense that the metal to be removed

is instantaneously vaporized with explosive results. Away from the actual centre of the

explosion, the metal is torn into fragments which may themselves be melted by the

intense heat. The dielectric medium, usually paraffin oil, pumped into the gap between

the tool electrode and the workpiece, has the tendency to quench the explosion and to

wipe away metallic vapor and molten particles.

The amount of work that can be affected in the system is a function of the energy of the

individual sparks and the frequency at which they occur.

The shape of the tool electrode is a mirror image of the passage to be machined in the

workpiece and to maintain a constant work gap, the electrode is fed into the workpiece as

erosion is effected.

11. Composite Materials And Sandwich Casings

High power to weight ratio and low component costs are very important considerations in

the design of any aircraft gas turbine engine, but when the function of such an engine is

to support a vertical take-off aircraft during transition, or as an auxiliary power unit, then

the power to weight ratio becomes extremely critical.

In such engines, the advantage of composite materials allows the designer to produce

structures in which directional strengths can be varied by directional lay-up of fibers

according to the applied loads.

Composite materials have and will continue to replace casings which in previous

generation of engines, would have been produced in steel or titanium. By-pass duct

assemblies comprising of three casings are currently being produced up to 4ft-7in in

diameter and 2ft-0in in length using pr-cured composite materials for the casing fabric.

Flanges and mounting bosses are added during the manufacturing process, which are then

drilled for both location and machined for peripheral feature attachment on C.N.C

machining centers. Conventional cast and fabricated casings and cowlings are also being

replaced by casings of sandwich construction which provide strength allied with lightness

and also act as noise suppression medium. Sandwiched construction casings comprise

honeycomb structure of Aluminium or stainless steel interposed between layers of

dissimilar material. The materials employed depend upon the environment in which they

are used.

12. Special Purpose manufacturing process, Manufacturing of aero engine components and assembly require high degree of

accuracy and quality special purpose machines are used. A few are described below.

Typical automated manufacturing of compressor blades

Because of high degree rotational speed accurate balancing of rotating assembly is

necessary. A typical dynamic balancing machine for indicating the magnitude and

angular position of the unbalance in each plane is shown below

Balancin

g machine

Curvic grinding machine

Nozzle guide vane of an engine

Special machine for NGV grinding

Typical Specification of

Cnc 7 Axes Nozzle Guide Vane Grinding Machine

Control system: - sinumeric 840 d with 611 digital servo drives.

Type: - mgc 130.65.65

X- axis stroke 1300 mm (longitudinal)

Y- axis stroke 650 mm (cross)

Z- axis stroke 650 mm (vertical)

Two axes (a&b) nc rotary table

Y-axis wheel head mounted dressing device.

W-axis-automaticall compensated waterspout axis

Grinding spindle drive power 50 kw

Grinding spindle drive speed stepless variable upto 63m/s

(peripheral speed)

Grinding wheel diameter: --- 450 mm

bore : --- 127 mm

Automatic diamond roll dressing equipment: -

(fully integrated with cnc)

Rroll diameter 160 mm

Length of roll 167 mm

Bore diameter 52mm

Travel distance 115 mm

Automatic grinding wheel balancing device:

Felsomat robotized loading system with three axes:

y, z and c (rotary)

Felsomat indexing table with 4 mecatool pallets station.

Typical view of plasma arc spray

13. Cellular ManufacturingTraditionally, manufacturing systems have been segregated into two categories based on

their physical layout. The first category is the line (product) layout where the machines

are organized in a serial manner to process a single type of part or a very limited family

of parts. The second category is the functional layout (process or job shop) where the

machines are organized into groups according to capabilities.

A third category for the physical distribution of machines in a manufacturing

facility is cellular manufacturing. Cellular manufacturing is a subset and derivative of

group technology. Group technology can be defined as the bringing together and

organizing of common concepts, principles, problems and tasks (technology) to improve

productivity. Productivity can be defined in a multitude ways, but generally is thought of

as being an increase in output pr unit of production time or a decrease in cost per unit

produced.

Cellular manufacturing is the physical devising of the manufacturing facilities

machinery into production cells. Each cell is designed to produce a part family. a part

family is defined as a set of parts that require similar machinery, tooling, machine

operations, and/or jigs and fixtures. Usually, the manufacturing facility cannot be

completely divided into specialized cells. Rather, a portion of the facility remains as a

large functional job shop which has been termed the remainder cell.

The control of the CM system can be divided into two activities: cell loading and

cell scheduling. Cell loading is the determination of which cell, among the feasible cells,

the part will be assigned to. Cell scheduling is the internal control of the jobs within each

cell. Scheduling, by definition, is the determination of the order of he jobs onto each

machine and the determination of the precise start time and completion time of each job

on each machine. In reality, most viable control schemes do not perform cell scheduling

but rather employ cell sequencing. Sequencing is limited to the determination of the order

of the jobs onto each machine, and does not address timing.

Advantages and Disadvantages

It is appropriate to review the advantages and disadvantages associated with a cellular

manufacturing system.

The advantages of cellular manufacturing control as follows:

1. implied reduction of necessary control;

2. reduced material handling

3. reduced set-up time

4. reduced tooling;

5. reduced in-process inventory;

6. increased operator expertise;

7. reduced expediting;

8. improved human relations.

The disadvantages of cellular manufacturing control are as follows:

1. reduced shop flexibility

2. possible reduced machine utilization

3. possible extended job flow times;

4. Possible increased job tardiness.

Conventional product layout

Conventional Process lay out

Process flow in cellular manufacturing.

14. QUALITY ASSURANCE

During the process of manufacture, component parts need to be checked to ensure defect

free engines are produced. Using automated machinery and automated inspection,

dimensional accuracy is maintained by using multi-directional applied probes that record

sizes and position of features. The C.N.C inspection machine can inspect families of

components at pre-determined allotted intervals without further operator intervention. In

the chip machining (i.e. turning, boring, milling etc.) and metal forming processes C.N.C

machine tools enable consistency of manufacture which can be statistically inspected i.e.

one in ten. Component integrity is achieved by use of ultrasonic, radiological, magnetic

particle and penetrate inspection techniques, as well as electrolytic and acid etching to

ensure all material properties are maintained to both laboratory and quality acceptance

standards.

CMM (Co ordinate measuring machine)

15. Engine assemblyThe engine can be built in the vertical or horizontal position using the ram or stand.

Assembly of engine sub assemblies or module is done in separate area to minimize the

build time on the stand.

Compressor rotor under build

Vertical engine assembly

16. Engine Testing & despatch

Aero engines, especially military engines are put through their extreme limits of

performance and spend a higher proportion of their time off the wings than their civil

counterparts. It is therefore absolutely critical to achieve fast throughput in a test facility

and the engine should be returned to service as quickly as possible. Historically, test beds

were designed and built by engine developer for their testing needs.

Aero engine testing starts with the evaluation of design, development testing, type

testing for certification to testing newly manufactured and overhauled engine prior to

introduction to service. Aero engines undergo extensive development testing prior to

production. Test requirements for a developed engine are well defined and are far simpler

compared to development test requirements.

Early test beds were designed for specific testing requirements of specific

engines. Developmental test beds are flexible in nature with lots instrumentation to cater

to changing nature of test requirements and large data collection. Production test beds are

designed and built to satisfy specific test requirements. With the cost of an engine test

facility brought down to the cost of a single engine, these facilities can be acquired by

every operator thereby reducing testing cost and low off-wing times.

A modern test facility comprises of an acoustic enclosure with intake and exhaust,

a test stand and adapter frame, a preparation area for pre-rigging the engines, a fuel

measurement and delivery system and a control room and console with fully

computerized advanced data processing system giving online color video monitoring

displays with graphical plotting and tabular data recording. Engine mounting time is

reduced by using quick pre-rigged adapter frames located by self-aligning arrangement.

The single most important innovation in test process is the incorporation of

automatic service coupling plates which connects all the fluid and electric supplies

including fuel and low pressure air for engine starting. When the engine adapter is

offered to the test bed, all 200 to 250 connections can be made simultaneously.

With advancements in test bed design with neutral test cell configurations,

maintenance and overhaul units of aero engines can utilize the facility to the maximum

extent. The engines can be accurately and quickly tested in an advanced facility after

maintenance or overhaul and inducted into service with more confidence.

Development engines are required to be type tested for certification. These tests need not

be done for the certified production engine. Test results of these tests may be used for

improvement in design. Typical vibration test for the certification is shown below.

Vibration testing of small gas turbine engine

On completion of assembly every production engines must be tested in a test cell. In test

cell engine is run at ambient temperature and pressure condition and resultant

performance figure is corrected to International standard atmosphere (I.S.A) sea level

condition

Turbo shaft engines are tested for the shaft power output. Engine is usually coupled to

dynamometer, which absorbs the power.

Testing rig for Developmental Turbo shaft engine

In Turbo jet engine testing thrust is on of the measure parameter to be evaluated. .

Engine is mounted on a floating frame. Floating frame movement is resisted by load cell.

Thrust is measured by load cell.

Testing of low power turbojet engine

Mechanical simulation comprises supplying the engine inlet with and accurately

controlled mass flow; reduce the noise level transmission, fuel supply and loading of the

engine is done at test bed

Floor Mounted jet engine test bed

Arrangement is made for the engine mounting and dismounting in the test cell by

providing the crane in the test cell. Engine is normally prepared in the preparation stand

for the test prior to mounting in the test cell.

The picture below shows a test cell view of high by pass engine test facility.

Testing of High bypass engine

The test facility may be of different type. An overhang facility in which engine is over

hanged in thrust stand has better suited for testing of different verities of engines in same

test facility

Jet engine test bed

Engine performance parameters are indicated on the control console. Reading of different

performance parameter are noted/ acquired and analyzed to evaluate the engine

performance and pass it for use in the service

Typical engine test bed console

Storage and dispatch

Usually storage and transportation of the aircraft engines calls for special treatment to

preserve the engine. To resist the corrosion during storage, the fuel system is inhibited by

special oil. The engine is enclosed in a reusable bag or special container in which specific

amount of desiccant is inserted.

Typical Specification ofCnc 7 Axes Nozzle Guide Vane Grinding MachineGrinding spindle drive power 50 kwAutomatic diamond roll dressing equipment: -Length of roll 167 mmTravel distance 115 mm