Manufacture - Manufacturing strategy

MANUFACTURING STRATEGY 
9. Manufacturing is changing and will continue to change to meet the increasing demands of aeroengine components for fuel efficiency, cost and weight reductions and being able to process the materials required to meet these demands.
10. With the advent of micro-processors and extending the use of the computer, full automation of components considered for in house manufacture are implemented in line with supply groups manufac- turing strategy, all other components being resourced within the world-wide supplier network.
11. This automation is already applied in the manufacture of cast turbine blades with the seven cell and computer numerical controlled (C.N.C.) grinding centres, laser hardfacing 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 electrochemical machining (E.C.M.) while inspection and processing are being automated using the computer.

12. Tolerances between design and manufacturing are much closer when the design specification is matched by the manufacturing proven capability.

Rolls-Royce contra-rotating fan (concept) | Armstrong Siddeley Sapphire

Armstrong Siddeley Sapphire

        

Rolls-Royce contra-rotating fan (concept)

   The Sapphire originated in 1946 with the Metrovick F9, which was handed over to Armstrong-Siddeley when Metropolitan- Vickers withdrew from aviation in 1947. The Sapphire first ran in October 1948 and the engine was flight tested in Meteor, Hastings and Canberra aircraft; before going into production for the Gloster Javelin and Hawker Hunter F2.

Rolls-Royce RB211-535C | Metrovick G2

Rolls-Royce RB211-535C

Metrovick G2

Following the successful operation at sea of the Metrovick F2-based 2500 hp Gatric marine gas  turbine,  the  Royal  Navy  ordered  four larger sets with a maximum operational rating of 4500 shp. Developed from the Metrovick F2/4 Beryl axial-flow aircraft engine; the G2s were installed in the Motor Gunboats 'Bold Pioneer1  and  'Bold  Pathfinder; the  former going to sea in 1951.

SEALING, A hypothetical turbine cooling and sealing arrangement,A generator cooling system,Labyrinth seals.

Fig. 9-5 A hypothetical turbine cooling and sealing arrangement.

SEALING :
12. Seals are used to prevent oil leakage from the engine bearing chambers, to control cooling airflows

and to prevent ingress of the mainstream gas into the turbine disc cavities.
13. Various sealing methods are used on gas turbine engines. The choice of which method is
dependent upon the surrounding temperature and pressure, wearability, heat generation, weight, space
available, ease of manufacture and ease of installa- tion and removal. Some of the sealing methods are
described in the following paragraphs. A hypothetical turbine showing the usage of these seals is shown in
fig. 9-5.

Bearing chamber cooling, Development of high pressure turbine blade cooling,High pressure nozzle guide vane construction and cooling ,Accessory cooling

Fig. 9-3 Development of high pressure turbine blade cooling.

Bearing chamber cooling


9. Air cooling of the engine bearing chambers is not normally necessary since the lubrication system
(Part 8) is adequate for cooling purposes.

Turbine cooling,Nozzle guide vane and turbine blade cooling arrangement

Turbine cooling :


5. High thermal efficiency is dependent upon high turbine entry temperature, which is limited by the
turbine blade and nozzle guide vane materials. Continuous cooling of these components allows their

Fig. 9-2 Nozzle guide vane and turbine blade cooling arrangement.

environmental operating temperature to exceed the material's melting point without affecting the blade
and vane integrity. Heat conduction from the turbine blades to the turbine disc requires the discs to be
cooled and thus prevent thermal fatigue and uncon- trolled expansion and contraction rates.

A centrifugal breather, A thread-type oil filter,A typical pressure and scavenge filter,LUBRICATING OILS

Fig. 8-11 A thread-type oil filter.

27. In some engines, to minimize the effect of the dynamic loads transmitted from the rotating
assemblies to the bearing housings, a 'squeeze film' type of bearing is used (fig. 8-9). They have a small
clearance between the outer race of the bearing and housing with the clearance being filled with oil. The
oil film dampens the radial motion of the rotating assembly and the dynamic loads transmitted to the
bearing housing thus reducing the vibration level of the engine and the possibility of damage by fatigue.

28. To prevent excessive air pressure within the oil tank, gearboxes and bearing chambers, a vent to
atmosphere is incorporated within the lubrication system. Any oil droplets in the air are separated out
by a centrifugal breather prior to the air being vented overboard. Some breathers may incorporate a
porous media, forming de-aerator segments, which improves the efficiency of the oil separation (fig, 8-10)

Principle of a gear pump

20. The most common type of oil distribution device is a simple orifice which directs a metered amount of
oil onto its target. These jet orifices are positioned as close to the target area as possible to overcome the
possibility of the local turbulent environment deflecting the jet of oil. The smallest diameter of a jet
orifice is 0.04 inch which allows a flow of 12 gallons per hour when operating at a pressure of 40 lb. per
sq. in. The use of restrictors upstream can reduce the flow rate if required.

Fig. 8-5 Principle of a gear pump.

21. All engines transfer heat to the oil by friction, churning and windage within a bearing chamber or
gearbox. It is therefore common practice to fit an oil cooler in recirculatory oil systems. The cooling
medium may be fuel or air and, in some instances, both fuel-cooled and air-cooled coolers are used.

22. Some engines which utilize both types of cooler may incorporate an electronic monitoring system

which switches in the air-cooled cooler only when it is necessary. This maintains the ideal oil temperature

and improves the overall thermal efficiency. 

An oil tank,OIL SYSTEM COMPONENTS

OIL SYSTEM COMPONENTS :


12. The oil tank (fig. 8-4) is usually mounted on the engine and is normally a separate unit although it
may also be an integral part of the external gearbox. It must have provision to allow the lubrication system
to be drained and replenished. A sight glass or a system contents to be checked. The filler can be

Fig. 8-4 An oil tank.

either the gravity or pressure filling type; on some engines both types are fitted. Provision is also made
for a continuous supply of oil to be made available in aircraft which are designed to operate during
inverted flight conditions. Since air is mixed with the oil in the bearing chambers, a de-aerating device is
incorporated within the oil tank which removes the air from the returning oil.

Internal air system,General internal airflow pattern,COOLING

 Internal air system :



INTRODUCTION

Fig. 9-1 General internal airflow pattern.

1. The engine internal air system is defined as those airflows which do not directly contribute to the engine thrust. The system has several important functions to perform for the safe and efficient operation of the engine. These functions include internal engine and accessory unit cooling, bearing chamber sealing prevention of hot gas ingestion into the turbine disc cavities, control of bearing axial loads, control of turbine blade tip clearances (Part 5) and engine anti-icing (Part 13). The system also supplies air for the aircraft services. Up to one fifth of the total engine core mass airflow may be used for these various functions.

ENERGY TRANSFER FROM GAS FLOW TO TURBINE

Fig. 5-6 A typical turbine blade showing
twisted contour.

ENERGY TRANSFER FROM GAS FLOW TO TURBINE
6. From the description contained in para. 1, it will be seen that the turbine depends for its operation on the transfer of energy between the combustion gases and the turbine. This transfer is never 100 per cent because of thermodynamic and mechanicallosses, (para. 11). 

Fig. 5-7 Gas flow pattern through nozzle and blade

  7.when the gas is expanded by the combustion process (Part 4), it forces its way into the discharge nozzles of the turbine where, because of theirconvergent shape, it is accelerated to about the speed of sound which, at the gas temperature, isabout 2,500 feet per second. At the same time thgas flow is given a ’spin’ or ’whirl’ in the direction o rotation of the turbine blades by the nozzle guid vanes. On impact with the blades and during th subsequent reaction through the blades, energy is absorbed, causing the turbine to rotate at high speed and so provide the power for driving the turbine shaft and compressor.

An electronically operated bleed valve system

Fig. 3-18 An electronically operated bleed valve system.

Fig. 3-19 Typical types of fan blades.

32. For casing designs the need is for a light but rigid construction enabling blade tip clearances to be accurately maintained ensuring the highest possible efficiency. These needs are achieved by using aluminium at the front of the compression system followed by .alloy steel as compression temperature increases. Whilst for the final stages of the compression system, where temperature requirements possibly exceed the capability of the best steel, nickel based alloys may be required. The use of titanium in .preference to aluminium and steel is now more common; particularly in military engines where its high rigidity to density ratio can result in significant weight reduction. With the development of new manufacturing methods component costs can now be maintained at a more acceptable level in spite of high initial material costs.

A typical rotor blade showing twisted contour

Fig. 3-12 A typical rotor blade showing
twisted contour.

26. The rotor blades are of airfoil section (fig. 3-12) and usually designed to give a pressure gradient along their length to ensure that the air maintains a reasonably uniform axial velocity. The higher pressure towards the tip balances out the centrifugal action of the rotor on the airstream. To obtain these conditions, it is necessary to ’twist’ the blade from root to tip to give the correct angle of incidence at each point. Air flowing through a compressor creates two boundary layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slow air in the boundary layer a localized increase in blade camber both at the blade tip and root has been introduced. The blade extremities appear as ifformed by bending over each corner, hence the term’end-bend’.Stator vanes
27. The stator vanes are again of airfoil section and are secured into the compressor casing or into stator vane retaining rings, which are themselves secured to the casing (fig. 3-13). The vanes are often assembled in segments in the front stages and may be shrouded at their inner ends to minimize the vibrational effect of flow variations on the longer vanes. It is also necessary to lock the stator vanes in such a manner that they will not rotate around the casing.
OPERATING CONDITIONS
28. Each stage of a multi-stage compressor possesses

Methods of securing blades to disc and Rotors of drum and disc construction

Fig. 3-11 Methods of securing blades to disc.

Fig. 3-10 Rotors of drum and disc
construction.

23. The construction of the compressor centres around the rotor assembly and casings. The rotor shaft is supported in ball and roller bearings and coupled to the turbine shaft in a manner that allows for any slight variation of alignment. The cylindrical casing assembly may consist of a number of cylindrical casings with a bolted axial joint between each stage or the casing may be in two halves with a bolted centre line joint. One or other of these construction methods is required in order that the casing can be assembled around the rotor. Rotors

Principles of operation - Pressure and velocity changes through an axial compressor.

Fig. 3-9 Pressure and velocity changes
through an axial compressor.

19. During operation the rotor is turned at high speed by the turbine so that air is continuously induced into the compressor, which is then accelerated by the rotating blades and swept rearwards onto the adjacent row of stator vanes. The pressure rise results from the energy imparted to the air in the rotor which increases the air velocity. The air is then decelerated (diffused) in the following stator passage and the kinetic energy translated into pressure. Stator vanes also serve to correct the deflection given to the air by the rotor blades and to present the air at the correct angle to the next stage of rotor blades. The last row of stator vanes usually act as air straighteners to remove swirl from the air prior to entry into the combustion system at a reasonably uniform axial velocity. Changes in pressure and velocity that occur in the airflow through the compressor are shown diagrammatically in fig. 3-9. The changes are accompanied by a progressive increase in air temperature as thepressure increases.
20. Across each stage the ratio of total pressures of outgoing air

THE AXIAL FLOW COMPRESSOR

Fig. 3-7 Typical axial flow compressors.

THE AXIAL FLOW COMPRESSOR
13. An axial flow compressor (fig. 3-7 and fig. 3-8 consists of one or more rotor assemblies that carry blades of airfoil section. These assemblies are mounted between bearings in the casings which incorporate the stator vanes. The compressor is a multi-stage unit as the amount of pressure increase by each stage is small; a stage consists of a row of rotating blades followed by a row of stator vanes. Where several stages of compression operate in series on one shaft it becomes necessary to vary the stator vane angle to enable the compressor to operate effectively at speeds below the design condition. As the pressure ratio is increased the incorporation of variable stator vanes ensures that the airflow is directed onto the succeeding stage of rotor blades at an acceptable angle, ref. para. 30, Airflow Control.

Fig. 3-8 Typical triple spool compressor.

14. From the front to the rear of the compressor, i.e. from the low to the high pressure end, there is a

Construction - Typical impellers for centrifugal compressors

Construction
10. The construction of the compressor centres around the impeller, diffuser and air intake system. The impeller shaft rotates in ball and roller bearings and is either common to the turbine shaft or split in the centre and connected by a coupling, which is usually designed for ease of detachment. Impellers
11. The impeller consists of a .forged, disc with integral, radially disposed vanes on one or both sides (fig. 3-5) forming convergent passages in conjunction with the compressor casing. The vanes may be swept back, but for ease of manufacture straight radial vanes are usually employed. To ease the air from axial flow in the entry duct on to the rotating impeller, the vanes in the centre of the impeller are curved in the direction of rotation. The curved sections may be integral with the radial vanes or formed separately for easier and more accurate manufacture.Diffusers
12. The diffuser assembly may be an integral part of the compressor casing or a separately attached assembly. In each instance it consists of a number of vanes formed tangential to the impeller. The vane passages are divergent to convert the kinetic energy into pressure energy and the inner edges of the vanes are in line with the direction of the resultant airflow from the impeller (fig. 3-6). The clearance between the impeller and the diffuser is an important factor, as too small a clearance will set up aerodynamic buffeting impulses that could be transferred to the impeller and create an unsteady airflow and vibration.

Fig. 3-6 Airflow at entry to diffuser.

 

 

Fig. 3-5 Typical impellers for centrifugal
compressors.

THE CENTRIFUGAL FLOW COMPRESSOR

THE CENTRIFUGAL FLOW COMPRESSOR

Fig. 3-3 Pressure and velocity changes
through a centrifugal compressor.

5. Centrifugal flow compressors have a single or double-sided impeller and occasionally a two-stage, single sided impeller is used, as on the Rolls-Royce Dart. The impeller is supported in a casing that also contains a ring of diffuser vanes. If a double-entry impeller is used, the airflow to the _rear side is reversed in direction and a plenum chamber is required. Principles of operation
6. The impeller is rotated at high speed by the turbine and air is continuously induced into the centre of the impeller. Centrifugal action causes it to flow radially outwards along the vanes to the impeller tip, thus accelerating the air and also causing a rise in pressure to occur. The engine intake duct may contain vanes that provide an initial swirl to the air entering the compressor.

Fig. 3-4 Impeller working clearance and
air leakage.

Compressors - INTRODUCTION

INTRODUCTION
1. In the gas turbine engine, compression of the air before expansion through the turbine is effected by one of two basic types of compressor, one giving centrifugal flow and the other axial flow. Both types are driven by the engine turbine and are usually coupled direct to the turbine shaft.

Fig. 3-1 A typical centrifugal flow compressor.

2. The centrifugal flow compressor (fig. 3-1) is a single or two stage unit employing an impeller to accelerate the air and a diffuser to produce the required pressure rise. The axial flow compressor (fig. 3-7 and fig. 3-8) is a multi-stage unit employing alternate .rows of rotating (rotor) blades and stationary (stator) vanes, to accelerate and diffuse the air until the required pressure rise is obtained. In some cases, particularly on small engines, an axial compressor is used to boost the inlet pressure to the centrifugal.

Fig. 3-2 Specific fuel consumption and
pressure ratio.

3. With regard to the advantages and disadvantages of the two types, the centrifugal compressor is usually more robust than the axial compressor and is also easier to develop and manufacture. The axial compressor however consumes far more air than a centrifugal

Rolls-Royce RB211-22B

Rolls-Royce RB211-22B

De Havilland H1 Goblin

 Development of the de Havilland Goblin began in 1941 with the Halford H1 with a design thrust of 3000 lb. The engine passed a 25 hr special category test in September 1942 and was cleared for flight at 2000 lb thrust. This took place in a Gloster Meteor on 5 March 1943 and was also the first flight of that aircraft type.In September 1943 the first flight of a de Havilland DH100 Vampire was made with a Goblin of 2300 lb thrust.