Abstract:
A boundary layer control arrangement comprises a pulse generator communicating with a surface having a fluid boundary layer thereacross. The boundary layer control arrangement further includes a fluid supply means for supplying a fluid to the surface via the pulse generator. The pulse generator is constructed such that fluid acts on the pulse generator to cause the fluid to pulse. Pulsing fluid passes from the pulse generator to the surface.

Description:
TECHNICAL FIELD 
   This invention relates to boundary layer control arrangements. More specifically, but not exclusively, the invention relates to boundary layer control arrangements for use in gas turbine engines. 
   BACKGROUND OF THE INVENTION 
   The passage of air over various components in gas turbine engines is influenced by the nature of the boundary layer of air across various surfaces. In some circumstances, it is necessary to ensure that the boundary layer remains on the surface across which the air is flowing, in other circumstances it is necessary to disrupt the boundary layer and prevent it flowing adjacent to the surface. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one aspect of this invention, there is provided a boundary layer control arrangement comprising a pulse generator communicating with a surface having a fluid boundary layer thereacross, and a fluid supply means for supplying a fluid to the surface via the pulse generator, wherein the pulse generator is constructed such that the fluid acts on the pulse generator to cause the fluid to pulse, whereby pulsing fluid passes from the pulse generator to the surface. 
   Preferably, the action of the fluid on the pulse generator creates said pulses in the fluid. 
   The pulse generator is preferably a passive pulse generator. Conveniently, the pulse generator comprises a wave generator for creating waves in the fluid. Preferably, the pulse generator comprises a sound wave generator for creating sound waves in the fluid. Desirably, the pulse generator can establish pulses in the fluid which are in the form of sound waves. The pulses may be at a predetermined substantially constant frequency. 
   In one embodiment, the pulse generator may comprise a chamber to receive at least some of the fluid from the fluid supply means. The chamber is conveniently configured such that a standing wave is created in the fluid in the chamber. In this embodiment, the pulse generator may comprise a fluid splitting member to split fluid from the fluid supply means, such that some of said fluid passes into the chamber, and some of said fluid passes to the surface. 
   The pulse generator may define an aperture via which the fluid supply means communicates with the surface. Preferably, the chamber extends from the aperture. The splitting member may be provided at the aperture. Preferably, the chamber is elongate. 
   In another embodiment, the pulse generator may comprise a chamber having a first sub-chamber, a second sub-chamber and a pressure responsive barrier between the first and second sub-chambers. 
   The pulse generator may define an aperture via which fluid from said chamber can pass to the surface. Pulse generator may further comprise an exit conduit extending to said aperture, whereby fluid can pass from the first sub-chamber to the surface via said exit conduit. The exit conduit may be split into a plurality of sub-conduits to provide a plurality of outlets of the surface. The pressure responsive barrier is preferably movable between an open condition to allow fluid to pass into the exit conduit, and a closed condition to prevent fluid entering the exit conduit. 
   Preferably, the fluid supply means supplies said fluid to the first sub-chamber to cause the barrier to respond and allow said fluid to pass through the conduit. Preferably, the barrier responds by deforming to allow fluid in the sub-chamber to enter the conduit. Preferably, the barrier is configured to respond when the pressure of fluid in the first sub-chamber reaches a pre-determined limit, the predetermined limit is desirably a pressure greater than the pressure in the second sub-chamber. Desirably the barrier is constructed to move to a deformed condition to open the conduit when the pressure in the first sub-chamber reaches the predetermined limit. Preferably, when fluid enters the conduit, the barrier moves to the non-deformed condition to close the conduit. 
   Alternatively, the barrier may comprise a flexible diaphragm or a piston, and urging means, such as a spring. The urging means may provide a force to urge the diaphragm or piston to close the conduit until the pressure in the first chamber overcomes the force applied by the urging means. 
   The fluid supply means may comprise a restrictor to restrict fluid into the first sub-chamber. 
   The boundary layer control arrangement may comprise a fluid supply regulator to regulate the supply of said fluid. The fluid supply regulator may comprise a valve, which may be configured to have an on condition and an off condition. Alternatively, the fluid supply regulator may comprise a valve, and may be configurable to vary the supply of said fluid continuously or in stepped changes. The arrangement may comprise a controller means to control the fluid supply regulator. The controller may be an electronic controller. Preferably, the controller means controls the valve. 
   The boundary layer control arrangement may comprise adjustment means to adjust the nature of the pulses in the fluid. Preferably, the adjustment means can adjust the frequency of the pulses in the fluid. 
   In the first embodiment, the adjustment means may comprise a wall of the chamber which may be movable along said chamber to alter the frequency of the standing wave in the chamber. Preferably, the wall is a wall opposite the fluid supply means. 
   In the second embodiment, the adjustment means may comprise an adjustment aperture for the second sub-chamber to allow a fluid to flow into or out of the second sub-chamber, thereby adjusting the pressure in the second sub-chamber. Thus, by adjusting the pressure in the second sub-chamber, the predetermined pressure at which the barrier deforms to allow fluid to pass into the conduit is also changed. 
   In another embodiment, the pulse generator comprises a vibratable member to receive fluid from the fluid supply means. The vibratable member is preferably vibratable by the action of fluid from the fluid supply means thereon. The vibratable member may comprise a reed. 
   According to another aspect of this invention, there is provided a boundary layer control system comprising a plurality of boundary layer control arrangements as described above. Preferably, the boundary layer control system comprises fluid distribution means to distribute fluid to the respective boundary layer control arrangements. The fluid distribution means may comprise a manifold. 
   According to another aspect of this invention, there is provided a gas flow conduit of an engine, said gas flow conduit comprising a boundary layer control arrangement as described above. 
   Preferably, the gas flow conduit comprises a gas intake for the engine. The engine may comprise a gas turbine engine and the intake may comprise a nacelle. 
   Preferably, the conduit may comprise a boundary layer control arrangement on the inner surface, and may also comprise a boundary layer control arrangement on the outer surface. Preferably, the intake comprises a boundary layer control system as described above. 
   In another embodiment, the conduit may comprise a duct in an engine, such as a gas turbine engine. 
   Preferably, the characteristics of the boundary layer control arrangement are pre-selected to match the conditions of the conduit. The conduit may comprise a plurality of boundary layer control arrangements. 
   According to another aspect of this invention, there is provided an aerofoil comprising a boundary layer control arrangement to control the boundary layer of fluid flowing across the aerofoil. 
   The boundary layer control, arrangement may be described as above. Alternatively, the boundary layer control arrangement may comprise an active boundary layer control arrangement. Conveniently, such active arrangements comprise active systems such as micro-electro-mechanical systems (MEMS), virtual jets. 
   In one embodiment, the active boundary layer control arrangement comprises a piston and cylinder arrangement, which may communicate with the surface of the aerofoil via an aperture control means, which may be provided to control the rate of reciprocation of the piston in the cylinder, thereby providing a pulsed jet of air into and out of the cylinder through the aperture. 
   In another embodiment, the active boundary layer control arrangement may comprise a conduit to supply air to the surface of the aerofoil via an aperture. An oscillatable valve may be provided in the conduit to provide pulsed air out of the aperture. The valve may be oscillatable between open and closed conditions to provide said pulsed air. 
   Control means may be provided to control the piston and cylinder arrangement and/or the oscillatable valve. The control means may include sensors to sense the condition of the boundary layer and thereby provide an appropriate frequency of the pulses. 
   In one embodiment, the aerofoil may comprise a fan blade of a fan of a gas turbine engine. In another embodiment, the aerofoil may comprise a vane of a rotary component of a gas turbine engine. The vane may comprise a stator vane of a compressor, or a nozzle guide vane of a turbine. 
   In the first embodiment, the boundary layer control arrangement may be arranged towards the trailing edge of the fan blade. Thus, in this embodiment, the boundary layer control arrangement disrupts the air towards the trailing edge, thereby disrupting the interaction of the air in the wake with further aerofoil downstream of the blade. This has the advantage of reducing noise from the engine, by reducing the interaction between the wake and the outlet guide vanes. 
   In another embodiment, the noise reduction effect can be achieved by alternately operating boundary layer control arrangements in the compression and suction sides of the blade. This has the advantage, in this embodiment, of providing an alternating effect at the trailing edge, which turns/interrupts the air in opposite directions in order to produce more disruption of the wake and hence more noise reduction. 
   In the second embodiment, the boundary layer control arrangement may comprise a boundary layer control arrangement as described above. Alternatively, the boundary layer control arrangement may comprise a blowing means for blowing air off the trailing edge of the blade. The blade can be any blade in the engine, such as a fan blade, a compressor blade or a turbine blade. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: 
       FIG. 1  is a flow diagram of a boundary layer control arrangement; 
       FIG. 2  is a diagrammatic view of one embodiment of a boundary layer control arrangement; 
       FIG. 3A  is a diagrammatic view of a second embodiment of a boundary layer control arrangement; 
       FIG. 3B  is a diagrammatic view of a modified version of the second embodiment of the boundary layer control device shown in  FIG. 3A ; 
       FIG. 3C  is a diagrammatic view of another modified version of the second embodiment of the boundary layer control device shown  FIG. 4A  is a schematic diagram of a boundary layer control system; 
       FIG. 4B  is a schematic diagram of a modified version of the boundary layer control system shown in  FIG. 4A ; 
       FIG. 5  is a diagrammatic view of a third embodiment of a boundary layer control arrangement; 
       FIG. 6  is a diagrammatic view of a fourth embodiment of a boundary layer control arrangement; 
       FIG. 7  is a diagrammatic view of a boundary layer control system arranged in the nacelle of a gas turbine engine. 
       FIG. 8  is a diagrammatic view of a boundary layer control arrangement arranged in an S shaped inlet of a gas turbine engine; 
       FIG. 9  is a diagrammatic view of a nozzle guide vane incorporating a boundary layer control arrangement; 
       FIG. 10  is a diagrammatic view of a further nozzle guide vane incorporating a boundary layer control arrangement; 
       FIG. 11  is a diagrammatic view of the arrangement of a boundary layer control system on a nozzle guide vane shown in  FIGS. 9 and 10 ; 
       FIG. 12  is a diagrammatic sectional side view of the nozzle guide vane in  FIG. 11  showing the arrangement of the boundary layer control arrangements in  FIG. 11 . 
       FIG. 13  shows a section of a gas turbine engine incorporating a boundary layer control arrangement; 
       FIG. 14  is a sectional side view of a fan blade incorporating a boundary layer control system; 
       FIG. 15  shows a section of a fan blade with a boundary layer control device; and 
       FIG. 16  shows a section of a fan blade with a further boundary layer control device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In gas turbine engines, where gas flows over various surfaces, for example aerofoils and ducts, it is often important to be able to control the boundary layer of the flow of air at that surface.  FIG. 1  shows a schematic flow diagram of a boundary layer control arrangement  10 . The boundary layer control arrangement  10  comprises supply means  12  for supplying air to an air flow modulator or a pulse generator  14 . A valve  16  is provided between the air supply means  12  and the pulse generator  14  to turn on or off the flow of air to the pulse generator  14 . The valve  16  is an on-off valve and is controlled by suitable electronic controller  18  which supplies a suitable signal  19  to turn the valve on or off. 
   Air from the pulse generator  14  passes therefrom to ducts/holes in a surface  20  across which air F is flowing. The boundary layer of the air flow at the surface is controlled by the pulsed air B from the pulse generator  14 . 
   Referring to  FIG. 2 , there is shown an embodiment of a pulse generator  14  for controlling a boundary layer of a flow of air F across a surface  20 . The pulse generator  14  shown in  FIG. 2  comprises a conduit  22  for supplying air, and a chamber  23 . The pulse generator  14  also includes a splitting member  24  at an aperture  25  in the surface  20  for splitting flow of air into two flows; a first flow A into the chamber  23 , and a second flow B out of the aperture  25 . 
   The chamber  23  has a predetermined length L to enable a standing wave to be established in the air within the chamber  23 . The standing wave causes the air exiting out of the aperture  25  to pulse at the same frequency as the standing wave in the chamber  23 , thereby disrupting or energising the boundary layer of the flow F of air across the surface  20 . 
     FIG. 3A  shows an alternative embodiment of a pulse generator  16  comprising a chamber  26  having a first sub-chamber  28  and a second sub-chamber  30 . A flexible membrane  32  extends between the first and second sub-chambers  28 ,  30 . An inlet conduit  34  allows air to be supplied to the first sub-chamber  28 . A restrictor  35  restricts the flow rate of air to the first sub-chamber  28 . 
   As more air enters the first sub-chamber  28 , the pressure inside the first sub-chamber  28  increases until the flexible membrane  32  deforms to the position shown in broken lines in  FIG. 3A . When this happens, the air passes into an exit conduit  36  which leads to the aperture  25  in the surface  20 . The air passes through the aperture  25 , as shown by the arrow C. As the air passes through the exit conduit  36 , the pressure in the first sub-chamber  28  reduces thereby causing the membrane  32  to move back to its non-deformed condition shown in unbroken lines in  FIG. 3A . 
   It will be appreciated that with a continuous supply of air into the first sub-chamber conduit  28 , there will be a vibration set up in the membrane  32  and this vibration will be dependent upon the pressure inside the second sub-chamber  30 . Thus, the air C exiting via the exit conduit  36  is pulsed having a frequency equal to the frequency of vibration of the membrane  32 . 
     FIG. 3B  shows a modified version of the embodiment shown in  FIG. 3A . In  FIG. 3B , the chamber  26  houses a piston  29  having a sealing member  31 . that engages against the exit conduit  36  the sub-chamber  28  is provided above the sealing member  31 , as shown in  FIG. 3B . Urging means in the form of a spring, shown schematically at  33  urges the piston  29  in the direction indicated by the arrow X. The spring  33  provides a predetermined force to urge the sealing member  31  against the exit conduit  36 . As air is pumped into the sub-chamber  28  as shown, the pressure in the sub-chamber  28  increases until the force downwardly on the sealing member  31  exceeds the predetermined force on the sealing member  31  by the spring  33 , at which time the piston is urged by the pressure in the sub-chamber  28  in the opposite direction to the direction indicated by the arrow X. Fluid thus passes into the exit conduit  36 . 
   Hence, in the same way as explained above with reference to  FIG. 3A  continuous pumping of air into the sub chamber  28  causes a vibration to be set up in the movement of the piston  29 , and as a result air exiting via the exit conduit  36  has a tonal frequency equal to the frequency of vibration f the piston  29 . 
   The version shown in  FIG. 3C  is similar to the version shown in  FIG. 3B , but the version shown in  FIG. 3C  comprises a flexible diaphragm  37  which is urged into the sealing engagement with the exit conduit  36  by urging means in the form of a spring  33 . The flexible diaphragm is held in sealing engagement with the exit conduit  36  by the spring  33  to prevent air in the sub-chamber  28  passing into the exit conduit  36 . In the same way as explained above the increase in pressure in the sub-chamber  28  eventually creates a greater force on the diaphragm than the urging force of the spring  33 , causing the diaphragm to move in the opposite direction to the direction indicated by the arrow X. As a result, in the same way as described above the diaphragm is caused to vibrate and a tonal frequency is set up in the air passing through the exit conduit  36 . 
   Referring to  FIG. 4 , there is shown a boundary layer control system  40  comprising a plurality of boundary layer control arrangements  10  linked together by a manifold  42  to which air is supplied from the air supply means  12 . On-off valves  16  are each connected to one of two control means  44 ,  46 , depending upon the nature of the operating conditions, or upon the different degrees of air flow turning required. As can be seen some of the pulse generators  14  are connected to a first control means  44  in the form of a first electronic controller. The other pulse generators  14  are connected to a second control means  46  in the form of a second electronic controller. Conduits  48  lead to a plurality of apertures  50  in the surface  20  to which pulsed air is delivered. 
   In the system  40  shown in  FIG. 4  air is supplied by the air supply means  12  to the manifold  42 . Air from the manifold then passes via on-off valves  16  that are switched “ON” to the respective pulse generators  14 . Thereafter pulsed jets of air are delivered via the respective conduits  50  to the surface  20  to influence the boundary if the flow of air F across the surface  20 . Alternatively, each valve  16  could allow air to be supplied to a plurality of pulse generators  14 , as shown in  FIG. 4B . 
   Referring to  FIG. 5 , there is shown a modification to the pulse generator  14  shown in  FIG. 2 . The pulse generator  14  in  FIG. 5  comprises many of the same features as shown in  FIG. 2  and these have been designated with the same reference numeral. 
   The pulse generator  14 , in  FIG. 5  differs from that shown in  FIG. 2  by the provision of a movable end wall  52  remote from the conduit  22 . The end wall  52  is movable as shown by the arrow D to adjust the effective length L 1  of the chamber  23 . By selecting suitable effective lengths L 1  of the chamber  23  the frequency of vibration of a standing wave created in the chamber  23  can be varied, thereby varying the frequency of the pulsed air B exiting via the aperture  25 . 
   Referring to  FIG. 6 , there is shown a modification of the pulse generator  14  shown in  FIG. 3 . The pulse generator  14  shown in  FIG. 6  comprises many of the same features as shown in  FIG. 3 , these have been designated with the same reference numerals. 
   The pulse generator  14  shown in  FIG. 6  differs from that shown in  FIG. 3  by the provision of a pressure adjusting aperture  54  in the wall of the chamber  26  that leads to the second sub-chamber  30 . Suitable pressure adjusting means  56  is provided to supply air to, or remove air from, the second sub-chamber  30 . In this way the frequency of vibration of the membrane  32  is varied, thereby varying the frequency of the pulsed jet C passing through the exit conduit  36  and the aperture  25 . 
   Referring to  FIG. 7 , there is shown a diagrammatic sectional view of the front of a gas turbine engine showing a nacelle  50  and a fan  48  at which a boundary layer control system  40  has been provided. The boundary layer control system  40  comprises a plurality of boundary layer control arrangements  10  as described above arranged to provide pulsed air via apertures  25 A at internally of the nacelle  51  and via apertures  25 B externally of the nacelle  51  to control the boundary layer in those respective regions. 
   A sensor  50  can be provided to sense the condition of the boundary layer. 
   An advantage of this arrangement is that it maintains the boundary layer during periods of excessive side wind or during rotation of the aircraft, or during descent or windmilling when air must be shed around the outside of the nacelle. 
     FIG. 8  shows a diagrammatic view of an S-shaped inlet  53  for a gas turbine engine for example, in an aircraft where stealth applications are important. In the inlet  53  air flows in the direction of the arrow D. A boundary layer control system  40 , comprising a plurality of boundary layer control arrangements  10  is provided. The pulsed air emitted from the boundary layer control arrangements  10  via the apertures  25 , to the surface  20 , controls the boundary layer flowing over the surface of the S-shaped inlet maintaining the boundary layer in contact with the surface, as shown at  55  in  FIG. 8 . 
   Referring to  FIG. 9 , there is shown a diagrammatic sectional side view of a nozzle guide vane  56  of a turbine in a gas turbine engine. A boundary layer control system  40  comprising a plurality of boundary layer control arrangements  10  is provided to deliver pulsed air through apertures  25  in a first surface  20 A of the nozzle guide vane  56 . 
   The control arrangements  10  can be as described above and have many of the features of  FIGS. 1 to 3 , which are designated in the same reference numerals as in  FIGS. 1 to 3 . 
   The arrow G indicates the direction of flow of air across the surface  20 A in the absence of pulsed jets of air. The arrow H indicates the direction of flow of air across the surface  56  in the presence of the pulsed jets of air. Alternatively, the flow of air indicated by the arrow G could be caused by the presence of pulsed jets of air, and the flow of air indicated by the arrow H could be caused by the absence of pulsed jets of air, depending upon the operational requirements. 
   Similarly,  FIG. 10  shows the nozzle guide vane  56  having a plurality of pulse generators  14 , as described above, for directing pulsed air through apertures  25  onto a second surface  20 B of the nozzle guide vane  56 . The control arrangement  10  can be as described above and have many of the features of  FIGS. 1 to 3 , which are designated with the same reference numerals. The arrow J indicates in  FIG. 10  the direction of flow of air in the absence of the pulsed jets from the apertures  25 , and the arrow I indicates the direction of flow of air in the presence of the pulsed jets of air from the apertures  25 . Alternatively, depending upon the operational requirements, the arrow J could represent the flow of air in the presence of pulsed jets of air, and the arrow I could represent the flow of air in the absence of pulsed jets of air. Similar arrangements can be used on other aerofoils such as on blades or other vanes. 
   Referring to  FIGS. 11 and 12 , there is shown the orientation of the pulse generators  14  in the nozzle guide vane  54  shown in  FIGS. 9 and 10 .  FIG. 11  shows the surface  20  (which can be either the first surface  20 A ( FIG. 9 ) or the second surface  20 B ( FIG. 10 )). A plurality of obliquely arranged boundary control arrangements  10  are provided in groups of three across the surface  20 .  FIG. 12  shows that the respective chamber  23  of each of the pulse generators  14  is arranged at an angle X to the surface  20 . 
     FIG. 13  shows a compressor region  58  of the gas turbine engine, which supplies compressed air to a combustor  60  in which the compressed air is combusted in the presence of fuel. The combustion products expand to drive the turbines  62 . 
   As can be seen from  FIG. 13  compressed air is taken from the compressor  58  via first and second conduits  64 ,  66  to the turbines  62 , by passing the combustor  60 . Air in the first conduits  64  is passed to an on-off valve  68  and thereafter to a first boundary layer control arrangement  10 A in nozzle guide vane  54 A of the turbines  62 . 
   Air in the second conduit  66  is passed through an air cooler  70  and a pump  72  splits the air into two streams via conduits  74 ,  76 . The air in the conduits  74 ,  76  is passed through respective on-off valves  78 ,  80  and thereafter to a boundary layer control arrangement  10  B in a nozzle guide vane  54 B of the turbines  62 . 
   The valves  68 ,  78 ,  80  are controlled by control signals  68 A,  78 A,  80 A from suitable controllers (not shown in  FIG. 13 ). 
   Referring to  FIG. 14 , there is shown a fan blade  82  having a boundary layer control system  40  comprising a plurality of boundary layer control arrangements  10  arranged towards the trailing edge of the fan blade  82 . The boundary layer control system  40  comprises a manifold  42  for supplying air to each of the boundary layer control arrangements  10 . Air is supplied to the manifold  42  via an on-off valve  84  which is controlled by a control signal  86 . Each of the boundary layer control arrangements may be as described above, or may be another device and is used to disrupt the flow of air at the trailing edge of the fan blade. 
   This has the advantage in that it disrupts air in the wake of the fan thereby influencing the interaction of the air with the outlet guide vane  88 , and reducing the noise of the engine. 
   As an alternative to the boundary layer control system  40 , the fan blade  82  may comprise a control system that comprises a plurality of active boundary layer control arrangements such as those shown in  FIGS. 15 and 16 . 
   In  FIG. 15 , the fan blade  82  is provided with a plurality of first active boundary layer control arrangements  90  (only one of which is shown for clarity). Each first active boundary layer control arrangement  90  comprises a piston and cylinder arrangement provided  92  within the fan blade  82 . The surface of the fan blade  82  defines an aperture  94  at each piston and cylinder arrangement  92 . 
   The piston and cylinder arrangement  92  is moved reciprocally as shown by the double headed arrow K at a desired frequency. This results in a pulse of air into and out of the aperture  94  (as shown by the arrows L 1 , L 2 ) thereby disrupting or energising the boundary layer across the fan blade  82 . 
   The piston  92  is controlled by an actuator  96  which, in turn, is connected to control means  98  which in turn is connected to appropriate sensors  99  on the surface of the fan blade  82 . The sensors  99  determine the conditions of the boundary layer and thereby the frequency of oscillation of the piston. 
   In  FIG. 16 , the blade  82  is provided with a plurality of second active boundary layer control arrangements  100 , each of which comprises a conduit  102  which extends within the fan blade  82  to an outlet aperture  104  in the surface of the fan blade  82 . Air is supplied along the conduit  102  to exit therefrom onto the surface via the outlet aperture  104  as shown by the arrow M. 
   A high speed valve  106  is provided within the conduit  102 . The valve  106  oscillates at high frequency between open and closed conditions to create pulses in the air exiting out of the aperture  104 . 
   The rate at which the valve  106  oscillates is controlled by suitable control means  108  and is dependent upon the boundary layer conditions. Suitable sensors  109  on the surface of the fan blade  82  are connected to the control means  108  for this purpose. 
   Each of the pulse generators arranged in the fan is supplied with air via a valve which is connected to a suitable electronic control means, as shown by the broken line. 
   Air can be supplied to the fan blade by means of centrifugal force provided by the rotating blade, or by other means such as bleeds from other parts of the engine, or using an auxiliary pump. 
   Various modifications can be made without departing from the scope of the invention. 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.