Patent Publication Number: US-2018030894-A1

Title: Gas turbine engine comprising an adjustable spinner

Description:
The present disclosure concerns gas turbine engines and methods for their operation. The disclosure may be particularly relevant to higher bypass ratio aero turbofan engines, but is not limited to such applications and may also find application in lower bypass ratio turbofans and turbojets for aero applications and/or turbofans or turbojets for non-aero applications such as industrial power generation. 
     With increasing rotation speed a compressor can operate with increasing pressure ratio across it. Where however the rotation speed falls too low with respect to the pressure ratio across the compressor it may stall and/or surge (temporary reversal of flow direction). The margin between pressure ratio across a turbine for a given rotation speed and the pressure ratio at which a surge would occur at that rotation speed is called the surge margin. 
     Maintaining sufficient surge margin under various operation conditions is a known and significant constraint in terms of compressor and whole engine design in the field of gas turbine engines. Furthermore, all else being equal, the trend in civil aviation turbofan engine design to improve efficiency by using ever higher bypass ratio and ever lower fan speed tends to increase fan loading and reduce surge and flutter margin. 
     Various approaches have been considered for tackling fan stability issues in such circumstances (e.g. variable area nozzle and variable pitch fan) but these require relatively complicated actuation systems and increased weight. 
     According to a first aspect there is provided a gas turbine engine comprising a compressor and an intake leading to the compressor, the compressor having a spinner, at least part of the spinner being adjustable to alter the size of the smallest flow area provided within the intake. The at least part of the spinner may therefore be used to alter the axial velocity of a main airflow reaching the compressor in use. This in turn offers the possibility of altering the working line of the compressor to increase its surge and flutter margins. Specifically at slower compressor rotation speeds the at least part of the spinner may be used to increase the axial flow velocity, thereby reducing the compressor loading and increasing its stability. Reducing the compressor loading may also reduce a wake created by the compressor and thereby potentially reduce broadband noise created by wake interaction with downstream components (e.g. outlet guide vanes and/or engine section stators). Another potential effect of varying the size of the smallest flow area may be to reduce/prevent choking of the compressor roots. Specifically increasing the size of the smallest flow area may increase diffusion of fluid towards the compressor blade roots. This may be desirable where root choking is more likely, e.g. where the air is of higher density or during take-off. 
     In some embodiments the gas turbine engine comprises a control system arranged to adjust the at least part of the spinner to alter the smallest flow area provided within the intake. The control system may be arranged to adjust the at least part of the spinner to decrease the smallest flow area where the compressor is rotating slower and increase the smallest flow area where the compressor is rotating faster. A decrease in the smallest flow area may be used to increase the axial velocity of a main airflow reaching the compressor and thereby increase the stall margin where it would otherwise be decreased (e.g. by slower rotation of the compressor). An increase in the smallest flow area may decrease the axial velocity of the main airflow reaching the compressor and thereby increase efficiency where the compressor is at reduced risk of stalling (e.g. where there is faster rotation of the compressor). 
     In some embodiments the control system comprises an actuator that selectively adjusts the at least part of the spinner to decrease and/or increase the smallest flow area. The actuator may be at least partially housed within the spinner and/or a hub of the compressor and/or a drive transmission shaft for the compressor. The actuator may be controlled by an engine electronic controller of the engine and control system, which may adjust the at least part of the spinner in response to sensed data (e.g. airflow velocity entering the intake and/or rotation rate of the compressor). Alternatively the actuator may be controlled by a dedicated spinner controller. The spinner controller may be at least partially housed within the spinner and/or the hub of the compressor and/or a drive transmission shaft for the compressor. Further the spinner controller may adjust the at least part of the spinner in response to one or more dedicated sensors that may be provided in or on the spinner. One such dedicated sensor may be an accelerometer provided inside the spinner. The actuator may be or any suitable design (e.g. mechanical, electric, hydraulic, pneumatic, magnetic or thermal). 
     In some embodiments the control system comprises one/or more resilient bodies biasing the at least part of the spinner towards one of reducing and increasing the smallest flow area. In this embodiment the actuator (where provided) may selectively overcome this bias in adjusting the at least part of the spinner in the opposite sense to the one or more biasing bodies. Alternatively adjustment of the at least part of the spinner in the opposite sense to the one or more biasing bodies may be achieved passively as a consequence of increased airflow velocity entering the intake and impinging on the at least part of the spinner. The resilient body or bodies may be elastic and could for example comprise deformable elastic walls of the spinner or one or more springs acting between the at least part of the spinner and a support structure. 
     In some embodiments an outer wall of the intake is shaped so as the flow area varies in an axial direction and the control system adjusts the at least part of the spinner by re-locating it with respect to the outer wall. It may be for instance that the at least part of the spinner is axially translated by the control system and that by moving the at least part of the spinner into or out of alignment with a portion of the intake having a smaller flow area, the size of the smallest flow area in the intake is adjusted. 
     Additionally or alternatively the control system adjusts the at least part of the spinner by altering its shape and/or extent. This may arise through deformation and/or reorientation/translation of an external wall of the spinner. It may be for instance that the control system selectively deforms the spinner external wall by porting pressurised gas or liquid into a cavity inside the spinner and behind the external wall. In this case the gas or liquid may inflate the at least part of the spinner until it is allowed to flow away by the control system. An alternative example is deforming the external wall through the action of the actuator providing a force thereon. The external wall may for instance simply deflect under the force exerted by the actuator. Alternatively the external wall may be provided with panels that are rotatably joined, (e.g. by means of a hinge) thereby allowing deformation of the external wall through rotation of one or more of the panels with respect to one another. A further example would be to form the external wall from shape memory alloy. In this case the control system may actively heat the external wall in order to return it to a default shape following deformation by the actuator. Alternatively the temperature of the shape memory alloy may be allowed to vary naturally in accordance with normal operation of the engine. A suitably selected shape memory alloy may allow the external wall to return to a default shape following actuator deformation at desired times in an operation cycle of the engine. 
     Where the control system supplies fluid (e.g. to drive the actuator or to inflate the spinner) it may be delivered via one or more passages through a drive arm connecting the compressor and the drive transmission shaft. Alternatively fluid may be supplied from inside the drive transmission shaft. The fluid may be compressed air bled from a core flow of the gas turbine engine, or alternatively a liquid such as oil, fuel, water or glycol. Where the control system sends a signal (e.g. that adjusts the actuator) this may be done wirelessly using a suitable transmitter and receiver, or may be achieved via a wired link passing through the drive transmission shaft. Where however a dedicated spinner controller is provided, signal transmission may be simplified as there may be no need to cross a static rotating boundary. 
     In some embodiments the control system is arranged to allow continuous adjustment of the at least part of the spinner. In this way the smallest flow area provided within the intake may be selected to be at various sizes between a maximum and minimum selectable using adjustment of the at least part of the spinner. It may be for instance that various intermediate axial positions of the at least part of the spinner are selectable between axial extreme positions that are also selectable. A further example would be selection of various degrees of partial inflation of the at least part of the spinner between maximum and minimum inflations which are also selectable. Alternatively it may be that the control system is arranged to allow only binary adjustment of the at least part of the spinner corresponding to minimum and maximum smallest flow area. 
     In some embodiments at least part of the spinner external wall has a conic or domed shape. Such a portion may exaggerate alteration in the smallest flow area, particularly where the adjustment is caused by re-location of the at least part of the spinner with respect to the outer wall of the intake duct shaped to vary the flow area in the axial direction. 
     In some embodiments the gas turbine engine may be a turbofan engine and the compressor may be a fan of turbofan engine. Further a gearbox may be provided in a drive path between the drive transmission shaft and the fan. 
     The gas turbine engine may be an aero gas turbine engine. Alternatively the gas turbine engine may be an industrial power generation engine. 
     According to a second aspect of the invention there is provided a method of operating the gas turbine engine of the first aspect comprising tending to adjust the at least part of the spinner to reduce the size of the smallest flow area provided within the intake when the compressor is rotating at slower speeds and increasing the size of the smallest flow area when the compressor is rotating at higher speeds. Additionally or alternatively the method may comprise operating the gas turbine engine of the first aspect to tend to adjust the at least part of the spinner to increase the size of the smallest flow area provided within the intake with increasing likelihood of choking of a root of the compressor given the conditions under which it is operating. It may be for instance that where the air compressed by the compressor is of higher density or is travelling at higher velocity, the likelihood of compressor root choking increases and can be compensated for by increasing the size of the smallest flow area and therefor flow diffusion. As will be appreciated control logic may be used to produce a compromise size of the smallest flow area where control based on compressor speed (or otherwise designed to improve compressor stability/surge margin) is in conflict with control based on reducing/preventing compressor root choking. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a sectional side view of a gas turbine engine; 
         FIG. 2  is a cross-sectional view of part of a gas turbine engine in accordance with an embodiment of the invention; 
         FIG. 3  is a cross-sectional view of part of a gas turbine engine in accordance with an embodiment of the invention; 
         FIG. 4  is a cross-sectional view of part of a gas turbine engine in accordance with an embodiment of the invention; 
         FIG. 5  is a cross-sectional view of part of a gas turbine engine in accordance with an embodiment of the invention; 
         FIG. 6  is a cross-sectional view of part of a gas turbine engine in accordance with an embodiment of the invention; 
         FIG. 7  is a cross-sectional view of part of a gas turbine engine in accordance with an embodiment of the invention. 
     
    
    
     With reference to  FIG. 1 , a gas turbine engine is generally indicated at  10 , having a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , an intermediate pressure turbine  18 , a low-pressure turbine  19  and an exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and the exhaust nozzle  20 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high  17 , intermediate  18  and low  19  pressure turbines drive respectively the high pressure compressor  15 , intermediate pressure compressor  14  and fan  13 , each by suitable interconnecting shaft. 
     Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
     Referring now to  FIG. 2  a forward portion of a turbofan engine is generally shown at  30 . A nacelle  32  of the gas turbine engine defines an intake  34  leading to a compressor (in this case a fan  36 ). The intake  34  has an outer wall  38 . The outer wall  38  is shaped so as the flow area therein varies in an axial direction. The nacelle  32  forms an intake lip  40  at its leading edge, from which the outer wall  38  converges in a downstream direction from the intake lip  40  towards a throat  42 . The throat  42  defines the smallest flow area provided within the intake  34 . From the throat  42  the outer wall  38  diverges in a downstream direction towards the fan  36 . 
     The fan  36  has a spinner  44  positioned at its centre and projecting upstream into the intake  34 . The spinner  44  has a conic fore portion  46  oriented so as the apex  48  of the cone is furthest upstream and a cylindrical aft portion  50 . In the region of the spinner  44  the intake  34  has an annular shape defined between an external wall  52  of the spinner  44  and the outer wall  38 . 
     Blades  54  of the fan  36  are coupled via a drive arm  56  to a drive transmission shaft  58 . The drive transmission shaft  58  connects the fan  36  to a low pressure turbine (not shown). Secured to the drive transmission shaft  58  is a cylinder  60  of an actuator  62 . The actuator  62  forms part of a control system that adjust the axial position of spinner  44 , thereby relocating it with respect to the outer wall  38 . A piston  64  of the actuator  62  is connected to the spinner  44  at a bracing plate  66  spanning the interior of the cylindrical aft portion  50 . The fan  36  (including its spinner  44 ), drive transmission shaft  58  and actuator are connected so as to be fixed relative to each other. The spinner  44  rotates with the rest of the fan  36  when it rotates. 
     The actuator  62  is hydraulically operated and is controlled by an engine electronic controller (not shown). The engine electronic controller selectively actuates the piston  64  to alter its position within the cylinder  60 , thereby axially translating the spinner  44  within the intake  34 . The engine electronic controller controllers the actuator via means of a wireless link provided by a suitable transmitter and receiver. At a maximum extension of the piston  64  from the cylinder  60  the spinner  44  is located at its furthest possible upstream extent (i.e. furthest from the rest of the fan  36 ). This position of the spinner  44  is shown in shadow in  FIG. 2  and corresponds to an adjustment to the smallest flow area provided within the intake such that it is at a minimum. This reduction in the smallest flow area is caused by a larger diameter part of the conic fore portion  46  being aligned with the throat  42 . At a minimum extension of the piston  64  from the cylinder  60  the spinner  44  is located at its furthest possible downstream extent (i.e. nearest to the rest of the fan  36 ). This position of the spinner  44  is shown in  FIG. 2  and corresponds to an adjustment to the smallest flow area provided within the intake such that it is at a maximum. This increase in the smallest flow area is caused by a smaller diameter part of the conic fore portion  46  being aligned with the throat  42 . 
     In use the control system adjusts the spinner  44  in accordance with data indicative of the actual or predicted surge margin for the fan  36 . In this case specifically, the engine electronic controller monitors one or more sensed or otherwise determined parameters that are indicative of the airflow velocity entering the intake  34 . It then sets a desired spinner  44  axial position between its furthest possible upstream and downstream extents using signal data to which the actuator  60  responds accordingly. The controller adjusts the axial position of the spinner  44  using continuously variable control (although in other embodiments simple on/off style control (i.e. full forward, full back) may be used). In other embodiments one or more additional parameters impacting on surge margin may be sensed/determined and accounted for by the engine electronic controller e.g. load placed on a spool of the gas turbine engine by a generator. In further alternative embodiments the engine electronic controller may be a multi-variable controller that accounts for the impact of changes in the spinner axial position other than on surge margin. In this case the desired spinner  44  axial position set may be a compromise in view of additional operational constraints and/or desires e.g. extent/risk of fan root choking. 
     In general the engine electronic controller of the present embodiment sets the desired spinner  44  axial position such that as the axial velocity of the main airflow entering the intake  34  and reaching the fan  36  decreases, so the spinner  44  is moved further upstream and vice versa. This correspondingly and respectively decreases or increases the smallest flow area provided in the intake and so respectively increases or decreases the axial velocity of the main airflow reaching the fan  36 . In this way the controller can adjust the working line of the compressor to increase its surge and/or flutter margins. The exact algorithm or scheduling used by the engine electronic controller will vary from embodiment to embodiment. Nonetheless by way of example it may be that the scheduled relationship between a sensed parameter and the outputted spinner  44  axial position could be a straight line with equation y=mx+c or may be a curve having the form of a quadratic, cubic, or higher order polynomial equation. Alternatively the relationship may be more complex, especially where additional operational constraints/desires are addressed. 
     Although in the present embodiment the engine electronic controller is responsible for controller the spinner  44  axial position, in alternative embodiments a separate and/or dedicated spinner controller may be provided for spinner control. Such a spinner controller may be at least partially housed within the spinner and/or a hub of the compressor and/or the drive transmission shaft for the compressor. Further the spinner controller may adjust the at least part of the spinner in response to one or more dedicated sensors that may be provided in or on the spinner. One such dedicated sensor may be an accelerometer provided inside the spinner. 
     Referring now to  FIG. 3  a forward portion of a turbofan engine  130  that is similar to the arrangement of  FIG. 2  is shown. The difference concerns the control system and specifically the replacement of the actuator  62 . Similar features are provided with similar reference numerals in the series  100 . 
     A nacelle  132  of the gas turbine engine defines an intake  134  leading to a compressor (in this case a fan  136 ). The intake  134  has an outer wall  138 . The outer wall  138  is shaped so as the flow area therein varies in an axial direction. The nacelle  132  forms an intake lip  140  at its leading edge, from which the outer wall  138  converges in a downstream direction from the intake lip  140  towards a throat  142 . The throat  142  defines the smallest flow area provided within the intake  34 . From the throat  142  the outer wall  138  diverges in a downstream direction towards the fan  136 . 
     The fan  136  has a spinner  144  positioned at its centre and projecting upstream into the intake  134 . The spinner  144  has a conic fore portion  146  oriented so as the apex  148  of the cone is furthest upstream and a cylindrical aft portion  150 . In the region of the spinner  144  the intake  134  has an annular shape defined between an external wall  152  of the spinner  144  and the outer wall  138 . 
     Blades  154  of the fan  136  are coupled via a drive arm  156  to a drive transmission shaft  158 . The drive transmission shaft  158  connects the fan  136  to a low pressure turbine (not shown). A bracing plate  166  spanning the interior of the cylindrical aft portion  150  is also provided. 
     The drive arm  156  comprises a plurality of passages  168  provided there through allowing fluid communication between an interior cavity of the cylindrical aft portion  150  and air delivery passage (not shown) for delivering compressed air from a compressor bleed (not shown) to the spinner  144 . The air delivery passage is valve controlled such that the quantity of compressed air delivered to the interior cavity of the cylindrical aft portion  150  is controllable. Additionally an air dump passage (not shown) is provided in fluid communication with the air delivery passage. The air dump passage allows selective, valve controlled dumping of pressurised fluid from the interior cavity of the cylindrical aft portion  150  back through the passages  168  and out to atmosphere. 
     A resilient body, in this case a spring  170 , acts between a circumferential lip  172  extending radially outwards from the cylindrical aft portion  150  and an internal circumferential rebate  174  of a shroud  176  that is connected to the fan  136 . The shroud  176  provides a substantially continuous surface with the conic fore portion  146  that shrouds the cylindrical aft portion  150 . The spring  170  urges the spinner  144  towards being located at its furthest possible downstream extent (i.e. nearest to the rest of the fan  136 ). This position of the spinner  144  is shown in  FIG. 3  and corresponds to an adjustment to the smallest flow area provided within the intake  134  such that it is at a maximum. This increase in the smallest flow area is caused by a smaller diameter part of the conic fore portion  146  being aligned with the throat  142 . 
     In contrast air supplied to the interior cavity of the cylindrical aft portion  150  urges the spinner  144  towards being located at its furthest possible upstream extent (i.e. furthest from the rest of the fan  136 ). This is achieved as a consequence of the force the compressed air exerts on the bracing plate  166 , which overcomes the bias provided by the spring  170 . The furthest possible upstream extent of the spinner  144  location is shown in shadow in  FIG. 3  and corresponds to an adjustment to the smallest flow area provided within the intake  134  such that it is at a minimum. This decrease in the smallest flow area is caused by a larger diameter part of the conic fore portion  146  being aligned with the throat  142 . 
     The position of the spinner  144  is controlled using similar inputs and control logic as the  FIG. 2  embodiment, but with the engine electronic controller of the control system selectively actuating the valves of the air delivery passage and air dump passage to vary the quantity of pressurised fluid inside the cylindrical aft portion  150  and so the spinner  144  axial position. 
     Referring now to  FIG. 4  a forward portion of a turbofan engine  230  that is similar to the arrangement of  FIG. 2  is shown. The difference concerns the control system and specifically the positioning and effect of the actuator. Similar features are provided with similar reference numerals in the series  200 . 
     A nacelle  232  of the gas turbine engine defines an intake  234  leading to a compressor (in this case a fan  236 ). The intake  234  has an outer wall  238 . The outer wall  238  is shaped so as the flow area therein varies in an axial direction. The nacelle  232  forms an intake lip  240  at its leading edge, from which the outer wall  238  converges in a downstream direction from the intake lip  240  towards a throat  242 . The throat  242  defines the smallest flow area provided within the intake  34 . From the throat  242  the outer wall  238  diverges in a downstream direction towards the fan  236 . 
     The fan  236  has a spinner  244  positioned at its centre and projecting upstream into the intake  234 . The spinner  244  has a configuration in which it is conical in shape and is oriented so as the apex  248  of the cone is furthest upstream. In the region of the spinner  244  the intake  234  has an annular shape defined between an external wall  252  of the spinner  244  and the outer wall  238 . 
     Blades  254  of the fan  236  are coupled via a drive arm  256  to a drive transmission shaft  258 . The drive transmission shaft  258  connects the fan  236  to a low pressure turbine (not shown). A bracing plate  266  spans the interior of the spinner  244  and is connected to the transmission shaft  258 . 
     The external wall  252  if the spinner  244  is provided by inner  278  and outer  280  walls. Each of the inner  278  and outer  280  walls has a plurality of fore  282  and a plurality of aft  284  plate segments. Each fore plate segment  282  is hingedly connected at its upstream end to a nose portion  286  of the spinner  244  and at its downstream end to an upstream end of one of the aft  284  plate segments, the latter occurring at a joint  288 . Each aft plate segment  284  is hingedly connected at its downstream end to a peripheral region of the bracing plate  266 . The inner  278  and outer  280  walls are offset with respect to each other such the fore  282  and aft  284  plates of the inner wall  278  are circumferentially misaligned with similar fore  282  and aft  284  plates of the outer wall  280 . Specifically it may be that discontinuities between fore plates  282  of the outer wall  280  overlay the circumferential centres of fore plates  282  of the inner wall  278 . 
     Secured to the bracing plate  266  is a cylinder  290  of an actuator  292 . The actuator  292  forms part of a control system that adjusts the shape of the spinner  244 , thereby altering the location and size of the smallest flow area in the intake  234 . A piston  294  of the actuator  292  is connected to the nose portion  286  of the spinner  244 . When the piston  294  is at its furthest limit of travel out of the cylinder  290  it locates the nose portion  286  at the limit of its travel in an upstream direction. In this position of the nose portion  286  shown in  FIG. 4 , each fore panel  282  forms a substantially continuous flat surface with the aft panel  284  to which it is hingedly connected, thus giving the spinner  244  a conic shape. With the spinner  244  in this configuration the smallest flow area provided within the intake  234  is at a maximum. 
     In contrast when the piston  282  is fully retracted with respect to the cylinder  278 , it locates the nose portion  286  at the limit of its travel in a downstream direction. 
     This adjustment in the nose portion  286  position is accommodated in part by rotation of hingedly joined fore  282  and aft  284  panels with respect to one another about their respective joints  288 . In particular the joints  288  move outwards thereby decreasing the rake of the fore  282  panels (shown in shadow). 
     In view of this adjustment discontinuities between each adjacent fore panel  282  and each adjacent aft panel  284  increase in size. Nonetheless in view of the offset of the inner  278  and outer  280  walls, the panels  282 ,  284  of the inner wall  278  substantially block the discontinuities, larger though they are, between the panels  282 ,  284  of the outer wall  280 . Thus the spinner  244  continues to present a substantially continuous outer surface. With the spinner  244  in this configuration the smallest flow area provided within the intake  234  is at a minimum, as provided between the joints  288  and the outer wall  238 . 
     The position of the nose portion  286  and therefore the shape of the spinner  244  is controlled using similar inputs and control logic as the  FIG. 2  embodiment. Specifically the same inputs and control logic are used, but the actuator  280  is used to adjust the shape of the spinner  244 . 
     Referring now to  FIG. 5  a forward portion of a turbofan engine  330  that is similar to the arrangement of  FIG. 4  is shown. In the  FIG. 5  embodiment however, the segmented inner and outer walls of the spinner are replaced by a continuous surface  396  that is flexed in a predictable manner (between conic and domed shapes) by operation of the piston on the nose portion. With the spinner in the conic configuration, the smallest flow area provided within the intake is at a minimum, whereas with the spinner in the domed configuration, the smallest flow area provided within the intake is at a maximum. In an alternative interpretation of  FIG. 5  the outer wall of the spinner comprises shape memory alloy material supported in part by the piston of an unpowered actuator. In such an embodiment the outer wall may be selectively heated or cooled in order to change the shape of the shape memory alloy and so alter the smallest flow area provided within the intake. By way of example selective heating may be achieved by delivering hot compressed air to an interior cavity of the spinner (see for example the manner in which compressed air is delivered as described with respect to the  FIG. 6  arrangement below). 
     Referring now to  FIG. 6  a forward portion of a turbofan engine  430  is shown which combines a similar spinner structure to that of the  FIG. 5  arrangement with pneumatic control similar to the  FIG. 3  arrangement. Similar features are provided with similar reference numerals in the series  400 . 
     A nacelle  432  of the gas turbine engine defines an intake  434  leading to a compressor (in this case a fan  436 ). The intake  434  has an outer wall  438 . The outer wall  438  is shaped so as the flow area therein varies in an axial direction. The nacelle  432  forms an intake lip  440  at its leading edge, from which the outer wall  438  converges in a downstream direction from the intake lip  440  towards a throat  442 . The throat  442  defines the smallest flow area provided within the intake  1034 . From the throat  442  the outer wall  438  diverges in a downstream direction towards the fan  436 . 
     The fan  436  has a spinner  444  positioned at its centre and projecting upstream into the intake  434 . The spinner  444  has a configuration in which it is conical in shape and is oriented so as the apex  448  of the cone is furthest upstream. In the region of the spinner  444  the intake  434  has an annular shape defined between an external wall  452  of the spinner  444  and the outer wall  438 . 
     Blades  454  of the fan  436  are coupled via a drive arm  456  to a drive transmission shaft  458 . The drive transmission shaft  458  connects the fan  436  to a low pressure turbine (not shown). A bracing plate  466  spanning the interior of the cylindrical aft portion  450  is also provided. 
     The drive arm  456  and bracing plate  466  comprise a plurality of passages  468  provided there through allowing fluid communication between an interior cavity of the spinner  444  and an air delivery passage (not shown) for delivering compressed air from a compressor bleed (not shown) to the spinner  444 . The air delivery passage is valve controlled such that the quantity of compressed air delivered to the interior cavity of the spinner  444  is controllable. Additionally an air dump passage (not shown) is provided in fluid communication with the air delivery passage. The air dump passage allows selective, valve controlled dumping of pressurised fluid from the interior cavity of the spinner  444  back through the passages  468  and out to atmosphere. 
     A resilient body, in this case an elastic external wall  498  of the spinner  444  biases the spinner  444  towards having a conical shape. This configuration of the spinner  444  gives rise to the smallest flow area provided within the intake  434  being at its maximum. 
     In contrast compressed air supplied to the interior cavity of the spinner  444  tends to inflate the spinner  444  by overcoming the bias created by the elastic external wall  498 . The elastic external wall  498  is inflated through the ingress of compressed air into a chamber  499  located immediately adjacent the elastic external wall  498 . The chamber  499  is formed by the elastic external wall  498  on one side and a support wall  499   a  on the other. Chamber passages  499   b  are provided through the support wall  499   a  to allow fluid communication between the chamber  499  and the interior cavity of the spinner  444 . When fully inflated the spinner  444  has a domed shape that is commensurate with the smallest flow area provided within the intake  434  being at a minimum. 
     The shape of the spinner  444  is controlled using similar inputs and control logic as the  FIG. 2  embodiment, but with the engine electronic controller of the control system selectively actuating the valves of the air delivery passage and air dump passage to vary the quantity of pressurised fluid inside the spinner  444  and so the extent of its inflation. 
     Referring now to  FIG. 7  a forward portion of a turbofan engine  1030  is shown which similar to the arrangement of  FIG. 6 , but modified to allow more convoluted inflated spinner shapes. Similar features are provided with similar reference numerals in the series  1000 . 
     A nacelle  1032  of the gas turbine engine defines an intake  1034  leading to a compressor (in this case a fan  1036 ). The intake  1034  has an outer wall  1038 . The outer wall  1038  is shaped so as the flow area therein varies in an axial direction. The nacelle  1032  forms an intake lip  1040  at its leading edge, from which the outer wall  1038  converges in a downstream direction from the intake lip  1040  towards a throat  1042 . The throat  1042  defines the smallest flow area provided within the intake  1034 . From the throat  1042  the outer wall  1038  diverges in a downstream direction towards the fan  1036 . 
     The fan  1036  has a spinner  1044  positioned at its centre and projecting upstream into the intake  1034 . The spinner  1044  has a configuration in which it is conical in shape and is oriented so as the apex  1048  of the cone is furthest upstream. In the region of the spinner  1044  the intake  1034  has an annular shape defined between an external wall  1052  of the spinner  1044  and the outer wall  1038 . 
     Blades  1054  of the fan  1036  are coupled via a drive arm  1056  to a drive transmission shaft  1058 . The drive transmission shaft  1058  connects the fan  1036  to a low pressure turbine (not shown). A bracing plate  1066  spanning the interior of the cylindrical aft portion  1050  is also provided. 
     The drive arm  1056  and bracing plate  1066  comprise a plurality of passages  1068  provided there through allowing fluid communication between an interior cavity of the spinner  1044  and an air delivery passage (not shown) for delivering compressed air from a compressor bleed (not shown) to the spinner  1044 . The air delivery passage is valve controlled such that the quantity of compressed air delivered to the interior cavity of the spinner  1044  is controllable. Additionally an air dump passage (not shown) is provided in fluid communication with the air delivery passage. The air dump passage allows selective, valve controlled dumping of pressurised fluid from the interior cavity of the spinner  1044  back through the passages  1068  and out to atmosphere. 
     A resilient body, in this case an elastic external wall  1098  of the spinner  1044  biases the spinner  1044  towards having a conical shape. This position of the spinner  1044  gives rise to the smallest flow area provided within the intake  1034  being at its maximum. 
     In contrast compressed air supplied to the interior cavity of the spinner  1044  tends to inflate the spinner  1044  by overcoming the bias created by the elastic external wall  1098 . The elastic external wall  1098  is inflated through the ingress of compressed air into discrete chambers  1100  located immediately adjacent the elastic external wall  1098 . The chambers  1100  are formed by the elastic external wall  1098  on one side, a manifold wall  1102  on the other and side walls  1104  which separate adjacent chambers  1100 . Chamber passages  1106  are provided through the manifold wall  1102  to allow fluid communication between each chamber  1100  and the interior cavity of the spinner  1044 . When fully inflated the spinner  1044  has a distorted dome shape, the distortion arising in view of the length of each side wall  1104  having been selected so as to restrain the inflation of the elastic external wall  1098  in a manner so as to give that overall shape. When the spinner  1044  is fully inflated the smallest flow area provided within the intake  1034  is at a minimum. 
     The shape of the spinner  1044  is controlled using similar inputs and control logic as the  FIG. 2  embodiment, but with the engine electronic controller of the control system selectively actuating the valves of the air delivery passage and air dump passage to vary the quantity of pressurised fluid inside the spinner  1044  and so the extent of its inflation. 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the appended claims. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.