Abstract:
A variable area nozzle for a gas turbine engine is provided that has a circumferential outer boundary (which may be formed at least in part by the nacelle of a turbofan engine) that has a fixed portion and a movable portion. The movable portion is movable by an actuator both upstream and downstream from a datum position. This allows the exit area of the variable area nozzle to be adjusted according to flight conditions. When the movable portion is at or upstream of the datum position, there is no flow path between the fixed and movable portions. This enables the nozzle exit area to be optimized throughout flight, for example during changing cruise conditions, without inducing unwanted and inefficient flow structures between the fixed and movable portions and without allowing flow to leak out of the outer boundary of the nozzle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from British Patent Application Number 11178241 filed 17 Oct. 2011, the entire contents of which are incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention is concerned with variable area nozzles, and in particular with variable area nozzles for gas turbine engines. 
         [0004]    A ducted fan gas turbine engine  10  is shown in  FIG. 1  on the wing  30  of an aircraft. The gas turbine engine  10  is attached to the wing  30  using a pylon  35 .  FIG. 2  shows a cross-section through the gas turbine engine  10 . 
         [0005]    The gas turbine engine  10  has a principal and rotational axis X-X. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , and intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . The ducted fan gas turbine engine  10  has a bypass duct  22 . A bypass exhaust nozzle  25  is defined between the trailing edge of a bypass duct casing  23  and a core casing  24 . 
         [0006]    The gas turbine engine  10  works in a conventional manner so that air entering through the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow A into the intermediate pressure compressor  13  and a second air flow B which passes through the bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow A directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
         [0007]    The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  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  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  16 ,  17 ,  18  respectively drive the high and intermediate pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting shafts. 
         [0008]    In the example shown in  FIGS. 1 and 2 , the bypass exhaust nozzle  25  is defined by a fixed bypass duct casing  23  and a fixed core casing  24 . However, the optimum exit area of the bypass exhaust nozzle  25  depends on the operating condition of the gas turbine engine  10 . For example, the optimum exit area of the bypass exhaust nozzle  25  may depend on the flight phase of an aircraft to whose wing  30  the gas turbine engine  10  is attached. Thus, for example, the fixed geometry of the bypass duct casing  23  and the core casing  24  in the  FIGS. 1 and 2  example may typically be a compromise between providing the optimum bypass exhaust nozzle  25  area at take-off, landing, and cruise. 
         [0009]    2. Description of the Related Art 
         [0010]    Some gas turbine engines therefore provide so-called variable area nozzles. Such variable area nozzles have movable geometry which enables the area of the bypass exhaust nozzle  25  to be varied. For example, the geometry may move so as to provide a larger bypass exhaust nozzle  25  exit area at take-off/landing than at cruise. 
         [0011]    An example of movable geometry that may be employed to change the exit area of the bypass exhaust nozzle is shown in  FIGS. 3   a  and  3   b . In the  FIGS. 3   a  and  3   b  example, the bypass flow B is shown through the bypass duct  22  (the arrow B is a schematic representation of the bypass flow and may not represent the precise bypass flow direction). The bypass duct  22  is formed between the core casing  24  and a bypass duct casing  40 . The bypass duct casing  40  comprises a first, fixed portion  42  (which may be part of what is commonly referred to as a nacelle). The bypass duct casing  40  also comprises a second portion  44  that is movable in an axially rearward direction (or downstream direction) C relative to the closed position shown in  FIG. 3   a . The mechanism for moving the second portion  44  is not shown in  FIGS. 3   a  and  3   b.    
         [0012]    The core casing  24  in the  FIGS. 3   a  and  3   b  example has a profile which includes a “bump”  26 . This bump  26  extends in a radial direction relative to the engine axis X-X. Due to the shape and position of the bump  26 , rearward axial translation C of the second portion  44  of the bypass duct casing  40  (from the configuration shown in  FIG. 3   a  to the configuration shown in  FIG. 3   b ) results in an increase in the exit area of the bypass exhaust nozzle  25 . This arrangement also opens a secondary nozzle flow path D when the second portion  44  is translated axially rearward. The secondary flow path D effectively further increases the nozzle exit area. 
         [0013]    Thus, the arrangement shown in  FIGS. 3   a  and  3   b  can be used to increase the exit area of the bypass exhaust nozzle  25  from a datum position, for example for use during take-off/landing. However, the  FIGS. 3   a  and  3   b  example is unable to provide an optimized nozzle geometry for all flight phases. For example, the optimum exit area of the nozzle  25  changes during cruise depending on conditions (such as flight speed, air conditions and weight, which changes as fuel is burned during flight). As such, in order to provide the optimum area of the nozzle  25  at all cruise conditions, for example, the second portion  44  of the bypass duct casing  40  would in some cases need to be moved rearward relative to the closed position shown in  FIG. 3   a . The rearward movement would typically be less than that shown in  FIG. 3   b , which is intended to represent a take-off/landing configuration. 
         [0014]    A close-up of a configuration that might be required to provide optimum nozzle  25  exit area during certain cruise conditions is shown in  FIG. 4 . In the  FIG. 4  configuration, the second portion  44  of the bypass duct casing  40  has been shifted slightly axially rearward in order to generate the optimum nozzle exit area. As a result, there is a discontinuity, gap, or step, between the first (fixed) portion  42  and the second portion of the bypass duct casing  40 . This results in flow losses, for example due to leakage flow L through a gap  46  and/or recirculation flow M, which may be a standing circumferential vortex, and/or disturbance of the external flow (i.e flow around the outside of the engine  10 ) due to the step created on the outer surface of the nacelle. Such losses result in reduced engine thrust and/or efficiency and/or drag, for example increased nacelle drag. As such, with the  FIGS. 3 and 4  configuration the bypass exhaust nozzle  25  either has to present a sub-optimal outlet area to the bypass flow B at some operating conditions (by retaining the closed configuration shown in  FIG. 3   b  throughout cruise), or the nozzle outlet area can be optimized, but at the expense of presenting a loss-inducing discontinuity to the flow (as in the  FIG. 4  configuration). Thus, the configuration of  FIGS. 3 and 4  cannot provide optimal nozzle flow at all cruise conditions. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0015]    According to an aspect of the invention, there is provided a variable area nozzle for a gas turbine engine. The variable area nozzle comprises an inner boundary and an outer boundary defining a generally annular nozzle flow path therebetween. The outer boundary comprises a fixed portion and a movable portion. The variable area nozzle comprises an actuator configured to move the movable portion in either an upstream direction or a downstream direction relative to a datum position. A biasing element configured to provide a biasing force to at least a part of the movable portion in the downstream direction when the movable portion is moved to an upstream position (relative to datum) is provided. An exit area of the nozzle flow path is defined between the movable portion (for example the downstream edge thereof) and the inner boundary. The inner boundary is shaped such that the exit area is dependent on the position of the movable portion. When the movable portion is at the datum position or upstream thereof, the fixed portion and the movable portion are arranged so as to have no flow path therebetween. 
         [0016]    The outer boundary, or a part thereof (such as the fixed portion), may be a part of the nacelle. The inner boundary may be formed by the core shroud, or cowl. The nozzle flow path may be part of a bypass flow through the gas turbine engine, which may be a turbofan. The upstream-downstream direction may be with respect to the direction of travel of the engine and/or the general flow direction through the nozzle flow path. The upstream-downstream direction may be parallel to an axial direction X-X of the engine. 
         [0017]    According to this arrangement, a movable portion of a variable area nozzle can move both upstream and downstream of a datum position. When moved to an upstream direction, there is no flow path between the movable and fixed portions. As such, the outer boundary may be sealed when the movable portion is in the datum position or upstream thereof. This may mean that when the movable portion is in a datum position or upstream thereof, at least a part of the fixed portion and at least a part of the movable portion are engaged, for example are in contact, for example at an interface. This may allow the geometry of the nozzle to be adjusted to suit all flight conditions, for example all cruise conditions. For example, such an arrangement may allow the nozzle area to be optimized for all cruise conditions and/or climb conditions, whilst minimizing/eliminating losses created by a lack of seal between the movable portion and the fixed portion. 
         [0018]    The datum position may simply be the most downstream position of the moveable portion at which there is no flow path between the moveable portion and the fixed portion. The datum position may, for example, correspond to the most downstream position of the moveable portion during cruise (and optionally climb) that is required to ensure that the nozzle exit area is optimized throughout cruise (and optionally climb). Thus, by way of example, if the moveable portion is moved upstream during cruise to maintain the optimum exit area, the datum position may correspond to the position of the moveable portion at the start of cruise. By way of further example, if the moveable portion is moved downstream during cruise to maintain the optimum exit area, the datum position may correspond to the position of the moveable portion at the end of cruise. Whether the moveable portion is moved upstream or downstream during cruise to maintain the optimum exit area may depend on a number of factors such as, for example, the shape of the inner boundary. 
         [0019]    Shaping the inner boundary such that the exit area is dependent on the position of the moveable portion means that the exit area can be varied simply be changing the position of the moveable portion (for example axially) whilst retaining sealing (or no flow path) between the moveable portion and the fixed portion (for example an air tight seal to prevent the relatively high pressure air in the nozzle escaping through the outer boundary, for example between the moveable portion and the fixed portion), at least when the moveable portion is at the datum position or upstream thereof. The arrangement may thus allow simultaneous improvements in engine efficiency (for example a reduction in specific fuel consumption) both by varying the exit area (of the nozzle) and by preventing high pressure air escaping, for example trough the outer boundary. 
         [0020]    The actuator may thus be arranged to move the moveable portion in a substantially axial direction, and the exit area of the nozzle flow path may be dependent on the axial position of the moveable portion. This may be a particularly effective way of ensuring a seal whilst varying the area of the nozzle. 
         [0021]    When the movable portion is at the datum position or upstream thereof, the outer boundary may form a substantially continuous outer surface of the annular nozzle flow path. This may mean that the outer surface (for example a substantially cylindrical outer surface) formed by the outer boundary forming the nozzle flow path may have substantially no discontinuities, steps, or gaps, even between the fixed portion and the movable portion. This may further help to reduce/minimize any losses that may otherwise be generated through unwanted flow disturbance, such as that explained above in relation to  FIG. 4 . 
         [0022]    The inner boundary may be shaped such that the exit area increases as the movable portion is moved in the downstream direction from the datum position. The inner boundary may be shaped such that the exit area decreases as the movable portion is moved in the upstream direction from the datum position. Such an arrangement may be formed, for example, using an appropriately positioned “bump” on the inner boundary. According to this feature, the nozzle exit area may be increased from the datum position in the downstream direction, for example for take-off/landing. The nozzle area may be decreased, for example to accommodate changing cruise conditions during flight, by moving the movable portion in the upstream direction from the datum position. The optimum nozzle area may dependent on a wide range of variable, such as flight Mach Number, air pressure, altitude, weight and/or throttle position, amongst others. Purely by way of example, in some flight missions, it may be desirable to have a reduced nozzle area during the climb phase compared with the cruise phase, and/or to gradually reduce the nozzle area through the cruise. The nozzle walls (inner and outer boundaries) may remain substantially airtight, or sealed (for example there may be no gap between the moveable portion and the fixed portion) throughout at least some (for example all) of the cruise and/or climb phases. 
         [0023]    The movable portion and the fixed portion may be in contact with each other at an interface when the movable portion is in the datum position and upstream thereof. The biasing element may comprise a flexible membrane. The flexible membrane may be provided at the interface. The flexible membrane may be configured to deform as the movable portion is moved from the datum position to an upstream position. 
         [0024]    Such a flexible membrane may be a particularly convenient and effective way of ensuring that there is no flow path between the movable and fixed section and/or that a suitable seal is formed therebetween when the movable portion is in the datum position and upstream thereof. The flexible membrane may be elastically deformable. The flexible membrane may provide a biasing force in the downstream direction when the movable portion is upstream of its datum position. 
         [0025]    The biasing element may comprise a hinged portion provided at the interface between the fixed and movable portions. The hinged portion may be configured to rotate as the movable portion is moved from the datum position to an upstream position. The hinge may have a spring, for example a torsion spring, to bias the hinged portion towards a closed position. Such a closed position may be the position of the hinged portion in the absence of any force acting on it from the movable portion. This may be, for example, when the movable portion is at or downstream of the datum position. In some embodiments, the biasing element may comprise both a hinged portion and a flexible membrane. 
         [0026]    The variable area nozzle may be arranged such that (depending on the particular arrangement) when the movable portion is in the datum position or in a position downstream thereof, the flexible membrane is in an undeformed state and/or the hinged portion is in a closed position. 
         [0027]    Any suitable arrangement of flexible membrane and/or hinged portion may be used. For example, the flexible membrane or hinged portion may be provided on the fixed portion of the outer boundary. For example, the flexible membrane or hinged portion may be provided on a rear, or downstream side of the fixed portion. This may be the region of the fixed portion that engages with the movable portion, for example when the movable portion is at the datum position or upstream thereof. The deformation/movement of the flexible membrane/hinged portion may then by due to the force, in the upstream direction, provided by the movable portion. 
         [0028]    Alternatively (or additionally), the flexible membrane or hinged portion may be provided on the movable portion of the outer boundary, for example on an upstream region of the movable portion, which may engage with the fixed portion, for example when the movable portion is at the datum position or upstream thereof. The deformation/movement of the flexible membrane/hinged portion may then be due to the force, in the downstream direction, provided by the fixed portion. 
         [0029]    When the movable portion is in the datum position or in a position downstream thereof, the fixed portion of the outer boundary may have a baseline shape in which the flexible membrane or hinged portion form a seal with the rest of the portion (i.e. fixed portion or movable portion) of which it is a part. Thus, the baseline shape may have a substantially continuous surface. However, the surface may have an opening to allow the actuator to pass through. 
         [0030]    In examples having a biasing element, the biasing element may comprise a spring, When the movable portion is moved in the upstream direction from the datum position, the spring may be compressed. In this way, the spring may provide a biasing force, for example to the movable portion. 
         [0031]    The movable portion may comprise an upstream element and a downstream element. 
         [0032]    The upstream element and the downstream element may be connected together by a spring. The downstream element may be movable upstream relative to the upstream element from the datum position through compression of the spring. In such an arrangement, both the upstream element and the downstream element may move when the actuator moves the movable portion in the downstream direction, but only one element (e.g. the downstream element) may move when the actuator moves the movable element in the upstream direction. As such, the upstream element may remain stationary when downstream element moves upstream from the datum position under the action of the actuator. 
         [0033]    The upstream element may be connected to the fixed portion by a spring. The downstream element may be movable downstream relative to the upstream element from the datum position. Both the upstream element and the downstream element may be movable upstream relative to the fixed portion through compression of the spring. In such an arrangement, only the downstream element may move when the actuator moves the downstream movable portion in the downstream direction, but both elements may move when the actuator moves the downstream movable element in the upstream direction. As such, the upstream element may remain stationary when downstream element moves downstream from the datum position under the action of the actuator. 
         [0034]    The variable area nozzle may further comprise a circumferential seal, which may be arranged to form a substantially airtight seal between the fixed portion and the movable portion. The circumferential seal may be biased such that the substantially airtight seal between the fixed portion and the movable portion is maintained regardless of the position of the movable portion. Additionally or alternatively, the circumferential seal may be biased such that the substantially airtight seal between the fixed portion and the movable portion is maintained when the movable portion moves in the upstream direction from the datum position. The circumferential seal may be, for example, a hinged seal and/or a resiliently biased seal. The circumferential seal may form a seal on the outer boundary, and/or on the freestream facing surfaces between the fixed and movable portions. 
         [0035]    In some arrangements, when the movable portion is moved in the downstream direction from the datum, a secondary exit flow area is formed between the fixed portion and the movable portion. 
         [0036]    The actuator, or a moving part thereof or attached thereto, may pass through a downstream surface of the fixed portion and/or an upstream surface of the movable portion. For example, the actuator, or a moving part thereof or attached thereto, may pass through a flexible membrane or hinged portion, where these elements are present. As such, the actuator (or a part thereof) may be located in the fixed portion, which may have packaging advantages. 
         [0037]    The variable area nozzle described above and herein in relation to the invention may be used in any suitable application. For example, the variable area nozzles may be used in a gas turbine engine (such as, by way of example only, turbojet, turboprop or turbofan engines), for example for use on an aircraft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Embodiments of the invention will now be described by way of example only, with reference to the accompanying diagrammatic drawings, in which: 
           [0039]      FIG. 1  is a schematic showing a gas turbine engine mounted on a the wing of an aircraft; 
           [0040]      FIG. 2  is a cross-section through a gas turbine engine such as that shown in  FIG. 1 ; 
           [0041]      FIG. 3   a  is a cross-section through a variable area nozzle with the nozzle in a closed configuration; 
           [0042]      FIG. 3   b  is a cross-section through the variable area nozzle of  FIG. 3   a  with the nozzle in an open configuration; 
           [0043]      FIG. 4  is an enlarged view of a cross-section through the variable area nozzle of  FIGS. 3   a  and  3   b  with the nozzle in a partially open configuration; 
           [0044]      FIG. 5   a  shows a cross section through geometry for a variable area nozzle according to the invention with a movable outer boundary portion in the datum configuration; 
           [0045]      FIG. 5   b  shows the variable area nozzle of  FIG. 5   a  with the movable portion moved to a downstream position relative to the datum position; 
           [0046]      FIG. 5   c  shows the variable area nozzle of  FIG. 5   a  with the movable portion moved to an upstream position relative to the datum position; 
           [0047]      FIG. 6  shows a cross section through a variable area nozzle according to the invention showing both the inner and outer boundary walls of the nozzle; 
           [0048]      FIG. 7   a  shows a cross section through geometry for a variable area nozzle according to another example of the invention with a movable outer boundary portion in the datum configuration; 
           [0049]      FIG. 7   b  shows the variable area nozzle of  FIG. 7   a  with the movable portion moved to an upstream position relative to the datum position; 
           [0050]      FIG. 8   a  shows a cross section through geometry for a variable area nozzle according to another example of the invention with a movable outer boundary portion in the datum configuration; 
           [0051]      FIG. 8   b  shows the variable area nozzle of  FIG. 8   a  with the movable portion moved to a downstream position relative to the datum position; 
           [0052]      FIG. 8   c  shows the variable area nozzle of  FIG. 8   a  with the movable portion moved to an upstream position relative to the datum position; 
           [0053]      FIG. 9   a  shows a cross section through geometry for a variable area nozzle according to another example of the invention with a movable outer boundary portion in the datum configuration; 
           [0054]      FIG. 9   b  shows the variable area nozzle of  FIG. 8   a  with the movable portion moved to an upstream position relative to the datum position; and 
           [0055]      FIG. 9   c  shows the variable area nozzle of  FIG. 8   a  with the movable portion moved to a downstream position relative to the datum position. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]    As explained above, a problem with some variable area nozzles is that they are not able to provide optimal nozzle outlet area throughout the flight phase (for example throughout cruise) without generating nozzle surfaces that cause increased drag and/or engine losses. An example of such a surface that may generate loss, for example during some cruise conditions, has been described above in relation to  FIG. 4 . 
         [0057]      FIG. 5   a  shows an outer boundary  100  for a variable area nozzle in accordance with the invention.  FIG. 5   a - 5   c  (and other examples described herein) may be a cross section through an axisymmetric outer boundary  100  (about the engine axis X-X), which may, for example, be substantially cylindrical or conical. The outer boundary  100  has a fixed portion  110  and a movable portion  120 . The outer boundary  100  has a flexible membrane  130 . The flexible membrane  130  may be referred to as a biasing element  130 . 
         [0058]    In the  FIG. 5   a  configuration, the movable portion  120  is shown in a datum position. In this datum configuration, the movable portion  120  is immediately adjacent, or abutting, the flexible membrane  130 . The movable portion may be in contact with the flexible membrane  130 . The movable portion may be in a position at which it does not exert any force on the flexible membrane  130 . As such, the flexible membrane  130  (or at least a deformable portion thereof), in its undeformed state, may have the same shape as the corresponding portion (which may be a leading edge portion) of the movable portion  120 . The flexible membrane  130  and the movable portion  120  may tessellate, for example when the movable portion is in the datum position or upstream thereof. The datum position shown in  FIG. 5   a  may be used at an appropriate flight condition. For example, the nozzle geometry may typically be set such that the datum position is used at the start of cruise (for example just after the climb phase). However, nozzle geometry may be set such that the datum position is used at other times (for example dependent on the flight mission, aircraft, and/or engine), for example the end of cruise (for example just before decent), or at a typical midpoint during cruise. 
         [0059]      FIG. 5   b  shows the  FIG. 5   a  variable area nozzle outer boundary  100  with the movable portion  120  moved in the downstream direction. In  FIG. 5   b , the movable portion  120  has moved away from the fixed portion  110 . The movement of the movable portion  120  may be said to be along the engine axis (or longitudinal engine axis) X-X. In  FIG. 5   b , the movable portion  120  has been moved a distance x in a downstream axial direction. In some embodiments, translation of the movable portion  120  may not be purely axial, i.e. the translation may have a component that is non-axial. The distance x may be any suitable distance. The distance x may change depending on the flight phase, for example x may be different for take-off and landing, and indeed for different phases within take-off and landing. The distance x at full extent may be, for example, in the range of from 0 mm to 500 mm, for example 50mm to 400 mm, for example 100 mm to 300 mm, for example 150 mm to 200 mm. The distance x at full extent may vary depending on various factors, including the type of engine, and in some embodiments may be outside these ranges. 
         [0060]    Moving the movable portion  120  to a downstream position, as shown in  FIG. 5   b , may result in an increase in the nozzle exit area. This may be due to, for example, the shape of the inner boundary of the nozzle. Such an effect will be appreciated from  FIG. 6 , which shows a cross section through a nozzle  200  including the outer boundary  100  of  FIGS. 5   a  to  5   c . In  FIG. 6 , the movable portion  120  is shown in the datum position. In the datum position, the nozzle exit area is indicated by the dashed arrow labeled NA. The nozzle exit is formed between the trailing edge  125  of the movable portion  120  and the inner boundary  140  (which may be formed, for example, by a core cowl). 
         [0061]      FIG. 6  illustrates an actuator  160  that may be used to move the movable portion  120  relative to the fixed portion  110 . It will be appreciated that any appropriate actuator  160  could be used, In the  FIG. 6  example, the actuator  160  has a portion  162  attached to the fixed portion  110  and a portion  164  attached to the movable portion  120 . The actuator  160  also has an actuator arm  165  that may slide within the portion  162  attached to the fixed portion  110  of the outer boundary  100 , as indicated by the arrow p, for example in the upstream/downstream direction. The actuator arm  165  may pass through the fixed portion  110  and the movable portion  120 . The actuator arm  165  may pass through the flexible membrane  130 . One or more seals may be provided to seal gaps between the actuator arm  165  and one or more of the fixed portion  110 , the movable portion  120  and the flexible membrane  130 . One or more actuators  160  and actuator arms  165  may be provided. For example, a circumferential array of actuators  160  and/or actuator arms  165  may be provided. The actuator  160  described herein, or any other suitable actuator, could be used with any embodiment, whether or not explicitly described herein. 
         [0062]    Downstream movement of the actuator arm  165  may result in the movable portion  120  moving downstream to a configuration such as that shown in  FIG. 5   b . In this configuration, the trailing edge  125  of the movable portion  120  may also move downstream, to a position labeled  125   d  in  FIG. 6 . In this downstream position (relative to the datum position), the nozzle exit area is indicated by the dashed arrow NM. This is representative of the nozzle area produced by the  FIG. 5   b  configuration. 
         [0063]    As can be seen in  FIG. 6  (which, like the other Figures, may not necessarily be to scale), the nozzle area NAd when the movable portion  120  is downstream of the datum position is bigger, in this embodiment, than the nozzle area NA when the movable portion  120  is in the datum position. For example, the nozzle area NAd may be up to, for example, at least 30% bigger than the nozzle area NA, for example 25% bigger, 20% bigger, 15% bigger, 10% bigger, 5% bigger, 2% bigger or less than 2% bigger. This is due to the shape of the inner boundary  140 . In particular, the inner boundary  140  has a hump  150 , or radially extending bump, that extends in the radial direction. The axial position of the maximum radial extent of the hump  150  is upstream of the trailing edge  125  of the movable portion  120  when in the datum position. This means that the downstream axial movement of the movable portion  120  (for example from the datum position) results in a greater radial separation between the trailing edge  125   d  and the inner boundary  140 , and thus a greater nozzle exit area NAd. 
         [0064]    Similarly, due to the axial position of the hump  150  on the inner boundary  140  described above, upstream movement of the movable portion  120  (for example from the datum position) results in a smaller radial separation between the trailing edge  125   u  and the inner boundary  140 , and thus a smaller nozzle exit area NAu. Such a configuration is shown in  FIG. 5   c , with the trailing edge of the movable portion  120  being at the position labeled  125   u  in  FIG. 6 . Again, any suitable actuator, such as the actuator  160  shown in  FIG. 6 , may be used to move the movable portion upstream, for example from the datum position. 
         [0065]    In  FIG. 5   c , the movable portion  120  has been moved a distance y, in an upstream axial direction. In some embodiments, the movement may not be entirely axial, i.e. it may have a component in a non-axial direction. The distance y may be any suitable distance. The distance y may change depending on the flight phase, for example y may vary during cruise. The distance y at full extent may be, for example, in the range of from 2 mm to 50 mm, for example 5 mm to 40 mm, for example 10 mm to 25 mm. The distance y at full extent may vary depending on various factors, including the type of engine, and in some embodiments may be outside these ranges. 
         [0066]    The nozzle exit area NAu with the movable portion  120  in the upstream position shown in  FIG. 5   c  may be up to, for example 10% smaller, for example around 5% smaller, for example around 2% smaller than the nozzle exit area NA when the movable portion  120  is in the datum position shown in  FIG. 5   a.    
         [0067]    Alternative configurations of nozzle geometry may be employed. Purely by way of example, the inner boundary  140  may have an alternative shape that is arranged such that downstream movement of the movable portion  120  results in a decrease in nozzle area, and upstream movement of the movable portion  120  results in an increase in nozzle area. An example of such an alternative shape of inner boundary  140 ′ is shown as the dashed line  140 ′ in  FIG. 6 . It will be appreciated that many other shapes of inner and outer boundary walls could be used to give the desired increase/decrease in nozzle area with movement of the movable portion  120 . 
         [0068]    The description herein relating to the relationship between the axial position of the movable portion  120  and the nozzle area NA may apply to any embodiment whether or not explicitly described herein. For example, any description relating to the shape of the inner boundary  140 ,  140 ′ may apply to any embodiment, whether or not explicitly described herein. 
         [0069]    In the  FIG. 5  embodiment, a flexible membrane  130  is provided on a downstream, or trailing edge, portion of the fixed portion  110  in order to accommodate upstream movement of the movable portion  120 , such as that shown in  FIG. 5   c . As the movable portion  120  moves upstream, an upstream portion of the movable portion  120  remains in contact with, and/or forms a seal with, the flexible membrane  130 . The flexible membrane  130  flexes, for example deforms, for example elastically deforms, under the action of the movable portion  120  as it is moved to an upstream position shown in  FIG. 5   c.    
         [0070]    Other configurations of embodiments including a flexible membrane  130  may be used. For example, a flexible membrane may be provided on the upstream side of the movable portion  120 , for example at least over the portion where the downstream portion  120  engages the upstream portion  110 . 
         [0071]      FIGS. 7   a  and  7   b  show a configuration in which at least one hinged portion is used to allow the downstream element  120  to move upstream from a datum position. The downstream element  120  is shown in the datum position in  FIG. 7   a , and in an upstream position relative to the datum position in  FIG. 7   b . The  FIGS. 7   a  and  7   b  configuration comprises a hinged portion  170  which is mounted on, and may be considered to be a part of, the fixed portion  110 . The hinged portion  170  in this embodiment is a rotatable hinged portion  170  that is configured to rotate about a hinge  175 . 
         [0072]    As shown in  FIG. 7   b , as the movable portion  120  is moved upstream of the datum position, in the direction of the arrow q, the hinged portion  170  is configured to rotate about the hinge  175 . This enables part of the movable element  120  to move inside the fixed element  110 . A seal may be maintained between the fixed portion  110  and the movable portion  120  as the movable portion is moved upstream from the datum position, for example between the movable portion  120  and the hinged portion  170 . As such, no flow may pass between the movable portion  120  and the fixed element  110  when the movable portion is in the datum position or upstream thereof. The hinged portion  170  may be biased towards its closed position shown in  FIG. 7   a  when it is in the rotated position shown in  FIG. 7   b . For example, the hinged portion  170  may be biased towards its closed position by the hinge  175 , which may be a spring loaded hinge  175 . 
         [0073]    The  FIGS. 7   a  and  7   b  embodiment may comprise more than one hinged portion  170 . The hinged portions  170  (or at least a part of the hinged portions) may circumferentially overlap with neighbouring hinged portions  170 . This may allow the hinged portions to move in the direction of arrow r, by rotating about hinge  175 , without a gap forming between neighbouring hinged portions. 
         [0074]    When the movable portion  120  is moved in the downstream direction relative to the datum configuration shown in  FIG. 7   a  (not shown in the Figures), the hinged portion  170  may remain in the closed position shown in  FIG. 7   a . As with the  FIG. 5  embodiment, a secondary flow path may open between the movable portion  120  and the fixed portion  110  when the movable portion is moved downstream of the datum position. 
         [0075]    In alternative arrangements, for example, the hinged portion may be provided on an upstream part of the movable portion  120  rather than on the fixed portion  110 . 
         [0076]      FIGS. 8   a ,  8   b , and  8   c  show a configuration in which the movable portion  120  comprises a spring  180 . The spring  180  could be any compressible element, such as (by way of non-limitative example) a coil spring (which may extend circumferentially) or a resilient element, such as an elastically deformable foam element. The movable portion  120  comprises an upstream element (or upstream movable portion)  122  and a downstream element (or downstream movable portion)  124 . In the  FIG. 8  arrangement, the upstream element  122  is connected to the downstream element  124  via the spring  180 . Thus, the spring  180  is disposed between the upstream element  122  and the downstream element  124 . 
         [0077]      FIG. 8   a  shows the movable portion  120  in the datum position. In the datum position, the upstream element  122  of the movable portion  120  engages with the fixed portion  110 , such that there is substantially no flow path therebetween. In  FIG. 8   b , the movable portion  120  has moved downstream. This may mean that the upstream element  122  and the downstream element  124  of the movable portion  120  have moved downstream together from the datum position, with substantially no relative movement between the upstream element  122  and the downstream element  124 . 
         [0078]    In  FIG. 8   c , the downstream element  124  has moved upstream. The downstream element  124  has moved upstream relative to both the upstream element  122  (which may remain stationary relative to the fixed portion  110 ) and relative to the fixed portion  110 . The upstream element  122  may remain stationary relative to the fixed portion  110 . In order to accommodate the upstream movement of the downstream element  124 , the spring  180  is compressed. As such, the axial spacing between the downstream element  124  and the upstream element  122  decreases. This means that the trailing edge of the movable portion  120  moves upstream, and thus the nozzle exit area changes, for example decreases as explained above in relation to  FIGS. 5 and 6 . Thus, the movable portion  120  in the  FIG. 8  embodiment may be said to be compressible. 
         [0079]    The gap between the upstream and downstream elements  122 ,  124  in which the spring is located is sealed with a sealing flap  190  in the  FIG. 8  arrangement. The sealing flap  190  may comprise a radially inner sealing flap and a radially outer sealing flap. Each of the radially inner and radially outer sealing flaps may comprise a plurality of sealing flap leaves, parts of which may overlap neighbouring sealing flap leaves in some configurations, for example the configuration shown in  FIG. 8   c . The sealing flap or sealing flap leaves  190  are configured so as to provide a smooth flow path for the nozzle flow past the outer boundary and/or so as to ensure that no nozzle flow passes through the movable portion  120 . The or each sealing flap  190  may be biased so as to maintain the seal between the upstream and downstream elements  122 ,  124 . For example, the or each sealing flap  190  may be mounted on the upstream element  122  and radially biased towards the downstream element  124 , or mounted on the downstream element  124  and radially biased towards the upstream element  122 . The bias of the sealing flap(s) may be provided, for example, by a hinge, for example a sprung hinge. Sealing flaps  190  such as those described above in relation to  FIG. 8  may be provided to any suitable embodiment, including the embodiment shown in  FIG. 9  and discussed below. 
         [0080]      FIGS. 9   a ,  9   b , and  9   c  show another configuration in which the moveable portion comprises a spring or compressible member  180 , such as that described above in relation to  FIGS. 8   a - 8   c.  The configuration of  FIGS. 9   a - 9   c  includes an upstream movable portion  126  and a downstream movable portion  128 , which together may be referred to as the movable portion  120 . The upstream movable portion  126  is connected to the fixed portion  110  by the spring  180 . The downstream movable portion  128  is connected to the fixed portion  110  by the actuator arm  165 . The upstream movable portion  126  and the downstream movable portion  128  may not be directly connected to each other. 
         [0081]      FIG. 9   a  shows the arrangement in the datum position. In the  FIG. 9   a  configuration, the spring  180  may be undeformed, i.e. the spring  180  may be neither compressed nor expanded. In the  FIG. 9   a  configuration, there may be no gap, or substantially no gap between the upstream movable portion  126  and the downstream movable portion  128 . 
         [0082]      FIG. 9   b  shows the arrangement when the upstream portion  126  and the downstream portion  128  have been moved in the upstream direction, for example under action of the actuator  160 , thereby compressing the spring  180 . This means that the trailing edge of the movable portion  120  moves upstream relative to the  FIG. 9   a  datum configuration, and thus the nozzle exit area changes, for example decreases as explained above in relation to  FIGS. 5 and 6 . 
         [0083]      FIG. 9   c  shows the arrangement when the downstream movable portion  128  has been moved in the downstream direction, for example under action of the actuator  160 . This means that the trailing edge of the movable portion  120  moves downstream relative to the  FIG. 9   a  datum configuration, and thus the nozzle exit area changes, for example increases as explained above in relation to  FIGS. 5 and 6 . The spring  180  remains undeformed in this configuration, as in the  FIG. 9   a  configuration. 
         [0084]    Thus, the  FIG. 9   a - 9   c  configuration may be said to have a split movable portion  120 . The downstream movable portion  128  may move both upstream and downstream from the datum configuration. The upstream movable portion  126  may only move upstream, but not downstream, from the datum configuration. 
         [0085]    It will be appreciated that many alternative configurations and/or arrangements of the variable area nozzle  200  and/or the outer boundary  100  of the variable area nozzle  100  and components/parts thereof other than those described herein may fall within the scope of the invention. For example, alternative arrangements of movable portion  120 , fixed portion  110  and elements, such as biasing elements, interacting therewith and/or components/parts thereof may fall within the scope of the invention and may be readily apparent to the skilled person from the disclosure provided herein. Furthermore, any feature described and/or claimed herein may be combined with any other compatible feature described in relation to the same or another embodiment.