fig1 illustrates a turbofan aeroengine as an example of the application of the described subject matter , which includes an outer bypass duct or nacelle 10 , a core casing 13 , a low pressure spool assembly seen generally at 12 which includes a fan assembly 14 , a low pressure compressor assembly 16 and a low pressure turbine assembly 18 , and a high pressure spool assembly seen generally at 20 which includes a high pressure compressor assembly 22 and a high pressure turbine assembly 24 . the core casing 13 surrounds the low and high pressure spool assemblies 12 and 20 in order to define a main fluid path ( not numbered ) therethrough including a combustor 26 . it should be noted that the terms axial , radial and circumferential are defined with respect to a main engine axis 28 . the terms downstream and upstream are defined with respect to the direction of an air flow indicated by arrow 30 , entering into and passing through the engine . referring to fig1 - 3 , the nacelle 10 of the aeroengine according to one embodiment , surrounds at least the fan assembly 14 and the low pressure and high pressure compressor assemblies 16 , 22 and may be configured to provide a variable geometry inlet system 11 a for the aeroengine . the nacelle 10 may include a main portion 32 in a stationary relationship with the engine , for example by being connected to the core casing 13 by a plurality of circumferentially spaced struts 34 ( see fig1 ). the main portion 32 of the nacelle 10 may be connected by a support structure ( not shown ) to an aircraft ( not shown ). the main portion 32 may have an annular outer skin 36 and an inner barrel 38 radially spaced apart from each other . the inner barrel 38 may include a front section 40 extending forward out of a front edge 42 of the annular outer skin 36 , thereby positioning the front section 40 upstream of the front edge 42 of the annular outer skin 32 . the nacelle 10 may include an inlet cowl 44 which has an annular outer skin 48 and an annular inner wall 50 radially spaced apart from each other . the inlet cowl 44 may have a lip portion 46 at the upstream end thereof which is connected with the annular outer skin 48 and the inner wall 50 . the inlet cowl 44 may be disposed upstream of the main portion 32 of the nacelle 10 and may be operatively connected to the main portion 32 , for example by means of a track system ( not shown ) which is known in the art , such that the inlet cowl 44 is translatable between a retracted position as shown in fig2 in which the outer skins 48 , 36 of the respective inlet cowl 44 and the main portion 32 of the nacelle 10 are positioned immediately axially adjacent each other while the annular inner wall 50 of the inlet cowl 44 and the inner barrel 38 of the main portion 32 are inserted one into the other , and an extended position as shown in fig3 in which the outer skins 48 , 36 of the respective inlet cowl 44 and the main portion 32 of the nacelle 10 are positioned axially spaced apart from each other while the inner wall 50 of the inlet cowl 44 and the inner barrel 38 of the main portion 32 are inserted one into the other less than the insertion in the retracted position as shown in fig2 . therefore , the annular inner wall 50 of the inlet cowl 44 and the inner barrel 38 ( including its front section 40 ), in combination form a length - variable inlet duct . the variable geometry inlet duct has a front opening 52 defined by the annular lip portion 46 of the inlet cowl 44 for intake of the airflow 30 . the inlet duct directs the air flow 30 towards and to pass the fan rotor 14 . a front section ( not numbered ) of the inlet duct defined by the annular inner wall 50 of the inlet cowl 44 may be movable between the extended position and the retracted position with respect to a rear section ( not numbered ) of the inlet duct defined by the inner barrel 38 of the main portion 32 , to thereby define a variable axial length of the air inlet passage which is continuous from the front opening 52 to the fan rotor 14 without any secondary inlet opening being formed between the front and rear sections of the inlet duct , regardless of the position of the front section of the inlet duct ( the position of the inlet cowl 44 ). the annular inner wall 50 of the inlet cowl 44 according to one embodiment , may be affixed to the lip portion 46 and may be slidingly inserted into a front section 40 of the inner barrel 38 of the main portion 32 , to form a telescoping configuration . the annular inner wall 50 of the inlet cowl 44 may define a front diameter corresponding to an inner diameter of the lip portion 46 of the inlet cowl 44 and may form a rear diameter thereof which may be greater than the front diameter of the annular inner wall 50 but slightly smaller than the inner diameter of the front section 40 of the inner barrel 38 of the main portion 32 . the front section 40 of the inner barrel 38 may have a substantially consistent diameter along the length thereof to thereby be substantially cylindrical . this telescoping configuration allows the annular wall 50 to be fully inserted into the front section 40 of the inner barrel 38 of the main portion 32 . in such a case , the annular front section 40 of the inner barrel 38 is fully received within an annular space between the outer skin 48 and the inner wall 50 of the inlet cowl 44 when the inlet cowl 44 is translated from the extended position to the retracted position . in this telescoping configuration the annular inner wall 50 of the inlet cowl 44 and the front section 40 of the inner barrel 38 may each be made of a respective metal ring of a single piece component . in the retracted position , the outer skin 48 of the inlet cowl 44 may be positioned immediately axially adjacent the front edge 42 of the annular outer skin 36 of main portion 32 , without a substantial axial gap therebetween , to thereby provide a low drag profile of the nacelle 10 for flight conditions such as cruise flight and take - off . in aircraft approach operations drag is less important for fuel consumption and therefore , the inlet cowl 44 can be translated to its extended position to increase the length / diameter ratio of the inlet duct formed within the nacelle 10 , which may help with reduction of noise levels propagated through the inlet duct . in order to further increase noise attenuation , the inner wall 50 of the inlet cowl 44 and the front section 40 of the inner barrel 38 of the main portion 32 may be provided with acoustic treatment capabilities , for example , by providing perforations therethrough or noise absorption material thereon to define a variable - geometry acoustic treatment area on the inner surface of the length - variable inlet duct formed within the nacelle 10 . when the inlet cowl 44 is in the extended position the acoustic treatment area defined by the inner surface of the front section 40 of the inner barrel 38 , is exposed and thus the total acoustic treatment area ( provided by the inner surface of both the inner wall 50 and the front section 40 of the inner barrel 38 ) is increased , in contrast to the total acoustic treatment area substantially defined by only the inner surface of the inner wall 50 of the inlet cowl 44 when the inlet cowl 44 is in the retracted position . optionally , one or more actuators 54 may be provided , for example being positioned in a space between the outer skin 36 and the inner barrel 38 of the main portion 32 and being supported on a stationary structure ( not numbered ) of the engine and may be operatively connected to the inlet cowl 44 for moving the inlet cowl 44 between the extended and retracted positions . referring to fig1 and 4 - 7 , the nacelle 10 of the aeroengine may be configured to provide a variable geometry inlet system 11 b according to another embodiment . the description of the variable geometry inlet system 11 b below will be focussed on the structures and features which are different from those of the variable geometry inlet system 11 a described above and illustrated in fig2 - 3 , and like structures and features will be indicated by like reference numerals and will not be redundantly described below . the fan assembly 14 may include a fan casing ( not numbered ) surrounding a fan rotor ( not numbered ). the fan casing according to this embodiment may be part of the inner barrel 38 of the main portion 32 of the nacelle 10 and may form a front section 40 ′ of the inner barrel 38 , positioned upstream of the front edge 42 of the annular outer skin 36 of the main portion 32 . the fan casing , at least a section thereof , thereby forms the stationary rear section of the inlet duct defined by the nacelle 10 . a plurality of circumferentially distributed plates 56 each may be pivotally connected about a tangential axis 58 ( shown as a pivoting point in fig4 and 5 ) at a front edge thereof , to an inside of the lip potion 46 of the inlet cowl 44 , to define an annular inner wall 50 ′. the circumferentially distributed plates 56 may overlap or interweave in the circumferential direction at adjacent side edges thereof . the interweaving or overlap of the plates 56 may be configured such that the annular inner wall 50 ′ defined by the plates 56 may be substantially free of gaps or ridges between adjacent plates 56 . an actuation system 60 may be provided to the plates 56 to actuate a pivotal motion about their respective tangential axis 58 such that the annular inner wall 50 ′ defined by the plates 56 is configured as a petal configuration having a truncated conical profile . the truncated conical profile may have a fixed diameter defined by the front edges of the plates 56 which corresponds to the inner diameter of the lip portion 46 of the inlet cowl 44 , and may have a variable conical angle resulting in a variable diameter at the rear edges ( not numbered ) of the plates opposite to the respective front edges of the respective plates . as the inlet cowl 44 is axially translated for example by the actuator 54 ( only shown in fig2 and 3 ), between the retracted and extended positions , the petal configuration of the annular inner wall 50 ′ may allow the conical angle thereof to be adjusted such that the annular front edge of the front section 40 ′ ( the front edge of the fan casing in this embodiment ) of the inner barrel 38 of the main portion 32 , is in contact with the respective plates 56 , thereby preventing formation of an abrupt step normal to the airflow 30 , between the plates 56 and the annular front section 40 ′ of the inner barrel 38 of the main portion 32 . this petal configuration also provides such a length - variable inlet duct by changing the conical angle thereof to allow a variable axial portion of the front section 40 ′ of the inner barrel 38 of the main portion 32 , to be inserted into the truncated conical profile of the plates 56 . when the inlet cowl 44 is in the retracted position and the front section 40 ′ of the inner barrel 38 is inserted deepest into the conical profile of the plates 56 , an axial portion of the plates 56 may be received in the annular space defined between the front section 40 ′ of the inner barrel 38 ( the van casing in this embodiment ) and the outer skin 48 of the inlet cowl 44 , as shown in fig4 . the plates 56 may be supported by a track system ( which may be similar to those used for aircraft flaps ) to ensure that the plates maintain their correct radial position throughout the translation of the plates 56 when the inlet cowl 44 moves between the retracted and extended positions . the actuation system 60 may include a series of individual actuators or a reduced number of actuators connected by a unison linkage system ( not shown ). the actuation system 60 may be made with electric , hydraulic or pneumatic means including an air motor or jack system which may be supplied by a branch from an inlet lip anti - icing system ( not shown ). the plates 56 may be provided with perforations therethrough or may have noise absorbing material applied on the inner surface thereof to thereby provide a variable - geometry acoustic treatment area which may increase when the inlet cowl 44 moves to the extended position to expose more inner surface of the plates 50 ′ ( which form a portion of the axial length of the front section of the inlet duct ) to the noise propagation through the inlet duct . it should be noted that the above embodiments of the described subject matter may be used to increase the inlet length / diameter ratio and thus the acoustic treatment area under aircraft landing approach conditions in order to achieve reduction of community noise . nevertheless , the above - described subject matter may also be applicable to make a low drag inlet system for high bypass engines . in order to minimize drag with clean inlet conditions at high speed flight operation , the inlet and forward cowl should be short . however , during low speed flight operation , for example in landing approach , conditions such as cross winds and ground vortices can result in fan inlet flow distortion in the inlet . the variable - geometry inlet may be able to provide a high length / diameter ratio for those conditions and a short inlet with low forward cowl drag in high speed flight operation . a secondary outer skin 49 as shown in fig2 and 3 may be optionally provided . the secondary skin 49 is a downstream extension of the annular outer skin 48 of the inlet cowl 44 , being stored within the nacelle 10 under the outer skin 36 of the main portion 32 when in the retracted position , and extending to cover a gap between the annular outer skin 48 and the and or outer skin 36 when in the extended position , in order to reduce external noise resulting from turbulence due to the discontinuity in the outer nacelle skin . the optional secondary outer skin 49 may be employed in situations where drag or turbulence need to be minimized for either noise or performance concerns . the above description is meant to be exemplary only , and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the disclosed subject matter . for example , a turbofan aeroengine is described as an application of the described embodiments , however aeroengines of other types , such as pusher turboprop aeroengines or other may also be suitable for application of the described subject matter . any suitable number of inlet sections may be provided . any suitable relative motion , or combination of motions , may be used to apply the teachings hereof . still other modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art , in light of a review of this disclosure , and such modifications are intended to fall within the appended claims .