Patent Publication Number: US-2021179286-A1

Title: Aircraft hybrid propulsion system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from United Kingdom patent application number GB 1918281.5 filed on Dec. 12, 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure concerns a parallel hybrid propulsion system for an aircraft and an aircraft comprising the propulsion system. 
     Description to the Related Art 
     Parallel hybrid aircraft have been proposed, in which an internal combustion engine is combined with one or more electric motors to drive one or more propulsors. Parallel hybrid systems can be distinguished from so-called “serial hybrid” systems, in that in a parallel hybrid system, a mechanical connection is provided by the internal combustion engine and at least one propulsor, with at least one electric motor driving either the same propulsor as that driven by the internal combustion engine, or a further propulsor. 
     SUMMARY 
     According to a first aspect there is provided an aircraft hybrid propulsion system comprising;
         an internal combustion engine;   an electric motor;   a propulsor;   a combining gearbox, the internal combustion engine being coupled to a first input of the combining gearbox, the electric motor being coupled to a second input of the combining gearbox, and the propulsor being coupled to an output of the combining gearbox, such that the propulsor is driveable in use by either or both of the internal combustion engine and the electric motor; wherein   each of the internal combustion engine and the electric motor is coupled to its respective input by a respective clutch.       

     Advantageously, a single propulsor can be driven by either or both of the internal combustion engine and the electric motor. Furthermore, in the event of a failure of either the electric motor or internal combustion engine, the propulsor can continue to be driven by the other input, in view of the clutches. 
     One or both of the clutches may comprise any of an overrunning clutch, and an activated clutch such as a friction clutch or fluidic clutch. 
     The electric motor may comprise one of a permanent magnet motor and an induction motor. The inventors have found that the present disclosure is suitable for a wide variety of motor types, but it particularly suitable for permanent magnet motors. In the event of a permanent magnet motor failure, it is important that the electric motor does not continue to turn. By providing an overrunning clutch, the propulsor can continue to turn, while preventing the motor from turning. 
     The combining gearbox may comprise a reduction gearbox, wherein at least one of the inputs is configured to turn at a higher speed than the output in use. Advantageously, a relatively fast turning internal combustion engine and/or electric motor can be used in combination with a relatively slow turning propulsor. Such an arrangement provides for relatively efficient, compact electric motors and internal combustion engines, as well as an efficient propulsor. 
     The combining gearbox may comprise a parallel-axis gearbox comprising first and second input gears, and an output gear, wherein teeth of the first and second input gears mesh with teeth of the output gear. 
     The combining gearbox may comprise an epicyclic gearbox comprising a sun gear, at least one planet gear, and a ring gear. The propulsor may be coupled to the ring gear, the gas turbine engine may be coupled to one of the sun gear and a planet gear, and the electric motor may be coupled to the other of the planet gear and the sun gear. Advantageously, a large gear reduction can be achieved in a single stage, while a wide range of gear reductions can be provided between the motor and the internal combustion engine. Furthermore, the arrangement may be relatively compact, light and robust. 
     The combining gearbox may comprise a multi-stage gearbox. 
     The combining gearbox may comprise a first stage comprising one of a first parallel-axis gearbox and a first epicyclic gearbox, and a second stage comprising one of a second parallel-axis gearbox, and a second epicyclic gearbox. 
     The epicyclic gearbox may comprise one of a planetary gearbox and a star gearbox. The output gear of the first step-aside gearbox may be coupled to a sun gear of the epicyclic gearbox, and one of a planet carrier and a ring gear of the epicyclic gearbox may be coupled to the propulsor. 
     The aircraft hybrid propulsion system may comprise one or more of an electric energy storage device and a generator configured to provide electrical power to the electric motor. The generator may be coupled to the internal combustion engine. 
     The internal combustion engine may comprise a gas turbine engine. The gas turbine engine may comprise a compressor coupled to a first turbine, and may comprise a second turbine. The second turbine may be de-coupled from an engine compressor. The first and/or second turbine may be coupled to one or both of the generator and the combining gearbox. 
     According to a second aspect there is provided an aircraft comprising the propulsion system of the first aspect. 
     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. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a plan view of a first aircraft comprising a parallel hybrid propulsion system; 
         FIG. 2  is a schematic diagram of a parallel hybrid propulsion system for the aircraft of  FIG. 1 ; 
         FIG. 3  is a schematic front view of part of a combining gearbox of the hybrid propulsion system of  FIG. 2 ; 
         FIG. 4  is a schematic front view of an overrunning clutch of the propulsion system of  FIG. 2 ; 
         FIG. 5  is a schematic front view of an electric motor of the propulsion system of  FIG. 2 ; 
         FIG. 6  is a schematic side view of an alternative clutch for the hybrid propulsion system of  FIG. 2 ; 
         FIG. 7  is a schematic side view of a further alternative clutch for the hybrid propulsion system of  FIG. 2 ; 
         FIG. 8  is a schematic side view of an alternative propulsion system for the aircraft of  FIG. 1 ; and 
         FIG. 9  is a schematic side view of further alternative propulsion system for the aircraft of  FIG. 1 . 
     
    
    
     With reference to  FIG. 1 , an aircraft  1  is shown. The aircraft  1  is of conventional configuration, having a fuselage  2 , wings  3 , tail  4  and a pair of propulsion systems  5 . One of the propulsion systems  5  is shown in figure detail in  FIG. 2 . 
       FIG. 2  shows the propulsion system  5  schematically. The propulsion system  5  includes an internal combustion engine in the form of a gas turbine engine  10 . The gas turbine engine  10  comprises, in axial flow series, a compressor  14 , combustion equipment  16  and high and low-pressure turbines  18 ,  20 . 
     DETAILED DESCRIPTION 
     The gas turbine engine  10  works in the conventional manner so that air flows through the compressor  14  where it is compressed, before delivering that air to 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 turbines  18 ,  20  before being exhausted through a nozzle. The high  18  and low-pressure turbines  20  drive respectively the compressor  14  and a propulsor  12  in the form of a propeller or fan, each by suitable interconnecting shaft  22 ,  24 . The low pressure shaft  24  is coupled to the propulsor  12  via a first clutch in the form of an overrunning clutch  50  and a combining gearbox  36 , which will be described in further detail later. 
     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. three) 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. 
     The propulsion system  5  further comprises one or more electrical machines driving the propulsor  12 . In particular, the system  5  comprises an electric motor  28 . The motor  28  is of a conventional type, such as an induction or permanent magnet electric machine, and is configured to drive a propulsor such as the fan  12 . In the present embodiment, the motor  28  comprises a permanent magnet AC motor, and is coupled to the fan  12  via a first motor shaft  38 , second clutch in the form of an overrunning clutch  52 , second motor shaft  56  and a combining gearbox  36 , which will be described in further detail later. 
     The electric motor  28  is shown in further detail in  FIG. 5 . The motor  28  is of conventional construction, comprising a stator  70  comprising a plurality of stator coils  72 , which are energised in use to produce a rotating magnetic field. This rotating magnetic field crosses an air gap  74  to link with a magnetic field produced by permanent magnets  76   a ,  76   b  of a rotor  78 . Consequently, the motor  28  acts as a motor when the stators  72  are energised. On the other hand, where the rotor  78  is rotated, the permanent magnets  76   a ,  76   b  produce a rotating magnetic field, which links with the stator windings  72  to produce an electric current, and so may act as a generator under some circumstances. 
     The electric motor  28  is coupled to an energy storage device  30  in the form of one or more of a chemical battery, fuel cell, and capacitor, which provides the electric motor  28  with electrical power during operation. In some cases, multiple energy storages systems, which may be of different types (chemical battery, fuel cell etc.) may be provided for each propulsion system  5 . In other cases, a common energy storage device  30  may be provided for multiple propulsion systems. 
     The propulsion system further comprises a generator  32 , which is electrically coupled to one or both of the motor  28  and the energy storage device  30 , such that additional electrical energy can be provided in operation. The generator  32  is typically driven by the low-pressure shaft  24  of the gas turbine engine  10 . 
     A controller  34  is provided, which is configured to control at least the motor  28  and energy storage device  30 , to control the torque provided by the motor  28 , and the charging/discharging of the energy storage device  30 . The controller  34  may also be configured to control operation of the generator  32 , to control electrical power produced by the generator  32 . 
     As briefly mentioned, the gas turbine engine  10  and electric motor  28  are each coupled to the propulsor via respective first and second overrunning clutches  50 ,  52 , and a combining gearbox  36 . 
     The overrunning clutches  50 ,  52  are typically in the form of Sprag clutches, although other overrunning clutch types are known. 
     The sprag clutch comprises an input comprising the low pressure shaft  24  of the gas turbine engine  10 . As will be understood, the corresponding component for the clutch  52  will be the motor shaft  38 . An output shaft  54  is provided, which is provided radially inward of the input shaft  24 . Between the input  24  and output shaft  24 ,  54 , is a plurality of sprags  57 , which engage against the shafts  24 ,  54  when they are relatively rotated in a first direction X, and disengage when rotated in a second direction Y. The clutch  24  is arranged such that the shafts  24 ,  54  are locked together to transfer torque when the speed of the input shaft  24  is equal to the speed of the output shaft, while the output shaft  54  rotates freely, such that no torque is transmitted back to the input shaft  24 , when the output shaft  54  rotates at a higher speed than the input shaft  24 . Consequently, torque is transferred from the input shaft  24  to the output shaft  54  only, and not the other way around. Similarly, in view of the second sprag clutch  52 , torque is transferred from the first motor shaft  38  to the second motor shaft  56 , and not the other way around. Consequently, the motor  28  and gas turbine engine low pressure shaft  24  rotate independently of one another, but both transfer torque to the propulsor  12 . Consequently, the fan  12  may be powered by either or both of the gas turbine engine  10  via the low-pressure turbine  20 , and the motor  28 . 
     As is well known, a clutch is a device configured to couple rotating devices, which can allow for decoupling. It will be understood that other clutch types could be employed. For example, non-overrunning, switchable clutches could be used. 
       FIG. 6  shows a switchable clutch  250  in the form of a friction clutch, as will be well known to the skilled person. The clutch  250  could replace either of the clutches  50 ,  52  in the propulsion system  5 , and comprises a first friction plate  80  splined to an input shaft (which will comprise either the low-pressure turbine shaft  24  or the motor shaft  38 ) and a second friction plate  82  splined to an output shaft (shaft  54  or  56 ). Axial movement of the first friction plate  80  or second friction plate  82  engages and disengages the clutch, to allow for rotation of the input and output shafts together, or independent rotation of the shafts. 
     Typically, the actuation mechanism (not shown) comprises one or more of an electro-magnetic solenoid, and a hydraulic actuator. 
     As will be understood, the clutch  250  requires actuation in order to operate, in contrast to the overrunning clutches  50 ,  52 . Consequently, failure of an actuation mechanism to disengage one of the clutches may result in damage to the propulsion system in the event of failure of the internal combustion engine or motor. On the other hand, the clutches may be designed such that such a possibility is sufficiently remote. 
     One advantage of providing an actuable clutch, is that the electric motor  28  can be used as a generator, to provide electrical power to charge the energy storage device  30  where the thrust requirements are low. Consequently, the engine  10  can be run at a relatively constant speed, with any reduction in thrust requirements being balanced by increased power generation. Such a system may be more efficient, since internal combustion engines (and gas turbine engines in particular) are more efficient at high speed and high load than at low speed/load. 
     A further alternative clutch type comprises a fluidic clutch/fluidic coupling.  FIG. 7  shows a typical fluidic coupling  350 . The coupling  350  comprises an input shaft  24  coupled to an input impeller  380 , an output shaft  54  coupled to an output turbine  382 , an optional stator  386 , which may be provided between the impeller  380  and turbine  382 , and a fluid filled housing  388  which houses the impeller  380 , turbine  382  and stator  386 . The system could also include a mechanical “lockup” clutch (not shown). 
     The operation of such a fluid coupling is well known. In operation, rotation of the input shaft  24  causes rotation of the impeller  380 , which causes swirling of the fluid within the housing  388 . This swirling fluid is guided by the stator  386  onto the turbine  382 , which causes the turbine  382  to rotate. The relative speeds of the impeller  380  and turbine  382  can be altered by adjusting the angle of the blades of the stator  386 , thus providing a torque converter. 
     In normal use, therefore, rotation of the input shaft  24  causes rotation of the output shaft  54 , with a small amount of slip in the fluid coupling  250  causing a slight variation in relative speed of the shafts. However, in the event that one of the shafts  24 ,  54  becomes locked (such as, for example, failure of the gas turbine engine  10  or electric motor  28 ), the fluid coupling stalls, thereby preventing significant torque from being transferred. The system may comprise a brake (not shown) configured to brake the input shaft  24 , in order to stall the fluid coupling  350 , to thereby effectively decouple the shafts  24 ,  54 . Alternatively or in addition, an actuator could be provided to rotate the vane angle of the stator  386 , such that rotation of the input impeller  380  does not impart a rotational torque on the turbine  382 . A further alternative way to decouple the input and output shafts, is to drain the housing of oil, thereby reducing or eliminating the turning force provided by the impeller. 
     The combining gearbox  36  is in the form of a multi-stage gearbox, comprising first and second stages  58 ,  60 . 
     The first stage  58  of the combining gearbox  36  comprises a step-aside gearbox comprising first and second input gears  62 ,  64 , which each mesh with an output gear  66 . The first input gear  62  is coupled to the gas turbine engine  10  (in particular, to the low pressure shaft  24  of the gas turbine engine  10 ) via the output shaft  54 . Consequently, the first input gear  62  is coupled to the low pressure shaft  24  via the overrunning clutch  50 . Similarly, the second input gear  64  is coupled to the electric motor  28  via the second motor shaft  56 . Consequently, the second input gear  64  is coupled to the motor  28  via the overrunning clutch  52 . 
     The first and second input gears  62 ,  64  typically have fewer teeth and are of a smaller diameter than the output gear  66 , such that the step-aside gearbox provides a speed reduction between the electric motor  28  and gas turbine engine low pressure shaft  24 , and the output gear  66 . Since the gas turbine engine  10  and electric motor  28  are coupled to the step-aside gearbox  58  via separate gears  62 ,  64 , different reduction gearing can be used for the motor  28  and gas turbine engine  10 , such that these can operate at different speeds. 
     This is advantageous for several reasons. Firstly, both the power density and efficiency of gas turbine engines and electric motors can typically be increased by operating these at higher speeds. Typically, turbines (especially those of relatively low power) need to operate at much higher speeds than is optimal for small electric motors. Consequently, by providing separate input gears having different numbers of teeth for the combining gearbox  36 , the rotational speed of the gas turbine  10  and motor  28  can be separately optimised. Secondly, propulsors typically operate more efficiently at relatively low rotational speeds. Operation at low rotational speeds also reduces noise. Consequently, each of the gas turbine  10 , motor  28  and propulsor  12  can be configured to rotate at an optimal speed. 
     The output gear  66  of the first stage of the gearbox  36  is coupled to an input of a second stage of the gearbox  36 . The second stage of the gearbox comprises an epicyclic gearbox in the form of a star gearbox  60 . The star gearbox is shown in further detail in  FIG. 3 , and comprises an input gear in the form of a sun gear  40 , one or more planet gears  42  which mesh with the sun gear, a planet carrier  44  configured to rotatably mount the planet gears  42 , and a ring gear  46  which meshes with the planet gears  42 . The sun gear  40  is provided at a radially inner position, the ring gear  46  is provided at a radially outer position, and the planet gears  42  are provided therebetween. 
     In the configuration of this embodiment, the second stage  60  is configured as a star gearbox, in which each of the sun gear  40 , planet gears  42  and ring gear  46  are mounted for rotation about their respective axes, while the planet carrier  44  is statically mounted. Suitable bearing arrangements (not shown) are provided, to allow for rotation of each component, while providing the necessary support. The sun gear  40  is utilised as the first input, and so is coupled to the output gear  66  of the first stage of the combining gearbox  36 . The ring gear  46  is utilised as the output, and so is coupled to the fan  12  via an output shaft  68 . Consequently, the fan  12  is driveable via either or both of the gas turbine engine, and the electric motor  28 , while the combining gearbox  36  also provides for reduction gearing, with the ratios being dependent on the relative number of teeth of the gears  62 ,  64 ,  66 ,  40 ,  42 ,  46 . 
     It will be appreciated that the second stage  60  of the gearbox  36  could instead be configured as a planetary gearbox, in which the ring gear  46  is held static, while the planet carrier  44  is allowed to rotate. In this case, the fan  12  would be coupled to the planet carrier  44 , and the ring gear  46  would be mounted to a static component. Similarly, the second stage could be replaced with a step-aside gearbox, similar to the first stage  58 . 
     In view of the arrangement of the motor  28 , gas turbine engine  10 , combining gearbox  36 , clutches  50 ,  52  and propulsor  12 , the propulsion system  5  permits several operational modes. 
     In a first operational mode, the gas turbine engine  10  is used to drive the propulsor  12  alone. In this mode, the windings  72  of the motor  28  are not energised with electrical power from either the generator  32  or the batteries  30 . In this case, the sprag clutch  50  engages in view of the torque input from the low pressure shaft  24 , and the load imposed by the propulsor  12 . Consequently, rotation of the low pressure shaft  24  drives the propulsor  12  via the reduction gearbox  36 . However, in this mode, the sprag clutch  52  disengages, in view of the rotation of the second motor shaft  56 , and the non-rotation of the first motor shaft  38 . Consequently, the electric motor  38  is not driven. This is particularly advantageous where the electric motor  28  comprises a permanent magnet motor, since rotation of the rotor  78  would normally generate electrical current. This may not be desirable, as this will reduce the amount of power available to drive the propulsor  12 , and will also generate additional heat. This mode of operation is also useful where the motor  28  has suffered a fault. Consequently, the motor  28  does not have to be of a “fault tolerant” type, which may reduce cost and weight. 
     In second operational mode, the motor  28  is used to drive the propulsor  12  alone. In this mode, the gas turbine engine  10  is shut-down. In this case, the sprag clutch  52  engages in view of the torque input from the motor  28 , and the load imposed by the propulsor  12 . Consequently, rotation of the motor  28  drives the propulsor  12  via the reduction gearbox  36 . However, in this mode, the sprag clutch  50  disengages, in view of the rotation of the shaft  54 , and the non-rotation of the low pressure shaft  24 . Consequently, the gas turbine engine  10  is not driven. This mode of operation allows the gas turbine engine  10  to be shut-down in flight. Since gas turbine engines are only efficient when operated at high power, this may reduce overall fuel usage. Furthermore, the propulsor  12  can continue to be driven by the electric motor  28  in the event of a failure of the gas turbine engine  10 . Again, this increases safety, and improves operational flexibility. 
     In a third first operational mode, the gas turbine engine  10  and the motor are used to drive the propulsor  12  together. In this mode, the windings  72  of the motor  28  are energised with electrical power from either the generator  32  or the batteries  30 , and the gas turbine engine  10  is also operated to generate power. In this case, both sprag clutches  50 ,  52  engage in view of the torque input from the low pressure shaft  24  and the motor  28 . Consequently, rotation of the low pressure shaft  24  and the motor  28  drives the propulsor  12  via the reduction gearbox  36 . In this mode, load is shared between the gas turbine engine  10  and motor  28 . This mode is particularly advantageous for operation at high power levels, such as take-off, where increased power is required. 
     Consequently, the present arrangement describes a lightweight, reliable aircraft propulsion system, which is flexible, efficient, and tolerant of failures. 
     Other alternative combining gearboxes could be provided.  FIG. 8  illustrates one such example. 
       FIG. 8  shows an alternative hybrid propulsion system  405 . The system  405  comprises a gas turbine engine  410 , which is similar to the engine  10 , and comprises an output shaft  424 , which is coupled to a propulsor  412  via a combining gearbox. The system  405  also comprises an electric motor  428 , which is again similar to the motor  28 , and is also coupled to the propulsor  412  via the combining gearbox  436 . Each of the motor  428  and gas turbine engine output shaft  424  are coupled to the combining gearbox  436  via respective clutches  450 ,  452 , which could be of any of the types shown in  FIGS. 4 to 7 . 
     The combining gearbox  436  comprises a single stage epicyclic gearbox  436 . The epicyclic gearbox comprises an input gear in the form of a sun gear  440 , one or more planet gears  442  which mesh with the sun gear, a planet carrier  444  configured to rotatably mount the planet gears  442 , and a ring gear  446  which meshes with the planet gears  442 . The sun gear  440  is provided at a radially inner position, the ring gear  446  is provided at a radially outer position, and the planet gears  442  are provided therebetween. 
     The sun gear  440  is coupled to the gas turbine engine output shaft  424  via the clutch  450 . One of the planet gears  442  is coupled to the motor  428  via the clutch  45   s . The planet carrier  444  is statically mounted, and the ring gear  446  is mounted to the propulsor  412 . 
     Consequently therefore, two inputs are provided into the gearbox  436 , one from the gas turbine engine  410 , and one from the electric motor  428 . Optionally, more than one electric motor  428  could be provided, with each motor being mounted to a respective planet gear  442 . Consequently, two inputs are provided, which are combined to a single output. 
     Again, the gearbox  436  provides reduction gearing between at least the gas turbine engine shaft  424  and the propeller  412 . Typically, the reduction ratio between the motor  428  and the propeller  412  will be lower than the reduction ratio between the engine shaft  424  and propeller  412 , in view of the different connections. Consequently, the reduction ratios for the motor  418  and gas turbine engine shaft  424  can be separately optimised, by controlling the number of teeth of the sun  440 , planet  442  and ring gears  446 . 
     Again, in view of the clutches  450 ,  452 , either the engine  410  or the motor  428  can be operated independently of the other, while still allowing for rotation of the propulsor  412 . 
       FIG. 9  shows a further alternative hybrid propulsion system  405 . The system  505  comprises a gas turbine engine  410 , which is similar to the engine  10 , and comprises an output shaft  524 , which is coupled to a propulsor  512  via a combining gearbox  536 . The system  505  also comprises an electric motor  528 , which is again similar to the motor  28 , and is also coupled to the propulsor  512  via the combining gearbox  536 . Each of the motor  528  and gas turbine engine output shaft  524  are coupled to the combining gearbox  536  via respective clutches  550 ,  552 , which could be of any of the types shown in  FIGS. 4 to 7 . 
     The combining gearbox  536  is of a different form to those of the previous embodiment, and is in the form of a two-stage epicyclic gearbox  536 . 
     A first stage epicyclic gearbox  558  comprises an input gear in the form of a sun gear  566 , one or more planet gears  542  which mesh with the sun gear  566 , a planet carrier  544  configured to rotatably mount the planet gears  542 , and a ring gear  546  which meshes with the planet gears  442 . The sun gear  566  is provided at a radially inner position, the ring gear  546  is provided at a radially outer position, and the planet gears  542  are provided therebetween. 
     The sun gear  540  is coupled to the gas turbine engine output shaft  524  via the clutch  550 . The planet carrier  544  comprises a toothed outer annulus, such that the planet carrier  544  also acts as a gear. A motor output gear  562  is provided, which meshes with the planet carrier teeth, such that the planet carrier  544  is driven by one or both of the gas turbine engine  510  via the sun  566  and planet gears  542 , and the motor  528 . The respective clutches  550 ,  552  are provided upstream in the drive train. 
     The ring gear  546  is statically mounted, and the planet carrier  544  is coupled to an input of a second stage epicyclic gearbox  560 . The second stage epicyclic gearbox  560  is similar to the first stage, and again comprises an input gear in the form of a sun gear  540 , one or more planet gears  541  which mesh with the sun gear  540 , a planet carrier  568  configured to rotatably mount the planet gears  541 , and a ring gear  547  which meshes with the planet gears  541 . The sun gear  540  is provided at a radially inner position, the ring gear  547  is provided at a radially outer position, and the planet gears  541  are provided therebetween. The planet carrier  568  is mounted to the propulsor  512 . 
     Consequently therefore, two inputs are provided into the gearbox  536 , one from the gas turbine engine  510 , and one from the electric motor  528 . Optionally, more than one electric motor  528  could be provided, with each motor being mounted at a different position around the planet carrier  542 . Consequently, two inputs are provided, which are combined to a single output. 
     In this arrangement, the gas turbine engine input shaft  424  and motor  428  are coupled to different elements of the gearbox  536 . Consequently, the reduction gearing ratio provided between the engine  510  and the propeller  512 , and between the motor  528  and the propeller  512  can be controlled independently of one another. For example, the ratio between the motor  428  and propeller  512  can be chosen by defining the diameter of the motor output gear  562  and the diameter of the planet carrier  544 . Since these diameters have no effect on the reduction gear ratio provided between the engine  510  and the propeller  512  (which is defined by the relative sun gear  566 ,  540  and planet gear  542 ,  542  diameters), a designer can optimise each of these components to run at their most efficient speed, without having to compromise any of the components. Such an arrangement is also relatively compact, with relatively little width or length added to the arrangement by the addition of the motor  528  and gearing arrangement, relative to a conventional geared turboprop engine. 
     Again, in view of the clutches  550 ,  552 , either the engine  510  or the motor  528  can be operated independently of the other, while still allowing for rotation of the propulsor  512 . 
     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 concepts described herein. For example, different gas turbine configurations could be used. Examples include single shaft gas turbine engines, and three-shaft gas turbine engines. The gas turbine engine could be replaced by a different type of internal combustion engine, such as a piston or Wankel rotary engines. 
     Different combining gearbox arrangements could be provided. For instance, either the epicyclic gearbox stage could be omitted, making the gearbox single-stage. As a further example, the gearbox could comprise one or more bevel gears, hypoid gears, or any other suitable gear type. 
     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.