TECHNICAL FIELD

[0001] The present invention relates to an aircraft power system, an aircraft landing gear drive system, an aircraft, a method of operating an aircraft and other aircraft power systems.

BACKGROUND

[0002] EP 4005920A1 discloses a hybrid aircraft power system comprising a hydraulic reservoir, a bidirectional hydraulic pump for pumping hydraulic fluid to and from the reservoir, and an electric motor. The electric motor is connectable to a first driveable component (one or more landing gear wheels) of the aircraft such that the electric motor is arranged to drive the first driveable component of the aircraft. The hydraulic pump is connectable to the first driveable component of the aircraft such that the hydraulic pump is arranged to pump hydraulic fluid from the reservoir to drive the first driveable component of an aircraft. Thus, in a first driveable mode of operation, the first driveable component is driven by both the electric motor and the hydraulic pump.

[0003] Such a system allows for both hydraulic and electric power to be utilised to drive a driveable component of the aircraft. Thus, the driveable component can be driven by both the hydraulic pump and the electric motor at the same time. This enables the electric motor to be designed for a "steady state" or normal power requirement of the driveable component, because the hydraulic pump can provide an additional power requirement to the driveable component on a temporary/ short-duration basis, when needed (e.g. to provide a peak power demand of 90 kW). This means that it is possible, in embodiments, for the electrical motor to be optimised for its most usual power output. The electric motor may also be designed for much lower required levels and so means that the system is much smaller than would otherwise be required. In embodiments, this enables maintenance, weight and cost savings, as well as a reduction in emissions and fuel bum.

[0004] However, it is desired to optimise such a system as far as possible to ensure the aircraft is as efficient as possible, in terms of overall weight. [0005] The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft power system.

SUMMARY

[0006] A first aspect of the present invention provides an aircraft power system comprising a first driveable component of the aircraft, and a mechanical flywheel, wherein the mechanical flywheel is connectable to the first driveable component such that the mechanical flywheel is arranged to drive the first driveable component of the aircraft, such that, in a first driveable mode of operation, the first driveable component is driven by the mechanical flywheel, and wherein the first driveable component is connectable to the mechanical flywheel such that the first driveable component is arranged to drive the mechanical flywheel, such that, in a first regenerative mode of operation, the mechanical flywheel is driven by the first driveable component.

[0007] Such a power system allows the mechanical flywheel to drive a driveable component of the aircraft (for example a landing gear wheel or wheels). This may provide an additional or alternative power source for the driveable component. Other power sources may comprise an electric power source (for example, including a motor, battery, APU, off board wired or wireless inductive power sources, and/or distribution network) and/or a hydraulic power source. Further, the power system allows the kinetic energy of a driveable component of the aircraft (for example, a landing gear wheel or wheels) to be stored by the mechanical flywheel. This allows the energy to be used later, for example for driving the driveable component or for being stored as a different energy type (e.g. electrical or hydraulic). This enables the power system to be flexible and able to efficiently store kinetic energy for later use. This allows the power system to be more lightweight and smaller than it otherwise might have been.

[0008] The mechanical flywheel may be lighter weight than other energy storage devices. The mechanical flywheel may be especially appropriate for harnessing high kinetic speed of the driveable component (e.g. the landing gear wheel or wheels soon after the aircraft has landed - i.e. in a deceleration phase). The mechanical flywheel arrangement may have a power range of up to around lOOkW. The mechanical flywheel arrangement may have a power density of greater than IkW per kg. The mechanical flywheel may have a torque density that is significantly lower than the hydraulic and/or electric systems.

[0009] The first driveable component may be one or more landing gear wheels. Hence, in the first driveable mode of operation the mechanical flywheel drives the landing gear wheels to move the aircraft over the ground (this is known as taxiing).

[0010] In a regenerating mode of operation (in the case where there are one or more driven landing gear wheels) it may be that the mechanical flywheel is externally driven by the landing gear wheel(s), for example after aircraft landing (e-taxi retardation) when the wheels are turning with excess kinetic energy. This mode may only occur during ground aircraft deceleration. In embodiments, this may help to reduce brake wear by slowing the aircraft by regenerative braking, as well as traditional carbon brakes. This may, as a result, reduce the cost of brake maintenance, as well as reduce the environmental impact of the brake manufacture required. In certain embodiments, it may be that the first driveable component comprises an open rotor propeller or a drive shaft of a high or low pressure turbine. Other driveable component may include: landing gear extension/retraction mechanisms, landing gear bay door mechanisms, cargo door mechanisms, landing gear braking, high lift device mechanisms (for example, flaps and/or slats), flight control surfaces or a hybrid propulsion system.

[0011] Preferably, the system further comprises a hydraulic reservoir and a bidirectional hydraulic pump for pumping hydraulic fluid to and from the reservoir.

[0012] The hydraulic reservoir may comprise a hydraulic fluid accumulator. Thus, the hydraulic fluid reservoir may be arranged to hold hydraulic fluid under pressure. Thus, the system allows hydraulic energy to be stored in the reservoir, for example in a hydraulic replenishing mode of operation. In such a mode of operation, the hydraulic pump may operate to store hydraulic power in the reservoir. This provides a means of storing surplus power as hydraulic power for later use to, for example drive an electric motor and/or drive a driveable component (for example, in circumstances where excess power is temporarily required by the driveable component of the aircraft). Hence, such a system is able to store surplus energy for later use.

[0013] The hydraulic arrangement may have a working pressure of up to around 400bar. The hydraulic arrangement may have an energy density of greater than 3.2Wh per kg. [0014] More preferably, the hydraulic pump is connectable to the first driveable component of the aircraft such that the hydraulic pump is arranged to drive the first driveable component, such that, in a second driveable mode of operation, the first driveable component is driven by both the mechanical flywheel and the hydraulic pump.

[0015] The hydraulic pump may be connectable to the mechanical flywheel in parallel such that, in the second driveable mode of operation, hydraulic pump is arranged to drive the driveable component independently of the mechanical flywheel. Thus, in the second driveable mode of operation, the mechanical flywheel and hydraulic pump may each be capable of supplying, and arranged to supply, power to the driveable component independently of the other. This ensures that, if one of the mechanical flywheel and hydraulic pump should fail, power is still provided to the driveable component by the other. It also means that each can be efficiently designed for their respective power outputs, rather than for the combined power output. Thus, the aircraft power system may be configured to operate in the second driveable mode of operation. Operating the aircraft power system in the second driveable mode of operation may comprise connecting the mechanical flywheel and the hydraulic pump to the first driveable component (for example, by use of the transfer unit). It may be that the hydraulic pump is configured to operate (for example, in the second driveable mode of operation) as a hydraulic motor under the action of hydraulic fluid flowing from the hydraulic fluid reservoir.

[0016] There may be another driveable mode of operation in which the first driveable component is driven by the hydraulic pump (and not by the mechanical flywheel).

[0017] Preferably, the hydraulic pump is connectable to the first driveable component of the aircraft such that the hydraulic pump is arranged to be driven by the first driveable component, such that, in a second regenerative mode of operation, the mechanical flywheel and the hydraulic pump are both driven by the first driveable component.

[0018] This mode may only occur during ground aircraft deceleration (i.e. the deceleration phase).

[0019] There may be another regenerative mode of operation in which the first driveable component drives the hydraulic pump (and not the mechanical flywheel).

[0020] The system may further comprise an electric motor. The electric motor may be configured to be driven as a generator. In the second regenerative mode of operation, the electric motor may be disconnected from the first driveable component, mechanical flywheel and/or hydraulic pump.

[0021] Preferably, the hydraulic pump is connectable to the mechanical flywheel such that the hydraulic pump is arranged to be driven by the mechanical flywheel, such that, in a first extended regenerative mode of operation, the hydraulic pump is driven by the mechanical flywheel.

[0022] This mode may occur during the entire flight cycle when the wheel taxiing is in standby mode (the first driveable component being one or more landing gear wheels).

[0023] The first driveable component may be disconnected from the mechanical flywheel and/or hydraulic pump in the first extended generative mode.

[0024] The system may further comprise an electric motor. The electric motor may be configured to be driven as a generator. In the first extended regenerative mode of operation, the electric motor may be disconnected from the mechanical flywheel and/or hydraulic pump. [0025] The system may further comprise an electric motor. The electric motor may be configured to be driven as a generator. There may be another extended regenerative mode of operation in which the hydraulic pump is driven by both the mechanical flywheel and the electric motor. This mode may occur during the entire flight cycle when the wheel taxiing is in standby mode (the first driveable component being one or more landing gear wheels). Thus, in this mode of operation, electrical energy (for example, from a battery or an electric distribution network) is used to store hydraulic power by driving the hydraulic pump to pump hydraulic fluid to the hydraulic reservoir. This may be through a transfer unit, if present. Operating the aircraft power system in this mode of operation may comprise connecting the electric motor to the hydraulic pump (for example, by use of the transfer unit). Operating the aircraft power system in this mode of operation may comprise disconnecting the electric motor and the hydraulic pump from the first driveable component (for example, by use of the transfer unit). Alternatively, or additionally, the hydraulic pump may be electrically connectable to a second electric motor such that the second electric motor is arranged to drive the hydraulic pump to pump hydraulic fluid into the reservoir. The different electric motor may be provided with electrical power from the aircraft APU, a hydrogen fuel cell, a battery, off board wired or wireless inductive power sources, or the aircraft engines. The first driveable component may be disconnected from the mechanical flywheel, hydraulic pump and/or electric motor in this extended generative mode. [0026] The system may further comprise an electric motor. The electric motor may be configured to be driven as a generator. There may be another extended regenerative mode of operation in which the hydraulic pump is driven by the electric motor (and not by the mechanical flywheel). This mode may occur during the entire flight cycle when the wheel taxiing is in standby mode. Thus, in this mode of operation, electrical energy (for example, from a battery or an electric distribution network) is used to store hydraulic power by driving the hydraulic pump to pump hydraulic fluid to the hydraulic reservoir. This may be through a transfer unit, if present. Operating the aircraft power system in this mode of operation may comprise connecting the electric motor to the hydraulic pump (for example, by use of the transfer unit). Operating the aircraft power system in this mode of operation may comprise disconnecting the electric motor and the hydraulic pump from the first driveable component (for example, by use of the transfer unit). Alternatively, or additionally, the hydraulic pump may be electrically connectable to a second electric motor such that the second electric motor is arranged to drive the hydraulic pump to pump hydraulic fluid into the reservoir. The different electric motor may be provided with electrical power from the aircraft APU, a hydrogen fuel cell, a battery, off board wired or wireless inductive power sources, or the aircraft engines. The first driveable component may be disconnected from the mechanical flywheel, hydraulic pump and/or electric motor in this extended generative mode. The mechanical flywheel may be disconnected from the electric motor and/or hydraulic pump in this extended generative mode. The first driveable component may be disconnected from the mechanical flywheel, hydraulic pump and/or electric motor in this extended generative mode. [0027] Preferably, the system further comprises an electric motor and, preferably, wherein the electric motor is configured to be driven as a generator.

[0028] The electric motor may be connected to an electrical power source, such as a battery of electrical distribution network, aircraft APU or a hydrogen fuel cell. Thus, the system allows electrical energy to be used to drive the electric motor and, for example, to drive a driveable component (for example, in circumstances where power is required by the driveable component of the aircraft). This may be a in a steady state situation, i.e. when the aircraft is being taxied at a constant speed on level ground (the first driveable component being one or more landing gear wheels). [0029] The electric motor arrangement may have a torque density of greater than 40NM per kg at 1000RPM and greater than 18NM per kg at 7500RPM. The electric motor arrangement may have a power density of greater than 7kW per kg.

[0030] The aircraft power system may further comprise an electric battery and wherein electrical energy generated by the electric motor is used to replenish the battery. This allows the electrical energy generated to be stored for later use. Hence, it allows a hydraulic energy store to be converted to an electrical energy store.

[0031] The aircraft power system may further comprise an electric distribution network and wherein electrical energy generated by the electric motor is distributed by the electric distribution network. For example, the electrical energy could be distributed around the aircraft to a variety of different electric devices, such as the aircraft APU or a hydrogen fuel cell.

[0032] More preferably, the electric motor is connectable to the first driveable component of the aircraft such that the electric motor is arranged to drive the first driveable component, such that, in a third driveable mode of operation, the first driveable component is driven by both the mechanical flywheel and the electric motor.

[0033] Here, in this mode, the mechanical flywheel is used to boost the power from the electric motor, for example if needed or desired in a “constant speed/steady state” taxiing scenario, or during “acceleration” or “breakaway”.

[0034] The aircraft power system may further comprise an electric battery and wherein the electric motor is driven using electrical energy of the battery. Alternatively or additionally, the aircraft power system may further comprise an electric distribution network and wherein the electric motor is driven using electrical energy of the distribution network. The electric motor may alternatively or additionally be provided with electrical power from the aircraft APU, a hydrogen fuel cell, off board wired or wireless inductive power sources, or the aircraft engines.

[0035] The electric motor may be connectable to the mechanical flywheel in parallel such that, in the third driveable mode of operation, the electric motor is arranged to drive the driveable component independently of the mechanical flywheel. Thus, in the third driveable mode of operation, the electric motor and mechanical flywheel may each be capable of supplying, and arranged to supply, power to the driveable component independently of the other. This ensures that, if one of the electric motor and mechanical flywheel should fail, power is still provided to the driveable component by the other. It also means that each can be efficiently designed for their respective power outputs, rather than for the combined power output. Thus, the aircraft power system may be configured to operate in the third driveable mode of operation. Operating the aircraft power system in the third driveable mode of operation may comprise connecting the electric pump and the mechanical flywheel to the first driveable component (for example, by use of the transfer unit).

[0036] There may be another driveable mode of operation in which the first driveable component is driven by the electric motor (and not by the mechanical flywheel).

[0037] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. There may be another driveable mode of operation in which the first driveable component is driven by the electric motor and hydraulic pump (and not by the mechanical flywheel). Such a system allows for both hydraulic and electric power to be utilised to drive a driveable component of the aircraft. Thus, the driveable component can be driven by both the hydraulic pump and the electric motor at the same time. Here, in this mode, the hydraulic pump is used to boost the power from the electric motor, for example if needed or desired in a “breakaway” scenario, or during “acceleration” or “constant speed/steady state”. This enables the electric motor to be designed and optimised for a “steady state” or normal power requirement of the driveable component (e.g. for driving the landing gear wheels at a constant velocity over level ground), because the hydraulic pump can provide an additional power requirement to the driveable component on a temporary/ short-duration basis (e.g. for driving the landing gear wheels on an uphill slope, or when the landing gear wheels are to accelerate (for example, when starting to taxi the aircraft), when needed (e.g. to provide a peak power demand of 90 kW). This means that it is possible, in embodiments, for the electrical motor to be optimised for its most usual power output. The electric motor may also be designed for much lower required levels and so means that the system is much smaller than would otherwise be required. In embodiments, this enables maintenance, weight and cost savings, as well as a reduction in emissions and fuel bum.

[0038] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. In the third driveable mode of operation, the hydraulic pump may be driven by the electric motor and/or mechanical flywheel. Here, surplus energy from the mechanical flywheel and electric motor are used to provide hydraulic energy, for example for use with other aircraft systems on the aircraft. This mode may be used during a “constant speed” taxiing phase, or a “breakaway” or “acceleration” phase.

[0039] Preferably, the electric motor is connectable to the first driveable component of the aircraft such that the electric motor is arranged to be driven by the first driveable component, such that, in a third regenerative mode of operation, the mechanical flywheel and the electric motor are both driven by the first driveable component.

[0040] There may be another regenerative mode of operation in which the first driveable component drives the electric motor (and not the mechanical flywheel).

[0041] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. There may be another regenerative mode of operation in which the first driveable component drives the electric motor and hydraulic pump (and not the mechanical flywheel).

[0042] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. In the third regenerative mode of operation, the hydraulic pump may be disconnected from the first driveable component, mechanical flywheel and/or electric motor.

[0043] Preferably, the electric motor is connectable to the mechanical flywheel such that the electric motor is arranged to be driven by the mechanical flywheel, such that, in a second extended regenerative mode of operation, the electric motor is driven by the mechanical flywheel.

[0044] This mode may occur during engine taxiing or during wheel taxiing. The first driveable component may be disconnected from the mechanical flywheel and/or electric motor in the second extended generative mode.

[0045] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. In the second extended regenerative mode of operation, the hydraulic pump may be disconnected from the mechanical flywheel and/or electric motor.

[0046] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. There may be another extended regenerative mode of operation in which the electric motor is driven by both the mechanical flywheel and the hydraulic pump. This mode may occur during engine taxiing or during wheel taxiing. The first driveable component may be disconnected from the mechanical flywheel, hydraulic pump and/or electric motor in this extended generative mode. [0047] The system may further comprise a hydraulic reservoir and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir. There may be another extended regenerative mode of operation in which the electric motor is driven by the hydraulic pump (and not by the mechanical flywheel). It may be that this occurs and for example is configured to so occur, when the first driveable component does not need to be driven by the electric motor. This mode may occur during engine taxiing or during wheel taxiing. The mechanical flywheel may be disconnected from the electric motor and/or hydraulic pump in this extended generative mode. The first driveable component may be disconnected from the mechanical flywheel, hydraulic pump and/or electric motor in this extended generative mode. [0048] Preferably, the hydraulic pump is connectable to the first driveable component of the aircraft such that the hydraulic pump is arranged to drive the first driveable component, and wherein the electric motor is connectable to the first driveable component of the aircraft such that the electric motor is arranged to drive the first driveable component, such that, in a fourth driveable mode of operation, the first driveable component is driven by the mechanical flywheel, hydraulic pump and the electric motor.

[0049] Such a system allows for mechanical, hydraulic and electric power to be utilised to drive a driveable component of the aircraft. Thus, the driveable component can be driven by the mechanical flywheel, hydraulic pump and the electric motor at the same time. This mode is known as a “hybrid boost wheel taxi mode”. In this mode, peak power or peak torque can be provided to the driveable component (e.g. one or more landing gear wheels), for example, whenever needed during taxiing (e.g. for uphill) or when peak acceleration is required (at the start of taxiing (known as “breakaway”). This enables the electric motor to be designed for a “steady state” or normal power requirement of the driveable component, because the hydraulic pump and/or mechanical flywheel can provide an additional power requirement to the driveable component on a temporary/short-duration basis, when needed (e.g. to provide a peak power demand of 90 kW). This means that it is possible, in embodiments, for the electrical motor to be optimised for its most usual power output (e.g. constant speed, level ground). The electric motor may also be designed for much lower required levels and so means that the system is much smaller than would otherwise be required. In embodiments, this enables maintenance, weight and cost savings, as well as a reduction in emissions and fuel bum.

[0050] The mechanical flywheel, electric motor and hydraulic pump may be connectable to each other in parallel such that, in the fourth driveable mode of operation, all are arranged to supply power to the driveable component independently of each other.

[0051] Preferably, the hydraulic pump is connectable to the first driveable component of the aircraft such that the hydraulic pump is arranged to be driven by the first driveable component and wherein the electric motor is connectable to the first driveable component of the aircraft such that the electric motor is arranged to be driven by the first driveable component, such that, in a fourth regenerative mode of operation, the mechanical flywheel, hydraulic pump and the electric motor are all driven by the first driveable component.

[0052] This mode may only occur during ground aircraft deceleration. It may be that operating in this fourth regenerative mode of operation comprises prioritising energy recovery by the hydraulic pump over energy recovery by the electric motor. Thus, it may be that energy recovered by operating in the regenerating mode is preferentially stored as hydraulic energy (for example, stored in the hydraulic fluid reservoir), rather than electrical energy (for example, stored in the battery). It may be that more than 50%, preferably more than 60%, more preferably more than 70%, yet more preferably more than 80% of the recovered energy is stored as hydraulic energy.

[0053] Prioritising energy recovery by the hydraulic pump over energy recovery by the electric motor can enable recovery of a greater percentage of the kinetic energy of the first driveable component. The effectiveness of electrical energy recovering using the electric motor is limited by the characteristics of the electric motor. For example, energy recovery using the electric motor requires a minimum commutation speed of the electric motor shaft. Furthermore, energy recovery using the electric motor is also constrained by the size and power rating of the electric motor. By contrast, it is possible for energy recovery using the hydraulic pump to be effective at all rotor speeds. Therefore, in embodiments, prioritising hydraulic energy recovery over electrical energy recovery enables a more efficient and complete recovery of kinetic energy from the first driveable component.

[0054] The aircraft power system may further comprise a hydraulic sensor (for example, a pressure sensor) configured to determine a state of charge of the hydraulic fluid reservoir. The hydraulic sensor may be configured to generate hydraulic sensor data indicating the determined state of charge of the hydraulic fluid reservoir. The aircraft power system may further comprise a battery sensor configured to determine a state of charge of the battery. The battery sensor may be configured to generate battery sensor data indicating the determined state of charge of the battery. The aircraft power system may be configured to operate on the basis of one or both of the hydraulic sensor data and the battery sensor data. Such data may, at least in part, be provided as digital data. Such data may, at least in part, be provided as analogue signals.

[0055] The aircraft power system may be configured to prioritise energy recovery by one of the hydraulic pump and the electric motor (for example, on the basis of one or both of the hydraulic sensor data and the battery sensor data). The aircraft power system may be configured to preferentially recover energy via the hydraulic pump (for example, unless or until the hydraulic sensor data indicates that the determined state of charge of the hydraulic fluid reservoir exceeds a predetermined threshold). The aircraft power system may be configured to preferentially recover energy via the hydraulic pump by recovering energy using only the hydraulic pump. The aircraft power system may be configured to recovery energy only via the hydraulic pump until the state of charge of the hydraulic fluid reservoir exceeds a predetermined threshold. It may be that, once the state of charge of the hydraulic fluid reservoir exceeds the predetermined threshold, the aircraft power system ceases to recover energy using the hydraulic pump. The aircraft power system may subsequently recover energy using the electric motor. Thus, the prioritisation of hydraulic energy recovery may be performed by exclusively recovering energy via the hydraulic pump for a first period of time before exclusively recovering energy via the electric motor for a second period of time. It will be appreciated that the first and second periods of time may be defined in terms of a quantity of energy recovered, rather than as fixed time periods.

[0056] It may be that the aircraft power system is configured in certain embodiments to simultaneously recover energy electrically and hydraulically whilst prioritising hydraulic energy recovery. In such cases, it may be that the transfer box is configured to perform torque splitting to direct the kinetic energy from the first driveable component to the hydraulic pump and the electric motor according to a predetermined torque splitting ratio. For example, the transfer box may be configured to perform torque splitting such that 70% the kinetic energy of the first driveable component is directed to the hydraulic pump (i.e. a torque splitting ratio between the hydraulic pump and the electric motor of 7:3). It will be appreciated by the skilled person that other torque split ratios are also equally possible. The torque splitting ratio may be in the range of 95:5 to 55:45, for example, in favour of prioritising hydraulic energy recovery - for at least some of the time. It may be that the aircraft power system is configured to adjust the torque splitting ratio (for example, on the basis of the hydraulic sensor data and the battery sensor data). The aircraft power system may be configured to alter the torque splitting ratio on the basis of one or both of the hydraulic sensor data and the battery sensor data. For example, the aircraft power system may be configured to alter the torque splitting ratio in response to the hydraulic sensor data indicating that the hydraulic fluid reservoir has reached a predetermined state of charge. In such a case, it may be that the predetermined state of charge is fully charged, and the alteration to the torque splitting ratio is such that the aircraft power system ceases to recover energy via the hydraulic pump.

[0057] The aircraft power system may comprise a processor and associated memory. The processor may be configured to (for example, by execution of instructions stored in the associated memory) control operation of the aircraft power system (including, for example, operation of the hydraulic sensor and the battery sensor).

[0058] As has been described, the first driveable component may be arranged to drive the mechanical flywheel, such that, in a first regenerative mode of operation, the mechanical flywheel is driven by the first driveable component. It is also possible, for example during a priming mode, for the mechanical flywheel to be driven (in addition, or alternatively) by one or both of the hydraulic pump and electric motor. This priming mode could be used prior to the fourth driveable mode of operation, where the first driveable component is driven by all three of the mechanical flywheel, hydraulic pump and the electric motor. It could also be used prior to the second (mechanical flywheel and hydraulic pump) or third (mechanical flywheel and the electric motor) driveable modes of operation. Hence, the priming mode charges the mechanical flywheel to provide an additional source of drive power for the first driveable component. It could also be used prior to the first (just mechanical flywheel) driveable mode of operation. Hence, the priming mode charges the mechanical flywheel to provide a source of drive power for the first driveable component.

[0059] Preferably, the mechanical flywheel and first driveable component are connectable to each other by a transfer unit. [0060] Hence, a transfer unit connects the mechanical flywheel to the first driveable component. The transfer unit may be connectable to a first driveable component of the aircraft. It will be appreciated that such a connection enables the transfer of mechanical power. [0061] An electric motor and/or a hydraulic pump may also be connected to the same transfer unit. Thus, it may be that the transfer unit is able to connect all of the mechanical flywheel, first driveable component, electric motor and the hydraulic pump.

[0062] The transfer unit may be configured to connect and disconnect the hydraulic pump from at least one of the mechanical flywheel, electric motor and the first driveable component. The transfer unit may be configured to connect and disconnect the electric motor from at least one of the mechanical flywheel, hydraulic pump and the first driveable component. Thus, the transfer unit may be configured to enable each of the mechanical flywheel, hydraulic pump, the electric motor, and the first driveable component to be independently connected and disconnected from the others.

[0063] More preferably, the transfer unit controls the various connections based on sensor data and/or status of the aircraft.

[0064] In other words, the transfer unit controls the coupling and decoupling of the various connections.

[0065] It may do this based upon the speed of the first driveable component (when not being driven). For example, if the first driveable component is rotating at a high speed, this energy may be arranged to be recovered by the mechanical flywheel (instead of or as a priority over the electric or hydraulic recovery) by coupling the mechanical flywheel to the first driveable component. This is done as the mechanical flywheel is optimised for harnessing such high speed kinetic energy (more so than the other electric or hydraulic energy recovery). [0066] It may do this based upon the speed and/or desired speed of the first driveable component (when being driven). For example, if the first driveable component is rotating at a low or zero speed and is required to be rotated faster, more than one energy source (electric, hydraulic and mechanical) may be arranged to be coupled to the first driveable component.

[0067] For example, the transfer unit may select a regenerative mode of operation in which the hydraulic pump is disconnected, based on sensor data that the hydraulic reservoir is already full, or nearly full, based on sensor data that the mechanical flywheel is at or near maximum speed and/or based on sensor data that the electric battery is empty or low on charge. [0068] For example, the transfer unit may select the mode (e.g. first driveable component - one or more landing gear wheels - being connected to and driven by electric motor) based on the status of the aircraft (for example, if the aircraft is being taxi driven by the wheels along level ground at constant speed). This is because the electric motor may have been optimised for providing this power level. The other two sources may be used when additional power is required, on a temporary basis.

[0069] For example, the transfer unit may select a mode in which the first driveable component is disconnected, for example when the aircraft status does not require the first driveable component to be driven (e.g. if the first driveable component is one or more landing gear wheels, when the engines are on prior to take off).

[0070] The transfer unit may comprise a switch arrangement, and wherein the switch arrangement is controllable to select a certain mode based on sensor data and/or status of the aircraft.

[0071] According to a second aspect of the invention there is also provided an aircraft landing gear drive system comprising the aircraft power system of the first aspect, wherein the first driveable component comprises one or more landing gear wheels of the landing gear drive system and wherein the mechanical flywheel is connectable to the one or more landing gear wheels, such that the mechanical flywheel is able to drive the one or more landing gear wheels and also connectable such that the one or more landing gear wheels are able to drive the mechanical flywheel.

[0072] According to a third aspect of the invention there is also provided an aircraft comprising the aircraft power system of the first aspect or the landing gear drive system of the second aspect.

[0073] It may be that the aircraft is a passenger aircraft.

[0074] According to a fourth aspect of the invention there is also provided a method of operating an aircraft, wherein the method comprises during a first mode of operation, connecting a mechanical flywheel to a first driveable component and using the mechanical flywheel to drive the first driveable component of the aircraft, and during a second mode of operation, connecting the first driveable component to the mechanical flywheel and using the first driveable component to drive the mechanical flywheel. [0075] According to a fifth aspect of the invention there is also provided a method of operating an aircraft, the aircraft being as per the third aspect (or comprising the aircraft power system of the first aspect or the landing gear drive system of the second aspect).

[0076] According to a sixth aspect of the invention there is also provided an aircraft power system comprising a mechanical flywheel, a hydraulic reservoir, and a bi-directional hydraulic pump for pumping hydraulic fluid to and from the reservoir, wherein the mechanical flywheel is connectable to the hydraulic pump such that the mechanical flywheel is arranged to drive the hydraulic pump such that, in a first mode of operation, the hydraulic pump is driven by the mechanical flywheel.

[0077] Such a system allows excess kinetic energy of the mechanical flywheel to be converted to hydraulic energy that can be more easily stored and used later on the aircraft. Thus, in this mode of operation, kinetic energy of the mechanical flywheel can be used to store hydraulic power by driving the hydraulic pump to pump hydraulic fluid to the hydraulic reservoir. This may be through a transfer unit, if present. Operating the aircraft power system in this mode of operation may comprise connecting the mechanical flywheel to the hydraulic pump (for example, by use of the transfer unit). Operating the aircraft power system in this mode of operation may comprise disconnecting the mechanical flywheel and the hydraulic pump from a first driveable component and/or an electric motor (for example, by use of the transfer unit).

[0078] According to a seventh aspect of the invention there is also provided an aircraft power system comprising a mechanical flywheel, and an electric motor and wherein the electric motor may be configured to be driven as a generator, wherein the mechanical flywheel is connectable to the electric motor such that the mechanical flywheel is arranged to drive the electric motor such that, in a first mode of operation, the electric motor is driven by the mechanical flywheel.

[0079] Such a system allows excess kinetic energy of the mechanical flywheel to be converted to electrical energy that can be more easily stored and used later on the aircraft. This may be through a transfer unit, if present. Operating the aircraft power system in this mode of operation may comprise connecting the mechanical flywheel to the electric motor (for example, by use of the transfer unit). Operating the aircraft power system in this mode of operation may comprise disconnecting the mechanical flywheel and the electric motor from a first driveable component and/or a hydraulic pump (for example, by use of the transfer unit). [0080] It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0082] Figure 1 shows a schematic view of an aircraft landing gear according to a first example embodiment of the invention;

[0083] Figure 2 shows a schematic view of the aircraft landing gear of Figure 1 in a first driving mode of operation;

[0084] Figure 3 shows a schematic view of the aircraft landing gear of Figure 1 in a second driving mode of operation;

[0085] Figure 4 shows a schematic view of the aircraft landing gear of Figure 1 in a third driving mode of operation;

[0086] Figure 5 shows a schematic view of the aircraft landing gear of Figure 1 in a first regenerative mode of operation;

[0087] Figure 6 shows a schematic view of the aircraft landing gear of Figure 1 in a second regenerative mode of operation;

[0088] Figure 7 shows a schematic view of the aircraft landing gear of Figure 1 in a third regenerative mode of operation;

[0089] Figure 8 shows a schematic view of the aircraft landing gear of Figure 1 in a fourth regenerative mode of operation;

[0090] Figure 9 shows a schematic view of the aircraft landing gear of Figure 1 in a fifth regenerative mode of operation;

[0091] Figure 10 shows a schematic view of the aircraft landing gear of Figure 1 in a first replenishing mode of operation;

[0092] Figure 11 shows a schematic view of the aircraft landing gear of Figure 1 in a second replenishing mode of operation; and [0093] Figure 12 shows a schematic view of an aircraft having the landing gear of Figure 1.

DETAILED DESCRIPTION

[0094] Figure 1 shows a schematic view of an aircraft landing gear 700 having a hybrid power system 800 according to a first example embodiment of the invention.

[0095] The landing gear 700 comprises a landing gear leg 110. An axle 140 is mounted on and extends outwards from the landing gear leg 110. A first landing gear wheel 120 is mounted for rotation on a first end of the axle 140. A second landing gear wheel 130 is mounted for rotation on an opposite second end of the axle 140. When the landing gear 700 is supporting the weight of an aircraft on the ground, the rotation of the first and second landing gear wheels 120, 130 facilitates movement of the aircraft across the ground.

[0096] The landing gear 700 further comprises a power system 800.

[0097] The power system 800 comprises an electric motor 230. The electric motor 230 is connected to the landing gear wheels 120, 130 via a transfer box 220. A mechanical connection 231 connects the electric motor 230 to the transfer box 220. The transfer box 220 is connected to the axle 140 (and thereby to the wheels 120, 130) by a drive shaft 221. Mechanical connection 231, transfer box 220, drive shaft 221, and axle 140 together enable the transfer of kinetic energy from the electric motor 230 to the wheels 120, 130.

[0098] Thus, the electric motor 230 can drive the wheels 120, 130. The electric motor 230 is therefore capable of converting electrical energy (for example, from an aircraft auxiliary power unit (APU), a hydrogen fuel cell, a battery, or the aircraft engines) into kinetic energy in the form of rotation of the wheels 120, 130. The electric motor 230, mechanical connection 231, transfer box 220, drive shaft 221, and axle 140 can all therefore be considered to form part of a landing gear drive system.

[0099] The power system 800 further comprises a hydraulic fluid reservoir 210, configured to contain a store of hydraulic fluid. The hydraulic fluid reservoir 210 is connected to a bi-directional hydraulic pump 212 by a hydraulic line 211, such that the hydraulic line 211 provides fluid communication between the hydraulic fluid reservoir 210 and the hydraulic pump 212. Thus, the hydraulic pump 212 is operable to pump hydraulic fluid to and from the hydraulic fluid reservoir 210. [0100] A mechanical connection 213 connects the hydraulic pump 212 to the transfer box 220. Thus, the hydraulic pump 212 is also connectable to the wheels 120, 130. Mechanical connection 213, transfer box 220, drive shaft 221, and axle 140 together enable the transfer of kinetic energy from the hydraulic pump 212 to the wheels 120, 130.

[0101] The hydraulic fluid reservoir 210 comprises a high-pressure accumulator, such that hydraulic fluid stored in the hydraulic fluid reservoir 210 is held under pressure. Thus, the hydraulic fluid reservoir 210 acts as a store of hydraulic power (for example, for use in driving the first driveable component). The hydraulic fluid reservoir 210 also comprises a valve (not shown) which is operable to control the release of hydraulic fluid from the hydraulic fluid reservoir 210. When stored hydraulic fluid is released from the hydraulic fluid reservoir 210 (for example, by opening the valve), it is driven from the hydraulic fluid reservoir 210 by action of the pressure at which it is held. Thus, the hydraulic fluid reservoir 210 is operable to provide a pressurised stream of hydraulic fluid.

[0102] Pressurised hydraulic fluid released from the hydraulic fluid reservoir 210, can be used to drive one or more driveable components of the aircraft. In particular, the flow of pressurised hydraulic fluid from the hydraulic fluid reservoir 210 can be used to turn the hydraulic pump 212. The bi-directional hydraulic pump 212 thereby acts as a hydraulic motor, converting the hydraulic power from the flow of hydraulic fluid through the hydraulic pump 212 into kinetic energy, which is transferred by the connection 213, transfer box 220, drive shaft 221, and axle 140 to the wheels 120, 130. Thus, bi-directional hydraulic pump 212 and mechanical connection 213 can also be considered to form part of a landing gear drive system. [0103] The power system 800 further comprises a mechanical flywheel 250, configured to store kinetic energy. The flywheel is a high power density storage device that can store kinetic energy in the form of rotational movement of the flywheel. The flywheel is mounted on bearings (conventional or magnetic) and made be made of steel or a more lightweight material such as carbon fibre, for example. The flywheel is mounted in a housing. The flywheel may be able to provide a power range of up to lOOkW, for example. The flywheel may have a power density of greater than IkW/kg, for example.

[0104] A mechanical connection 251 connects the mechanical flywheel 250 to the transfer box 220. Thus, the mechanical flywheel 250 is also connectable to the wheels 120, 130. Mechanical connection 251, transfer box 220, drive shaft 221, and axle 140 together enable the transfer of kinetic energy from the mechanical flywheel 250 to the wheels 120, 130. Thus, the mechanical flywheel 250 and mechanical connection 251 can also be considered to form part of a landing gear drive system.

[0105] The transfer box 220 is operable to selectively connect and disconnect the mechanical connections 213, 231, 251 and the drive shaft 221. In this case, the transfer box comprises a clutching mechanism (not shown) connected to each of the mechanical connections 213, 231, 251 and the drive shaft 221. Thus, by operating the clutching mechanisms the mechanical connections 213, 231, 251 and the drive shaft 221 can each be independently connected and disconnected. Thus, the transfer box 220 controls the transfer of energy between the electric motor 230, the hydraulic pump 212, mechanical flywheel 250 and the wheels 120, 130. Thus, the electric motor, hydraulic pump and mechanical flywheel can be said to be connected to each other in parallel such that the electric motor, the hydraulic pump and the mechanical flywheel can each supply power to the wheels 120, 130 independently of the other. The power system 800 can therefore be said to be a hybrid power system (in that the system utilises both kinetic, hydraulic and electrical power). The transfer box 220 is also operable to connect the electric motor 230 and mechanical flywheel 250 to the bi-directional pump, such that the electric motor 230 and mechanical flywheel 250 can be used to drive the bi-directional pump (for example, to pump hydraulic fluid to or from the hydraulic fluid reservoir 210).

[0106] The hydraulic pump 212 is also connectable to one or more further driveable components (not shown) of the aircraft. By pumping hydraulic fluid to and from the wheels 120, 130, it is possible to operate the wheels 120, 130. Similarly, the hydraulic pump 212 is operable to pump hydraulic fluid from the reservoir to drive the one or more further driveable components. Examples of such driveable components include landing gear extension/retraction mechanisms, landing gear bay door mechanisms, cargo door mechanisms, landing gear braking, high lift device mechanisms (for example, flaps and/or slats), and flight control surfaces.

[0107] The hydraulic fluid reservoir 210 is shown as comprising a hydraulic fluid reservoir 801 separate from a hydraulic accumulator 803. Hydraulic sensor 240 is configured to monitor a state of charge of the hydraulic accumulator 803. The hydraulic sensor is configured to determine a state of charge of the hydraulic fluid reservoir 210 and to generate hydraulic sensor data indicating the determined state of charge of the hydraulic fluid reservoir 210. The power system 800 also comprises a battery sensor (not shown). The battery sensor is configured to determine a state of charge of a battery and to generate battery sensor data indicating the determined state of charge of the battery. The power system 800 further comprises a processor (not shown) and associated memory (not shown). The processor is configured to execute instructions stored in the associated memory to control operation of the power system 800 (including, for example, operation of the hydraulic sensor 240 and the battery sensor).

[0108] This landing gear 700 is similar to the system described in relation to Figures 6a to 6d in EP 4005920A1. However, there are some important differences.

[0109] In EP 4005920A1, the power system 800 only uses hydraulic and electrical power. Here, there is also a mechanical flywheel 250, as described above, to store and use kinetic energy.

[0110] In EP 4005920A1, only on-board energy storage/ supply is considered. Here, off- board energy storage/supply (indicated by box 261) is provided. This off-board energy storage/supply comprises an electrical battery 262 and an electrical capacitance device 263.

[0111] The electrical motor 230 is connected to the off-board energy storage/supply 261 by a connection 260. This connection 260 is a wireless (inductive) connection.

[0112] The power system 800 is capable of operating in three modes of operation: a driving mode, a regenerative mode, and a replenishing mode. These modes of operation include other functions and variables in the present embodiment, than in EP 4005920A1. For example, the addition of a flywheel 250 increases the options in terms of driving and regeneration/replenishment of energy. Also, the power system 800 is able to operate in an extended regenerative mode, as will be described.

[0113] Operation of the hybrid power system 800, and in particular the mode of the hydraulic system, in the first driveable, regenerating, and replenishing modes of operation is controlled by use of a three-position switch 805. A first (middle) position of the switch 805 causes the hybrid power system 800 to operate in the operation (i.e. where at least the hydraulic system is providing power e.g. to drive the wheels). A second (right hand side) position of the switch 805 (shown in Figure 1) causes the hybrid power system 800 to operate in the regenerating mode of operation (i.e. where the hydraulic system (reservoir) is replenished). A third (left hand side) position of the switch 805 causes the hybrid power system 800 to operate in the replenishing mode of operation (i.e. where the hydraulic system (reservoir) is replenished and where hydraulic energy is supplied to an additional hydraulic system of the aircraft).

[0114] In the driving mode of operation, the wheels 120, 130 are driven by one or more of the electric motor 230, the hydraulic pump 212 and mechanical flywheel 250. Thus, in the driving mode, one, two or all three of the electric motor 230, the hydraulic pump 212 and mechanical flywheel 250 are connected to the wheels 120, 130 by the transfer box 220.

[0115] Figure 2 shows a schematic view of the aircraft landing gear of Figure 1 in a first driving mode of operation.

[0116] Here, all three (electric motor 230, the hydraulic pump 212 and mechanical flywheel 250) are connected to the wheels 120, 130 (indicated by arrows 901, 902, 903, 904). The electric motor 230 delivers electric drive power 902 to the transfer box 220, the hydraulic pump 212 delivers hydraulic drive power 903 to the transfer box 220 and the mechanical flywheel 250 delivers kinetic energy 904 to the transfer box 220. The transfer box 220 delivers the combined drive power 901 to the drive shaft 221 to turn the wheels 120, 130, enabling the aircraft to taxi.

[0117] This mode is known as a hybrid boost wheel taxi mode and is intended to be used when peak power or torque at the wheels is required. This could be any time during a taxi phase of the aircraft but most likely during a peak acceleration phase, such as breakaway (when the aircraft is moved from stationary using the drive system 700) or during acceleration. [0118] Figure 3 shows a schematic view of the aircraft landing gear of Figure 1 in a second driving mode of operation.

[0119] Here, the electric motor 230, and mechanical flywheel 250 are connected to the wheels 120, 130 (indicated by arrows 901, 902, 904). The electric motor 230 delivers electric drive power 902 to the transfer box 220 and the mechanical flywheel 250 delivers kinetic energy 904 to the transfer box 220. The transfer box 220 delivers the combined drive power 901 to the drive shaft 221 to turn the wheels 120, 130, enabling the aircraft to taxi. The hydraulic pump 212 is disconnected from the transfer box 220, shown by cross 923 (or is depleted/in standby).

[0120] This mode is known as the hybrid boost wheel taxi electric motor & high-speed mechanical flywheel mode and is intended to be used in an acceleration phase of taxiing. However, it could also be used during breakaway or when at a constant speed. [0121] Figure 4 shows a schematic view of the aircraft landing gear of Figure 1 in a third driving mode of operation.

[0122] Here, the electric motor 230, and mechanical flywheel 250 are connected to the wheels 120, 130 (indicated by arrows 901, 902, 904). The electric motor 230 delivers electric drive power 902 to the transfer box 220 and the mechanical flywheel 250 delivers kinetic energy 904 to the transfer box 220. The transfer box 220 delivers the combined drive power 901 to the drive shaft 221 to turn the wheels 120, 130, enabling the aircraft to taxi. The hydraulic pump 212 is also connected to the transfer box 220 (and the switch 805 is in the third position) and also receives power from the transfer box 220, indicated by arrow 913, to replenish the hydraulic fluid in the accumulator 803 and supply hydraulic energy to one or more additional hydraulic systems of the aircraft.

[0123] This mode is known as the hybrid boost wheel taxi electric motor & high-speed mechanical flywheel & hydraulic supply mode and is intended to be used in a constant speed phase of taxiing. However, it could also be used during breakaway or an acceleration phase of taxiing.

[0124] Other driving modes of operation are also possible.

[0125] For example, a wheel taxi electric motor mode where the mechanical flywheel and hydraulic pump are disconnected from the transfer box 220 (or are depleted) and the electric motor (on its own) drives the wheels 120, 130. Typically, this mode would occur during a constant speed phase of taxiing. However, it could also be applied to a breakaway or acceleration phase of taxiing.

[0126] For example, a hybrid boost wheel taxi electric motor & hydraulic pump/motor mode where the mechanical flywheel is disconnected from the transfer box 220 (or is depleted) and the electric motor and hydraulic pump drive the wheels 120, 130. Typically, this mode would occur during a breakaway phase. However, it could also be applied to an acceleration or constant speed phase of taxiing.

[0127] For example, a hybrid boost wheel taxi electric motor (without mechanical) & hydraulic supply mode where the mechanical flywheel is disconnected from the transfer box 220 (or is depleted/in standby). The electric motor provides power to drive the wheels and also to provide power to the hydraulic system. Typically, this mode would occur during a constant speed phase of taxiing. However, it could also be applied to an acceleration or breakaway phase of taxiing. [0128] In the regenerative mode of operation, the power system 800 operates to recover kinetic energy from the wheels 120, 130. The energy recovery can be performed when the electric motor is externally driven by the wheels 120, 130 (for example, when the aircraft is braking). In the regenerative mode, the wheels 120, 130 are connected by the transfer box 220 to one, two or three of the electric motor 230, the hydraulic pump 212 and mechanical flywheel 250.

[0129] Which of the electric motor 230, the hydraulic pump 212 and mechanical flywheel 250 the wheels 120, 130 are connected to is determined by the desired means for storing the recovered energy.

[0130] When in the regenerative mode of operation, the kinetic energy 400 of the rotation of the wheels 120, 130 is transferred to the transfer box 220. The transfer box 220 then transfers the kinetic energy to one, two or three of the electric motor 230, the hydraulic pump 212 and mechanical flywheel 250. When the wheels 120, 130 are connected to the electric motor 230, the kinetic energy of the wheels is transferred (represented by arrow 912) to the rotor of the electric motor 230. The electric motor 230 operates as a generator to convert the kinetic energy into electrical energy. This electrical energy can be stored (for example, in a battery) for later use, either by the electric motor 230 or by another aircraft subsystem. When the wheels 120, 130 are connected to the hydraulic pump 212, the kinetic energy of the wheels is transferred (represented by arrow 913) to the hydraulic pump 212. The hydraulic pump 212 is driven, using the kinetic energy, to pump hydraulic fluid into the hydraulic fluid reservoir 210. The kinetic energy is thereby stored as hydraulic energy in the hydraulic fluid reservoir 210 for later use, either to drive the wheels 120, 130 or another driveable component of the aircraft. When the wheels 120, 130 are connected to the mechanical flywheel 250, the kinetic energy of the wheels is transferred (represented by arrow 914) to the mechanical flywheel to be stored as kinetic energy.

[0131] Thus, the power system 800 enables the recovery and storage (electrically and/or hydraulically and/or kinetically) of kinetic energy from the wheels 120, 130. Further, performing regenerative braking in such a way reduces the braking forces that must be applied by the actual brakes, and therefore reduces the wear on those brakes.

[0132] Figure 5 shows a schematic view of the aircraft landing gear of Figure 1 in a first regenerative mode of operation. [0133] Here, the electric motor 230, hydraulic pump 212 and mechanical flywheel 250 are connected to the wheels 120, 130 (indicated by arrows 911, 912, 913, 914). The wheels provide power to the transfer box 220. The electric motor 230 then receives electric power 912 from the transfer box 220, the hydraulic pump receives hydraulic power 913 from the transfer box 220 and the mechanical flywheel 250 receives kinetic energy 914 from the transfer box 220.

[0134] This mode provides the control of the aircraft deceleration on the ground. The kinetic energy from the wheels is stored in all three ways. During this mode, it is also possible to redistribute the kinetic energy stored in the hydraulic and electrical systems to the other aircraft systems which would enable additional kinetic energy recovery.

[0135] Figure 6 shows a schematic view of the aircraft landing gear of Figure 1 in a second regenerative mode of operation.

[0136] Here, the hydraulic pump 212 and mechanical flywheel 250 are connected to the wheels 120, 130 (indicated by arrows 911, 913, 914). The wheels provide power to the transfer box 220. The hydraulic pump then receives hydraulic power 913 from the transfer box 220 and the mechanical flywheel 250 receives kinetic energy 914 from the transfer box 220. The electric motor 230 is disconnected/in standby (indicated by cross 922) from the transfer box 220.

[0137] This mode is known as the regenerative hydraulic and mechanical mode and is used to control the deceleration of the aircraft on the ground via storage of the kinetic energy with the hydraulic and mechanical systems. During this mode, it is also possible to redistribute the kinetic energy stored in the hydraulic system to the other aircraft systems which would enable additional kinetic energy recovery.

[0138] Figure 7 shows a schematic view of the aircraft landing gear of Figure 1 in a third regenerative mode of operation.

[0139] Here, the electric motor 230 and mechanical flywheel 250 are connected to the wheels 120, 130 (indicated by arrows 911, 912, 914). The wheels provide power to the transfer box 220. The electric motor 230 then receives electric power 912 from the transfer box, and the mechanical flywheel 250 receives kinetic energy 914 from the transfer box 220. The hydraulic pump 212 is disconnected/in standby (indicated by cross 923) from the transfer box 220. [0140] This mode is known as the regenerative electrical motor & mechanical mode and is used to control the deceleration of the aircraft on the ground via storage of the kinetic energy with the electrical and mechanical systems. During this mode, it is also possible to redistribute the kinetic energy stored in the electrical to the other aircraft systems which would enable additional kinetic energy recovery.

[0141] Figure 8 shows a schematic view of the aircraft landing gear of Figure 1 in a fourth regenerative mode of operation.

[0142] Here, the wheels 120, 130 are disconnected from the transfer box 220 (indicated by cross 921). The electric motor 230, hydraulic pump 212 and mechanical flywheel 250 are all connected to the transfer box 220 (indicated by arrows 912, 903, 904). The hydraulic pump 212 and mechanical flywheel 250 provide power (903, 904) to the transfer box 220. The electric motor 230 then receives electric power 912 from the transfer box.

[0143] This mode is known as the extended regenerative hydraulic & mechanical mode and provides the ability to extend the energy recovery phase by using the hydraulic system and mechanical system to provide power to the electrical system. During this mode, it is also possible to redistribute the kinetic energy stored in the electrical system to the other aircraft systems which would enable additional kinetic energy recovery. This mode is typically used during engine taxiing (i.e. when the aircraft is being propelled on the ground by its engines). However, it may also be used during wheel taxiing.

[0144] Figure 9 shows a schematic view of the aircraft landing gear of Figure 1 in a fifth regenerative mode of operation.

[0145] Here, the wheels 120, 130 are disconnected from the transfer box 220 (indicated by cross 921). The hydraulic pump 212 is also disconnected (or in standby) from the transfer box 220 (indicated by cross 923). The electric motor 230 and mechanical flywheel 250 are both connected to the transfer box 220 (indicated by arrows 912, 904). The mechanical flywheel 250 provides power 904 to the transfer box 220. The electric motor 230 then receives electric power 912 from the transfer box.

[0146] This mode is known as the extended regenerative mechanical mode and provides the ability to extend the energy recovery phase by using the mechanical system to provide power to the electrical system. During this mode, it is also possible to redistribute the kinetic energy stored in the electrical system to the other aircraft systems which would enable additional kinetic energy recovery. This mode is typically used during engine taxiing (i.e. when the aircraft is being propelled on the ground by its engines). However, it may also be used during wheel taxiing.

[0147] Other regenerative modes of operation are also possible.

[0148] For example, a regenerative electrical motor & hydraulic mode where the kinetic energy of the wheels can be distributed to the hydraulic and electrical systems while the mechanical flywheel is disconnected (or in standby). This mode is used to control the deceleration of the aircraft on the ground via storage of the kinetic energy with the electrical and hydraulic systems to extend the energy recovery phase by using the wheels to provide power to the electrical and hydraulic systems. During this mode, it is also possible to redistribute the kinetic energy stored in the electrical system to the other aircraft systems which would enable additional kinetic energy recovery.

[0149] For example, the power system 800 is also capable of operating in an extended regenerative mode. In the extended regenerative mode the wheels 120, 130 and mechanical flywheel are mechanically disconnected from the transfer box 220, but the electric motor 230 and the hydraulic pump 212 are connected.

[0150] The hydraulic pump 212 is driving the electric motor 230 as a generator using high pressure hydraulic fluid from the accumulator 803. The hydraulic energy is transferred (represented by arrow 620) to the transfer box 220. The transfer box 220 transfers (represented by arrow 630) that energy on to the electric motor 230. Thus, in the extended regenerative mode, the electric energy stored in a battery, for example, can be replenished by the hydraulic fluid.

[0151] The second position of the switch 805 is used in the extended regenerative mode of operation.

[0152] The extended regenerative mode is typically used when the aircraft is being propelled on ground with the aircraft engines. This is because, after an aircraft deceleration phase, the hydraulic energy within the accumulator is high and so can be used to then drive the electrical generator/motor, via the hydraulic motor/pump. The electrical energy can be stored in a battery and/or can be used to provide additional power to an aircraft electrical distribution network etc.

[0153] In the replenishing mode of operation, the wheels 120, 130 are mechanically disconnected from the electric motor 230, the hydraulic pump 212 and the mechanical flywheel 250 by the transfer box 220. [0154] The electric motor 230 and/or mechanical flywheel 250 can drive the hydraulic pump 212 to pump hydraulic fluid into the hydraulic fluid reservoir 210, thereby “replenishing” the store of hydraulic fluid.

[0155] Figure 10 shows a schematic view of the aircraft landing gear of Figure 1 in a first replenishing mode of operation.

[0156] Here, the wheels 120, 130 are disconnected from the transfer box 220 (indicated by cross 921). The electric motor 230, hydraulic pump 212 and mechanical flywheel 250 are all connected to the transfer box 220 (indicated by arrows 902, 913, 904). The electric motor 230 provides electric power 902 to the transfer box. The mechanical flywheel 250 provides power 904 to the transfer box 220. The hydraulic pump 212 then receives power 913 from the transfer box.

[0157] This mode is known as the hydraulic supply mode and provides power from the electrical and mechanical systems to drive the hydraulic pump to replenish the hydraulic accumulator. This mode is performed throughout the entire flight cycle when the wheel taxiing is in standby mode. During this mode, it is also possible to redistribute the hydraulic energy stored in the hydraulic system to the other aircraft systems which would enable additional kinetic energy recovery.

[0158] Figure 11 shows a schematic view of the aircraft landing gear of Figure 1 in a second replenishing mode of operation.

[0159] Here, the wheels 120, 130 are disconnected from the transfer box 220 (indicated by cross 921). The electric motor 230 is also disconnected (or in standby) from the transfer box 220 (indicates by cross 922). The hydraulic pump 212 and mechanical flywheel 250 are both connected to the transfer box 220 (indicated by arrows 913, 904). The mechanical flywheel 250 provides power 904 to the transfer box 220. The hydraulic pump 212 then receives power 913 from the transfer box.

[0160] This mode is known as the hydraulic supply mode with mechanical mode and provides power from the mechanical system to drive the hydraulic pump to replenish the hydraulic accumulator. This mode is performed throughout the entire flight cycle when the wheel taxiing is in standby mode. During this mode, it is also possible to redistribute the hydraulic energy stored in the hydraulic system to the other aircraft systems which would enable additional kinetic energy recovery.

[0161] Other replenishing modes of operation are also possible. [0162] For example, a hydraulic supply mode with electric motor mode. This provides power from the electrical system to drive the hydraulic pump to replenish the hydraulic accumulator. The wheels are disconnected from the transfer box and the mechanical fly wheel is either also disconnected or in standby mode. This mode is performed throughout the entire flight cycle when the wheel taxiing is in standby mode. During this mode, it is also possible to redistribute the hydraulic energy stored in the hydraulic system to the other aircraft systems which would enable additional kinetic energy recovery.

[0163] Figure 12 shows an aircraft 1000 comprising the landing gear 700. The operation of the landing gear 700 and the hybrid power system 800 is now described.

[0164] Whilst the aircraft is on the ground before take-off, the power system operates in a first driveable mode of operation, driving the wheels 120, 130 using all of the electric motor 230, the hydraulic pump 212 and mechanical fly wheel 250 to enable the aircraft to taxi. The aircraft may also drive the wheels 120, 130 using only one or two of these (for example, once the aircraft is in motion across the ground and travelling at a constant speed, only drive power from the electrical system may be needed).

[0165] Once the aircraft is in the air, the power system 800 may operate in the replenishing mode of operation, wherein the wheels are disconnected and the electric motor 230 and/or mechanical flywheel 250 drives the hydraulic pump 212 to pump hydraulic fluid into the hydraulic fluid reservoir 210. Optionally, the power system 800 may also operate the hydraulic pump 212 to provide hydraulic power to one or more other driveable components of the aircraft 1000 (for example, by controlling the electric motor 230 and/or mechanical flywheel 250 to drive the hydraulic pump 212 to provide hydraulic power to the other driveable components).

[0166] After the aircraft lands (when braking during taxiing), the power system may operate in the regenerating mode of operation, wherein the wheels 120, 130 are used to drive at least one (or two or three) of the electric motor 230, the hydraulic pump 212 and mechanical flywheel 250 to recover and store kinetic energy (as electrical, hydraulic and kinetic energy, respectively). This stored energy can then be used in subsequent operation (for example, in the driving mode or to operate the one or more further driveable components). By using the stored energy to drive the wheels 120, 130, the hybrid power system 800 can recover energy during braking and subsequently use the recovered energy to accelerate again. Such operation can enable more efficient taxiing (for example, where the aircraft is taxiing in a queue, so is repeatedly braking and accelerating).

[0167] When the aircraft is being propelled on the ground by its engines and/or when the hydraulic energy store is full or nearly full, hydraulic energy within the accumulator is high and so can be used to then drive the electrical generator/motor, via the hydraulic motor/pump. The electrical energy can be stored in a battery and/or can be used to provide additional power to an aircraft electrical distribution network, for example. By using the stored hydraulic energy to drive the electric motor 230, the hybrid power system 800 can recover electrical energy and subsequently use the recovered energy. Such operation can enable more efficient power and energy use.

[0168] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[0169] The driving modes may alternatively, or additionally be used for reverse movement of the aircraft (known as pushback), as well as forward taxiing. One or more of the wheels may be driven in a reverse direction. There may be provided additional sensors and/or systems to ensure the aircraft does not tip back during pushback.

[0170] The driving modes may additionally be used for steering the aircraft by driving the wheels at different speeds (differential speed control), which may include driving wheels in opposite directions).

[0171] During, and/or before take-off of the aircraft, for example when the aircraft is being driven by its engines and/or not being driven by the wheels, wheel driving may be inhibited or blocked by the arrangement.

[0172] The accumulator and/or mechanical flywheel may be discharged in situations where peak acceleration or deceleration performance is required. This provides optimised performance of the hydraulic pump, mechanical flywheel and/or motor.

[0173] The off-board electrical storage/supply 261 may alternatively or additionally comprise any suitable electrical storage or supply, including a supply connected to a mains supply.

[0174] The off-board energy supply/storage 261 may have a wired connection 260 to the electrical motor 230, a wireless (inductive) connection or both. [0175] The above embodiments are to be understood as illustrative examples of the invention. Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

[0176] It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

[0177] It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

[0178] It should be noted that throughout this specification, “or” should be interpreted as “and/or”.

[0179] Although the invention has been described above mainly in the context of a fixed-wing aircraft application, it may also be advantageously applied to various other applications, including but not limited to applications on vehicles such as helicopters, drones, trains, automobiles and spacecraft.