Patent Description:
Typically, an aircraft taxis to and from a runway using thrust from its engines. In an effort to reduce fuel usage, there has been an effort to develop alternative means to enable aircraft taxiing. It is well known in the prior art to use an aircraft power system to drive a driveable component of an aircraft (for example, landing gear extension/retraction mechanisms, high lift device mechanisms, or landing gear brakes). Such driveable components are typically hydraulically actuated, with the aircraft power system providing hydraulic power to actuate the driveable component.

It has been proposed to use a hydraulic aircraft power system to drive the wheels of an aircraft to enable the aircraft to taxi. For example, Chinese patent no. <CIT> describes a hydraulic aircraft power system for driving the wheels of the aircraft to enable taxiing. The hydraulic aircraft power system includes a hydraulic fluid reservoir, which provides a store of hydraulic power. The aircraft wheels are each connected to a hydraulic motor, which is connected to the hydraulic reservoir such that the hydraulic aircraft power system can be used to drive the hydraulic motor (and thereby the wheels) to enable taxiing. As hydraulic fluid from the reservoir is used to drive the hydraulic motors, the reservoir is depleted. The hydraulic aircraft power system therefore also includes an electric motor connected to a hydraulic pump. Using electrical power from an aircraft electrical power system, the electric motor drives the hydraulic pump to pump hydraulic fluid back into the reservoir. Thus, the electric motor and hydraulic pump operate to convert electrical power into hydraulic power. In addition, the hydraulic motors are configured to also operate as hydraulic pumps. Thus, when the aircraft is braking, the hydraulic motors use the kinetic energy of rotation of the wheels to pump hydraulic fluid into the reservoir. Thus, the hydraulic motor also operates to recover kinetic energy to recharge the hydraulic reservoir.

In recent years, it has also been proposed to enable taxiing by driving the wheels of the aircraft using an electric motor and an electric aircraft power system (so-called "e-taxi" systems). However, whilst electric motors are well-suited to maintaining the wheels at a constant rotation speed (for example, when an aircraft is already in motion), they are less suited to initiating a change in rotation speed (for example, when a stationary aircraft begins taxiing) or when going up a slope. Thus, the electric motor must be oversized in order to enable electric taxiing across the full range of expected wheel rotation drive requirements, increasing the weight and power consumption of the electric motor.

The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft power system.

<CIT> describes a high-lift actuation system for differentially actuating a plurality of high-lift surfaces of an aircraft. The high-lift actuation system includes a plurality of independent drive devices for individually actuating the plurality of high-lift surfaces. The independent drive devices may include a power drive unit (PDU) arranged between an inboard actuator and an outboard actuator of a respective high-lift surface and an interconnecting driveline. The PDU of at least one independent drive device may include two motors having a respective motor output coupled together in a torque summing arrangement. A controller may be implemented with two independent control channels respectively coupled to one of the two motors.

The present invention provides, according to a first aspect, an aircraft power system comprising: a hydraulic reservoir, a bi-directional 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 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 act as a hydraulic motor to drive the first driveable component. In a first driveable mode of operation, the first driveable component is driven by both the electric motor and the hydraulic pump. The electric motor and hydraulic pump are connected to each other in parallel such that, in the first driveable mode of operation, both are arranged to supply power to the driveable component independently of the other. The electric motor is connectable to the hydraulic pump such that, during a replenishing mode of operation, the electric motor is arranged to drive the hydraulic pump to pump hydraulic fluid into the reservoir.

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 <NUM> 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 burn.

The hydraulic fluid 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 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 drive the 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.

The electric motor and hydraulic pump are connected to each other in parallel. Thus, in the first driveable mode of operation, the electric motor 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 electric motor 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 a first driveable mode of operation. Operating the aircraft power system in the first driveable mode of operation may comprise connecting the electric pump 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 first driveable mode of operation) as a hydraulic motor under the action of hydraulic fluid flowing from the hydraulic fluid reservoir.

It may be that the electric motor and hydraulic pump are connected to a transfer unit. The transfer unit may be connectable to the driveable component. Thus, it may be that the transfer unit connects the electric motor to the hydraulic pump. It will be appreciated that such a connection enables the transfer of mechanical power. The transfer unit may be configured to connect and disconnect the hydraulic pump from at least one (for example, both) of the electric motor and the first driveable component. The transfer unit may be configured to connect and disconnect the electric motor from at least one (for example, both) of the hydraulic pump and the first driveable component. Thus, the transfer unit may be configured to enable each of the hydraulic pump, the electric motor, and the first driveable component to be independently connected and disconnected from the other two of the hydraulic pump, the electric motor, and the first driveable component.

The electric motor is connected to the hydraulic pump such that, during a replenishing mode of operation, the electric motor is arranged to drive the hydraulic pump to pump hydraulic fluid into the reservoir. Thus, in the replenishing mode or operation, electrical energy (for example, from a battery) is used to pump hydraulic fluid to the hydraulic reservoir. The same electric motor can be used to store hydraulic power by driving the hydraulic pump to pump hydraulic fluid to the hydraulic reservoir in the regenerating mode of operation. This may be through the transfer unit, if present. In certain embodiments, the electric motor and the hydraulic pump may be connected to one another directly. Thus, the aircraft power system may be configured to operate in a replenishing mode of operation. Operating the aircraft power system in the replenishing 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 the replenishing 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).

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, or the aircraft engines.

The electric motor may also function as a generator. In such a case, it may be that, in a regenerating mode of operation, one or both of the electric motor and the hydraulic pump is externally driven by a moveable component of the aircraft (for example, the first driveable component) to store the kinetic energy from the moveable component. generate electricity. Hence, in the replenishing mode, the electricity generated is used to pump hydraulic fluid to the hydraulic reservoir. The moveable component may be the first driveable component. Thus, the aircraft power system may be configured to operate in a regenerating mode of operation. Operating the aircraft power system in the regenerating mode of operation may comprise connecting at least one (for example, both) of the electric motor and the hydraulic pump to the first driveable component (for example, by use of the transfer unit). Operating the aircraft power system in the regenerating 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).

It may be that operating in the regenerating 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 <NUM>%, preferably more than <NUM>%, more preferably more than <NUM>%, yet more preferably more than <NUM>% of the recovered energy is stored as hydraulic energy. Energy recovery using the hydraulic pump will be henceforth referred to as hydraulic energy recovery. Energy recovery using the electric motor will be henceforth referred to as electrical energy recovery.

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.

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.

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.

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 <NUM>% 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 <NUM>:<NUM>). 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 <NUM>:<NUM> to <NUM>:<NUM>, 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.

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).

The hydraulic pump may also be connectable to other driveable components of the aircraft. Hence, in a second driveable mode of operation, the hydraulic pump can provide hydraulic power to one or more other driveable components. These other driveable components 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.

The electric motor may be connected to the hydraulic pump such that the electric motor is arranged to drive the hydraulic pump to pump hydraulic fluid out of the reservoir. Hence, in the second driveable mode of operation, the electric motor may drive the hydraulic pump to provide hydraulic power to the other driveable components. 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.

The first driveable component may comprise one or more landing gear wheels. Hence, in the first driveable mode of operation the electric motor (and hydraulic pump) drives the landing gear wheels to move the aircraft over the ground (this is known as e-taxiing). Here, the electric motor can be optimised for driving the landing gear wheels at a constant velocity over level ground, for example. In such embodiments, if there is an uphill slope, or when the landing gear wheels are to accelerate (for example, when starting to e-taxi the aircraft), it may then be the case that the hydraulic pump is used to provide additional power.

In a regenerating mode of operation (in the case where there are one or more driven landing gear wheels) it may be that the electric motor 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. 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.

According to a second aspect of the invention there is also provided an aircraft landing gear drive system comprising the aircraft power system as described above, wherein the first driveable component comprises one or more landing gear wheels of the landing gear drive system. The electric motor and the hydraulic pump are both connected to the one or more landing gear wheels, such that the electric motor and the hydraulic pump are both able to drive the one or more landing gear wheels.

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. It may be that the aircraft is a passenger aircraft.

According to a fourth 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). In a first driveable mode of operation, the first driveable component is driven by both the electric motor and the hydraulic pump.

It may be that, in a replenishing mode of operation, the electric motor drives the hydraulic pump to pump hydraulic fluid into the reservoir.

It may be that, in a second driveable mode of operation, the hydraulic pump provides hydraulic power to one or more other driveable components.

It may be that, in the second driveable mode of operation, the electric motor drives the hydraulic pump to provide hydraulic power to the other driveable components.

<FIG> shows a schematic view of an aircraft landing gear <NUM> according to a first embodiment of the invention. The landing gear <NUM> comprises a landing gear leg <NUM>. An axle <NUM> is mounted on and extends outwards from the landing gear leg <NUM>. A first landing gear wheel <NUM> is mounted for rotation on a first end of the axle <NUM>. A second landing gear wheel <NUM> is mounted for rotation on an opposite second end of the axle <NUM>. When the landing gear <NUM> is supporting the weight of an aircraft on the ground, the rotation of the first and second landing gear wheels <NUM>, <NUM> facilitates movement of the aircraft across the ground.

The landing gear <NUM> further comprises a power system <NUM>. The power system <NUM> comprises an electric motor <NUM>. The electric motor <NUM> is connected to the landing gear wheels <NUM>, <NUM> via a transfer box <NUM>. A mechanical connection <NUM> connects the electric motor <NUM> to the transfer box <NUM>. The transfer box <NUM> is connected to the axle <NUM> (and thereby to the wheels <NUM>, <NUM>) by a drive shaft <NUM>. Mechanical connection <NUM>, transfer box <NUM>, drive shaft <NUM>, and axle <NUM> together enable the transfer of kinetic energy from the electric motor <NUM> to the wheels <NUM>, <NUM>.

Thus, the electric motor <NUM> can drive the wheels <NUM>, <NUM>. The electric motor <NUM> 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 <NUM>, <NUM>. The electric motor <NUM>, mechanical connection <NUM>, transfer box <NUM>, drive shaft <NUM>, and axle <NUM> can all therefore be considered to form part of a landing gear drive system.

The power system <NUM> further comprises a hydraulic fluid reservoir <NUM>, configured to contain a store of hydraulic fluid. The hydraulic fluid reservoir <NUM> is connected to a bi-directional hydraulic pump <NUM> by a hydraulic line <NUM>, such that the hydraulic line <NUM> provides fluid communication between the hydraulic fluid reservoir <NUM> and the hydraulic pump <NUM>. Thus, the hydraulic pump <NUM> is operable to pump hydraulic fluid to and from the hydraulic fluid reservoir <NUM>.

A mechanical connection <NUM> connects the hydraulic pump <NUM> to the transfer box <NUM>. Thus, the hydraulic pump <NUM> is also connectable to the wheels <NUM>, <NUM>. Mechanical connection <NUM>, transfer box <NUM>, drive shaft <NUM>, and axle <NUM> together enable the transfer of kinetic energy from the hydraulic pump <NUM> to the wheels <NUM>, <NUM>. The hydraulic fluid reservoir <NUM> comprises a high-pressure accumulator, such that hydraulic fluid stored in the hydraulic fluid reservoir <NUM> is held under pressure. Thus, the hydraulic fluid reservoir <NUM> acts as a store of hydraulic power (for example, for use in driving the first driveable component). The hydraulic fluid reservoir <NUM> also comprises a valve (not shown) which is operable to control the release of hydraulic fluid from the hydraulic fluid reservoir <NUM>. When stored hydraulic fluid is released from the hydraulic fluid reservoir <NUM> (for example, by opening the valve), it is driven from the hydraulic fluid reservoir <NUM> by action of the pressure at which it is held. Thus, the hydraulic fluid reservoir <NUM> is operable to provide a pressurised stream of hydraulic fluid.

Pressurised hydraulic fluid released from the hydraulic fluid reservoir <NUM>, 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 <NUM> can be used to turn the hydraulic pump <NUM>. The bi-directional hydraulic pump <NUM> thereby acts as a hydraulic motor, converting the hydraulic power from the flow of hydraulic fluid through the hydraulic pump <NUM> into kinetic energy, which is transferred by the connection <NUM>, transfer box <NUM>, drive shaft <NUM>, and axle <NUM> to the wheels <NUM>, <NUM>. Thus, bi-directional hydraulic pump <NUM> and mechanical connection <NUM> can also be considered to form part of a landing gear drive system.

The transfer box <NUM> is operable to selectively connect and disconnect the mechanical connections <NUM>, <NUM> and the drive shaft <NUM>. In this case, the transfer box comprises a clutching mechanism (not shown) connected to each of the mechanical connections <NUM>, <NUM> and the drive shaft <NUM>. Thus, by operating the clutching mechanisms the mechanical connections <NUM>, <NUM> and the drive shaft <NUM> can each be independently connected and disconnected. Thus, the transfer box <NUM> controls the transfer of energy between the electric motor <NUM>, the hydraulic pump <NUM>, and the wheels <NUM>, <NUM>. Thus, the electric motor and hydraulic pump can be said to be connected to each other in parallel such that both the electric motor and the hydraulic pump can each supply power to the wheels <NUM>, <NUM> independently of the other. The power system <NUM> can therefore be said to be a hybrid power system (in that the system utilises both hydraulic and electrical power). The transfer box <NUM> is also operable to connect the electric motor <NUM> to the bi-directional pump, such that the electric motor <NUM> can be used to drive the bi-directional pump (for example, to pump hydraulic fluid to or from the hydraulic fluid reservoir <NUM>).

The hydraulic pump <NUM> is also connectable to one or more further driveable components (not shown) of the aircraft. By pumping hydraulic fluid to and from the wheels <NUM>, <NUM>, it is possible to operate the wheels <NUM>, <NUM>. Similarly, the hydraulic pump <NUM> 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.

The power system <NUM> comprises a hydraulic sensor <NUM>. The hydraulic sensor is configured to determine a state of charge of the hydraulic fluid reservoir <NUM> and to generate hydraulic sensor data indicating the determined state of charge of the hydraulic fluid reservoir <NUM>. The power system <NUM> also comprises a battery sensor (not shown). The battery sensor is configured to determine a state of charge of the battery and to generate battery sensor data indicating the determined state of charge of the battery. The power system <NUM> 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 <NUM> (including, for example, operation of the hydraulic sensor <NUM> and the battery sensor).

The power system <NUM> is capable of operating in three distinct modes of operation: a driving mode, a regenerative mode, and a replenishing mode.

<FIG> shows the power system <NUM> operating in the driving mode of operation. In the driving mode of operation, the wheels <NUM>, <NUM> are driven by one or both of the electric motor <NUM> and the hydraulic pump <NUM>. Thus, in the driving mode, one or both the electric motor <NUM> and the hydraulic pump <NUM> are connected to the wheels <NUM>, <NUM> by the transfer box <NUM>. When the electric motor <NUM> is connected to the wheels <NUM>, <NUM> and the hydraulic pump <NUM> is disconnected from the wheels <NUM>, <NUM>, the electric motor <NUM> delivers electric drive power <NUM> to the transfer box <NUM>, which delivers the electric drive power to the drive shaft <NUM> to turn the wheels. When the electric motor <NUM> is disconnected from the wheels <NUM>, <NUM> and the hydraulic pump <NUM> is connected to the wheels <NUM>, <NUM>, the hydraulic pump <NUM> delivers el hydraulic drive power <NUM> to the transfer box <NUM>, which delivers the hydraulic drive power to the drive shaft <NUM> to turn the wheels. When both the electric motor <NUM> and the hydraulic pump <NUM> are connected to the wheels <NUM>, <NUM>, the electric motor delivers electric drive power <NUM> to the transfer box <NUM> and the bi-directional hydraulic pump <NUM> delivers hydraulic drive power <NUM> to the transfer box <NUM>. The transfer box <NUM> delivers the combined drive power <NUM> to the drive shaft <NUM> to turn the wheels <NUM>, <NUM>, enabling the aircraft to taxi (represented by item <NUM>).

The ratio of power delivered to the wheels <NUM>, <NUM> from the electric motor <NUM> to that from the hydraulic pump <NUM> varies depending on a current demand on the landing gear drive system. For example, when the aircraft is already in motion and the landing gear drive system is required only to maintain the current speed, the driving power delivered to the wheels <NUM>, <NUM> may be primarily (for example, entirely) provided by the electric motor <NUM>. When a higher demand is placed of the landing gear drive system (for example, when the aircraft is stationary and the landing gear drive system is required to accelerate the aircraft) the power from the electric motor <NUM> may be supplemented by additional power provided by the hydraulic pump <NUM>. Thus, in this situation, the wheels are driven by both the electric motor and the hydraulic pump <NUM> together. The hydraulic pump <NUM> can therefore be used to provide additional power at times of high demand. Enabling the hydraulic pump <NUM> to supplement the electric motor <NUM> in this way means that the electric motor <NUM> can be optimised for operation at a constant speed, removing the need to oversize the electric motor <NUM> in order to enable it to meet the increased demands of accelerating the aircraft. This provides a reduction in the weight of the aircraft and thereby a reduction in emissions and fuel consumption.

<FIG> shows the power system <NUM> operating in the regenerative mode of operation. In the regenerative mode of operation, the power system <NUM> operates to recover kinetic energy from the wheels <NUM>, <NUM>. The energy recovery can be performed when the electric motor is externally driven by the wheels <NUM>, <NUM> (for example, when the aircraft is braking). In the regenerative mode, the wheels <NUM>, <NUM> are connected by the transfer box <NUM> to one or both of the electric motor <NUM> and the hydraulic pump <NUM>. Which of the electric motor <NUM> and the hydraulic pump <NUM> the wheels <NUM>, <NUM> are connected to is determined by the desired means for storing the recovered energy. The transfer box <NUM> can operate to connect the electric motor <NUM> to the wheels <NUM>, <NUM> and disconnect the hydraulic pump <NUM> from the electric motor <NUM> and the wheels <NUM>, <NUM>. Alternatively, the transfer box <NUM> can operate to connect the hydraulic pump <NUM> to the wheels <NUM>, <NUM> and disconnect the electric motor <NUM> from the hydraulic pump <NUM> and the wheels <NUM>, <NUM>. The transfer box <NUM> can also operate to connect the electric motor <NUM> and the hydraulic pump <NUM> to the wheels <NUM>, <NUM>.

When in the regenerative mode of operation, the kinetic energy <NUM> of the rotation of the wheels <NUM>, <NUM> is transferred to the transfer box <NUM>. The transfer box <NUM> then transfers the kinetic energy to one or both of the electric motor <NUM> and the hydraulic pump <NUM>. When the wheels <NUM>, <NUM> are connected to the electric motor <NUM>, the kinetic energy of the wheels is transferred (represented by arrow <NUM>) to the rotor of the electric motor <NUM>. The electric motor <NUM> 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 <NUM> or by another aircraft subsystem. When the wheels <NUM>, <NUM> are connected to the hydraulic pump <NUM>, the kinetic energy of the wheels is transferred (represented by arrow <NUM>) to the hydraulic pump <NUM>. The hydraulic pump <NUM> is driven, using the kinetic energy, to pump hydraulic fluid into the hydraulic fluid reservoir <NUM>. The kinetic energy is thereby stored as hydraulic energy in the hydraulic fluid reservoir <NUM> for later use, either to drive the wheels <NUM>, <NUM> or another driveable component of the aircraft. It will be appreciated that the wheels <NUM>, <NUM> can be connected to both the electric motor <NUM> and the hydraulic pump <NUM> at the same time, allowing recovered kinetic energy to be stored both electrically and hydraulically simultaneously. At other times, only one of the electric motor <NUM> and the hydraulic pump <NUM> is connected to the wheels <NUM>, <NUM>.

Thus, the power system <NUM> enables the recovery and storage (electrically and/or hydraulically) of kinetic energy from the wheels <NUM>, <NUM>. 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.

In this particular embodiment, the power system <NUM> is configured to prioritise energy recovery via the hydraulic pump <NUM> over energy recovery via the electric motor <NUM>. In this example embodiment, the power system <NUM> is configured to prioritise energy recovery via the hydraulic pump <NUM> by performing energy recovery using only the hydraulic pump <NUM> (i.e. not using electric motor <NUM>) until such time as hydraulic sensor data from hydraulic sensor <NUM> indicates that the hydraulic reservoir is fully charged. However, it will be appreciated that other schemes for managing energy recovery in the regenerating mode of operation (for example, as described above) are equally usable.

<FIG> shows the power system <NUM> operating in the third replenishing mode of operation. In the replenishing mode of operation, the wheels <NUM>, <NUM> are mechanically disconnected (represented by cross <NUM>) from the electric motor <NUM> and the hydraulic pump <NUM> by the transfer box <NUM>, but the electric motor <NUM> and the hydraulic pump <NUM> are connected. The electric motor <NUM> converts electrical energy into kinetic energy (in the form of rotation of the rotor) which is transferred (represented by arrow <NUM>) to the transfer box <NUM>. The transfer box <NUM> transfers (represented by arrow <NUM>) that kinetic energy on to the hydraulic pump <NUM>, where it is used to operate the hydraulic pump <NUM> to pump hydraulic fluid.

The electric motor <NUM> can drive the hydraulic pump <NUM> to pump hydraulic fluid into the hydraulic fluid reservoir <NUM>, thereby "replenishing" the store of hydraulic fluid. The electric motor is also operable to drive the hydraulic pump <NUM> to pump hydraulic fluid out of the hydraulic fluid reservoir <NUM> (for example, in order to operate one or more further driveable components connected to the hydraulic pump <NUM>). Thus, in the replenishing mode, the electric motor <NUM> can be said to be operable to control one or more further driveable components hydraulically connected to the hydraulic pump <NUM>.

<FIG> shows an aircraft <NUM> comprising the landing gear <NUM>.

The operation of the landing gear <NUM> and the hybrid power system <NUM> is now described. Whilst the aircraft is on the ground before take-off, the power system operates in a first driveable mode of operation, driving the wheels <NUM>, <NUM> using both the electric motor <NUM> and the hydraulic pump <NUM> to enable the aircraft to taxi. The aircraft may also drive the wheels <NUM>, <NUM> using only one of the electric motor <NUM> and the hydraulic pump <NUM> (for example, once the aircraft is in motion across the ground and only drive power sufficient to maintain the motion is required). Once the aircraft is in the air, the power system <NUM> may operate in the replenishing mode of operation, wherein the electric motor <NUM> drives the hydraulic pump <NUM> to pump hydraulic fluid into the hydraulic fluid reservoir <NUM>. Optionally, the power system <NUM> may also operate the hydraulic pump <NUM> to provide hydraulic power to one or more other driveable components of the aircraft <NUM> (for example, by controlling the electric motor <NUM> to drive the hydraulic pump <NUM> to provide hydraulic power to the other driveable components). When the aircraft lands (or when braking during taxiing), the power system may operate in the regenerating mode of operation, wherein the wheels <NUM>, <NUM> are used to drive at least one (for example, both) of the electric motor <NUM> and the hydraulic pump <NUM> to recover and store kinetic energy (as electrical and hydraulic 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 <NUM>, <NUM>, the hybrid power system <NUM> 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).

<FIG> shows a schematic view of an aircraft landing gear <NUM> having a hybrid power system <NUM> according to a second example embodiment of the invention. The same reference numerals as in respect of the first embodiment have been used to label corresponding elements of landing gear <NUM> and hybrid power system <NUM>. The landing gear <NUM> and hybrid power system <NUM> of the second embodiment are the same as those of the first embodiment but for the following differences.

In this example embodiment, the hydraulic fluid reservoir <NUM> is shown as comprising a hydraulic fluid reservoir <NUM> separate from a hydraulic accumulator <NUM>. Hydraulic sensor <NUM> is configured to monitor a state of charge of the hydraulic accumulator <NUM>. Operation of the hybrid power system <NUM> in the first driveable, regenerating, and replenishing modes of operation is controlled by use of a three-position switch <NUM>. A first position of the switch <NUM> (shown in <FIG>) causes the hybrid power system <NUM> to operate in the first driveable mode of operation. A second position of the switch <NUM> (shown in <FIG>) causes the hybrid power system <NUM> to operate in the regenerating mode of operation. A third position of the switch <NUM> (shown in <FIG>) causes the hybrid power system <NUM> to operate in the replenishing mode of operation. In <FIG>, the flow of high pressure (H. P) hydraulic fluid is shown with the use of solid black arrows, and the flow of lower pressure (L. P) hydraulic fluid is shown with the use of solid grey arrows.

<FIG> is a diagram <NUM> illustrating schematically a method of operating the aircraft of <FIG>. Thus, <FIG> shows a first driveable mode of operation <NUM> in which the landing gear wheels <NUM> are driven by both the electric motor <NUM> and the hydraulic pump <NUM>. <FIG> also shows a replenishing mode of operation <NUM> in which the electric motor <NUM> drives the hydraulic pump <NUM> to pump hydraulic fluid into the hydraulic reservoir <NUM>. <FIG> also shows a second driveable mode of operation <NUM>, in which the electric motor <NUM> drives the hydraulic pump <NUM> so that it provides hydraulic power to one or more other driveable components <NUM>. <FIG> also shows a regenerating mode of operation <NUM> in which the landing gear wheels <NUM> are externally driven to drive the electric motor <NUM> and/or the hydraulic pump <NUM>.

Whilst, in the embodiments described above, the first driveable component comprises the wheels of the aircraft, in other embodiments of the invention the first driveable component comprises another part of the aircraft. For example, the first driveable component could be an open rotor propeller. In such a case, it may be that the aircraft is a passenger aircraft. The aircraft may have a capacity of less than <NUM> passengers, preferably less than <NUM> passengers, more preferably less than <NUM> passengers, yet more preferably less than <NUM> passengers. It may be that the aircraft has a range of less than <NUM> nautical miles, preferably less than <NUM> nautical miles, more preferably less than <NUM> nautical miles, yet more preferably less than <NUM> nautical miles. It will also be appreciated by the skilled person that the present invention could be applied to any actuated aircraft part from which recovery of kinetic energy is possible.

In other embodiments of the invention, the power system <NUM> may further comprise a second electric motor. The second electric motor may be connectable to the hydraulic pump <NUM>. This enables the simultaneous replenishment of the hydraulic fluid reservoir <NUM> whilst taxiing using the first electric motor <NUM>. The second electric motor may be powered from an aircraft APU, a hydrogen fuel cell, a battery, or the aircraft engines.

It will also be appreciated that, in some embodiments of the invention, the power system <NUM> may also comprise an unpressurised (or low-pressure) hydraulic fluid tank, which provides (i) a source of hydraulic fluid to pump into the hydraulic fluid reservoir <NUM> and (ii) a destination for hydraulic fluid released from the hydraulic fluid reservoir <NUM> in order to drive a driveable component of the aircraft.

Claim 1:
An aircraft power system (<NUM>) comprising:
i) a hydraulic fluid reservoir (<NUM>),
ii) a bi-directional hydraulic pump (<NUM>) for pumping hydraulic fluid to and from the reservoir, and
iii) an electric motor (<NUM>),
wherein:
the electric motor is connectable to a first driveable component 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 act as a hydraulic motor to drive the first driveable component, such that, in a first driveable mode of operation, the first driveable component is driven by both the electric motor and the hydraulic pump;
the electric motor and hydraulic pump are connected to each other in parallel such that, in the first driveable mode of operation, both are arranged to supply power to the driveable component independently of the other;
characterised in that:
the electric motor is connectable to the hydraulic pump such that, during a replenishing mode of operation, the electric motor is arranged to drive the hydraulic pump to pump hydraulic fluid into the reservoir.