Electricity generator

Abstract

An electricity supply unit is disclosed. The electricity supply unit 100 forms part of an unmanned aerial vehicle and comprises a generator unit 110 connectable to, and capable of supplying electricit

Description

Technical Field The present invention relates to electricity generators, and more particularly to supply units that are configured as unmanned aerial vehicles and methods of operating such generators. Background The provision of electrical power to remote locations can be a major limiting factor in the success of humanitarian relief efforts, as well as military and other operations. To support such operations, electricity generators often need to be transported to terrain which is difficult to reach (e.g. inaccessible by road) and in hostile environmental conditions. The generators used for these types of operations may need to be of sufficient size power a variety of different electrical loads, including but not limited to: batteries, computer systems, communication links, lights, ground control stations, and medical appliances. Components such as these may all need to be powered simultaneously and may have high power requirements. Generators which have large enough power output to meet this demand are typically bulky and heavy, and need a sufficient supply of fuel in order to run for long periods. This often makes such generators unsuitable for transportation to remote and hostile terrain. Whilst full-scale manned aircraft may have the capacity to transport such generators to the site of the operation, the costs associated with manning and running such aircraft are large. Unmanned aerial vehicles (UAVs) are now used to support a variety of military and civilian operations. However, the payload and distance range of a typical UAV is small, and insufficient for transporting and running electricity generators of the sort needed to provide the significant amount of power that is often required for humanitarian relief or military operations. Summary of Invention Embodiments of the present invention aim to address the above problems and others by providing an electricity supply unit, such as an electricity generator, which is air-mobile. This may be achieved by providing an electricity supply unit which is, itself, configured as an unmanned aerial vehicle (UAV). The engine of the generator also powers flight of the UAV. A first aspect of the disclosure provides an electricity supply unit configured as an unmanned aerial vehicle, the electricity supply unit comprising: a generator unit connectable to one or more external electrical devices, the generator unit arranged to supply electrical power to the one or more external electrical devices; lifting means operable to transport the generator unit by flight; and an engine arranged to power the lifting means and to provide mechanical power to drive the generator unit. The engine may be configured to operate in a first mode at a speed and / or power output selected to match power requirements of the generator unit. Additionally or alternatively, the engine may be configured to operate in a second mode in which it is controllable for providing unmanned flight. In the first mode, the power output of the engine may be selected based on the electrical load, for example the load of the one or more external electrical devices. For example, the power output of the engine may be adjusted in response to a change in power demand placed on the generator unit (e.g. by electrical load). In the second mode, the engine may be configured to operate at a speed and / or power output selected based on flight conditions and / or in response to commands issued by a flight control system. The engine speed in the first mode may be different from the engine speed in the second mode. For example, the engine speed in the first mode may be less than the engine speed in the second mode. The speed of the engine may be remotely controllable. The engine may be configured to run on a heavy fuel, such as aviation fuel or diesel. The generator unit may be configured to generate electrical power only when the UAV is not in flight. The unit is typically configured to prevent operation of the lifting means during operation of the generator. The electricity supply unit may comprise a first gearing arranged to convert torque from the engine into torque for powering the lifting means. The electricity supply unit may additionally or alternatively comprise a second gearing, separate from the first gearing, arranged to convert torque from the engine into torque for powering the generator unit. The generator unit may be configured to supply at least 1kW of electrical power to the one or more electrical devices. For example the generator unit may be configured to supply up to 7.5kW of electrical power. The generator unit may have a maximum power output of up to 60kW, for example up to 30kW. The generator unit may have a maximum power output which is between 1 % and 50% of the maximum power of the engine, for example between 30% and 40% of the maximum power of the engine. The lifting means may comprise a helicopter rotor head. The lifting means may in particular comprise a counter-rotating coaxial rotor head, and two or more sets of counter rotating rotor blades. The electricity supply unit may comprise an engagement mechanism coupled to a mechanical output of the engine. The mechanical output of the engine may be provided by an output shaft. The engagement mechanism may be configured to engage and disengage the generator unit from the mechanical output of the engine. The engagement mechanism may be further configured to engage and / or disengage the lifting means from the mechanical output of the engine. A first end of the output shaft may provide a first mechanical output and a second end of the output shaft may provide the second mechanical output. The electricity supply unit may comprise a gearbox coupled between the engine and the lifting means. The gearbox may house the engagement mechanism. The electricity supply unit may further comprise a second engagement mechanism coupled to a second mechanical output of the engine. The second engagement mechanism may be configured to engage and / or disengage the lifting means from the mechanical output of the engine. The second engagement mechanism, configured to engage and / or disengage the lifting means from the mechanical output of the engine, may instead be coupled to the same mechanical output of the engine as the (first) engagement mechanism, e.g. the engagement mechanism that is configured to engage and disengage the generator unit from the mechanical output of the engine. The electricity supply unit may further comprise a fuel tank arranged to provide fuel to the engine. The mass of the fuel tank when filled and the generator unit together may be at least a third of the total payload or lifting capacity of the electricity supply unit, for example at least half of the total payload or lifting capacity of the electricity supply unit, for example at least 70%, for example at least 80%, for example at least 90%, for example substantially all of the payload or lifting capacity of the electricity supply unit. The mass of the fuel tank when filled and the generator unit together may be at least a third of the total mass of the electricity supply unit, for example at least half of the total mass of the electricity supply unit. For example, the maximum payload or lifting capacity of the electricity supply unit may be at least a third of the total mass of the electricity supply unit, for example at least half of the total mass of the electricity supply unit. The generator unit may comprise a connection for electrically grounding the generator unit. The electricity supply unit may comprise an engagement means which operates to provide an interlock which prevents operation of the lifting means when the generator is engaged with the mechanical output of the engine. The electricity supply unit may comprise a plurality of engagement means which operate together to provide the interlock. The unit may comprise a controller, such as an engine control unit (ECU). The controller may be controlled by a flight computer or other controlling means. The controller may be configured to adjust the speed of the engine, and / or to switch the engine between the first mode and the second mode, e.g. by providing control signals to any of the engagement mechanisms described, the engine, the generator unit, and / or the lifting means / gearbox. The controller may be configured to operate the interlock. Another aspect of the disclosure provides a method of operating an electricity supply unit configured as an unmanned aerial vehicle for providing electrical power to one or more external devices, the method comprising: operating an engine to fly the vehicle; switching into a generator mode; in the generator mode: operating the engine to power the one or more external devices. The method may further comprise, in the generator mode, controlling the engine based on the electrical load of the one or more external devices. The method may further comprise electrically connecting the external device to the electricity supply unit. Another aspect provides an engine control unit (ECU) for an unmanned aerial vehicle (UAV) comprising control means configured to perform any of the above methods. Another aspect provides a computer program product comprising program instructions configured to program a controller to perform any of the above methods. Brief Description of Figures Some examples of the present disclosure will now be described with reference to the figures, in which: Figure 1 shows an example air-mobile electricity supply unit; Figure 2 is a schematic illustration of a first configuration of an electrical supply unit; Figure 3 is a schematic illustration of a second configuration of an electricity supply unit; Figure 4 is a flow chart illustrating an example method of operating an electricity supply unit; In the drawings like reference numerals are used to indicate like elements. Specific Description Described below with reference to the figures are examples of electricity supply units which are configured as unmanned aerial vehicles. These electricity supply units comprise generators which are powered from an engine which also powers the means for lifting the vehicle. Such electricity supply units may be distinct from other UAVs in that their primary purpose is the generation of electricity when stationary. Figure 1 illustrates an example electricity supply unit 100 comprising a generator configured as an unmanned aerial vehicle. The unit 100 comprises a body 102 and lifting means in the form of a counter-rotating coaxial rotor head 104, which acts as a lifting means to lift the unit 100 and transport it by flight. The rotor head 104 comprises a first set of rotor blades 106 configured to rotate in a first direction, and a second set of rotor blades 108 configured to rotate in an opposite direction, thereby providing lift to the unit 100. The first set 106 and second set 108 of rotor blades share an axis of rotation. The counterrotation of the two sets of blades 106, 108 means that the unwanted torque exerted on the aircraft body 102 by one set of blades is offset by the other, thereby avoiding the need for any additional rotor about another axis of the unit 100. The unit 100 further comprises an electricity generator unit 110, arranged within and / or attached to the body 102. The generator unit 110 is arranged to provide an electrical connection 112 to one or more electrical devices or loads 114a-e. Purely by way of example, devices 114a-e may include a battery or other energy storage means, computing devices such as a laptop or server, communications devices such as satellite communications devices such as satellite dishes, wireless local-area network (WLAN) routers, lights, medical devices, and any other devices which require electrical power. It will be appreciated that the number of devices shown as connected to the unit 100 is merely illustrative. Each of the devices 114-122 may be separately connected to the generator unit 110, e.g. via a plurality of connection ports provided on the electricity generator unit 110. The unit 100 also comprises an engine 120. The engine is 120 is mechanically coupled to provide power to the lifting means, e.g. rotor head 104. The engine is also mechanically coupled to provide mechanical power to drive the generator unit 110. In particular, the unit 100 may be operable in a first mode in which the engine 120 provides mechanical power to the generator unit 110 to drive the generator to produce electrical power for providing an electrical power supply. This may be used to supply electrical power to connected external devices 114-122. The unit 100 may also be operable in a second mode to drive the lifting means, e.g. rotor head 104, to transport the unit by flight. In operation, the unit 100 may be flown to a location in the second mode, by using the engine to power the lifting means to transport the unit by flight. During flight, the engine is disengaged from powering the generator unit 110. Electrical power for onboard flight systems may be provided from a battery or an alternator separate from the generator. Upon arrival at the location, the unit 100 may land and then switch into the first mode - in which the engine is disengaged from powering the lifting means and operated only to drive the generator unit 110. Specific examples of different arrangements the unit 100 shown in Figure 1 are now described with reference to Figures 2 and 3. Figure 2 is a schematic illustration of a first example unit 200 comprising a generator configured as an unmanned aerial vehicle. The unit 200 comprises an engine 202, which may for example be a liquid-injection piston engine. The engine is arranged to provide a mechanical output to an output shaft 206 to which it is coupled, and is configured to apply a rotational force to the output shaft 210 when operated, in order to drive both a generator unit and lifting means of the unit 200 as discussed in more detail below. The unit further comprises a fuel store 206 that is arranged to store fuel and supply it to the engine 202 when the engine is operational. The fuel store 206 is connected to an input of the engine 202 via a conduit 208. The fuel store 206 may have a capacity such that, when filled, a large proportion of the total mass of the unit consists of the combined mass of the fuel store 206 and the generator unit 212 (discussed in more detail below), for example up to half of the total mass of the unit 200. The engine 202 is configured in at least some examples to run off heavy fuel, such as diesel or aviation fuel, and the fuel store 206 in those examples is configured to contain such fuels. The unit 200 comprises an electricity generator unit 212, which is configured to convert mechanical energy derived from the engine 204 into electrical energy, and to provide electrical power to external electrical loads or devices which can be connected to the generator unit. The generator unit 212 is capable of delivering a total output of over 1kW of electrical power to supply to the connected electrical devices, and in at least some examples is capable of delivering 7.5kW electrical power or more, and may have a maximum power output of 60kW, or up to 60kW, for example 30kW. The generator unit 212 may have a maximum power output which is between 1% and 50% of the maximum power of the engine 204, for example between 30% and 40% of the maximum power of the engine. The generator unit 212 comprises a connection interface comprising a plurality of connection ports 214a-c. The connection ports 214a-c are arranged for connecting to electrical devices to be charged via corresponding power or charging cables 216a-c. The connection interface comprises a variety of different types of connection ports, and it will be appreciated that the number of ports shown in Figure 2 is merely exemplary and that a greater or fewer number of ports may be provided. The generator unit 212 may also include a port for electrically grounding the generator unit 212. At least one of the ports 214a-c may be arranged to provide a mains electricity supply output, for example a single-phase AC output at a voltage such as 110V, 220V, 240V. In some examples, a plurality of such ports are provided, with each configured to provide an AC supply at a different one of these voltage. In some examples, at least one of the connection ports may be arranged to provide three phase AC power, for example at 480V and / or 600V. In some examples, the ports 214a-c also include one or more DC connections for providing DC power to connected devices. The generator unit 212 comprises a generator component configured to convert the mechanical power from the engine into electrical output. The generator component may comprise an armature configured to be rotationally driven by the engine 101. The armature and / or the stator in which it is disposed may comprise one or more permanent magnets, such as rare-earth magnets, arranged to induce a current in one or more wire windings upon relative rotation of the magnets and windings. In some examples, the generator unit 212 comprises two or more generator components. In such examples, each generator component may be selectively connected and disconnected from the input and / or output of the generator unit 212, e.g. switched in and out, so as to adjust the total output power provided by the generator unit 212 to the connected external devices. The generator unit 212 may further comprise power electronics configured to convert the electrical output from the generator component(s) into a form suitable for the devices connected to the ports 214a-c, e.g. a mains supply. In particular, the power electronics are configured to convert the electrical output of the generator component into one or more of the different types of AC and / or DC power. Where multiple types of AC or DC output are provided, each may be output at a corresponding port 214a-c. For example, the types may correspond to the AC / mains and DC examples mentioned above. The unit 200 may further comprise a controller, configured to detect the electrical requirements of the connected devices, e.g. to sense the power demand or electrical load of the connected devices, and adjust the speed of the engine 202 based on the detected power demand. For example, the controller may be configured to increase the speed of the engine in response to an increased power demand (e.g. in response to another device being connected to one of the ports 214a-c), and to decrease the speed of the engine in response to a decreased power demand (e.g. in response to a device being removed from one of the ports 214a-c). In some examples, the controller’s functionality may be provided by or integrated into the generator unit 212, for example into the power electronics of the generator unit 212. The controller may be configured to adjust the speed of the engine 202 based on the detected power demand. In some examples, the generator unit 212, e.g. the power electronics, may further be configured to communicate with, e.g. send and receive signals to, the one or more of the devices coupled to the 214a-c. The controller may thereby, in response to a signal from one or more of the connected devices, e.g. from a CAN bus of the connected device, control and / or adjust the electrical power supplied to said devices, and the speed of the engine. The unit 200 further comprises a gearbox 220. The gearbox 220 is mechanically connected between the engine 202 and the lifting means of the unit 200, which may be specifically a helicopter rotor head such as that described above with reference to Figure 1. The gearbox 220 is configured to convert the rotational speed and torque of the engine 202 into a speed and torque for driving the rotor blades. In particular, the gearbox 220 comprises a plurality of gears with ratios to convert a relatively high speed, low torque input from the engine 202 into low speed, high torque output for powering the rotor blades. The unit 200 comprises a drive shaft 222 coupled between an output of the gearbox 220 and the rotor head (not shown) to drive the rotor blades. In particular, as discussed above, the rotor head is a counter-rotating coaxial rotor head. Therefore, the drive shaft 222 in this example comprises a pair of coaxially aligned shafts each coupled to one set of rotor blades and arranged to rotate in opposite directions (clockwise and anticlockwise), with one shaft arranged to rotate substantially inside the other. The mechanical connection between the engine 202 and both the generator unit 212 and the gearbox 220 (and thereby also the lifting means) is now described in more detail. The engine output shaft 210 is coupled at one end (a first end) to an output of the engine 202, and at its other end (its second end) to a first engagement mechanism 224. The first engagement mechanism 224 is coupled to an intermediate shaft 226, and is configured to engage and disengage the first end of the engine output shaft 210 from the intermediate shaft 226. The first engagement mechanism 224 is operable to connect and disconnect the engine 202 from the downstream drivetrain and load of e.g. the lifting means and / or generator unit 212, For example the first engagement mechanism 224 may be operable to disconnect the engine from the drivetrain when the engine 202 is starting up or idling, and then to engage the engine with the drivetrain once the engine reaches a certain threshold speed, e.g. after starting up. This may thereby enable a smooth transition in engine speed and torque as the engine increases speed and the first engagement mechanism 224 engages. In particular, the first engagement mechanism 224 may itself be speed-dependent, for example a speed-dependent clutch such as a centrifugal clutch. Such a speed-dependent mechanism may be configured to automatically engage the intermediate shaft 226 with the engine output shaft 210 as the rotational speed of the engine output shaft 210 reaches a certain level, and to disengage the intermediate shaft 226 from the engine output shaft 210 as the rotational speed of the engine output shaft 210 drops below a certain level. The second end of the intermediate shaft 226 is coupled to a second engagement mechanism 228. The second engagement mechanism 228 is thus coupled to a mechanical output of the engine, and is configured to engage and disengage both the generator unit 212 and the lifting means from the mechanical output of the engine 202. In particular, the second engagement mechanism 228 is arranged to selectively engage and disengage the intermediate shaft 226 from a gearbox shaft 230 and a generator shaft 232. The second engagement mechanism 228 may comprise a clutch such as a dog clutch, for example comprising interlocking teeth on the end of the intermediate shaft 226 and an end of each of the gearbox shaft 230 and the generator shaft 232. The second engagement mechanism 228 is operable to engage and disengage the intermediate shaft 226 from the gearbox shaft 230 thereby connecting and disconnecting the lifting means (e.g. rotor blades) of the unit 200 from the engine 202. Similarly, the second engagement mechanism 228 is operable to engage and disengage the intermediate shaft 226 from the generator shaft 232 thereby connecting and disconnecting the generator unit 212 from the engine 202. As such, operation of the second engagement mechanism 228 enables the unit to switch between a mode of operation in which the engine 202 is used to power the lifting means of the unit 200, and another mode in which the engine 202 is used to power the generator 12. In particular, the second engagement mechanism 228 is configured to disengage the intermediate shaft 226 from the generator shaft 232, and then engage the intermediate shaft 226 with the gearbox shaft 230 and vice versa. As such, the second engagement mechanism 228 may also be configured such that the intermediate shaft 232 is disengaged from both the gearbox shaft 230 and the generator shaft 232, so that the engine 202 is not disconnected from both the lifting means and the generator unit 212, e.g. when the engine 202 may be idling. In some examples, the second engagement mechanism 228, e.g. dog clutch, may be integrated into the gearbox 220. The unit 200 further comprises a generator gearing 234 arranged between the generator shaft 232 and the generator unit 212. In some examples the generator gearing 234 is integrated into the generator unit 212, or the generator unit 212 may have has its own internal gearbox. The generator gearing 234 comprises gears with a particular gear ratio, to convert the rotational speed of the engine 202 and generator shaft 232 into a selected or desirable input speed for the generator unit 212. In operation, the engine 202 may be initially at rest. When the engine 202 is switched on, it draws fuel from the fuel store 206 and operates to drive the engine output shaft 210. In examples where first engagement mechanism 224 is speed dependent, e.g. a centrifugal clutch, the output shaft 210 is initially disengaged from the intermediate shaft 226. Once the engine 202 and output shaft 210 reaches the threshold rotational speed, the first engagement mechanism brings the output shaft 210 into engagement with the intermediate shaft 226 to drive the intermediate shaft 226. The second engagement mechanism 228 can then be operated to bring the intermediate shaft into engagement with either the gearbox shaft 230, to drive the lifting means, or into engagement with the generator shaft 232 to drive the generator unit 212, so that the apparatus 200 is arranged to run in either a flight mode or a generator mode respectively. It will be appreciated that in other examples the second engagement mechanism 228 may be operated in this way prior to engagement of the first engagement mechanism 224 so that the intermediate shaft 226 is already in engagement with either the gearbox shaft 230 or the generator shaft 232 before the intermediate shaft 226 is driven by the engine 202. In other examples, the second engagement mechanism may already be configured such that the intermediate shaft 226 is in engagement with either the gearbox shaft 230 or the generator shaft 232, before the engine is turned on. In the case where the second engagement mechanism 228 is configured so that the intermediate shaft 226 is in engagement with the gearbox shaft 230 (e.g. in a flying mode), the second engagement mechanism 228 can be operated to disengage the intermediate shaft 226 with the gearbox shaft 230 and then engage the intermediate shaft 226 from the generator shaft 212 to drive the generator. In the case where the second engagement mechanism 228 is configured so that the intermediate shaft 226 is in engagement with the generator shaft 212 (e.g. in a generator mode), the second engagement mechanism 228 can be operated to disengage the intermediate shaft 226 from the generator shaft 212and then engage the intermediate shaft 226 with the gearbox shaft 230 to drive the gearbox 220 and lifting means. A controller may be connected to and provide control signals to control operation of the second engagement mechanism and cause the engagement and disengagement as described above. Figure 3 is a schematic illustration of another example unit 300 comprising a generator configured as an unmanned aerial vehicle. The unit comprises an engine 202 and fuel tank 206 as in the example described above in relation to Figure 2. The unit 300 further comprises a gearbox 220, lifting means, and a generator unit 212. The engine 202, the generator unit 212, the gearbox 220 and the lifting means are configured substantially as described above in relation to the Figure 2 example. The example shown in Figure 3 differs from that shown in Figure 2 in that the gearbox 220 for the lifting means and the generator unit 212 are connected to different outputs of the engine 202, e.g. different ends of a single engine shaft 310. A first mechanical output of the engine shaft 310 is coupled to an engagement mechanism 318. The engagement mechanism 318 is a clutch e.g. a dog clutch, coupled to the generator unit via a shaft 322. The engagement mechanism 318 is configured to engage and disengage the generator unit 212 from the mechanical output the engine 202. Upon engagement the unit is configured such that the engine 202 can be operated to provide power to the generator unit 212. In some other examples, the engagement mechanism 318 is instead integrated into the generator itself, e.g. such that the generator unit 212 has an inbuilt mechanical or electromagnetic clutch. A second mechanical output of the engine may be provided by the other end of the shaft 310, which may be arranged on an opposite side of the engine 202 from the first mechanical output, is coupled to a second engagement mechanism 312, which may be in the form of a clutch, e.g. a centrifugal clutch. The second engagement mechanism 312 is coupled to the gearbox 220 via a shaft 316 to drive the lifting means, e.g. a helicopter rotor head. The second engagement mechanism 312 is configured to engage and disengage the gearbox 220 from the second mechanical output of the engine 202. In so doing, the second engagement mechanism 312 is configured to engage and disengage the lifting means from the engine 202. In the example unit 300, the engagement mechanisms 312 and 318 are configured to prevent the engine shaft 310 being engaged to drive with both the generator unit 212 and the gearbox 220 at the same time. For example, one or both of the mechanisms 312, 318 further comprises a locking mechanism, e.g. an interlock, configured to prevent the drive shaft 310 from engaging the gearbox shaft (and thereby preventing the lifting means from engaging with the engine) when the engine 202 is coupled to and driving the generator unit 212, and vice versa. In the examples shown in both Figures 2 and 3 described above, the units 200, 300 are thus configured to operate in two modes. In particular, the engine 202 is configured to operate in a first mode, e.g. a generator mode, in which it is coupled to and powers the generator unit, and in a second mode, e.g. a flying mode, in which it is coupled to and powers the lifting means. In the first mode, the engine 202 is configured to operate at a speed and / or provide a power output that is selected to match the power requirements of the generator unit 212. As such the engine 202 may be configured to operate at a different setpoint of speed and / or power in the first mode than in the second mode. In general, the rotational speed of the engine 202 is less in the first mode when driving the generator unit 212, than in the second mode when driving the lifting means. The generator unit 212 may be configured to operate most efficiently at a particular rotational speed, typically in the range of 1,800 to 4,000 rpm. Therefore, in the first mode, the engine 202 may be configured to operate at a speed, e.g. an engine setpoint, that provides such a preferred speed to the input of the generator unit 212, subject to the adjustment in rotational speed provided by the gearing 234 or the generator unit’s internal gearbox as discussed above. The power required by the generator unit 212 from the engine 202 may be based on the size of the electrical load of the external electrical devices which are connected to the generator unit 212. For example the required power may vary as the size of the of the electrical load of the external electrical devices which are connected to the generator unit 212. The power output of the engine 202 may thereby be based, at least in part, on the electrical load of the connected external electrical devices. In the second mode, the units 200, 300 and in particular the engine 202 is controllable for providing unmanned flight. The units 200, 300 may therefore also comprise a controller such as a throttle and / or flight computer for controlling the speed of the engine 202. As discussed above, the units 200, 300 are configured as unmanned aerial vehicles, and so the units 200, 300, and specifically the speed of the engine 202, may be remotely controllable. In such examples, the units 200, 300 further comprise means for communicating with a remote controller, e.g. that is operated by a user on the ground, and which is configured to obtain instruction signals to adjust the speed of the engine 202, e.g. via control of the flight computer, throttle, and / or or other flight control means, based on flight conditions. As described above, the lifting means comprise a helicopter rotor head, in particular a coaxial counterrotating rotor head comprising two sets of rotor blades arranged to rotate in opposite direction about a common axis. The rotor blades may be detachable from the units described above. For example, each set of rotor blades may be coupled to a rotor hub, and the rotor head may comprise a plurality of release mechanisms configured to -15 - quickly release each rotor blade from its corresponding hub. As such, the rotor blades can be removed from the units when they are operated in the above-described generator mode. Alternatively, the rotor head as a whole, or each hub and associated blades as a whole, may be detached from the unit 100. Alternatively or additionally, the rotor blades may be configured such that they can be folded away, rather than detached, upon operation of a release mechanism. With reference to Figure 4, a particular method 400 of operating an electricity supply unit configured as an unmanned aerial vehicle, such as the units 200 and 300 described above, will now be briefly described. The method comprises as a first step 401, operating the engine to fly the unit. This may for example involve operating the unit as in a flying mode as discussed above in which the engine provides power to the lifting means of the unit. In this mode the unit is arranged as a UAV to fly a generator to a particular location to deliver electrical power. During this mode of operation a battery or alternator carried by the unit may provide electrical power for onboard electrical systems, separate from the generator. The method further comprises a step 402 of switching the unit into a generator mode. This step may be carried out after the unit has stopped operating in the flying mode, e.g. once the unit has landed. Step 402 may involve operating one or more of the engagement mechanisms described above so that the engine is now configured to provide power to the generator unit and prevented from driving the lifting means. The method further comprises at step 403, operating the engine in the generator mode in order to drive the electricity generator unit and power the one or more external devices that are connected to the generator unit, as described above. The engine speed may be controlled in the generator mode based on the power requirements of the generator unit, which in turn may be based on the electrical load of the one or more external devices. In particular, in this mode the generator unit is configured to sense the power demand of the connected external devices, and the electronics of the generator unit and / or wider electricity supply unit are configured to select and set an engine speed based on this demand, e.g. so that the power demand is met. The generator unit is also configured to sense a change in the power demand of the connected external devices, e.g. due to a connection or disconnection of a device, and the electronics generator unit and / or wider electricity supply unit are configured to change the speed of the engine based on the change in power demand. It will be appreciated that the method may also include further optional steps not shown. For example, the method may further comprise the step of landing the unit. The method may also include a subsequent step of ceasing electricity generation and switching the unit back into the flight mode. This may involve operating one or more of the engagement mechanisms described above so that the engine is now configured to provide power to the lifting means of the unit. The method may also include a step of electrically connecting one or more external devices to the generator unit, e.g. by plugging the devices in to one or more of the ports on the generator unit described above. The units described above may comprise a controller, such as an engine control unit (ECU) which may be controlled by a flight computer or other controlling means. The controller may comprise control means configured to perform the above methods. Additionally or alternatively, the units may comprise a controller, and a computer program product comprising program instructions may be configured to program the controller to perform the above methods. For example, the controller may be configured with one or more set points for operating the engine in the generator mode, and for adjusting operation of the engine according to electrical load on the generator. It may be further configured with a different set of engine operating conditions, which it applies when the unit is operating in the flight mode. It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and / or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit. For example the control and / or sensing functions described herein may be provided by integrated control and / or sensing functionality and / or by separate controllers and sensors. For example, it will be appreciated that the first engagement mechanism, e.g. the first engagement mechanism 224, is merely optional, and that in other examples the engine output shaft is directly coupled to the gearbox or to what has been described above as the second engagement mechanism. The first engagement mechanism can in other examples be provided by a component other than a centrifugal clutch, for example a hydraulically operated clutch or torque converter. In addition, the second engagement mechanism, e.g. second engagement mechanism 228, can in other examples be provided by a component other than a dog-clutch. For example, a standard automotive clutch, comprising a friction pad configured to engage and disengage with rotating flywheel, may instead be used. Some embodiments above have been described as including a speed-dependent clutch or engagement mechanism (e.g. the first engagement mechanism 224 in Figure 2). It will be appreciated however that it is not necessary for this clutch to be speed dependent, and that any actuatable clutch may be used, the control of which may or may not be speed dependent. In other examples, an engagement mechanism configured to engage and disengage the engine from the lifting means may instead be arranged between the generator unit and the gearbox in the drive chain. In such examples, a mechanical output of the engine, such as a drive shaft, may be coupled to a mechanical input of the generator unit, and the same output, e.g. drive shaft, may be coupled to the gearbox to drive the lifting means. In this example, a first engagement mechanism, configured to engage and disengage the generator unit from the engine output, may be coupled between the engine output and the generator unit, and a second engagement mechanism, configured to engage and disengage the lifting means from the engine output may be arranged between the first engagement mechanism and the gearbox. In such examples, the second engagement mechanism is not speed-dependent, e.g. in order to avoid the engagement of the first engagement mechanism causing engagement of the second engagement mechanism. In some examples, the generator unit comprises its own integrated engagement mechanism, e.g. an electromagnetic clutch. For example, the first engagement mechanism in the example described directly above may be provided by such an integrated engagement mechanism. In the example shown in Fig. 3 such an integrated engagement mechanism could be provided in place of the engagement mechanism 318. In any of the examples described above, the generator unit may comprise such an engagement mechanism in addition to the engagement mechanisms described. In other examples, at least one of the engagement mechanisms described, e.g. second engagement mechanism 228, may instead be integrated into the gearbox, e.g. gearbox 220. In such examples, the gearbox may comprise two output shafts - a first output shaft coupled to the lifting means, and a second output shaft coupled to the input of the generator unit. The engagement mechanism, which may for example be two separate mechanisms e.g. clutches, is configured to selectively engage the mechanical input of the gearbox with either the first or the second output shaft. That is, the engagement mechanism is configured to disengage the mechanical input of the gearbox from the first output shaft so as to disengage the lifting means from the engine, and, once the lifting means has been disengaged, to engage the mechanical input of the gearbox with the second output shaft so as to engage the generator with the engine to drive the generator. The engagement mechanism is similarly configured to disengage the mechanical input of the gearbox from the second output shaft so as to disengage the generator unit from the engine, and, once the generator unit has been disengaged, to engage the mechanical input of the gearbox to the second output shaft so as to engage the lifting means with the engine to drive the lifting means. In certain examples a controller described herein may be configured to perform any of the methods, or particular steps of said methods. The activities and apparatus outlined herein may be implemented using controllers and / or processors which may be provided by fixed logic such as assemblies of logic gates or programmable logic such as software and / or computer program instructions executed by a processor. Other kinds of programmable logic include programmable processors, programmable digital logic (e.g., a field programmable gate array (FPGA), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM)), an application specific integrated circuit, ASIC, or any other kind of digital logic, software, code, electronic instructions, flash memory, optical disks, CD-ROMs, DVD ROMs, magnetic or optical cards, other types of machine-readable mediums suitable for storing electronic instructions, or any suitable combination thereof. As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any features of any aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example, method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination with, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention. Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

:

1. An electricity supply unit configured as an unmanned aerial vehicle, the electricity supply unit comprising:a generator unit connectable to one or more external electrical devices, the generator unit arranged to supply electrical power to the one or more external electrical devices;lifting means operable to transport the generator unit by flight;an engine arranged to power the lifting means and provide mechanical power to drive the generator unit; andan engagement mechanism coupled to a mechanical output of the engine, and configured to engage and disengage the generator unit from the mechanical output of the engine.

2. The electricity supply unit of claim 1, wherein the engine is configured to operate in a first mode at a speed and / or power output selected to match requirements of the generator unit, and to operate in a second mode in which it is controllable for providing unmanned flight.

3. The electricity supply unit of claim 2, wherein in the first mode, the power output of the engine is based on the electrical load of the one or more external electrical devices.

4. The electricity supply unit of claim 2 or 3, wherein in the second mode, the engine is configured to operate at a speed and / or power output selected based on flight conditions.

5. The electricity supply unit of any of claims 2 to 4, wherein the engine speed in the first mode is different from the engine speed in the second mode.

6. The electricity supply unit of any preceding claim, wherein the speed of the engine is remotely controllable.

7. The electricity supply unit of any preceding claim, comprising a first gearing arranged to convert torque from the engine into torque for powering the lifting means, and a second gearing, separate from the first gearing, arranged to convert torque from the engine into torque for powering the generator unit.

8. The electricity supply unit of any preceding claim, wherein the generator unit is configured to supply at least 1 kW of electrical power to the one or more electrical devices.28 03 259. The electricity supply unit of any preceding claim, wherein the lifting means comprises a helicopter rotor head.

10. The electricity supply unit of claim 9, wherein the lifting means comprises a counterrotating coaxial rotor head.

11. The electricity supply unit of any preceding claim, wherein the engagement mechanism is further configured to engage and disengage the lifting means from the mechanical output of the engine.

12. The electricity supply unit of any preceding claim, further comprising a gearbox coupled between the engine and the lifting means, wherein the gearbox houses the engagement mechanism.

13. The electricity supply unit of any preceding claim, further comprising a second engagement mechanism coupled to a second mechanical output of the engine, wherein the second engagement mechanism is configured to engage and disengage the lifting means from the second mechanical output of the engine.

14. The electricity supply unit of any preceding claim, further comprising a fuel tank arranged to provide fuel to the engine, wherein the mass of the fuel tank when filled and the generator unit together is at least a third of the total mass of the electricity generator configured as an unmanned aerial vehicle.

15. The electricity supply unit of any preceding claim, wherein the generator unit comprises a connection for electrically grounding the generator unit.

16. A method of operating an electricity supply unit configured as an unmanned aerial vehicle for providing electrical power to one or more external devices, the method comprising:operating an engine to fly the vehicle;switching into a generator mode;in the generator mode:operating the engine to power the one or more external devices;wherein switching into the generator mode comprises engaging the generator unit with a mechanical output of the engine.

17. The method of claim 16, comprising, in the generator mode, controlling the engine based on the electrical load of the one or more external devices.

18. The method of claim 16 or 17, further comprising electrically connecting the external device to the supply unit.

19. The method of any of claims 16 to 18, wherein the electricity supply unit is the electricity supply unit of any of claims 1 to 15.

20. An engine control unit (ECU) for an unmanned aerial vehicle (UAV) comprising control means configured to perform the method of any of claims 16 to 18.

21. A computer program product comprising program instructions configured to program a controller to perform the method of any of claims 16 to 18.28 03 25

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