Patent Publication Number: US-2023159178-A1

Title: Aerial drone

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. patent application Ser. No. 17/174,208 filed Feb. 11, 2021, which is a Continuation of U.S. patent application Ser. No. 15/571,258 filed Nov. 1, 2017, which is a 371 National Stage of International Patent Application No. PCT/GB2016/051254 filed Apr. 29, 2016, which claims priority to Great Britain patent application no. 1507534.4 filed May 1, 2015, and Great Britain patent application no. 1519214.9 filed Oct. 30, 2015, the disclosure of all of which are incorporate by reference herein in their entirety. 
    
    
     The present application relates to aerial vehicles and, in particular, to unmanned aerial vehicles. 
     Aerial vehicles, or aircraft, may be powered or unpowered. Unpowered aircraft include gliders and some lighter-than-air vehicles, such as balloons. Powered aircraft generally include planes, helicopters or other rotorcraft, microlights, and other lighter-than-air vehicles, such as airships. 
     Unmanned aerial vehicles (UAVs) have many applications including reconnaissance, remote sensing and providing an airborne base for a telecommunications transceiver. 
     UAVs are typically smaller than manned aircraft and may weigh between a few grams and 20 kilograms, for example. The expression “unmanned aerial vehicle” as used herein is intended to encompass aerial vehicles not capable of conveying a pilot. 
     UAVs typically require power in order to provide propulsion, which may in some cases be necessary in order for a UAV to remain airborne for a prolonged period, and power for auxiliary functions such as image or video capture, signal telemetry, or other on-board systems. In addition, for many applications the computing power required on-board the vehicle in order to provide the necessary functionality may represent a significant power demand. This is particularly the case in autonomous UAV applications in which an onboard computer may make decisions regarding flight path and the deployment of auxiliary functions. Such an autonomous UAV is pilotless in a strict sense. Alternatively, a UAV may be piloted remotely and so although the vehicle itself is unmanned, it is still under human control. 
     Some conventional UAVs use primary cells to provide power, although it is now more common to use secondary cells such as lithium-ion batteries. A problem with conventional UAVs is that the flight time may be limited because of the relatively high-power demands of the propulsion and other on-board systems. In recent years, photovoltaic panels have been provided on UAVs in order to extend the flight range in comparison to UAVs that have only primary or secondary cells. However, the power generating capacity of a photovoltaic panel depends on the ambient weather condition and the time of day and so photovoltaic panels may not be appropriate for use in all circumstances. In addition, the power generation capacity of photovoltaic panels may be inadequate for some applications in which either high power (speed) propulsion is required or the on-board systems of the UAV that provide its functionality are particularly heavy or demand substantial electrical power. 
     DISCLOSURE 
     According to one aspect of the disclosure there is provided an unmanned aerial vehicle comprising a plurality of at least one type of fuel cell system component distributed about the vehicle. 
     The at least one type of fuel cell system component may be a fuel cell. The at least one type of fuel cell system component may comprise at least one of: a fuel cell, a fuel generator; a coolant structure; a fuel reservoir; and a reactant reservoir. 
     Disclosed herein are systems, methods and devices of ariel vehicles having fuel cell system components forming at least a portion of the aerial vehicle&#39;s support structure including propulsion modules affixed to wings, a fuel cell having vertical plates oriented from top to bottom surface of each wing, air inlets for each fuel cell provided at the forward surface of each wing, a fuel cell system component forming a portion of the body and wherein the air inlets are unblocked during flight, each propulsion module is configured to provide air as an oxidant to a fuel cell via the air inlets, a fuel cell system component provides a structural load bearing component of the aerial vehicle; and the fuel cells form a fuel cell stack which generates power. In some instances, the fuel cells are formed as unitary components with the wings of the ariel vehicle and the surfaces of end plates of each of the fuel cells are aerodynamically shaped to at least partially provide the functionality of the wing structure. In some instances the fuel cells are formed as unitary components with the upright portion of the tailplane configured with air inlets for said fuel cell provided at the forward surface of the tailplane and the surfaces of end plates of each of the fuel cells are aerodynamically shaped to at least partially provide the functionality of the tailplane structure. 
     Disclosed herein are systems, methods and devices of ariel vehicles having fuel cell system components forming at least a portion of the aerial vehicle&#39;s support structure including propulsion modules affixed to wings, a fuel cell having one of bipolar and monopolar vertical plates oriented from top to bottom surface of each wing, air inlets for each fuel cell provided at the forward surface of each wing, a fuel cell system component forming a portion of the body and wherein the air inlets are unblocked during flight, each propulsion module is configured to provide air as an oxidant to a fuel cell via the air inlets, a fuel cell system component provides a structural load bearing component of the aerial vehicle; and the fuel cells form a fuel cell stack which generates power. In some instances, the fuel cells are formed as unitary components with the wings of the ariel vehicle. In some instances, the fuel cell system component is one of a fuel cell, a fuel generator, a coolant structure, a fuel reservoir, and a reactant reservoir. In some instances, a plurality of at least one type of fuel cell system component distributed about the vehicle. In some instances, the fuel cell component is a fuel supply module. In some instances, a separate fuel supply module is provided to each fuel cell. In some instances, a centralized fuel supply module is used to supply fuel to the fuel cells. 
     Disclosed herein are systems, methods and devices of ariel vehicles having fuel cell system components forming at least a portion of the aerial vehicle&#39;s support structure including propulsion modules affixed to wings, a fuel cell having one of bipolar and monopolar vertical plates oriented from top to bottom surface of each wing, air inlets for each fuel cell provided at the forward surface of each wing, a fuel cell system component forming a portion of the body and wherein the air inlets are unblocked during flight, each propulsion module is configured to provide air as an oxidant to a fuel cell via the air inlets, a fuel cell system component provides a structural load bearing component of the aerial vehicle; and the fuel cells form a fuel cell stack which generates power. In some instances, the fuel cells are formed as unitary components with the wings of the ariel vehicle. In some instances, the fuel cell system component is one of a fuel cell, a fuel generator, a coolant structure, a fuel reservoir, and a reactant reservoir. In some instances, a plurality of at least one type of fuel cell system component distributed about the vehicle. In some instances, the air inlets are fanless and rely on the motion of the vehicle through the air when in flight to direct air into the forward-facing air inlets. In some instances, the ariel vehicle further comprising airflow from propellers to direct air into the forward-facing air inlets. In some instances, the system further comprising airflow from propellers to direct air into the forward-facing air inlets of a portion of the fuel cells distributed on the vehicle. 
     The body may comprise a reactant or fuel reservoir “FR” and/or a fuel cell. The fuel cell may provide a structural component of the vehicle. Each fuel cell may comprise a plurality of fuel cell plates. The plates are orientated to be substantially aligned with each other so that the plates are vertical in use. A vertical air flow path may be provided through the plates. 
     Each propulsion module is associated with a respective fuel cell. An air inlet of each of the fuel cells may be associated with a respective propulsion module. Each propulsion module may be configured to provide oxidant and/or coolant to the associated fuel cell. 
     Each propulsion module may be the only active source of oxidant and/or coolant to the associated fuel cell. Each propulsion module may have a propeller or rotor. The fuel cell may be integral with a surface of the vehicle. The fuel cells may provide power for propulsion of the vehicle. The fuel cells may provide power for auxiliary or on-board functions of the vehicle. 
     The unmanned aerial vehicle may comprise a controller. The controller may be configured to receive electrical power from the fuel cells. The controller may be configured to distribute the electrical power to the propulsion modules. 
     According to a further aspect of the disclosure there is provided an unmanned aerial vehicle (UAV) comprising a fuel cell, wherein an air inlet of the fuel cell is associated with a propulsion module, and wherein the propulsion module is configured to provide oxidant and/or coolant to the fuel cell. The propulsion module may be the only active source of oxidant and/or coolant to the fuel cell. 
     According to a further aspect of the disclosure there is provided an unmanned aerial vehicle (UAV) comprising a body and a plurality of propulsion modules connected to the body by respective struts, in which one or more of the struts comprises a fuel cell system component. 
     According to a further aspect of the disclosure there is provided an unmanned aerial vehicle (UAV) comprising a fuel cell, wherein the fuel cell provides a structural component of the vehicle. 
     Any feature described with reference to one of the aspects may be provided in combination with the features of another of the aspects. 
     The use of an electrochemical fuel cell as a power source for an unmanned aerial vehicle is particularly advantageous because fuel cells can offer improved power/weight and 35 power/volume ratio performance compared to some prior art power supplies. In addition, a fuel cell can provide the level of power demanded in modern UAV applications. By incorporating the fuel cell into a structural component of the UAV, the relative weight added by the fuel cell can be reduced because the fuel cell performs both its primary purpose of power generation and provides a structural support required by the vehicle. As such, the efficiency and performance of the UAV can be improved compared to prior art solutions. 
     The fuel cell may be a planar fuel cell. The fuel cell may comprise a plurality of fuel cell plates. The plates may be orientated to be aligned with, or transverse to, a mechanical load associated with use of the vehicle, such as a direction of thrust from a propulsion unit of the vehicle. The vehicle may comprise a plurality of fuel cells. The fuel cells may be provided as a fuel cell stack. The fuel cells may be distributed about the vehicle. The vehicle may comprise a plurality of propulsion modules. Each propulsion module may be associated with one of the plurality of fuel cells. An air inlet of the, or each, fuel cell may be associated with the, or a particular, propulsion module. The, or each, propulsion module may be configured to provide oxidant and/or coolant to the, or the associated fuel cell. The, or each, propulsion module may be the only active source of oxidant and/or coolant to the, or the associated, fuel cell. The, or each propulsion module may have a propeller or rotor. The fuel cell may be integral with a surface of the vehicle. The fuel cell may provide power for propulsion of the vehicle or for auxiliary or on-board functions of the vehicle. 
     According to a further aspect of the disclosure there is provided an unmanned aerial vehicle comprising a fuel cell, wherein an air inlet of the fuel cell is associated with a propulsion module, and wherein the propulsion module is configured to provide oxidant and/or coolant to the fuel cell. The propulsion module may be the only active source of oxidant and/or coolant to the fuel cell. 
     According to a further aspect of the disclosure there is provided an unmanned aerial vehicle comprising a plurality of fuel cells. The fuel cells may be distributed about the vehicle. The vehicle may comprise a plurality of propulsion modules. Each propulsion module may be associated with a respective one of the plurality of fuel cells. 
     According to a further aspect of the disclosure there is provided an aerial vehicle comprising a fuel cell. 
     The vehicles according to the any aspects may comprise any of the features described with regard to any other aspect or features otherwise described herein. 
    
    
     
       Exemplars of the present disclosure will now be described by way of example and with reference to the accompanying drawings in which: 
         FIG.  1    illustrates a schematic view of a glider comprising fuel cells; 
         FIG.  2    illustrates a schematic view of a plane comprising fuel cells; 
         FIG.  3    illustrates a schematic view of another plane comprising fuel cells; 
         FIG.  4   a    illustrates a schematic view of a rotorcraft comprising a plurality of fuel cells; 
         FIG.  4   b    illustrates a schematic longitudinal cross section through a strut of the rotorcraft of  FIG.  4     a;    
         FIG.  5   a    illustrates a rotorcraft comprising a plurality of fuel cells and a plurality of fuel generators; 
         FIG.  5   b    illustrates a schematic longitudinal cross section through a strut of the rotorcraft of  FIG.  5   a    in which a fuel generator, a reactant cartridge and a fuel cell are located within the strut; 
         FIG.  5   c    illustrates a schematic longitudinal cross section through a strut in which a fuel generator, a first reactant reservoir and a fuel cell are located within the strut and a second reactant reservoir is located within the body; 
         FIG.  5   d    illustrates a schematic longitudinal cross section through a strut in which a fuel generator and a fuel cell are located within the strut and first and second reactant reservoirs are located within the body; 
         FIG.  5   e    illustrates a schematic longitudinal cross section through a strut comprising a fuel cell and a fuel reservoir; 
         FIG.  6   a    illustrates a rotorcraft comprising a plurality of coolant structures; and  FIG.  6   b    illustrates a schematic longitudinal cross section through a strut of the rotorcraft of  FIG.  6     a.    
     
    
    
     All callouts are hereby incorporated by this reference as if fully set forth herein. 
     Throughout the present specification, the descriptors relating to relative orientation and position, such as “horizontal”, “vertical”, “top”, “bottom” and “side”, are used in the sense of the orientation of the unmanned aerial vehicle as presented in the drawings. However, such descriptors are not intended to be in any way limiting to an intended use of the described or claimed invention. Corresponding series of reference numerals are used in the figures in order to refer to similar or corresponding features between different figures. 
     Electrochemical fuel cells convert fuel and oxidant, generally both in the form of gaseous streams, into electrical energy and a reaction product. A common type of electrochemical fuel cell for reacting hydrogen and oxygen comprises a polymeric ion transfer membrane, also known as a proton exchange membrane (PEM), within a membrane-electrode assembly (MEA), with fuel and air being passed over respective sides of the membrane. 
     Protons (i.e. hydrogen ions) are conducted through the membrane, balanced by electrons conducted through a circuit connecting the anode and cathode of the fuel cell. To increase the available voltage, a stack can be formed comprising a number of series-connected MEAs arranged with separate anode and cathode fluid flow paths. Such a stack is typically in the form of a block comprising numerous individual fuel cell plates held together by end plates at either end of the stack. In general, the expression “fuel cell” is used herein to encompass either a single fuel cell or a plurality of individual fuel cells assembled in series to form a fuel cell stack. 
       FIG.  1    illustrates a glider, which is an example of an unpowered aerial vehicle  100 . The vehicle  100  has a body  102 , a pair of wings  104 ,  106  and a tailplane  108  with an upright portion. Each of the wings  104 ,  106  and upright portion of the tailplane  108  comprises a fuel cell  109 ,  111 ,  113 . The fuel cells  109 ,  111 ,  113  may be configured to provide power to on-board systems such as telecommunication systems. 
     The fuel cells  109 ,  111 ,  113  each provide a structural component of the vehicle  100  in that they are integrated with a structural component of the vehicle. A structural component may provide a function of the vehicle  100 . In this example, the fuel cells  109 ,  111 ,  113  are formed as unitary components with the wings  104 ,  106  and tailplane  108 . External surfaces of end plates of each of the fuel cells  109 ,  111 ,  113  may be aerodynamically shaped in order to at least partially provide the functionality of the structure with which they are integrated. By providing the fuel cells as part of structural components, as opposed to providing additional components within the body  102  of the vehicle  100  or providing an additional layer such as a photovoltaic skin on top of structural components, the overall weight of the vehicle  100  may be reduced. In some examples, one or more of the fuel cells  109 ,  111 ,  113  may be configured to bear a mechanical load of the vehicle  100 . 
     Examples of mechanical loads include those associated with flight or landing. 
     The fuel cells  109 ,  111 ,  113  each have an air inlet  110 ,  112 ,  114  in order to draw in oxidant and/or coolant. Such an arrangement may be particularly advantageous for air cooled, open cathode fuel cells. Fans may be provided at the air inlets  110 ,  112 ,  114  in order to draw in the air. Alternatively, the air inlets  110 ,  112 ,  114  may be fanless (without fans) and rely on the motion of the vehicle  100  through the air when in flight to draw air into the forward-facing fans. Each fuel cell  109 ,  111 ,  113  also has an air outlet (not shown), which may be provided downstream of the air inlet  110 ,  112 ,  114 . 
     An advantage of distributing the plurality of separate fuel cells  109 ,  111 ,  113  around the vehicle  100  is that, in the event of a failure of a particular fuel cell  109  due to, for example, the impact of a foreign object such as a bird with its inlet  110 , the remaining fuel cells  111 ,  113  located at distal parts of the vehicle  100  may continue to operate and provide power as normal. A controller may be provided in order to manage power production by the fuel cells  109 ,  111 ,  113  in accordance with the demands of the vehicle  100 . 
     A separate fuel supply module may be provided to each of the fuel cells  109 ,  111 ,  113 . 
     Alternatively, a central fuel supply module may serve the fuel cells  109 ,  111 ,  113 . 
       FIG.  2    illustrates a plane, which is an example of a powered aerial vehicle  200 . The vehicle  200  is generally similar to the vehicle described with reference to  FIG.  1   , and in addition comprises a propulsion module  215 . The propulsion module  215  is configured to propel the vehicle  200  during flight. It will be appreciated that a wide variety of types of propulsion module are available. In this example, the propulsion module  215  comprises a motor configured to drive a propeller  216 . In use, the propeller rotates in order to propel air over the aerodynamic surfaces of the vehicle in order to generate both thrust and lift. 
     The propulsion module and air inlets  210 ,  212 ,  214  of the fuel cells  209 ,  211 ,  213  in this example are arranged on the wings  204 ,  206  and upright portion of the tailplane  208  such that air is propelled by the propulsion module into the air inlets  210 ,  212 ,  214  in order to provide oxidant and/or coolant to the fuel cells  209 ,  211 ,  213 . 
     The fuel cells  209 ,  211 ,  213  may provide power for propulsion of the vehicle in addition to power for other on-board systems, which in the context of a powered flight vehicle may be considered to provide auxiliary functions. The propulsion module  215  may be an electric motor powered by the fuel cells  209 ,  211 ,  213 . 
       FIG.  3    illustrates another vehicle  300 . The vehicle  300  is similar to that described with reference to  FIG.  2    and includes a plurality of propulsion modules  318 ,  320 . 
     A first propulsion module  318  is associated with an air inlet  310  of a first fuel cell  309 . A second propulsion module  320  is associated with an air inlet  312  of a second fuel cell  311 . 
     The first and second propulsion modules  318 ,  320  are configured to provide oxidant and/or coolant to the respective first and second fuel cells  309 ,  311 . 
     Each air inlet  310 ,  312  may have an open face that is transverse to, or substantially normal to, a direction of thrust from the propulsion module  318 ,  320  with which it is associated. 
     The air inlets  310 ,  312  of the first and second fuel cells  309 ,  311  may be fanless and rely solely on the propulsion modules  318 ,  320  to draw air into the first and second fuel cells  309 ,  311 . That is, the propulsion modules  318 ,  320  may be the only active source of oxidant and/or coolant for the first and second fuel cells  309 ,  311 . An advantage of such an arrangement is that the additional spatial volume, weight, and material cost associated with additional fans for drawing air into the fuel cells  309 ,  311  may be eliminated. 
     The first fuel cell  309  may be configured to provide power to the first propulsion module  318 . The second fuel cell  311  may be configured to provide power to the second propulsion module  320 . In this way, the power supply for the first propulsion module  318  may be provided separately from the power supply for the second propulsion module  320 . 
     Alternatively, power from both the first and second fuel cells  309 ,  311  may be provided to both the first and second propulsion modules  318 ,  320 . One or more controllers may be provided in order to control operation of the fuel cells  309 ,  311  in accordance with the demands of the first and second propulsion modules  318 ,  320  and, optionally, in accordance with the power demand of any other on-board systems on the vehicle  300 . 
     As in the vehicle of  FIG.  2   , the vehicle  300  comprises a third fuel cell  313  provided in the tailplane  308 . An air inlet  314  of the third fuel cell  313  is independent of the airflow directly driven by the first and second propulsion modules  318 ,  320 . A fan may be provided at the air inlets  314  in order to draw in air to the fuel cell  313 . Alternatively, the air inlet  314  may be fanless and rely on air to be drawn into the forward-facing air inlet  313  by the motion of the vehicle  300  through the air when in flight. 
       FIGS.  4  to  6    provide examples of unmanned aerial vehicles that comprise a fuel cell system with a plurality of at least one type of fuel cell system component. Each type of fuel cell system component is distributed about the vehicle so that each component of a particular type is spatially separated from other components of that type. 
       FIGS.  4  to  6    each illustrate a quadcopter, which is an example of a rotorcraft  400 ,  500 ,  600 . Each rotorcraft  400 ,  500 ,  600  comprises a body  438  and a plurality of propulsion modules  440 - 443 . The body is located centrally with respect to the plurality of modules and may also be referred to as a central body  438 . A controller and/or other on-board systems of the rotorcraft  400  may be provided within the central body  438 . The propulsion modules  440 - 443  are coupled to the central body  438  by respective struts  444 - 447 , which may also be referred to as arms, or limbs, of the rotorcraft  400 ,  500 ,  600 . The struts  444 - 447  are examples of structural components of the rotorcraft  400 ,  500 ,  600 . Each propulsion module  440 - 443  comprises a motor that is configured to drive a respective rotor  448 - 451 . The rotors  448 - 451  provide thrust and lift for the rotorcraft  400 ,  500 ,  600  in a conventional manner. 
     In  FIG.  4   a   , the rotorcraft  400  comprises a plurality of fuel cells  452 - 455  distributed about the vehicle. Each of the struts  444 - 447  comprises a fuel cell  452 - 455  having an air inlet  456 - 459  on a top surface of the strut, an air outlet (not shown) on a bottom surface of the strut and an air flow path within the struts  444 - 447  between the respective air inlets  456 - 459  and air outlets. The fuel cells  452 - 455  comprise plates, such as monopolar and bipolar plates, that are aligned vertically within the struts  444 - 447 . Each fuel cell  452 - 455  may be provided by an individual fuel cell or fuel cell stack. 
     In some examples, each fuel cell  452 - 455  may provide one of the struts  444 - 447 . That is, each fuel cell  452 - 455  may be integrally formed with one of the struts  444 - 447 . Such fuel cells  452 - 455  are configured to bear a mechanical load placed on the struts  444 - 447  by the central body  438  and the propulsion modules  440 - 443 . Additional mechanical loads include those associated with flight or landing. The plates of the fuel cells  452 - 455  are orientated to be aligned with mechanical loads, such as the direction of thrust of the propulsion modules  440 - 443 , when the vehicle is in use. The plates of the fuel cells  452 - 455  may be especially rigid perpendicular to a plane of the plates and so resist a force applied in the vertical direction by the propulsion modules  440 - 443 . 
     In  FIG.  4   a   , each air inlet  456 - 459  is associated with a respective propulsion module  440 - 443  and each propulsion module  440 - 443  is configured to provide air as an oxidant and/or coolant to a respective fuel cell  452 - 455 . 
     As in the example described with reference to  FIG.  3   , the air inlets  456 - 459  of the fuel cells  452 - 455  may be fanless and rely solely on the propulsion modules  440 - 443  to draw air into the fuel cells  452 - 455 . That is, the propulsion modules  440 - 443  may be the only active source of air to provide oxidant and/or coolant for the fuel cells  452 - 455 . 
     The rotorcraft  400  may have a modular construction in which the struts  444 - 447  are detachable from the central body  438  using a clip-on arrangement, for example. Such an arrangement may enable the body  438  to be extensible in order to change the payload carrying capability of the rotorcraft  400 . An extended body may be able to accommodate a greater number of clip-on struts and so carry a greater weight. By providing a fuel cell system component in the strut  444 - 447 , the power generating capability of the rotorcraft  400  can be varied accordingly with the number of propulsion modules  440 - 443 . In addition, the provision of a modular construction of the rotorcraft  400  may be useful in reducing a volume of space occupied by the rotorcraft  400  in storage. 
       FIG.  4   b    illustrates a schematic longitudinal cross section through one of the struts  444  of the rotorcraft  400  of  FIG.  4   a   . In this view, the air inlet  457  and the air outlet  470  are visible on respective faces of the strut  444 . The air inlet  457  faces the rotor  448  in order to receive the oxidant and/or coolant for the fuel cell  452 . An air flow path  472  is provided from the rotor  448  of the propulsion device  440  to the inlet  457  of the fuel cell  452 , through the fuel cell  452  within the strut  444  and is exhausted from the outlet  470  on the reverse face of the strut  444 . The fuel cells  452 - 455  are orientated so that the air flow path  472  through the fuel cells is aligned with a downdraught produced by the propulsion modules  440  in order to reduce drag. 
       FIG.  5   a    illustrates a rotorcraft  500  comprising a plurality of fuel cells  552 - 555  and a plurality of fuel generators  560 - 563 . The plurality of fuel cells  552 - 555  and fuel generators  560 - 563  are each distributed about the rotorcraft  500 . Each of the struts  444 - 447  comprises one of the plurality of fuel cells  552 - 555  and one of the plurality of fuel generators  560 - 563 . Each of the plurality of fuel cells  552 - 555  has an inlet  556 - 559 . The arrangement of the fuel cells  552 - 555  and respective inlets  556 - 559  is generally similar to that described previously with reference to  FIG.  4     a.    
     The fuel generators  560 - 563  may be provided by known hydrogen generators that are configured to react a first reactant, such as sodium borohydride, with a second reactant, such as water, in order to generate fuel, such as hydrogen gas, for consumption by the fuel cells  552 - 555 . The fuel generators  560 - 563  may comprise a catalyst for catalyzing the reaction to generate hydrogen gas from the first and second reactants. Such reactions are typically exothermic. 
     The temperature that is reached in the fuel generators  560 - 563  during use may be lower than would be the case if the same volume of fuel generator was provided at a single, centralized location in the rotorcraft  500 , rather than distributed about the rotorcraft  500 . 
     As such, the requirements for cooling of the fuel generators  560 - 563  may be reduced. This is advantageous because cooling systems, such as fans and heat sinks, may add additional bulk and weight to the rotorcraft and so reduce its efficiency. 
     A fuel generator typically generates more heat that a fuel cell when in use and so in this example the fuel generators  560 - 563  are provided closer to the propulsion modules  440 - 443  than the fuel cells  552 - 555  in order that the fuel generators  560 - 563  are subject to more cooling from downdraft from the rotors  448 - 451  of the propulsion modules  440 - 443 . 
     Each fuel cell  552 - 555  may be associated with a respective propulsion module  440 - 443  such that each propulsion module  440 - 443  only receives electrical power from a particular fuel cell  552 - 555 , which may be the fuel cell  552 - 555  provided in the strut  444 - 447  that is connected to that particular propulsion module  440 - 443 . 
     Each fuel generator  560 - 563  may be provided with a respective reactant reservoir for one or more reactants. In order to avoid uneven depletion of the reactant reservoirs associated with the fuel generator  560 - 563 , it is advantageous to provide a controller that is configured to: distribute electrical power from the fuel cells  552 - 555  to the propulsion modules  440 - 443  in accordance with the requirements of the propulsion modules and a remaining reactant level in each of the reactant reservoirs; and additional or alternatively to redistribute the one or more reactants between the reactant reservoirs  560 - 563  during flight in accordance with variations in the reactant levels of the reactant reservoirs  560 - 563 . The redistribution of the one or more reactants may assist in maintaining an appropriate weight balance of the unmanned aerial vehicle and so ensure that its flight characteristics remain within expected parameters. The redistribution of the one or more reactants may be achieved by transferring the one or more reactants directly between the various reactant reservoirs  560 - 563 . In some cases, the controller may be configured to adjust a flying mode of the unmanned aerial vehicle, such as its direction, in order to change the fuel consumption from the various reactant reservoirs and so rebalance the relative distribution of the one or more reactants. 
     Various options for arranging fuel cell system components within one of the struts  444  and the body  438  are described below with reference to  FIGS.  5   b  to  5   e   . Similar arrangements may be provided in the other struts  445 - 447  described with reference to  FIG.  5     a.    
       FIG.  5   b    illustrates a schematic longitudinal cross section through a strut  444  of the rotorcraft of  FIG.  5   a   . In addition to the components described with reference to  FIG.  5   a   , the strut  444  has a bay for receiving and interfacing with a removable reactant cartridge  573 . The bay for the removable reactant cartridge  573  is situated adjacent to the fuel generator  560  within the strut  444 . 
     In this example, the reactant cartridge  573  provides a reservoir for at least one reactant. The reactant cartridge  573  may comprise a first reservoir for a first reactant and a second reservoir for a second reactant. Alternatively, the reactant cartridge  573  may comprise a single reservoir to store a mixture of the first and second reactants and a reaction retarding chemical. A catalyst may be provided in the reaction chamber  560  in such examples to overcome the reaction retarding effects of the chemical. Sodium hydroxide, for example, may be used as a reaction retarding chemical in the case where the first reactant is sodium borohydride and the second reactant is an aqueous solution such as water. Other examples of first reactants for use with an aqueous second reactant include other metal borohydrides, nano-silicon, aluminium and other metals made active for water splitting, lithium hydride, lithium aluminium hydride, sodium aluminium hydride, calcium hydride and sodium silicide. In other examples, a thermolysis fuel may be used in the least one reactant. Thermolysis fuels include ammonia borane, aluminium hydride (alane) and magnesium borohydride. There are also fuels that require the use of a reformer, such as methane or butane, for example. 
     Providing fuel cell system components such as the removable reactant cartridges within the struts  444 - 447 , as opposed to elsewhere in the drone, may reduce the drone surface area and volume because the struts would otherwise provide unoccupied space. 
     A first air flow path  572   a  and a second air flow path  572   b  are also shown in  FIG.  5   b   . The first air flow path passes through the fuel cell  452  in a similar manner to that described with reference to  FIG.  4   b   , although in this case the position of the fuel cell  452  is further offset from the propulsion module  444  along the length of the strut  444 . The second air flow path  572   b  flows around a portion of the strut  444  that houses the fuel generator  560  and the reactant cartridge  573 , rather than through the strut  444 . Cooling is therefore provided to the fuel generator  560  and the reactant cartridge  573  through a surface of the strut  444 . 
     The portion of the strut  444  that houses the fuel generator  560  and the reactant cartridge  573  may take a conventional aerodynamic design in order to avoid disturbing airflow and creating drag. 
       FIG.  5   c    illustrates a schematic longitudinal cross section through an alternative arrangement of the strut  444 . A fuel cell is not shown within the strut  444  in order to enable the other fuel cell system components to be shown more clearly. A fuel cell may be provided within the strut  444  of  FIG.  5   c    in a similar manner to that described with reference to  FIG.  5     b.    
     In this example, the reactant cartridge  573  provides a first reactant reservoir for a first reactant, such as sodium borohydride. A second reactant reservoir for holding a second reactant, such as water, is provided within the central body  438 . In this case, the strut  444  has a conduit  576  for providing the second reactant from the second reactive reservoir  574  in central body  438  to the fuel generator  560  within the strut  444 . The second reactant reservoir  574  may be provided as a cartridge or as a refillable container. The strut  444  also has an optional conduit  578  for transporting reactant by-product from the fuel generators  560 - 563  to an optional waste storage portion  580  of the central body  438 . 
     Typically, a reactant reservoir may be a relatively heavy component of the UAV when it is full of water. Providing heavy components closer to the centre of mass of the UAV reduces the rotational inertia of the UAV and so improves its agility and manoeuvrability. Further, heating of the water within the second reactant reservoir  574  may be avoided by providing the second reactant reservoir  574  distally from the fuel generator  560 . 
     As an alternative to the example shown in  FIG.  5   c   , the reactant cartridge  573  may be omitted and the second reactant reservoir  574  may be the only reactant reservoir for the fuel cell in the strut  444 . For example, a mixture of a first reactant, a second reactant and a reactant retarding chemical may be provided within the second reactant reservoir  574  as described previously. 
       FIG.  5   d    illustrates a schematic longitudinal cross section through a further alternative arrangement of the strut  444 . In this example, the fuel cell (not shown) and fuel generator  560  are provided within the strut  444  as described previously with reference to  FIGS.  5   b  and  5   c   . This example differs from that described with reference to  FIG.  5   c    in that the first reactant reservoir is provided within the central body  438 . In this example, the first reactant reservoir is provided as a reactant cartridge  573 . 
       FIG.  5   e    illustrates a schematic longitudinal cross section through a further alternative arrangement of the strut  444  which differs from the example described with reference to  FIG.  5   b    in that the fuel generator is omitted. The reactant cartridge of  FIG.  5   b    is replaced by a removable fuel cartridge  586  within the strut  444 . The removable fuel cartridge contains fuel, such as hydrogen gas, for a fuel cell  585  within the strut rather than precursor reactants for generating fuel for the fuel cell  585 . Typically, the fuel cell  585  may generate more heat than the fuel cartridge  586  when in use and so the fuel cell is positioned closer to the downdraught from the rotor  448  than the fuel cartridge  586 . Air flow from the rotor  448  flows over a surface of the fuel cartridge  586  in a similar manner to that described previously for airflow over the fuel generator and reactant cartridge in previous examples. 
     The fuel cartridge  586  may be conventionally aerodynamically shaped in order to avoid disturbing airflow and creating drag. 
       FIG.  6   a    illustrates a rotorcraft  600  comprising a fuel cell system with a plurality of coolant structures  646 ,  648 . The coolant structures  646 ,  648 , which may be provided by heat pipes, act as heat sinks for other components of the fuel cell system. The coolant structures  646 ,  648  are provided within some of the struts  445 ,  447  in this example. The coolant structures  646 ,  648  may provide structural components of the struts  445 ,  447 , that is, they may bear the load of the propulsion modules  441 ,  443  and the loads inflicted upon the rotorcraft  600  during flight. 
     Each of the coolant structures  646 ,  648  is associated with a different propulsion module  441 ,  443 , which acts as an active source of coolant air for the coolant structure  646 ,  648  so that the flow of air around or through the coolant structure cools the fuel cell components within the central body  438 . 
     In this example, the central body  438  comprises fuel cell system components including a fuel cell (not shown in  FIG.  6   a   ). The fuel cell system components within the central body  438  are thermally coupled to the coolant structures  646 ,  648  in order for excess heat to be conducted away from the fuel cell components and dissipated. An air inlet  650  for the fuel cell is shown on a top surface of the body  438 . The air inlet  650  of the fuel cell is arranged to receive ambient air or disturbed airflow from the propulsion modules  441 ,  443 . 
       FIG.  6   b    illustrates a schematic longitudinal cross section through one of the struts  441  of the rotorcraft  600  of  FIG.  6   a   . The body  438  has a bottom surface comprising an air outlet  688  provided on a reverse surface to the top surface that comprises the air inlet  650 . 
     A fuel cell  691  is provided within the central body  438 . A first air flow path  692  is provided through the body  438  from the air inlet  650 , through the fuel cell  691  and out of the air outlet  688 . The air exhausted from the air outlet  688  contributes to a downdraught of the rotorcraft  600 . 
     A second air flow path  690  is provided by air disturbed by the rotor  449  of the propulsion module  441 . The second air flow path  690  passes through a portion of the strut  445  composed of the coolant structure  646 . The second air flow path  690  cools the coolant structure  646  by convection and so dissipates heat. In this way, heat is drawn away from the fuel cell system components that are thermally coupled to the coolant structure  646  and so the overall temperature of the fuel cell system components is reduced. 
     In an alternative example, one or more coolant structures within the struts may be provided in an arrangement such as those described with reference to  FIGS.  5   a    to  5   e.    
     The following description of an unmanned aerial vehicle is also disclosed. The vehicle comprises: 
     (I) a fuel cell (FC) integrated with/near propeller module thereby providing air flow to FC with the propeller and/or airspeed without need of additional fans—this also provide more redundancy to the airframe in the event that some of it is not working. 
     There is, de facto, distributed power across the airframe so not all prop modules stop at the same time if one FC fails. The fuel may also be distributed or centralized in one or more fuel reservoir “FR” see  FIGS.  1  and  2   . 
     (II) FC as part of the structural struts and/or beam parts of the airframe itself (I) can be combined with (I) above if the FC is near the propellers, e.g. with beams connecting the main body of a quadcopter or such to the propeller/electrical motor module. A fuel cell stack is made of plates so is especially rigid in the plane perpendicular to the bipolar plates. 
     (III) a planar FC, with external and internal surfaces of the airframe (chassis, enclosure/any surfaces whether active of passive transformed into a power source. 
     This can be used either for main propulsion services or for hotel loads/APUs. 
     Further embodiments are intentionally within the scope of the accompanying claims.