Patent Description:
UAVs, which are also known as drones, are becoming increasingly popular for applications such as photography, surveillance, farm maintenance (e.g., pest control), atmospheric research, fire control, wildlife monitoring, package delivery, and military purposes. UAVs generally fall into two categories, namely multirotor UAVs used generally in commercial applications and fixed wing UAVs used for military applications. UAVs are equipped with navigation systems. The payloads in the UAVs vary depending on the end-application and may comprise video cameras, reconnaissance equipment, remote sensing devices, pesticides held in a suitable container that is capable of spraying, fire retardants, packages for delivery, and the like. UAVs are typically smaller than manned aircraft and may weigh, for example, between a few grams and dozens of kilograms.

UAVs require power to provide propulsion and to power auxiliary functions (e.g., operating payloads, such as image or video capture, signal telemetry, etc.) or other on-board systems. For many applications, the computing power required on-board the vehicle in order to provide necessary functionality may represent a significant power demand. This is particularly the case in autonomous UAVs in which an on-board control system may make decisions regarding flight path and the deployment of auxiliary functions. Although the vehicle itself is unmanned, a UAV may be piloted remotely and may still be under some form of human control.

Some UAVs use primary batteries to provide power, although it is now more common to use secondary (rechargeable) batteries such as lithium-ion batteries. When power is supplied only by batteries, the flight time of UAVs may be limited because of the power demands of the propulsion and other on-board systems. In recent years, photovoltaic panels have been used to extend the flight range of UAVs. However, the power generating capacity of a photovoltaic panel depends on the ambient weather conditions and the time of day, and, subsequently, 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. The flight time and range of UAVs are generally a function of payload (weight) and the energy (Watt-hours) available from a power supply. Other power supplies include jet engines fueled by fuels such as gasoline and jet fuel for fixed wing military applications and fuel cells fueled by hydrogen and other fuels, such as propane, gasoline, diesel, and jet fuel. The UAVs typically return home, that is, to a home station or home base, after a flight to recharge or refill the power supplies.

Fuel cells are attractive power supplies for UAVs, may exceed the energy provided by batteries, and may extend flight time (or range) in many instances. Fuel cells are electro-chemical energy conversion devices that convert an external source fuel into electrical current. Many fuel cells use hydrogen as the fuel and oxygen (typically from air) as an oxidant. The by-product for such a fuel cell is water, making the fuel cell a very low environmental impact device for generating power. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.

Even with the advantages of using fuel cells, in some instances a power level supplied by one FCPM may not be enough for a particular application. But as demanded power output from an FCPM grows, the size of the stacks becomes unwieldy. For example, it is very difficult if not impractical to package a single, big lump of a fuel cell stack such that it can be mounted on a UAV. Another issue with known UAV powering approaches is that when the power supply fails mid-flight the mission and/or payload are at considerable risk of being damaged via a crash landing. Positioning and orientation of the different components mounted onto a frame of the UAV may also pose CoG and/or weight-balancing issues.

Document <CIT> describes ground stations and methods for pre-flight health and safety check of PEM fuel cell powered unmanned aerial vehicles. The ground stations are described to be capable of reconditioning UAV fuel cell stacks and refilling the hydrogen supply. The methods are described to permit the checking and returning of UAVs back into flight service within a few minutes.

Document <CIT> describes a fuel cell power pack used as a power source in a multicopter which includes a fuel tank and a fuel cell stack for producing electrical energy using hydrogen supplied from the fuel tank and supplying the electrical energy to a battery. Since the fuel cell stack is disposed at a certain point of an arm extended from the aircraft body in the radius direction (a point affected by a descending air current generated by each rotating blade), the electrical energy can be produced using the descending air current generated by the rotating blade without configuring a separate blowing apparatus.

Document <CIT> describes an unmanned aerial vehicle which comprises at least one fuel cell. The fuel cell provides a structural component of the vehicle. The structural component may form struts linking propulsion modules to a central body. Air inlets are associated with the fuel cells to provide air as an oxidant and/or coolant to the fuel cells. The fuel cells may form other parts of an aircraft structure such as a fuel cell within a tailplane or a fuel cell forming part of a wing structure.

According to the present invention there is provided an unmanned aerial vehicle (UAV) according to Claim <NUM>. A preferred embodiment of the invention is defined in Claim <NUM>. In the following description, embodiments will be described. These embodiments fall within the scope of the present invention only if they are in accordance with Claim <NUM>.

Aspects of methods, systems and device disclosed herein for a mounting frame including but not limited to a payload,.

A plurality of fuel cell stacks operable in a predefined configuration, each of the plurality of stacks being in a separate package;.

Aspects of methods, systems and device disclosed herein for a mounting frame including but not limited to an unmanned aerial vehicle, having.

Aspects of methods, systems and device disclosed herein for a modular power supply for powering components of UAV, into which signal and power lines may be connected. Two or more fuel cell power modules (FCPMs) may be connected in series or parallel so that a power output is doubled and so that the end-user has a single communications port.

The power controller may be configured to communicate with the fuel cell stacks and other component(s) of the UAV. The controller may be configured to control at least one of the hydrogen supply, inert gas supply, electrical loads, and auxiliary power source.

In some instances, the power supplies are hybrid versions, wherein a combination of power supplies may be used. For example, when a fuel cell is used, any peak power requirement such as during take-off, may be supplemented using a battery. Fuel cells are attractive power supplies for UAVs, may exceed the energy provided by batteries, and may extend flight time (or range) in many instances. Fuel cells are electro-chemical energy conversion devices that convert an external source fuel into electrical current. Many fuel cells use hydrogen as the fuel and oxygen (typically from air) as an oxidant. The by-product for such a fuel cell is water, making the fuel cell a very low environmental impact device for generating power. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.

Other features and advantages of the present disclosure will be set forth, in part, in the descriptions which follow and the accompanying drawings, wherein the preferred aspects of the present disclosure are described and shown, and in part, will become apparent to those skilled in the art upon examination of the following detailed description taken in conjunction with the accompanying drawings or may be learned by practice of the present disclosure. The advantages of the present disclosure may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

The foregoing aspects and many of the attendant advantages of this disclosure will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:.

The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the disclosure. The failure to number an element in a figure is not intended to waive any rights. Unnumbered references may also be identified by alpha characters in the figures.

The following detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which some disclosed aspects may be practiced. These embodiments, which are also referred to herein as "examples" or "options," are described in enough detail to enable those skilled in the art to practice methods and devices disclosed. The embodiments may be combined, other embodiments may be utilized, or structural or logical changes may be made without departing from the scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the disclosure is defined by the appended claims.

Particular aspects of the disclosure are described below for the purpose of illustrating use of a plurality of fuel cells for powering UAVs. These fuel cells may be arranged in a series or parallel configuration, depending on particular use cases. Various modifications may be made, and the scope of the disclosure is not limited to the exemplary aspects described.

A schematic representation of an exemplary UAV <NUM> is shown in <FIG>. UAV <NUM> comprises several components, such as a fuel cell power supply <NUM>, which in turn comprises a plurality of fuel cell stack modules <NUM> (connected in series or parallel). The plurality of fuel cell stack modules <NUM> may each comprise fuel cell stack <NUM> and one or more fans <NUM>. The plurality of fuel cell stack modules <NUM> interface with one or more fuel cell power supply controllers <NUM>. Power controller <NUM> may interface communication signals and power with each of modules <NUM>. Power controller <NUM> may further communicate with one or more tanks <NUM> (e.g., to control a pump, line pressure, or otherwise adjust the flow of compressed hydrogen from the tank). UAV <NUM> includes other components, such as one or more motors <NUM>, one or motor rotors <NUM>, one or more motor controllers <NUM>, and payload <NUM>. Power supply controller <NUM> may feed power from modules <NUM> to motors <NUM> directly or indirectly via motor controller <NUM>.

UAV <NUM>, in <FIG>, may be a helicopter and comprise one or more propulsion systems coupled to frame <NUM> by one or more struts <NUM>, which may also be referred to the arms or limbs of the UAV. Each propulsion system may comprise motor <NUM> that is capable of driving respective rotor <NUM>. The number of propulsion systems in UAV <NUM> may vary depending on the aerodynamic design, payload, and flight time required.

Fuel cell power supply <NUM> may be removably coupled to frame <NUM> and electrically coupled to fuel cell power supply controller <NUM> via a suitable electrical adapter or plug <NUM>. The struts may provide mechanical support and also may provide for conduits to carry signals (e.g., cables) that provide electrical and control communication between modules <NUM>, power controller <NUM>, motor controller <NUM>, and each of the propulsion systems. The rotors <NUM> provide thrust and lift for UAV <NUM>. Exemplary UAV <NUM> may also comprise a plurality of leg members <NUM> to support the UAV during landing and to protect payload <NUM> during landing.

Hydrogen feed to fuel cell power supply <NUM> is supplied by hydrogen supply <NUM> (e.g., a tank or cylinder), which may be removably mounted on saddles that may be mechanically coupled to frame <NUM>. Hydrogen supply <NUM> may also be removably mounted to the frame <NUM> using brackets, ties, and the like. Hydrogen supply <NUM> may comprise a hydrogen connection assembly capable of mating with a first end of a hydrogen supply conduit using quick connect/disconnect fittings, magnetic couplings, and the like. The hydrogen connection assembly may comprise at least one of a pressure regulator, solenoid valve, shut off valve, and pressure relief valve to ensure that hydrogen at the desired flow rate and pressure is routed to power supply <NUM>. Hydrogen supply <NUM> may be configured to store compressed hydrogen at a pressure below <NUM> bar.

In some exemplary implementations, for UAV <NUM>, a selection between series and parallel configurations is made based on efficiency of fuel cell stack modules <NUM>. In some exemplary implementations, the efficiency is based on a power output of modules <NUM>. In some exemplary implementation, providing <NUM> Volts (V), known as <NUM>, to a propulsion system results in more efficient operation of motors <NUM> than if <NUM> V, known as <NUM>, were provided to a propulsion system.

The components that comprise the hydrogen connection assembly may be electrically actuated by a signal from motor controller <NUM> or from power supply controller <NUM>. The second end of the hydrogen supply conduit that is opposite the first end is capable of mating with fuel cell connection assembly <NUM>. Fuel cell connection assembly <NUM> may comprise at least one of a pressure regulator, solenoid valve, shut off valve, and pressure relief valve to ensure that hydrogen at the desired flow rate and pressure is routed to fuel cell power supply <NUM>. The components included in the fuel cell connection assembly <NUM> may be electrically actuated by a signal from controller <NUM> or from power supply controller <NUM>.

In some implementations, payload <NUM> may include one or more cameras and may be removably coupled to fuel cell power supply <NUM> or to frame <NUM> (<FIG>). Payload <NUM> is capable of communicating with at least one of controller <NUM>, controller <NUM>, and fuel cell power supply <NUM>.

Controller <NUM> may be configured to control at least one of propulsion systems, operation of payload <NUM>, and an auxiliary power supply, such as a rechargeable battery, which may be configured to store excess power generated by fuel cell power supply <NUM>. Controller <NUM> may be configured to control at least one of the propulsion systems, operation of fuel cell power supply <NUM>, operation of hydrogen supply <NUM>, operation of payload <NUM>, and the auxiliary power supply, such as the rechargeable battery.

In some exemplary implementations, auxiliary power supply, such as backup battery <NUM>, may be removably coupled to frame <NUM>. In some implementations, backup battery <NUM> is sized to provide a predetermined amount of peak power (e.g., for a known period of time, such as to recover from strong winds). In some exemplary implementations, backup battery <NUM> is a lithium-polymer battery.

The auxiliary power supply may also be used to power at least one of payload <NUM> and other component(s) of UAV <NUM> during a transient power period, such as take-off, or when fuel cell power supply <NUM> is producing less power than expected. Auxiliary power supplies may also comprise super capacitors and primary batteries. Exemplary systems and methods for operating a device using a fuel cell power supply and an auxiliary power supply to power a load (device such as UAV <NUM>) are described in commonly owned <CIT> and <CIT>.

Fuel cell power supply <NUM> may be provided in relation to fuel cell power supply controller <NUM>, in which case, controller <NUM> is capable of communicating with fuel cell power supply controller <NUM> in a bidirectional manner. Alternatively, fuel cell power supply controller <NUM> may be used to control the components in fuel cell connection assembly <NUM> and the hydrogen connection assembly instead of controller <NUM>. UAV <NUM> may return home after a flight, that is, to a home station or home base (not shown), after a flight to recharge or refill the power supplies.

In some exemplary implementations, two or more fuel cell stack modules <NUM> are linked in series or parallel, via a configuration facilitated by power controller <NUM>. By having modules <NUM> supplying power in series, a power output (e.g., to the propulsion system) may be doubled, while doubling the supply voltage, e.g., from modules <NUM>-<NUM> and <NUM>-<NUM> from at or around <NUM> V to between <NUM> V and <NUM> V (but this example is not intended to be limiting, as any suitable voltage byproduct of the series configuration of any suitable number of modules <NUM> may be used). In these or other implementations, the doubling may occur while keeping a current through each of the two or more modules <NUM> (e.g., modules <NUM>-<NUM>, <NUM>-<NUM>) the same as if the each module was operating independently.

In UAV implementations where two or more modules <NUM> are arranged in parallel, the power doubling may be based on an output voltage of each of the two or more modules <NUM> being the same as if the each stack was operating independently and on an output current from the two or more modules <NUM> being doubled. In UAV implementations where modules <NUM> are connected in parallel, a total output current greater than that available from one individual module <NUM> may be obtained. The parallel configuration of modules <NUM> within UAV <NUM> may also be beneficial by providing redundancy, enhancing reliability, avoiding PCB thermal issues and boosting system efficiency. In some exemplary implementations, power controller <NUM> may be configured to balance a current from each of modules <NUM>. That is, some exemplary implementations of modules <NUM> in the parallel configuration may be performed such that the load current is shared, e.g., to prevent one of modules <NUM> from shutting down before the required current is delivered. Some exemplary implementations may actively balance the output current from modules <NUM> using a control loop to compensate between modules <NUM>. To accomplish this, some implementations may monitor both the voltage and temperature via the control loop.

Power controller <NUM> of UAV <NUM> is configured to detect a fault or failure of one of modules <NUM> and to cause the one or more other modules <NUM> to continue operating such that the propulsion system (i.e., motor(s) <NUM> and rotor(s) <NUM>) is able to bring UAV <NUM> to a safe landing (e.g., without damaging payload <NUM> and/or any other component of UAV <NUM>). Power controller <NUM> of UAV <NUM> is further configured to be controlled, either remotely via a device on the ground or locally via a direct connection on-board the unmanned aerial vehicle, to breach a safety threshold related to fuel cell overheating such that payload <NUM> has a better probability of landing undamaged, when the fault is detected, due to prioritizing safety of payload <NUM> over survival of any other component on UAV <NUM> (e.g., motors <NUM>, modules <NUM>, etc.). In some exemplary implementations, use of backup battery <NUM> to at least temporarily power the propulsion system(s) may increase the probability of a safe landing, responsive to the fault being detected.

Fuel cell power supply <NUM> may comprise a plurality of fuel cell stack modules <NUM> (e.g., <NUM>-<NUM> and <NUM>-<NUM>, as shown in <FIG>). In some exemplary implementations, each of fuel cell stack modules <NUM> may be packaged independently and positioned separately around UAV <NUM>. In other implementations, fuel cell stack modules <NUM> may be packaged together inside fuel cell power supply <NUM>. As shown in <FIG>, fuel cell power supply <NUM> (which comprises modules <NUM>) may be located above hydrogen supply <NUM>, with reference to UAV <NUM> being in a stationary position on the ground. Alternatively, fuel cell power supply <NUM> may be located below hydrogen supply <NUM> (<FIG>). Alternatively, fuel cell power supply <NUM> and hydrogen supply <NUM> may be mounted adjacent to each other (<FIG>).

Depending on the total power requirement of UAV <NUM>, each of fuel cell stack modules <NUM> may output about <NUM> Watts (W) or about <NUM> W maximum continuous power, but any maximum continuous power output value is contemplated by the present disclosure. In some exemplary implementations, the maximum peak power output from each of modules <NUM> may be temporarily (e.g., for about <NUM> seconds or less) about <NUM> W or about <NUM> W. In some exemplary implementations, power modules <NUM> may be the same as each other. For example, module <NUM>-<NUM> may be identical to each of (if used) module <NUM>-<NUM>,. <NUM>-n (n being any natural number). In this or another example, each of modules <NUM> may be configured to generate a same amount of power and have a same efficiency rating. In some exemplary implementations, module <NUM>-<NUM> may produce a different maximum continuous power output from any other module <NUM> (e.g., module <NUM>-<NUM>). For example, a <NUM> W module may be configured in series with an <NUM> W module. In another example, a <NUM> W module may be configured in parallel with an <NUM> W module.

In some exemplary implementations, the double-headed arrows representing bidirectional communication may depict signals. These signals may convey communication data, e.g., command and control (e.g., a status) of each of fuel cell stacks <NUM>, hydrogen supply <NUM> (e.g., current fill level, pressure level in the lines, etc.), motors <NUM>, motor controller <NUM>, fans <NUM>, and/or payload <NUM>, to/from controller <NUM>.

In some exemplary implementations, fuel cell stack modules <NUM> may be connected in series only. For reasons related to being in a series configuration, the communication signals of each of modules <NUM> may be isolated from power controller <NUM>. In a parallel configuration, some or more of the same signals would not require isolation; rather, these signals may be multiplexed through to controller <NUM>.

In some exemplary implementations, when linking fuel cell stack modules <NUM> in series, UAV <NUM> may be prevented from having a virtual earth in the mid-rail. That is, some implementations may have connected a positive terminal of fuel cell stack module <NUM>-<NUM> to a negative terminal of fuel cell stack module <NUM>-<NUM>, and in this configuration module <NUM>-<NUM>'s ground becomes module <NUM>-<NUM>'s power. Presently disclosed are thus methods to galvanically isolate the communication signals, via optically coupled technology combined with an analog to digital converter (ADC). Further disclosed are methods for isolating a transformer (which is relatively heavy), a simple opto-isolator, hall effect sensor, or series connected capacitors to decouple the signals. Some implementations may generate a common earth/ground inside an isolation barrier. In some exemplary implementations, the communication signals are isolated with respect to each of modules <NUM>. Disclosed implementations thus overcome a problem of connecting modules <NUM> and/or controller <NUM>, whereby direct connection there would otherwise be a virtual earth in the mid-rail.

The required total power output from power supply <NUM> may depend on the mass and/or functionality of payload <NUM>. In some implementations, each of the fuel cell stack modules <NUM> may be an open cathode proton-exchange membrane fuel cell (PEMFC) stack module. A plurality of hydrogen supplies <NUM> may be employed depending on the flight time required and the mass budget that is available to the fuel supply for a given mass of payload <NUM>. Payload <NUM> may be coupled to frame <NUM>. In <FIG>, UAV <NUM> comprises a single fuel cell power supply <NUM>, which may include a plurality of separately-packaged fuel cell stacks <NUM> (connected in series or parallel) and a plurality of fans <NUM>.

In some exemplary implementations, one or more components (e.g., fuel cell stack modules <NUM>, hydrogen supplies <NUM>, payload <NUM>, power controllers <NUM>, motor controllers <NUM>, and battery <NUM>) of UAV <NUM> may be affixed onto frame <NUM>. In some exemplary implementations, manual pre-flight mechanical arrangement, power controller <NUM>, or another controller may be configured to adjust the center of gravity (CoG) of UAV <NUM> by adjusting, via frame <NUM>, a position or orientation of the one or more components.

While used to illustrate some different possible mounting configurations, the depictions of <FIG> are not intended to be limiting, as any configuration or orientation of the various components of UAV <NUM> is contemplated. And controllers <NUM> and <NUM> may be mounted on frame <NUM> at any suitable location for an optimal CoG, with respect to flight characteristics of UAV <NUM>. For example, these components may be mounted in a distributed fashion around frame <NUM> or at least some of the components may be lumped together. In some exemplary implementations, UAV <NUM> may have modules <NUM> distributed around frame <NUM> such that a center of mass of the vehicle is balanced and a manner in which the vehicle flies is controllably affected. In some exemplary implementations, the mounting placement and orientation of the components of UAV <NUM> may flexibly control the weight balance of the UAV as a whole. The mounting placement and orientation of these components may also be aerodynamically designed such that drag is minimized. By orientation, the present disclosure refers to rotating, flipping, or tilting one or more of the components of UAV <NUM>. In implementations where a plurality of hydrogen supplies <NUM> are used, supplies <NUM> may be repositioned to balance weight distribution (i.e., including CoG considerations with respect to the other components of UAV <NUM>). In these or other implementations, frame <NUM> may allow for both manual and automated reconfiguration. That is, power controller <NUM> or another component of UAV <NUM> may control positioning and orientation of supply <NUM>, power controller <NUM>, motor controller <NUM>, payload <NUM>, and each of fuel cell stack modules <NUM>.

The power output as a function of cumulative time of service from fuel cell stack modules <NUM> is dependent on various factors, such as the ambient temperature, humidity, and number of start/stops. To ensure reliable operation of fuel cell power supply <NUM>, it is desirable to check the condition (health) of fuel cell stack modules <NUM>, e.g., when UAV <NUM> returns to the home base using a ground station to either condition stacks <NUM> or replace one or more of fuel cell stack modules <NUM>. In particular, for long duration flights, it may be desirable to condition stacks <NUM> prior to take-off. In this disclosure, conditioning of stack <NUM> may include the conditioning of one or more fuel cells that comprise the stack.

<FIG> show series and parallel configurations, respectively, of two fuel cell power modules. But these exemplary implementations are not intended to be limiting in number, since three or more power modules may be connected in a series or parallel configuration. In <FIG>, fuel cell stack module <NUM>-<NUM> is connected in series with fuel cell stack module <NUM>-<NUM>, particularly, by connecting (i) its positive terminal to a "power" terminal of a resistive load, (ii) its negative terminal to the positive terminal of fuel cell stack module <NUM>-<NUM>, and (iii) the negative terminal of fuel cell stack module <NUM>-<NUM> to a "ground" terminal of the resistive load. By contrast, <FIG> depicts fuel cell stack module <NUM>-<NUM> connected in parallel with fuel cell stack module <NUM>-<NUM>, particularly, by connecting (i) its positive terminal to a "power" terminal of a resistive load and to the positive terminal of fuel cell stack module <NUM>-<NUM> and (ii) its negative terminal to a "ground" terminal of the resistive load and to the negative terminal of fuel cell stack module <NUM>-<NUM>. In these and/or other implementations, the resistive load may be motor controller <NUM>, motors <NUM>, payload <NUM>, and/or any electrical functionality associated with payload <NUM>.

While the methods and fuel cell power systems have been described in terms of what are presently considered to be the most practical and preferred implementations, it is to be understood that the disclosure need not be limited to the disclosed implementations. The present disclosure includes any and all implementations of the following claims.

It should also be understood that a variety of changes may be made without departing from the essence of the disclosure. It should be understood that this disclosure is intended to yield a patent covering numerous aspects of the disclosure. Further, each of the various elements of the disclosure and claims may also be achieved in a variety of manners. This disclosure should be understood to encompass each such variation, be it a variation of an implementation of any apparatus implementation, a method or process implementation, or even merely a variation of any element of these, insofar as the variation falls within the scope of the appended claims.

As used throughout this application, the word "may" is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The words "include", "including", and "includes" and the like mean including, but not limited to. As used herein, the singular form of "a," "an," and "the" include plural references unless the context clearly dictates otherwise. As employed herein, the term "number" means one or an integer greater than one (i.e., a plurality). As used herein, the statement that two or more parts or components are "coupled" means that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs. As used herein, "directly coupled" means that two elements are directly in contact with each other. As used herein, "fixedly coupled" or "fixed" means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other. As used herein, the word "unitary" means a component is created as a single piece or unit. That is, a component that includes pieces that are created separately and then coupled together as a unit is not a "unitary" component or body. As employed herein, the statement that two or more parts or components "engage" one another means that the parts exert a force against one another either directly or through one or more intermediate parts or components.

In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Directional phrases used herein, such as, for example and without limitation, above, top, bottom, below, left, right, upper, lower, front, back, and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein.

Claim 1:
An unmanned aerial vehicle, comprising:
a mounting frame (<NUM>) onto which at least a payload (<NUM>) is affixed;
a plurality of fuel cell stacks operable in a predefined configuration, each of the plurality of fuel cell stacks being in a separate package;
one or more tanks (<NUM>) configured to supply hydrogen to the plurality of fuel cell stacks;
a propulsion system configured to receive an output power generated from the plurality of fuel cell stacks; and
a power controller (<NUM>) configured to couple the plurality of fuel cell stacks in the predefined configuration;
wherein the propulsion system comprises one or more motors (<NUM>) and one or more rotors (<NUM>);
wherein the power controller (<NUM>) is further configured to detect a fault in one of the plurality of fuel cell stacks and to cause the other stack(s) to continue operating such that the propulsion system is able to bring the unmanned aerial vehicle to a safe landing; and
wherein the power controller (<NUM>) is further configured to be controlled, either remotely via a device on the ground or locally via a direct connection on-board the unmanned aerial vehicle, to breach a safety threshold related to fuel cell overheating such that the payload (<NUM>) has a non-negligible probability of landing undamaged, when the fault is detected, due to prioritizing safety of the payload (<NUM>) over survival of the stack(s) and/or of a motor (<NUM>) of the unmanned aerial vehicle.