Patent Description:
This invention relates to aerial vehicles, namely an aerial vehicle using hydrogen fuel at high altitude.

The world is transitioning away from the use of fossil fuels, and is in need of high energy density energy storage to support the transition of long range aviation to renewable energy sources. Air travel at high altitudes allows higher velocities and shorter flight times than possible in the lower atmosphere, while keeping landing and take-off speeds at modest levels that allow operation from a wider range of airports.

Liquid hydrogen can provide a suitable means for storing and transporting energy for use in aviation, it can be manufactured with high efficiency from distributed renewable sources, and, with the highest energy density of all potential aviation fuels, it is likely the best solution for long range flight. The prior art is illustrated by document <CIT>.

A high efficiency hydrogen fuel system for an aircraft at high altitude which utilizes compressors to compress air to a sufficiently high pressure for the fuel cell. Liquid hydrogen is pumped to higher pressure and then utilized in heat exchangers to cool the air during compression, maintaining the air at a temperature low enough for the fuel cell. The hydrogen is also used to cool the fuel cell as it is also expanded and cooled prior to its entry in the fuel cell cycle. A water condensation system allows for water removal from the airstream to reduce water vapor impacts to the atmosphere. The hydrogen fuel system may be used with VTOL aircraft, which may allow them to fly at higher elevations. The hydrogen fuel system may be used with other subsonic and supersonic aircraft, such as with asymmetric wing aircraft.

Among the goals of the current invention is to provide a hydrogen fueled high energy and power density propulsion system suited to propelling aircraft at high altitude over long distances. In the absence of exceptional increases in battery performance high altitude aviation will require the use of high density hydrogen or hydrogen containing fuels that react with atmospheric oxygen to produce water vapor, if this water is exhausted into the upper atmosphere then it will have average residence times that can extend up to years depending upon altitude, meaning that extensive operation of aviation at high altitude could lead to unacceptable accumulation of water vapor in the upper atmosphere. Water vapor is a powerful greenhouse gas, and can have impacts both as a gas and as ice crystals at high altitude and may thus have a significant impact upon climate if there are a lot of flights operating at high altitudes.

Condensation of water vapour into liquid water and ice releases energy that may be utilised to heat air that may usefully contribute to overall propulsive thrust via the Meridith or ram-jet effect wherein inlet air in a fast moving aircraft undergoes compression in the inlet, is then heated and undergoes greater expansion on outlet to produce a higher outlet velocity than the aircraft flight velocity and thereby a net propulsive effect. Condensing water and creating ice may thereby create a useful secondary propulsive boost and higher overall propulsive efficiency.

It is therefore another goal of the current invention to provide a means for reducing the amount of water vapor released into the upper atmosphere by long range, high altitude aviation. Large aircraft currently present the standard solution for long distance passenger flight owing to the advantages afforded by gas turbine propulsion systems that increase in efficiency, durability and power-to-weight as they increase in size. Aerodynamic efficiency also improves with size as the useful volume within the fuselage compared to the surface area of the aircraft increases with size.

Hydrogen has very low density and storing sufficient hydrogen to enable long distance flight requires substantial volumes of fuel to be stored, with correspondingly large heavy cryogenic tanks with significant thermal insulation requirements. These tanks can represent a significant proportion of the entire vehicle volume, leading to increased vehicle drag as the vehicle volume is increased to accommodate passengers and payload, tanks and other systems. This is particularly true in smaller vehicles where increasing internal volume comes with a greater drag price than for larger aircraft. To minimize the volume and mass of hydrogen required, as well as to reduce fuel costs it is desirable to have a high efficiency conversion scheme for converting the hydrogen into propulsive power. This system also needs to operate adequately over a wide range of atmospheric pressures and temperatures encountered from ground level to high altitude.

It is therefore a goal of the current invention to create a liquid hydrogen powered propulsion system suitable for small high speed aircraft operating at high altitude at high power levels with high efficiency and reliability. Liquifying hydrogen requires a lot of energy, but this energy can be partially recovered through the use of an appropriate thermodynamic process that in turn can further improve the energy density of the hydrogen fuel and usefully reduce the necessary tank volume and mass. This process can have a secondary useful effect of providing additional cooling as the energy extracted from the hydrogen gas expanding in a turbine or the like serves to further cool it and means that it can provide additional heat sinking for other components of the propulsive system.

Another goal of the current invention is to optionally integrate a means for creating propulsive power from the pressurization of liquid hydrogen fuel followed by heating of the liquid to convert it to a warm gas so that it may be expanded though one or more stages of turbine (being one or more of a list that includes rotary screw expander, rotary scroll expander, reciprocating piston expander or turbo-machinery turbine) optionally with reheating to heat the gas further after each stage of expansion, to thereby provide additional mechanical work to the propulsion system before the hydrogen is used as a fuel and oxidized to release energy in the propulsion system. The heating of the hydrogen can preferably provide useful cooling to elements of the propulsion system or other elements of the aircraft.

Proton Exchanger Membrane Fuel Cells (PEMFC) have a lot of advantages for aviation applications. They have high efficiencies and high power to weight ratios, but have disadvantages in that they need to be kept cool, typically in the range of <NUM>-<NUM>, and require substantial cooling to dissipate the heat from typically <NUM>-<NUM>% of the hydrogen oxidation energy that is not converted into electricity. Additionally, they need to be supplied with reactants at elevated pressure. Achieving this in a manner that maintains the advantages of high power to weight and high efficiency even while operating in the low ambient pressures found at high altitude and particularly in small sizes presents many challenges. Another goal of the current invention is to provide a fuel cell system that may maintain the fuel cell within its optimal operating temperature range with near optimal pressurization of air and hydrogen supplied to the fuel cell through the full range of operating altitudes, ambient air pressures and temperatures, while minimizing the drag incurred by the fuel cell propulsion system.

Preferably the air ingested for the purpose of operating and cooling the fuel cell is exhausted from the aircraft with a rearwards velocity approximately the same as the other propulsive actuators, such as propellers fans and the like, so as to optimally contribute to the overall driving thrust of the aircraft without wasting energy. Preferably the air provided to the fuel cell for cooling and to provide oxygen to react in the fuel cell will be accelerated by the action of a mechanically driven fan or the like so that it will exit the vehicle at a velocity similar to the air exiting other propulsive elements of the plane. In some aspects, the ingested air may be driven with a propeller or fan in order to speed up/raise the pressure of the inletted air to the system. In some aspects, the ingested air may be driven at such a high speed (or high pressure) that the outletted air from the air passage system is used as a propulsion source for the aircraft.

<FIG> illustrates a high efficiency hydrogen fueled high altitude thermodynamic fuel cell system <NUM> according to some embodiments of the present invention. In some aspects, the system <NUM> is adapted to deliver electricity to power an electrically powered aircraft, which may then use the electricity to power electric motors. In some aspects, the electrically powered aircraft may be a vertical take-off and landing aircraft. In some aspects, the aircraft may be an asymmetric wing aircraft. In other aspects, other aircraft configurations may utilize the high efficiency hydrogen fueled thermodynamic fuel cell system <NUM>. In some aspects, the system <NUM> additionally provides thrust as part of the air outletting from the system.

The hydrogen fueled thermodynamic fuel cell system may include a series of intertwined pathways, such as one or more pathways for intake air, a pathway for hydrogen, a pathway for byproducts from the fuel cell, and a pathway for water condensed out from the byproducts from the fuel cell.

An air inlet <NUM> allows for air entry into the hydrogen fueled thermodynamic fuel cell system. In some aspects, the fuel cell system provides electric power for electric motor driven propellers, which may be part of a VTOL aircraft. In some such systems, intake air may be routed through intake vents at various locations on the aircraft. In the case of a VTOL aircraft, for example, the intake vents may route the intake air through an intake fan <NUM> as part of the intake air routing. In some aspects, such as with a fan tube as described below, the inletted air may be air entering the fan tube, and the intake fan <NUM> may also be the thrust fan. In some aspects, the fuel cell system provides electric power for an electric motor driven fan system, and in some such systems the intake air may be routed in from within the fan tube. In some aspects, an inlet fan <NUM> is used to accelerate air into the air inlet <NUM>. The air pathway is seen highlighted separately in <FIG>. As the system may be used at very high altitudes where pressures are low, the air traveling through the system may be compressed through a series of compressors <NUM>, <NUM>, <NUM> which may compress the air up to <NUM> times higher than the inlet pressure, for example. After compression in a first compressor <NUM>, the compressed air may be cooled first in an air to air first intercooler <NUM> where it is cooled with inletted air not routed to the first compressor <NUM>. The compressed air is then further cooled by the liquid hydrogen flow at a second intercooler <NUM>. The cooled air is then sent to a second compressor <NUM>, and afterwards again is routed through a third intercooler <NUM>, where it is again cooled with inletted air, and a fourth hydrogen intercooler <NUM>, where it is again cooled further by the hydrogen flow. The cooled air is then sent to a third compressor <NUM>, after which it is routed to the fuel cell portion <NUM>. A portion of this compressed inlet air may be routed to a burner <NUM>. Using the cooling methods described above, the airflow was able to be compressed to a pressure high enough as needed for the fuel cell, while also able to be delivered at a temperature low enough as need for the fuel cell. In some aspects, there may be bypass systems which allow one or more of the compressors <NUM>, <NUM>, <NUM> to be bypassed such as when the ambient pressure is higher at lower altitudes to enable efficient operation throughout the full range of flight altitudes.

Inletted air, which may be very cold, is also routed to intercoolers, in addition to its separate routing to the compressors described above. The inlet air may be routed to a heat exchanger <NUM> in an ice maker <NUM> to assist in the freezing of water condensed from the fuel cell exhaust. This air path may then continue a seventh intercooler <NUM> where it cools the fuel cell exhaust, after which it may route to the system air outlet flow <NUM>. The inletted air may also route to the third intercooler <NUM> to cool compressed air coming from the second compressor <NUM>.

Liquid hydrogen may be stored in a liquid hydrogen tank <NUM> and is routed to a liquid hydrogen pump <NUM> where it is elevated in pressure. The hydrogen pathway is seen highlighted separately in <FIG>. The liquid hydrogen may be routed through the second intercooler <NUM> where it helps cool compressed air as it evaporates. The hydrogen is then routed through the fourth intercooler <NUM> where it similarly helps cool compressed air that is further down the air pathway. The hydrogen may then be expanded at a first expander to extract useful mechanical power and usefully cool the hydrogen, and then routed to the fuel cell. In some aspects, the hydrogen may be expanded at a first expander 313and then routed to the fuel cell <NUM>. In some aspects, the hydrogen may be expanded at a first expander, rerouted as cooling flow through the fuel cell, expanded further at a second expander, and then routed to the fuel cell.

A portion of the hydrogen flow may be routed to a burner <NUM>, which may provide heat in an exchanger that may flow to a series of turbochargers <NUM>, <NUM>, <NUM>, which may be used to assist in powering the compressors <NUM>, <NUM>, <NUM>, which may be primarily powered by electric motors <NUM>, <NUM>, <NUM>.

The exhaust <NUM> from the fuel cell <NUM> follows an exhaust flow path which is shown with more clarity highlighted in <FIG>. In some aspects, the exhaust from the fuel cell consists predominantly of partially oxygen depleted air and water vapor. The exhaust <NUM> routes to a sixth intercooler <NUM> and then a seventh intercooler <NUM>. Water in the exhaust in condensed out during this cooling and is separated out at a trap <NUM>. Some of the water flow <NUM> may be routed to a water reservoir <NUM>, while some water may instead be sent to an ice making unit <NUM>. Water flow is illustrated in <FIG>. The ice making unit <NUM> may be cooled by the very cold air inletted at high altitude, with the ice made then expelled from the aircraft. After leaving the trap <NUM>, the air flow may go to an eighth intercooler <NUM> wherein it is heated by exhaust from the burner <NUM>. As mentioned above, expelling the water as ice allows for the ice to descend to lower altitudes before the ice melts and adds water vapor to the atmosphere. In some aspects, water in the water reservoir <NUM> may also be routed to a water pump <NUM> which then routes to a water spray unit <NUM>, which expels water <NUM> into the air flow path prior to an eight intercooler <NUM> adapted to cool the fuel cell. The water spray <NUM> accentuates the cooling rate of the eighth intercooler <NUM>, which may be thermally coupled to a closed loop cooling system <NUM> adapted to cool the fuel cell with a thermal interchange portion <NUM>. The closed loop cooling system <NUM> may include a pump <NUM> and an appropriate coolant. As mentioned above, water may be condensed out of the flow from the fuel cell system at an exhaust water condenser <NUM>, which may then allow the condensed water to be frozen and ejected from the aircraft as solid ice. This may allow the ice to descend to a lower altitude before melting, thus not contributing water vapor to the highest flight areas of the aircraft. If liquid water is exhausted into the upper atmosphere, then it may have average residence times that can extend up to years depending upon altitude, meaning that extensive operation of aviation at high altitude could lead to unacceptable accumulation of water vapor in the upper atmosphere. Water vapor is a powerful greenhouse gas, and can have impacts both as a gas and as ice crystals at high altitude and may thus have a significant impact upon climate if there are a lot of flights operating at high altitudes. The implementation of this ice ejection system may significantly reduce or eliminate this impact.

The use of a Rankine cycle, pumping liquid hydrogen to high pressure before heating, and using available heat sources and expanding through mechanical turbines to recover extra energy from the liquid hydrogen prior to consumption by the fuel cell provides improvements over any previous systems. Condensing water from the exhaust of the propulsion system provides another advantage in that the heat transferred into cooling air may produce a useful propulsive thrust benefit via the Meredith or ram-jet effect transferring otherwise significant wasted heat energy into useful propulsive force. Also, the operation of the fuel cell at elevated pressures preferably above 2bar absolute where a larger portion of the output water will be in water form at the operating temperature of the fuel cell. Another improvement is to utilize a recuperative counter-flow heat exchange system wherein much of the heat sinking required to cool the fuel cell exhaust down and condense the water vapor is provided by the exhaust, with water removed, being heated back up to near the fuel cell operating temperature, with supplementary cooling to condense the water vapor coming from some combination of additional inletted cooling air that is not used to react with the hydrogen in the fuel cell, or hydrogen that is afterwards used to power the propulsion system.

In an exemplary embodiment, the aircraft is flying at an altitude of <NUM> at <NUM>/s, with the air at a temperature of <NUM>, and a pressure of <NUM> kPa. The inlet air passes through an intake fan and is at <NUM> kPa and <NUM>. Some of this air at this condition may be bypass routed to the first and third intercoolers, but the inlet air proceeds at to a first compressor where it exits at <NUM>. 3kPa and <NUM>. That air then enters the first intercooler where it is cooled with the inlet air routed through the bypass to the first intercooler. That inlet air exits the first intercooler at <NUM> kPa and <NUM>. This air then enters the second intercooler where it warms the liquid hydrogen, and this air exits the second intercooler at <NUM> and <NUM> kPa, where it enters a second compressor. The air exits the second compressor at <NUM> and <NUM> kPa, where it enters a third intercooler where it exchanges heat with the original inlet air. It exits this third intercooler at <NUM> and <NUM> kPa and proceeds to the fourth intercooler where it exchanges heat with the hydrogen. The air exits the fourth intercooler at <NUM> and <NUM> kPa, and then enters a third compressor. It exits the third compressor at <NUM> and <NUM> MPa, and is now in a condition to enter the fuel cell, with a portion that may routed to a burner. On its routing to the fuel cell, it has twice warmed the hydrogen, and twice been cooled by the bypassed original cold inlet air.

In this same exemplary embodiment, the hydrogen is stored as a liquid in a fuel tank at <NUM> and at <NUM>. The liquid hydrogen is first routed to a compressor at <NUM>/s where it exits at <NUM> and 2MPa. The hydrogen then enters the second intercooler where it exits at <NUM> and <NUM> MPa. The hydrogen then enters the fourth intercooler where it exits at <NUM> and <NUM> MPa. The hydrogen then enters one or more expanders to realize a temperature of <NUM> and a pressure of <NUM> MPa. The fuel cell is a PEMFC with an efficiency of <NUM>% and generates 300kW.

In this exemplary embodiment, the exhaust from the fuel cell is routed through a fifth, sixth, and seventh intercooler. The fuel cell exhaust may exit the fuel cell at <NUM>, and then route through the sixth and then the seventh intercooler, where it exits at <NUM> and <NUM> MPa. It then enters the water separator, where water may then be routed to the ice maker which is coupled to inlet air flow at <NUM>. The dried exhaust then proceeds to the sixth intecooler (recuperative condenser) and is then heated in the fifth intercooler with heat from the burner, where it exits at <NUM> and <NUM> MPa. This heated air can then be used to help power the air compressors, going to <NUM> and <NUM> kPa after routing through the first sequential turbine, <NUM> and <NUM> kPa after the second turbine, and <NUM> and <NUM> kPa after the final turbine, where it is then routed out to the exhaust.

A vertical take-off and landing (VTOL) aircraft presents unusual circumstances when powered by a liquid hydrogen high efficiency thermodynamic fuel system according to embodiments of the present invention. A VTOL aircraft may have its highest power load during hover, take-off, and landing. The need for higher thrust during these operations may present the limiting case for the design of the power system, including the thermal management system. Although these operations typically occur at low altitude where the air is denser (thus not requiring as much inlet air compression, for example), they do occur where the air temperature may be much higher than would be seen at high altitude operation In order to avoid designing and sizing a cooling system around an operational mode which may, typically, last under a minute, a water assisted cooling mode may be added for use during hover, take-off, and landing, and as otherwise needed.

With the use of the water reservoir <NUM>, water can be sprayed <NUM> from a water spray unit <NUM> in the air flow upstream of an intercooler adapted for cooling the fuel cell. The water spray unit may also be downstream in the air flow from the routing of the inletted air into compression system leading to input into the fuel cell. In this way, a VTOL aircraft can have this extra, auxiliary, cooling for the VTOL operations and avoid the need for having a thermal system oversized for ordinary flight modes. Although water may be condensed from the exhaust flow from the fuel cell, as described herein, a water tank may be used to allow for the use of water cooling during initial flight operations, such as take-off, in advance of the collection of water from the fuel cell exhaust during flight. An example of such a VTOL aircraft is discussed below.

As shown in <FIG>, the tiltrotor aircraft <NUM> includes an airframe and a plurality of propulsion assemblies coupled to the airframe. The aircraft <NUM> is operable between a hover mode, wherein the plurality of propulsion assemblies <NUM> is arranged in a hover arrangement, and a forward mode, wherein the plurality of propulsion assemblies is arranged in a forward arrangement. The hover arrangement defines the position of each propeller <NUM> of the plurality of propulsion assemblies relative to each other propeller of the plurality of propulsion assemblies and the airframe during aircraft operation in the hover mode, and the forward arrangement likewise defines the relative position of each propeller to each other propeller and the airframe during operation in the forward mode. The airframe can include a left wing, a right wing, a fuselage, and an empennage, wherein the left and right wings are coupled to the fuselage <NUM> and positioned between forward of the empennage118. Each propulsion assembly includes a propeller, a tilt mechanism, and an electric motor. Each propulsion assembly is operable, preferably by the tilt mechanism <NUM> associated therewith but alternatively in any other suitable manner, between a hover configuration and a forward configuration as described in further detail below. The tiltrotor aircraft can additionally include an electric power source, flight control surfaces and actuators, and any other suitable components.

The tiltrotor aircraft <NUM> functions to provide an aerial vehicle operable between a hover mode (e.g., rotary-wing mode) and a forward mode (e.g., fixed-wing mode). The hover mode can include vertical takeoff, vertical landing, and/or substantially stationary hovering of the aircraft <NUM>; however, the hover mode can additionally or alternatively include any suitable operating mode wherein vertically-directed thrust is generated by one or more of the plurality of propulsion assemblies. The forward mode can include forward flight, horizontal takeoff, and/or horizontal landing of the aircraft <NUM> (e.g., conventional take-off and landing / CTOL); however, the forward mode can additionally or alternatively include any suitable operating mode wherein horizontally-directed thrust is generated by one or more of the plurality of propulsion assemblies. Thus, the hover mode and forward mode are not mutually exclusive, and the tiltrotor aircraft <NUM> can operate in a superposition of the hover mode and the forward mode (e.g., wherein the plurality of propulsion assemblies <NUM> is arranged in a superposition of the hover arrangement and the forward arrangement defined by a liminal configuration of each of the plurality of propulsion assemblies between the hover configuration and the forward configuration). The tiltrotor aircraft <NUM> can also function to provide an aerial vehicle that is stable in hover mode (e.g., maximally stable, stable within a defined stability window or envelope of flight conditions, stable up to a stability threshold magnitude of various control inputs to the aircraft <NUM>, etc.) and efficient (e.g., aerodynamically efficient, power efficient, thermodynamically efficient, etc.) in forward mode. The tiltrotor aircraft <NUM> can also function to provide airborne transportation to passengers and/or cargo. However, the tiltrotor aircraft <NUM> can additionally or alternatively have any other suitable function.

The tiltrotor aircraft <NUM> is operable between a plurality of modes, including a hover mode and a forward mode. In the hover mode, the plurality of propulsion assemblies can be arranged in the hover arrangement. In the hover arrangement, each of the plurality of propellers is preferably arranged in the hover configuration. In the forward mode, the plurality of propulsion assemblies can be arranged in the forward arrangement. In the forward arrangement, each of the plurality of propellers is preferably arranged in the forward configuration. However, each of the plurality of propellers can be arranged in any suitable state between the forward and hover configurations, independently of one another, and/or in any suitable orientation in the hover mode of aircraft <NUM> operation; furthermore, each of the plurality of propellers can be arranged in any suitable state between the forward and hover configurations, independently of one another, and/or in any other suitable orientation in the forward mode of aircraft <NUM> operation. Furthermore, the tiltrotor aircraft <NUM> can be operated in any suitable liminal mode between the hover mode and forward mode, wherein a component of thrust generated by one or more propulsion assemblies <NUM> is directed along both the vertical axis and the longitudinal axis (e.g., and/or the lateral axis).

Though the aircraft <NUM> is referred to herein as a tiltrotor aircraft <NUM>, the terms "propeller" and "rotor" as utilized herein can refer to any suitable rotary aerodynamic actuator, commonly referred to as a rotor, a propeller, a rotating wing, a rotary airfoil, and the like. While a rotor can refer to a rotary aerodynamic actuator that makes use of an articulated or semi-rigid hub (e.g., wherein the connection of the blades to the hub can be articulated, flexible, rigid, and/or otherwise connected), and a propeller <NUM> can refer to a rotary aerodynamic actuator that makes use of a rigid hub (e.g., wherein the connection of the blades to the hub can be articulated, flexible, rigid, and/or otherwise connected), no such distinction is explicit or implied when used herein, and the usage of propeller can refer to either configuration, and any other possible configuration of articulated or rigid blades, and/or any other possible configuration of blade connections to a central member or hub. Accordingly, the tiltrotor aircraft <NUM> can be referred to as a tilt-propeller aircraft <NUM>, a tilt-prop aircraft <NUM>, and/or otherwise suitably referred to or described. In the context of an electric motor, which in some variations can include a stator and rotor, the rotor of the electric motor <NUM> can refer to the portion of the motor that rotates as electrical potential energy is converted to rotational kinetic energy in operation of the electric motor.

The tiltrotor aircraft <NUM> includes a plurality of propulsion assemblies coupled to the airframe at a corresponding plurality of propulsion assembly attachment points. Each propulsion assembly preferably includes a propeller, a tilt mechanism, and an electric motor. The propulsion assembly functions to house and collocate the propeller, the tilt mechanism, the electric motor, and any other suitable components related to the propeller and electromechanical drive thereof. The tiltrotor aircraft <NUM> preferably includes an even number of propulsion assemblies, and more preferably includes six propulsion assemblies; however, the tiltrotor aircraft <NUM> can additionally or alternatively include an odd number of propulsion assemblies, eight propulsion assemblies, and any other suitable number of propulsion assemblies <NUM>.

The propeller <NUM> of the propulsion assembly <NUM> functions to convert rotational kinetic energy supplied by the electric motor <NUM> to aerodynamic forces (e.g., for propelling the aircraft <NUM> in the hover mode, the forward mode, etc.). The propeller <NUM> can include a number of propeller <NUM> blades (e.g., blades, airfoils, etc.), a head (e.g., a hub and associated linkages), and any other suitable components. The propeller <NUM> is preferably a variable-pitch propeller <NUM> (e.g., wherein the pitch of each propeller <NUM> blade is variable in coordination such as via collective control, wherein the pitch of each propeller <NUM> blade is independently variable such as via cyclic control, etc.), but can additionally or alternatively be a fixed-pitch propeller. In some variations, the aircraft <NUM> can include both variable-pitch and fixed-pitch propeller <NUM> associated with different propulsion assemblies <NUM> of the plurality of propulsion assemblies <NUM>. In additional or alternative variations, the propeller <NUM> can be articulated into a negative angle of attack condition, which can function to produce reverse thrust without changing the direction of rotation of the propeller. The propeller <NUM> preferably includes five blades per propeller, but can additionally or alternatively include any suitable number of blades per propeller <NUM> (e.g., two, three, four, six, etc.). The propeller <NUM> can define any suitable disc area (e.g., propeller disc, disc, etc.), and each blade can define any suitable cross section and/or twist angle as a function of blade span.

In a specific example, each propeller of the plurality of propulsion assemblies <NUM> includes a set of propeller blades attached to the hub by a variable pitch linkage that rotates each propeller blade about a long axis of the propeller blade and constrains propeller blade motion normal to the disc plane (e.g., the propeller blade does not substantially articulate forward or backward from the disc plane).

The electric power source <NUM> functions to power the propulsion assembly and any other electrically-powered components of the aircraft coupled thereto (e.g., motorized linkages, flight control surface actuators, and any other electrical actuators, sensors, transducers, displays, etc.). The electric power source may include one or more batteries, but can additionally or alternatively include an electrical generator (e.g., a combustion-driven generator, a fuel cell, a photovoltaic generator, etc.). In variations including a fuel cell, air inlets <NUM> may be located at various locations around the aircraft, such as on the leading edges of the wings. The air inlets <NUM> provide inlet air for the thermodynamic fuel system described above. As seen in <FIG>, downstream from the inlet the may be a fan <NUM> which increases the pressure of the airflow. As described above, some of this inletted air enters a multi-stage compressor system and is delivered to the fuel cell. As also described above, some of this inletted air provides fuel cell cooling at an eighth intercooler <NUM>. In the case of this VTOL aircraft, extra cooling may be needed during VTOL operations. A water spray unit <NUM> may provide water <NUM> into the airflow upstream of the eight intercooler <NUM>. Exhaust outlets <NUM> for the fuel cell system and for the inletted cooling air may be located at appropriate locations along the wings or on the aircraft body. The tiltrotor aircraft <NUM> can include a power distribution system that couples the electric power source <NUM> to each electrically-powered component (e.g., including each electric motor). The power distribution system can include an electrical power transmission bus that distributes power from a plurality of electric power sources to components of the aircraft <NUM> requiring electrical power. Each propulsion assembly <NUM> is preferably connected to at least one associated electric power source <NUM> that powers the electric motor assembly of the propulsion assembly. However, the electric power sources can additionally or alternatively be interconnected to one another and/or to one or more propulsion assemblies <NUM> such that any propulsion assembly <NUM> (or other powered component) can draw electrical power from any suitable subset of electric power sources of the aircraft <NUM>, with any suitable relative power draw between electric power sources.

In some embodiments of the present invention, as seen in <FIG>, another VTOL aircraft <NUM> is adapted for flight with a pilot and six passengers, for example. The VTOL aircraft <NUM> is adapted for flight over <NUM>,<NUM> feet altitude with a cruising speed of <NUM> knots. The VTOL aircraft <NUM> uses a high efficiency hydrogen powered fuel cell system as described above.

In other embodiments of the present invention, the high efficiency hydrogen fueled thermodynamic fuel cell system may be utilized with a fan tube propulsion system. A cross-sectional view of such a fan tube propulsion system is seen in <FIG> and discussed further below. The fan tube propulsion system is seen in an exemplary embodiment of the thrust units <NUM>, <NUM>, <NUM> in <FIG>, seen in support of an asymmetric wing aircraft adapted for high speeds and high altitudes.

In some embodiments of the present invention, as seen in <FIG>, a multi-segment oblique wing aircraft <NUM> includes a center segment <NUM>, a left wing segment <NUM>, and a right wing segment <NUM>. The center segment <NUM> is substantially thicker in the Zb direction (as defined below), and is thick enough to allow for passengers in a passenger area <NUM>. A plurality of thrust units <NUM>, <NUM>, <NUM> may use pivoting pylons <NUM>, <NUM>, <NUM> which allow for thrusting in different forward flight configurations. The rotation of the thruster units will change the sweep of the oblique wing aircraft, both due to the change in thrust direction and also due to a rudder effect of the pylons. There may be further trimming and control surfaces and devices which assist in the sweep change.

In some embodiments of the present invention, as seen in <FIG>, a multi-segment oblique wing aircraft <NUM> includes a center segment <NUM>, a left wing segment <NUM>, and a right wing segment <NUM>. The center segment <NUM> has a leading edge 210a and a trailing edge 210b. Although there may be variations along their lengths, the leading edge 210a and the trailing edge 210b of the center segment <NUM> are substantially parallel. The center segment <NUM> may be substantially thicker than the other segments and may be adapted to contain pilots and passengers of the aircraft. Although illustrated without propulsion units shown, it is understood the multi-segment oblique wing aircraft <NUM> may be powered similarly to the aircraft <NUM> discussed above.

The left wing segment <NUM> has a leading edge 212a and a trailing edge 212b. The left wing segment <NUM> tapers as it routes outboard from the center segment <NUM>, in that the chord length lessens along the span of the wing segment. The left wing segment <NUM> may be substantially thinner in the vertical direction Zb than the center segment <NUM>. The right wing segment <NUM> has a leading edge 211a and a trailing edge 211b. The right wing segment <NUM> tapers as it routes outboard from the center segment <NUM>, in that the chord length lessens along the span of the wing segment. The right wing segment <NUM> may be substantially thinner in the vertical direction Zb than the center segment <NUM>.

<FIG> introduces coordinate systems which illustrate aspects of the system. A prevailing wind coordinate system <NUM> includes the prevalent airflow across the wing as a composite of Xw and Yw, with Xw being the airflow direction seen in forward flight directly into the wind. A body coordinate system <NUM> is set to remain constant with the body of the wing, with the Yb axis set approximately parallel to the composite average direction of the leading edges 212a, 211a of the wings. The Zb axis of the body coordinate system comes out of the view towards the viewer. A quarter chord coordinate system <NUM> sets Yl as parallel to the quarter chord tangent at that point, and Xl as perpendicular to the quarter chord at that point. The body coordinate system <NUM> remains fixed with regard to the aircraft. The prevailing wind coordinate system <NUM> is a product of the environment and is independent of the wing, and the quarter chord coordinate system <NUM> a function of the wing design but alters relative to which point on the wing is being referenced.

The multi-segment wing may be viewed as having a transition from the left wing segment <NUM> to the center segment <NUM> at a reference line <NUM>, and as having a transition from the right wing segment <NUM> to the center segment <NUM> at a reference line <NUM>. Within the reference lines <NUM>, <NUM>, the leading edge 210a and the trailing edge 210b of the center segment <NUM> are substantially parallel.

The thrust units <NUM>, <NUM>, <NUM> may be electrically powered fan units with an internal fan. In some aspects, each of the electrically powered fan units may be powered by a plurality of fuel cells. As seen in cross-section in <FIG>, an exemplary fan tube <NUM> has an inlet <NUM> which allow for an inlet air flow <NUM>. Within the fan tube <NUM> is a fan <NUM>. The fan <NUM> may have an electric motor and be electrically coupled to a aircraft power system which is turn coupled to one or more fuel cells. Downstream from the fan <NUM> may be one or more air system inlets <NUM> which are adapted to route air to the series of compressors used to compress the air prior to delivery to the fuel cell, and which are also adapted to route air to other various cooling pathways as described above. Also in the fan tube <NUM> may be exhaust ducts <NUM> which route the fuel cell exhaust and the inletted air which has been routed through cooling pathways back into the air tube <NUM> prior to the nozzle area <NUM>, which is then joined with the other thrusted air from the fan <NUM> into the exit flow <NUM>.

In some aspects, as seen in <FIG>, a more complex air tube configuration may also include heat exchangers <NUM> downstream from the fan <NUM>. A water spray unit <NUM> may be placed upstream from the heat exchangers <NUM> to introduce water from the water reservoir into the air flow to enhance cooling. Although illustrated in <FIG> for a fan tube, it is understood that in the thermodynamic system of the VTOL aircraft, as described above, a similar configuration of an intake fan, a water spray unit, and a downstream heat exchanger will be used even when this portion of the system is not a primary thrust producing aspect of the aircraft, as discussed above. <FIG> illustrates an embodiment where the heat exchanger <NUM> is upstream from the fan <NUM>.

In some aspects of methods according to the present invention, and as seen in <FIG>, a method of providing a high efficiency hydrogen fueled high altitude thermodynamic fuel cell system powered aircraft comprising the steps of inletting air <NUM>, routing the inletted air through an inlet air fan <NUM>, further routing the inletted air to a series of one or more compressors and also to routing some of the inletted air to a plurality of thermal system pathways <NUM>, <NUM>, compressing the inletted air in one or more compressors <NUM>, routing the compressed air to a fuel cell <NUM>, routing liquid hydrogen from a liquid hydrogen tank to a liquid hydrogen compressor <NUM>, heating the hydrogen with the compressed air at an intercooler <NUM>, expanding the hydrogen <NUM>, routing the expanded hydrogen to a fuel cell, generating electricity at the fuel cell <NUM>, powering electric thrust elements with the generated electricity <NUM>, routing the exhaust <NUM> through one or more intercoolers to condense the water in the fuel cell exhaust <NUM>, routing some or all of the condensed water to an ice maker <NUM>, and expelling the ice from the aircraft <NUM>.

In some aspects, a method of providing a high efficiency hydrogen fueled high altitude thermodynamic fuel cell system powered VTOL aircraft comprising the steps of inletting air, routing the inletted air through an inlet air fan, further routing the inletted air to a series of one or more compressors and also to routing some of the inletted air to a plurality of thermal system pathways, compressing the inletted air in one or more compressors, routing the compressed air to a fuel cell, routing liquid hydrogen from a liquid hydrogen tank to a liquid hydrogen compressor, heating the hydrogen with the compressed air at an intercooler, expanding the hydrogen, routing the expanded hydrogen to a fuel cell, providing electricity to a plurality of electrically powered VTOL rotor assemblies, and spraying water <NUM> into the inletted air upstream of a intercooler cooling a closed loop fuel cell thermal system.

In some aspects, a method of providing a high efficiency hydrogen fueled high altitude thermodynamic fuel cell system powered aircraft with fan tubes comprising the steps of inletting air into the fan tube, routing the inletted air through an air fan, further routing a portion of the inletted air to a series of one or more compressors and also to routing some of the inletted air to a plurality of thermal system pathways, compressing the inletted air in one or more compressors, routing the compressed air to a fuel cell, routing liquid hydrogen from a liquid hydrogen tank to a liquid hydrogen compressor, heating the hydrogen with the compressed air at an intercooler, expanding the hydrogen, routing the expanded hydrogen to a fuel cell, routing the exhaust through one or more intercoolers to condense the water in the fuel cell exhaust, the condensed water to a water reservoir, outletting the exhaust into the fan tube downstream of the air fan, and providing thrust to the aircraft from the fan tube. In some aspects, the method further comprises routing the air flow in the fan tube through heat exchanger that are part of the thermal system which are residing within the fan tube.

Claim 1:
A high efficiency hydrogen fueled thermodynamic fuel cell system (<NUM>) for a high altitude aircraft, said system (<NUM>) comprising:
an air inlet (<NUM>);
a fan (<NUM>) downstream from air inlet (<NUM>);
a first air compressor (<NUM>) fluidically coupled to said air inlet (<NUM>) downstream from said fan (<NUM>), said first compressor (<NUM>) adapted to compress inletted air into a fuel cell air pathway;
a second air compressor (<NUM>) fluidically coupled to said first air compressor (<NUM>) downstream from said first air compressor (<NUM>);
a third air compressor (<NUM>) fluidically coupled to said second air compressor (<NUM>) downstream from said second air compressor (<NUM>);
a liquid hydrogen reservoir (<NUM>);
a liquid hydrogen pump (<NUM>) coupled to said liquid hydrogen reservoir (<NUM>), said pump (<NUM>) adapted to pressurize liquid hydrogen through a hydrogen pathway;
a first heat exchanger (<NUM>) adapted to cool air downstream of said first air compressor (<NUM>) with inletted air downstream from said fan (<NUM>);
a second heat exchanger (<NUM>) adapted to cool air downstream of said first compressor (<NUM>) with hydrogen from said pump (<NUM>);
a third heat exchanger (<NUM>) adapted to cool air downstream of said second air compressor (<NUM>) with inletted air downstream from said fan (<NUM>);
a fourth heat exchanger (<NUM>) adapted to cool air downstream of said second compressor (<NUM>) with hydrogen from said pump (<NUM>) downstream along said hydrogen pathway from said second heat exchanger (<NUM>);
a hydrogen expander (<NUM>) along said hydrogen pathway downstream from said fourth heat exchanger (<NUM>), and
a fuel cell (<NUM>), said fuel cell (<NUM>) fluidically coupled to said hydrogen pathway downstream of said hydrogen expander (<NUM>), said fuel cell (<NUM>) fluidically coupled to said fuel cell air pathway downstream of said third compressor (<NUM>).