Patent Description:
Parallel hybrid electric propulsion systems for aerial vehicles typically include a combustion engine or other mechanically-driven powerplant that drives a generator to produce electrical power. The combustion engine may also drive a source of thrust for the aerial vehicle, such as a propeller. The electrical power generated by the generator is utilized to drive an additional source of thrust. For instance, the electrical power may be provided to an electric motor that utilizes the electrical power to drive the additional thrust source, such as a propeller on the other side of the aerial vehicle.

Such systems inherently contain single failures that, due to electrical coupling, can produce changes in vehicle thrust and thrust asymmetry that are more severe than non-hybrid systems. Moreover, rapid electrical load changes may cause significant and unsafe aircraft handling issues. Additionally, rapid changes to engine torque can lead to overspeed issues, among others.

Accordingly, a control system for a hybrid electric propulsion system for an aerial vehicle and methods therefore that address one or more of the challenges noted above would be useful.

<CIT> relates to a hybrid propulsion system for an aircraft, in which fuel flow may be reduced, to a combustion chamber of a turbomachine, in response to a failure condition. <CIT> relates to an externally fired combined cycle gas turbine system.

A hybrid electric propulsion system for an aerial vehicle is defined in claim <NUM>. A method for operating a hybrid electric propulsion system for an aerial vehicle is defined in claim <NUM>.

In one aspect, the present disclosure is directed to a hybrid electric propulsion system for an aerial vehicle. The hybrid electric propulsion system includes an engine and a first electric machine mechanically coupled with the engine and configured to generate electrical power when driven by the engine. The hybrid electric propulsion system also includes one or more power consuming loads electrically coupled with the first electric machine and configured to receive electrical power from the first electric machine. The one or more power consuming loads include a second electric machine, among other potential power consuming loads. Further, the hybrid electric propulsion system includes a propulsion assembly mechanically coupled with the second electric machine and configured to produce thrust when driven by the second electric machine. Moreover, the hybrid electric propulsion system includes one or more sensing devices positioned onboard the aerial vehicle for detecting one or more performance indicators indicative of an electrical load on the engine. In addition, the hybrid electric propulsion system includes one or more controllers communicatively coupled with the one or more sensors. The one or more controllers are configured to receive, from the one or more sensing devices, the one or more performance indicators indicative of the electrical load on the engine; determine whether an electrical load change is present based at least in part on the one or more performance indicators; and generate, if the rapid electrical load change is present, a control action in response to the electrical load change.

In another aspect, the present disclosure is directed to a method for operating a hybrid electric propulsion system for an aerial vehicle. The method includes receiving, from one or more sensing devices positioned onboard the aerial vehicle, one or more performance indicators indicative of an electrical load on the engine, the engine mechanically coupled with an electric machine configured to generate electrical power when driven by the engine, the electric machine electrically coupled with one or more power consuming devices, wherein at least one of the one or more power consuming devices are mechanically coupled with a propulsion assembly. The method also includes determining whether an electrical load decrease on the engine is present based at least in part on the one or more performance indicators. The method further includes generating, if the electrical load decrease on the engine is present, a control action in response to the electrical load decrease, wherein generating the control action comprises reducing a fuel flow to the engine.

In a further aspect, the present disclosure is directed to a hybrid electric propulsion system for an aerial vehicle. The hybrid electric propulsion system includes an engine. The hybrid electric propulsion system also includes a first propulsion assembly mechanically coupled with the engine and configured to produce thrust when driven by the engine. Further, the hybrid electric propulsion system includes a first electric machine mechanically coupled with the engine and configured to generate electrical power when driven by the engine. Moreover, the hybrid electric propulsion system includes a second electric machine electrically coupled with the first electric machine and configured to receive electrical power from the first electric machine. In addition, the hybrid electric propulsion system includes a second propulsion assembly mechanically coupled with the second electric machine and configured to produce thrust when driven by the second electric machine. The hybrid electric propulsion system also includes one or more sensing devices positioned onboard the aerial vehicle for detecting one or more performance indicators indicative of an electrical load on the engine. Moreover, the hybrid electric propulsion system includes one or more controllers communicatively coupled with the one or more sensors, the one or more controllers configured to: receive, from the one or more sensing devices, the one or more performance indicators indicative of the electrical load on the engine; determine whether an electrical load change is present based at least in part on the one or more performance indicators; and generate, if the electrical load change is present, a control action in response to the electrical load change.

Furthermore, as used herein, terms of approximation, such as "approximately," "substantially," or "about," refer to being within a ten percent (<NUM>%) margin of error. Further, as used herein, the terms "first", "second", and "third" may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components.

In general, the present disclosure is directed to a hybrid electric propulsion system and methods therefore. More particularly, the present disclosure is directed to control systems for hybrid electric propulsion systems for aerial vehicles that are configured for rapidly and automatically taking action in response to rapid electrical load changes on a torque source, such as an engine.

<FIG> provides a perspective schematic view of an exemplary aerial vehicle <NUM> having a hybrid electric propulsion system <NUM> in accordance with exemplary embodiments of the present disclosure. As shown, for this embodiment, the aerial vehicle <NUM> is a fixed-wing aircraft. In other embodiments, the aerial vehicle <NUM> may be other suitable types of aerial vehicles, such as a rotary aircraft, a vertical take-off and landing aircraft, tiltrotors, airships, unmanned aerial vehicles, etc. The aerial vehicle <NUM> extends between a first end <NUM> and a second end <NUM>, e.g., along a longitudinal axis L. The first end <NUM> is a forward end of the aerial vehicle <NUM> and the second end <NUM> is a rear or aft end of the aerial vehicle <NUM> in the depicted embodiment of <FIG>. The aerial vehicle <NUM> includes a fuselage <NUM> and a pair of wings <NUM> each extending laterally outward from the fuselage <NUM>. The aerial vehicle may include various control surfaces for controlling propulsion and movement of the aerial vehicle <NUM>. Example control surfaces include elevators, rudders, ailerons, spoilers, flaps, slats, air brakes, or trim devices, etc. Various actuators, servo motors, and other devices may be used to manipulate the various control surfaces and variable geometry components of the aerial vehicle <NUM>. Moreover, as noted above, the aerial vehicle <NUM> includes the hybrid electric propulsion system <NUM> for producing thrust. More particularly, for this embodiment, the hybrid electric propulsion system <NUM> is a parallel hybrid electric propulsion system.

As depicted in <FIG>, the hybrid electric propulsion system <NUM> includes an engine <NUM> mounted to one of the wings <NUM> of the aerial vehicle <NUM>. The engine <NUM> may be any suitable aeromechanical torque source. For instance, the engine <NUM> is a gas turbine engine in the depicted embodiment. The gas turbine engine may be configured as a turboprop (as shown in <FIG>), or other suitable types of gas turbine engines, e.g., turbofans, turbojets, turboshaft, etc. In alternative embodiments, the engine <NUM> may be a piston driven engine or some other type of internal combustion engine, such as a rocket engine.

A first electric machine <NUM> is mounted to one of wings <NUM> and is mechanically coupled with the engine <NUM>. The first electric machine <NUM> is configured to generate electrical power when driven by the engine <NUM>. To generate electrical power, as will be appreciated, the first electrical machine converts the rotational energy received from an output shaft of the engine <NUM> into electrical energy, which may be delivered to various components of the hybrid electric propulsion system <NUM> as described more fully below. Thus, the first electric machine <NUM> may serve as an electric generator. In some preferred embodiments, the first electric machine <NUM> may serve as a generator or a motor depending on the circumstances. A clutch <NUM> (<FIG>) or like feature may be provided to disengage the first electric machine <NUM> from the engine <NUM>, e.g., in the event of a complete failure of the first electric machine <NUM>.

The hybrid electric propulsion system <NUM> also includes a first propulsion assembly <NUM> mechanically coupled with the engine <NUM> and configured to produce thrust when driven by the engine <NUM>. For this embodiment, the first propulsion assembly <NUM> is a propeller or fan. The blades of the propeller may be adjustable in unison through a plurality of pitch angles, e.g., by activation of an actuation mechanism. Pitch adjustment of the blades may cause the propeller assembly to produce more or less thrust. In some embodiments, the first propulsion assembly <NUM> is mechanically coupled with the engine <NUM> in parallel with the first electric machine <NUM>, e.g., to avoid single fault failures of the system. In alternative embodiments, the first propulsion assembly <NUM> is mechanically coupled with the engine <NUM> in series with the first electric machine <NUM>. Some of the torque output from the engine <NUM> is directed to the first electric machine <NUM>, e.g., for power generation, and some of the torque output is supplied to the first propulsion assembly <NUM>, e.g., for propulsion of the aerial vehicle <NUM>.

A first power converter <NUM> is electrically coupled with the first electric machine <NUM>. The first power converter <NUM> provides an electronic interface between the first electric machine <NUM> and a power bus <NUM> of the propulsion system <NUM>. As one example, the first power converter <NUM> may be a rectifier configured to convert the alternating current (AC) generated by the first electric machine <NUM> to direct current (DC). The first power converter <NUM> may be a passive system that includes a plurality of diodes or an active system that includes various processing devices, semiconductor switches and other electronic components.

An energy storage device <NUM> is electrically coupled with the first power converter <NUM>, and thus the first electric machine <NUM>. The energy storage device <NUM> may be embodied as, for example, one or more superconducting energy storage devices, batteries, or battery packs. The energy storage device <NUM> may be mounted within the fuselage <NUM> or in another suitable location. As will be explained in more detailed herein, energy storage device <NUM> may receive electrical power from first electric machine <NUM>, and in some instances, may supply electrical power stored therein to various components of the propulsion system <NUM>, such as e.g., the first electric machine <NUM>, a second electric machine <NUM>, and other loads.

A second power converter <NUM> is electrically coupled with the first power converter <NUM> and the energy storage device <NUM>. The second power converter <NUM> is thus likewise also in electrical communication or electrically coupled with the first electric machine <NUM>. The second power converter <NUM> provides the electronics to interface the second electric machine <NUM> with the power bus <NUM> of the propulsion system <NUM>. As one example, the second power converter <NUM> may be an inverter configured to convert the DC current flowing through the power bus <NUM> to AC current, e.g., to control the speed or torque of the second electric machine <NUM>.

The second electric machine <NUM> is electrically coupled with the first electric machine <NUM> and is configured to receive electrical power from the first electric machine <NUM> (e.g., directly, indirectly via the energy storage device <NUM>, or both). Moreover, the second electric machine <NUM> is configured to convert the electrical power received from the first electric machine <NUM> into rotational energy, e.g., to rotate an output shaft of the second electric machine <NUM>. Thus, the second electric machine <NUM> may serve as an electric motor. In some embodiments, the second electric machine <NUM> may serve as a motor or generator depending on the circumstances. The second power converter <NUM> may control the amount of electrical power delivered to the second electrical machine <NUM>, e.g., to control the speed or torque output of the output shaft of the second electrical machine <NUM>.

A second propulsion assembly <NUM> is mechanically coupled with the second electric machine <NUM>, e.g., via a coupling of the output shaft and power gearbox of the second propulsion assembly <NUM>. The second propulsion assembly <NUM> is configured to produce thrust when driven by the second electric machine <NUM>. For this embodiment, like the first propulsion assembly <NUM>, the second propulsion assembly <NUM> is a propeller or fan. The blades of the propeller may be adjustable in unison through a plurality of pitch angles, e.g., by activation of an actuation mechanism. Pitch adjustment of the blades may cause the propeller assembly to produce more or less thrust. In some embodiments, the hybrid electric propulsion system <NUM> may include multiple electric machines each coupled with one or more propulsion assemblies that are powered by the first electric machine <NUM>. For instance, the hybrid electric propulsion system <NUM> may include a third electric machine that is electrically coupled with the first electric machine <NUM> and configured to receive electrical power therefrom. The third electric machine may be mechanically coupled with a third propulsion assembly for producing thrust. Further, the hybrid electric propulsion system <NUM> may include more electric machines and propulsion assemblies that are similarly configured as third electric machine and third propulsion assembly.

In some instances, the hybrid electric propulsion system <NUM> may experience thrust asymmetry or significant handling issues due to rapid electrical load changes, particularly during rapid electrical load drops or loss of electrical machine torque. For example, if the first electric machine <NUM> fails or drops offline due to a detected fault or other failure, the second electric machine <NUM> that relies on the first electric machine <NUM> for electrical power will cease producing an output torque to drive the second propulsion assembly <NUM> and the electrical load on the engine <NUM> will rapidly decrease or drop. Similarly, a sudden loss or failure of the second electric machine <NUM> will cause the load or counter torque on the first electrical machine <NUM> to rapidly drop to zero (<NUM>) and the electrical load on the engine <NUM> will rapidly decrease or drop. This may cause the first electric machine <NUM> and/or the engine <NUM> to overspeed. Further, in such instances, the thrust produced by the second propulsion assembly <NUM> will rapidly drop, and due to the removed electrical load on the first electric machine <NUM> and ultimately the engine <NUM>, the thrust produced by the first propulsion assembly <NUM> rapidly increases due to the counter torque on the first electrical machine <NUM> dropping to zero (<NUM>) with the torque output of the engine <NUM> remaining unchanged. Accordingly, the result is thrust asymmetry. That is, thrust rapidly increases on one side of the aircraft while thrust rapidly drops on the other side. In accordance with exemplary aspects of the present disclosure, the hybrid electric propulsion system <NUM> includes a control system <NUM> (<FIG>) that includes features that rapidly and automatically take action in the event of such rapid electrical load changes. In particular, the control system <NUM> of the hybrid electric propulsion system <NUM> is configured to rapidly and automatically account for electrical load changes on the order of microseconds while the engine <NUM> spools up or down to match the torque load of the electrical system on the engine <NUM>. The engine <NUM> may take several seconds to spool up or down. Thus, the control system <NUM> is configured to take rapid and automatic action during this transient period. An exemplary control system is provided below.

<FIG> provides a schematic view of the exemplary control system <NUM> for the hybrid electric propulsion system <NUM> of the aerial vehicle <NUM> of <FIG>. As shown, the control system <NUM> includes one or more sensing devices <NUM> positioned onboard the aerial vehicle <NUM> for detecting one or more performance indicators indicative of an electrical load on the engine <NUM>. For this embodiment, at least one sensing device <NUM> is positioned adjacent a shaft <NUM> mechanically coupling the engine <NUM> with the first electric machine <NUM>, e.g., to sense or measure a performance indicator between the engine <NUM> and the first electric machine <NUM>. The performance indicator may be an output torque of the engine <NUM> (or the input torque to the first electrical machine <NUM>), the rotational speed of the shaft <NUM>, or another suitable indicator. The performance indicator may be utilized to calculate, predict, or estimate the electrical load on the engine <NUM>. Further, at least one sensing device <NUM> is positioned adjacent a shaft <NUM> mechanically coupling the second electric machine <NUM> with the second propulsion assembly <NUM>, e.g., to sense or measure a performance indicator between the second electrical machine <NUM> and the second propulsion assembly <NUM>. The performance indicator may be an output torque of the second electric machine <NUM> (or the input torque to the second propulsion assembly <NUM>), the rotational speed of the shaft <NUM>, or another suitable parameter.

Other sensing devices <NUM> may include current, voltage, or speed sensors configured to measure various parameters of the first electrical machine <NUM> and/or the second electrical machine <NUM>. From such performance indicators, the electrical load or counter torque on the engine <NUM> may be calculated or estimated. Additionally, at least one sensing device <NUM> may be positioned adjacent to a shaft <NUM> mechanically coupling the engine <NUM> with the first propulsion assembly <NUM>, e.g., to sense or measure a performance indicator between the engine <NUM> and the first propulsion assembly <NUM>. The performance indicator may be an output torque of the engine <NUM> (or the input torque to the first propulsion assembly <NUM>), the rotational speed of the shaft <NUM>, or another suitable parameter. Like the other performance indicators, the performance indicator sensed or measured between the engine <NUM> and the first propulsion assembly <NUM> may be utilized to predict or estimate the electrical load on the engine <NUM>.

The control system <NUM> also includes one or more controllers <NUM> configured to control the various components of the hybrid electric propulsion system <NUM>. The one or more controllers <NUM> are communicatively coupled with various components of the hybrid electric propulsion system <NUM>. For this embodiment, the one or more controllers <NUM> are communicatively coupled with the one or more sensors <NUM>, the first electric machine <NUM>, the second electric machine <NUM>, the first power converter <NUM>, the second power converter <NUM>, the first propulsion assembly <NUM>, the second propulsion assembly <NUM>, the energy storage device <NUM>, the clutch <NUM>, and the engine <NUM>, and more particularly, one or more engine controllers <NUM> configured to control the engine <NUM>. The engine controller <NUM> may be, for example, an Electronic Engine Controller (EEC) or an Electronic Control Unit (ECU) equipped with Full Authority Digital Engine Control (FADEC). The engine controller <NUM> includes various components for performing various operations and functions, such as e.g., for controlling various variable geometry components and controlling a fuel flow to the combustor.

The one or more controllers <NUM> and one or more engine controllers <NUM> can each include one or more processor(s) and one or more memory device(s). The one or more processor(s) can include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, logic device, and/or other suitable processing device. The one or more memory device(s) can include one or more computer-readable media, including, but not limited to, non-transitory computer-readable media, RAM, ROM, hard drives, flash drives, and/or other memory devices.

The one or more memory device(s) can store information accessible by the one or more processor(s), including computer-readable instructions that can be executed by the one or more processor(s). The instructions can be any set of instructions that when executed by the one or more processor(s) cause the one or more processor(s) to perform operations. The instructions can be software written in any suitable programming language or can be implemented in hardware. Additionally, and/or alternatively, the instructions can be executed in logically and/or virtually separate threads on processor(s).

The memory device(s) can further store data that can be accessed by the one or more processor(s). For example, the data can include sensor data collected from the various sensing devices <NUM> of the hybrid electric propulsion system <NUM>. Specifically, the data can include one or more performance indicators indicative of the electrical load on the first electric machine <NUM>. The data can also include other data sets, parameters, outputs, information, etc. shown and/or described herein.

The one or more controllers <NUM> and engine controller <NUM> can each include a communication interface used to communicate, for example, with the other components of the aerial vehicle <NUM> and each other. The communication interface can include any suitable components for interfacing with one or more network(s), including for example, transmitters, receivers, ports, controllers, antennas, network interface components, and/or other suitable components. The one or more controllers <NUM> and one or more engine controllers <NUM> may be communicatively coupled with a communication network of the aerial vehicle <NUM>. Communication network can include, for example, a local area network (LAN), a wide area network (WAN), SATCOM network, VHF network, a HF network, a Wi-Fi network, a WiMAX network, a gatelink network, and/or any other suitable communications network for transmitting messages to and/or from the aerial vehicle <NUM> such as to a cloud computing environment and/or the off board computing systems. Such networking environments may use a wide variety of communication protocols. The communication network can include a data bus or a combination of wired and/or wireless communication links. The communication network can also be coupled to the one or more controllers <NUM>, <NUM> by one or more communication cables or by wireless means.

As noted above, the one or more controllers <NUM> are configured to control the various components of the hybrid electric propulsion system <NUM> to take correction action when a rapid electrical load change on the engine <NUM> occurs. The rapid electrical load change may be a load decrease or a load increase. In the event of a load decrease, one or more electrical components of the hybrid electric propulsion system <NUM> may fail or otherwise be controlled to drop offline. For instance, the first electric machine <NUM> may fail, the second electric machine <NUM> may fail, etc. When this occurs, the electrical load on the engine <NUM> rapidly decreases, or stated another way, the torque countering the rotation of the output shaft of the engine <NUM> rapidly decreases. Such rapid changes may lead to a number of problems as noted previously, such as overspeed of the first electric machine <NUM>, thrust asymmetry, etc. In the event of a load increase, the electrical power demanded by the power consuming devices or loads electrically coupled with the first electric machine <NUM> cannot be delivered by the first electric machine <NUM>. That is, the demanded power is greater than the available power capable of being generated by the first electric machine <NUM>. Exemplary manners in which the control system <NUM> may take correction action when a rapid electrical load change occurs are provided below.

The one or more controllers <NUM> are configured to receive, from the one or more sensing devices <NUM>, the one or more performance indicators indicative of the electrical load on the engine <NUM>. For instance, the sensing device <NUM> positioned adjacent the shaft <NUM> mechanically coupling the engine <NUM> with the first electric machine <NUM> may sense a performance indicator indicative of the torque input to the first electric machine <NUM>. Another sensing device <NUM> positioned adjacent the shaft <NUM> mechanically coupling the second electric machine <NUM> with the second propulsion assembly <NUM> may sense a performance indicator indicative of the torque output of the second electric machine <NUM>. Based on these performance indicators, and potentially others, the one or more controllers <NUM> determine the electrical load on the engine <NUM>.

Next, the one or more controllers <NUM> are configured to determine whether an electrical load change is present based at least in part on the one or more performance indicators. In some embodiments, the one or more controllers <NUM> are configured to determine an electrical load rate indicative of the electrical load on the engine <NUM> over a predetermined time interval based at least in part on the one or more performance indicators. Thereafter, the one or more controllers <NUM> are configured to compare the electrical load rate with a predetermined rate threshold. In such embodiments, if the electrical load rate exceeds the predetermined rate threshold, the one or more controllers <NUM> determine that the electrical load change is present. In other embodiments, the one of more controllers <NUM> are configured to receive, from controllers <NUM> and/or <NUM>, status information indicative of a failure.

<FIG> provides a graph of a torque load on the engine as a function of time and depicts one example manner in which it may be determined whether an electrical load change is present based at least in part on the one or more performance indicators. As shown, for a first predetermined time interval Int. <NUM> spanning from time T1 to time T2, the one or more controllers <NUM> determine an electrical load rate indicative of the electrical load on the engine <NUM> based at least in part on the one or more performance indicators. The first predetermined time interval Int. <NUM> may be a single time step or multiple time steps of the one or more controllers <NUM>, e.g., on the order of milliseconds or microseconds. In this example, the various torque inputs are provided to the controllers <NUM>, and based on the torque inputs, the controllers <NUM> calculate the total electrical load on the engine <NUM>. During the first predetermined time interval Int. <NUM>, the electrical load on the engine <NUM> remained constant, and thus, no electrical load change on the engine <NUM> was present. Accordingly, the control system <NUM> need not take corrective action during the first predetermined time interval Int.

For a second predetermined time interval Int. <NUM> spanning from time T2 to time T3, the one or more controllers <NUM> once again determine an electrical load rate indicative of the electrical load on the engine <NUM> based at least in part on the one or more performance indicators. The second predetermined time interval Int. <NUM> may span the same amount of time as the first predetermined time interval Int. Based on the torque inputs, the controllers <NUM> calculate the total electrical load on the engine <NUM>. During the second predetermined time interval Int. <NUM>, the electrical load on the engine <NUM> decreases, and thus, a load decrease on the engine <NUM> is present. Thus, the one or more controllers <NUM> compare the electrical load rate with a predetermined rate threshold RT. For this embodiment, the predetermined rate threshold RT is set such that if the electrical load rate does not exceed the threshold, an electrical load decrease on the engine will not cause overspeed damage to the first electric machine <NUM> and any resulting thrust asymmetry will not cause a significant danger to the aerial vehicle <NUM>. As shown in <FIG>, the torque load decrease on the engine <NUM> exceeds the predetermined rate threshold RT (the rate of the torque load on the engine <NUM> decreases at a faster rate than permitted by the predetermined rate threshold RT). Accordingly, in this example, the one or more controllers <NUM> determine that there is an electrical load change present as the electrical load rate exceeds the predetermined rate threshold RT. It will be appreciated that other methods are possible for determining that an electrical load change is present. For example, determination can occur based on other performance parameters such as bus electrical power, engine / motor / propeller speed, or status indications provided by controllers <NUM> or <NUM>.

Thereafter, the one or more controllers <NUM> are configured to generate, if the rapid electrical load change is present, a control action in response to the electrical load change. As will be explained further below, the appropriate control action response to the electrical load change on the engine <NUM> depends on whether the electrical load change is a load decrease (e.g., as shown in <FIG>) or a load increase.

Referring again to <FIG>, in the event of a load decrease or load drop, the one or more controllers <NUM> can generate a number of different control actions in accordance with exemplary aspects of the present disclosure. As one example, as shown in <FIG>, the control system <NUM> includes a fuel control device <NUM> configured to selectively control a fuel flow FF to the engine <NUM>. For instance, the fuel control device <NUM> may be positioned along a fuel line between a combustor assembly of the engine <NUM> and a fuel tank <NUM>. The fuel control device <NUM> is communicatively coupled with the one or more controllers <NUM>. Based on one or more controls signals from the one or more controllers <NUM>, the fuel control device <NUM> may be moved between a closed position and an open position. The fuel control device <NUM> may be movable between an infinite number of open positions and the closed position, e.g., by use of proportional control valves, or may be switchable between a single open position and a closed position. If the electrical load change is a load decrease on the engine <NUM>, in generating the control action the one or more controllers <NUM> are configured to activate the fuel control device <NUM> to reduce the fuel flow FF to the engine <NUM>. For instance, the one or more controllers <NUM> may activate the fuel control device <NUM> to move to the closed position based on one or more control signals, e.g., to reduce the fuel flow FF to the engine <NUM>. By reducing the fuel flow to the engine <NUM>, the engine <NUM> output torque is decreased, allowing the engine output torque to drop to match the torque load (or the electrical load) on the engine <NUM> more rapidly.

In some embodiments, additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the control system <NUM> includes an electrical braking system <NUM>. For the depicted embodiment of <FIG>, the electrical braking system <NUM> includes a load or resistor bank <NUM> comprised of a plurality of resistors and/or other dissipating elements. The resistor bank <NUM> may be positioned within the fuselage <NUM> or some other suitable location. The electrical braking system <NUM> also includes an electric switching device <NUM> for selectively electrically coupling the first electric machine <NUM> (or the DC Bus) and the resistor bank <NUM>. For this embodiment, the electrical switching device <NUM> is positioned within the first power converter <NUM>. In this way, the first power converter <NUM> may direct all, a portion, or some of the electrical power generated by the first electric machine <NUM> to the resistor bank <NUM>.

In such embodiments, if the electrical load change is a load decrease on the engine <NUM>, in generating the control action the one or more controllers are configured to activate the electric switching device <NUM> to electrically couple the first electric machine <NUM> and the resistor bank <NUM> for a transient time period to direct electrical power from the first electric machine <NUM> to the resistor bank <NUM>. In this way, the resistive electrical load on the engine <NUM> may be rapidly and automatically increased to accommodate the loss or drop in generator electrical load on the engine <NUM>. Notably, the electrical switching device <NUM> may be switched nearly instantaneously (e.g., within microseconds) upon the controllers <NUM> determining that an electrical load decrease is present. Thus, excess electrical power generated by the first electric machine <NUM> may be dissipated via the resistor bank <NUM>. This may, for example, prevent overspeed of the first electric machine <NUM> and reduce thrust asymmetry. Although the output torque of the second electric machine <NUM> may decrease a certain degree, the overall thrust asymmetry is reduced by transiently sinking electrical power from engine <NUM>. Particularly, electrical power may be directed to the resistor bank <NUM> for a transient time period, which may be a time sufficient for the engine <NUM> to reduce its torque output to match the torque load of the electrical system on the engine <NUM>. For instance, the transient time period may be several seconds.

In yet other embodiments, additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the control system <NUM> includes a physical braking system <NUM> mechanically coupled with the engine <NUM>. For the depicted embodiment of <FIG>, the physical braking system <NUM> includes a disc brake <NUM> mechanically coupled with the engine <NUM>. Although the disc brake <NUM> is shown positioned rear or aft of the engine <NUM> in <FIG>, the disc brake <NUM> may be positioned in other suitable locations so long as the disc brake <NUM> is mechanically coupled with the engine <NUM>. For instance, the disc brake <NUM> may be positioned forward of the engine <NUM>, e.g., between the engine <NUM> and the first electric machine <NUM>. In such embodiments, if the rapid electrical load change is a load decrease on the engine <NUM>, in generating the control action the one or more controllers are configured to activate the physical braking system <NUM> to reduce a torque output of the engine <NUM> for a transient time period. By applying a load on the engine <NUM> via the physical braking system <NUM>, the engine <NUM> output torque is decreased, allowing the engine output torque to drop to match the torque load (or the electrical load) on the engine <NUM> more rapidly. Further, the physical braking system <NUM> may prevent overspeed of the engine <NUM> and/or the first electric machine <NUM>.

In some further embodiments, additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the energy storage device <NUM> electrically coupled with the first electric machine <NUM> may be utilized by the control system <NUM> to account for rapid electrical load changes. In some embodiments, if the rapid electrical load change is a load decrease on the engine, the energy storage device <NUM> is configured to receive an amount of excess electrical power from the first electric machine <NUM> for a transient time period. The transient time period may be a time sufficient for the engine <NUM> to reduce its torque output to match the torque load of the electrical system on the engine <NUM>. The first power converter <NUM> may include a switching element that directs all, a portion, or some of the electrical power generated by the first electric machine <NUM> to the energy storage device <NUM>. For instance, some of the electrical power may be directed to the electrical braking system <NUM> and some of the electrical power may be directed to the energy storage device <NUM>. In other embodiments, a switching element may be positioned along the power bus <NUM> for selectively directed excess electrical power to the energy storage device <NUM>.

In yet further embodiments, additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the control system <NUM> may utilize one or more variable geometry components <NUM> of the engine <NUM> to account for rapid electrical load changes, e.g., by reducing the torque output of the engine <NUM>. The one or more variable geometry components <NUM> of the engine <NUM> may include variable guide vanes, such as variable inlet guide vanes and variable outlet guide vanes, as well as other variable airfoil surfaces that alter or affect the mass flow through a gas path of the engine. Other variable geometry components may include bleed valves, one or more High Pressure Turbine Active Clearance Control (HPTACC) valves, Low Pressure Turbine Active Clearance Control (LPTACC) valves, Core Compartment Cooling (CCC) valves, Booster Anti-Ice (BAI) valves, Nacelle Anti-Ice (NAI) valves, Start Bleed Valves (SBV), Transient Bleed Valves (TBV), Modulated Turbine Cooling (MTC) valves and/or combined valves. In such embodiments, if the rapid electrical load change is a load decrease on the engine <NUM>, the one or more controllers <NUM> are configured to activate one or more variable geometry components <NUM> of the engine <NUM> so that the engine <NUM> is operated in a less efficient manner for a transient time period.

For instance, upon determining that a rapid load decrease has occurred, the one or more controllers <NUM> may communicate with the one or more engine controllers <NUM>, e.g., to instruct the one or more engine controllers <NUM> to actuate one or more of the variable geometry components <NUM> of the engine <NUM> in a less efficient manner. By operating the engine <NUM> in a less efficient manner, the torque output of the engine <NUM> will decrease, and accordingly, the torque output of the engine <NUM> will decrease more rapidly so that it more rapidly matches the torque load on the engine <NUM>. The transient time period may be a time sufficient for the engine <NUM> to reduce its torque output to match the torque load of the electrical system on the engine <NUM>. When the torque output of the engine <NUM> matches the torque load of the electrical system on the engine <NUM>, the one or more controllers <NUM> may communicate with the engine controllers <NUM> to control the one or more variable geometry components <NUM> to operate in a more efficient manner, e.g., to increase the efficiency of the engine <NUM>.

In yet other embodiments, additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the control system <NUM> may utilize one or more of the propulsion assemblies <NUM>, <NUM> to account for rapid electrical load changes, e.g., by reducing the torque load on the engine <NUM>. In such embodiments, if the rapid electrical load change is a load decrease on the engine <NUM>, the one or more controllers <NUM> are configured to control the first propulsion assembly <NUM>, the second propulsion assembly <NUM>, some other propulsion assembly mechanically coupled with an electric machine of the electrical system, or some combination thereof in a less efficient manner for a transient time period. For instance, in some embodiments, the first propulsion assembly <NUM> and the second propulsion assembly <NUM> are variable pitch propeller assemblies each having a plurality of blades adjustable in unison through a plurality of pitch or blade angles, e.g., by activation of an actuation mechanism. Pitch adjustment of the blades may cause the propeller assemblies to produce more or less thrust. In the event of a load decrease on the engine <NUM>, the controllers <NUM> may communicate directly or indirectly with the activation mechanism of the propulsion assemblies <NUM>, <NUM> to pitch the blades to a more coarse or flat angle. In this manner, the torque load on the engine <NUM> may be increased and thus some of the excess torque output from the engine <NUM> may be accounted for by the propulsion assemblies <NUM>, <NUM>. Advantageously, adjustment of one or more propulsion assemblies may prevent engine/electric machine overspeed.

Additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the control system <NUM> decreases the efficiency of the first electric machine <NUM>, the second electric machine <NUM>, or some other power consuming device or load electrically coupled with the first electric machine <NUM> to account for the drop in electrical load on the engine <NUM>. If the rapid electrical load change is a load decrease on the engine <NUM>, the one or more controllers <NUM> are configured to control at least one of the first electric machine <NUM>, the second electric machine <NUM>, other power consuming devices electrically coupled with the first electrical machine <NUM> (e.g., power converters <NUM>, <NUM>, other electric machines, etc.), or some combination thereof to operate in a less efficient manner for a transient time period. In this manner, the transient excess power produced by the first electric machine <NUM> may be dissipated as heat via one or more of the electric machines or power consuming devices electrically coupled with the first electric machine <NUM>. By way of example, the phase or current advance angle, the amplitude of phase current, or some other parameter know to affect the efficiency of one or more of the electric machines may be controlled to operate one or more of the electric machines in a reduced-efficiency mode during the excess power transient. The transient time period may be a time sufficient for the engine <NUM> to reduce its torque output to match the torque load of the electrical system on the engine <NUM>. By operating the first electric machine <NUM>, the second electric machine <NUM>, or some other power consuming device or load in a less efficient or high-power manner, an electrical load may be placed on the engine <NUM> to account for the load decrease. Such transient electrical load may be placed on the engine <NUM> nearly instantaneously (e.g., within microseconds of a determination that there is a load decrease on the engine <NUM>).

In some embodiments, the first electrical machine <NUM> is an electric generator (e.g., <NUM> of <FIG>) and the second electric machine <NUM> is an electric motor (e.g., <NUM> of <FIG>) electrically coupled with the electric generator <NUM>. If the rapid electrical load change is a load decrease on the engine <NUM> (or <NUM> of <FIG>), the one or more controllers <NUM> (or <NUM> of <FIG>) are further configured to control at least one of the electric motor <NUM> or other power consuming loads (which may also be electric motors) to operate in a less efficient or high-power manner.

In some embodiments, additionally to reducing the fuel flow FF to the engine <NUM> via the fuel control device <NUM>, the control system <NUM> may direct transient excess electrical power to one or more accessory loads <NUM> of the aerial vehicle <NUM> (<FIG>). Example accessory loads <NUM> may include an air conditioning unit of the aerial vehicle <NUM>, pumps or fans, displays, data processing units, communication units, other sub-systems, some combination thereof, etc. The control system <NUM> of the hybrid electric propulsion system <NUM> includes an electric switching device <NUM> for selectively electrically coupling the first electric machine <NUM> and the one or more accessory loads <NUM> of the aerial vehicle <NUM>. For this embodiment, the electrical switching device <NUM> is positioned within the second power converter <NUM>. In this way, the second power converter <NUM> may direct all, a portion, or some of the electrical power generated by the first electric machine <NUM> to the one or more accessory loads <NUM>, e.g., as AC current. In such embodiments, if the rapid electrical load change is a load decrease on the engine <NUM>, the one or more controllers <NUM> are configured to activate the electric switching device <NUM> to electrically couple the first electric machine <NUM> and the one or more accessory loads <NUM> for a transient time period to direct electrical power from the first electric machine <NUM> to the one or more accessory loads <NUM>. By utilizing the accessories <NUM> of the aerial vehicle <NUM> to increase the electrical load on the engine <NUM> during the transient time period, the engine <NUM> may more rapidly match its torque output to the torque load of the electrical system on the engine <NUM>. It will be appreciated that electrical switching device <NUM> may be positioned in other suitable locations of the electrical system and that power switching and routing can be accomplished in any suitable fashion and in any suitable physical locations.

In the event of a load increase, the one or more controllers <NUM> can generate a number of different control actions in accordance with exemplary aspects of the present disclosure. Whether a load increase is present may be determined by the one or more controllers <NUM>, e.g., by determining an electrical load rate indicative of the electrical load on the engine <NUM> over a predetermined time interval based at least in part on the one or more performance indicators and then comparing the electrical load rate with a predetermined rate threshold. In such embodiments, if the electrical load rate exceeds the predetermined rate threshold (e.g., by having a greater slope than the slope of the predetermined threshold slope), the one or more controllers <NUM> determine that the electrical load change is present, and particularly, that a load increase is present. It will be appreciated that other suitable methods are possible for determining that an electrical load change is present. For example, determination can occur based on other performance parameters such as bus electrical power, engine / motor / propeller speed, or status indications provided by controllers <NUM> or <NUM>.

In some embodiments, if the rapid electrical load change is a load increase on the engine <NUM> and the load increase is caused by a failure of the first electric machine <NUM>, the one or more controllers <NUM> are configured to control the delivery of electrical power from the energy storage device <NUM> to the second electric machine <NUM> for a transient time period. In this way, despite the failure of the first electric machine <NUM>, electrical power may still be delivered to the second electric machine <NUM> so that the second electric machine <NUM> may drive the second propulsion assembly <NUM>. Thus, thrust asymmetry is reduced or eliminated.

In other embodiments, if the rapid electrical load change is a load increase on the engine <NUM> and the load increase is intentionally demanded, the one or more controllers <NUM> are likewise configured to control the delivery of electrical power from the energy storage device <NUM> to the second electric machine <NUM> for a transient time period. For instance, if the electrical power demanded and consumed by second electric machine <NUM> to drive the second propulsion assembly <NUM> spikes (e.g., during a vertical takeoff or landing, a hover maneuver, etc.), causing the electrical load on the engine <NUM> to rapidly increase, the one or more controllers <NUM> control the energy storage device <NUM> to deliver electrical power to the second electric machine <NUM> for a transient time period to achieve the desired power demand. In this way, the control system <NUM> may facilitate meeting the power demanded by the electrical system of the hybrid electric propulsion system <NUM>.

Further, in some embodiments, the first electric machine <NUM> may be mechanically coupled with the first propulsion assembly <NUM> (e.g., as shown in <FIG>). The first electric machine <NUM> may be directly mechanically coupled with the first propulsion assembly <NUM> or an additional clutch (not shown) may be provided therebetween to disengage the first electric machine <NUM> from the first propulsion assembly <NUM>. In the event of a catastrophic failure of the engine <NUM>, clutch <NUM> may be controlled to disengage the first electric machine <NUM> from the engine <NUM>, and first electric machine <NUM> may receive electrical power from the energy storage device <NUM>. The first electric machine <NUM>, acting as an electric motor, may drive the first propulsion assembly <NUM>. Accordingly, in some embodiments, the hybrid electric propulsion system <NUM> may switch to a fully electric system.

Moreover, in some instances, the first electric machine <NUM> may fail. Upon a failure of the first electric machine <NUM>, the amount of electrical power going to the second electric machine <NUM> decreases while the torque load on the engine <NUM> also decreases. In some embodiments, the electrical power shortfall to second electric machine <NUM> can be compensated-for by sourcing power from energy storage device <NUM>. Other methods, already addressed, can be utilized to prevent overspeed of engine <NUM> and first propulsion assembly <NUM>.

<FIG> provides a schematic view of another exemplary hybrid electric propulsion system <NUM> for an aerial vehicle in accordance with exemplary embodiments of the present disclosure. The hybrid electric propulsion system <NUM> of <FIG> is configured for driving propulsion and supplying electrical power to any suitable aerial vehicle, such as e.g., an unmanned aerial vehicle configured for vertical take-off and landing and hovering maneuvers. The exemplary hybrid electric propulsion system <NUM> of <FIG> is configured in a similar manner as the hybrid electric propulsion system <NUM> of <FIG> and <FIG> except as noted below.

In contrast with the hybrid electric propulsion system <NUM> of <FIG> and <FIG>, the hybrid electric propulsion system <NUM> of <FIG> includes a torque source or engine <NUM> that is not mechanically coupled with a propulsion assembly. Rather, for this embodiment, the engine <NUM> is configured to drive one or more electric generators <NUM>. More particularly, the engine <NUM> is configured to drive a pair of electric generators <NUM> each mechanically coupled with the engine <NUM>. Like the first electric machine <NUM> of <FIG> and <FIG>, the electric generators <NUM> are configured to generate electrical power when driven by the engine <NUM>. In some alternative embodiments, the hybrid electric propulsion system <NUM> may include one electric generator or more than two (<NUM>) electric generators.

As further shown in <FIG>, a plurality of power consuming devices <NUM> are electrically coupled with the electric generators <NUM> via power buses <NUM>. For this embodiment, the power consuming devices <NUM> include a plurality of electric motors configured to receive electrical power from one of the electric generators <NUM> and generate an output torque to drive a propulsion assembly <NUM> mechanically coupled thereto. The propulsion assemblies <NUM> may be fans, rotors, or another suitable propulsion device, for example. Moreover, for this embodiment, some of the power consuming devices <NUM> are powered by the electric generators <NUM> but are not mechanically coupled with propulsion assemblies <NUM>. For instance, such power consuming devices <NUM> may power a display, a controller <NUM> of the hybrid electric propulsion system <NUM>, avionics systems, etc. Energy storage devices <NUM> are also electrically coupled with the electric generators <NUM> and the power consuming devices <NUM>, e.g., to receive electrical power therefrom and deliver electrical power thereto. Although not shown, various power converters may be included in the hybrid electric system <NUM>, e.g., to convert the generated electrical power into desired form.

A plurality of sensing devices <NUM> are positioned to detect one or more performance indicators indicative of the electrical load or torque load on the engine <NUM>. As one example, one or more of the sensing devices <NUM> may be speed sensors configured to sense the rotational speed of the shaft or rotation component that they are positioned proximate. As another example, one or more of the sensing devices <NUM> may be torque sensors configured to measure the torque of the shaft or rotational component that they are positioned proximate. As yet another example, one or more of the sensing devices <NUM> may be configured to measure or sense an electrical parameter indicative of the torque load on the engine <NUM>, such as the current flowing through one of the electric machines, the voltage across the machine, or some other parameter. Such performance indicators may be routed to the controller <NUM>, which may be a plurality of controllers or a single controller, so that the torque load or electrical load on the engine <NUM> may be calculated or estimated. In accordance with the exemplary aspects disclosed herein, the hybrid electric propulsion system <NUM> includes a control system <NUM> configured to take rapid and automatic corrective action in the event of a rapid electric load change on the engine <NUM>, e.g., in a similar as noted above with respect to the control system <NUM> of the hybrid electric system <NUM> of <FIG> and <FIG>. For instance, the controller <NUM> of the control system <NUM> may control the various components of the hybrid electric propulsion system <NUM> in the same or similar manner as noted above. Some or all of the components noted above with respect to the control system <NUM> of the hybrid electric system <NUM> of <FIG> and <FIG> may be incorporated into the control system <NUM> of the hybrid electric propulsion system <NUM>.

<FIG> provides a flow diagram of an exemplary method (<NUM>) for operating a hybrid electric propulsion system for an aerial vehicle. For instance, the hybrid electric propulsion system and the aerial vehicle may be the hybrid electric propulsion system <NUM> and the aerial vehicle may be the aerial vehicle <NUM> of <FIG> and <FIG>. Method (<NUM>) is also applicable to the hybrid electric propulsion system <NUM> of <FIG>. For context, reference numerals utilized to describe the hybrid electric propulsion system <NUM> and the aerial vehicle <NUM> noted above and their various features will be utilized below.

At (<NUM>), the method (<NUM>) includes receiving, from one or more sensing devices positioned onboard the aerial vehicle, one or more performance indicators indicative of an electrical load on the engine, the engine mechanically coupled with an electric machine configured to generate electrical power when driven by the engine, the electric machine electrically coupled with one or more power consuming devices, wherein at least one of the one or more power consuming devices are mechanically coupled with a propulsion assembly. For instance, the sensing devices <NUM> positioned onboard the aerial vehicle <NUM> may sense one or more performance indicators indicative of the electrical load on the engine <NUM>. The performance indicators may be torque or speed of a rotational component, an electrical parameter of an electric machine or power consuming device (e.g., current, voltage, etc.), or some other indicator of the electrical load or torque load on the engine <NUM>. Some signals or measurements may then be routed to the one or more controllers <NUM> for processing. In this way, the electrical load on the engine <NUM> may be determined. A combination of measurements and estimates may be used to calculate the electrical load on the engine <NUM>.

At (<NUM>), the method (<NUM>) includes determining whether an electrical load decrease on the engine is present based at least in part on the one or more performance indicators. For instance, in some implementations, the one or more controllers <NUM> are configured to determine an electrical load rate indicative of the electrical load on the engine <NUM> over a predetermined time interval based at least in part on the one or more performance indicators. Thereafter, the one or more controllers <NUM> are configured to compare the electrical load rate with a predetermined rate threshold. In such embodiments, if the electrical load rate exceeds the predetermined rate threshold, the one or more controllers <NUM> determine that the electrical load change is present. <FIG> and the accompanying text illustrate one exemplary manner in which it may be determined whether a rapid electrical load decrease is present. In some further implementations, the torque demanded by the electrical system on the engine <NUM> may also be considered. Thus, in addition to checking if the electrical load decrease is present based on comparing the electrical load on the engine <NUM> and the predetermined rate threshold, the demanded torque of the system may be utilized to determine whether an electrical load decrease is present.

At (<NUM>), the method (<NUM>) includes generating, if the electrical load decrease on the engine is present, a control action in response to the electrical load decrease, wherein generating the control action comprises reducing a fuel flow to the engine. For instance, as noted previously, the control system <NUM> of the hybrid electric propulsion system <NUM> may include a fuel cutoff or fuel control device <NUM> configured to selectively control fuel flow FF to the engine <NUM> (e.g., as shown in <FIG>). In the event a load decrease is determined to be present, the controllers <NUM> communicatively coupled with the fuel control device <NUM> active the fuel control device <NUM> to move to toward the closed position to cut off fuel flow FF to the engine <NUM>. By reducing the fuel flow to the engine <NUM>, the engine <NUM> output torque is decreased, allowing the engine output torque to drop more rapidly to match the torque load (or the electrical load) on the engine <NUM>.

In some implementations of method (<NUM>), the control action further includes delivering an amount of excess electrical power from the electric machine to an electrical braking system for a transient time period. For instance, in some implementations, the control system <NUM> includes the electrical braking system <NUM> of <FIG>, which includes resistor bank <NUM> comprised of a plurality of resistors and/or other dissipating elements. The electrical braking system <NUM> also includes electric switching device <NUM> for selectively electrically coupling the first electric machine <NUM> and the resistor bank <NUM>. In the event a load decrease is determined to be present, the controllers <NUM> communicatively coupled with the electric switching device <NUM> activate the electric switching device <NUM> to electrically couple the electric machine and the resistor bank <NUM> for a transient time period to direct electrical power from the electric machine to the resistor bank <NUM>. In this way, the electrical load on the engine <NUM> may be rapidly and automatically increased to accommodate the loss or drop in electrical load on the engine <NUM>. Thus, excess electrical power generated by the electric machine may be dissipated via the resistor bank <NUM>. The transient time period that the resistor bank <NUM> receives electrical power may be time in which the engine <NUM> spools down to match its output torque to the electrical load on the engine <NUM>.

In some implementations of method (<NUM>), the control action further includes delivering an amount of excess electrical power from the electric machine to one or more accessory loads of the aerial vehicle for a transient time period. For instance, in some implementations as shown in <FIG>, one or more accessory loads <NUM> are electrically coupled with the electric machine, which may be the first electric machine <NUM> of the hybrid electric propulsion system <NUM> of <FIG>, for example. Accessory loads could be any power consuming load of the aerial vehicle <NUM> (<FIG>). Example accessory loads <NUM> may include an air conditioning unit of the aerial vehicle <NUM>, pumps or fans, displays, data processing units, communication units, other sub-systems, some combination thereof, etc. In some embodiments, a power management device is electrically coupled with the electric machine and is configured to selectively distribute the excess loads to the accessory loads <NUM> of the aerial vehicle <NUM>. In the event a load decrease is determined to be present, the controllers <NUM> communicatively coupled with the power management device may direct the power management device to distribute the excess electrical power to the one or more accessory loads <NUM> of the aerial vehicle <NUM>. In this way, the electrical load on the engine <NUM> may be rapidly and automatically increased to accommodate the loss or drop in electrical load on the engine <NUM>. The transient time period that the accessory loads <NUM> receive electrical power may a time period in which the engine <NUM> spools down to match its output torque to the electrical load on the engine <NUM>.

In some implementations of method (<NUM>), the control action further includes controlling one or more variable geometry components of the engine to move such that the engine is operated in a less efficient manner for a transient time period. For instance, in some implementations as shown in <FIG>, the engine <NUM> may include one or more variable geometry components <NUM>, such as e.g., variable guide vanes positioned along one or more gas paths of the engine <NUM>. In such implementations, upon determining that a rapid load decrease on the engine <NUM> has occurred at (<NUM>), based on one or more command signals from the controllers <NUM>, the one or more engine controllers <NUM> control one or more variable geometry components <NUM> of the engine <NUM> to actuate or move such that the engine <NUM> is operated in a less efficient manner for a transient time period. For instance, inlet guide vanes of a compressor of a gas turbine engine may be moved such that the vanes impart a less efficient whirling motion to airflow passing across the vanes, thereby reducing the torque output of the engine more rapidly. As noted previously, by operating the engine <NUM> in a less efficient manner, the torque output of the engine <NUM> will decrease, and accordingly, the torque output of the engine <NUM> will decrease more rapidly so that it more rapidly matches the torque load on the engine <NUM>. The transient time period may be a time sufficient for the engine <NUM> to reduce its torque output to match the torque load of the electrical system on the engine <NUM>.

In some implementations of method (<NUM>), the control action further includes controlling the electric machine and/or at least one of the one or more power consuming devices or loads to operate in a less efficient or high-power manner for a transient time period. For instance, the electric machine may be the first electric machine <NUM> of <FIG> and <FIG> and the power consuming devices may include the second electric machine <NUM> and other loads. In the event a load decrease is determined to be present, the one or more controllers <NUM> control at least one of the first electric machine <NUM>, the second electric machine <NUM>, other power consuming devices electrically coupled with the first electrical machine <NUM> (e.g., power converters <NUM>, <NUM>, other electric machines, etc.), or some combination thereof to operate in a less efficient or high-power manner for a transient time period. In this manner, the excess power produced during the transient period by the first electric machine <NUM> may be dissipated as heat via one or more of the electric machines or power consuming devices electrically coupled with the first electric machine <NUM>. The transient time period may be a time sufficient for the engine <NUM> to reduce its torque output to match the torque load of the electrical system on the engine <NUM>.

In some further implementations of method (<NUM>), the control action further includes controlling the propulsion assembly in a less efficient manner for a transient time period. For instance, the propulsion assembly may be the first propulsion assembly <NUM> of <FIG> and <FIG>. In the event a load decrease is determined to be present, the one or more controllers <NUM> may control the first propulsion assembly to operate less efficiently, e.g., by adjusting the pitch of the blades to a more coarse or flat angle (i.e., an angle opposite the feathering angle). In this manner, the torque load on the engine <NUM> may be increased and thus some of the excess torque output from the engine <NUM> may be accounted for by the propulsion assemblies <NUM>. Of course, more than one propulsion assembly may be operated less efficiently at one time, such as in the case of the propulsion assemblies <NUM> of <FIG>. Advantageously, adjustment of one or more propulsion assemblies may prevent engine/electric machine overspeed.

Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only.

Claim 1:
A hybrid electric propulsion system (<NUM>) for an aerial vehicle (<NUM>), the hybrid electric propulsion system (<NUM>) comprising:
an engine (<NUM>);
a first propulsion assembly (<NUM>) mechanically coupled with the engine and configured to produce thrust when driven by the engine;
an electrical system comprising a first electric machine (<NUM>) mechanically coupled with the engine (<NUM>) and configured to generate electrical power when driven by the engine (<NUM>), and one or more power consuming loads electrically coupled with the first electric machine (<NUM>) and configured to receive electrical power from the first electric machine (<NUM>), the one or more power consuming loads comprising a second electric machine (<NUM>);
a second propulsion assembly (<NUM>) mechanically coupled with the second electric machine (<NUM>) and configured to produce thrust when driven by the second electric machine (<NUM>);
one or more sensing devices (<NUM>) positionable onboard the aerial vehicle (<NUM>) for detecting one or more performance indicators indicative of an electrical load on the engine (<NUM>);
a fuel control device (<NUM>) configured to selectively control a fuel flow to the engine (<NUM>);
one or more controllers (<NUM>) communicatively coupled with the one or more sensing devices (<NUM>) and communicatively coupled with the fuel control device (<NUM>), the one or more controllers (<NUM>) configured to:
receive, from the one or more sensing devices (<NUM>), the one or more performance indicators indicative of the electrical load on the engine (<NUM>);
determine, based at least in part on the one or more performance indicators, an occurrence of an electrical load change indicating a thrust asymmetry, the electrical load change comprising a load decrease on the engine (<NUM>); and
generate, responsive to the occurrence of the electrical load change being a load decrease on the engine, a control action comprising (i) activating the fuel control device (<NUM>) to reduce the fuel flow to the engine (<NUM>) and (ii) decreasing an efficiency of the first and/or second electric machine (<NUM>);
wherein the control action to decrease the efficiency of the first and/or second electric machine is performed for a transient time period sufficient for the engine (<NUM>) to reduce its torque output to match the torque load of the electrical system on the engine (<NUM>).