Patent Publication Number: US-2023139190-A1

Title: Hybrid electric single engine descent energy management

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/273,559 filed Oct. 29, 2021, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The subject matter disclosed herein generally relates to rotating machinery and, more particularly, to a method and an apparatus for hybrid electric single engine descent energy management. 
     In a hybrid gas turbine engine, an electric motor can be available to assist the gas turbine engine operation by adding rotational force to a spool of the gas turbine engine while fuel flow to the gas turbine engine is reduced below idle or shut off. Such a configuration can result in non-intuitive control from a pilot perspective, depending on how the two energy sources, fuel and electricity, are expected to be managed through the range of aircraft operation. 
     BRIEF DESCRIPTION 
     According to one embodiment, a system of a hybrid aircraft includes a first gas turbine engine, a second gas turbine engine, a power source, and a controller. The first gas turbine engine includes a first electric machine. The second gas turbine engine includes a second electric machine. The controller is operable to determine an operating condition of the first gas turbine engine and the second gas turbine engine and to detect a change in a thrust command for the hybrid aircraft. The controller is further operable to determine an adjustment to the second electric machine to compensate for the change in the thrust command while the first gas turbine engine is operating in a fuel-driven mode and the second gas turbine engine is operating in an electrically-driven mode. At least a portion of electric power to perform the adjustment to the second electric machine is extracted from the power source. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the first gas turbine engine is fuel-driven, and fuel combustion is inhibited in the second gas turbine engine while a single engine descent mode is active. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the first electric machine is configured in a generator mode of operation to provide electric power to the power source and/or to the second electric machine to control a rate of thrust change in response to the thrust command. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the controller is operable to determine that the hybrid aircraft is operating in cruise, and a variation within a predetermined bandwidth of the change in the thrust command is handled by adjusting a command to the second electric machine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the first electric machine is a first high spool electric machine and the second electric machine is a second high spool electric machine, and the system can further include a first low spool electric machine of the first gas turbine engine and a second low spool electric machine of the second gas turbine engine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include an energy storage system, where the controller is operable to use a combination of two or more of the first high spool electric machine, the second high spool electric machine, the first low spool electric machine, and/or the second low spool electric machine to change a thrust of the hybrid aircraft in response to the change in the thrust command. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the power source includes an energy storage system, and the controller can be operable to provide at least a portion of power extracted to the energy storage system from one or more of the first high spool electric machine, the second high spool electric machine, the first low spool electric machine, and/or the second low spool electric machine in response to the change in the thrust command. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the controller is operable to blend a thrust response to the change in the thrust command between the first gas turbine engine and the second gas turbine engine by controlling a distribution of power between the first gas turbine engine, the second gas turbine engine, and the power source. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the change in the thrust command is accommodated by the second electric machine at a first change rate, a response to the thrust command is accommodated in part by the first gas turbine engine at a second change rate, and the first change rate is faster than the second change rate. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the controller is operable to change a designation of the first gas turbine engine and the second gas turbine engine between flights of the hybrid aircraft. 
     According to an embodiment, a method includes determining, by a controller, an operating condition of a first gas turbine engine and a second gas turbine engine of a hybrid aircraft. The first gas turbine engine includes a first electric machine, and the second gas turbine engine includes a second electric machine. The controller can detect a change in a thrust command for the hybrid aircraft. The controller can determine an adjustment to the second electric machine to compensate for the change in the thrust command while the first gas turbine engine is operating in a fuel-driven mode and the second gas turbine engine is operating in an electrically-driven mode. At least a portion of electric power can be extracted to perform the adjustment to the second electric machine from a power source of the hybrid aircraft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include combusting fuel by the first gas turbine engine and inhibiting fuel combustion in the second gas turbine engine while the single engine descent mode is active. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include determining that the hybrid aircraft is operating in cruise, and a variation within a predetermined bandwidth of the change in the thrust command is handled by adjusting a command to the second electric machine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include using a combination of two or more of the first high spool electric machine, the second high spool electric machine, the first low spool electric machine, and/or the second low spool electric machine to change a thrust of the hybrid aircraft in response to the change in the thrust command. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the power source includes an energy storage system, and at least a portion of power extracted is provided to the energy storage system from one or more of the first high spool electric machine, the second high spool electric machine, the first low spool electric machine, and/or the second low spool electric machine in response to the change in the thrust command. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include blending a thrust response to the change in the thrust command between the first gas turbine engine and the second gas turbine engine by controlling a distribution of power between the first gas turbine engine, the second gas turbine engine, and the power source. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include changing a designation of the first gas turbine engine and the second gas turbine engine between flights of the hybrid aircraft. 
     A technical effect of the apparatus, systems and methods is achieved by performing hybrid electric single engine descent energy management. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG.  1    is a schematic diagram of a hybrid aircraft, in accordance with an embodiment of the disclosure; 
         FIG.  2    is a schematic diagram of a hybrid electric propulsion system, in accordance with an embodiment of the disclosure; 
         FIG.  3    is a schematic diagram of control signal paths of a hybrid electric propulsion system, in accordance with an embodiment of the disclosure; and 
         FIG.  4    is a flow chart illustrating a method, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
       FIG.  1    schematically illustrates a hybrid aircraft  10  that includes a pair of hybrid electric propulsion systems  100 A,  100 B (also referred to as hybrid gas turbine engines  100 A,  100 B or hybrid propulsion systems  100 A,  100 B). Each of the hybrid electric propulsion systems  100 A,  100 B includes a gas turbine engine  20  (e.g., a first gas turbine engine  20 A and a second gas turbine engine  20 B) with a low speed spool  30  configured to drive rotation of a fan  42 . Each gas turbine engine  20 A,  20 B also includes a high speed spool  32  that operates at higher speeds and pressures than the low speed spool  30 . A low spool electric machine  12 A can be configured to augment rotational power of the corresponding gas turbine engine  20 A,  20 B, for instance, by driving rotation of the low speed spool  30  and fan  42  in a motor mode. The low spool electric machine  12 A can be configured to extract power from the low speed spool  30  and output electrical power in a generator mode. In some embodiments, one or more of the gas turbine engines  20 A,  20 B can include a high spool electric machine  12 B configured to drive the high speed spool  32  in a motor mode. The high spool electric machine  12 B can be configured to extract rotational power from the high speed spool  32  of the corresponding gas turbine engine  20 A,  20 B and produce electric power. At least one power source  16  of the hybrid aircraft  10  can provide at least a portion of electrical power to the electric machines  12 A,  12 B of the gas turbine engines  20 A,  20 B and/or other components of the hybrid aircraft  10 . The power source  16  can be a stored energy source or a generator driven by an engine. For example, the power source  16  can include one or more of a battery, a supercapacitor, an ultracapacitor, a fuel cell, a flywheel, and the like. Where the hybrid aircraft  10  includes an additional thermal engine (not depicted), such as an auxiliary power unit or a supplemental power unit, the power source  16  can be a generator driven by the thermal engine. 
     Further, electric machines  12 A,  12 B of one of the hybrid electric propulsion systems  100 A,  100 B can provide power to the other hybrid electric propulsion systems  100 A,  100 B and/or power to the power source  16 . For example, if the hybrid electric propulsion system  100 A is combusting fuel, the hybrid electric propulsion system  100 B may operate without burning fuel and can drive the low speed spool  30  and fan  42  based on either or both of the electric machines  12 A,  12 B of the hybrid electric propulsion system  100 B receiving electric power from either or both of the electric machines  12 A,  12 B of the hybrid electric propulsion system  100 A and/or the power source  16 . Further, if the hybrid electric propulsion system  100 B is combusting fuel, the low speed spool  30  of the hybrid electric propulsion system  100 A can be driven based on either or both of the electric machines  12 A,  12 B of the hybrid electric propulsion system  100 A receiving electric power from either or both of the electric machines  12 A,  12 B of the hybrid electric propulsion system  100 B and/or the power source  16 . 
     While the example of  FIG.  1    illustrates a simplified example of the gas turbine engines  20 A,  20 B, it will be understood that any number of spools, and inclusion or omission of other elements and subsystems are contemplated. Further, rotor systems described herein can be used in a variety of applications and need not be limited to gas turbine engines for aircraft applications. For example, rotor systems can be included in power generation systems, which may be ground-based as a fixed position or mobile system, and other such applications. Further, each of the electric machines  12 A,  12 B can be separated and implemented as a separate electric motor and a generator rather than switching each of the electric machines  12 A,  12 B between a motor mode and a generator mode. 
       FIG.  2    illustrates a hybrid electric propulsion system  100  (also referred to as hybrid gas turbine engine  100  or hybrid propulsion system  100 ) as a further example of the hybrid electric propulsion system  100 A,  100 B of  FIG.  1   . In the example of  FIG.  2   , the hybrid electric propulsion system  100  includes gas turbine engine  20  operably coupled to an electrical power system  210  as part of a hybrid electric aircraft, such as hybrid aircraft  10  of  FIG.  1   . One or more mechanical power transmissions  150  (e.g.,  150 A,  150 B) can be operably coupled between the gas turbine engine  20  (e.g., first gas turbine engine  20 A, second gas turbine engine  20 B) and the electrical power system  210 . The gas turbine engine  20  includes one or more spools, such as low speed spool  30  and high speed spool  32 , each with at least one compressor section and at least one turbine section operably coupled to a shaft (e.g., low pressure compressor  44  and low pressure turbine  46  coupled to inner shaft  40  and high pressure compressor  52  and high pressure turbine  54  coupled to outer shaft  50 ). The electrical power system  210  can include a low spool electric machine  12 A configured to augment rotational power of the low speed spool  30  and a high spool electric machine  12 B configured to augment rotational power of the high speed spool  32 . The low spool electric machine  12 A can control thrust by driving rotation of the fan  42 , and the high spool electric machine  12 B can act as a motor in driving rotation of the high speed spool  32 . 
     Although two electric machines  12 A,  12 B are depicted in  FIG.  2   , it will be understood that there may be only a single electric machine (e.g., only high spool electric machine  12 B) or additional motors (not depicted). Further, the electric machines  12 A,  12 B can be electric motors/generators or alternate power sources may be used, such as hydraulic motors, pneumatic motors, and other such types of motors known in the art. In some embodiments, the electrical power system  210  can include a low spool generator  213 A configured to convert rotational power of the low speed spool  30  to electric power and/or a high spool generator  213 B configured to convert rotational power of the high speed spool  32  to electric power. For example, where the low spool electric machine  12 A is implemented as an electric motor, the low spool generator  213 A can be a separate component, and/or where the high spool electric machine  12 B is implemented as an electric motor, the high spool generator  213 B can be a separate component. The combination of low spool electric machine  12 A and low spool generator  213 A can be collectively referred to as an electric machine or a motor-generator, and similarly, the combination of high spool electric machine  12 B and high spool generator  213 B can be collectively referred to as an electric machine or a motor-generator. Further, although two electric generators  213 A,  213 B (generally referred to as generators  213 ) are depicted in  FIG.  2   , it will be understood that there may be only a single electric generator (e.g., only electric generator  213 B) or additional electric generators (not depicted). In some embodiments, one or more of the electric machines  12 A,  12 B can be configured as a motor or a generator depending upon an operational mode or system configuration, and thus one or more of the electric generators  213 A,  213 B may be omitted. 
     In the example of  FIG.  2   , the mechanical power transmission  150 A includes a gearbox operably coupled between the inner shaft  40  and a combination of the low spool electric machine  12 A and low spool generator  213 A. The mechanical power transmission  150 B can include a gearbox operably coupled between the outer shaft  50  and a combination of the high spool electric machine  12 B and high spool generator  213 B. In embodiments where the electric machines  12 A,  12 B are configurable between a motor and generator operating mode, the mechanical power transmission  150 A,  150 B can include a clutch or other interfacing element(s). 
     The electrical power system  210  can also include electric machine drive electronics  214 A,  214 B operable to condition current to/from the electric machines  12 A,  12 B. The electrical power system  210  can also include rectifier electronics  215 A,  215 B operable to condition current from the electric generators  213 A,  213 B (e.g., AC-to-DC converters). The electric machine drive electronics  214 A,  214 B and rectifier electronics  215 A,  215 B can interface with an energy storage management system  216  that further interfaces with an energy storage system  218 . The energy storage management system  216  can be a bi-directional DC-DC converter that regulates voltages between energy storage system  218  and electronics  214 A,  214 B,  215 A,  215 B. The energy storage system  218  can include one or more energy storage devices, such as a battery, a supercapacitor, an ultracapacitor, and the like. The energy storage management system  216  can facilitate various power transfers within the hybrid electric propulsion system  100 . The energy storage management system  216  may also transfer power to/from one or more electric machines on the engine, or to external loads  217  and receive power from one or more external power sources  219  (e.g., power source  16  of  FIG.  1   , aircraft power, auxiliary power unit power, supplemental power unit, cross-engine power, and the like). 
     A power conditioning unit  220  and/or other components can be powered by the energy storage system  218 . The power conditioning unit  220  can distribute electric power to support actuation and other functions of the gas turbine engine  20 . For example, the power conditioning unit  220  can power an integrated fuel control unit  222  to control fuel flow to the gas turbine engine  20 . The power conditioning unit  220  can also power a plurality of actuators (not depicted), such as bleed actuators, vane actuators, and the like. 
     One or more accessories  70  can also be driven by or otherwise interface with the gas turbine engine  20 . Examples of accessories  70  can include oil pumps, fuel pumps, and other such components. As one example, the accessories  70  include an oil pump driven through gearing, such as mechanical power transmission  150 B, in response to rotation of the high speed spool  32  and/or the high spool electric machine  12 B. Alternatively, accessories  70  can be electrically driven through power provided by the energy storage management system  216  or other such sources of electrical power. 
     Engagement and operation of the low spool electric machine  12 A, low spool generator  213 A, high spool electric machine  12 B, and high spool generator  213 B can change depending upon an operating state of the gas turbine engine  20  and any commands received. Collectively, any effectors that can change a state of the gas turbine engine  20  and/or the electrical power system  210  may be referred to as hybrid electric system control effectors  240 . Examples of the hybrid electric system control effectors  240  can include the electric machines  12 A,  12 B, electric generators  213 A,  213 B, integrated fuel control unit  222 , and/or other elements (not depicted). 
       FIG.  3    is a schematic diagram of control signal paths  250  of the hybrid electric propulsion system  100  of  FIG.  2    and is described with continued reference to  FIGS.  1  and  2   . A controller  256  can interface with the electric machine drive electronics  214 A,  214 B, rectifier electronics  215 A,  215 B, energy storage management system  216 , integrated fuel control unit  222 , accessories  70 , and/or other components (not depicted) of the hybrid electric propulsion system  100 . In embodiments, the controller  256  can control and monitor for fault conditions of the gas turbine engine  20  and/or the electrical power system  210 . For example, the controller  256  can be integrally formed or otherwise in communication with a full authority digital engine control (FADEC) of the gas turbine engine  20 . Alternatively, the controller  256  can be an aircraft level control or be distributed between one or more systems of the hybrid aircraft  10  of  FIG.  1   . In embodiments, the controller  256  can include a processing system  260 , a memory system  262 , and an input/output interface  264 . The controller  256  can also include various operational controls, such as a hybrid engine control  266  that controls the hybrid electric system control effectors  240  further described herein, for instance, based on a thrust command  270 . The thrust command  270  can be a throttle lever angle or a command derived based on a throttle lever angle control of the hybrid aircraft  10  of  FIG.  1   . 
     The processing system  260  can include any type or combination of central processing unit (CPU), including one or more of: a microprocessor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. The memory system  262  can store data and instructions that are executed by the processing system  260 . In embodiments, the memory system  262  may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and algorithms in a non-transitory form. The input/output interface  264  is configured to collect sensor data from the one or more system sensors and interface with various components and subsystems, such as components of the electric machine drive electronics  214 A,  214 B, rectifier electronics  215 A,  215 B, energy storage management system  216 , integrated fuel control unit  222 , accessories  70 , and/or other components (not depicted) of the hybrid electric propulsion system  100 . The controller  256  provides a means for controlling the hybrid electric system control effectors  240  using a hybrid engine control  266  that can be dynamically updated during operation of the hybrid electric propulsion system  100 . The means for controlling the hybrid electric system control effectors  240  can be otherwise subdivided, distributed, or combined with other control elements. 
     The controller  256  with hybrid engine control  266  can apply control laws and access/update models to determine how to control and transfer power between the low speed spool  30  and high speed spool  32 , as well as power transfers between multiple gas turbine engines  20 . For example, sensed and/or derived parameters related to speed, flow rate, pressure ratios, temperature, thrust, and the like can be used to establish operational schedules and transition limits to maintain efficient operation of the gas turbine engine  20 , as well as for the hybrid aircraft  10  collectively. For instance, an operating mode of the gas turbine engine  20 , such as idle, takeoff, climb, cruise, and descent can have different power settings, thrust requirements, flow requirements, and temperature effects. With respect to the hybrid aircraft  10  of  FIG.  1   , each of the gas turbine engines  20 A,  20 B can have different settings and splits between electric and fuel-burn based operations in one or more of the operating modes. The hybrid engine control  266  can control electric current provided to the low spool electric machine  12 A and high spool electric machine  12 B in a motor mode and loading effects of the low spool electric machine  12 A and high spool electric machine  12 B in a generator mode. The hybrid engine control  266  can also determine a power split between delivering fuel to the combustor  56  and using the low spool electric machine  12 A and/or high spool electric machine  12 B to power rotation within the gas turbine engine  20 . 
     In embodiments, the controller  256  can blend the power distribution between the hybrid electric system control effectors  240  and fuel burn in the combustor  56 . From a pilot&#39;s perspective, the setting of a throttle lever angle produces thrust command  270  without the pilot having to distinguish between whether motor-based thrust or fuel burn based thrust is needed, although the pilot may control whether fuel is on or off. With respect to the hybrid aircraft  10 , the hybrid electric propulsion systems  100 A,  100 B can be independently controlled such that one of the hybrid electric propulsion systems  100 A,  100 B is operating in a fuel burning mode while the other of the hybrid electric propulsion systems  100 A,  100 B is operated using the low spool electric machine  12 A and/or the high spool electric machine  12 B. Such a mixed operating mode may be used, for instance, during descent of the hybrid aircraft  10 , where thrust is desired from both gas turbine engines  20 A,  20 B, but only one of the gas turbine engines  20 A,  20 B actively burns fuel. 
     In embodiments, the controller  256  can perform thrust balancing between a first gas turbine engine  20 A of the first hybrid electric propulsion system  100 A and a second gas turbine engine  20 B of the second hybrid electric propulsion system  100 B prior to activation of single engine descent mode and after activation of single engine descent mode. During descent of the hybrid aircraft  10  of  FIG.  1   , both gas turbine engines  20 A,  20 B may combust fuel to maintain a desired airspeed. As thrust demand is reduced for the hybrid aircraft  10 , one of the gas turbine engines  20 A,  20 B may have a sufficient capacity to produce thrust without the other gas turbine engine  20 A,  20 B producing thrust. However, operating only one of the gas turbine engines  20 A,  20 B through combusting fuel can result in an imbalance and could result in performance issues if the fuel combusting engine experiences a fault while the depowered engine takes time to get up to speed for relighting. Embodiments can operate the fuel burning engine with an operating margin to extract power and provide electric power to either or both of the low spool electric machine  12 A and the high spool electric machine  12 B of the gas turbine engine  20  not combusting fuel such that the resulting thrust from being electrically driven substantially balances the thrust produced by the gas turbine engine  20  that is fuel driven. Supplemental electrical power can be provided from the energy storage system  218  and/or one or more external power sources  219  as needed. 
     The controller  256  can perform energy management and balance power extraction from the high speed spool  32 , low speed spool  30 , energy storage system  218 , and/or external power sources  219  to power either or both of the low spool electric machine  12 A and the high spool electric machine  12 B of the gas turbine engine  20  being electrically driven. 
     In some aspects, power transfers are performed for relatively short durations, such as the energy storage system  218  and/or one or more external power sources  219  providing electric current to either or both of the low spool electric machine  12 A and/or the high spool electric machine  12 B of the second gas turbine engine  20 B in an electrically-driven mode responsive to a change in the thrust command  270 . Similarly, the energy storage system  218  and/or one or more external power sources  219  can absorb power for relatively short durations, where excess electric current is available from one or more of the low spool electric machine  12 A and/or the high spool electric machine  12 B of the first gas turbine engine  20 A and/or second gas turbine engine  20 B operating in a generator mode. 
     As one example, if the low spool electric machine  12 A and/or the high spool electric machine  12 B of the second gas turbine engine  20 B are operating in a motor mode and have a change in electric current demand, rather than using only the energy storage system  218  and/or one or more external power sources  219  to meet the demand, the energy storage system  218  and/or one or more external power sources  219  can initially source current. The first gas turbine engine  20 A can spool up to increase power output and use either or both of the low spool electric machine  12 A and/or the high spool electric machine  12 B in a generator mode to output current to the power the low spool electric machine  12 A and/or the high spool electric machine  12 B of the second gas turbine engine  20 B in a motor mode. 
     As another example, the first gas turbine engine  20 A can be driven to operate at a localized optimized speed or thrust setting. Changes to the thrust command  270  can occur as temporary adjustments, oscillations, or longer-term changes. The energy storage system  218  and/or one or more external power sources  219  can be used to provide power to the low spool electric machine  12 A and/or the high spool electric machine  12 B of the second gas turbine engine  20 B to add thrust while maintaining the first gas turbine engine  20 A at the localized operating set point until another condition change is detected. For instance, there can be one or more energy usage thresholds defined that upon consuming stored energy from the energy storage system  218  down to a lower threshold, a set point of the first gas turbine engine  20 A is adjusted to produce extra power for use by the low spool electric machine  12 A and/or the high spool electric machine  12 B in a generator mode. Keeping the first gas turbine engine  20 A operating at the localized optimized speed or thrust setting for as long as possible can enhance fuel savings. Transitions between power sources can be tapered or blended to avoid rapid changes. Further, changes in thrust demand can be distributed between both the first gas turbine engine  20 A and the second gas turbine engine  20 B with different rates of change to absorb the impact. Since rapid changes in operation can have a great impact on thermal conditions, clearances, and component life, blending changes in the thrust command  270  between multiple components can reduce the impact on any one component. 
     As a further example, energy management can be performed using different approaches at various flight phases. For instance, during cruise or other phases having a substantially steady-state thrust command, the controller  256  can be configured to run to the most efficient point, stay there as long as possible and only move off the efficient point if needed, or a low power condition of the energy storage system  218  and/or external power sources  219  occurs or is predicted. For example, the fuel-driven engine can run very efficiently and may provide 90% or more of the thrust needed for the electrically-driven engine. The fuel-driven engine can be operated at a local minimum of an efficiency curve. The controller  256  can push the fuel-driven engine to operate at the local minimum, and the energy storage system  218  and/or external power sources  219  can be used to offset as needed. 
     One approach to implement response smoothing is to use filtering such that higher rate of change commands pass through to an electric component, such as the low spool electric machine  12 A and/or the high spool electric machine  12 B of the second gas turbine engine  20 B to make thrust changes in a motor mode. Slower rate of changes in the thrust command  270  can be filtered and passed to the first gas turbine engine  20 A for more gradually implemented changes. The conditions can be managed through transfer functions implemented within the controller  256  and need not be explicitly checked through conditional logic. 
     To provide electrical power from the gas turbine engine  20  being fuel-driven, fuel flow to the gas turbine engine  20  may increase to accommodate the loading of the low spool electric machine  12 A and/or the high spool electric machine  12 B operating in a generator mode. As one example, the high spool electric machine  12 B of the gas turbine engine  20  being fuel-driven may be initially used as the primary source of electrical power as the gas turbine engine  20  is driven above idle during descent. If the speed of the fan  42  of the gas turbine engine  20  being fuel-driven results in excess thrust, the low spool electric machine  12 A of the gas turbine engine  20  being fuel-driven can be used to extract power and add loading. The controller  256  can be configured as a dynamic multi-variable control that determines power distribution based on thrust and/or speed of the fan  42  (N 1 ) to maintain minimum fuel flow and airflow requirements and provide electrical power for the gas turbine engine  20  being electrically driven. The electrical power can be distributed between the low spool electric machine  12 A and/or the high spool electric machine  12 B of the gas turbine engine  20  being electrically driven. For example, the high spool electric machine  12 B of the gas turbine engine  20  being electrically driven may be the primary motor to maintain relight conditions for rapid restarting. The low spool electric machine  12 A of the gas turbine engine  20  being electrically driven can be directly driven to make more rapid thrust adjustments. Control loops can balance electrical production by the gas turbine engine  20  being fuel driven with electrical demand by the gas turbine engine  20  being electrically driven and electrical power available from the energy storage system  218  and/or external power sources  219 . 
     Referring now to  FIG.  4    with continued reference to  FIGS.  1 - 3   ,  FIG.  4    is a flow chart illustrating a method  400  for providing hybrid electric single engine descent energy management, in accordance with an embodiment. The method  400  may be performed, for example, by the hybrid aircraft  10  through the hybrid electric propulsion systems  100 A,  100 B of  FIG.  1   . For purposes of explanation, the method  400  is described primarily with respect to the hybrid electric propulsion system  100  of  FIG.  2   ; however, it will be understood that the method  400  can be performed on other configurations (not depicted). 
     Method  400  pertains to the controller  256  executing embedded code for power extraction, energy management, transfer, and thrust control using hybrid engine control  266  along with other control functions, where the controller  256  can be an aircraft-level control or distributed between aircraft and engine system levels of control. At block  402 , the controller  256  can determine an operating condition of the first gas turbine engine  20 A and the second gas turbine engine  20 B. A first electric machine  12  of the first gas turbine engine  20 A can be a first high spool electric machine  12 B, and a second electric machine  12  of the second gas turbine engine  20 B can be a second high spool electric machine  12 B. The first gas turbine engine  20 A can include a first low spool electric machine  12 A, and the second gas turbine engine  20 B can include a second low spool electric machine  12 A. 
     The controller  256  can receive a thrust command  270  for each gas turbine engine  20 A,  20 B, where each gas turbine engine  20 A,  20 B includes a low speed spool  30 , a high speed spool  32 , and a combustor  56 . The thrust command  270  can be different between the first and second gas turbine engines  20 A,  20 B, or the thrust command  270  can be the same for both the first and second gas turbine engines  20 A,  20 B. At block  404 , the controller  256  can detect a change in a thrust command  270  for the hybrid aircraft  10 . 
     Energy management for either or both of the first low spool electric machine  12 A and the first high spool electric machine  12 B of the first gas turbine engine  20 A can be performed while a single engine descent mode is active and in other modes of operation. While single engine descent mode is active, fuel combustion can be commanded as a complete shut off of fuel flow to prevent fuel burn depending upon an operating state of the second gas turbine engine  20 B. The first gas turbine engine  20 A of hybrid electric propulsion system  100 A can combust fuel, and fuel combustion can be inhibited in the second gas turbine engine  20 B of hybrid electric propulsion system  100 B for at least a portion of the time while the single engine descent mode is active and in other modes of operation. For example, the controller  256  can output a command of no fuel, fuel flow off, and/or otherwise effectively disable or reduce fuel flow as targeted. The operating state can depend on a combination of commands, conditions, and modes, such as an e-taxi mode, a starting mode, a ground idle mode, a takeoff mode, a climb mode, a cruise mode, an in-flight idle mode, a descent mode, a landing mode, and other such modes. The controller  256  can determine an allocation of the thrust command  270  between commanding fuel flow to the combustor  56  and electric current to the low spool electric machine  12 A and/or high spool electric machine  12 B based on the operating state of the first and second gas turbine engines  20 A,  20 B and a throttle lever angle, where the throttle lever angle can be received from a pilot control, an auto-pilot control, or other such source on the hybrid aircraft  10 . In a motor mode, the low spool electric machine  12 A and/or the high spool electric machine  12 B can be powered by one or more of a generator, an energy storage system  218 , and a power source  16  external to the gas turbine engine  20 . The single engine descent mode can be active based on determining that the first gas turbine engine  20 A has a thrust generation capacity to maintain a targeted airspeed during descent of the hybrid aircraft  10 . 
     The power source  16  can include an energy storage system  218  and/or one or more external power sources  219 . The controller  256  can be operable to provide at least a portion of the power extracted from either or both of the first low spool electric machine  12 A and the first high spool electric machine  12 B to the energy storage system  218 . The controller  256  can be operable to power either or both of the second low spool electric machine  12 A and the second high spool electric machine  12 B at least in part by one or more of the energy storage system  218  and/or another power source external to the second gas turbine engine  20 B (e.g., external power source  219 ). 
     At block  406 , the controller  256  can determine an adjustment to the second electric machine  12  to compensate for the change in the thrust command  270  while the first gas turbine engine  20 A is operating in a fuel-driven mode, and the second gas turbine engine  20 B is operating in an electrically-driven mode. Electric power can be provided to either or both of the second low spool electric machine  12 A and the second high spool electric machine  12 B while the single engine descent mode is active to balance thrust between the first gas turbine engine  20 A and the second gas turbine engine  20 B. 
     At block  408 , the controller  256  can extract at least a portion of electric power to perform the adjustment to the second electric machine  12  from the power source  16 . As an example, the controller  256  can control a high spool electric machine  12 B to accelerate the high speed spool  32  and augment rotational power of the high speed spool  32 , while the low spool electric machine  12 A can control thrust produced by the low speed spool  30 . The controller  256  can be operable to provide power to maintain relight readiness of the second gas turbine engine  20 B. 
     Power source selection can depend on the available power and allocation of power between systems of the hybrid aircraft  10 . For instance, using electric power from one of the gas turbine engines  20  burning fuel can allow that engine to operate at a higher thermal efficiency by using a higher power setting. A greater amount of battery power or other stored energy from the energy storage system  218  may be available after a recharge event. Some embodiments can support recharging during operation of the hybrid aircraft  10 , such as during cruise. 
     In embodiments, the first electric machine  12  can be configured in a generator mode of operation to provide electric power to the power source  16  and/or to the second electric machine to control a rate of thrust change in response to the thrust command  270 . The controller  256  can be operable to determine that the hybrid aircraft  10  is operating in cruise, and a variation within a predetermined bandwidth of the change in the thrust command  270  can be handled by adjusting a command to the second electric machine  12 . The controller  256  can be operable to use a combination of two or more of the first high spool electric machine  12 B, the second high spool electric machine  12 B, the first low spool electric machine  12 A, and/or the second low spool electric machine  12 A to change a thrust of the hybrid aircraft  10  in response to the change in the thrust command  270 . The controller  256  can be operable to provide at least a portion of power extracted to the energy storage system  218  from one or more of the first high spool electric machine  12 B, the second high spool electric machine  12 B, the first low spool electric machine  12 A, and/or the second low spool electric machine  12 A in response to the change in the thrust command  270 . The controller  256  can be operable to blend a thrust response to the change in the thrust command  270  between the first gas turbine engine  20 A and the second gas turbine engine  20 B by controlling a distribution of power between the first gas turbine engine  20 A, the second gas turbine engine  20 B, and the power source  16 . Further, the change in the thrust command  270  can be accommodated by the second electric machine  12  at a first change rate. A response to the thrust command  270  can be accommodated in part by the first gas turbine engine  20 A at a second change rate, and the first change rate can be faster than the second change rate. 
     A designation of the first gas turbine engine  20 A and the second gas turbine engine  20 B can be changed between flights of the hybrid aircraft  10  to alternate which engine is burning fuel when single engine descent mode is active while the other operates on electric power. The designation needed not change for each flight and may be based on various selection criteria, such as deterioration, in order to optimize fleet management. 
     Embodiments of the invention can provide a number of advantages and benefits. For instance, compared to conventional descent, fuel burn can be reduced. Using the energy storage system  218  with recharging during cruise can support the use of stored energy collected nearer to cruise efficiency to power descent. Driving rotation of the fan  42  of both gas turbine engines  20 A,  20 B can reduce a yawing moment and improve aerodynamics of the hybrid aircraft  10  during descent as compared to fully shutting down one of the gas turbine engines  20 A,  20 B. This can also improve engine thermal efficiency of the gas turbine engine  20  that is fuel-driven by continuing to burn fuel with higher power operation and improve engine restarting by keeping components of the electrically-driven gas turbine engine  20  rotating. Further, one or more accessories  70  of the first gas turbine engine  20 A and one or more accessories  70  of the second gas turbine engine  20 B can be powered while the single engine descent mode is active. 
     While the above description has described the flow process of  FIG.  4    in a particular order, it should be appreciated that unless otherwise specifically required in the attached claims that the ordering of the steps may be varied. Also, it is clear to one of ordinary skill in the art that, the asymmetric hybrid aircraft idle described herein can be combined with aircraft and propulsion system control features, such as fuel flow control, power management, emergency operation, and the like. 
     The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.