Dual hybrid propulsion system for an aircraft having a cross-connecting clutch

A propulsion system for an aircraft is disclosed, and includes a first propeller, a second propeller, a first hybrid propulsion system, a second hybrid propulsion system, and a cross-connecting clutch. The first hybrid propulsion system includes a first motor coupled to a first engine by a first overrunning clutch, where the first hybrid propulsion system is operably coupled to drive the first propeller. The second hybrid propulsion system includes a second motor coupled to a second engine by a second overrunning clutch, where the second hybrid propulsion system is operably coupled to drive the second propeller. The cross-connecting clutch is operably coupled to both the first hybrid propulsion system and the second hybrid propulsion system and configured to actuate into an engaged position.

INTRODUCTION

The present disclosure relates to a propulsion system for an aircraft. More particularly, the present disclosure is directed towards an aircraft propulsion system having a first hybrid propulsion system operably coupled to a first propeller, a second hybrid propulsion system operably coupled to a second propeller, and a cross-connecting clutch.

BACKGROUND

A commuter aircraft is typically smaller in size, carries a limited number of passengers, and may be used to travel relatively short distances. A commuter aircraft may either be propeller-driven or, alternatively, driven by gas turbines, which are also referred to as jet engines. In general, propeller-driven aircraft are usually employed when flying shorter distances while gas turbines are employed for longer distances. For example, a propeller-driven aircraft may be used to transport super-commuters between their place of residence and their work. Some super-commuters may live in a different geographic region than their workplace, while other super-commuters live outside or on the outskirts of a major metropolitan area where their workplace is located. Thus, a propeller-driven aircraft may be used frequently to transport commuters back and forth between their residence and work.

SUMMARY

According to several aspects, a propulsion system for an aircraft is disclosed. The propulsion system comprises a first propeller and a second propeller. The propulsion system also includes a first hybrid propulsion system including a first motor coupled to a first engine by a first overrunning clutch, where the first hybrid propulsion system is operably coupled to drive the first propeller. The propulsion system also includes a second hybrid propulsion system including a second motor coupled to a second engine by a second overrunning clutch, where the second hybrid propulsion system is operably coupled to drive the second propeller. The propulsion system also includes a cross-connecting clutch operably coupled to both the first hybrid propulsion system and the second hybrid propulsion system and configured to actuate into an engaged position.

In another aspect, a method of operating a propulsion system for an aircraft is disclosed. The method includes monitoring, by a control module, operation of a first engine and a second engine. The first engine is part of a first hybrid propulsion system that is operably coupled to a first propeller and the second engine is part of a second hybrid propulsion system operably coupled to a second propeller. The method includes determining, by the control module, either the first engine or the second engine is not outputting a required amount of torque to maintain a desired phase of flight. The first hybrid propulsion system includes a first motor coupled to the first engine by a first overrunning clutch and the second hybrid propulsion system includes a second motor coupled to the second engine by a second overrunning clutch. In response to determining either the first engine or the second engine is not outputting the required torque, the method includes instructing a cross-connecting clutch to actuate from a disengaged position into an engaged position.

In yet another aspect, an aircraft is disclosed. The aircraft includes a nacelle, a first propeller, and a second propeller, where the first propeller and the second propeller are attached to and located on opposing sides of the nacelle. The aircraft also includes a first hybrid propulsion system including a first motor coupled to a first engine by a first overrunning clutch, where the first hybrid propulsion system is operably coupled to drive the first propeller. The aircraft also includes a second hybrid propulsion system including a second motor coupled to a second engine by a second overrunning clutch, where the second hybrid propulsion system is operably coupled to drive the second propeller. The aircraft also includes a cross-connecting clutch operably coupled to both the first hybrid propulsion system and the second hybrid propulsion system and configured to actuate into an engaged position. Engaging the cross-connecting clutch results in the first hybrid propulsion system driving the second propeller when the second engine does not output a required torque to maintain a desired phase of flight of the aircraft, and engaging the cross-connecting clutch results in the second hybrid propulsion system driving the first propeller when the first engine does not output the required torque to maintain the desired phase of flight of the aircraft.

DETAILED DESCRIPTION

The present disclosure is directed towards a propulsion system for a propeller-driven aircraft. The aircraft includes a first hybrid propulsion system that is operably coupled to and drives a first propeller. The aircraft also includes a second hybrid propulsion system that is operably coupled to and drives a second propeller. The aircraft also includes a cross-connecting clutch that is operably coupled to both the first hybrid propulsion system and the second hybrid propulsion system. The first hybrid propulsion system drives both the first propeller and the second propeller when the cross-connecting clutch is in the engaged position. Similarly, the second hybrid propulsion system drives both the first propeller and the second propeller when the clutch is in the engaged position. Accordingly, in the event one of the two hybrid propulsion systems become inoperable, both propellers may still be driven.

Additionally, each hybrid system includes an engine, a motor, and an overrunning clutch that operably couples the engine to the motor. The overrunning clutch is configured to engage when the torque produced by the engine exceeds the torque produced by the motor such as, for example, when the engine is initially started. Moreover, the engine of each hybrid propulsion system is sized to accommodate the power requirements of the aircraft during the cruise phase of flight. For phases of flight other than cruise, such as during the climb phase of flight, the motor may be utilized to augment the torque produced by the engine such that the torque supplied to the propeller is a sum of the torque provided by the engine and the motor. In contrast, conventional systems may have engines that are sized to accommodate the climb phase of flight.

Referring toFIG. 1, a propulsion system10for an aircraft12is shown. The aircraft12is a propeller-driven aircraft, and therefore includes a first propeller20A and a second propeller20B that are attached to and located on opposing sides14of a nacelle16. The propulsion system10includes a first hybrid propulsion system22A, a second hybrid propulsion system22B, a cross-connecting clutch26, an exhaust system28, a tail cone fan30, and a control module32in electronic communication with the cross-connecting clutch26. The first hybrid propulsion system22A, the second hybrid propulsion system22B, the cross-connecting clutch26, and the exhaust system28are all located within the nacelle16of the aircraft12. The first hybrid propulsion system22A is operably coupled to and drives the first propeller20A. Similarly, the second hybrid propulsion system22B is operably coupled to and drives the second propeller20B.

The first hybrid propulsion system22A includes a first engine36A, a first motor38A, a first overrunning clutch40A, and a first turbocharger42A. The first overrunning clutch40A couples the first engine36A and the first motor38A to one another when engaged and decouples the first engine36A from the first motor38A when disengaged. When the first overrunning clutch40A is engaged, the first engine36A and the first motor38A are part of a torque summing arrangement. The first overrunning clutch40A and the second overrunning clutch40B are one-way overrunning clutches such as sprag clutches.

The first overrunning clutch40A is connected to an output44A of the first engine36A and an input (not shown) of the first motor38A. An output46A of the first motor38A is connected to a first final drive gearbox48A. Similarly, the second hybrid propulsion system22B includes a second engine36B, a second motor38B, a second overrunning clutch40B, and a second turbocharger42B. The second overrunning clutch40B is connected to an output44B of the second engine36B and an input (not shown) of the second motor38B. An output46B of the second motor38B is connected to a second final drive gearbox48B.

In the non-limiting embodiment as shown inFIG. 1, the first engine36A and the second engine36B are illustrated as turbocharged six-cylinder inline diesel engines. In one specific embodiment, the engines36A and36B are compression ignition (i.e., diesel) four-stroke engines, as a compression ignition engine is more efficient than a spark ignition engine. However, it is to be appreciated that the engines36A,36B are not limited to compression ignition engines. In an alternative embodiment, the engines36A,36B are spark ignition engines. Furthermore, although four-stroke engines are mentioned, a two-stroke engine may also be used. Two-stroke engines tend to have lower maintenance costs, as a two-stroke engine has a more simplified valve train when compared to a four-stroke engine. The first motor38A and the second motor38B are both electric motors such as, but not limited to, permanent magnet direct current (DC) motors.

Sometimes either the first hybrid propulsion system22A or the second hybrid propulsion system22B are unable to transmit sufficient power to the respective propeller20A,20B to maintain the aircraft12in the desired phase of flight. Specifically, the control module32is in electronic communication with both the first engine36A and the second engine36B, and monitors operation of the first engine36A and the second engine36B in real-time. The control module32may refer to, or be part of an electronic circuit, a combinational logic circuit, a field programmable gate array (FPGA), a processor (shared, dedicated, or group) that executes instructions, code, or a combination of some or all of the above, such as in a system-on-chip.

In operation, the control module32determines when either the first engine36A or the second engine36B is not outputting a required amount of torque. For example, the first engine36A or the second engine36B may become non-operational due to a loss in oil pressure. In response to determining either the first engine36A or the second engine36B is not outputting the required amount of torque, the control module32instructs the cross-connecting clutch26to actuate from a disengaged position into the engaged position. The first hybrid propulsion system22A is operably coupled to both the first propeller20A and the second propeller20B when the cross-connecting clutch26is engaged. Therefore, the first hybrid propulsion system22A drives both propellers20A,20B when the cross-connecting clutch26is engaged. Likewise, the second hybrid propulsion system22B is operably coupled to drive both the first propeller20A and the second propeller20B when the cross-connecting clutch26is engaged. In one exemplary embodiment, the cross-connecting clutch26is a disk clutch that is coupled to the first propeller20A and the second propeller20B by respective gearboxes48A,48B,50A,50B, and is described in greater detail below.

Operation of the overrunning clutches40A and40B are now described. The first overrunning clutch40A and the second overrunning clutch40B engage based on a difference between the torque produced by the respective engine36A,36B and motor38A,38B. Specifically, the first overrunning clutch40A is engaged when an engine torque of the first engine36A is greater than a motor torque of the first motor38A. For example, the engine torque of the first engine36A is greater than the motor torque of the first motor38A when the first engine36A is turned on. Thus, the first overrunning clutch40A remains engaged when the aircraft12operates in a climb phase of flight. Similarly, the second overrunning clutch40A is engaged when the engine torque of the second engine36B is greater than the motor torque of the second motor38B, such as when the second engine36A is turned on, or when the aircraft12is in the climb phase of flight. The first overrunning clutch40A and the second overrunning clutch40B are both configured to disengage when the engine torque is zero, or substantially zero, such as when the rotational speed of the engine output44A,44B is less than a rotational speed of a shaft of the respective motor38A,38B such as when the first and second engines36A or36B are turned off.

The control module32is in electronic communication with both motors38A,38B. The control module32sends a mode signal58A,58B to the respective motors38A,38B, which instructs the motors38A,38B to operate in either a power mode or a regeneration mode. The control module32instructs the motors38A,38B to operate in either the power mode or the regeneration mode based on the phase of flight of the aircraft12, which, in one example may be inferred by the throttle position of the engines36A,36B, the angle of the stabilizer, and the like. For example, a thirty percent throttle position would be considered the cruise phase of flight, while eighty percent throttle would be considered the takeoff or climb phase of flight. For example, when the control module32determines that the operator desires to operate the aircraft12in the climb phase of flight, based on the throttle position, the control module32transmits respective mode signals58A,58B to the motors38A and38B to shift from a regeneration mode to the power mode. In the power mode, the torque produced by the motors38A,38B are summed with the torque produced by the respective engines36A,36B to drive the respective propellers20A,20B.

The motors38A,38B operate in the power mode when the aircraft12requires excess thrust or power, such as when the aircraft12operates in a climb phase of flight. When excess thrust or power is not required, the control module32transmits sends the mode signal58A,58B to the motors38A and/or38B to shift from the power mode to the regeneration mode. In the regeneration mode, the motors38A,38B function as generators to charge the battery packs72A,72B. Moreover, the torque produced solely by the respective engines36A,36B is used to drive the respective propellers20A,20B. Accordingly, the first motor38A and the second motor38B operate in the power mode when commanded by the control module32, such as when the control module32determines that the aircraft12is operating in the climb phase of flight. The first motor38A and the second motor38B operate in the regeneration mode when commanded by the control module32, such as when the control module32determines that the aircraft12is being commanded to operate in the cruise phase of flight.

In an exemplary embodiment, the control module32may determine that the pilot has commanded a change in the phase of flight. For example, the pilot may command a change in the phase of flight by repositioning the throttle lever and/or moving a stabilizer (not shown) of the aircraft12. In response to determining a change in the phase of flight, the control module32determines the current torque being output by engines36A and/or36B is not the required amount of torque to maintain the current phase of flight, or to achieve the newly requested phase of flight. In another embodiment, the control module32may determine that either first engine36A or the second engine36B is non-operational and thus the aircraft12is unable to maintain the current phase of flight.

It is to be appreciated that the engines36A,36B receive assistance from the motors38A,38B when the motors38A,38B operate in the power mode. Accordingly, the engines36A,36B are not sized based on the power requirements of the propellers20A,20B during the climb phase of flight. Instead, the engines36A,36B are sized to accommodate the power requirements of the propellers20A,20B during the cruise phase of flight. Since the cruise phase of flight requires less power when compared to the climb phase of flight, the engines36A,36B are smaller and weigh less when compared to conventional engines that are sized to accommodate the power requirements during the climb phase of flight.

FIG. 2is a schematic diagram of the propulsion system10. As seen inFIG. 2, the first hybrid propulsion system22A and the second hybrid propulsion system22B are arranged in a parallel configuration. Therefore, the first engine36A and the first motor38A provide power to the first propeller20A through torque summing when the first motor38A is operating in the power mode. Similarly, the second engine36B and the second motor38B provide power to the second propeller20B through torque summing when the second motor38B is operating in the power mode. It is to be appreciated that the parallel configuration shown inFIG. 2results in improved power transfer efficiency between the engines36A,36B and the respective propellers20A,20B when compared to a series arrangement, since the series configuration requires energy conversion.

The propulsion system10further includes a first battery pack72A, a second battery pack72B, one or more first inverters74A, and one or more second inverters74B. The first battery pack72A is electrically coupled to the first motor38A and another system within the aircraft12such as, for example, an environmental control system (ECS)76. The second battery pack72B is electrically coupled to the second motor38B and yet another system within the aircraft12such as, for example, a hydraulic system78.

The first battery pack72A and the second battery pack72B both include a plurality of individual batteries80that are rechargeable as shown inFIG. 3. For example, the individual batteries80may be lithium ion batteries. The first battery pack72A provides electrical power to the first motor38A when the first motor38A operates in the power mode. Specifically, the first battery pack72A creates DC current that is converted into alternating current (AC) by the first inverters74A. It is to be appreciated that the first battery pack72A and the second battery pack72B are both sized for takeoff assist to the respective engines36A,36B. Accordingly, in one alternative embodiment the first battery pack72A and the second battery pack72B provide the power to the propellers20A,20B during climb and the first engine36A and the second engine36B are not operating during climb. This approach may be used to reduce emissions or reduce noise nearby an airport.

The first motor38A operates as a generator to charge the first battery pack72A when operating in the regeneration mode, such as during the cruise phase of flight. Recharging the first battery pack72A during the cruise phase of flight provides various benefits. Specifically, when the first battery pack72A is charged during the cruise phase of flight, the aircraft12does not require re-charging after landing. The second battery pack72B is also charged by the second motor38B when operating in the regeneration mode, such as during the cruise phase of flight.

FIGS. 3A and 3Billustrate an exemplary approach for switching an arrangement of the battery packs72A,72B when the motors38A,38B operate in the regeneration mode, such as during the cruise phase of flight. It is to be appreciated that in the exemplary embodiment, the voltage generated by each battery pack72A,72B may be reduced by approximately fifty percent once the aircraft12finishes the climb phase of flight when compared to the voltage of the battery packs72A,72B at the beginning of the climb phase of flight. It is also to be appreciated that the voltage generated by the motors38A,38B during the regeneration mode is greater than the voltage generated by each battery pack72A,72B, to enable successful charging during flight and to also enable the motors38A and38B to provide power to operate the tail cone fan30as described in more detail below. Therefore, the arrangement of individual batteries80of the battery packs72A,72B is changed between the power mode, where the battery packs72A,72B provide power, and the regeneration mode, where the battery packs72A,72B are re-charged. The first battery pack72A and the second battery pack72B both include a plurality of individual batteries80that are recharged during the regeneration mode.

FIG. 3Ais an exemplary arrangement of the individual batteries80during climb. In the embodiment as shown inFIG. 1, the plurality of individual batteries80are connected to one another in a first arrangement, which may also be referred to as a series arrangement, during the power mode.FIG. 3Bis an exemplary arrangement of the individual batteries80during the power mode. In the embodiment as shown inFIG. 3B, the plurality of individual batteries80are connected to one another in a second arrangement, which may also be referred to as a combination series and parallel arrangement, during the regeneration mode.

In the non-limiting embodiment as shown, each battery pack72A,72B includes ten individual batteries80that each produce an individual voltage Vn, however, it is to be appreciated that any number of individual batteries80may be used. In the embodiment as shown inFIG. 3A, the plurality individual batteries80are each arranged in a series configuration to maximize voltage. In contrast, the arrangement of individual batteries80inFIG. 3Breduces the voltage to a value that is less than the voltage produced by the motors38A,38B (FIGS. 1 and 2) during the regeneration mode.

Referring specifically toFIG. 3A, the voltage produced by each battery pack72A,72B is determined by adding the individual voltage Vnproduced by each individual battery80together. For example, if each individual battery80produced 30 Volts at 20% state of charge as the aircraft12finishes the climb phase of flight (i.e., Vn=30 Volts), then the voltage produced by each battery pack72A,72B is 300 Volts. However, if the voltage produced by each motor38A,38B during the regeneration mode is only 280 Volts, then the motors38A,38B are unable to provide charge. Accordingly, as seen inFIG. 3B, half of the individual batteries80are arranged in a series configuration and the remaining half of the individual batteries80are arranged in a parallel configuration. The individual batteries80arranged in a series configuration are placed in parallel with the individual batteries arranged in the parallel configuration, which results in an overall voltage of 150 Volts.

Referring back toFIG. 1, the tail cone fan30is disposed along a fore end90of the nacelle16and includes a plurality of rotating blades (not visible) that create airflow. An electric motor56is disposed within the tail cone fan30and is powered by the first motor38A and the second motor38B when the aircraft12is operating in the cruise phase of flight, where both motors38A,38B are operating in the regeneration mode and are being driven by the respective engines36A,36B. It is to be appreciated that the tail cone fan30also provides propulsive power to the aircraft12. Specifically, the tail cone fan30provides propulsive power for two reasons. First, the tail cone fan30reduces ram air drag around the nacelle16. Specifically, ram air is passed through the nacelle16and out of the tail cone fan30, where a ram air flow R is shown inFIG. 1. Second, the tail cone fan30boosts or increases the mass of outside air A that is introduced into the respective combustion air inlets94A,94B of the turbochargers42A,42B. The tail cone fan30also improves thrust by ingesting boundary air around the nacelle16. It is to be appreciated that boundary air increases drag.

The first turbocharger42A is fluidly connected to a first air intake96A of the first engine36A and the second turbocharger42B is fluidly connected to a second air intake96B of the second engine36B. The turbocharges42A,42B compress the outside air A. The compressed outside air A is then provided to the air intakes96A,96B of the respective engine36A,36B. The turbochargers42A,42B may improve the efficiency of an internal combustion engine by forcing more outside air A into the respective internal combustion engines36A,36B. The tail cone fan30further increases the outside air A that is forced over the engines36A,36B.

The exhaust system28includes a first exhaust conduit98A fluidly connected to an exit manifold100A of the first engine36A. The exhaust system28further includes a second exhaust conduit98B fluidly connected to an exit manifold100B of the second engine36B. Exhaust gases G that exit the first engine36A and the second engine36B travel within their respective exhaust conduits98A,98B and towards the fore end90of the nacelle16, and toward the tail cone fan30. The tail cone fan30is fluidly connected to receive exhaust gases from the first exhaust conduit98A and the second exhaust conduit98B, and is driven by the exhaust gases during all flight phases, except for cruise.

In an embodiment, the first hybrid propulsion system22A further includes a first air-oil heat exchanger110A. Likewise, the second hybrid propulsion system22B includes an air-oil heat exchanger110B. The first air-oil heat exchanger110A is fluidly connected to an air intake112A disposed along an aft end92of the nacelle16, and the second air-oil heat exchanger is fluidly connected to an air intake112B also disposed along the aft end92of the nacelle16. The air-oil heat exchangers110A,110B are configured to provide cooling to the respective engines36A,36B. In addition to cooling the engines36A,36B, the air-oil heat exchangers110A,110B also provide heated air to the respective exhaust conduits98A,98B and also extract thermal energy from the fuel used to power the engines36A,36B.

It is to be appreciated that in the exemplary embodiment the engines36A,36B operate at a rotational speed of approximately 2000 RPM. Furthermore, it is also to be appreciated that the rotational speed of the propellers20A,20B also operate at the same rotational speed as the engines36A,36B. Accordingly, it is to be appreciated that the rotational speed of the first engine36A controls the rotational speed of the first propeller20A and the rotational speed of the second engine36B controls the rotational speed of the second propeller20B. Thus, it is also to be appreciated that unlike some hybrid propulsion systems that require a planetary gearset to decouple the rotational speed of the engine and the output, the disclosed propulsion system10only require gearboxes48A,48B,50A,50B for accommodating the geometry or packaging layout of the propellers20A,20B.

FIG. 4is an enlarged view of one or the final drive gearboxes48A,48B. Referring toFIGS. 1 and 4, the first final drive gearbox48A and the second final drive gearbox48B both include three bevel gears122A,122B,122C. It is to be appreciated that bevel gears transmit motion between two shafts that are not aligned with one another. Moreover, the bevel gears122A,122B,122C are meshingly engaged with one another at a 1:1 ratio to drive the propellers20A,20B at the same rotational speed as the engines36A,36B. A driving beveled gear122A is driven by the output46A,46B of either the first motor38A or the second motor38B. The driving beveled gear122A is meshed with the remaining two driven beveled gears122B,122C. The driven bevel gear122B drives the cross-connecting clutch26through a shaft128A,128B. The driven bevel gear122C drives a respective propeller20A,20B thought a connecting shaft130A,130B.

Referring back toFIG. 1, the gearboxes50A,50B also include bevel gears132A,132B for transmitting motion between the connecting shafts130A,130B and a corresponding propeller shafts140A,140B. It is to be appreciated that the connecting shafts130A,130B are arranged perpendicular with respect to their corresponding propeller shaft140A,140B. A driving bevel gear132A is driven by the respective connecting shaft130A,130B. A driven bevel gear132B is driven by the driving bevel gear132A, where the driven bevel gear132B is connected to the respective propeller shaft140A,140B.

Referring now toFIGS. 5A and 5B, exemplary process flow diagrams illustrating methods200and300of operating the propulsion system10for the aircraft12. Specifically, method200shown inFIG. 5Aillustrates operation of the cross-connecting clutch26, andFIG. 5Billustrates operation of the overrunning clutches40A,40B. Referring now toFIGS. 1 and 5A, the method200begins at block202. In block202, the control module32monitors operation of the first engine36A and the second engine36B, where the first engine36A is part of the first hybrid propulsion system22A that is operably coupled to the first propeller20A, and the second engine36B is part of the second hybrid propulsion system22A operably coupled to the second propeller20B. The method200then proceeds to block204.

In block204, the control module32determines either the first engine36A or the second engine36B is not outputting the required amount of torque to maintain the desired phase of flight. As mentioned above, the first hybrid propulsion system22A includes the first motor38A coupled to the first engine36A by the first overrunning clutch40A, and the second hybrid propulsion system22B includes the second motor38B coupled to the second engine36B by a second overrunning clutch40B. The method200then proceeds to block206.

In block206, in response to determining either the first engine36A or the second engine36B is not outputting the required amount of torque, the control module32instructs the cross-connecting clutch26to actuate from the disengaged position into the engaged position. Accordingly, engaging the cross-connecting clutch26results in the first hybrid propulsion system22A driving the second propeller20B when the second engine36A does not output the required torque to maintain the desired phase of flight of the aircraft12. Similarly, engaging the cross-connecting clutch26results in the second hybrid propulsion system22B driving the first propeller20A when the first engine36A does not output the required torque to maintain the desired phase of flight of the aircraft12. The method200then terminates.

Referring now toFIGS. 1 and 5B, the method300begins at blocks302A and302B. It is to be appreciated that blocks302A and302B are performed simultaneously. In block302A, the first overrunning clutch40A is engaged when the engine torque of the first engine36A is greater than the motor torque of the first motor38A, such as when the first engine36A is started. Thus, during normal operation, the first engine36A and the first motor38A are coupled together by the overrunning clutch40A during all phases of flight of the aircraft12. Similarly, in block302B, the second overrunning clutch40B is engaged when the engine torque of the second engine36B is greater than the motor torque of the second motor38B, such as when the second engine36B is started. Therefore, the second engine36B and the second motor38B are also coupled together by the second overrunning clutch40B during all phases of flight of the aircraft12.

As mentioned above, because the respective engine36A,36B and motor38A,38B are coupled together by the respective overrunning clutch40A,40B during all phases of flight, the additional torque generated by the motors38A,38B during the climb phase of flight enables the engines36A,36B to be sized specifically for the cruise phase of flight when the additional torque provided by the motors38A,38B is not required. This results in a smaller, lighter engine when compared to conventional engines that are typically sized for the climb phase of flight.

Referring generally to the figures, the disclosed propulsion system provides various technical effects and advantages. Specifically, the cross-connecting clutch is engaged when one of the two engines do not generate the torque required to maintain a desired phase of flight. Therefore, in the event one of the two hybrid propulsion systems become inoperable, both propellers may still be driven. Moreover, this may also reduce maintenance costs associated with the engines. Moreover, when the engine torque is greater than the motor torque, such as when the aircraft is in the climb phase of flight, the overrunning clutches are engaged to couple the engine and motor to one another. As a result, the engine may be smaller and less expensive than other conventional engines that are sized to provide all of the power required when the aircraft is operating in the climb phase of flight. It is also to be appreciated that an internal combustion engine provides improved efficiency, which may be especially advantageous if the propeller-driven aircraft is used frequently. Finally, since the battery packs are charged during the cruise portion of flight, then the batteries re-charged as the aircraft lands. Accordingly, the aircraft does not need re-charging after completing the flight.