IL313869A

IL313869A - Propulsion system

Info

Publication number: IL313869A
Application number: IL313869A
Authority: IL
Inventors: Abramov Danny; Gaby Vaisman
Original assignee: Israel Aerospace Ind Ltd; Abramov Danny; Gaby Vaisman
Priority date: 2024-06-24
Filing date: 2024-06-24
Publication date: 2026-01-01
Also published as: WO2026003832A1

Description

0302674312- PROPULSION SYSTEM TECHNOLOGICAL FIELD The presently disclosed subject matter relates to propulsion systems for air vehicles, in particular for fixed-wing air vehicles having VTOL capabilities.

BACKGROUND A class of air vehicle includes fixed wing vehicles having a propulsion system for providing VTOL capabilities, and another propulsion system for providing forward thrust for powered aerodynamic flight. In some cases, the VTOL propulsion system includes a number of VTOL propulsion units that are in fixed spatial relationship with respect to the air vehicle, i.e., the thrust vector of the respective thrust generated by each VTOL propulsion unit is in single fixed spatial relationship with respect to the air vehicle to provide a vertical, or near vertical thrust. In such cases, the thrust vector cannot be moved with respect to the air vehicle, for example for providing horizontal thrust for aerodynamic flight. The VTOL propulsion system is configured for providing VTOL capabilities and can thereby be operated to generate thrust for vertical take-off, hover, and vertical landing, but is switched off to generate zero thrust during powered aerodynamic flight. In many such cases, the VTOL propulsion units are turned by electric motors, which require heavy batteries or a generator to provide the required electrical power.

In some examples of such air vehicles, the VTOL propulsion units are configured as variable rpm fixed pitch VTOL propulsion units, while in other examples of such air vehicles, the VTOL propulsion units are configured as variable pitch constant rpm VTOL propulsion units.

Such air vehicles are often operated to include a transition between vectored thrust flight and powered aerodynamic forward flight. 0302674312- By way of non-limiting example, US 11,834,167 discloses a VTOL drone aircraft that can include a rechargeable battery, a primary processor, lift propellers, an internal combustion engine with a thrust propeller, a generator, and a power regulation controller. The generator can receive power from the internal combustion engine and deliver electrical power to the rechargeable battery, the lift propellers, or both. The power regulation controller can regulate dynamically power delivery from the internal combustion engine to the thrust propeller and the generator, and from the generator to the rechargeable battery based upon changing conditions during flight. The power regulation controller can prevent operation of the generator when peak power is needed from the internal combustion engine for the thrust propeller. The power regulation controller can also control a clutch coupled to the thrust propeller to regulate the delivery of power to the thrust propeller when the internal combustion engine is active.

Also by way of non-limiting example, US 11,628,933 discloses a compound aircraft that embodies an array of rotors for vertical flight positioned on support booms and wing elements for cruise flight coupled to a central fuselage housing avionics and a pusher propeller for forward propulsion. The aircraft accommodates a cargo-carrying container with mating of the surfaces between container and fuselage and latching mechanisms for attaching and detaching the container and vehicle.

Also by way of non-limiting example, US 10,131,426 relates to an aircraft which can both take off and land vertically and can hover and also fly horizontally at a high cruising speed. The aircraft has a support structure, a wing structure, at least three and preferably at least four lifting rotors and at least one thrust drive. The wing structure is designed to generate a lifting force for the aircraft during horizontal motion. To achieve this the wing structure has at least one mainplane provided with a profile that generates dynamic lift. The wing structure is preferably designed as a tandem wing structure. Each of the lifting rotors is fixed to the support structure, has a propeller and is designed to generate a lifting force for the aircraft by means of a rotation of the propeller, said force acting in a vertical direction. The thrust drive is designed to generate a thrust force on the support structure, said force acting in a horizontal direction.

Also by way of non-limiting example, CN 111196357 relates to a fuel oil power variable-rotating-speed control composite wing unmanned aerial vehicle. The fuel oil 0302674312- power variable-rotating-speed control composite wing unmanned aerial vehicle comprises a vehicle body; wing mechanisms and rotor wing mechanisms are arranged on the vehicle body; a propelling propeller is arranged at the tail of the vehicle body; the wing mechanisms are symmetrically arranged in an X shape; the four rotor wings are located at the ends of the wings respectively; the rotor wings are directly driven by a fuel oil power system arranged in the vehicle body; a speed adjusting motor and a torque adjusting motor cooperate with a differential planetary gear train to change the rotating speed of each rotor wing, so that flight control can be conducted.

Also by way of non-limiting example, CN 105539834 discloses a composite-wing vertical take-off and landing unmanned aerial vehicle. A conventional fixed-wing aerodynamic layout is adopted to be combined with an X-shaped four-axis layout, and the aerial vehicle states such as vertical take-off and landing, hovering and high-speed cruising are achieved. The yaw control moment is increased through a variable-pitch propeller installed on a vertical tail and a four-axis motor which is installed on a wing and has a tilt angle, and the robustness and the control precision of the large-rotational-inertia composite-wing unmanned aerial vehicle in the low-speed flying state.

Also by way of non-limiting example, CN 106927036 discloses a foldable composite oil drive high-speed four-rotor-wing unmanned aerial vehicle which mainly comprises a power transmission system, a rotor wing control system, a vehicle body and a folding system.

Also by way of non-limiting example, CN 105752331 relates to a single-power internal combustion engine-driven multi-rotor wing unmanned aerial vehicle based on variable pitch control, and belongs to the field of air vehicles. The single-internal combustion engine power multi-rotor wing unmanned aerial vehicle based on variable pitch control comprises a variable pitch control part and a single-power transmission part. 0302674312- GENERAL DESCRIPTION According to a first aspect of the presently disclosed subject matter, there is provided a propulsion system for an air vehicle comprising: a first engine system comprising at least one first engine; a second engine system comprising at least one second engine; a VTOL rotor system; a horizontal thrust generating system; a coupling system; wherein the first engine system is configured for being mechanically coupled with respect to the VTOL rotor system; and wherein the second engine system is selectively mechanically coupled with respect to each one of the VTOL rotor system and the horizontal thrust generating system via the coupling system; and wherein the coupling system is configured for enabling the second engine system to be selectively coupled or decoupled with respect to the VTOL rotor system, and, for independently enabling the second engine system to be selectively coupled or decoupled with respect to the horizontal thrust generating system.

For example, each said first engine is a liquid fuel engine.

Additionally or alternatively, for example, each said first engine is an internal combustion engine.

Additionally or alternatively, for example, each said second engine is a liquid fuel engine.

Additionally or alternatively, for example, each said second engine is an internal combustion engine.

Additionally or alternatively, for example, the propulsion system is configured for being operated in each one of vectored thrust flight mode, aerodynamic forward flight mode, and transition mode. For example, the first propulsion system and the second propulsion system are configured for together generating sufficient power to the VTOL rotor system to enable the VTOL rotor system to generate sufficient thrust for enabling at least said vectored thrust flight mode. Additionally or alternatively, for example, the 0302674312- second propulsion system is configured for generating sufficient power to the horizontal thrust generating system to enable the horizontal thrust generating system to generate sufficient thrust for enabling at least said aerodynamic forward flight mode.

Additionally or alternatively, for example, the first propulsion system comprises a single said first engine, and wherein the second propulsion system comprises a single said second engine.

Additionally or alternatively, for example, the coupling system is configured for having a first coupling mode and a first decoupling mode, and a second coupling mode and a second decoupling mode, wherein: - in said first coupling mode the coupling system enables the second engine system to be mechanically coupled with respect to the VTOL rotor system, and wherein in said first decoupling mode the coupling system the second engine system to be mechanically decoupled with respect to the VTOL rotor system; and wherein - in said second coupling mode the coupling system enables the second engine system to be mechanically coupled with respect to the horizontal thrust generating system, and wherein in said second decoupling mode the coupling system enables the second engine system to be mechanically decoupled with respect to the horizontal thrust generating system.

Additionally or alternatively, for example, the coupling system comprises a first clutch arrangement and a second clutch arrangement, the first clutch arrangement being configured for selectively and alternately providing each one of the first coupling mode and the first decoupling mode, and the second clutch arrangement being configured for selectively and alternately providing each one of the second coupling mode or the second decoupling mode. For example, the first clutch arrangement is distinct from the second clutch arrangement. Additionally or alternatively, for example, the second engine system comprises a first power output shaft end and a second power output shaft end, each one of the first power output shaft end and the second power output shaft end being configured for selectively transmitting power generated by the second engine system. For example, the second clutch arrangement is operatively coupled to the second power output shaft end of the second engine system, and operatively coupled to the horizontal thrust 0302674312- generating system. Additionally or alternatively, for example, the first clutch arrangement is operatively coupled to the first power output shaft end, and operatively coupled to the VTOL rotor system via the first engine system. For example, the first engine system and the second engine system are selectively coupled to one another in series via the first clutch system.

Additionally or alternatively, for example, the first clutch arrangement is operatively coupled to the first power output shaft end, and operatively coupled to the VTOL rotor system independently of the first engine system. For example, the first engine system and the second engine system are selectively coupled to the VTOL rotor system in parallel.

Additionally or alternatively, for example, the VTOL rotor system comprises a plurality of VTOL rotors configured to be coupled to at least the first engine system via a transmission system, each VTOL rotor configured for generating a respective vertical thrust when turned by power generated by the first engine system and the second engine system. For example, the transmission system comprises a torque interface mechanically coupled to the first engine arrangement, and further comprises a plurality of drive shafts operatively interconnecting the VTOL rotors to the torque interface. Additionally or alternatively, for example, the propulsion system is configured for providing constant rpm to the VTOL rotors, and wherein the VTOL rotors are configured as variable pitch rotors to thereby enable control of thrust generated by each said VTOL rotor. For example, the first engine system and the second engine system are configured for providing constant rpm to the VTOL rotors in said vectored thrust flight mode and in said transition mode. Additionally or alternatively, for example, the VTOL rotor system comprises four said VTOL rotors in polygonal arrangement with respect to one another.

Additionally or alternatively, for example, the horizontal thrust generating system comprises at least one horizontal rotor, said horizontal rotor configured for generating a respective horizontal thrust when turned by power generated by the second engine system.

Additionally or alternatively, for example, the propulsion system comprises a controller operatively coupled to at least each one of the first engine system, the second engine system, said coupling system, and said VTOL rotor system. For example, said 0302674312- controller is configured for selectively operating the propulsion system in any one of said vectored thrust flight mode, said aerodynamic forward flight mode, and said transition mode. For example, in said vectored thrust flight mode, the controller operates to cause the first engine system to become coupled with the second engine system via the coupling system, and concurrently operates to cause the second engine system to become decoupled with respect to the horizontal thrust generating system via the coupling system. Additionally or alternatively, for example, in said vectored thrust flight mode, the controller operates to cause the VTOL rotor system to become coupled with the second engine system via the coupling system, and concurrently operates to cause the second engine system to become decoupled with respect to the horizontal thrust generating system via the coupling system. Additionally or alternatively, for example, in said aerodynamic forward flight mode, the controller is configured to operate to cause the first engine system to be decoupled with respect to the second engine system via the coupling system, and concurrently to operate to cause the second engine system to become coupled with respect to the horizontal thrust generating system via the coupling system. Additionally or alternatively, for example, in said transition mode from vectored thrust flight mode to forward flight mode, the controller is configured to operate to cause the VTOL rotor system to become coupled with respect to the second engine system via the coupling system, and concurrently to operate to cause the second engine system to become coupled with respect to the horizontal thrust generating system via the coupling arrangement. Additionally or alternatively, for example, in said transition mode from forward flight mode to vectored thrust flight mode, the controller is configured to operate to cause the VTOL rotor system to become coupled with respect to the second engine system via the coupling system, and concurrently to operate to cause the second engine system to become decoupled with respect to the horizontal thrust generating system via the coupling arrangement.

Additionally or alternatively, for example, the first engine system and the second engine system are distinct from one another.

Additionally or alternatively, for example, the first engine system is fixedly coupled with respect to the VTOL rotor system. Alternatively, the propulsion system further comprises an auxiliary coupling system, wherein the auxiliary coupling system is 0302674312- configured for enabling the first engine system to be selectively coupled or decoupled with respect to the VTOL rotor system.

Additionally or alternatively, for example, said first engine system and said second engine system are integrally included in a unified engine, wherein the unified engine is a liquid fuel internal combustion engine comprising an engine casing accommodating a first set of pistons and a second set of pistons, wherein said first set of pistons and said second set of pistons are reciprocally and rotatably mounted to a common crankshaft within the engine casing, and wherein the first set of pistons corresponds to the first engine system, and the second set of pistons corresponds to the second engine system, and wherein the unified engine is configured for selectively decoupling the first set of pistons from providing power to the crankshaft so that no power or torque is transmitted by the set of second pistons to the VTOL rotor system. For example, at least during aerodynamic forward flight mode, the propulsion system operates to cause the first set of pistons to be selectively decoupled from providing power to the crankshaft, and the coupling system decouples the unified engine with respect to the VTOL rotor system, and wherein the coupling system couples the unified engine to the horizontal thrust generating system. Additionally or alternatively, for example, at least during vectored thrust flight mode, the propulsion system operates to cause the first set of pistons to be coupled to provide power to the crankshaft, and the coupling system couples the unified engine to the VTOL rotor system, and wherein the coupling system decouples the unified engine with respect to the horizontal thrust generating system.

According to a second aspect of the presently disclosed subject matter there is provided an air vehicle drive system for selectively driving a VTOL rotor system and a horizontal thrust generating system to provide a propulsion system for an air vehicle, the drive system comprising: a first engine system comprising at least one first engine; a second engine system comprising at least one second engine; a coupling system; wherein the first engine system is configured for being mechanically coupled with respect to the VTOL rotor system; and 0302674312- wherein the second engine system is configured for being selectively mechanically coupled with respect to each one of the VTOL rotor system and the horizontal thrust generating system via the coupling system; and wherein the coupling system configured for enabling the second engine system to be selectively coupled or decoupled with respect to the VTOL rotor system, and, for independently enabling the second engine system to be selectively coupled or decoupled with respect to the horizontal thrust generating system.

According to a third aspect of the presently disclosed subject matter there is provided an air vehicle comprising a propulsion system as defined according to the first aspect of the presently disclosed subject matter.

For example, the air vehicle comprises a fixed wing arrangement, fuselage and empennage. For example, the air vehicle comprises a pair of booms attached to the fixed wing arrangement, wherein the VTOL rotor system is at least partially mounted with respect to the booms.

According to a fourth aspect of the presently disclosed subject matter there is provided a method for operating a propulsion system, comprising: - providing the propulsion system as defined according to the first aspect of the presently disclosed subject matter; - operating the propulsion system to operate in at least one of the vectored thrust flight mode, the aerodynamic forward flight mode, and the transition mode.

A feature of at least some examples of the presently disclosed subject matter is that the propulsion system including the first engine system and the second system is more efficient for cruise, for example, than an analogous system in which the first engine system and the second engine system is replaced with a single engine to provide full power for VTOL and partial power for cruise.

Another feature of at least some examples of the presently disclosed subject matter is that the propulsion system enables the air vehicle to hover for significantly longer time periods than is expected in analogous air vehicles having electrically powered VTOL rotor systems. 0302674312- Another feature of at least some examples of the presently disclosed subject matter is that the propulsion system provides the air vehicle with improved endurance compared to that expected in analogous air vehicles having electrically powered VTOL rotor systems.

Another feature of at least some examples of the presently disclosed subject matter is that the propulsion system enables the air vehicle to have a weight fraction (dry weight/ all up weight) that is superior to that expected in analogous air vehicles having electrically powered VTOL rotor systems, in particular due to the lack of heavy batteries.

Another feature of at least some examples of the presently disclosed subject matter is that the propulsion system enables the air vehicle to attain greater forward speeds and/or greater altitudes as compared to the forward speeds and/or altitudes expected to be attained in analogous air vehicles that are electrically powered.

Another feature of at least some examples of the presently disclosed subject matter is that the propulsion system enables the horizontal rotor to be decoupled from the second engine system during vectored thrust mode, thereby avoiding rotation of the horizontal rotor when the second engine system can be idling, thereby avoiding generation of forward thrust by the horizontal rotor, and thereby enabling the control system for the VTOL rotor system to be improved and simplified as compared to that expected to be provided in analogous air vehicles in which the horizontal rotor is permanently coupled to an engine or motor and thus rotates during vectored thrust mode and thereby introducing a horizontal thrust. 0302674312- BRIEF DESCRIPTION OF THE DRAWINGS In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 shows in isometric view an air vehicle according to a first example of the presently disclosed subject matter. Fig. 2 shows in top view the example of Fig. 1. Fig. 3 schematically illustrates a propulsion system according to a first example of the presently disclosed subject matter. Fig. 4 schematically illustrates an alternative variation of the example of Fig. 3. Fig. 5 schematically illustrates another alternative variation of the example of Fig. 3. Fig. 6 schematically illustrates another alternative variation of the example of Fig. 3. Fig. 7 schematically illustrates another alternative variation of the example of Fig. 3.

Fig. 8 schematically illustrates another alternative variation of the example of Fig. 3.

Fig. 9 schematically illustrates another alternative variation of the example of Fig. 3. 0302674312- DETAILED DESCRIPTION Referring to Fig. 3, a propulsion system for a VTOL fixed wing air vehicle (also interchangeably referred to herein as "fixed wing air vehicle" or "air vehicle") according to a first example of the presently disclosed subject matter is generally designated with reference numeral 600 , and comprises a first engine system 200 , a second engine system 300 , a coupling system 700 , a VTOL rotor system 400 , and a horizontal thrust generating system 800 .

As will become clearer herein, the first engine system 200 is mechanically coupled with respect to the VTOL rotor system 400 , and the second engine system 300 is selectively mechanically coupled with respect to each one of the VTOL rotor system 400and the horizontal thrust generating system 800 via the coupling system 700 . Furthermore, the coupling system 700 is configured for enabling the second engine system 300 to be selectively coupled or decoupled with respect to the VTOL rotor system 400 , and, for independently enabling the second engine system 300 to be selectively coupled or decoupled with respect to the horizontal thrust generating system 800 .

The propulsion system 600 is configured for selectively providing vertical thrust to the fixed wing air vehicle such as to provide the fixed wing air vehicle with VTOL capabilities, and thus enable the fixed wing air vehicle to operate in vectored thrust flight mode VFM . The propulsion system 600 is also configured for selectively providing horizontal thrust to the fixed wing air vehicle such as to enable the fixed wing air vehicle to operate in aerodynamic forward flight mode FFM . The propulsion system 600 is also configured such as to enable the fixed wing air vehicle to operate in transition mode TRM between vectored thrust flight mode VFM and aerodynamic forward flight mode FFM .

By "vectored thrust flight mode" is meant that the respective air vehicle is being operated such that the weight thereof is being balanced, partially or fully, or such that the weight thereof is being exceeded, by an upward force that is exclusively a thrust force generated by the respective VTOL rotor system, and that no part of the upward force is generated aerodynamically by the fixed wings of the air vehicle. Vectored thrust flight mode thus includes hover, vertical take-off, vertical landing, vectored thrust climb, vectored thrust descent, vectored thrust maneuvering in one or more of pitch, roll and yaw, and so on. 0302674312- By "aerodynamic forward flight mode" is meant that the respective air vehicle is being operated such that the weight thereof is being balanced, partially or fully, or such that the weight thereof is being exceeded, by an upward force that is exclusively generated aerodynamically by the fixed wings of the air vehicle, and that no part of the upward force is a thrust force generated by the respective VTOL rotor system.

By "transition mode" is meant when the air vehicle is transitioning between vectored thrust flight mode and aerodynamic forward flight mode, including from vectored thrust flight mode (for example hover) to aerodynamic forward flight mode, or, from aerodynamic forward flight mode to vectored thrust flight mode (for example hover), and wherein at least during part of the transition mode the respective air vehicle is being operated such that the weight thereof is being balanced, partially or fully, or such that the weight thereof is being exceeded, by an upward force that is provided in part by a thrust force generated by the VTOL rotor system, and that is provided in complementary part by the aerodynamic lift generated aerodynamically by the fixed wings of the air vehicle.

Referring to Figs. 1 and 2, at least one example of such a fixed wing air vehicle, generally designated with reference numeral 1 , comprises a fuselage 2, lift-generating wings 3, empennage 4, and accommodates the propulsion system 600 .

The air vehicle 1 further comprises an air vehicle controller 9 configured for controlling operation of the propulsion system 600 , including during at least each one of: aerodynamic forward flight mode FFM ; vectored thrust flight mode VFM ; and transition mode TRM ; as will become clearer herein. In at least this example, the controller 9 is also configured for controlling operation of the air vehicle 1 , for example for controlling operation of control surfaces of the wings 3 and empennage 4 , for controlling magnitude of the angle of attack of the air vehicle, and so on, for each one of: aerodynamic forward flight mode FFM ; vectored thrust flight mode VFM ; and the transition mode TRM . In alternative variations of this example, a different controller can be provided for controlling operation of the respective air vehicle; such a controller can optionally also be operatively coupled to controller 9 .

As disclosed above, the propulsion system 600 comprises the VTOL rotor system 400 . The propulsion system 600also includes a transmission system 500 , which in at least some examples can be provided as part of the VTOL rotor system 400 , or which in at least 0302674312- some other examples can be provided as part of the first engine system 200 , or which in at least some other examples can be provided as a separate component from the VTOL rotor system 400 or from the first engine system 200 . The rotor system 400 is configured for generating vertical thrust when driven by the first engine system 200 and the second engine system 300 via the transmission system 500 , as will become clearer herein.

The rotor system 400is thus configured for selectively providing at least vertical thrust (i.e., vertical propulsion) and also for selectively generating control moments to the air vehicle 1. For example, the propulsion system 600 via the rotor system 400can be used to provide the air vehicle 1with VTOL capability, i.e., enable the air vehicle 1 to operate in vectored thrust flight mode VFM , including providing vertical takeoff capability, and/or vertical climb capability, and/or vertical landing capability, and/or vertical descent capability, and/or hover capability in vectored flight mode VFM . Furthermore, the rotor system 400can be used to provide the air vehicle 1with control moments in pitch, roll and yaw, at least when the air vehicle 1is in vectored flight mode VFM , or in transition mode TRM between aerodynamic forward flight mode FFM and vectored flight mode VFM , in all environmental conditions for which the air vehicle 1is designed to operate in during vectored thrust flight mode VFM or during transition mode TRM .

In at least this example, the air vehicle 1 , comprises a pair of pods 110associated with rotor system 400 . One pod 110is mounted to the port wing 3, and the other pod 110is mounted to the starboard wing 3. Each pod 110comprises a pod forward section 112that extends forward of the leading edge of the respective wing 3, a pod aft section 114that extends aft of the trailing edge of the respective wing 3, and a central pod section 115connecting between the respective pod forward section 112and the respective pod aft section 114. In at least this example, the central pod section 115is configured for enabling the respective pod 110to be mounted to the air vehicle 1. For example, the central pod section 115is configured for mounting to a wing pylon or strut 8 , to enable the pod 110to be mounted to a respective wing 3.

In at least this example, the starboard-mounted pod 110 is essentially a mirror-image of the port-mounted pod 110 , with respect to a longitudinal-vertical plane through the centerline or longitudinal axis LA of the air vehicle 1 . 0302674312- The rotor system 400comprises a plurality of rotors 450 which, when driven by the first engine system 200 and the second engine system 300 via the transmission system 500 , each generates a vertical thrust T .

In at least this example, the rotor system 400comprises four rotors 450 , two rotors 450 being rotatably mounted to one pod 110, and two rotors 450 being rotatably mounted to the other pod 110. Furthermore, in at least this example, and referring in particular to Fig. 2, the plurality of rotors 450 are arranged in a polygonal arrangement (in top view) such as to enclose therein the center of gravity CG of the air vehicle 1. The position of the center of gravity CG can vary within a center of gravity envelope CGE . However, in at least some alternative variations of this example, each pod can have a different number of rotors 450 , for example three, four, or more than four rotors 450 .

In yet other alternative variations of this example, the respective plurality of rotors (including three or more rotors, for example 3, 4, 5, 6, 7, 8 or more than 8 rotors) are rotatably mounted to the air vehicle (for example fixed with respect to the fuselage and/or with respect to the wings and/or with respect to the empennage) via alternative arrangements, for example via struts, pylons, or the like, and the pods can be optionally be omitted. Additionally or alternatively, each rotor (of the three or more rotors) can be rotatably mounted with respect to the fuselage and/or wings and/or empennage for example by being embedded or otherwise integrated into the fuselage and/or wings and/or empennage.

In at least this example, the rotors 450 are each rotatably mounted with respect to the respective boom 110 about a respective motor turning axis RA via a respective pod installation 130 . Each pod installation 130 is configured for rotatably mounting the respective rotor 450 thereto in vertical mode, i.e., in a single fixed vertical spatial relationship, with respect to an air vehicle structure, for example with respect to the respective pod 110 . For example, each pod installation 130 includes a respective rotor shaft (not shown), coaxial with the respective rotor turning axis RA , and rotatably mounted to the respective pod via bearings (not shown), the respective rotor 450 being fixed to the rotor shaft.

By "vertical mode" is meant that the respective rotor 450provides an exclusively vertical thrust to the air vehicle, or that the respective rotor 450provides a predominantly vertical thrust, i.e., with a possible horizontal thrust component (for example in a forward or 0302674312- aft direction, and/or in a sideways direction) of significantly lesser magnitude than the predominantly vertical thrust. For example, the vertical thrust can have a thrust vector, parallel to the rotor turning axis RA , that is inclined by an inclination angle to the vertical, wherein at least in this example such an inclination angle can be about 5°, for example.

In at least this example, each rotor 450 comprises a single set of rotor blades, the respective set of rotor blades including two rotor blades 455 , the two rotor blades 455 being diametrically arranged and co-planarly fixed to the respective rotor shaft. In at least some alternative variations of this example, each respective rotor can instead comprise three or more than three rotor blades.

For example, the forward-starboard and aft-port rotors can rotate in the same direction with respect to one another, and in the opposite direction to the rotational direction of the forward-port and aft-starboard rotors.

In at least some other alternative variations of this example, each respective rotor can instead comprise a plurality of sets of rotor blades, each set having two or more rotor blades; for example, each respective rotor can have two sets of rotor blades, co-rotating or counter-rotating with respect to one another, about the respective rotor turning axis. Optionally, each set of rotor blades can be controlled to vary the thrust generated by the respective set of rotor blades independently of the other sets of rotor blades of the same rotor and/or of the other rotors.

In at least this example, the two-blade feature of each rotor 450 can enable the VTOL rotor system 400in some situations to minimize drag when the VTOL rotor system 400is not being used to generate thrust, for example in aerodynamic forward flight mode FFM . For example, and as illustrated in Fig. 2, and when the VTOL rotor system 400is not being operated, for example when the air vehicle is in aerodynamic forward flight mode FFM , the respective VTOL rotor system 400can be locked in a position where the two rotor blades 455of each rotor 450 are aligned parallel to the pod longitudinal axis PLA , or parallel to the longitudinal axis LA (also generally parallel to the direction of forward flight), thereby minimizing the frontal cross-sectional area of the VTOL rotor system 400 , and thus minimizing drag. 0302674312- In at least this example, the VTOL rotor system 400is configured as a constant rpm, variable pitch rotor system. Accordingly, each rotor 450 is configured for enabling the pitch of the respective rotor blades 455 to be changed, independently of the other rotors 450 , to thereby control the thrust generated by each rotor 450independently of the other rotors 450 . Thus, the rotor blades 455 are pivotably mounted to the respective rotor shaft via pivot axes (typically orthogonal to the rotor turning axis RA ) to enable the blade pitch to be controllably varied. Such variable pitch rotor systems are well-known in the art, and can include, for example, an actuator (not shown) for each rotor 450 , operatively connected to the controller 9 , and coupled to the blades 455 via links (not shown), such that selective operation of each of the actuators pivots the respective rotor blades 455 , during rotation thereof about the respective rotor turning axis RA , to thereby control the respective blade pitch angle.

Nevertheless, the level of rpm for the rotors 450can optionally be adjusted, for example according to conditions at different parts of the flight envelope and/or according to different loading conditions, for maximizing performance of the air vehicle.

In at least this example, and referring again to Fig. 3, the VTOL rotor system 400 is mechanically coupled to the first engine system 200via the transmission system 500 , which as disclosed above can be considered to part of the VTOL rotor system 400 and/or part of the first engine system 200 , or, which can be considered to be a different components from the VTOL rotor system 400 and the first engine system 200 . In particular, each rotor 450 is mechanically coupled to the first engine system 200via the transmission system 500 .

The coupling of each of the rotors 450 to the first engine system 200 via the transmission system 500is such that all the rotors 450are synchronized to concurrently turn together at the same rotational speed, and furthermore such that the angular dispositions of the rotor blades are the same at least when parallel to the pod longitudinal axis or to the longitudinal axis of the air vehicle.

In at least this example, the first engine system 200 , the second engine system 300 , the coupling system 700 , and the horizontal thrust generating system 800 are accommodated by the fuselage 2 , while the transmission system 500 is partially accommodated by the fuselage 2 , and partially accommodated by the wings 3and pods 110 . 0302674312- In at least this example, the transmission system 500is configured for transmitting and distributing the output torque and power generated by the first engine system 200 and the second engine system 300 together (when mechanically coupled to one another via the coupling system 700 ) to the VTOL rotor system 400 , in particular to the rotors 450 .

The transmission system 500 comprises a torque interface 510 mechanically coupled to the first engine arrangement 200 , in particular mechanically coupled to the output shaft end 210 of the first engine arrangement 200 .

In at least this example, the transmission system 500 comprises a plurality of drive shafts operatively interconnecting the rotors 450 to the torque interface 510 . In at least this example, the transmission system 500 comprises: a port transverse power shaft 530P and a starboard transverse power shaft 530S ; a port longitudinal power shaft 540P and a starboard longitudinal power shaft 540S ; a port 90 degree gearbox 550P and a starboard 90 degree gearbox 550S .

In at least this example, the output shaft end 210 is generally parallel to the longitudinal axis LA of the air vehicle 1 , and the torque interface 510 is coupled to a central degree gearbox 590 that is configured for transmitting the rotation, power and torque of the outputs shaft 210 via the torque interface 510 by 90 degrees to the port transverse power shaft 530P and to the starboard transverse power shaft 530S . Similarly, the port 90 degree gearbox 550P is configured for transmitting the rotation and torque of the port transverse power shaft 530P by 90 degrees to port longitudinal power shaft 540P . Similarly, the starboard 90 degree gearbox 550S is configured for transmitting the rotation and torque of the starboard transverse power shaft 530S by 90 degrees to starboard longitudinal power shaft 540S .

The forward and aft ends of the port longitudinal power shaft 540P are rotatably coupled with the forward and aft rotors 450 of the port pod 110 via suitable gear arrangement, for example via respective 90 degree gearboxes 115 .

The forward and aft ends of the starboard longitudinal power shaft 540S are rotatably coupled with the forward and aft rotors 450 of the starboard pod 110 via suitable gear arrangement, for example via respective rotor 90 degree gearboxes 115 . 0302674312- Thus, the output torque and power from the first engine system 200 and the second engine system 300 together (when mechanically coupled to one another via the coupling system 700 ) is transmitted to the port rotors 450 via the torque interface 510 , central degree gearbox 590 , port transverse power shaft 530P , port 90 degree gearbox 550P , the port longitudinal power shaft 540P , and the port 90 degree gearboxes 115 , and to the starboard rotors 450 via the torque interface 510 , central 90 degree gearbox 590 , starboard transverse power shaft 530S , starboard 90 degree gearbox 550S , the starboard longitudinal power shaft 540S , and the starboard 90 degree gearboxes 115 .

In at least this example, each one of the central 90 degree gearbox 590 , port 90 degree gearbox 550P , starboard 90 degree gearbox 550S and the respective rotor 90 degree gearboxes 115 , is configured for effectively transmitting torque from a respective input rotational axis to a respective output rotational axis, wherein the respective input rotational axis and the respective output rotational axis are at 90 degrees to one another.

For example, each one of the central 90 degree gearbox 590 , port 90 degree gearbox 550P , starboard 90 degree gearbox 550S and the respective rotor 90 degree gearboxes 115 , can include any one of: straight bevel gears; spiral bevel gears; worm gears; Miter gears; screw gears; and so on.

In examples in which the respective rotors 450 are required to rotate at rpm that is different from the rpm of the first engine system 200 and of the second engine system 300 , the respective propulsion system 600 can further include a reduction drive or reduction gearbox (not shown), for example as part of the transmission system 500 , to accommodate these differences. For example such a reduction drive or reduction gearbox can be operated to reduce the relatively high rpm of the first engine system 200 and the second engine system 300 , to the relatively lower rpm of the rotors 450 . Such a reduction drive or reduction gearbox can also serve to increase the available torque to the rotors.

In at least some alternative variations of this example, each one of the central degree gearbox 590 , port 90 degree gearbox 550P , starboard 90 degree gearbox 550S and the respective rotor 90 degree gearboxes 115 , is configured for effectively transmitting torque from a respective input rotational axis to a respective output rotational axis, wherein the respective input axis and the respective output axis are at an inclination angle to one another that is different from 90 degrees (for example in the range between 30 degrees and 0302674312- 150 degrees), and/or one or more of the various transmission power shafts include universal joints, as needed depending on the relative spatial dispositions of the rotors 450 with respect to the first engine system 200 .

In at least some alternative variations of this example, the port transverse power shaft 530P and the starboard transverse power shaft 530Scan be provided as a single transverse power shaft. Additionally or alternatively, the port longitudinal power shaft 540P can be split into a forward port longitudinal power shaft and an aft port longitudinal power shaft, and/or, the starboard longitudinal power shaft 540S can be split into a forward starboard longitudinal power shaft and an aft starboard longitudinal power shaft.

In at least some alternative variations of the above examples, the respective transmission system instead comprises a plurality of flexible drive shafts mechanically and rotatably coupling or otherwise interconnecting each of the rotors 450 to the torque interface 510 , and configured for transmitting power and torque from the torque interface 510 to the rotors 450 via the flexible drive shafts.

In yet other alternative variations of the above examples, the respective transmission system instead comprises a plurality of belt pulley arrangements mechanically and rotatably coupling each of the rotors 450 to the torque interface 510,and configured for transmitting power and torque from the torque interface 510 to the rotors 450 via the belt pulley arrangements.

The first engine system 200 , the second engine system 300 , the coupling system 700 , and optionally part or all of the transmission system 500 , form an air vehicle drive system 900 , and said air vehicle drive system 900 can be coupled to the VTOL rotor system 400 and to the horizontal thrust generating system 800 to provide the propulsion system 600 .

The second engine system 300is operable independently of the first engine system 200 . Said differently, the second engine system 300is configured for operating to generate power independently of whether or not the first engine system 200is operating to generate power. In at least this example, the first engine system 200 and the second engine system 300 are provided as separate and distinct mechanical entities.

In at least this example, the first engine system 200 comprises a first engine 250 . The first engine 250 is a liquid fuel engine, and in particular an internal combustion engine that 0302674312- is fueled by liquid hydrocarbon fuels. In at least some alternative variations of these examples, the first engine can be a turboshaft jet engine, or gas fuel internal combustion engine. According to another aspect of the presently disclosed subject matter, the first engine can be an electrical motor.

Furthermore, the single first engine 250 of the first engine system 200 includes the output shaft end 210 that is mechanically coupled to the torque interface 510 , to thereby transmit the power and torque generated by the first engine system 200 to the VTOL rotor system 400 via the transmission system 500 .

The first engine system 200 , and in particular the first engine 250 , is permanently coupled to the VTOL rotor system 400 via the transmission system 500 , i.e., the first engine system 200 , and in particular the first engine 250 , remains coupled to the VTOL rotor system 400 via the transmission system 500 at least during the entirety of all phases and modes of flight, including aerodynamic forward flight mode FFM , vectored thrust flight mode VFM and transition mode TRM . In operation of the propulsion system 600 , the first engine system 200 , and in particular the first engine 250 , is selectively switched on to generate rotational power during the vectored thrust flight mode VFM and also during a portion of the transition mode TRM ; conversely, the first engine system 200 , and in particular the first engine 250 , is selectively switched off to cease generation of rotational power to the VTOL rotor system 400during the aerodynamic forward flight mode FFM In at least this example, the second engine system 300 comprises a second engine 350 . The second engine 350 is in particular an internal combustion engine that is fueled by liquid hydrocarbon fuels. In at least some alternative variations of these examples, the second engine can be a turboshaft jet engine, or gas fuel internal combustion engine. According to another aspect of the presently disclosed subject matter, the second engine can be an electrical motor.

In at least this example, the second engine system 300 , and in particular the second engine 350 , is configured for being selectively coupled to the VTOL rotor system 400 via the coupling system 700 (and via the transmission system 500 ). In particular, in at least this example, the second engine system 300 , and in particular the second engine 350 , is configured for being selectively coupled to the VTOL rotor system 400 in an indirect 0302674312- manner, i.e., via the transmission system 500 , the coupling system 700 and the first engine system 200 .

Thus, in at least this example, the first engine system 200 and the second engine system 300are selectively coupled to one another in series, via the coupling system 700 .

In at least this example, the coupling system 700 is configured for enabling selectively coupling and selectively decoupling the VTOL rotor system 400 and the second engine system 300 , indirectly, between a first coupled mode CM1 and a first decoupled mode DCM1 . In particular, in at least this example, the coupling system 700 is configured for enabling selectively coupling and selectively decoupling the first engine system 200 and the second engine system 300 , between the first coupled mode CM1 and the first decoupled mode DCM1 .

In at least this example, the coupling system 700 is also configured for enabling selectively coupling and selectively decoupling the second engine system 300 and the horizontal thrust generating system 800 between a second coupled mode CM2 and a second decoupled mode DCM2 .

In at least this example, and as will become clearer herein, the coupling system 700 is further configured for ensuring that: - the coupling system 700 is in the first coupling mode CM1 and in the second decoupling mode DCM2 during vectored thrust flight mode VFM ; - the coupling system 700 is in the first decoupling mode DCM1 and in the second coupling mode CM2 during aerodynamic forward flight mode FFM ; - the coupling system 700 is in the first coupling mode CM1 and in the second coupling mode CM2 during transition mode TRMfrom vectored thrust flight mode VFM to forward flight mode FFM ; - the coupling system 700 is in the first coupling mode CM1 and in the second decoupling mode DCM2 during transition mode TRMfrom forward flight mode FFM to vectored thrust flight mode VFM Thus, in the first coupled mode CM1 , the first engine system 200 and the second engine system 300are mechanically coupled to one another via the coupling system 700 . In vectored thrust flight mode VFM the coupling system is also in the second decoupling mode 0302674312- DCM2 , and thus the combined power and torque generated by the first engine system 200 and the second engine system 300are outputted to the output shaft end 210 , and thus to the torque interface 510 . In this manner, in at least this example, the full power and torque output of the first engine system 200 and the second engine system 300combined are transmitted to the VTOL rotor system 400 , enabling the air vehicle 1 to be flown in vectored thrust flight mode VFM , including vertical take-off, vectored thrust climb, hover, vectored thrust descent and vertical landing. In addition, in vectored thrust flight mode VFM , the VTOL rotor system 400 can be controlled to generate control moments in pitch and/or roll and/or yaw, while maintaining the required total thrust TT required for any one of vertical take-off, vectored thrust climb, hover, vectored thrust descent and vertical landing.

For example, during each portion of vectored thrust flight mode VFM the total thrust TT requirements can vary: for example during hover, the total thrust TT equals the weight W of the air vehicle 1 ; during vertical take-off and vectored thrust climb the total thrust TT exceeds the weight W of the air vehicle 1 ; during vectored thrust descent and vertical landing the total thrust TT is less than the weight W of the air vehicle 1 .

When no control moments are generated during vectored thrust flight mode VFM , the rotors 450 each generate the same thrust T to one another, such that the total (vertical) thrust TT is the summation of the individual (vertical) thrusts T generated by the rotors: TT =  ( T ) = 4 * T During a pitch down maneuver in vectored thrust flight mode VFM , the aft-mounted rotors 450 are controlled by the controller 9 to increase thrust each by a thrust increase +  T , while the fore-mounted rotors 450 are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T . Conversely, for a pitch up maneuver in vectored thrust flight mode VFM , the fore-mounted rotors 450 are controlled by the controller 9 to increase thrust each by a thrust increase +  T , while the aft-mounted rotors 450 are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T .

Thus, the total thrust TT remains the same during the pitch maneuvers: TT =  ( T ) = T +  T + T +  T + T -  T + T -  T = 4 * T In at least this example, in which the VTOL rotor system 400 is configured with constant rpm, variable pitch rotors 450 , the change in thrust required for the pitch maneuver 0302674312- is accomplished by correspondingly varying the pitch of the respective rotor blades of the rotors; the torque and power distribution between the rotors 450 changes according to the thrust being generated by the individual rotors 450 , but the total thrust remains the same. The power and torque output provided by the combination of the first engine system 200 and the second engine system 300 thus is unchanged during the pitch maneuver.

Similarly, during a roll (counter-clockwise, as seen from the front) maneuver in vectored thrust flight mode VFM , the port-mounted rotors 450 are controlled by the controller 9 to increase thrust each by a thrust increase +  T , while the starboard-mounted rotors 450 are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T . Conversely, for a roll (clockwise) maneuver in vectored thrust flight mode VFM , the starboard-mounted rotors 450 are controlled by the controller 9 to increase thrust each by a thrust increase +  T , while the port-mounted rotors 450 are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T .

Thus, the total thrust TT remains the same during the roll maneuvers: TT =  ( T ) = T +  T + T +  T + T -  T + T -  T = 4 * T In at least this example, in which the VTOL rotor system 400 is configured with constant rpm, variable pitch rotors 450 , the change in thrust required for the roll maneuver is accomplished by correspondingly varying the pitch of the respective rotor blades of the rotors; the torque and power distribution between the rotors 450 changes according to the thrust being generated by the individual rotors 450 , but the total thrust remains the same. The power and torque output provided by the combination of the first engine system 200 and the second engine system 300 thus is unchanged during the roll maneuver.

During a yaw (to starboard) maneuver in vectored thrust flight mode VFM , the port-forward-mounted rotor 450 and the starboard-aft mounted rotor 450 (in examples in which the rotors are rotating in a counter clockwise direction when observed from above) are controlled by the controller 9 to increase thrust each by a thrust increase +  T , while the starboard-forward-mounted rotor 450 and the port-aft mounted rotor 450 are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T . Conversely, for a yaw (to port) maneuver in vectored thrust flight mode VFM , the starboard-forward-mounted rotor 450 and the port-aft mounted rotor 450 (in examples in which the rotors are rotating in a 0302674312- clockwise direction when observed from above) are controlled by the controller 9 to increase thrust each by a thrust increase +  T , while the port-forward-mounted rotor 450 and the starboard-aft mounted rotor 450 are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T .

Similarly, in at least some other examples, during a yaw (to starboard) maneuver in vectored thrust flight mode VFM , the port-forward-mounted rotor 450 and the starboard-aft mounted rotor 450 (in examples in which the rotors are rotating in a clockwise direction when observed from above) are controlled by the controller 9 to decrease thrust each by a thrust increase -  T , while the starboard-forward-mounted rotor 450 and the port-aft mounted rotor 450 are controlled by the controller 9 to increase thrust each by a thrust decrease +  T . Conversely, for a yaw (to port) maneuver in vectored thrust flight mode VFM , the starboard-forward-mounted rotor 450 and the port-aft mounted rotor 450 (in examples in which the rotors are rotating in a counter clockwise direction when observed from above) are controlled by the controller 9 to decrease thrust each by a thrust decrease -  T , while the port-forward-mounted rotor 450 and the starboard-aft mounted rotor 450 are controlled by the controller 9 to increase thrust each by a thrust increase +  T .

In any case, in examples in which the rotor turning axes RA are inclined to the vertical by a non-zero inclination angle (for example by 5º), a horizontal component of the differential thrusts can also add or detract from the rolling moment generated by the rotors 450 , depending on the relative direction of the horizontal component with respect to the center of gravity of the air vehicle.

Thus, the total thrust TT remains the same during the yaw maneuvers: TT =  ( T ) = T +  T + T +  T + T -  T + T -  T = 4 * T In at least this example, in which the VTOL rotor system 400 is configured with constant rpm, variable pitch rotors 450 , the change in thrust required for the yaw maneuver is accomplished by correspondingly varying the pitch of the respective rotor blades of the rotors; the torque and power distribution between the rotors 450 changes according to the thrust being generated by the individual rotors 450 , but the total thrust remains the same. The power and torque output provided by the combination of the first engine system 200 and the second engine system 300 thus is unchanged during the yaw maneuver. 0302674312- Similarly, during any combination of pitch and/or roll and/or yaw, the total thrust provided by the generation of power and torque output of the combination of the first engine system 200 and the second engine system 300 remains unchanged during the maneuver as before or after, and is only changed according to whether the air vehicle is transitioning between taking off, climbing, hovering, descending, or landing. However, the power and torque output of the first engine system 200 and the second engine system 300 can nevertheless change according to various factors – for example the altitude, ascent rate or decent rate of the air vehicle, and so on.

Accordingly, the first engine system 200 and the second engine system 300 are together rated for enabling the VTOL rotor system 400 to generate the maximum thrust needed for all parts of the flight envelope in which the air vehicle is operating in vectored thrust flight mode VFM , for example maximum vertical climb acceleration, without the need to further increase the thrust to take account of pitch/roll/yaw maneuvering.

In at least this example, the coupling system 700 comprises a first clutch arrangement 720 and a second clutch arrangement 740 .

In at least this example, the first clutch arrangement 720 selectively couples the first engine system 200 and the second engine system 300to one another in the first coupled mode CM1 , and selectively uncouples the first engine system 200 and the second engine system 300from one another in the first decoupled mode DCM1 .

In at least this example, the second clutch arrangement 740 selectively couples the second engine system 300 and the horizontal thrust generating system 800to one another in the second coupled mode CM2 , and selectively uncouples the second engine system 300and the horizontal thrust generating system 800 from one another in the second decoupled mode DCM2 .

For example, the first clutch arrangement 720 can include for example any suitable mechanical clutch, that is controllably and selectively actuable between the first coupled mode CM1 and the first decoupled mode DCM1 via friction pads, for example via an actuator operatively coupled to the controller 9 . Alternatively, for example, the first clutch arrangement 720 can include for example any electromagnetic clutch, that is controllably and selectively actuable between the first coupled mode CM1 and the first decoupled mode 0302674312- DCM1 via magnetically susceptible powder, for example via suitable magnetic field generator (for example coils) operatively coupled to the controller 9 .

Similarly, for example, the second clutch arrangement 740 can include for example any suitable mechanical clutch, that is controllably and selectively actuable between the second coupled mode CM2 and the second decoupled mode DCM2 via friction pads, for example via an actuator operatively coupled to the controller 9 . Alternatively, for example, the second clutch arrangement 740 can include for example any electromagnetic clutch, that is controllably and selectively actuable between the second coupled mode CM2 and the second decoupled mode DCM2 via magnetically susceptible powder, for example via suitable magnetic field generator (for example coils) operatively coupled to the controller 9 .

In at least this example, the single first engine 250of the first engine system 200 has a power input shaft end 230 as well as the output shaft end 210 , and the single second engine 350of the second engine system 300 has a first power output shaft end 310 and a second power output shaft end 320 .

For example, the power input shaft end 230 and the output shaft end 210 can be opposed ends of a crankshaft of the first engine 250 , while the first power output shaft end 310 and the second power output shaft end 320can be opposed ends of a crankshaft of the second engine 350 .

The first power output shaft end 310 and the power input shaft end 230are each engaged with opposite sides of the first clutch arrangement 720 , such that in the first coupled mode CM1 the first power output shaft end 310 and the power input shaft end 230are operatively and mechanically connected to one another and thus both rotate together, and such that in the first decoupled mode DCM1 the first power output shaft end 310 and the power input shaft end 230are operatively and mechanically disconnected to one another and thus do not rotate together.

It is to be noted that in at least this example, the first engine system 200 and the second engine system 300 , in particular the first engine 250 and the second engine 350 , operate at identical rpm one to the other when coupled to one another in the first coupled mode CM1 . For example, the first engine 250 and the second engine 350can be the same type and model of engine. However, in at least some alternative variations of these examples, 0302674312- in which the first engine system 200 and the second engine system 300 , in particular the first engine 250 and the second engine 350 , operate at different rpm one from the other the other particularly when coupled to one another, a suitable reduction drive or reduction gearbox can be provided (interposed between the first engine system 200 and the second engine system 300 ) to thereby synchronize rpms when the first engine system 200 and the second engine system 300 , in particular the first engine 250 and the second engine 350 , are coupled together via the coupling system 700 . Furthermore, when coupled together, the first engine system 200 and the second engine system 300 are controlled by the controller 9 such as to operate together in a synchronized manner.

Referring again to Figs. 1, 2 and 3, in at least this example, the horizontal thrust generating system 800 comprises a horizontal rotor 850 , rotatably mounted with respect to the air vehicle 1 via a turning shaft 820 having a turning axis TA . In at least this example, the turning axis TA is parallel to the longitudinal axis LA of the air vehicle. In at least this example the horizontal thrust generating system 800 comprises a plurality of rotors 850 , for example two rotors, connected to the turning shaft 820 . However, in at least some alternative variations of these examples, the turning axis TA can be inclined to the longitudinal axis LA of the air vehicle, for example to thereby provide a vertical thrust component (upwards or downwards).

When turned by the second engine system 300 via the coupling system 700(in the second coupled mode CM2 ), the horizontal thrust generating system 800 generates sufficient (horizontal) thrust to enable operating the air vehicle 1 in aerodynamic forward flight mode FFM and in transition mode TRM from vertical thrust flight mode VFM to forward flight mode FFM .

In particular, in at least this example, the second power output shaft end 320 and the turning shaft 820 are each engaged with opposite sides of the second clutch arrangement 740 , such that in the second coupled mode CM2 the second power output shaft end 320 and the turning shaft 820 are operatively and mechanically connected to one another and thus both rotate together, and such that in the second decoupled mode DCM2 the second power output shaft end 320 and the turning shaft 820 are operatively and mechanically disconnected to one another and thus do not rotate together. 0302674312- Thus, in the second coupled mode CM2 , the second engine system 300and the horizontal thrust generating system 800 are mechanically coupled to one another via the coupling system 700 . In aerodynamic forward flight mode FFM the coupling system 700 is also in the first decoupling mode DCM1 , and thus only the power and torque generated by the second engine system 300is outputted to the second output shaft end 320 , and thus to the horizontal thrust generating system 800 via the turning shaft 820 . Concurrently, the first engine system 200 can be shut down, and no power or torque is transmitted to the VTOL rotor system 400 ; the rotors 450 are generally stopped and locked in position during aerodynamic forward flight mode FFM . In this manner, the second engine system 300can be optimized for aerodynamic forward flight mode FFM , for example for cruise, and can thus be significantly smaller than if sized for exclusively providing the required power and torque for vectored thrust flight mode VFM .

In examples in which the horizontal rotor 850of the horizontal thrust generating system 800 is required to rotate at rpm that is different from the rpm of the second engine system 300 , the respective propulsion system 600 can further include a reduction drive or reduction gearbox (not shown), to accommodate these differences. For example such a reduction drive or reduction gearbox can be operated to reduce the relatively high rpm of the second engine system 300 , to the relatively lower rpm of the horizontal rotor 850 . Such a reduction drive or reduction gearbox can also serve to increase the available torque to the horizontal rotor 850 .

During transition mode TRMfrom vectored thrust flight mode VFM to forward flight mode FFM , the coupling system 700 is in the first coupled mode CM1 , and thus the first engine system 200 and the second engine system 300are mechanically coupled to one another, and, concurrently, the coupling system 700 is also in the second coupled mode CM2 , and thus the second engine system 300and the horizontal thrust generating system 800 are also mechanically coupled to one another. Accordingly, the first engine system 200 and the second engine system 300 together provide the thrust required to generate a forward speed via the horizontal thrust generating system 800 , and thus at some point enable the wings 3 to generate aerodynamic lift; at the same time the first engine system 200 and the second engine system 300also provide power to the VTOL rotor system 400which can generate sufficient thrust to maintain hover for example. When the air vehicle has reached a sufficient forward speed in which the wings generate sufficient aerodynamic lift to support 0302674312- the weight of the air vehicle, the air vehicle has fully transitioned to forward flight mode FFM , and the coupling system 700 is in the first decoupled mode DCM1 .

During transition mode TRMfrom forward flight mode FFM to vectored thrust flight mode VFM , the coupling system 700 is in the first coupled mode CM1 , and thus the first engine system 200 and the second engine system 300are mechanically coupled to one another. Concurrently, the coupling system 700 is also in the second decoupled mode DCM2 , and thus the second engine system 300and the horizontal thrust generating system 800 are mechanically uncoupled from one another. Accordingly, the first engine system 200 and the second engine system 300 together enable the VTOL rotor system 400to generate sufficient thrust to maintain hover, for example, while at the same time the air vehicle decelerates in the forward direction. The level of thrust generated by the VTOL rotor system 400 is regulated by the controller 9to compensate for the loss in aerodynamic lift as the air vehicle decelerates to hover. As the air vehicle decelerates to zero forward speed, or at least to stall speed, in which the wings no longer generate aerodynamic lift, the VTOL rotor system 400 generates all the necessary vertical thrust to support the weight of the air vehicle, and the air vehicle has fully transitioned to vectored thrust flight mode VFM .

The first engine system 200 and the second engine system 300 are configured for enabling suitable thrust to be generated for the vectored thrust flight mode VFM , as well as for forward flight mode FFMand transition mode TRM(in either transition direction).

Thus, the maximum power rating P1 of the first engine system 200 and the maximum power rating P2 of the second engine system 300 together (including a suitable safety margin) sum up to provide the maximum vertical power PV required for generating the maximum vertical thrust required in the vectored thrust flight mode VFM and in transition mode TRM , i.e.: PV = P1 + P2 The maximum vertical power PV is thus determinable from the flight and mission requirements for the air vehicle.

Concurrently, the second engine system 300 is also configured for enabling suitable thrust to be generated for the aerodynamic forward flight mode FFMand in transition mode TRM . Thus, the power rating P2 of the second engine system 300 (including a suitable 0302674312- safety margin) is also such as to provide the maximum horizontal power PH required for generating the maximum horizontal thrust required in the aerodynamic forward flight mode VFM or transition mode TRM , and is thus determinable from the flight and mission requirements for the air vehicle.

Once the maximum vertical power PV and the maximum horizontal power PH are determined for the air vehicle, the power P1 required from the first engine system 200 can be determined by the difference between the maximum vertical power PV and the maximum horizontal power PH , i.e.: P1 = PV - PH In at least this example, the power P1 generated by the first engine system and the power P2 generated by the second engine system 300 are the same, and the ratio P1 / P2 is unity, i.e.: P1 = P2 However, in at least some alternative variations of this example and in some other examples, the respective first engine system can instead be configured to generate more power than the respective second engine system (while still complying with the horizontal power PH and the vertical power PV requirements), i.e.: P1 > P2 For example, the ratio P1 / P2 in such cases can be, for example, any one of: 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.6, 1.8, 1.9, 2.0.

Conversely, in at least some other alternative variations of this example and in yet some other examples, the respective first engine system can instead be configured to generate less power than the respective second engine system (while still complying with the horizontal power PH and the vertical power PV requirements), i.e.: P2 > P1 For example, the ratio P2 / P1 in such cases can be, for example, any one of: 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.6, 1.8, 1.9, 2.0.

As discussed above, in the example of Fig. 3, the second engine system 300 is configured for being selectively coupled to the VTOL rotor system 400 indirectly via the 0302674312- transmission system 500 , the coupling system 700 and the first engine system 200 , and furthermore the first engine system 200 and the second engine system 300are selectively coupled to one another in series, via the coupling system 700 .

However, and referring to Fig. 4 for example, in at least some alternative variations of the above examples, the first engine system 200 and the second engine system 300are instead selectively coupled to one another in parallel, via the coupling system 700 . For example, the first engine system 200 is coupled to the VTOL rotor system 400 via the transmission system 500 , but the first engine system 200 is not directly coupled to the second engine system 300 . Rather, the second engine system 300 is instead selectively coupled to the VTOL rotor system 400 via the transmission system 500 and the first clutch arrangement 720 of the coupling system 700 . Thus, the transmission system 500 is configured for receiving in parallel the torque and power generated by the first engine system 200 and the second engine system 300 when the first clutch arrangement 720 is in the first coupling mode CM1 . In the first decoupling mode DCM1 , the second engine system 300 is decoupled directly from the transmission system 500 via the first clutch arrangement 720 . However, as with the example of Fig. 3 mutatis mutandis, the example of Fig. 4 also operates to selectively couple or decouple the second clutch arrangement 740 of the coupling system 700 with respect to the horizontal thrust generating system 800 , according to whether the coupling system 700 is in the second coupling mode CM2or second decoupling mode DCM2 , respectively.

As discussed above, in the examples of Fig. 3 or Fig. 4, the coupling system 700 comprises a first clutch arrangement 720 and a second clutch arrangement 740 that are distinct one from the other.

However, and referring to Fig. 5 for example, in at least some alternative variations of the above examples, the coupling system 700 can be provided instead as a unified mechanism. For example, the first engine system 200 is coupled to the VTOL rotor system 400 via the transmission system 500 but not directly to the second engine system 300 . Rather, the second engine system 300 is selectively coupled to the VTOL rotor system 400 via the transmission system 500 and the coupling system 700 , which is also selectively coupled to the horizontal thrust generating system 800 . The unified coupling system 700 is therefore configured for selectively operating in each one of the following operations modes: 0302674312- - to directly couple the second engine system 300 to the transmission system 500 (and thus to the VTOL rotor arrangement 400 ) and concurrently decouple the horizontal thrust generating system 800from the second engine system 300 , the coupling system 700 thus being in the first coupling mode CM1 and in the second decoupling mode DCM2 ; - to decouple the second engine system 300 from the transmission system 500 (and thus from the VTOL rotor arrangement 400 ) and concurrently couple the horizontal thrust generating system 800to the second engine system 300 , the coupling system 700thus being in the first decoupling mode DCM1and in the second coupling mode CM2 ; - to directly couple the second engine system 300 to the transmission system 500 (and thus to the VTOL rotor arrangement 400 ) and concurrently couple the horizontal thrust generating system 800to the second engine system 300 , the coupling system 700 thus being in the first coupling mode CM1 and in the second coupling mode CM2 ; - to decouple the second engine system 300 from the transmission system 500 (and thus from the VTOL rotor arrangement 400 ) and concurrently decouple the horizontal thrust generating system 800from the second engine system 300 , the coupling system 700thus being in the first decoupling mode DCM1and in the second decoupling mode DCM2 .

Thus, the transmission system 500 is configured for receiving in parallel the torque and power generated by the first engine system 200 and the second engine system 300 when the unified coupling arrangement 700 is in the first coupling mode CM1 . In the first decoupling mode DCM1 , the second engine system 300 is decoupled directly from the transmission system 500 via the unified coupling arrangement 700 . However, as with the examples of Fig. 3 and Fig. 4 mutatis mutandis, the example of Fig. 5 also operates to selectively couple or decouple the unified coupling system 700 with respect to the horizontal thrust generating system 800 , according to whether the coupling system 700is in the second coupling mode CM2or second decoupling mode DCM2 , respectively. 0302674312- As discussed above, in the examples of Fig. 3, Fig. 4 or Fig. 5, the first engine system 200 can comprise a single first engine 250 , and the second engine system 300 can comprise a single second engine 350 . However, in at least some alternative variations of the above examples, the first engine system 200 comprises a plurality of first engines 250 , and/or the second engine system 300 comprises a plurality of second engines 350 .

For example, in such cases the plurality of first engines 250 can be coupled to one another such that the combined torque and power output is provided at a single output shaft end 210 of the first engine arrangement 200 , which is then transmitted to the transmission system 500 , for example as disclosed above for the example of Fig. 3, mutatis mutandis.

Alternatively, and referring to Fig. 6, in at least some alternative variations of the examples, the plurality of first engines 250 can be coupled to one another in two engine groups, marked 200A and 200B in Fig. 6, each engine group including one or more first engine 250 , and such that the combined torque and power output of each engine group is provided at a respective output shaft end 210A, 210B of the first engine arrangement 200 . The respective output shaft ends 210A, 210B can be coupled to the transmission system 500 to transmit power and torque to the VTOL rotor system 400 in a similar manner to the example of Fig. 3, mutatis mutandis. Alternatively, in the example of Fig. 6, the transmission system 500 can be separated into a first transmission system 500A and a second transmission system 500B , each respective output shaft end 210A, 210B being coupled to a different one of the first transmission system 500A and the second transmission system 500B . In the example of Fig. 6, the first transmission system 500A is coupled only to the starboard rotors 450 , and the second transmission system 500B is coupled only to the port rotors 450 . However in at least some alternative variations of this example, the first transmission system 500A is coupled only to the fore rotors 450 , and the second transmission system 500B is coupled only to the aft rotors 450 , for example. In yet at least some other alternative variations of this example, the first transmission system 500A is coupled only to the fore port and aft starboard rotors 450 , and the second transmission system 500B is coupled only to the fore starboard and aft port rotors 450 , for example.

While in the example of Fig. 6, the plurality of first engines 250 are coupled to the second engine system 300 in series via the coupling system 700 , in at least some alternative variations of this example, the plurality of first engines 250 are coupled to the second engine 0302674312- system 300 in parallel, for example in a similar manner to the example of Fig. 4, mutatis mutandis.

Similarly, for example, in such cases the plurality of second engines 350 can be coupled to one another such that selectively part or all of the combined torque and power output is provided at a single first power output shaft end 310 of the second engine arrangement 300 , which is then transmitted to the transmission system 500 , via the coupling system 700 , and such that selectively part or all of the combined torque and power output is provided at a single second power output shaft end 320 of the second engine arrangement 300 , which is then transmitted to the horizontal thrust generating system 800 via the coupling system 700 , for example as disclosed above for the example of Fig. 3, mutatis mutandis.

Alternatively, and referring to Fig. 7, in at least some alternative variations of these examples, the plurality of second engines 350 can be coupled to one another in two engine groups, marked 300A and 300B in Fig. 7, each engine group including one or more second engine 350 , and such that the combined torque and power output of each engine group is provided at a respective first power output shaft end 310A, 310B and/or at a respective second power output shaft end 320A, 320B of the second engine arrangement 300 .

The respective first power output shaft end 310A, 310B can be selectively coupled to the transmission system 500 via the coupling system 700 to transmit power and torque to the VTOL rotor system 400 in a similar manner to the example of Fig. 3 or Fig. 4, mutatis mutandis. Similarly, the respective second power output shaft end 320A, 320B can be selectively coupled to the horizontal thrust generating system 800 via the coupling system 700 to transmit power and torque to the horizontal thrust generating system 800 in a similar manner to the example of Fig. 3 or Fig. 4, mutatis mutandis.

Alternatively, in the example of Fig. 7, the coupling system 700 can be separated into a port coupling system 700A and a starboard coupling system 700B , each respective first power output shaft end 310A, 310B being coupled to the respective first clutch arrangement 720 of a different one of the port coupling system 700A and the starboard coupling system 700B . Similarly, each respective second power output shaft end 320A, 320B being coupled to the respective second clutch arrangement 740 of a different one of the port coupling system 700A and the starboard coupling system 700B . Further in the example of Fig. 7, the horizontal thrust generating system 800 can also be separated into a 0302674312- port horizontal thrust generating system 800A and a starboard horizontal thrust generating system 800B , each being coupled to the respective second clutch arrangement 740 of a different one of the port coupling system 700A and the starboard coupling system 700B . While in the example of Fig. 7, the first engine system 200 is coupled to the second engine system 300 in series via the port coupling system 700A and the starboard coupling system 700B , in at least some alternative variations of this example, the first engine system 200 is coupled to the second engine system 300 in parallel, for example in a similar manner to the example of Fig. 4, mutatis mutandis.

As discussed in the above examples of Figs. 3 to 7, the first engine system 200 and the second engine system 300 in at least these examples are distinct one from the other.

However, and referring to Fig. 9 for example, in at least some alternative variations of the above examples, the functions of the first engine system 200 and the second engine system 300can be provided instead by a unified engine 2300 that is configured for providing the functions of both the first engine system 200 and the second engine system 300 . For example, such a unified engine 2300is a liquid fuel internal combustion engine comprising an engine casing accommodating a first set of pistons 200Aand a second set of pistons 300A , both sets of pistons being reciprocally and rotatably mounted to a common crankshaft 2310 within the engine casing. The first set of pistons 200A corresponds to the first engine system 200 , and the second set of pistons 300Acorresponds to the second engine system 300 . The unified engine 2300 thus essentially integrally includes the first engine system 200 and the second engine system 300 .

Furthermore, in at least example, the coupling system 700 also comprises a first clutch arrangement 720 and a second clutch arrangement 740 , for example as disclosed herein for the example of Fig. 3 but with some differences, mutatis mutandis, as will become clearer herein.

The first clutch arrangement 720 is interposed between the unified engine 2300and the VTOL rotor arrangement 400 , in particular between the unified engine 2300and the transmission system 500 . In the respective first coupling mode CM1 , the first clutch arrangement 720 mechanically couples the unified engine 2300 to the VTOL rotor arrangement 400 (via the transmission system 500 ), while in the respective first decoupling 0302674312- mode DCM1 , the first clutch arrangement 720 mechanically decouples the unified engine 2300from the VTOL rotor arrangement 400 (via the transmission system 500 ).

The second clutch arrangement 740 is interposed between the unified engine 2300and the horizontal thrust generating system 800 , in a similar manner, mutatis mutandis, to the example of Fig. 3, for example. In the respective second coupling mode CM2 , the second clutch arrangement 740 mechanically couples the unified engine 2300 to the horizontal thrust generating system 800 , while in the respective second decoupling mode DCM2 , the second clutch arrangement 740 mechanically decouples the unified engine 2300from the horizontal thrust generating system 800 .

The unified engine 2300 is configured for selectively decoupling the first set of pistons 200Afrom providing power to the crankshaft 2310 (for example by stopping flow of liquid fuel to the first set of pistons 200A ), for example during aerodynamic forward flight mode FFM or during part of the transition mode TRM , so that no power or torque is transmitted by the set of second pistons 200Ato the VTOL rotor system 400 .

Similarly, the unified engine 2300 is optionally configured for selectively decoupling the second set of pistons 300A from providing power to the crankshaft (for example by stopping flow of liquid fuel to the second set of pistons 300A ), so that power and torque are transmitted to the horizontal thrust generating system 800 .

Examples of such a unified engine are known in the art of liquid fuel internal combustion engines, for example, and some such examples are referred to as having "variable cylinder management" (VCM), "cylinder deactivation technology", or "cylinder on demand" (CoD).

Furthermore, for example during aerodynamic forward flight mode FFM or during part of the transition mode TRM , the first set of pistons 200A is selectively decoupled from providing power to the crankshaft 2310 , and the first clutch arrangement 720 is in the first decoupling mode DCM1 , so that no power is provided by the unified engine 2300 to the VTOL rotor system 400 . Concurrently, the second clutch arrangement 740 is in the second coupling mode CM2 enabling the power generated by the second set of pistons 300A to be transmitted to the horizontal thrust generating system 800 . Optionally, and with the first clutch arrangement 720 in the first decoupling mode DCM1 and the second clutch 0302674312- arrangement 740 in the second coupling mode CM2 , the first set of pistons 200A can be selectively coupled to provide power to the crankshaft 2310 , thereby enabling the power generated by the first set of pistons 200A and by the second set of pistons 300A to be together transmitted to the horizontal thrust generating system 800 to thereby enable the horizontal thrust generating system 800 to generate additional thrust.

Similarly, for example during vectored thrust flight mode VFM or part of transition mode TRM , the first set of pistons 200A is also coupled to provide power to the crankshaft 2310 , and the first clutch arrangement 720 is in the first coupling mode CM1 , enabling full power from both sets of pistons to be provided by the unified engine 2300 to the VTOL rotor system 400 . Concurrently, the second clutch arrangement 740 is in the second decoupling mode DCM2 preventing power generated by the unified engine 2300 to be transmitted to the horizontal thrust generating system 800 .

As discussed above, for example in the example of Fig. 3, the first engine system 200 , and in particular the first engine 250 , is permanently coupled to the VTOL rotor system 400 via the transmission system 500 , i.e., the first engine system 200 , and in particular the first engine 250 , remains coupled to the VTOL rotor system 400 via the transmission system 500 at least during the entirety of all phases and modes of flight, including aerodynamic forward flight mode FFM , vectored thrust flight mode VFM and transition mode TRM .

However, and referring to Fig. 9 for example, in at least some alternative variations of the above examples, the first engine system 200 is instead selectively coupled to the transmission system 500 , and thus to the VTOL rotor system 400 , via an auxiliary coupling system 750 . The auxiliary coupling system 750 comprises a third clutch arrangement 760 .

In at least this example, the third clutch arrangement 760 selectively couples the first engine system 200 and the VTOL rotor system 400(via the transmission system 500 ) to one another in the third coupled mode CM3 , and selectively uncouples the first engine system 200 and the VTOL rotor system 400(via the transmission system 500 ) from one another in the third decoupled mode DCM3 .

For example, the third clutch arrangement 760 can include for example any suitable mechanical clutch, that is controllably and selectively actuable between the third coupled mode CM3 and the third decoupled mode DCM3 via friction pads, for example via an 0302674312- actuator operatively coupled to the controller 9 . Alternatively, for example, the third clutch arrangement 760 can include for example any electromagnetic clutch, that is controllably and selectively actuable between the third coupled mode CM3 and the third decoupled mode DCM3 via magnetically susceptible powder, for example via suitable magnetic field generator (for example coils) operatively coupled to the controller 9 .

Also in at least this example, the first engine system 200 (in particular the first engine 250) has an output shaft end 210 , and the transmission system 500 comprises a torque interface 510 .

The output shaft end 210 and the torque interface 510 are each engaged with opposite sides of the third clutch arrangement 760 , such that in the third coupled mode CM3 the output shaft end 210 and the torque interface 510 are operatively and mechanically connected to one another and thus both rotate together, and such that in the third decoupled mode DCM3 the output shaft end 210 and the torque interface 510 are operatively and mechanically disconnected to one another and thus do not rotate together.

Thus, in the third coupled mode CM3 the example of Fig. 9 operates substantially as the example of Fig. 3, mutatis mutandis.

However, in the third decoupled mode DCM3 , it is possible to also selectively concurrently couple the second engine system 200 with the first engine system 300via the coupling system 700 , to thereby boost the power to the horizontal rotor 800 in forward flight mode FFM , which enables the air vehicle to reach higher speeds and/or higher attitudes, albeit at the cost of less endurance, as comparted with operating the air vehicle in the third coupled mode CM3 .

The third clutch arrangement 760 of the example of Fig. 9 can optionally also be incorporated in the examples of Fig. 6 and Fig. 7, in a similar manner to that of Fig. 9, mutatis mutandis.

In the method claims that follow, alphanumeric characters and Roman numerals used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps. 0302674312- Finally, it should be noted that the word “comprising ” as used throughout the appended claims is to be interpreted to mean “including but not limited to ”.

While there has been shown and disclosed examples in accordance with the presently disclosed subject matter, it will be appreciated that many changes may be made therein without departing from the scope of the presently disclosed subject matter as set out in the claims.

Claims

- 41 - 0302674312- CLAIMS:

1. A propulsion system for an air vehicle comprising: a first engine system comprising at least one first engine; a second engine system comprising at least one second engine; a VTOL rotor system; a horizontal thrust generating system; a coupling system; wherein the first engine system is configured for being mechanically coupled with respect to the VTOL rotor system; and wherein the second engine system is selectively mechanically coupled with respect to each one of the VTOL rotor system and the horizontal thrust generating system via the coupling system; and wherein the coupling system is configured for enabling the second engine system to be selectively coupled or decoupled with respect to the VTOL rotor system, and, for independently enabling the second engine system to be selectively coupled or decoupled with respect to the horizontal thrust generating system.

2. The propulsion system according to claim 1, wherein each said first engine is a liquid fuel engine.

3. The propulsion system according to any one of claims 1 to 2, wherein each said first engine is an internal combustion engine.

4. The propulsion system according to any one of claims 1 to 3, wherein each said second engine is a liquid fuel engine.

5. The propulsion system according to any one of claims 1 to 4, wherein each said second engine is an internal combustion engine.

6. The propulsion system according to any one of claims 1 to 5, wherein the propulsion system is configured for being operated in each one of vectored thrust flight mode, aerodynamic forward flight mode, and transition mode.

7. The propulsion system according to claim 6, wherein the first propulsion system and the second propulsion system are configured for together generating sufficient power to the VTOL rotor system to enable the VTOL rotor system to generate sufficient thrust for enabling at least said vectored thrust flight mode.

8. The propulsion system according to any one of claims 6 to 7, wherein the second propulsion system is configured for generating sufficient power to the horizontal thrust - 42 - 0302674312- generating system to enable the horizontal thrust generating system to generate sufficient thrust for enabling at least said aerodynamic forward flight mode.

9. The propulsion system according to any one of claims 1 to 7, wherein the first propulsion system comprises a single said first engine, and wherein the second propulsion system comprises a single said second engine.

10. The propulsion system according to any one of claims 1 to 9, wherein the coupling system is configured for having a first coupling mode and a first decoupling mode, and a second coupling mode and a second decoupling mode, wherein: - in said first coupling mode the coupling system enables the second engine system to be mechanically coupled with respect to the VTOL rotor system, and wherein in said first decoupling mode the coupling system the second engine system to be mechanically decoupled with respect to the VTOL rotor system; and wherein - in said second coupling mode the coupling system enables the second engine system to be mechanically coupled with respect to the horizontal thrust generating system, and wherein in said second decoupling mode the coupling system enables the second engine system to be mechanically decoupled with respect to the horizontal thrust generating system.

11. The propulsion system according to any one of claims 1 to 10, wherein the coupling system comprises a first clutch arrangement and a second clutch arrangement, the first clutch arrangement being configured for selectively and alternately providing each one of the first coupling mode and the first decoupling mode, and the second clutch arrangement being configured for selectively and alternately providing each one of the second coupling mode or the second decoupling mode.

12. The propulsion system according to claim 11, wherein the first clutch arrangement is distinct from the second clutch arrangement.

13. The propulsion system according to any one of claims 11 to 12, wherein the second engine system comprises a first power output shaft end and a second power output shaft end, each one of the first power output shaft end and the second power output shaft end being configured for selectively transmitting power generated by the second engine system. - 43 - 0302674312-

14. The propulsion system according to claim 13, wherein the second clutch arrangement is operatively coupled to the second power output shaft end of the second engine system, and operatively coupled to the horizontal thrust generating system.

15. The propulsion system according to any one of claims 13 to 14, wherein the first clutch arrangement is operatively coupled to the first power output shaft end, and operatively coupled to the VTOL rotor system via the first engine system.

16. The propulsion system according to claim 15, wherein the first engine system and the second engine system are selectively coupled to one another in series via the first clutch system.

17. The propulsion system according to any one of claims 13 to 14, wherein the first clutch arrangement is operatively coupled to the first power output shaft end, and operatively coupled to the VTOL rotor system independently of the first engine system.

18. The propulsion system according to claim 17, wherein the first engine system and the second engine system are selectively coupled to the VTOL rotor system in parallel.

19. The propulsion system according to any one of claims 1 to 18, wherein the VTOL rotor system comprises a plurality of VTOL rotors configured to be coupled to at least the first engine system via a transmission system, each VTOL rotor configured for generating a respective vertical thrust when turned by power generated by the first engine system and the second engine system.

20. The propulsion system according to claim 19, wherein the transmission system comprises a torque interface mechanically coupled to the first engine arrangement, and further comprises a plurality of drive shafts operatively interconnecting the VTOL rotors to the torque interface.

21. The propulsion system according to any one of claims 19 to 20, wherein the propulsion system is configured for providing constant rpm to the VTOL rotors, and wherein the VTOL rotors are configured as variable pitch rotors to thereby enable control of thrust generated by each said VTOL rotor.

22. The propulsion system according to claim 21, wherein the first engine system and the second engine system are configured for providing constant rpm to the VTOL rotors in said vectored thrust flight mode and in said transition mode.

23. The propulsion system according to any one of claims 19 to 22, wherein the VTOL rotor system comprises four said VTOL rotors in polygonal arrangement with respect to one another. - 44 - 0302674312-

24. The propulsion system according to any one of claims 1 to 23, wherein the horizontal thrust generating system comprises at least one horizontal rotor, said horizontal rotor configured for generating a respective horizontal thrust when turned by power generated by the second engine system.

25. The propulsion system according to any one of claims 1 to 24, comprising a controller operatively coupled to at least each one of the first engine system, the second engine system, said coupling system, and said VTOL rotor system.

26. The propulsion system according to claim 25, wherein said controller is configured for selectively operating the propulsion system in any one of said vectored thrust flight mode, said aerodynamic forward flight mode, and said transition mode.

27. The propulsion system according to claim 26, wherein in said vectored thrust flight mode, the controller operates to cause the first engine system to become coupled with the second engine system via the coupling system, and concurrently operates to cause the second engine system to become decoupled with respect to the horizontal thrust generating system via the coupling system.

28. The propulsion system according to claim 26, wherein in said vectored thrust flight mode, the controller operates to cause the VTOL rotor system to become coupled with the second engine system via the coupling system, and concurrently operates to cause the second engine system to become decoupled with respect to the horizontal thrust generating system via the coupling system.

29. The propulsion system according to any one of claims 26 to 28, wherein in said aerodynamic forward flight mode, the controller is configured to operate to cause the first engine system to be decoupled with respect to the second engine system via the coupling system, and concurrently to operate to cause the second engine system to become coupled with respect to the horizontal thrust generating system via the coupling system.

30. The propulsion system according to any one of claims 26 to 29, wherein in said transition mode from vectored thrust flight mode to forward flight mode, the controller is configured to operate to cause the VTOL rotor system to become coupled with respect to the second engine system via the coupling system, and concurrently to operate to cause the second engine system to become coupled with respect to the horizontal thrust generating system via the coupling arrangement.

31. The propulsion system according to any one of claims 26 to 30, wherein in said transition mode from forward flight mode to vectored thrust flight mode, the controller is - 45 - 0302674312- configured to operate to cause the VTOL rotor system to become coupled with respect to the second engine system via the coupling system, and concurrently to operate to cause the second engine system to become decoupled with respect to the horizontal thrust generating system via the coupling arrangement.

32. The propulsion system according to any one of claims 1 to 31, wherein the first engine system and the second engine system are distinct from one another.

33. The propulsion system according to any one of claims 1 to 32, wherein the first engine system is fixedly coupled with respect to the VTOL rotor system.

34. The propulsion system according to any one of claims 1 to 32, further comprising an auxiliary coupling system, wherein the auxiliary coupling system is configured for enabling the first engine system to be selectively coupled or decoupled with respect to the VTOL rotor system.

35. The propulsion system according to any one of claims 1 to 31, wherein said first engine system and said second engine system are integrally included in a unified engine, wherein the unified engine is a liquid fuel internal combustion engine comprising an engine casing accommodating a first set of pistons and a second set of pistons, wherein said first set of pistons and said second set of pistons are reciprocally and rotatably mounted to a common crankshaft within the engine casing, and wherein the first set of pistons corresponds to the first engine system, and the second set of pistons corresponds to the second engine system, and wherein the unified engine is configured for selectively decoupling the first set of pistons from providing power to the crankshaft so that no power or torque is transmitted by the set of second pistons to the VTOL rotor system.

36. The propulsion system according to claim 35, wherein at least during aerodynamic forward flight mode, the propulsion system operates to cause the first set of pistons to be selectively decoupled from providing power to the crankshaft, and the coupling system decouples the unified engine with respect to the VTOL rotor system, and wherein the coupling system couples the unified engine to the horizontal thrust generating system.

37. The propulsion system according to any one of claims 35 to 36, wherein at least during vectored thrust flight mode, the propulsion system operates to cause the first set of pistons to be coupled to provide power to the crankshaft, and the coupling system couples the unified engine to the VTOL rotor system, and wherein the coupling system decouples the unified engine with respect to the horizontal thrust generating system. - 46 - 0302674312-

38. An air vehicle drive system for selectively driving a VTOL rotor system and a horizontal thrust generating system to provide a propulsion system for an air vehicle, the drive system comprising: a first engine system comprising at least one first engine; a second engine system comprising at least one second engine; a coupling system; wherein the first engine system is configured for being mechanically coupled with respect to the VTOL rotor system; and wherein the second engine system is configured for being selectively mechanically coupled with respect to each one of the VTOL rotor system and the horizontal thrust generating system via the coupling system; and wherein the coupling system configured for enabling the second engine system to be selectively coupled or decoupled with respect to the VTOL rotor system, and, for independently enabling the second engine system to be selectively coupled or decoupled with respect to the horizontal thrust generating system.

39. An air vehicle comprising a propulsion system as defined in any one of claims to 37.

40. The air vehicle according to claim 39, comprising a fixed wing arrangement, fuselage, and empennage.

41. The air vehicle according to claim 40, comprising a pair of booms attached to the fixed wing arrangement, and wherein the VTOL rotor system is at least partially mounted with respect to the booms.

42. A method for operating a propulsion system, comprising: - providing the propulsion system as defined in any one of claims 1 to 37; - operating the propulsion system to operate in any one of the vectored thrust flight mode, the aerodynamic forward flight mode, and the transition mode. - 47 - 0302674312- For the Applicants, REINHOLD COHN AND PARTNERS By: