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Title:
DISPLACEMENT CONTROL HYDROSTATIC PROPULSION SYSTEM FOR MULTIROTOR VERTICAL TAKE OFF AND LANDING AIRCRAFT
Document Type and Number:
WIPO Patent Application WO/2019/074859
Kind Code:
A2
Abstract:
A hydraulic propulsion system is disclosed. The system includes one or more input interfaces configured to receive mechanical power from a power source, four or more variable displacement pumps coupled to the one or more input interfaces adaptable to generate a controlled variable quantity of fluid to be pumped out of each of the variable displacement pumps in response to a control input from a corresponding control interface, and four or more positive displacement motors each fluidly coupled to a corresponding variable displacement pump and configured to receive the pumped fluid, wherein each motor is configured to be mechanically coupled to one or more aerodynamic rotors of a multi-rotor vertical take-off and landing aircraft to control thrust and attitude.

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Inventors:
SHANG, Lizhi (Office of Technology Commercialization, 1801 Newman RoadWest Lafayette, IN, 47906, US)
Application Number:
US2018/054890
Publication Date:
April 18, 2019
Filing Date:
October 08, 2018
Export Citation:
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Assignee:
PURDUE RESEARCH FOUNDATION (Office of Technology Commercialization, 1801 Newman RoadWest Lafayette, IN, 47906, US)
International Classes:
B63H11/02
Attorney, Agent or Firm:
PIROOZI, Hamid R. (Piroozi-IP, LLC8500 E. 116th St., #638, Fishers IN, 46038, US)
Download PDF:
Claims:
Claims:

1. A hydraulic propulsion system, comprising:

one or more input interfaces configured to receive mechanical power from a power source;

four or more variable displacement pumps coupled to the one or more input interfaces adaptable to generate a controlled variable quantity of fluid to be pumped out of each of the variable displacement pumps in response to a control input from a corresponding control interface; and

four or more positive displacement motors each fluidly coupled to a corresponding variable displacement pump and configured to receive the pumped fluid, wherein each motor is configured to be mechanically coupled to one or more aerodynamic rotors of a multi-rotor vertical take-off and landing aircraft to control thrust and attitude.

2. The system of claim 1, wherein the power source is one or more internal combustion engines.

3. The system of claim 1, wherein the power source is one or more electric motors.

4. The system of claim 1, wherein the power source is one or more turbine engines.

5. The system of claim 1, wherein the positive displacement motors are fixed displacement motors.

6. The system of claim 1, wherein the positive displacement motors are variable

displacement motors adapted to further change the rotor speed for the corresponding pump fluid flow and wherein the motor displacement is controlled by a motor

displacement control device which is one of an electro-hydraulic displacement control device, mechanical displacement control device, electro-mechanical displacement control device, and a combination thereof.

7. The system of claim 1, wherein the quantity of fluid to be pumped out of each of the variable displacement pumps is controlled by a pump displacement control device which is one of an electro-hydraulic displacement control device, mechanical displacement control device, electro-mechanical displacement control device, and a combination thereof.

8. The system of claim 1, wherein the control input is provided from one of a flight control computer, a pilot, and a combination thereof.

9. The system of claim 1, further comprising a flight control computer coupled to each of the variable displacement pumps and configured to control the quantity of fluid to be pumped from each.

10. The system of claim 9, further configured to receive a signal corresponding to the speed of each motor and to provide the speed information as speed feedback signals to the flight control computer.

11. The system of claim 10, further comprising a closed-loop control arrangement using the speed feedback signals.

12. The system of claim 10, further configured to receive a signal corresponding to the

displacement of each positive displacement motors and to provide the displacement information as displacement feedback signals to the flight control computer.

13. The system of claim 12, further comprising a closed-loop control arrangement using the displacement feedback signals.

14. The system of claim 10, the flight control computer further configured to receive signals corresponding to one or more of position, attitude, and motion of the aircraft and control fluid flow from the variable displacement pumps accordingly to achieve a desired position, attitude and motion of the aircraft.

15. The system of claim 7, further comprising a charge pump adapted to provide power for the pump displacement control device.

16. The system of claim 1, further comprising a fluid cooling device adapted to cool fluid used therein.

17. The system of claim 1, wherein the variable displacement pumps are coupled to each other in series manner.

18. The system of claim 1, wherein the variable displacement pumps are coupled in pairs in a series manner, and each pair is coupled to at least one other pair in a parallel manner.

19. The system of claim 18, wherein each pair is coupled to a dedicated power source.

20. The system of claim 18, wherein each pair is coupled to the one or more input interfaces.

21. The system of claim 20, wherein the one or more input interfaces is a gearbox.

22. The system of claim 18, wherein each pair is coupled to a dedicated input interface which is coupled to a dedicated power source.

23. The system of claim 1, wherein the fluid is a compressible fluid.

24. The system of claim 23, wherein the compressible fluid is air.

25. The system of claim 1, wherein the fluid is an incompressible fluid.

26. The system of claim 24, wherein the incompressible fluid is one of hydraulic oil, water, fuel, antifreeze, and a combination thereof.

Description:
DISPLACEMENT CONTROL HYDROSTATIC PROPULSION SYSTEM FOR MULTI- ROTOR VERTICAL TAKE OFF AND LANDING AIRCRAFT

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application is related to and claims the priority benefit of U.S.

Provisional Patent Application Serial No. 62/571,183 filed October 11, 2017, U.S. Provisional Patent Application Serial No. 62/571,192 filed October 11, 2017, and a counterpart international application to be filed the same day as the present disclosure having the title Aviation Hydraulic Propulsion System Utilizing Secondary Controlled Drives, the contents of each of which are hereby incorporated by reference in their entirety into the present disclosure.

TECHNICAL FIELD

[0002] The present disclosure relates to a hydraulic propulsion system for rotary-wing aircrafts.

BACKGROUND

[0003] This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.

[0004] Multi-rotor vertical take-off and landing (VTOL) aircrafts are becoming commonplace.

These VTOL aircrafts have to be equipped with a minimum of four independently controlled rotors to control motion of the aircraft including pitch, roll, and yaw as well as attitude and translational motion without inclusion of any other motion controlling device. Two important limitations associated with these aircrafts include weight and ability to independently control each rotor. Various propulsion systems are used, such as electrical, mechanical, and

electromechanical. However, each suffer from excessive weight and/or lack of responsiveness limiting their utility. In particular, in order to dynamically control each rotor independently so that a desired attitude can be achieved for the aircraft, a number of complicated devices are typically used which are both heavy and require constant maintenance. [0005] Therefore, there is an unmet need for a novel approach for propulsion of VTOL aircrafts.

SUMMARY

[0006] A hydraulic propulsion system is disclosed. The system includes one or more input interfaces configured to receive mechanical power from a power source, four or more variable displacement pumps coupled to the one or more input interfaces adaptable to generate a controlled variable quantity of fluid to be pumped out of each of the variable displacement pumps in response to a control input from a corresponding control interface, and four or more positive displacement motors each fluidly coupled to a corresponding variable displacement pump and configured to receive the pumped fluid. Each motor is configured to be mechanically coupled to one or more aerodynamic rotors of a multi-rotor vertical take-off and landing aircraft to control thrust and attitude.

[0007] According to one embodiment of the system, the power source is one or more internal combustion engines.

[0008] According to one embodiment of the system, the power source is one or more electric motors.

[0009] According to one embodiment of the system, the power source is one or more turbine engines.

[0010] According to one embodiment of the system, the positive displacement motors are fixed displacement motors.

[0011] According to one embodiment of the system, the positive displacement motors are variable displacement motors adapted to further change the rotor speed for the corresponding pump fluid flow and wherein the motor displacement is controlled by a motor displacement control device which is one of an electro-hydraulic displacement control device, mechanical displacement control device, electro-mechanical displacement control device, and a combination thereof.

[0012] According to one embodiment of the system, the quantity of fluid to be pumped out of each of the variable displacement pumps is controlled by a pump displacement control device which is one of an electro-hydraulic displacement control device, mechanical displacement control device, electro-mechanical displacement control device, and a combination thereof. [0013] According to one embodiment of the system, the control input is provided from one of a flight control computer, a pilot, and a combination thereof.

[0014] According to one embodiment of the system, the system further includes a flight control computer coupled to each of the variable displacement pumps and configured to control the quantity of fluid to be pumped from each.

[0015] According to one embodiment of the system, the system is further configured to receive a signal corresponding to the speed of each motor and to provide the speed information as speed feedback signals to the flight control computer.

[0016] According to one embodiment of the system, the system further includes a closed-loop control arrangement using the speed feedback signals.

[0017] According to one embodiment of the system, the system is further configured to receive a signal corresponding to the displacement of each positive displacement motors and to provide the displacement information as displacement feedback signals to the flight control computer.

[0018] According to one embodiment of the system, the system further includes a closed-loop control arrangement using the displacement feedback signals.

[0019] According to one embodiment of the system, the flight control computer further configured to receive signals corresponding to one or more of position, attitude, and motion of the aircraft and control fluid flow from the variable displacement pumps accordingly to achieve a desired position, attitude and motion of the aircraft.

[0020] According to one embodiment of the system, the system further includes a charge pump adapted to provide power for the pump displacement control device.

[0021] According to one embodiment of the system, the system further includes a fluid cooling device adapted to cool fluid used therein.

[0022] According to one embodiment of the system, the variable displacement pumps are coupled to each other in series manner.

[0023] According to one embodiment of the system, the variable displacement pumps are coupled in pairs in a series manner, and each pair is coupled to at least one other pair in a parallel manner.

[0024] According to one embodiment of the system, each pair is coupled to a dedicated power source. [0025] According to one embodiment of the system, each pair is coupled to the one or more input interfaces.

[0026] According to one embodiment of the system, the one or more input interfaces is a gearbox.

[0027] According to one embodiment of the system, each pair is coupled to a dedicated input interface which is coupled to a dedicated power source.

[0028] According to one embodiment of the system, the fluid is a compressible fluid.

[0029] According to one embodiment of the system, the compressible fluid is air.

[0030] According to one embodiment of the system, the fluid is an incompressible fluid.

[0031] According to one embodiment of the system, the incompressible fluid is one of hydraulic oil, water, fuel, antifreeze, and a combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

[0032] FIGs. 1A and IB are schematic representations of directional movements of an aircraft from a top view (FIG. 1A) and a side view (FIG. IB).

[0033] FIG. 2A is a schematic of an embodiment of a hydraulic propulsion system according to the present disclosure.

[0034] FIG. 2B is a schematic of another embodiment of a hydraulic propulsion system according to the present disclosure.

[0035] FIG. 2C is a schematic of yet another embodiment of a hydraulic propulsion system according to the present disclosure.

[0036] FIG. 2D is a schematic of yet another embodiment of a hydraulic propulsion system according to the present disclosure.

[0037] FIGs. 3A and 3B are schematics of a top view (FIG. 3A) and a side view (FIG. 3B) of an exemplary embodiment of the hydraulic propulsion system of any one of FIGs. 2A, 2B, 2C, or 2D.

[0038] FIGs. 3C and 3D are top view schematics of alternative embodiments with six rotors (FIG. 3C) and eight rotors (FIG. 3D).

[0039] FIGs. 3E and 3F are side view schematics of alternative embodiments with four rotors, according to the present disclosure. [0040] FIGs. 3G, 3H, and 31 are side view schematics of alternative embodiments based on one or more fixed displacement motors driving one or more rotors directly or via a gearbox.

[0041] FIG. 4A is a schematic of a propulsion control system according to the present disclosure which can be used in conjunction with one or more of the embodiments disclosed herein.

[0042] FIG. 4B is a schematic of another propulsion control system according to the present disclosure which can be used in conjunction with one or more of the embodiments disclosed herein.

[0043] FIG. 4C is a control block diagram according to the present disclosure which can be used in conjunction with one or more of the embodiments disclosed herein.

[0044] FIG. 5 is a schematic of another embodiment of a hydraulic propulsion system according to the present disclosure.

[0045] FIG. 6A is a schematic of another propulsion control system according to the present disclosure which can be used in conjunction with one or more of the embodiments disclosed herein.

[0046] FIG. 6B is a schematic of another propulsion control system according to the present disclosure which can be used in conjunction with one or more of the embodiments disclosed herein.

[0047] FIG. 6C is a control block diagram according to the present disclosure which can be used in conjunction with one or more of the embodiments disclosed herein.

[0048] FIG. 7 is a schematic of another embodiment of a hydraulic propulsion system according to the present disclosure.

DETAILED DESCRIPTION

[0049] For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.

[0050] In the present disclosure, the term "about" can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range. [0051] In the present disclosure, the term "substantially" can allow for a degree of variability in a value or range, for example, within 90%, within 95%, or within 99% of a stated value or of a stated limit of a range.

[0052] The propulsion system according to the present disclosure is related to a family of multi- rotor aircrafts that include at least four rotors which are independently speed controlled. The propulsion system according to the present disclosure is used to control the different speeds of a minimum of four rotors to achieve motion in pitch, roll, and yaw directions as shown in FIGs. 1A and IB. The thrust generated from the aerodynamic rotors is also used to overcome gravity and drag and to control the attitude of the aircraft. In FIGs. 1A and IB, schematics of a top view and side view, respectively, of the aircraft is shown. Yaw is defined as the rotational movement about a z-axis that passes vertically through the center of the aircraft. Roll is defined as the rotational movement about an x-axis that passes horizontally through the center of the aircraft. Pitch is defined as the rotational movement about a y-axis that passes horizontally through the center of the aircraft.

[0053] The propulsion system according to the present disclosure utilizes at least four hydrostatic transmissions to distribute and transmit mechanical power from a power source (e.g., single or multiple internal combustion engines, or turbine engines, or electric motors) to the rotors (which can be single or multiple propellers, fans, or compressors). Each hydrostatic transmission contains a variable displacement hydraulic pump, at least one fixed or variable displacement motor, and the pipes and hoses that connect the pump and the motor. The rotor speed is controlled by the displacement of the pump. The displacement of the pump is controlled by a control arrangement including electrical, mechanical, electromechanical, hydraulic, electrohydraulic, mechanical-hydraulic actuators, by human power through appropriate linkages, or any combination thereof, further described below.

[0054] This hydrostatic propulsion system of the present disclosure is configured to control the speed of each individual rotor with faster response and lower weight in comparison with prior art propulsion system counterpart owing to the bandwidth of the displacement control and the compactness of hydraulic units. As a result, a more stable flight and more useful payload capability can be achieved. The reliability of the aircraft increases due to the highly reliable nature of hydraulic systems. Furthermore, the aircraft power source (e.g., internal combustion engine) can be arranged to run at relatively constant speed, which extends the lifetime of such power source. Furthermore, since hydraulic components are made of metal, the propulsion system of the present disclosure can be made with less cost and is further readily recyclable.

[0055] The present disclosure is related to a counterpart application to be filed the same day as the present disclosure having the title Aviation Hydraulic Propulsion System Utilizing Secondary

Controlled Drives. The difference between the present disclosure and this counterpart application lies in control strategies. In the present disclosure the control scheme is based on primary or displacement control in which the hydraulic propulsion system controls the rotational speed of the hydraulic motor by changing the pump displacement. As such, the motors can be fixed displacement or variable displacement. In case of using variable displacement motor, the motor displacement changes only to assist the pump to achieve improved overall performance. In the displacement control hydraulic propulsion system of the present disclosure, the bandwidth of the thrust is substantially determined by the bandwidth of the pump. According to the present disclosure, in order to control four propeller speeds independently, at least 4 pumps and 4 motors are required for the displacement control hydraulic propulsion system. In contrast, the disclosure found in the counterpart application is based on a secondary control hydraulic propulsion system which controls the output (i.e., speed of the propellers) by changing the motor displacement. As such, the pumps can be fixed displacement or variable displacement. In case of using fixed displacement pump, the system pressure is adjusted by utilizing a valve network. In case of using variable displacement pump, the pump displacement changes to adjust the system pressure. In the secondary control hydraulic propulsion system, the bandwidth of the thrust is substantially determined by the bandwidth of the motor. In case of multiple motors, each motor speed can be controlled independently. Therefore, in order to control, e.g., 4 propeller speeds independently, at least 1 pump and 4 motors are required for the secondary control hydraulic propulsion system.

Comparing to the counterpart application, the present disclosure benefits from the control strategy simplicity and the lightweight owing to the fixed displacement motors.

[0056] Referring to FIG. 2A, a schematic of an embodiment of a propulsion system 1000A according to the present disclosure is shown. An engine 48 is mechanically coupled and powers directly via a driveshaft 50 four variable displacement pumps 35, 36, 37, and 38 disposed in a series manner such that rotational speed of each pump 35-38 (also, the driveshaft 50) is substantially the same. IT should be appreciated that while an engine (engine 48) is shown in

FIG. 2A, other power sources such as electric motors, can be substituted for the engine 48. Hydraulic lines 39, 40, 41, 42, 43, 44, 45, and 46 couple the variable displacement pumps 35, 36, 37, and 38 with fixed displacement motors 7, 8, 9, and 10. Each motor 7, 8, 9, and 10 is in mechanical communication and drives aerodynamic rotors 3, 4, 5, and 6 which convert power transmitted by the driveshaft 50 into lift and thrust. Each variable displacement pump 35, 36, 37, and 38 can be a positive displacement machine such as an axial, radial piston or vane-type machine, or combinations thereof which generally comprise an array of displacement elements arranged radial or axial to the driveshaft 50. For example, in axial piston machines, one or more piston-cylinder combinations are arranged axially in a cylinder block. An example is shown in US publication 20120079936 for Ivantysynova et al., incorporated by reference herein in its entirety. For example, by linear movement of a piston within a cylinder, the piston which is coupled to a swash plate controls the swash plate angle which controls the output flow of the variable displacement pump which can in turn control the speed of the fixed displacement motor, to thereby control the speed of the aerodynamic rotor driven by the fixed displacement motor. By adjusting the inclination of the swash plate to one extreme, e.g., a vertical position, the displacement of the pump can be decreased to about zero. The angle of the swash plate can be controlled by electrical, mechanical, electromechanical, hydraulic, electrohydraulic, mechanical- hydraulic actuators, human power though appropriate linkages, or any combination thereof. By adjusting the swash plate inclination independently and separately, each of the variable displacement pumps 35, 36, 37, and 38 can be configured to communicate different amounts of flow to a respective fixed displacement motor 7, 8, 9, and 10 causing each of the coupled rotors 3, 4, 5, and 6 to rotate at a desired speed. By doing so, the desired directional pitch, roll, and yaw and translational motion can be achieved such that the aircraft can move from a Cartesian position X1Y1Z1 with a movement defined by a vector Al to a Cartesian position X2Y2Z2 with a movement defined by a vector A2.

[0057] Referring to FIG. 2B, a schematic of another embodiment of a propulsion system 1000B according to the present disclosure is provided. The embodiment shown in FIG. 2B differs from the embodiment shown in FIG. 2A, in that the engine 48 is coupled to a gearbox 47 via a driveshaft 49 prior to being coupled to the driveshafts 50A and 50B. Additionally, the variable displacement pumps 37 and 38 are provided in series and the combination is provided in parallel to the series coupled variable displacement pumps 35 and 36 such that rotational speed of the drive shaft 50A is the same for the variable displacement pumps 37 and 38 and the rotational speed of the drive shaft 50B is the same for the variable displacement pumps 35 and 36. Given the gear ratios of the gearbox 47, rotational speed of driveshafts 50A and 50B can be set to be substantially the same or different. By adjusting the displacement of each pump 35, 36, 37, and 38 the propulsion system 1000B can be configured to communicate different amounts of flow to a respective fixed displacement motor 7, 8, 9, and 10 causing each of the coupled rotors 3, 4, 5, and 6 to rotate at a desired/selective speed.

[0058] Referring to FIG. 2C, a schematic of yet another embodiment of a propulsion system lOOOC according to the present disclosure is provided. The embodiment shown in FIG. 2C differs from the embodiment shown in FIG. 2A, in that two engines 48A and 48B are coupled to the driveshafts 50A and 50B which are coupled to the variable displacement pumps 37 and 38 which are provided in series and the combination is provided in parallel to the series coupled variable displacement pumps 35 and 36 such that rotational speed of the drive shaft 50A is the same for variable displacement pumps 37 and 38 and the rotational speed of the drive shaft 50B is the same for the variable displacement pumps 35 and 36. The rotational speed of driveshafts

50A and 50B can be set to be substantially the same or different by operating the respective engines 48A and 48B at substantially the same or different speeds. By adjusting the displacement of each variable displacement pump 35, 36, 37, and 38 the propulsion system lOOOC can be configured to communicate different amounts of flow to a respective fixed displacement motor 7,

8, 9, and 10 causing the coupled rotor 3, 4, 5, and 6 to each rotate at a desired/selective speed. In

FIG. 2C, each engine powers part of the propulsion system, advantageously allowing placement of the engines 48A and 48B and the variable displacement pumps 37, 38 and 35, 36 closer to the fixed displacement motors 7, 8 and 9, 10, and the propeller 3, 4 and 5, 6, respectively.

[0059] Referring to FIG. 2D, a schematic of yet another embodiment of a propulsion system

1000D according to the present disclosure is provided. The embodiment shown in FIG. 2D differs from the embodiment shown in FIG. 2B, in that two engines 48 A and 48B are coupled to a gearbox 47 via driveshafts 49 A and 49B prior to being coupled to the driveshafts 50A and 50B, respectively. These driveshafts are in turn coupled to the variable displacement pumps 37 and 38 which are provided in series and the combination is provided in parallel to the series coupled variable displacement pumps 35 and 36 such that rotational speed of the drive shaft 50A is the same for pumps 37 and 38 and the rotational speed of the drive shaft 50B is the same for pumps

35 and 36. The rotational speed of driveshafts 50A and 50B can be set to be substantially the same or different by operating the respective engines 48A and 48B at substantially the same or different speeds or by choosing different gear ratios in the gearbox 47. By adjusting the displacement of each variable displacement pump 35, 36, 37, and 38 the propulsion system 1000D can be configured to communicate different amounts of flow to the respective fixed displacement motor 7, 8, 9, and 10 causing the coupled rotor 3, 4, 5, and 6 to each rotate at a desired/selective speed. In FIG. 2D, all the engines power the propulsion system together The advantage of the propulsion system 1000D is the redundancy of multiple engines such that when one engine 48A or 48B has experienced a complete or partial engine failure, the propulsion system will work at s reduced power level.

[0060] Referring to FIGs. 3 A and 3B, schematics of a top view and a side view of an exemplary embodiment of the propulsion system 1000A, 1000B, lOOOC, or 1000D according to the present disclosure are provided. Four individually controlled rotors 3, 4, 5, and 6 are driven by four fix displacement motors 7, 8, 9, and 10. The center compartment 2 contains the common components that are shared by all the rotors, such as a flight control computer and a shared power source. The payload may also be located at the center of the aircraft. For manned aircraft, the center compartment 2 also includes the cockpit and the cabin (not shown). The aircraft is further defined by a frame 31.

[0061] The propulsion system according to the present disclosure can also have more than four rotors. Schematics of exemplary embodiments with 6 and 8 rotors are shown in FIGs. 3C and

3D, respectively. In addition to rotors 3, 4, 5, and 6, in FIG. 3C rotors 11 and 12 are also provided. Rotors 11 and 12 are coupled to motors 13 and 14, in a similar manner as discussed above. In addition to rotors 3, 4, 5, 6, 11, and 12, in FIG. 3D rotors 15 and 16 are also provided.

Rotors 15 and 16 are coupled to motors 17 and 18 in a similar manner as discussed above. Other implementations with different number of rotors are also possible. For example, 5 or 7 rotors may be implemented, or more than 8 rotors may be implemented in a similar fashion.

[0062] Besides the arrangements shown in FIGs. 3C and 3D, other arrangements with respect to the position of the rotors may also be possible. Referring to FIGs. 3E and 3F schematics of different rotor layouts are provided as alternative layouts to those shown in other exemplary embodiments of the propulsion system of the present disclosure. In addition to motor 8 being coupled to rotor 4, representing a first set of motor-rotor combination, in FIG. 3E, motor 24 is also provided and is coupled to rotor 20, representing a second set of motor-rotor combination. In addition, motor 9 being coupled to rotor 5, representing a third set of motor-rotor combination, in FIG. 3E, motor 25 is also provided and is coupled to rotor 21, representing a fourth set of motor-rotor combination. Alternatively, in addition to motor 8 being coupled to rotor 4, representing a first set of motor-rotor combination, in FIG. 3F, motor 32 is also provided and is coupled to rotor 28, representing a second set of motor-rotor combination, such that the two sets are coupled to each other in a parallel manner. In addition, motor 9 being coupled to rotor 5, representing a third set of motor-rotor combination, in FIG. 3F, motor 33 is coupled to rotor 29, representing a fourth set of motor-rotor combination, such that the third and fourth sets are coupled to each other in a parallel manner.

[0063] In another embodiment, according to the present disclosure, with reference to FIG. 3G, a schematic is provided of a single fix displacement motor 7 driving two or more rotors 58 and 59 via a gearbox 54.

[0064] In yet another embodiment, according to the present disclosure, with reference to FIG. 3H, a schematic is provided of two or more fix displacement motors 7 and 66 coupled in series driving one rotor 3.

[0065] In still yet another embodiment, according to the present disclosure, with reference to FIG. 31, a schematic is provided of one fix displacement motor 7 driving one rotor 3 via a gearbox 67.

[0066] With reference to FIG. 4A, a schematic of a propulsion system 100A according to the present disclosure is provided that can be used in conjunction with one or more of the

embodiments disclosed herein. The system of FIG. 4A is a closed circuit system with an open- loop speed control provision. A single or multiple variable displacement pumps 35 are used to control flow to a single or multiple hydraulic fixed displacement motors 7 to control the speed of the aerodynamic rotor 3. As described above, the speed of the hydraulic motor 7 and the aerodynamic rotor 3 is controlled by the displacement of the pump 35 which is driven by the engine 48 via a shaft 49 and a distributing gearbox 47. A displacement control device 70 is used to adjust the pump displacement according to a command signal generated from the pilot or a flight control computer in an open-loop manner. According to one embodiment of the present disclosure, the hydraulic drive of single or multiple rotors contains an optional cooling device 72 for cooling the hydraulic fluid used therein. In FIG. 4A, a reservoir 71 is shown in fluid communication with the low-pressure side of the system. The reservoir 71 can be a pressurized tank, bootstrap reservoir, or a low-pressure hydro-pneumatic accumulator.

[0067] With reference to FIG. 4B, a schematic of a propulsion system 100B according to the present disclosure is provided that can also be used in conjunction with one or more of the embodiments disclosed herein. Differences between FIGs. 4B and 4A include a charge pump 85 and valve 87. The charge pump 85 can also be used to provide the power for the displacement control device 70 of the variable displacement pump 35. According to the embodiment shown in FIG. 4B, a charge pump 85 is used with each variable displacement pump 35; or in some cases, a charge pump 85 is shared by a number of variable displacement pumps 35. Valve 87 isolates the charge pump 85 from the hydraulic motor and prevents unwanted rotation of the rotor. In any of these cases, the charge pump is in fluid communication with the reservoir 71 to obtain fluid therefrom. In FIG. 4B, the reservoir 71 is also optionally shown to be fluidly coupled to the case train line connection of both the variable displacement pumps 35 and the fixed (or variable) displacement motor 7.

[0068] With reference to FIG. 4C, a control scheme 200 is shown that can be used to control one or more of the embodiments, and in particular the propulsion system 100A shown in FIG. 4A. The control scheme 200 starts with a desired attitude for the aircraft as an input from a computer or a pilot. The desired attitude is computationally combined with signals from attitude sensors (e.g., yaw, pitch, and roll sensors), and fed to the flight control computer to generate the desired pump displacement for the variable displacement pumps to correct the error between the desired and the actual attitude due to the change of the desired attitude and the disturbance (e.g., a gust of wind or the shift of the center of mass). The actual attitude with error is thus sensed with the attitude sensors (described above) and mathematically measured against the desired attitude. The flight control computer generates the associated control signals for each of the variable displacement pumps (e.g., 35, 36, 37, and 38 shown in e.g., FIG. 2A). These signals are fed to displacement control devices (e.g., swash plate control devices described above) which thereby control the rotational speed of the propeller. Selective rotational speed of each rotor then change the actual attitude to reduce the attitude error .

[0069] With reference to FIG. 5, another aspect of the present disclosure includes an open loop speed control implementation of the propulsion system. A flight control computer 78 provides control signals to displacement control devices 70, 75, 76, and 77 (e.g., swash plate control devices described above) which in turn control displacement of the variable displacement pumps 35, 36, 37, and 38 driven by the engine 48 via a run-through driveshaft 50. The variable displacement pumps 35, 36, 37, and 38 are coupled and deliver flow to fixed displacement motors 7, 8, 9, and 10, through high pressure hydraulic lines 39, 41, 43, and 45.

[0070] With reference to FIG. 6A, a schematic of a propulsion system 300A according to the present disclosure is provided that can be used in conjunction with one or more of the embodiments disclosed herein. The propulsion system 300A of FIG. 6A is a closed circuit with a close-loop speed control provisions. The embodiment shown in FIG. 6A includes a speed sensor 73 and speed controller 74 providing a closed loop system for controlling speed of the fixed displacement motor 7 and the rotor 3. In another aspect of the present disclosure, a pilot can replace the flight control computer 78. Alternatively, the variable displacement pump 35 according to one embodiment of the present disclosure includes a mechanical feedback device for its displacement that can be used to determine actual displacement of the variable displacement pump 35 vs. desired displacement. According to another aspect of the present disclosure, the variable displacement pump 35 contains an electric sensor to measure the displacement and a micro controller to compare the actual displacement to the displacement commanded from the pilot or flight control computer and to adjust the pump displacement accordingly.

[0071] With reference to FIG. 6B, a schematic of a propulsion system 300B according to the present disclosure is provided that can also be used in conjunction with one or more of the embodiments disclosed herein. Differences between FIGs. 6B and 6A include a charge pump 85 and valve 87. According to the embodiment shown in FIG. 6B, a charge pump 85 is used with each variable displacement pump 35; or in some cases, a charge pump 85 is shared by a number of variable displacement pumps 35. The charge pump 85 provides the hydraulic power for the displacement control device 70 of the variable displacement pumps 35. Valve 87 isolates the charge pump 85 from the hydraulic motor and prevents unwanted rotation of the rotor. In any of these cases, the charge pump is in fluid communication with the reservoir 71 to obtain fluid therefrom. In FIG. 6B, the reservoir 71 is also optionally shown to be fluidly coupled to the case train line connection of both the variable displacement pumps 35 and the fixed (or variable) displacement motor 7. [0072] With reference to FIG. 6C, a control scheme 400 is shown that can be used to control one or more of the embodiments, and in particular the propulsion system 300A shown in FIG. 6A. The control scheme 400 starts with a desired attitude for the aircraft as an input from a computer or a pilot. The desired attitude is computationally combined with signals from attitude sensors (e.g., yaw, pitch, and roll sensors), and fed to the flight control computer to generate the desired speed for each of the rotors to correct the error between the desired and the actual attitude due to the change of the desired attitude and the disturbance (e.g., a gust of wind or the shift of the center of mass). The actual attitude with error is thus sensed with the attitude sensors (described above) and mathematically measured against the desired attitude. Associated sensors for each rotor provide feedback signals that can be computationally added (compared) to the commanded rotor speed. This comparison provides a way to compensate for the speed of each rotor by controlling the variable displacement pumps. The flight control computer generates the associated control signals for each of the variable displacement pumps (e.g., 35, 36, 37, and 38 shown in e.g., FIG. 2A). These signals are fed to displacement control devices (e.g., swash plate control devices described above) which thereby control the output for each variable displacement pump which can then generate selective rotor speed control. Selective rotational speed of each rotor then change the actual attitude to reduce the attitude error.

[0073] With reference to FIG. 7, another aspect of the present disclosure which is based on a closed loop control system is presented. The flight control computer 78 generates command signals for motor speed and each motor speed is close loop controlled by the speed controllers 74, 79, 80, and 81, and the displacement control devices 70, 75, 76, and 77 (e.g., swash plate control devices described above) using the signal from the speed sensor 73, 82, 83, and 84.

According to another aspect of the present disclosure, the speed controllers 74, 79, 80, and 81 are integrated into the flight control computer 78. According to yet another aspect of the present disclosure, the speed controllers 74, 79, 80, and 81 are integrated into the variable displacement pumps 35, 36, 37, and 38.

[0074] It should be appreciated that each motor described herein can be a fixed or variable displacement motor. In the variable displacement embodiments of the motor, the displacement of the motor is changed in order to further change the rotor speed based on the corresponding pump flow. [0075] Those having ordinary skill in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.