HYBRID MULTIROTOR PROPULSION SYSTEM FOR AN AIRCRAFT

Abstract

A hybrid multirotor propulsion system for an aircraft includes a plurality of propulsion units, each propulsion unit having a propeller, an electromotor and a peripheral differential gearbox; a plurality of driving elements, each of which is coupled to a respective one of the plurality of propulsion units; a mechanical power source; a main distributor gearbox; at least one electric machine; and a power management unit. The power management unit is configured according to a predetermined operating mode, which causes the mechanical power source to output first and second mechanical power components; and distributing the first mechanical power component to provide each driving element with a direct mechanical propeller power; and causes the electric machine to convert the second mechanical power component into electric power, part of which provides each electromotor with an electric propeller power. The direct mechanical propeller power causes each electromotor to convert the electric propeller power into an indirect mechanical propeller power, outputted to the peripheral differential gearbox; and causes the peripheral differential gearbox of each propulsion unit to aggregate the direct mechanical propeller power and the indirect mechanical propeller power to a total mechanical propeller power which drives the propeller of each propulsion unit.

Claims

1. A hybrid multirotor propulsion system for an aircraft, comprising: a plurality of propulsion units (PU), each propulsion unit (PU) comprising a propeller (Pr), an electromotor (EU) and a peripheral differential gearbox (PDGU); a plurality of driving elements (PDS), each of which being coupled to a respective one of said plurality of propulsion units (PU); a mechanical power source (MPS); a main distributor gearbox (MDG); at least one electric machine (EM); and a power management unit (PMU); wherein said power management unit (PMU) is configured according to a predetermined operating mode: to cause said mechanical power source (MPS) to output total mechanical power and to cause said total mechanical power to be split into first and second mechanical power components; to cause said main distributor gearbox (MDG) to distribute said first mechanical power component to said plurality of driving elements (PDS) for providing each driving element (PDS) with a direct mechanical propeller power; to cause said electric machine (EM) to convert said second mechanical power component into electric power and to cause at least a part of said electric power to be distributed to said plurality of electromotors (EU) for providing each electromotor (EU) with an electric propeller power; to cause each driving element (PDS) to output said direct mechanical propeller power to said peripheral differential gearbox (PDGU) of said propulsion unit (PU) to which said driving element (PDS) is coupled; to cause each electromotor (EU) to convert said electric propeller power into an indirect mechanical propeller power and to output said indirect mechanical propeller power to said peripheral differential gearbox (PDGU) of said propulsion unit (PU) comprising said electromotor (EU); and to cause said peripheral differential gearbox (PDGU) of each propulsion unit (PU) to aggregate said direct mechanical propeller power and said indirect mechanical propeller power to a total mechanical propeller power and to drive said propeller (Pr) of each propulsion unit (PU) based on said total mechanical propeller power.

2. The hybrid multirotor propulsion system according to claim 1, further comprising an electrical energy storing device which is connected to said electric machine (EM) and to said plurality of electromotors (EU), wherein said power management unit (PMU) is configured to cause said electrical energy storing device to store said electric power generated by said electric machine (EM) and to distribute at least a part of said stored electric power to said plurality of electromotors (EU).

3. The hybrid multirotor propulsion system according to claim 2, wherein said electrical energy storing device comprises at least one capacitor unit (CU) and/or at least one rechargeable battery.

4. The hybrid multirotor propulsion system according to claim 1, wherein said mechanical power source (MPS) comprises first and second power outputs, and wherein said power management unit (PMU) is configured to cause said mechanical power source (MPS) to output said first mechanical power component from said first power output to said main distributor gearbox (MDG) and to cause said mechanical power source (VIPS) to output said second mechanical power component from said second power output to said electric machine (EM).

5. The hybrid multirotor propulsion system according to claim 1, wherein said mechanical power source (MPS) comprises a single power output, wherein said power management unit (PMU) is configured to cause said mechanical power source (MPS) to output said total mechanical power from said single power output to one of said main distributor gearbox (MDG) and said electric machine (EM), and wherein said power management unit (PMU) is configured to cause said one of said main distributor gearbox (MDG) and said electric machine (EM) to split said total mechanical power into said first and second mechanical power components.

6. The hybrid multirotor propulsion system according to claim 1, wherein said main distributor gearbox (MDG) comprises a plurality of power outputs, each of which being coupled to a respective one of said plurality of drive elements.

7. The hybrid multirotor propulsion system according to claim 6, wherein said electric machine (EM) is coupled to one of said plurality of power outputs of said main distributor gearbox (MDG).

8. The hybrid multirotor propulsion system according to claim 6, wherein said main distributor gearbox (MDG) comprises an additional power output, and wherein said electric machine (EM) is coupled to said additional power output of said main distributor gearbox (MDG).

9. The hybrid multirotor propulsion system according to claim 1, wherein said power management unit (PMU) is configured to control each peripheral differential gearbox (PDGU) based on a variable ratio of said direct and indirect mechanical propeller powers.

10. The hybrid multirotor propulsion system according to claim 1, wherein said power management unit (PMU) is configured to vary said indirect mechanical propeller power for fine tuning said total mechanical propeller power, said indirect mechanical propeller power being smaller than said direct mechanical propeller power.

11. The hybrid multirotor propulsion system according to claim 1, wherein said electric machine (EM) is configured to be operated both in a generator mode for converting said second mechanical power component into electric power and in an electromotor mode for converting electric power stored in said electrical energy storing device into mechanical power.

12. The hybrid multirotor propulsion system according to claim 1, wherein said predetermined operating mode is a standard hybrid power mode in which said power management unit (PMU) is configured to cause said electric machine (EM) to be operated in said generator mode.

13. The hybrid multirotor propulsion system according to claim 12, further comprising a boost hybrid power mode differing from said standard hybrid power mode in that said power management unit (PMU) is configured to cause said second mechanical power component to be zero and said electric machine (EM) to be operated in an idle mode.

14. The hybrid multirotor propulsion system according to claim 12, further comprising a total boost hybrid power mode differing from said standard hybrid power mode in that said power management unit (PMU) is configured to cause said second mechanical power component to be zero and said electric machine (EM) to be operated in said electromotor mode for providing additional mechanical power increasing said first mechanical power component.

15. The hybrid multirotor propulsion system according to claim 11, further comprising a start power mode in which said power management unit (PMU) is configured to operate said electric machine (EM) in said electromotor mode to output mechanical starting power for starting said mechanical power source (MPS).

16. The hybrid multirotor propulsion system according to claim 2, further comprising a mechanical power source failure mode in which said power management unit (PMU) is configured to cause said electrical energy storing device to output electric power to said plurality of electromotors (EU).

17. The hybrid multirotor propulsion system according to claim 1, wherein said main distributor gearbox (MDG) comprises a root gearbox (M) which has a gearbox input coupled to said mechanical power source (MPS) and a plurality of gearbox outputs, each of which being directly or indirectly coupled to a respective one of said plurality of driving elements (PDS).

18. The hybrid multirotor propulsion system according to claim 17, wherein said main distributor gearbox (MDG) has a branched gearbox configuration defining successive branching levels for indirectly coupling said root gearbox to said driving elements (PDS), wherein a lowest branching level is defined by said root gearbox and at least one higher branching level is defined by a plurality of secondary gearboxes, wherein each secondary gearbox comprises a gearbox input coupled to a respective one of said gearbox outputs assigned to lower branching level, and wherein each secondary gearbox comprises at least two gearbox outputs, each of which being coupled to a respective one of said gearbox inputs assigned to a higher branching level or being coupled to a respective one of said driving elements (PDS).

19. The hybrid multirotor propulsion system according to claim 1, wherein said mechanical power source comprises at least one internal combustion engine.

20. Method for controlling power in a hybrid multirotor propulsion system of an aircraft, said hybrid multirotor propulsion system comprising: a plurality of propulsion units (PU), each propulsion unit (PU) comprising a propeller, an electromotor (EU) and a peripheral differential gearbox (PDGU); a plurality of driving elements (PDS), each of which being coupled to a respective one of said plurality of propulsion units (PU); a mechanical power source (MPS); a main distributor gearbox (MDG); and at least one electric machine (EM); wherein said method comprises the following steps: causing said mechanical power source (VIPS) to output total mechanical power and splitting said total mechanical power into first and second mechanical power components; causing said main distributor gearbox (MDG) to distribute said first mechanical power component to said plurality of driving elements (PDS) (PDS) for providing each driving element (PDS) with a direct mechanical propeller power; causing said electric machine (EM) to convert said second mechanical power component into electric power and distributing at least a part of said electric power to said plurality of electromotors (EU) for providing each electromotor (EU) with an electric propeller power; causing each driving element (PDS) to output said direct mechanical propeller power to said peripheral differential gearbox (PDGU) of said propulsion unit (PU) to which said driving element (PDS) is coupled; causing each electromotor (EU) to convert said electric propeller power into an indirect mechanical propeller power and to output said indirect mechanical propeller power to said peripheral differential gearbox (PDGU) of said propulsion unit (PU) comprising said electromotor (EU), and causing said peripheral differential gearbox (PDGU) of each propulsion unit to aggregate said direct mechanical propeller power and said indirect mechanical propeller power to a total mechanical propeller power and to drive said propeller of each propulsion unit based on said total mechanical propeller power.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0038] Hereinafter, specific embodiments are described with reference to the drawings in which:

[0039] FIG. 1 is a block diagram showing a hybrid multirotor propulsion system according to an embodiment;

[0040] FIG. 2 is a block diagram illustrating power flows from the mechanical power source to the propellers according to the embodiment shown in FIG. 1;

[0041] FIG. 3 is a block diagram illustrating a standard hybrid power mode of the hybrid multirotor propulsion system shown in FIG. 1;

[0042] FIG. 4 is a block diagram illustrating a boost hybrid power mode of the hybrid multirotor propulsion system shown in FIG. 1;

[0043] FIG. 5 is a block diagram illustrating a total boost hybrid power mode of the hybrid multirotor propulsion system shown in FIG. 1;

[0044] FIG. 6 is a block diagram illustrating a MPS power failure mode of the hybrid multirotor propulsion system shown in FIG. 1;

[0045] FIG. 7 is a block diagram illustrating a start MPS power mode of the hybrid multirotor propulsion system shown in FIG. 1;

[0046] FIG. 8 is a block diagram illustrating several options for locating the electric machine within the hybrid multirotor propulsion system;

[0047] FIG. 9 is a diagram illustrating a star-like configuration of the distributor gearbox according to an embodiment; and

[0048] FIG. 10 is a diagram illustrating two tree-like configurations of the distributor gearbox according to another embodiment.

DETAILED DESCRIPTION

[0049] The block diagram according to FIG. 1 shows a hybrid multirotor propulsion system 100 according to an embodiment of the present invention. The hybrid multirotor propulsion system 100 comprises a plurality of propulsion units PU.sub.1, PU.sub.2, . . . , PU.sub.n, wherein each of the propulsion units PU.sub.1, PU.sub.2, . . . , PU.sub.n includes a propeller, an electromotor unit EU.sub.1, EU.sub.2, . . . EU.sub.n, and a peripheral differential gearbox unit PDGU.sub.1, PDGU.sub.2, . . . , PDGU.sub.n.

[0050] The hybrid multirotor propulsion system 100 shown in FIG. 1 further comprises a mechanical power source MPS, a main distributor gearbox MDG, and an electric machine EM. The mechanical power source MPS may include one or more internal combustion engines (ICE). In the specific embodiment shown in FIG. 1, the mechanical power source MPS has a first output shaft MPS-OUT1 and a second output shaft MPS-OUT2. The mechanical power source MPS is coupled via the first output shaft MPS-OUT1 to an input shaft MDG-IN of the main distributor gearbox. Further, the mechanical power source MPS is coupled via the second output shaft MPS-OUT2 to an input shaft EM-IN of the electric machine EM. Further, the main distributor gearbox MDG comprises a plurality of output shafts PDS.sub.1, PDS.sub.2, . . . PDS.sub.n, each of which being coupled to a respective one of the peripheral differential gearbox units PDGU.sub.1, PDGU.sub.2, . . . , PDGU.sub.n. The output shafts PDS.sub.1 to PDS.sub.n form driving elements which are used to drive the plurality of propellers as described below.

[0051] The hybrid multirotor propulsion system 100 shown in FIG. 1 further comprises an electrical energy storing device which may be formed by a rechargeable electric power bank REPB. The rechargeable electric power bank REPB may include e.g. at least one capacitor unit and at least one rechargeable battery. The electrical energy storing device is electrically connected to the electric machine EM and to the plurality of electromotor units EU.sub.1, EU.sub.2, . . . EU.sub.n.

[0052] The hybrid multirotor propulsion system 100 further comprises a power management unit PMU which may be formed by a processor, said processor being used for controlling each of the propulsion units PU.sub.1, PU.sub.2, . . . , PU.sub.n, in particular the electromotor units EU.sub.1, EU.sub.2, . . . EU.sub.n thereof, for controlling the mechanical power source MPS, for controlling the electric machine, and for controlling the electrical energy storing device including the rechargeable electric power bank REPB.

[0053] The hybrid multirotor propulsion system 100 provides for a standard hybrid power mode in which the power management unit PMU controls powering of the plurality of propulsion units PU.sub.1, PU.sub.2, . . . , Pu.sub.n as explained hereinafter.

[0054] Under control of the power management unit PMU, the mechanical power source MPS operates to generate a total mechanical power being composed of a first mechanical power component output to the main distributor gearbox MDG and a second mechanical power component output to the electric machine EM. Accordingly, the power management unit PMU causes mechanical power source MPS to split the total mechanical power into the afore-mentioned first and second mechanical power components. For efficiency reasons already discussed above, the first mechanical power component is preferably larger than the second mechanical power component. Just as an example, one may assume a splitting ratio of 80% to 20%.

[0055] The main distributor gearbox MDG distributes the first mechanical power component through the driving elements PDS.sub.1 to PDS.sub.n to the plurality of propulsion units PU.sub.1 to PU.sub.n. Specifically, each peripheral differential gearbox unit PDGU.sub.1 to PDGU.sub.n receives a fraction of the first mechanical power component representing a direct mechanical propeller power. The term “direct” refers to the fact that the afore-mentioned fraction of the first mechanical power component is transmitted to the respective peripheral differential gearbox unit PDGU.sub.1 to PDGU.sub.n without applying any conversion from mechanical to electrical and back to mechanical power.

[0056] The power management unit PMU controls the electric machine EM such that the electric machine EM converts the second mechanical power component into electric power which is output to the electrical energy storing device comprising the rechargeable electric power bank REPB. A part of the electric power generated by the electric machine EM is distributed to the plurality of electromotor units EU.sub.1 to EU.sub.n. Accordingly, each electromotor unit EU.sub.1 to EU.sub.n is provided with electric propeller power. The electric propeller power is converted by the respective electromotor unit EU.sub.1 to EU.sub.n into an indirect mechanical propeller power which is output to the corresponding peripheral differential gearbox unit PDGU.sub.1 to PDGU.sub.n. The term “indirect” refers here to the fact that the fraction of the second mechanical power component received by the peripheral differential gearbox unit PDGU.sub.1 to PDGU.sub.n is generated by utilizing a conversion from mechanical to electric and back to mechanical power.

[0057] The peripheral differential gearbox unit PDGU.sub.1 to PDGU.sub.n of the respective propulsion unit PU.sub.1 to PU.sub.n aggregates the first mechanical propeller power output from the main distributor gearbox MDG and the second mechanical propeller power output by the respective electromotor unit EU.sub.1 to EU.sub.n to a total mechanical propeller power and outputs this total mechanical propeller power to the propeller through an output shaft coupling the propeller to the peripheral differential gearbox unit PDGU.sub.1 to PDGU.sub.n.

[0058] As can be seen from the above, a major fraction of the total mechanical power generated by the mechanical power source MPS, namely the first mechanical power component is transferred from the mechanical power source MPS through the main distributor gearbox MDG and the plurality of driving elements PDS.sub.1 to PDS.sub.n to the propellers of the propulsion units PU.sub.1 to PU.sub.n. Thus, a major part of the power transmission is implemented by means of a direct mechanical chain which is beneficial in terms of power efficiency. Thus, as explained above, ICE systems are capable to provide a higher power to weight ratio.

[0059] On the other hand, it is rather difficult to precisely control ICE systems on a level which is required for multirotor operations. Thus, multirotor operations often require rapid changes in propeller rotation during flight which is very difficult to realize by only using a direct mechanical chain. Therefore, a minor fraction of the total mechanical power generated by the mechanical power source, namely the second mechanical power component, is transferred to the propellers via an indirect mechanical chain which allows a precise control as required in multirotor operations for realizing rapid propeller rotation changes.

[0060] The splitting into a direct and an indirect mechanical chain, is illustrated once again in the block diagram of FIG. 2. Thus, FIG. 2 shows the respective power/energy flows starting from the mechanical power source MPS.

[0061] The power control explained above refers to the standard hybrid power mode. The standard hybrid power mode is a continuous power mode in which the electric machine EM is used for converting the first mechanical power component which is utilized at least in part for establishing an indirect transmission chain. In addition to the standard hybrid power mode, the hybrid multirotor propulsion system 100 may comprise further operating modes deviating in some aspects from the standard hybrid power mode. Hereinafter, these additional operating modes are compared to the standard hybrid power mode referring to FIGS. 3 to 7, wherein FIG. 3 illustrates once again the standard hybrid power mode.

[0062] The different operating modes can be distinguished from each other in particular when comparing the flows of mechanical and electric power. In FIGS. 3 to 7, the flow of the first mechanical power component is denoted as “FMPC”, the flow of the second mechanical power component is denoted as “SMPC”, the flow of the respective first mechanical propeller power is denoted as “FMPP.sub.1 . . . n”, the flow of the electric power output from the electric machine EM and the rechargeable electric power bank REPB, respectively, is denoted as “EP”, and the flow of the respective electric propeller powers is denoted as “EPP.sub.1 . . . n”.

[0063] As shown in FIG. 3, according to the standard hybrid power mode, the flow FMPC of the first mechanical power component is directed from the mechanical power source MPS to the main distributor gearbox MDG. Likewise, the flow SMPC of the second mechanical power component is directed from the mechanical power source MPS to the electric machine EM. The power flow FMPP.sub.1 . . . n of the first mechanical propeller powers is directed from the main distributor gearbox MDG to the plurality of propulsion units PU.sub.1 to PU.sub.n. Further, the power flow EP of the electric power is directed from the electric machine EM to the rechargeable electrical power bank REPB (or to the electrical energy storing device in general). Finally, the power flow EPP.sub.1 . . . n of the electric propeller power is directed from the main distributor gearbox MDG to the plurality of propulsion units PU.sub.1 to PU.sub.n.

[0064] FIG. 4 illustrates a boost hybrid power mode which differs from the standard hybrid power mode shown in FIG. 3 in terms of the power flow SMPC of the second mechanical power component and in terms of the power flow EP of electric power. Thus, according to the boost hybrid power mode being a short term operating mode, the electric machine EM is operated at least nearly in an idle mode so that the electric machine EM does not generate any electric power. Accordingly, the power flow SMPC of the second mechanical power component is zero, and the power flow EP of the electric power is zero. In the boost hybrid power mode of FIG. 4, the total mechanical power provided by the mechanical power source MPS is fully available for direct mechanical transmission up to the plurality of propulsion units PU.sub.1 to PU.sub.n. Accordingly, the rechargeable electric power bank REPB is not charged by the electric machine EM. However, as in the standard hybrid power mode, the rechargeable electrical power bank REPB is discharged to output the electric propeller powers to the plurality of propulsion units PU.sub.1 to PU.sub.n.

[0065] FIG. 5 illustrates a total boost hybrid power mode which is a short term operating mode and deviates from the standard hybrid power mode in terms of the power flow SMPC of the second mechanical power component and in terms of the power flow EP of the electric power. As in the boost hybrid power mode, the total mechanical power generated by the mechanical power source MPS is fully available for providing the first mechanical power component in order to establish a powerful direct transmission chain to the plurality of propulsion units PU.sub.1 to PU.sub.n. Accordingly, it is assumed that the second mechanical power component is zero in the total boost hybrid power mode. In contrast to the boost hybrid power mode, the electric machine EM is not operated idling in the total boost hybrid power mode. Rather, the electric machine EM is operated in electromotor mode receiving electric power from the rechargeable electrical power bank REPB. Thus, the electric machine EM provides additional mechanical power AMPC which is used to increase the first mechanical power component directly output to the propulsion units PU.sub.1 to PU.sub.n.

[0066] FIG. 6 illustrates an MPS failure power mode deviating from the standard hybrid power mode in terms of all power flows except from the flow EPP.sub.1 . . . n of the electric propeller power. Thus, the MPS failure power mode refers to an emergency situation in which the mechanical power source MPS breaks down so that no mechanical power is available for driving the propulsion units PU.sub.1 to PU.sub.n. In such an emergency situation, the power management unit PMU causes the rechargeable electric power bank REPB to take over full responsibility for powering the plurality of propulsion units PU.sub.1 to PU.sub.n. Thus, the electrical energy stored in the rechargeable electric power bank REPB is output to the propulsion units PU.sub.1 to PU.sub.n, in order to continue operation of the system during a period which is required for emergency landing.

[0067] FIG. 7 illustrates a start MPS power mode in which the electric machine EM is used as a starter for the mechanical power source MPS. Thus, it is assumed that the mechanical power source MPS has not yet started to generate mechanical power, i.e. the first and second mechanical power components are assumed to be zero in FIG. 7. The electric machine EM is operated in electromotor mode for outputting mechanical starting power (SMPC) to the mechanical power source MPS.

[0068] The block diagram of FIG. 8 illustrates exemplary configurations regarding the location of the electric machine EM within the hybrid multirotor propulsion system 100. In principle, the electric machine EM may be inserted and coupled anywhere in the mechanical power transmission line between the mechanical power source MPS and the propulsion units PU.sub.1 to PU.sub.n. It is to be noted that the electric machine EM may be a multipurpose machine of any type which is suitable to be operated by the power management unit PMU. Further, a plurality of electric machines EM may be integrated instead of only one electric machine. In case that the mechanical power source comprises a plurality of ICEs, each ICE might be associated with a separate electric machine EM.

[0069] FIG. 8a shows a configuration in which the mechanical power source MPS comprises a first power output coupled to the main distributor gearbox MDG and a second power output coupled to the electric machine EM. The second power output may be formed by a back shaft of the mechanical power source MPS for connecting the electric machine EM thereto. Whereas the first power output of the mechanical power source MPS serves to supply the first mechanical power component to the main distributor gearbox MDG, the second mechanical output of the mechanical power source MPS serves to supply the second mechanical power component to the electric machine EM. The configuration shown in FIG. 8a corresponds to the embodiment shown in FIG. 1. It is to be noted that the power management unit PMU and the rechargeable electric power bank REPB are omitted in FIG. 8a for sake of simplicity. This applies also to FIGS. 8b to 8d.

[0070] FIG. 8b shows a configuration in which the electric machine EM is coupled between the mechanical power source MPS and the main distributor gearbox MDG. Thus, the mechanical power source MPS outputs the total mechanical power in the amount of the first and second mechanical power components via a single power output. The electric machine EM derives from the total mechanical power the second mechanical power component in order to convert the second mechanical power component into electric energy supplied to the rechargeable electric power bank REPB (not shown in FIG. 8b). The first mechanical power component passes through the electric machine EM and is received from the main distributor gearbox MDG.

[0071] FIG. 8c shows a configuration in which the electric machine EM is coupled to the main distributor gearbox MDG. The mechanical power source MPS has a single power output supplying the main distributor gearbox MDG with the total mechanical power in the amount of the first and second mechanical power components. Apart from the plurality of power outputs of the main distributor gearbox MDG, each of which coupling the respective propulsion unit PU.sub.1 to PU.sub.n to the main distributor gearbox MDG, the main distributor gearbox MDG comprises an additional power output for coupling the electric machine EM to the main distributor gearbox MDG. Thus, the main distributor gearbox MDG splits the total mechanical power received from the mechanical power source MPS into the first mechanical power component and the second mechanical power component. Whereas the first mechanical power component is distributed to the plurality of propulsion units PU.sub.1 to PU.sub.n without applying power conversion, the additional power output of the main distributor gearbox MDG supplies the electric machine EM with the second mechanical power component for converting the second mechanical power component into electric power. Again, the electric power generated by the electric machine EM is transmitted to the rechargeable electric power bank (not shown in FIG. 8c).

[0072] FIG. 8d shows a configuration in which the electric machine EM is inserted in the power transmission line between the main distributor gearbox MDG and one of the plurality of propulsion units PU.sub.1 to PU.sub.n (unit with index n in the example of FIG. 8d). As in the case of the configurations shown in FIGS. 8a to 8c, the main distributor gearbox MDG distributes the first mechanical power component received from the mechanical power source MPS to the plurality of peripheral differential gearbox units PDGU.sub.1 to PDGU.sub.n of the propulsion units PU.sub.1 to PU.sub.n. However, according to the configuration shown in FIG. 8d, the main distributor gearbox MDG outputs the second mechanical power component received from the mechanical power source MPS to the single power transmission line connecting the main distributor gearbox MDG to the specific peripheral differential gearbox unit PDGU.sub.n. The electric machine EM converts the second mechanical power component received from the main distributor gearbox MDG into electric power and outputs the electric power to the rechargeable electric power bank REPB (not shown in FIG. 8d). From the rechargeable electric power bank REPB, the electric power is distributed to the plurality of electromotor units EU.sub.1 to EU.sub.n of the propulsion units PU.sub.1 to PU.sub.n.

[0073] Various configurations for implementing the main distributor gearbox MDG are conceivable. FIGS. 9 and 10 show exemplary configurations which may be considered as star-like and tree-like configuration, respectively.

[0074] According to a star-like configuration shown in FIG. 9, the main distributor gearbox MDG comprises a root gearbox M having an input coupled to the mechanical power source MPS. The root gearbox M comprises a plurality of outputs which are connected to the propulsion units PU.sub.1 to PU.sub.n trough the driving elements PDS.sub.1 to PDS.sub.n. The root gearbox M distributes the received first mechanical power component via the driving elements PDS.sub.1 to PDS.sub.n directly to the different propulsion units PU.sub.1 to PU.sub.n.

[0075] Whereas according to the embodiment shown in FIG. 9, the driving elements PDS.sub.n to PDS.sub.n directly connect the outputs of the root gearbox M to the propulsion units PU.sub.1 to PU.sub.n, tree-like configurations shown in FIGS. 10a and 10b provide for an indirect coupling between the root gearbox M and the plurality of propulsion units PU.sub.1 to PU.sub.n.

[0076] According to the embodiment shown in FIG. 10a, the main distributor gearbox MDG comprises a plurality of secondary gearboxes I.sub.1.1 to I.sub.1.n defining a first branching level and a plurality secondary gearboxes I.sub.2.1 to I.sub.2.n defining a second branching level. Each secondary gearbox I.sub.1.1 to I.sub.1.n of the first branching level has an input which is connected through a driving shaft D.sub.1 to D.sub.n to the respective output of the root gearbox M. Further, each secondary gearbox I.sub.1.1 to I.sub.1.n of the first branching level has two outputs, each of which being connected through a driving shaft IDE.sub.2.1 to IDE.sub.2.n to an input of one of the secondary gearboxes I.sub.2.1 to I.sub.2.n of the second branching level. Finally, each secondary gearbox I.sub.2.1 to I.sub.2.n of the second branching level has two outputs, each of these two outputs being connected with a respective one of the driving elements (not explicitly shown in FIG. 10a). In the configuration shown in FIG. 10a, the first mechanical power component received by the root gearbox M from the mechanical power source MPS is distributed in two branching levels to the plurality of propellers Pr.sub.1 to Pr.sub.n. When considering e.g. the propeller Pr.sub.1 in FIG. 10a, a portion of the first mechanical power component is transferred from the root gearbox M to the secondary gearbox I.sub.1.1 and from there to the secondary gearbox I.sub.2.1 which supplies the propeller Pr.sub.1 with the mechanical power.

[0077] The embodiment shown in FIG. 10a comprises a symmetrical configuration, i.e. each power transmission line from the root gearbox M to the respective propeller comprises two splitting stages for distributing the mechanical power received from the root gearbox M. However, it is also possible that the different power transmission lines comprise different numbers of splitting stages as exemplarily illustrated in FIG. 10b. Here, the power transmission line leading to the propeller Pr.sub.1 comprises two splitting stages, whereas the power transmission line leading to the propeller Pr.sub.x comprises three splitting stages.

[0078] According to the embodiments shown in FIGS. 9 and 10, the first mechanical power component received by the root gearbox M is distributed in accordance with fixed transmission ratios to the plurality of propellers Pr.sub.1 to Pr.sub.n. Accordingly, it is possible to used simple mechanical splitters instead of differential gearboxes.