Thrust producing unit with at least two rotor assemblies and a shrouding

A thrust producing unit for producing thrust in a predetermined direction, comprising at least two rotor assemblies and a shrouding that accommodates at most one of the at least two rotor assemblies, wherein the shrouding defines a cylindrical air duct that is axially delimited by an air inlet region and an air outlet region, and wherein the air inlet region exhibits in circumferential direction of the cylindrical air duct an undulated geometry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European patent application No. EP 17400008.3 filed on Feb. 27, 2017, the disclosure of which is incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The invention is related to a thrust producing unit for producing thrust in a predetermined direction, the thrust producing unit comprising at least two rotor assemblies and a shrouding. The invention is further related to a multirotor aircraft with at least one thrust producing unit for producing thrust in a predetermined direction, the thrust producing unit comprising at least two rotor assemblies and a shrouding.

2) Description of Related Art

Each one of these conventional multirotor aircrafts is equipped with two or more thrust producing units that are provided for producing thrust in a predetermined direction during operation of the multirotor aircraft. In general, each thrust producing unit includes one or more rotors or propellers and is, usually, designed for specific flight conditions. By way of example, a thrust producing unit that is designed as an airplane propeller operates at its optimum in cruise conditions, whereas a thrust producing unit that is designed as propeller of a compound helicopter is rather optimized for hover or forward flight conditions, while a thrust producing unit that implements e.g. a so-called Fenestron® tail rotor is particularly designed for hover conditions.

In all of these examples, the respective thrust producing unit is optimized for operation in axial air flow conditions, i.e. in an air flow direction that is oriented at least approximately along a rotor axis resp. rotation axis of the given one or more rotors or propellers and, therefore, referred to as an axial air flow direction. If, however, the respective thrust producing unit is operated in transversal air flow conditions, i.e. in an air flow direction that is oriented transverse to the rotor axis of the given one or more rotors or propellers and, therefore, referred to as a non-axial air flow direction, a respective efficiency of the thrust producing unit usually decreases considerably.

By way of example, in the case of operation of a multirotor aircraft with two or more thrust producing units, the thrust producing units will be subjected to axial air flow conditions e.g. during a vertical take-off phase. Subsequently, respective thrust vectors generated by the thrust producing units can be inclined in a predetermined direction, e.g. by rotating the thrust producing units accordingly, so that the multirotor aircraft gains velocity and leaves a previous hovering condition such that is converts to forward flight, wherein the thrust producing units are subjected to transversal air flow conditions. However, in the transversal air flow conditions, respective ducts or shrouds, which are beneficial in axial air flow conditions, are penalizing by generating a comparatively large amount of drag. In other words, an underlying advantage provided by the ducts or shrouds in hovering turns out to be a disadvantage in forward flight, which increases with increasing a respective advancing speed of the multirotor aircraft in forward flight.

Furthermore, it should be noted that in axial air flow conditions a ducted rotor or propeller, i.e. a rotor or propeller that is provided with a duct or shroud, is approximately 25% to 50% more efficient than an equivalent isolated or non-ducted rotor or propeller, i.e. a rotor or propeller without duct or shroud, which has comparable global dimensions, i.e. diameter and mean chord. In other words, the presence of a duct or shroud increases a respectively produced thrust of a given thrust producing unit at constant required power. Therefore, conventional thrust producing units are frequently provided with one or more rotors or propellers that is/are completely enclosed in an associated duct or shroud. This classical configuration uses a respective rotor or propeller induced velocity to generate thrust also from the duct or shroud.

In general, a duct or shroud is defined by an enclosed, annular surface that is arranged around a rotor or propeller in order to improve respective aerodynamics and performances of the rotor or propeller. A conventional duct or shroud is usually not rotatable, i.e. cannot be inclined, and has a height that is selected such that a given rotor or propeller is fully enclosed therein.

However, as the duct or shroud must have a certain height or length in order to enclose an associated rotor or propeller and is, thus, comparatively large in size, the duct or shroud increases an overall weight of a respective multirotor aircraft due to its size, and further increases drag e.g. during forward flight, i.e. in transversal air flow conditions, as the duct or shroud cannot be inclined for adjustment of an underlying thrust vector direction. The comparatively large size also leads to a comparatively large projection surface on which wind and/or wind gust may act. This leads to an increased overpower necessity for the respective multirotor aircraft. Furthermore, if two or more rotors or propellers are e.g. coaxially positioned atop of each other, a given duct or shroud that is provided for enclosing these rotors or propellers will even require a still larger height and be still heavier. Moreover, conventional ducts or shrouds are usually not actively rotated and must be designed comparatively stiff, as usually a minimum gap between rotors or propellers and duct or shroud surface is requested. In addition, conventional ducts or shrouds of respective thrust producing units are not suitable for enclosing differently configured rotors or propellers, i.e. rotors or propellers having differing inclinations, positioning and/or sizes resp. diameters.

In summary, in a conventional thrust producing unit with a duct or shroud, a thrust vector that is produced in operation in axial air flow conditions is aligned with a rotor axis of a respective rotor or propeller of the thrust producing unit and directed against a direction of a velocity field induced by the rotor or propeller in operation. The rotor or propeller accelerates a certain mass-flow through an associated rotor or propeller plane or disk. A resulting flow acceleration, which occurs when air traverses the rotor or propeller plane or disk, forms areas of under-pressure around a respective collector region of the duct or shroud, thus, generating additional thrust. This generation of additional thrust is an important advantage resulting from the use of the duct or shroud that is, however, strongly penalizing in forward flight, i.e. in transversal air flow conditions, due to additional drag generated by the duct or shroud. The additional drag is directly proportional to a respective frontal area that is defined by a product of height and width of the duct or shroud. Thus, by way of example, for a thrust producing unit having a counter-rotating rotor or propeller configuration with two rotors or propellers that are completely embedded into a single duct or shroud, the additional drag almost doubles compared with a thrust producing unit that is only provided with one rotor or propeller that is completely embedded into a single duct or shroud.

The document U.S. Pat. No. 5,150,857 A describes an unmanned aerial vehicle (UAV) having a toroidal fuselage that surrounds a pair of coaxial, multi-bladed, counterrotating rotors. The toroidal fuselage defines a duct or shroud and has an airfoil profile that is configured to provide high hover efficiency and to produce a pressure distribution that provide high lift forces. The airfoil profile is symmetrical and adapted to counteract the undesirable nose-up pitching moments experienced by ducted rotary-type UAVs in forward translational flight. However, the symmetrical duct or shroud that is defined by the toroidal fuselage exhibits the above-described disadvantages in forward flight, i.e. in transversal air flow conditions.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a new thrust producing unit, in particular for use with multirotor aircrafts, which exhibits improved aerodynamics and performances in transversal air flow conditions.

This object is solved by a thrust producing unit for producing thrust in a predetermined direction, the thrust producing unit comprising the features of claim1. More specifically, according to the present invention a thrust producing unit for producing thrust in a predetermined direction comprises at least two rotor assemblies and a shrouding that accommodates at most one of the at least two rotor assemblies. The shrouding defines a cylindrical air duct that is axially delimited by an air inlet region and an air outlet region, wherein the air inlet region exhibits in circumferential direction of the cylindrical air duct an undulated geometry.

It should be noted that the term “shrouding” should be understood as encompassing simultaneously the terms “duct” and “shroud”. In other words, in the context of the present invention, the term “shrouding” refers interchangeably to a duct or a shroud.

Advantageously, the inventive thrust producing unit is implemented as a shrouded multiple rotor assembly configuration that leads to a significantly reduced drag in transversal air flow conditions, e.g. in forward flight of a given multirotor aircraft that uses the inventive thrust producing unit. This significantly reduced drag results not only from the accommodation of the at most one of the at least two rotor assemblies in the shrouding, so that an overall height of the shrouding can be reduced significantly, but also from the inventive design of the shrouding itself, in particular the undulated geometry of the air inlet region in circumferential direction of the cylindrical air duct.

More specifically, the inventive shrouding and all associated elements are preferably axially non-symmetric, i.e. non-symmetric over the azimuth ψ of the shrouding. In other words, the shrouding is designed on the basis of a variable factor with respect to all associated elements, i.e.:

Air inlet region radius vs. Azimuth ψ,

Arrangement of additional lifting surfaces vs. Azimuth ψ.

In particular, the variable height of the shrouding enables significant advantages in the trade-off between vertical take-off and hovering, wherein an underlying efficiency increases with an increase of the height of the shrouding, and forward flight, wherein an underlying drag decreases with a decrease of the height of the shrouding, as this reduces a respective drag area of the shrouding.

Furthermore, the inventive thrust producing unit exhibits a significantly lower weight than a conventional shrouded thrust producing unit having a single shrouding that completely encloses two rotor or propeller assemblies, while having comparable performances in axial air flow conditions, i.e. in hover flight of the respective multirotor aircraft. In fact, it should be noted that a conventional shrouded thrust producing unit having a single shrouding that completely encloses two or more, preferentially counter-rotating rotor or propeller assemblies provides the same thrust versus power characteristics than e.g. a thrust producing unit having a much shorter shrouding that encloses only one of the two or more rotor or propeller assemblies, such as the inventive thrust producing unit, while leaving the other(s) unshrouded, i.e. exposed to the air. This is due to the fact that the above-mentioned additional thrust is generated by the air inlet region defined by the shrouding only, and not by the duct resp. shrouding itself. Moreover, a respective velocity field induced by the at least two rotor or propeller assemblies with the long and short shroudings is such that the under-pressure field generated on the air inlet region is also the same for the long and short shroud configurations. This likewise applies to a configuration featuring multiple rotor or propeller assemblies, each being enclosed in a single associated shrouding having a minimized height.

Preferably, the shrouding of the inventive thrust producing unit is used as an additional lifting device during hover and forward flight cases of a multirotor aircraft that features the inventive thrust producing unit and, thus, beneficially allows reduction of a respective power consumption of the at most one of the at least two rotor assemblies that is accommodated in the shrouding. Furthermore, the shrouding advantageously allows to reduce at least an underlying diameter of the at most one of the at least two rotor assemblies that is accommodated therein, since the shrouding increases its effectiveness. In addition, the shrouding beneficially provides for a shielding effect with respect to the at most one of the at least two rotor assemblies that is accommodated therein and, thus, advantageously allows to reduce a respective rotor noise footprint on ground.

According to one aspect, the inventive thrust producing unit can be provided with a foreign object protection, e.g. by being enclosed by a grid, in order to protect the at most one of the at least two rotor assemblies that is accommodated therein from foreign objects. Such a foreign object protection beneficially prevents misuse and accidents by and of individuals, e.g. by preventing them from getting their hands caught in rotating parts, thereby leading to an increased operational safety level of the inventive thrust producing unit.

Advantageously, by providing the inventive thrust producing unit with the at least two rotor assemblies that define different rotor planes, the rotor assemblies can be positioned above each other and rotated in a counter rotating manner, leading to a thrust producing unit that provides for an increased safety level and that allows reduction of the overall dimensions of an associated multirotor aircraft, resulting in a comparatively small aircraft, since the two or more rotor planes can be combined in a single thrust producing unit. Preferably, the at least two rotor assemblies of the inventive thrust producing unit, each of which defines an associated rotor plane or surface, are positioned on top of each other, either coaxially or with separate individual rotor axes, and can be inclined with respect to each other. Furthermore, the inventive thrust producing unit is adapted for providing torque individually as a result of its counter-rotating rotor assemblies, which can be used to maneuver a given multirotor aircraft that features the inventive thrust producing unit, e.g. with respect to yawing.

According to a preferred embodiment, the cylindrical air duct exhibits a height defined between the air outlet region and the air inlet region in axial direction of the cylindrical air duct that varies in circumferential direction of the cylindrical air duct, wherein the height that varies in the circumferential direction of the cylindrical air duct defines the undulated geometry of the air inlet region.

According to a further preferred embodiment, the cylindrical air duct comprises in circumferential direction a leading edge and a diametrically opposed trailing edge, and a board side lateral shoulder and a diametrically opposed star board side lateral shoulder, wherein the board side lateral shoulder and the star board side lateral shoulder are respectively arranged in the circumferential direction of the cylindrical air duct between the leading edge and the trailing edge, and wherein the height at the leading edge is smaller than the height at the board side lateral shoulder and/or the star board side lateral shoulder.

According to a further preferred embodiment, the height at the trailing edge is smaller than the height at the board side lateral shoulder and/or the star board side lateral shoulder.

According to a further preferred embodiment, the height at the trailing edge is smaller than the height at the leading edge.

According to a further preferred embodiment, the height at the board side lateral shoulder and/or the star board side lateral shoulder is selected in a range from 0.05*D to 0.5*D, wherein D defines a diameter of the cylindrical air duct.

According to a further preferred embodiment, the air inlet region of the cylindrical air duct exhibits an air inlet region radius that varies in the circumferential direction of the cylindrical air duct, wherein the air inlet region radius differs between at least two of the leading edge, the trailing edge, the board side lateral shoulder and the star board side lateral shoulder.

According to a further preferred embodiment, the air outlet region of the cylindrical air duct exhibits an air outlet region radius that varies in the circumferential direction of the cylindrical air duct, wherein the air outlet region radius differs between at least two of the leading edge, the trailing edge, the board side lateral shoulder and the star board side lateral shoulder.

According to a further preferred embodiment, the trailing edge of the cylindrical air duct is at least essentially open and provided with a stiffening element.

According to a further preferred embodiment, the trailing edge of the cylindrical air duct is equipped with a flap.

According to a further preferred embodiment, the leading edge of the cylindrical air duct is provided with an additional lifting surface.

According to a further preferred embodiment, a first rotor assembly of the at least two rotor assemblies is arranged outside of the cylindrical air duct and adjacent to the air inlet region of the cylindrical air duct, wherein the shrouding accommodates a second rotor assembly of the at least two rotor assemblies.

According to a further preferred embodiment, the first rotor assembly defines a first rotor axis and the second rotor assembly defines a second rotor axis, the first and second rotor axes being coaxially arranged.

According to a further preferred embodiment, the first and second rotor axes are inclined by associated inclination angles comprised in a range between −60° and +60°.

The present invention further relates to a multirotor aircraft comprising at least one thrust producing unit that is configured as described above.

Advantageously, the shrouding of the inventive thrust producing unit allows reducing respective overall dimensions of the inventive multirotor aircraft that features the inventive thrust producing unit. Furthermore, individuals approaching the shrouded thrust producing unit are protected against injury, foreign object damages of the thrust producing unit in operation, such as e.g. bird strike or wire strike, can securely and reliably be prevented, and the overall operational safety of the associated multirotor aircraft in case of air collisions can be improved.

Moreover, respective aerodynamics, acoustics and performances can be improved by reducing a respective rotor blade loading in operation, reducing an overall power consumption, reducing a respective noise emission and ameliorating functioning in hover and forward flight of the inventive multirotor aircraft. Furthermore, an underlying required diameter of the thrust producing unit can be reduced. Additionally, lift of the inventive multirotor aircraft is improved by the shrouding itself, potentially reducing the overall power required by the inventive multirotor aircraft.

It should be noted that although the inventive aircraft is described above with reference to a multirotor structure with multiple rotor assemblies, it could likewise be implemented as a multipropeller structure with multiple propeller assemblies or as a multipropeller and -rotor structure. More specifically, while rotors are generally fully articulated, propellers are generally not articulated at all. However, both can be used for generating thrust and, thus, for implementing the thrust producing units according to the present invention. Consequently, any reference to rotors or rotor structures in the present description should likewise be understood as a reference to propellers and propeller structures, so that the inventive multirotor aircraft can likewise be implemented as a multipropeller and/or multipropeller and -rotor aircraft.

In other words, the present invention principally relates to a multiple thrust configuration with rotors/propellers that define rotor/propeller planes, which can be selected to be positioned atop of each other individually, a shrouding for enclosing any rotating parts of at most one of the rotors/propellers, at least one electrical engine which drives each rotor/propeller, wherein each engine can be segregated in order to increase a provided safety level, and wherein a logic connection preferably exists between battery and electrical engines, the logic connection preferentially comprising a redundant design increasing the safety level in case of failure, and wherein preferably a battery redundancy layout with an appropriate safety level in case of failure is provided.

Advantageously, the inventive multirotor aircraft is designed for transportation of passengers and is, in particular, suitable and adapted for being certificated for operation within urban areas. It is preferably easy to fly, has multiple redundancies, meets the safety demands of the authorities, is cost efficient in design and only creates comparatively low noise. Preferably, the inventive multirotor aircraft has a comparatively small rotor diameter with a light weight design and a fixed angle of incident, and is nevertheless adapted for fulfilment of an emergency landing, although these rotor characteristics lead to a comparatively low inertia and a non-adjustable torque in operation.

According to one aspect, the inventive multirotor aircraft is capable of hovering and comprises a distributed propulsion system. It is further preferably designed with autorotation capability, which is necessary amongst other requirements in order to meet authority regulations, such as e.g. FAR and EASA regulations, regarding safety failure modes that amount up to approximately 1*10−7failures per flight hour for the entire multirotor aircraft. In the aeronautical sector, these safety levels are typically defined by the so-called Design Assurance Levels (DAL) A to D.

Preferably, the inventive multirotor aircraft fulfils the authorities' regulation safety level needed to transport passengers. This is preferentially achieved by a combination and correlation of:

at least two individual rotor assemblies per thrust producing unit,

a redundant, segregated battery layout,

a redundant power supply and harness layout,

a physical separation and segregation of an underlying power management,

redundant, segregated electrical engines, and pitch control and/or RPM control of the rotor assemblies.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a multirotor aircraft1with an aircraft airframe2according to the present invention. The aircraft airframe2defines a supporting structure that is also referred to hereinafter as the fuselage of the multirotor aircraft1.

The fuselage2has an extension in longitudinal direction1aand an extension in lateral direction1band preferably defines an internal volume2athat is at least adapted for transportation of passengers, so that the multirotor aircraft1as a whole is adapted for transportation of passengers. The internal volume2ais preferably further adapted for accommodating operational and electrical equipment, such as e.g. an energy storage system that is required for operation of the multirotor aircraft1.

It should be noted that exemplary configurations of the internal volume2athat are suitable for transportation of passengers, but also for accommodation of operational and electrical equipment, are readily available to the person skilled in the art and generally implemented to comply with applicable authority regulations and certification requirements regarding passenger transportation. Thus, as these configurations of the internal volume2aas such are not part of the present invention, they are not described in detail for brevity and conciseness.

According to one aspect, the multirotor aircraft1comprises a plurality of thrust producing units3. Preferably, the plurality of thrust producing units3comprises at least two and preferentially four thrust producing units3a,3b,3c,3d. The thrust producing units3a,3b,3c,3dare embodied for producing thrust (9inFIG. 3) in operation, such that the multirotor aircraft1is able to hover in the air as well as to fly in any forward or rearward direction.

Preferably, the thrust producing units3a,3b,3c,3dare structurally connected to the fuselage2. By way of example, this is achieved by means of a plurality of structural supports4. More specifically, the thrust producing unit3ais preferably connected to the fuselage2via a structural support4a, the thrust producing unit3bvia a structural support4b, the thrust producing unit3cvia a structural support4cand the thrust producing unit3dvia a structural support4d, wherein the structural supports4a,4b,4c,4ddefine the plurality of structural supports4.

Preferably, at least one of the thrust producing units3a,3b,3c,3dcomprises an associated shrouding in order to improve underlying aerodynamics and to increase operational safety. By way of example, a plurality of shrouding units6is shown with four separate shroudings6a,6b,6c,6d. Illustratively, the shrouding6ais associated with the thrust producing unit3a, the shrouding6bwith the thrust producing unit3b, the shrouding6cwith the thrust producing unit3cand the shrouding6dwith the thrust producing unit3d.

The shroudings6a,6b,6c,6dcan be made of a simple sheet metal. However, according to one aspect the shroudings6a,6b,6c,6dhave a complex geometry, such as e.g. described below with reference toFIG. 5.

Furthermore, the shroudings6a,6b,6c,6dcan be connected to the fuselage2together with the structural supports4a,4b,4c,4d, in order to reinforce the connection between the thrust producing units3a,3b,3c,3dand the fuselage2. Alternatively, only the shroudings6a,6b,6c,6dcan be connected to the fuselage2.

According to one aspect, at least one and, preferably, each one of the thrust producing units3a,3b,3c,3dis equipped with at least two rotor assemblies. By way of example, the thrust producing unit3ais equipped with two rotor assemblies7a,8a, the thrust producing unit3bis equipped with two rotor assemblies7b,8b, the thrust producing unit3cis equipped with two rotor assemblies7c,8cand the thrust producing unit3dis equipped with two rotor assemblies7d,8d. The rotor assemblies7a,7b,7c,7dillustratively define a plurality of upper rotor assemblies7and the rotor assemblies8a,8b,8c,8dillustratively define a plurality of lower rotor assemblies8.

The plurality of upper and lower rotor assemblies7,8is preferably connected to the plurality of structural supports4by means of a plurality of gearbox fairings5. Illustratively, the upper and lower rotor assemblies7a,8aare connected to the structural support4aby means of a gearbox fairing5a, the upper and lower rotor assemblies7b,8bare connected to the structural support4bby means of a gearbox fairing5b, the upper and lower rotor assemblies7c,8care connected to the structural support4cby means of a gearbox fairing5cand the upper and lower rotor assemblies7d,8dare connected to the structural support4dby means of a gearbox fairing5d.

Preferably, each one of the upper rotor assemblies7a,7b,7c,7ddefines an associated upper rotor plane (21inFIG. 7) and each one of the lower rotor assemblies8a,8b,8c,8ddefines an associated lower rotor plane (22inFIG. 7). Preferably, the upper and lower rotor assemblies7a,7b,7c,7d,8a,8b,8c,8ddefine pairs of upper and lower rotor assemblies7a,8a;7b,8b;7c,8c;7d,8dthat are accommodated in the shroudings6a,6b,6c,6d, respectively, so that the associated upper and lower rotor planes (21,22inFIG. 7) are located inside the shroudings6a,6b,6c,6dof the multirotor aircraft1.

According to one aspect, the multirotor aircraft1comprises an aircraft operating structure and a redundant security architecture. The aircraft operating structure is preferably adapted for operation of the multirotor aircraft1in failure-free operating mode and the redundant security architecture is preferably at least adapted for operation of the multirotor aircraft1in case of a failure of the aircraft operating structure. In particular, the redundant security architecture is provided to comply preferentially with applicable authority regulations and certification requirements regarding passenger transportation.

Preferably, the aircraft operating structure comprises at least a first part of the upper and lower rotor assemblies7a,7b,7c,7d,8a,8b,8c,8dand the redundant security architecture comprises at least a second part of the upper and lower rotor assemblies7a,7b,7c,7d,8a,8b,8c,8d. Preferentially, a first one of the upper and lower rotor assemblies7a,8a,7b,8b,7c,8c,7d,8dof each thrust producing unit3a,3b,3c,3dis associated with the aircraft operating structure, while a second one is associated with the redundant security architecture. By way of example, the upper rotor assemblies7a,7b,7c,7dare associated with the aircraft operating structure and the lower rotor assemblies8a,8b,8c,8dare associated with the redundant security architecture. Thus, at least in case of a failure of the upper rotor assemblies7a,7b,7c,7d, the lower rotor assemblies8a,8b,8c,8doperate the multirotor aircraft1in order to avoid e.g. a crash thereof.

It should, however, be noted that the above configuration, wherein the upper rotor assemblies7a,7b,7c,7dare associated with the aircraft operating structure and the lower rotor assemblies8a,8b,8c,8dare associated with the redundant security architecture, is merely described by way of example and not for limiting the invention thereto. Instead, alternative associations are likewise possible and contemplated. For instance, the rotor assemblies7a,7c,8b,8dcan be associated with the aircraft operating structure, while the rotor assemblies8a,8c,7b,7dare associated with the redundant security architecture. Alternatively, all upper and lower rotor assemblies7a,7b,7c,7d,8a,8b,8c,8dcan be associated with the aircraft operating structure and/or the redundant security architecture, and so on. As such alternative associations are readily available to the person skilled in the art, they are likewise contemplated and considered as being part of the present invention.

FIG. 2shows the multirotor aircraft1ofFIG. 1with the thrust producing units3a,3b,3c,3dthat are connected to the fuselage2. The thrust producing units3a,3b,3c,3drespectively comprise the upper and lower rotor assemblies7a,7b;7b,8b;7c,8c;7d,8d, which are preferably arranged in a side-by-side configuration with congruent rotor axes (12inFIG. 3andFIG. 4). Preferentially, the upper rotor assemblies7a,7b,7c,7dare arranged above the lower rotor assemblies8a,8b,8c,8dsuch that the upper and lower rotor assemblies7a,7b;7b,8b;7c,8c;7d,8dare stacked, i.e. arranged on top of each other with congruent rotor axes (12inFIG. 3andFIG. 4). However, alternative configurations are likewise contemplated, such as e.g. axially displaced rotor axes.

As can further be seen fromFIG. 2, the thrust producing units3a,3b,3c,3dare all exemplarily arranged laterally with respect to the fuselage2, i.e. on the left or right side of the fuselage2seen in its longitudinal direction1a. Illustratively, the left side corresponds to the lower side and the right side to the upper side of the fuselage2as shown inFIG. 2. Furthermore, the fuselage2is exemplarily embodied such that the laterally arranged thrust producing units3a,3b,3c,3ddefine at least approximately a trapezoidal shape.

However, it should be noted that this exemplary arrangement is only described by way of example and not for limiting the present invention thereto. Instead, other arrangements are also possible and likewise contemplated. For instance, two of the thrust producing units3a,3b,3c,3dcan respectively be arranged at a front and rear section of the fuselage2, and so on.

FIG. 3shows the multirotor aircraft1ofFIG. 1andFIG. 2in an exemplary failure-free operating mode. In this exemplary failure-free operating mode, the plurality of thrust producing units3produce airstreams in a thrust producing airstream direction9by means of the plurality of upper and/or lower rotor assemblies7,8, which are suitable to lift the multirotor aircraft1off ground10.

Each one of the plurality of upper rotor assemblies7defines a first rotor axis and each one of the plurality of lower rotor assemblies8defines a second rotor axis. Preferably, the first and second rotor axes are respectively congruent, i.e. coaxially arranged, so that the plurality of upper and lower rotor assemblies7,8define a plurality of coaxially arranged rotor axes12. Illustratively, the upper and lower rotor assemblies7c,8cdefine first and second congruent rotor axes, which are commonly referred to as the rotor axis12c, and the upper and lower rotor assemblies7d,8ddefine first and second congruent rotor axes, which are commonly referred to as the rotor axis12d.

However, other configurations are likewise contemplated. E.g. the rotor axes can be arranged in parallel to each other, and so on.

Preferably, the plurality of thrust producing units3is inclined in the longitudinal direction1aof the multirotor aircraft1by a plurality of longitudinal inclination angles11in order to increase the maneuverability of the multirotor aircraft1and to reduce an overall inclination in the longitudinal direction1aof the multirotor aircraft1during forward flight. The plurality of longitudinal inclination angles11is illustratively defined between a vertical reference line10aof the multirotor aircraft1and the plurality of coaxially arranged rotor axes12. Preferably, a possible and realized number of the plurality of longitudinal inclination angles11depends on an underlying number of provided thrust producing units.

More specifically, according to one aspect, at least one of the plurality of thrust producing units3is inclined in the longitudinal direction1aof the multirotor aircraft1by a first longitudinal inclination angle defined between a vertical reference line10aof the multirotor aircraft1and the first and second congruent rotor axes of this at least one of the plurality of thrust producing units3. The first longitudinal inclination angle is preferably comprised in a range between −45° and +80° and preferentially amounts to 7°.

Illustratively, the thrust producing unit3cof the plurality of thrust producing units3is inclined by a first longitudinal inclination angle11adefined between the vertical reference line10aand the rotor axis12c, wherein the first longitudinal inclination angle11ais preferably comprised in a range between −45° and +80°, and preferentially amounts to 7°. However, it should be noted that the thrust producing unit3aof the plurality of thrust producing units3ofFIG. 1andFIG. 2is preferably also inclined by the first longitudinal inclination angle11a.

According to one aspect, at least one of the plurality of thrust producing units3is inclined in the longitudinal direction1aof the multirotor aircraft1by a second longitudinal inclination angle defined between the vertical reference line10aand the first and second congruent rotor axes of this at least one of the plurality of thrust producing units3. The second longitudinal inclination angle is preferably also comprised in a range between −45° and +80°, and preferentially amounts to 7°.

Illustratively, the thrust producing unit3dof the plurality of thrust producing units3is inclined by a second longitudinal inclination angle11bdefined between the vertical reference line10aand the rotor axis12d, wherein the second longitudinal inclination angle11bis preferably comprised in a range between −45° and +80°, and preferentially amounts to 7°. However, it should be noted that the thrust producing unit3bof the plurality of thrust producing units3ofFIG. 1andFIG. 2is preferably also inclined by the second longitudinal inclination angle11b.

FIG. 4shows the multirotor aircraft1with the fuselage2ofFIG. 3, which illustratively comprises a width2b. The latter is defined as a maximum distance measured orthogonally to the longitudinal direction1aof the multirotor aircraft1between the respective outmost left hand and right hand side surfaces of the fuselage2.

The multirotor aircraft1is again exemplarily shown in the failure-free operating mode, wherein the plurality of thrust producing units3produce airstreams in the thrust producing airstream direction9by means of the plurality of upper and lower rotor assemblies7,8. The upper and lower rotor assemblies7c,8cdefine the rotor axis12cand the upper and lower rotor assemblies7d,8ddefine the rotor axis12das described above with reference toFIG. 3.

Furthermore, the upper and lower rotor assemblies7a,8aexemplarily define first and second congruent rotor axes, which are commonly referred to as the rotor axis12a, and the upper and lower rotor assemblies7b,8bdefine first and second congruent rotor axes, which are commonly referred to as the rotor axis12b. It should be noted that the rotor axes12a,12b,12c,12dare preferably implemented in order to reduce the overall complexity, system weight as well as geometrical size of the multirotor aircraft1.

Preferably, the plurality of thrust producing units3is inclined in the lateral direction1bof the multirotor aircraft1by a plurality of lateral inclination angles13in order to provide reduced gust sensitivity and to increase the maneuverability of the multirotor aircraft1. The plurality of lateral inclination angles13is illustratively defined between the vertical reference line10aof the multirotor aircraft1and the plurality of coaxially arranged rotor axes12. Preferably, a possible and realized number of the plurality of lateral inclination angles13depends on an underlying number of provided thrust producing units.

More specifically, according to one aspect, at least one of the plurality of thrust producing units3is inclined in the lateral direction1bof the multirotor aircraft1by a first lateral inclination angle defined between the vertical reference line10aof the multirotor aircraft1and the first and second congruent rotor axes of this at least one of the plurality of thrust producing units3. The first lateral inclination angle is preferably comprised in a range between −45° and +80°, and preferentially amounts to 5°.

Illustratively, the thrust producing unit3aof the plurality of thrust producing units3is inclined by a first lateral inclination angle13adefined between the vertical reference line10aand the rotor axis12a, wherein the first lateral inclination angle13ais preferably comprised in a range between −45° and +80°, and preferentially amounts to 5°. However, it should be noted that the thrust producing unit3cof the plurality of thrust producing units3ofFIG. 1andFIG. 2is preferably also inclined by the first lateral inclination angle13a.

According to one aspect, at least one of the plurality of thrust producing units3is inclined in the lateral direction1bof the multirotor aircraft1by a second lateral inclination angle defined between the vertical reference line10aof the multirotor aircraft1and the first and second congruent rotor axes of this at least one of the plurality of thrust producing units3. The second lateral inclination angle is preferably comprised in a range between −45° and +80°, and preferentially amounts to 5°.

Illustratively, the thrust producing unit3bof the plurality of thrust producing units3is inclined by a second lateral inclination angle13bdefined between the vertical reference line10aand the rotor axis12b, wherein the second lateral inclination angle13bis preferably comprised in a range between −45° and +80°, and preferentially amounts to 5°. However, it should be noted that the thrust producing unit3dof the plurality of thrust producing units3ofFIG. 1andFIG. 2is preferably also inclined by the second lateral inclination angle13b.

FIG. 5shows the thrust producing unit3dof the preceding figures, with its upper rotor assembly7d, its lower rotor assembly8d, its gearbox fairing5dand its shrouding6d, for further illustrating an exemplary configuration thereof. It should, however, be noted that the thrust producing units3a,3b,3cof the preceding figures preferably comprise similar configurations, so that the thrust producing unit3dis only described representative for all thrust producing units3a,3b,3c,3d, for brevity and conciseness.

Preferably, the shrouding6dis configured with a supporting structure16that can be made of a simple pressed, bended metal sheet. The supporting structure16is preferentially provided with an internal volume that can e.g. be used as storage volume for a battery system of the multirotor aircraft1of the preceding figures. Illustratively, the shrouding6dand, more specifically, the supporting structure16accommodates at most one and, exemplarily, the lower rotor assembly8d. Illustratively, the lower rotor assembly8dcomprises at least two and, exemplarily, three rotor blades19a,19b,19cfor producing thrust in operation. Similarly, the upper rotor assembly7dpreferably also comprises at least two and, exemplarily, three rotor blades18a,18b,18cfor producing thrust in operation.

Furthermore, preferably at least one first engine14ais provided for driving the rotor blades18a,18b,18c, i.e. the upper rotor assembly7d, in operation and at least one second engine14bis provided for driving the rotor blades19a,19b,19c, i.e. the lower rotor assembly8d, in operation. The at least one first engine14ais preferably associated with the aircraft operating structure described above with reference toFIG. 1, and the at least one second engine14bis preferably associated with the redundant security architecture described above with reference toFIG. 1. Illustratively, the at least one first and second engines14a,14bare arranged inside of and, thus, encompassed by the gearbox fairing5d.

It should be noted that optionally one or more gearboxes can be introduced between the at least one first and second engines14a,14band the rotor blades18a,18b,18crespectively19a,19b,19c. By such an optional introduction of one or more gearboxes, an operating efficiency of the at least one first and second engines14a,14bcan be increased since their rotational speed is increased.

It should further be noted that the at least one first and second engines14a,14bcan be implemented by any suitable engine that is capable of producing torque in operation, such as a turbine, diesel engine, Otto-motor, electrical engine and so on, and that can be connected to the rotor blades18a,18b,18crespectively19a,19b,19cfor rotating these rotor blades18a,18b,18crespectively19a,19b,19c, i.e. the upper and lower rotor assemblies7drespectively8d, in operation. However, as such engines are well-known to the person skilled in the art and not part of the present invention, they are not described in greater detail for brevity and conciseness.

Preferably, the upper rotor assembly7dis adapted to be rotated in a first rotation direction15aaround a first rotor axis12ein operation. Similarly, the lower rotor assembly8dis adapted to be rotated in a second rotation direction15baround the rotor axis12d, which illustratively defines a second rotor axis, in operation. Illustratively, the first and second rotation directions15a,15bare preferably opposed to each other.

According to one aspect, the first and second rotor axes12e,12dcan be inclined by associated inclination angles21a,22awith respect to a respective longitudinal direction of the shrouding6d, which illustratively corresponds to the second rotor axis12d. The associated inclination angles21a,22aare preferably comprised in a range between −60° and +60°. More specifically, the associated inclination angle21ais preferably comprised in a range between −10° and +45°, and the associated inclination angle22ais preferably comprised in a range between −5° and +5°. Illustratively, the first rotor axis12eand, thus, the upper rotor assembly7d, is inclined by the associated inclination angle21aof exemplarily approximately 30° with respect to the second rotor axis12dand, thus, the lower rotor assembly8d.

At least the upper rotor assembly7dand, more specifically, its rotor blades18a,18b,18c, may be provided with an optional pitch variation17. Similarly, the lower rotor assembly8d, i.e. its rotor blades19a,19b,19c, may also be provided with such an optional pitch variation. In this case, control of the produced airstream in the thrust producing airstream direction9ofFIG. 3andFIG. 4can either be achieved in operation by means of pitch variation, by means of RPM variation or by means of a combination of pitch and RPM variation.

In contrast, if the upper and lower rotor assemblies7d,8dare not provided with such an optional pitch variation, e.g. if the rotor blades18a,18b,18crespectively19a,19b,19care implemented as fixed pitch blades, control of the produced airstream in the thrust producing airstream direction9ofFIG. 3andFIG. 4in operation by means of pitch variation cannot by performed. In this case, only RPM variation can be used for control of the airstream in the thrust producing airstream direction9ofFIG. 3andFIG. 4that is produced by the upper and lower rotor assembly7d,8din operation.

Preferably, each one of the upper and lower rotor assemblies7d,8dis individually sized and comprises a diameter that ranges from 0.05 to 6 times of the fuselage width2bofFIG. 4, which is designated as W hereinafter for simplicity. In other words, the diameter of each one of the upper and lower rotor assemblies7d,8dpreferably ranges from 0.05*W to 6*W, and preferentially amounts to 1.5*W.

According to one aspect, the shrouding6ddefines a cylindrical air duct20, which is illustratively radially delimited by the supporting structure16. The cylindrical air duct20is preferably axially delimited by an air inlet region20eand an air outlet region20f. Outside of the cylindrical air duct20and preferably adjacent to the air inlet region20eof the cylindrical air duct20is preferably arranged the first rotor assembly7d.

It should be noted that the air duct20is only by way of example designated as a “cylindrical” air duct and not for limiting the present invention accordingly. In other words, while a “cylindrical” shaping of the air duct implies equal radii all along the air duct20from the air inlet region20eto the air outlet region20f, alternative configurations are likewise contemplated. For instance, the air duct20may exhibit the form of a frustum, such that its radius is e.g. greater at the air outlet region20fthan at the air inlet region20e, and so on. Therefore, is should be understood that the expression “cylindrical air duct” is meant to encompass also such alternative configurations of the air duct20.

The air inlet region20epreferably exhibits in circumferential direction of the cylindrical air duct20an undulated geometry. More specifically, this undulated geometry implies that when moving in circumferential direction of the cylindrical air duct20along the air inlet region20e, an undulated motion resp. a wave-shaped movement is performed.

Illustratively, the shrouding6d, i.e. the cylindrical air duct20, exhibits a leading edge20aand a trailing edge20b. Only for clarity, it should be noted that the leading edge20ais the edge of the shrouding6d, i.e. the cylindrical air duct20, that is arranged during forward flight of the multirotor aircraft ofFIG. 1toFIG. 4in an upstream position with respect to the trailing edge20b. Furthermore, the shrouding6d, i.e. the cylindrical air duct20, preferentially exhibits a board side lateral shoulder20cand a star board side lateral shoulder20dthat are located at the air inlet region20e.

More specifically, the leading edge20ais diametrically opposed to the trailing edge20bin circumferential direction of the shrouding6d, i.e. the cylindrical air duct20, and the board side lateral shoulder20cis diametrically opposed to the star board side lateral shoulder20d. Furthermore, the board side lateral shoulder20cand the star board side lateral shoulder20dare respectively arranged between the leading edge20aand the trailing edge20bin circumferential direction of the shrouding6d, i.e. the cylindrical air duct20.

FIG. 6shows the thrust producing unit3dofFIG. 5, with its upper rotor assembly7d, its lower rotor assembly8dand its shrouding6dthat defines the cylindrical air duct20, which is preferably axially delimited by the air inlet region20eand the air outlet region20f, for further illustrating the undulated geometry of the air inlet region20e.FIG. 6also further illustrates the inclination of the upper rotor assembly7dby the associated inclination angle21awith respect to the lower rotor assembly8d.

FIG. 7shows a schematic view of the thrust producing unit3dofFIG. 5andFIG. 6with the upper and lower rotor assemblies7d,8d, which preferentially rotate around their respective rotor axes12e,12d. Preferably, the upper and lower rotor assemblies7d,8ddefine separated rotor planes21,22in order to reach a required safety level and a satisfying flight mechanical behaviour. Illustratively, the rotor planes21,22are arranged on top of each other. Preferentially, a predetermined distance between the rotor planes21,22is comprised in a range between 0.01*DR and 2*DR, and preferably amounts to 0.17*DR, wherein DR defines a diameter of the second rotor assembly8d.

As described above, the shrouding6ddefines the cylindrical air duct20that is axially delimited by the air inlet region20eand the air outlet region20f. The lower rotor assembly8dis arranged inside of the shrouding6dand the upper rotor assembly7dis arranged outside of the shrouding6d, i.e. outside of the cylindrical air duct20and, preferably, adjacent to the air inlet region20e.

In operation of the thrust producing unit3d, the air inlet region20epreferably functions as an air collector and is, therefore, hereinafter also referred to as the “collector20e”, for simplicity and clarity. The air outlet region20fmay be embodied and function as a diffusor, but not necessarily, and is therefore hereinafter also referred to as the “diffusor20f”, for simplicity and clarity.

Part (A) ofFIG. 7illustrates an exemplary operation of the thrust producing unit3din axial air flow conditions, i.e. during vertical take-off and hovering of the multirotor aircraft1ofFIG. 1toFIG. 4. However, in contrast toFIG. 5andFIG. 6, the rotor axes12e,12dare exemplarily arranged coaxially to each other.

Illustratively, in the axial air flow conditions, an axial airstream23aenters the cylindrical air duct20via the collector20e, is accelerated by means of the upper and lower rotor assemblies7d,8dand exits the cylindrical air duct20via the diffusor20f. It should be noted that the airstream23ais referred to as an “axial” airstream as it is at least approximately oriented in parallel to the coaxially arranged rotor axes12e,12d.

The axial airstream23aby itself produces thrust and further produces an additional thrust by acting on the shrouding6d, i.e. the cylindrical air duct20. This will lead to a total thrust illustrated by a thrust vector23, which will allow the multirotor aircraft1ofFIG. 1toFIG. 4to be lifted. It should be noted that at the same thrust level, a respective amount of power needed by the at least one first and second engines14a,14bofFIG. 5andFIG. 6for driving the upper and lower rotor assemblies7d,8dwill be significantly lower than the power that would be needed to drive the upper and lower rotor assemblies7d,8dwithout the shrouding6d.

Part (B) ofFIG. 7illustrates an exemplary operation of the thrust producing unit3din transversal air flow conditions, i.e. during forward flight of the multirotor aircraft1ofFIG. 1toFIG. 4. The rotor axes12e,12dare illustratively still arranged coaxially to each other according to part (A), but now a transversal airstream23benters the cylindrical air duct20via the collector20e, is accelerated by means of the upper and lower rotor assemblies7d,8dand exits the cylindrical air duct20via the diffusor20f. It should be noted that the airstream23bis referred to as a “transversal” airstream, as it is at least approximately oriented in a direction transversal to the coaxially arranged rotor axes12e,12d.

In order to allow forward flight of the multirotor aircraft1ofFIG. 1toFIG. 2with the thrust producing unit3daccording to part (B), preferably RPM variation is used for control of the transversal airstream23bin the cylindrical air duct20. More specifically, the upper rotor assembly7dis preferably rotated around the rotor axis12ewith a higher rotational speed than the lower rotor assembly8daround the rotor axis12d. Thus, an underlying direction of the total thrust illustrated by the thrust vector23, which is still shown as in part (A), will be re-oriented as illustrated in part (C) in order to allow the forward flight of the multirotor aircraft1ofFIG. 1toFIG. 4.

Part (C) ofFIG. 7illustrates another exemplary operation of the thrust producing unit3din transversal air flow conditions, i.e. during forward flight of the multirotor aircraft1ofFIG. 1toFIG. 4according to the present invention, wherein the transversal airstream23baccording to part (B) enters the cylindrical air duct20via the collector20e, is accelerated by means of the upper and lower rotor assemblies7d,8dand exits the cylindrical air duct20via the diffusor20f. However, in contrast to part (B) the rotor axis12eis now inclined by the inclination angle21a, as described above with reference toFIG. 5andFIG. 6. Thus, the thrust vector23is re-oriented as exemplarily illustrated in order to allow for enhanced forward flight conditions of the multirotor aircraft1ofFIG. 1toFIG. 4.

FIG. 8shows another schematic view of the thrust producing unit3dofFIG. 5andFIG. 6with the shrouding6dthat defines the cylindrical air duct20, which is preferably axially delimited by the collector20eand the diffusor20fand which comprises the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d. However, for purposes of simplicity and clarity, illustration of the upper and lower rotor assemblies7d,8dwas omitted.

According to one aspect, the cylindrical air duct20exhibits a height defined between the diffusor20fand the collector20ein axial direction of the cylindrical air duct20that varies in circumferential direction of the cylindrical air duct20. This height varies in the circumferential direction of the cylindrical air duct20and, thus, defines the undulated geometry of the collector20eas described above with reference toFIG. 5.

More specifically, a height24aat the leading edge20ais preferably smaller than a height24cat the board side lateral shoulder20cand/or the star board side lateral shoulder20d. Furthermore, a height24bat the trailing edge20bis preferably smaller than the height24cat the board side lateral shoulder20cand/or the star board side lateral shoulder20d. Moreover, the height24bat the trailing edge20bis preferably smaller than the height24aat the leading edge20a. According to one aspect, the height24cat the board side lateral shoulder20cand/or the star board side lateral shoulder20dis selected in a range from 0.05*D to 0.5*D, wherein D defines a diameter, preferably an inner diameter (20ginFIG. 10), of the cylindrical air duct20.

According to one aspect, the collector20eof the cylindrical air duct20exhibits a radius that varies in the circumferential direction of the cylindrical air duct20. In other words, the collector20eis preferably not provided with a flat upper edge, i.e. its edge that points away from the diffusor20f, but with a rounded upper edge. Preferentially, the radius of the collector20e, which is hereinafter also referred to as the “collector radius” for simplicity and clarity, differs between at least two of the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d.

Preferably, a collector radius25aat the leading edge20ais selected in a range from 0.01*D to 0.25*D, a collector radius25bat the trailing edge20bis selected in a range from 0 to 0.25*D, and a collector radius25cat the board side lateral shoulder20cand/or the star board side lateral shoulder20dis selected in a range from 0.01*D to 0.25*D. As already mentioned above, D defines the diameter, preferably the inner diameter (20ginFIG. 10), of the cylindrical air duct20.

Likewise, the diffusor20fof the cylindrical air duct20may exhibit a radius that varies in the circumferential direction of the cylindrical air duct20. In other words, the diffusor20fis not necessarily provided as illustrated with a flat lower edge, i.e. its edge that points away from the collector20e, but with a rounded lower edge. Preferentially, the radius of the diffusor20f, which is hereinafter also referred to as the “diffusor radius” for simplicity and clarity, differs between at least two of the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d.

Preferably, a diffusor radius26aat the leading edge20ais selected in a range from 0 to 0.1*D, a diffusor radius26bat the trailing edge20bis selected in a range from 0 to 0.1*D, and a diffusor radius26cat the board side lateral shoulder20cand/or the star board side lateral shoulder20dis selected in a range from 0 to 0.1*D. Again, as already mentioned above, D defines the diameter, preferably the inner diameter (20ginFIG. 10), of the cylindrical air duct20.

FIG. 9shows the shrouding6dofFIG. 5toFIG. 8that defines the cylindrical air duct20, which is preferably axially delimited by the collector20eand the diffusor20fand which comprises the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d. According to one aspect, the leading edge20ais provided with an additional lifting surface27.

FIG. 10shows the shrouding6dofFIG. 5toFIG. 8that defines the cylindrical air duct20, which comprises the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d. Illustratively, a diameter and, more specifically, an inner diameter D of the cylindrical air duct20, is labeled with the reference sign20g. Furthermore, the azimuth ψ of the cylindrical air duct20, i.e. the shrouding6d, is labeled with the reference sign20h. By way of example, it is assumed that the azimuth ψ is defined in clockwise direction of the shrouding6das illustrated and starts turning from the trailing edge20bsuch that ψ=0 at the trailing edge20b.

FIG. 11shows four exemplary cross-sections of the shrouding6dthat defines the cylindrical air duct20, which is preferably axially delimited by the collector20eand the diffusor20fand which comprises the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d. Each cross-section corresponds to a sectional view of the shrouding6dat a given azimuth ψ ofFIG. 10.

More specifically, a first sectional view illustrates an exemplary cross-section of the shrouding6dat the azimuth ψ=180° seen in direction of the cut line A-A ofFIG. 10. This first sectional view illustrates the leading edge20aof the shrouding6dthat is provided with the additional lifting surface27ofFIG. 9. By way of example, the collector20eis provided at the leading edge20aas described above with reference toFIG. 8with a rounded upper edge, while the diffusor20fis illustratively provided with a flat lower edge.

A second sectional view illustrates an exemplary cross-section of the shrouding6dat the azimuth ψ=0° seen in direction of the cut line A-A ofFIG. 10. This second sectional view illustrates the trailing edge20bof the shrouding6d. By way of example and as described above with reference toFIG. 8, the collector20eis provided at the trailing edge20bwith a rounded upper edge and the diffusor20fis provided with a rounded lower edge.

A third sectional view illustrates an exemplary cross-section of the shrouding6dat the azimuth ψ=90° seen in direction of the cut line B-B ofFIG. 10. This third sectional view illustrates the board side lateral shoulder20cof the shrouding6d. By way of example, the collector20eis provided at the board side lateral shoulder20cas described above with reference toFIG. 8with a rounded upper edge, while the diffusor20fis illustratively provided with a flat lower edge.

A fourth sectional view illustrates an exemplary cross-section of the shrouding6dat the azimuth ψ=270° seen in direction of the cut line B-B ofFIG. 10. This fourth sectional view illustrates the star board side lateral shoulder20dof the shrouding6d. By way of example, the collector20eis provided at the star board side lateral shoulder20das described above with reference toFIG. 8with a rounded upper edge, while the diffusor20fis illustratively provided with a flat lower edge.

FIG. 12shows the thrust producing unit3dofFIG. 5andFIG. 6according to Part (C) ofFIG. 7, with the shrouding6dand the upper and lower rotor assemblies7d,8d. The upper rotor assembly7drotates in operation around the rotor axis12eand defines the rotor plane21, and the lower rotor assembly7drotates in operation around the rotor axis12dand defines the rotor plane22. The rotor axis12eis inclined with respect to the rotor axis12d, as described above.

More specifically,FIG. 12illustrates an exemplary control method for controlling the thrust producing unit3dby means of RPM variation. In other words, if e.g. the upper rotor assembly7dis operated with a rotational speed Ω2that is higher than a rotational speed Ω1of the lower rotor assembly8d, the thrust vector23is inclined with respect to an exemplary reference plane28aby an associated thrust orientation angle ε, which is labelled with the reference sign28. As long as the associated thrust orientation angle ε is smaller than 90°, i.e. ε<90°, as illustrated on the left-hand side ofFIG. 12, the multirotor aircraft1ofFIG. 1toFIG. 4is operated in forward flight. If, however, the associated thrust orientation angle ε is equal to 90°, i.e. ε=90°, as illustrated on the right-hand side ofFIG. 12, the multirotor aircraft1ofFIG. 1toFIG. 4is operated in hover or vertical take-off.

However, it should be noted that this functioning also depends on a particular implementation of the upper and lower rotor assemblies7d,8d. More specifically, the required rotational speed differences may e.g. vary depending on pitch differences between the upper and lower rotor assemblies or the inclination between the rotor axis12eand the rotor axis12d, and so on. However, the detailed functioning is considered to be readily available to the person skilled in the art and, as such, not subject of the present invention. Therefore, a more detailed description thereof is omitted for brevity and conciseness.

FIG. 13shows an exemplary RPM offset control diagram29illustrating operation of the multirotor aircraft1ofFIG. 1toFIG. 4. The diagram29illustratively comprises a flight mode axis29aand a rotational speed axis29b.

In diagram29, two graphs30,31are illustratively represented. The graph30exemplifies the rotational speed Ω2of the upper rotor assembly7dofFIG. 12, and the graph31exemplifies the rotational speed Ω1of the lower rotor assembly8dofFIG. 12.

When operation of the multirotor aircraft1ofFIG. 1toFIG. 4starts, the upper rotor assembly7dis preferably operated with a rotational speed Ω2that is lower than the rotational speed Ω1of the lower rotor assembly8d, as indicated with an arrow32a. Thus, the multirotor aircraft1ofFIG. 1toFIG. 4is operated in an associated hover mode, i.e. hovers.

Subsequently, the rotational speed Ω2of the upper rotor assembly7dis preferably increased and the rotational speed Ω1of the lower rotor assembly8dis preferably decreased. Then, when the upper rotor assembly7dis operated with a rotational speed Ω2that is higher than the rotational speed Ω1of the lower rotor assembly8d, the multirotor aircraft1ofFIG. 1toFIG. 4is operated in an associated forward flight mode, as indicated with an arrow32b.

FIG. 14shows the shrouding6dofFIG. 5toFIG. 12that defines the cylindrical air duct20, which comprises the leading edge20a, the trailing edge20b, the board side lateral shoulder20cand the star board side lateral shoulder20d. However, in contrast to the implementation of the shrouding6daccording toFIG. 5toFIG. 12, the trailing edge20bof the cylindrical air duct20is now at least essentially open and merely provided with a stiffening element33. Preferably, the cylindrical air duct20is open at the trailing edge20bover a predetermined opening angle33aof e.g. 30° to 60°, which corresponds to an extension angle of the stiffening element33.

FIG. 15shows the thrust producing unit3dofFIG. 5andFIG. 6according to Part (C) ofFIG. 7, with the shrouding6dand the upper and lower rotor assemblies7d,8d. The shrouding6dcomprises the leading edge20aand the trailing edge20b. The upper rotor assembly7drotates in operation around the rotor axis12eand defines the rotor plane21, and the lower rotor assembly7drotates in operation around the rotor axis12dand defines the rotor plane22.

The rotor axis12eis inclined with respect to the rotor axis12d, as described above. InFIG. 15, this inclination is clarified with respect to a horizontal reference plane34. More specifically, the rotor axis12eis inclined with respect to the horizontal reference plane34by an associated inclination angle α, which is labelled with the reference sign34a, and the rotor axis12dis illustratively perpendicular to the horizontal reference plane34, as illustrated by means of an associated inclination angle β, which is labelled with the reference sign34b.

Furthermore, according to one aspect and in contrast to the implementation of the shrouding6daccording toFIG. 5toFIG. 12, the trailing edge20bis now equipped with a flap35that is preferentially designed as an airfoil. The flap35is preferably rotatable around an associated rotation axis35dand illustrated with continuous lines in an exemplary hover position35a, as well as with dotted lines in an exemplary forward flight position35b.

FIG. 16shows the shrouding6dof the thrust producing unit3dofFIG. 15that is provided with the flap35at the trailing edge20b. Illustratively, the flap35is spanned, i.e. extends, over an extension angle35cat the trailing edge20bof the shrouding6d. By way of example, the flap35is shown in its exemplary hover position35aofFIG. 15.

FIG. 17shows the shrouding6dof the thrust producing unit3dofFIG. 15that is provided with the flap35at the trailing edge20b, which is spanned, i.e. extends, over an extension angle35cat the trailing edge20bof the shrouding6daccording to FIG.16. By way of example, the flap35is now shown in its exemplary forward fight position35bofFIG. 15.

Finally, it should be noted that modifications of the above described aspects of the present invention are also within the common knowledge of the person skilled in the art and, thus, also considered as being part of the present invention.

REFERENCE LIST

2aAircraft airframe internal volume

3Thrust producing units

3a,3b,3c,3dThrust producing unit

4Thrust producing units structural supports

4a,4b,4c,4dThrust producing unit structural support

14aUpper rotor assembly engine

14bLower rotor assembly engine

15aUpper rotor assembly rotation direction

15bLower rotor assembly rotation direction

18a,18b,18cUpper rotor assembly rotor blade

19a,19b,19cLower rotor assembly rotor blade

20cBoard side lateral shoulder

20dStar board side lateral shoulder

21Upper rotor assembly rotor plane

21aUpper plane inclination angle

22Lower rotor assembly rotor plane

22aLower plane inclination angle

23bForward flight airstream direction

24aTotal height of air duct leading edge (HL)

24bTotal height of air duct trailing edge (HT)

24cTotal height of air duct lateral shoulder (HS)

25aCollector radius at air duct leading edge (CRL)

25bCollector radius at air duct trailing edge (CRT)

25cCollector radius at air duct lateral shoulder (CRS)

26aDiffusor radius at air duct leading edge (DRL)

26bDiffusor radius at air duct trailing edge (DRT)

26cDiffusor radius at air duct lateral shoulder (DRS)

29RPM offset control diagram

29aFlight mode axis

32bForward flight mode

33aStiffening element extension angle

34Rotor assembly inclination reference plane

35bFlap forward flight position

35cFlap extension angle