WO2025114468A1 - Vertical take-off and landing aircraft - Google Patents

Abstract

The invention relates to a vertical take-off and landing aircraft comprising: at least four pairs of counter-rotating propellers (10); drive units (21, 22, 23) for rotating the propellers (11, 12), a flight control unit for controlling the drive units (21, 22, 23) so as to obtain, for each propeller (11, 12), a target thrust; and a detection system for detecting a failure of the drive units (21, 22, 23). Each of the pairs of propellers (10) is provided with three separately controllable drive units (21, 22, 23), namely a primary drive unit (21) for driving one of the propellers of the pair, and secondary and tertiary drive units (22, 23) for driving the other propeller, wherein the secondary and tertiary drive units (22, 23) are arranged such that their torques combine.

Description

"Vertical takeoff and landing aircraft"

TECHNICAL FIELD OF THE INVENTION

The invention relates to a vertical take-off and landing aircraft or “VTOL” aircraft (acronym for “Vertical Take-off and Landing”).

STATE OF THE ART

Many VTOL aircraft configurations have been studied in the past. Examples of known configurations are described in patent document EP 3393904 B1 and in Publication 1: “Local controllability and attitude stabilization of multirotor UAVs: Validation on a coaxial octorotor” by Majd Saied et al (Robotics and Autonomous Systems 91 (2107) 128-138).

In Publication 1, according to the so-called coaxial counter-rotating octorotor configuration, the aircraft comprises: four pairs of counter-rotating propellers to provide lift for the aircraft; power units to drive the propellers in rotation, each power unit comprising an electric motor and its electronic control system; and a flight control unit to control the power units so as to obtain, for each propeller, a target thrust. Compared to other configurations, the eight-rotor coaxial contra-rotating configuration has advantages in terms of compactness. In addition, since the helical flow of the first propeller in a pair is straightened by the second, propulsive efficiency is improved. Such a configuration therefore provides a maneuverable, stable aircraft that is well-suited for operating in confined areas. On the other hand, it is difficult to control the aircraft's behavior after the failure of one of the power units or, worse, two power units (see Table 10 of Publication 1).

The failure of one of the power units has a direct and immediate impact on the aircraft's balance and creates a major safety risk. In the worst case, the aircraft risks turning over and crashing. The problem becomes even more critical if two power units fail simultaneously. Hereinafter, we refer to a "double failure" to refer to the simultaneous failure of two power units belonging, respectively, to two pairs of propellers. While the simultaneous failure of three or more power units is a highly improbable event, the occurrence of a double failure is considered sufficiently probable to be taken into consideration in the aircraft design. This is therefore a critical point for airworthiness certification, particularly for certain categories of aircraft such as unmanned aerial vehicles (UAVs) weighing more than 25 kg when they must fly over urban areas or for VTOL aircraft intended for passenger transport.

Publications JP 6 487607 B2, CA 2 840 823 A1, US 2017/274984 A1, CN 112 093 042 A and WO 2022/150534 A1 describe examples of aircraft, but none provide a solution to the double failure problem.

There is therefore a need for a solution to better control the behavior of a multirotor aircraft with counter-rotating coaxial propellers in the event of a double failure. This solution must also be of relatively simple design and relatively limited mass, so as to guarantee a good level of aircraft performance, particularly in terms of payload and autonomy.

SUMMARY OF THE INVENTION

A vertical take-off and landing aircraft according to the invention comprises: at least four pairs of counter-rotating propellers for ensuring, at least in part, the lift of the aircraft; power units for driving the propellers in rotation, each power unit comprising an electric motor and its electronic control system; a flight control unit for controlling the power units so as to obtain, for each propeller, a target thrust; and a detection system to detect failure of the power units.

Each of the pairs of propellers comprises an upper propeller and a lower propeller rotating in opposite directions around a propeller axis substantially parallel to the yaw axis of the aircraft. The pairs of propellers are distributed symmetrically with respect to the roll and pitch axes of the aircraft. Each of the pairs of propellers is equipped with three separately controllable power units, namely a primary power unit for driving one of the propellers, called a single-engine propeller, and secondary and tertiary power units for driving the other propeller, called a twin-engine propeller, the secondary and tertiary power units being arranged so that their engine torques are added together.

When a dual failure is detected by the detection system, the flight control unit controls the secondary and tertiary power units of at least two propellers, called compensating propellers, chosen from among the dual-engine propellers, to drive each of the compensating propellers by means of its secondary and tertiary power units, so as to increase the thrust of each of the compensating propellers and compensate for the loss of thrust linked to the failure of the two power units.

The proposed solution therefore consists, for each pair of counter-rotating propellers, of providing a single-engine propeller and a twin-engine propeller and, in the event of a double failure, of using at least two twin-engine propellers as compensating propellers. Thanks to the action of the compensating propellers, it is possible to control the behavior of the aircraft in the event of a double failure, whatever the configuration of the double failure.

By doubling the power of only one of the propellers in each pair, a good compromise is obtained between the desired safety and the weight of the aircraft. It should be noted that another approach would consist of doubling the power of each of the two propellers in each pair of counter-rotating propellers. This solution, known as the "total redundancy solution", would also provide an answer to the problem of double failure of the power units. However, it would have the disadvantage of increasing the weight of the aircraft too significantly, so that the compromise between safety and weight would not be satisfactory.

The foregoing and other features and advantages will become apparent from the following detailed description. This detailed description refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings are schematic and are not necessarily to scale; they are intended primarily to illustrate the principles of the invention. In these drawings, from one figure (fig) to another, identical elements (or parts of elements) are identified by the same reference signs.

Figure 1 shows an example of a VTOL aircraft seen from the side.

Figure 2 shows the example VTOL aircraft in figure 1 seen from above.

Figure 3 schematically represents another example of a VTOL aircraft.

Figure 4 shows in detail an example of counter-rotating propellers with their drive units.

Figure 5 shows the aircraft of Figure 3 during a first example of a double failure.

Figure 6 shows the aircraft of Figure 3 during a second example of double failure.

Figure 7 shows the aircraft of Figure 3 during a third example of double failure.

Figure 8 shows the aircraft of Figure 3 during a fourth example of double failure.

DETAILED DESCRIPTION OF THE INVENTION

Particular embodiments of the proposed aircraft are described in detail below, with reference to the example shown in the accompanying drawings. These embodiments illustrate the characteristics and advantages of the invention. It is however recalled that the invention is not limited to these embodiments, nor to the example shown.

Generally, the multi-rotor VTOL aircraft comprises at least four pairs of counter-rotating propellers to provide, at least in part, the lift of the aircraft; power units to drive the propellers in rotation, each power unit comprising an electric motor and its electronic control system; a flight control unit to control the power units so as to obtain, for each propeller, a target thrust; and a detection system to detect a failure of the power units.

Figures 1 and 2 schematically represent an example of an aircraft 1 with four pairs of counter-rotating propellers 10. Figure 3 schematically represents another example of an aircraft 1 with eight pairs of counter-rotating propellers 10. The number of pairs of propellers is however not limited to these examples and the aircraft may comprise 4, 5, 6, 7, 8, 9, 10, etc. pairs of propellers. When the aircraft 1 comprises an odd number of pairs of propellers 10, at least one of the pairs of propellers 10 is arranged on the roll axis X of the aircraft.

The aircraft's roll X, pitch Y, and yaw Z axes are imaginary axes around which the aircraft rotates. These axes are oriented as follows:

- the roll axis X, or longitudinal axis, is parallel to the line extending from the front part (e.g., the nose) to the rear part (e.g., the tail) of the aircraft, through the central body (e.g., the fuselage) of the aircraft, and passes through the center of mass of the aircraft;

- the pitch axis Y, or lateral or transverse axis, is the axis perpendicular to the roll axis passing through the center of mass of the aircraft. When the aircraft has a main fixed wing plane, the pitch axis Y extends from one end of the main fixed wing plane to the other end of this plane; and

- the yaw axis Z, or vertical axis, passes through the center of mass G of the aircraft, from top to bottom, and is perpendicular to the other two axes X, Y.

These X, Y, Z axes are identified in the examples in figures 1 to 3. Front and rear, like upstream or downstream, are defined in relation to the normal direction of travel of the aircraft, identified by the arrow on the X axis.

Figure 4 schematically represents, in detail, an example of a pair of counter-rotating propellers 10. The pair of propellers 10 comprises a high propeller 11 and a low propeller 12 rotating in opposite directions around a propeller axis A. The propeller axis A is substantially parallel to the yaw axis Z of the aircraft 1. The expression “substantially parallel” means that the propeller axis A may not be strictly parallel to the yaw axis Z and form a slight angle (e.g., of less than 10°) with the Z axis, as long as this angle does not prevent the effect that the pair of propellers 10 is intended to produce.

The direction of rotation of each propeller is represented by a double arrow in the figures. The pairs of propellers 10 are distributed symmetrically with respect to the roll axes X and pitch Y of the aircraft, and surround the central body (e.g., the fuselage) of the aircraft 1. Each pair of propellers can be connected, for example by means of a fixing arm 15 or any other connecting element, to the central body of the aircraft or to an intermediate element connected to the fuselage such as a fixed wing plan.

In the example of Figures 1 and 2, the aircraft 1 comprises a fuselage 2 forming the central body of the aircraft, a propulsion system 5 at the front of the fuselage 2 and four pairs of counter-rotating propellers 10. The aircraft 1 also comprises three wing plans: a canard-type front wing plan 20 located at the front of the aircraft 1; a main wing plan 30 located in the middle part of the aircraft 1; and a tailplane-type rear wing plan 40 located at the rear of the aircraft 1. The main wing plan 30 is formed of a pair of wings 32 (i.e. a right wing and a left wing) joined together above the fuselage 2.

The four pairs of propellers 10 are distributed symmetrically on either side of the main wing plan 30 and on either side of the fuselage 2. In other words, two pairs of propellers 10 are located to the right of the fuselage 2, on either side (i.e., in front and behind) the right wing 32, and two pairs of propellers 10 are located to the left of the fuselage, on either side (i.e., in front and behind) the left wing 32. In this example, each pair of propellers 10 is structurally connected, i.e., is fixedly attached, to the main wing plan 30. Together, the four pairs of propellers 10 and the wing plans 10, 20, 30 provide lift for the aircraft.

In the example of Figure 3, the aircraft 1 comprises a central body 3 around which the pairs of propellers 10 are distributed. The pairs of propellers 10 are distributed symmetrically with respect to the roll axes X and pitch Y of the aircraft 1. Thus, there are two pairs of propellers 10 in each of the front right, front left, rear right and rear left sectors of the aircraft.

In some embodiments and in the examples of Figures 1 to 3, the upper propellers 11 symmetrical with respect to the roll axis X rotate in opposite directions, the upper propellers 11 symmetrical with respect to the pitch axis Y rotate in opposite directions, the lower propellers 12 symmetrical with respect to the roll axis X rotate in opposite directions, and the lower propellers 12 symmetrical with respect to the pitch axis Y rotate in opposite directions. Thus, in the example of Figure 2, the upper propellers 11 of the front right and rear left sectors rotate in the same direction.

As illustrated in Figure 4, each pair of propellers 10 is equipped with three separately controllable power units, namely a primary power unit 21 for driving one of the propellers, called the single-engine propeller, and secondary 22 and tertiary 23 power units for driving the other propeller, called the double-engine propeller. The secondary 22 and tertiary 23 power units are arranged so that their engine torques add up. In other words, if in a flight configuration the propeller 11 must provide thrust requiring a significant torque that only one of the power units 22, 23 cannot provide, starting the second power unit makes it possible to increase the total engine torque to obtain the required thrust. In the example of Figure 4, the upper propeller 11 is the dual-engine propeller and the lower propeller 12 is the single-engine propeller, but the reverse configuration is possible. Note that the use of the ordinal adjectives "primary", "secondary" and "tertiary" is not intended to imply or create an order of operation of motor units. The use of these adjectives simply serves to distinguish between motor units.

Each drive unit 21, 22, 23 comprises an electric motor and its electronic control system. Each drive unit 21, 22, 23 may also comprise an electronic power supply system and, in particular, an inverter. The drive units 21, 22, 23 may be powered by different types of electrical energy sources, such as an energy storage system (batteries, hydrogen), an energy generation system (thermal, hydrogen) or a mixed system (hybrid). When there are several electrical energy sources on board, the outputs of the electrical energy sources may be added together before being distributed to the drive units. The drive units 21, 22, 23 may be powered separately.

The aircraft 1 comprises a detection system for detecting a failure of each of the power units 21, 22, 23. Failure is understood to mean a fault or breakdown of the power unit, whether this fault or breakdown comes from the electric motor, its electronic control system or its electronic power supply system. In the present application, a propeller having a faulty power unit is referred to as a “faulty propeller”.

The detection system may include one or more sensors such as temperature probes, sensors for measuring the rotation speed of the motor, torque measurement, measurement of the various currents of the motor and/or its electronic circuit, measurement of leakage currents, partial discharge, or any other measurement allowing a health diagnosis (i.e., a condition diagnosis) of the power unit. The detection system may also use measurements and/or the estimation of one or more parameters of the aircraft and/or the flight condition.

When a simultaneous failure of two power units, or double failure, is detected by the detection system, the flight control unit controls the secondary and tertiary power units of at least two propellers, called compensating propellers, chosen from among the twin-engine propellers, to drive each of the compensating propellers by means of its secondary and tertiary power units, so as to increase the thrust of each of the compensating propellers and compensate for the loss of thrust linked to the shutdown of the two faulty power units.

In some embodiments, the thrust of each of the trim propellers is increased beyond the maximum thrust that would be available if the trim propeller were driven solely by its secondary power unit 22 or its tertiary power unit 23. In other words, the secondary and tertiary power units tertiary, when they are actuated together, make it possible to obtain a thrust, called "compensation thrust", greater than the thrust that can be obtained with only one of the two power units 22, 23. Thus, the secondary power unit 22 is not sized to obtain, on its own, said compensation thrust. The same applies to the tertiary power unit 23. This makes it possible to reduce the dimensions and mass of these power units 22, 23 which, typically, are only designed (or selected off the shelf) to ensure the nominal flight thrust.

In some embodiments, for each pair of propellers 11, 12 operating normally, the drive units 21, 22 and/or 23 are controlled so that the resistive torques of each of the propellers 11, 12 cancel each other out. The pair of propellers 11, 12 is thus torque balanced so as not to create a yaw torque.

In certain embodiments, the secondary 22 and tertiary 23 drive units are identical in terms of engine torque. This choice makes it possible to provide symmetrical processing in the management of failures of each of these drive units 22, 23.

In some embodiments, the upper propellers 11 symmetrical with respect to the roll X and pitch Y axes rotate in opposite directions, and the lower propellers 12 symmetrical with respect to the roll X and pitch Y axes rotate in opposite directions. This makes it possible to cancel the yaw moments produced by all of the upper propellers and the yaw moments produced by all of the lower propellers. Control of the aircraft via the central flight control unit is then facilitated.

In some embodiments, the flight control unit may be centralized. In some embodiments, the flight control unit is part of the aircraft's flight control system, or "FCS" (acronym for "Flight Control System"), this system being based on the control of one or more control members of one or more state parameters of the aircraft and/or measurements and/or estimation of one or more parameters of the aircraft and/or the flight condition. This flight control system is, in addition, provided with a control reconfiguration function. The flight control system also has a pilot alert function indicating the detection of one or more failures (faults or breakdowns) and the reconfiguration of the controls and the impact on the mission. Preferably, the flight control system includes a pilot assistance function offering the pilot an optimal trajectory and assistance in managing the trajectory following failure and reconfiguration of the controls.

In some embodiments, the flight control unit defines a compensation strategy, or compensation logic, based on the simultaneous number of failures to be considered. This compensation strategy can be defined from a table dealing with all possible failure cases and proposing for each case a compensation strategy (optimized or not). The compensation strategy can also be obtained by an onboard optimization algorithm operating in real time. The compensation strategy can be used by the flight control system's control reconfiguration function. The reconfiguration function will adapt the piloting laws and the servo gains according to the selected compensation strategy, the piloting mode and the flight configuration (speed, altitude, etc.).

Figures 5 to 8 illustrate different possible double failure cases. These figures schematically represent the aircraft example of Figure 3. The direction of rotation of the propellers 11, 12 is the same as in Figure 3. The single-engine propellers are the low propellers 12 and the twin-engine propellers are the high propellers 11, as in the example of Figure 4. However, the single-engine propellers could be the high propellers and the twin-engine propellers could be the low propellers, without this affecting the explanations which follow. The letters “D”, “C” and “E” are used in these figures following the propeller number (e.g. 11 C, 11 D, 12D, 12E) to designate, respectively, failed propellers (“D”), compensating propellers (“C”) and non-failed propellers voluntarily shut down (“E”). The aircraft’s roll X and pitch Y axes define between them four sectors: a right front FR sector, a left front FL sector, a right rear RR sector and a left rear RL sector. Several compensation strategies are defined below with reference to these figures.

In some embodiments, according to a first compensation strategy, when the two faulty power units are two primary power units driving, respectively, two single-engine propellers rotating in opposite directions, the compensating propellers comprise the two dual-engine propellers belonging, respectively, to the two pairs of propellers of which the two single-engine propellers are part. In other words, the faulty single-engine propellers and the compensating dual-engine propellers belong to the same pairs of propellers.

This first compensation strategy is illustrated by the example in Figure 5. In this example, the failed single-engine propellers are denoted 12D and marked with a cross. In the example, the failed 12D propellers are located, respectively, in the front right FR and front left FL sectors, but they could be located in other sectors or both be located in the same sector. The failed 12D single-engine propellers rotate in opposite directions. In such a case of double failure, the proposed compensation strategy consists of stopping the power units 21 of the failed propellers 12D and to use as compensating propellers the twin-engine propellers denoted 11C. In normal operation, these compensating propellers 11C can be driven by a single power unit, for example the secondary power unit 22. In the event of a double failure, the tertiary power units 23 are actuated so that each compensating propeller 11C is driven not only by its secondary power unit 22 but also by its tertiary power unit 23. Thus, the thrust of each compensating propeller 11C can be increased so as to compensate for the loss of thrust linked to the failure of the propellers 12D. Alternatively, in normal operation, the compensating propellers 11C can be driven by their two secondary 22 and tertiary 23 power units, these power units producing a nominal torque lower than their maximum available torque. In the event of a double failure, the secondary and tertiary power units 22, 23 are commanded to produce their maximum available torque (or a torque strictly greater than their nominal torque and less than or equal to their maximum available torque). Thus, the thrust of each compensating propeller 11C can be increased so as to compensate for the loss of thrust linked to the failure of the propellers 12D.

In some embodiments, according to a second compensation strategy, when the two faulty power units are two primary power units belonging to two adjacent sectors and driving, respectively, first and second single-engine propellers rotating in the same direction, the flight control unit commands the stopping of the two faulty power units and of the two primary power units located, respectively, in the two adjacent sectors and driving the two single-engine propellers symmetrical to the first and second single-engine propellers with respect to the roll axis X or pitch axis Y. In this case, the compensating propellers comprise the four double-engine propellers belonging, respectively, to the pairs of propellers of which the stopped power units are part.

This second compensation strategy is illustrated by the example in Figure 6. In this example, the failed single-engine propellers 12D are located in the front right FR and front left FL sectors, but they could be located in other adjacent sectors (e.g., the RL and FL sectors, the RL and RR sectors, or the RR and FR sectors). The failed single-engine propellers 12D rotate in the same direction. In such a case of double failure, the proposed compensation strategy consists of stopping the failed propellers 12D but also stopping the single-engine propellers denoted 12E which are located in the two adjacent sectors FL and FR and which are symmetrical to the two single-engine propellers 12D with respect to the roll axis X. The compensation strategy also consists of using as compensating propellers the four double propellers engine denoted 11C. These 11C compensating propellers are the twin-engine propellers belonging to the same pairs of propellers as the 12D and 12E propellers. In normal operation, the 11C compensating propellers can be driven by a single power unit, for example the secondary power unit 22. In the event of a double failure, the tertiary power units 23 are actuated so that each 11C compensating propeller is driven not only by its secondary power unit 22 but also by its tertiary power unit 23. Thus, the thrust of each 11C compensating propeller can be increased so as to compensate for the loss of thrust linked to the shutdown of the 12D and 12E propellers. Alternatively, in normal operation, the compensating propellers 11C can be driven by their two secondary 22 and tertiary 23 drive units, these drive units producing a nominal torque lower than their maximum available torque. In the event of a double failure, the secondary and tertiary drive units 22, 23 are controlled to produce their maximum available torque (or a torque strictly higher than their nominal torque and lower than or equal to their maximum available torque). Thus, the thrust of each compensating propeller 11C can be increased so as to compensate for the loss of thrust linked to the stopping of the propellers 12D and 12E.

In some embodiments, according to a third compensation strategy, when the two failed power units are a primary power unit and a secondary power unit driving, respectively, a first single-engine propeller and a first dual-engine propeller, the flight control unit commands: the stopping of the failed primary power unit; the stopping of a primary power unit driving a second single-engine propeller symmetrical to the first single-engine propeller with respect to the roll axis X or pitch axis Y; and the tertiary power unit of the first dual-engine propeller to compensate for the loss of thrust linked to the failure of the secondary power unit. According to this third compensation strategy, the compensating propellers comprise the two dual-engine propellers belonging, respectively, to the two pairs of propellers of which the first and second single-engine propellers are part.

This third compensation strategy is illustrated by the examples in Figures 7 and 8. In the example in Figure 7, the failed single-engine and twin-engine propellers 12D, 11 D are located in the same sector, namely the left front sector FL. In such a double failure case, the proposed compensation strategy consists of stopping the failed single-engine propeller 12D and the single-engine propeller 12E symmetrical to the first single-engine propeller with respect to the roll axis X. The proposed compensation strategy also consists of controlling the tertiary power unit 23 of the failed twin-engine propeller 11 D to compensate for the loss of thrust related to the failure of the unit secondary drive unit 22 of this propeller 11 D. According to this third compensation strategy, the compensating propellers 11C comprise the two twin-engine propellers belonging to the same pairs of propellers as the single-engine propellers 12D and 12E. In normal operation, the compensating propellers 11C can be driven by a single power unit, for example the secondary power unit 22. In the event of a double failure, the tertiary power units 23 are actuated so that each compensating propeller 11C is driven not only by its secondary power unit 22 but also by its tertiary power unit 23. Thus, the thrust of each compensating propeller 11C can be increased so as to compensate for the loss of thrust linked to the stopping of the propellers 12D and 12E. Alternatively, in normal operation, the compensating propellers 11C can be driven by their two secondary 22 and tertiary 23 drive units, these drive units producing a nominal torque lower than their maximum available torque. In the event of a double failure, the secondary and tertiary drive units 22, 23 are controlled to produce their maximum available torque (or a torque strictly higher than their nominal torque and lower than or equal to their maximum available torque). Thus, the thrust of each compensating propeller 11C can be increased so as to compensate for the loss of thrust linked to the stopping of the propellers 12D and 12E.

In some embodiments, the propellers of the device and their power units are designed to produce a nominal thrust (Tnom) and a maximum compensation thrust (Tmax). This variation in thrust can be obtained by varying the rotational speed of the propellers and/or their pitch. The maximum thrust (Tmax) is calculated based on the number of multiple failures that the system must manage. The nominal thrust (Tnom) and the maximum compensation thrust (Tmax) of each propeller correspond, respectively, to a nominal torque (Qnom) and a maximum torque (Qmax) required by the propeller.

In some embodiments, each of the power units (i.e., each of the motors) of the dual-motor propellers is sized to produce a nominal torque (Qnom/2) that represents half the nominal torque (Qnom) requested by the propeller, and a maximum torque (Qmax/2) that represents half the maximum torque requested (Qmax) by the propeller, in which case the power units are both used in normal operation to obtain the nominal torque (Qnom) requested by the propeller, and are both used in a compensation situation to obtain the maximum torque (Qmax) requested by the propeller. Furthermore, the maximum torque (Qmax/2) is greater than or equal to the nominal torque (Qnom) requested by the propeller, such that in the event of failure and shutdown of one of the two power units, the other power unit can alone produce the nominal torque (Qnom) required by the propeller. The use of a gearbox between the common shaft of the power unit engines and the propeller shaft can allow the characteristics of the engines to be better adapted to the required requirements.

The embodiments described in this disclosure are given for illustrative and non-limiting purposes, a person skilled in the art being able, in view of this disclosure, to easily modify these embodiments, or envisage others, while remaining within the scope of the invention.

In particular, a person skilled in the art will easily be able to envisage variants comprising only part of the characteristics of the embodiments previously described, if these characteristics alone are sufficient to provide one of the advantages of the invention. In addition, the various characteristics of these embodiments can be used alone or combined with each other. When combined, these characteristics can be as described above or differently, the invention not being limited to the specific combinations described in the present disclosure. In particular, unless otherwise specified, a characteristic described in relation to one embodiment can be applied in a similar manner to another embodiment.

Claims

1. Vertical take-off and landing aircraft comprising: at least four pairs of counter-rotating propellers (10) for ensuring, at least in part, the lift of the aircraft (1); power units (21, 22, 23) for driving the propellers (11, 12) in rotation, each power unit comprising an electric motor and its electronic control system; a flight control unit for controlling the power units (21, 22, 23) so as to obtain, for each propeller (11, 12), a target thrust; and a detection system for detecting a failure of the power units (21, 22, 23), wherein each of the pairs of propellers (10) comprises an upper propeller (11) and a lower propeller (12) rotating in opposite directions about a propeller axis (A) substantially parallel to the yaw axis (Z) of the aircraft (1), wherein the pairs of propellers (10) are distributed symmetrically with respect to the roll (X) and pitch (Y) axes of the aircraft (1), characterized in that each of the pairs of propellers (10) is equipped with three separately controllable power units (21, 22, 23), namely a primary power unit (21) for driving one of the propellers, called a single-engine propeller, and secondary and tertiary power units (22, 23) for driving the other propeller, called a dual-engine propeller, the secondary and tertiary drive units (22, 23) being arranged so that their drive torques add up, and in that when a simultaneous failure of two drive units (21, 22, 23) belonging, respectively, to two pairs of propellers (10) is detected by the detection system, the flight control unit controls the secondary and tertiary drive units (22, 23) of at least two propellers, called compensating propellers (11C), chosen from among the dual-engine propellers, to drive each of the compensating propellers (11C) by means of its secondary and tertiary drive units (22, 23), so as to increase the thrust of each of the compensating propellers (11C) and compensate for the loss of thrust linked to the failure of the two drive units. 2. An aircraft according to claim 1, wherein the thrust of each of the compensating propellers (11C) is increased beyond the maximum thrust that would be available if the compensating propeller (11C) were driven solely by its secondary power unit (22) or its tertiary power unit (23). 3. Aircraft according to claim 1 or 2, wherein the secondary (22) and tertiary (23) power units are identical in terms of engine torque. 4. Aircraft according to any one of claims 1 to 3, wherein, when the two faulty power units are two primary power units (21) driving, respectively, two single-engine propellers (12D) rotating in opposite directions, the compensating propellers (11C) comprise the two double-engine propellers belonging, respectively, to the two pairs of propellers (10) of which the two single-engine propellers (12D) are part. 5. Aircraft according to any one of claims 1 to 4, in which the roll (X) and pitch (Y) axes of the aircraft define between them four sectors (FL, FR, RL, RR), and in which, when the two faulty power units are two primary power units (21) belonging to two adjacent sectors and driving, respectively, first and second single-engine propellers (12D) rotating in the same direction, the flight control unit commands the stopping of the two faulty power units and of the two primary power units located, respectively, in the two adjacent sectors and driving the two single-engine propellers (12E) symmetrical to the first and second single-engine propellers (12D) relative to the roll (X) or pitch (Y) axis; and wherein the compensating propellers (11 C) comprise the four twin-engine propellers belonging, respectively, to the pairs of propellers (10) of which the stopped power units are part. 6. Aircraft according to any one of claims 1 to 5, wherein, when the two failed power units are a primary power unit and a secondary power unit driving, respectively, a first single-engine propeller (12D) and a first twin-engine propeller (11 D), the flight control unit commands: the stopping of the failed primary power unit; the stopping of the primary power unit driving a second single-engine propeller (12E) symmetrical to the first single-engine propeller (12D) relative to the roll (X) or pitch (Y) axis; the tertiary power unit (23) of the first twin-engine propeller (11 D) to compensate for the loss of thrust linked to the failure of the secondary power unit, and wherein the compensating propellers comprise the two twin-engine propellers (11C) belonging, respectively, to the two pairs of propellers (10) of which the first and second single-engine propellers (12D, 12E) are part. 7. Aircraft according to any one of claims 1 to 6, wherein, for each pair of propellers (10) operating normally, the power units (21, 22, 23) are controlled so that the resistive torques of each of the propellers (11, 12) cancel each other out. 8. Aircraft according to any one of claims 1 to 7, in which the upper propellers (11) symmetrical with respect to the roll (X) and pitch (Y) axes rotate in opposite directions, and in which the lower propellers (12) symmetrical with respect to the roll (X) and pitch (Y) axes rotate in opposite directions.