Method for controlling a hybrid helicopter in the event of an engine failure

Abstract

The present invention relates to a control method in case of engine failure of a hybrid helicopter having a power plant connected to at least one lift rotor and to at least one propeller, said lift rotor having a plurality of first blades and said at least one propeller having a plurality of second blades. The method comprises the following steps: (i) measuring a forward speed of the hybrid helicopter, (ii) on condition that said forward speed is greater than a first speed threshold and that each engine has failed, automatically implementing a first emergency piloting mode comprising a step for automatic reduction by an automatic piloting system of a pitch of said second blades toward an objective pitch making said at least one propeller produce a motive power which is transmitted to the lift rotor.

Claims

1. A method of controlling a hybrid helicopter, the hybrid helicopter having a power plant connected to at least one lift rotor and to at least one propeller, the power plant having at least one engine, the lift rotor having a plurality of first blades and the at least one propeller having a plurality of second blades, wherein the method comprises the following steps: measuring (STP1.1) a forward speed (TAS) of the hybrid helicopter, provided that (STP1.3) the forward speed (TAS) is greater than a first speed threshold (SV1) and that each engine of the at least one engine has failed, automatically implementing (STP2) a first emergency piloting mode (MOD1) comprising a step for automatic reduction (STP2.1) by an automatic piloting system of a pitch of the second blades up to an objective pitch making the at least one propeller produce a motive power which is transmitted to the lift rotor, and in that provided that the forward speed (TAS) is less than or equal to the first speed threshold (SV1) and that each engine of the at least one engine has failed, automatically implementing (STP3) a second emergency piloting mode (MOD2) comprising the following steps: automatically adjusting (STP3.1) an average pitch component (TCC) of the pitch of the second blades to an average pitch with zero thrust (TCC0) via the automatic piloting system, the average pitch with zero thrust (TCC0) being calculated by the automatic piloting system so that the at least one propeller exerts no thrust in the absence of a yaw movement order.

2. The method according to claim 1, wherein during the first emergency piloting mode (MOD1), the automatic piloting system determines a value of the objective pitch zeroing out a sum of a mechanical power (PWlr) implemented by the at least one propeller as well as a mechanical power (PWrp) consumed by the lift rotor and a mechanical power (PWinst) consumed by the power plant.

3. The method according to claim 2, said wherein the mechanical power (PWinst) consumed by the power plant can be equal to the sum of a mechanical power (PWprt) resulting from installation losses and a mechanical power (PWcons) consumed by at least one accessory of the power plant.

4. The method according to claim 1, wherein the method comprises determining, by the automatic piloting system, the objective pitch by solving the following equation: TCCdesynch=TCCcur−[(PWrp+PWlr+PWinst)/(dPWlr/dTCC)], with “TCCdesynch” which represents the objective pitch, “TCCcur” which represents a current average pitch component of the pitch of the second blades, “PWrp” which represents mechanical power consumed by the lift rotor, “PWlr” which represents the mechanical power implemented by the at least one propeller, “PWinst” which represents the mechanical power consumed by the power plant, “/” represents the division sign, “−” represents the subtraction sign, “+” represents the addition sign, “=” represents the equal sign, “dPWlr/dTCC” represents the derivative of the mechanical power consumed by the propeller with respect to the current average pitch component of the second blade pitch.

5. The method according to claim 1, wherein, upon initiation of the first emergency piloting mode (MOD1), a speed of rotation of the first blades is kept equal to a setpoint speed of rotation reached before the failure.

6. The method according to claim 1, wherein, during the first emergency piloting mode (MOD1), the automatic reduction by the automatic piloting system of the pitch of the second blades towards the objective pitch comprises the following step: slaving (STP2.3) of a speed of rotation (Nr) of the lift rotor at a setpoint speed of rotation (Nr*) by the automatic piloting system by regulating the pitch of the second blades.

7. The method according to claim 1, wherein, during the first emergency piloting mode (MOD1), the automatic reduction by the automatic piloting system of the pitch of the second blades to the objective pitch comprises the following steps: calculating the objective pitch and slaving of the pitch of the second blades to the objective pitch.

8. The method according to claim 1, wherein, during the first emergency piloting mode (MOD1), the automatic reduction by the automatic piloting system of the pitch of the second blades to the objective pitch comprises the following steps: calculating the objective pitch and slaving of the pitch of the second blades to a setpoint pitch equal to the objective pitch adjusted as a function of a current speed of rotation (Nr) of the lift rotor with respect to a setpoint speed of rotation (Nr*).

9. The method according to claim 1, wherein, on condition that the first emergency piloting mode (MOD1) is implemented, the method comprises a step (STP4) in which a pilot addresses a man-machine interface, the man-machine interface emitting a mode change signal which is transmitted to the automatic piloting system, the automatic piloting system applying, following reception of the change signal, the second emergency piloting mode (MOD2) maintaining the average pitch component of the pitch of the second blades equal to the average pitch with zero thrust (TCC0) when the pitch of the second blades reaches the average pitch with zero thrust (TCC0).

10. The method according to claim 1, wherein, on condition that the first emergency piloting mode (MOD1) is implemented, the method comprises the following steps: prompting (STP5) a collective pitch control by a pilot when the forward speed (TAS) is less than or equal to the first speed threshold (SV1), implementing the second emergency piloting mode (MOD2) via the automatic piloting system keeping the average pitch component of the pitch of the second blades equal to the average pitch with zero thrust (TCC0) when the pitch of the second blades reaches the average pitch with zero thrust (TCC0).

11. The method according to claim 1, wherein, on condition that the first emergency piloting mode (MOD1) is implemented, the method comprises the following steps (STP6): comparing the forward speed (TAS) with a second speed threshold (SV2) which is lower than the first speed threshold (SV1), automatically reducing a pitch of the first blades via the automatic piloting system and implementing the second emergency piloting mode (MOD2) via the automatic piloting system when the forward speed (TAS) is less than the second speed threshold (SV2).

12. The method according to claim 1, wherein, on condition that the first emergency piloting mode (MOD1) is implemented, the method comprises the following steps: detecting (STP7) a restart of the at least one engine, following the detection, automatically disengaging the first emergency piloting mode (MOD1) by the automatic piloting system.

13. The method according to claim 1, wherein the forward speed is true air speed (TAS) of the hybrid helicopter.

14. A hybrid helicopter, the hybrid helicopter having an automatic piloting system being configured to apply the method according to claim 1, the hybrid helicopter having the power plant connected to the at least one lift rotor and to the at least one propeller, the power plant having the at least one engine, the lift rotor having the plurality of first blades and the at least one propeller having the plurality of second blades, wherein the hybrid helicopter comprises a speed sensor for measuring the forward speed (TAS) of the hybrid helicopter and the automatic piloting system connected to the speed sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention and its advantages will emerge in more detail in the context of the description which follows with examples given by way of illustration with reference to the appended figures, which show:

(2) FIG. 1, a schematic view of a hybrid helicopter according to the invention,

(3) FIG. 2, a view illustrating the method according to a first embodiment,

(4) FIG. 3, a view illustrating the method according to a second embodiment,

(5) FIG. 4, a view illustrating the method according to a third embodiment,

(6) FIG. 5, a view illustrating a display during the application of the method according to the invention before a failure of a power plant,

(7) FIG. 6, a view illustrating a display during the application of the method according to the invention during the implementation of the first emergency piloting mode,

(8) FIG. 7, a view illustrating a display during the application of the method according to the invention illustrating a passage from the first emergency piloting mode to the second emergency piloting mode, and

(9) FIG. 8, a view illustrating a display during the application of the method according to the invention during the implementation of the second emergency piloting mode.

DETAILED DESCRIPTION

(10) Elements which are present in several distinct figures are assigned a single reference.

(11) FIG. 1 shows an example of a hybrid helicopter 1 according to the invention.

(12) This hybrid helicopter 1 comprises a cell 4 bearing at least one lift rotor 2. This lift rotor 2 is provided with several blades called “first blades 3” for convenience.

(13) In addition, the hybrid helicopter 1 is provided with one propeller 6 or more propellers 6. The expression “each propeller” is sometimes used subsequently whether the aircraft comprises a single propeller or several propellers for the sake of simplicity. The propeller(s) 6 each comprise several blades called “second blades 7” for convenience. The propeller(s) 6 can be arranged laterally with respect to the cell 4, possibly being borne by a support 5. Such a support 5 may optionally be aerodynamic, such as a wing for example. Thus, according to the variant, the hybrid helicopter 1 comprises a single propeller 6 or at least two propellers 6, and possibly at least two propellers 6 arranged laterally, optionally on either side of the cell 4.

(14) Furthermore, the hybrid helicopter 1 comprises a power plant 10 to supply power to the lift rotor 2 and to each propeller 6. This power plant 10 comprises at least one engine 12 for this purpose which is controlled by a standard engine computer 13. Such an engine computer 13 may be able to detect an engine failure by conventional techniques and by means of conventional sensors.

(15) The term “computer” refers hereinafter to a unit which may for example comprise at least one processor and at least one memory, at least one integrated circuit, at least one programmable system, at least one logic circuit, these examples not limiting the scope given to the expression “computer.” The term “processor” can denote a central processing unit known by the acronym CPU, a graphics processing unit GPU, a digital unit known by the acronym DSP, a microcontroller, etc.

(16) In addition, the power plant 10 may comprise, for example within an interconnection system, at least one power transmission box 11, 14, at least one shaft, and/or at least one connection member between two rotating parts, etc. For example, one or more engines 12 are mechanically connected by one or more mechanical connecting chains to a main power transmission box 11 which drives the lift rotor 2 in rotation. In addition, the main power transmission box 11 can be mechanically connected by at least one shaft to a lateral power transmission box 14 by propeller, which is therefore in turn connected to a propeller 6.

(17) In addition, the power plant 10 can set various accessories 16 in motion. For example, the main power transmission box 11 can set in motion a pump of a hydraulic circuit.

(18) Furthermore, the hybrid helicopter 1 can comprise various controls to be piloted by a human pilot.

(19) In particular, the hybrid helicopter 1 can comprise a system for collectively and cyclically controlling the pitch of the first blades 3. Such a system can for example include a set of swashplates 8. Thus, at each instant, the pitch of the first blades 3 can be equal to the sum of an identical collective pitch for all the first blades 3 and of a cyclic pitch which varies as a function of the azimuth of each first blade 3.

(20) Consequently, the hybrid helicopter 1 can comprise a collective pitch control 20 which acts on a mechanical and/or electrical control chain 21 to collectively vary the pitch of the first blades 3, optionally via the set of swashplates 8. Likewise, the hybrid helicopter 1 can comprise a cyclic pitch control 23 which acts on one or more mechanical and/or electrical control chains 24 to cyclically vary the pitch of the first blades 3, optionally via the set of swashplates 8.

(21) Usually, the hybrid helicopter 1 can comprise a system for controlling the pitch of the second blades 7. At each instant, the pitch of the second blades 7 of a propeller 6 can be equal to the sum of an average pitch component and of a differential pitch component or to the difference of this average pitch component and of the differential pitch component.

(22) Usually, the hybrid helicopter 1 may comprise a thrust control 26 which acts on one or more mechanical and/or electrical control chains 27 in order to vary the average pitch component of the pitch of the second blades 3 in order for example to pilot a forward speed of the hybrid helicopter 1. Likewise, the hybrid helicopter 1 may comprise a yaw control 29 which acts on one or more mechanical and/or electrical control chains 30 to vary the differential pitch component of the pitch of the second blades 7 in order for example to pilot a yaw movement of the hybrid helicopter 1.

(23) Reference will be made to the literature to obtain information on such an architecture, and for example to document FR 2946315.

(24) Furthermore, the hybrid helicopter 1 comprises an automatic piloting system 40 for applying the method of the invention. This automatic piloting system 40 can comprise an automatic piloting computer 41.

(25) Further, the automatic piloting system 40 can comprise at least one actuator arranged on one of the various control chains 21, 24, 27, 30 mentioned above so as to control the pitch of the first blades 3 and of the second blades 7. Thus, the automatic piloting computer 40 can be configured to issue commands, for example in the form of an electrical, digital, analog or optical signal, to at least one collective pitch actuator 22 making it possible to collectively modify the pitch of the first blades 3, at least one cyclic pitch actuator 25 making it possible to cyclically modify the pitch of the first blades 3, at least one thrust actuator 28 making it possible to modify the average pitch component of the pitch of the second blades 7 in the same way, and at least one yaw actuator 31 making it possible to modify the differential pitch component of the pitch of the second blades 7.

(26) Furthermore, the automatic piloting system 40 may comprise a man-machine interface 42 in wired or wireless communication, direct or indirect, with the automatic piloting computer 41. Such a man-machine interface 42 can be of the tactile, mobile, sound, etc. type. For example, such a man-machine interface 42 can comprise a button, a microphone, a touch screen, etc.

(27) According to another aspect, the automatic piloting system 40 may comprise a display 50.

(28) Furthermore, the automatic piloting system 40 may comprise or may cooperate with different measurement systems of the hybrid helicopter 1.

(29) Thus, a speed sensor 15 of the hybrid helicopter 1 can be in wired or wireless, direct or indirect communication, for example with the automatic piloting computer 41 to provide information relating to a forward speed of this hybrid helicopter 1. For example, the speed sensor 15 is a usual sensor making it possible to determine the true air speed of the hybrid helicopter 1, for example by means of a pressure difference.

(30) In addition, the hybrid helicopter 1 can comprise a first torque meter 61 and a first rotational speed sensor 62 which are arranged on a rotating member and for example a rotor mast of the lift rotor 2 or the like. A computer, or even the automatic piloting computer 41, can be connected to this first torque meter 61 and to this first rotational speed sensor 62 in order to determine a mechanical power PWrp consumed by the lift rotor 2, this mechanical power PWrp consumed by the lift rotor 2 possibly being transmitted to the automatic piloting computer 41 when calculated by another computer. The mechanical power PWrp consumed by the lift rotor 2 can be equal to the product of a speed measured by the first rotational speed sensor 62 and a torque measured by the first torque meter 61 and possibly a coefficient of proportionality depending on the respective locations of the first rotational speed sensor 62 and of the first torque meter 61.

(31) Optionally, the automatic piloting computer 41 can deduce therefrom, in the usual way, a mechanical power PWcons consumed by the accessories 16 and/or a mechanical power PWprt resulting from installation losses. The sum of the mechanical power PWcons consumed by the accessories 16 and the mechanical power PWprt resulting from installation losses gives a mechanical power PWinst consumed by the power plant 10 as such.

(32) In addition, the hybrid helicopter 1 can include at least one second torque meter 63 and at least one second rotational speed sensor 64 per propeller arranged on a rotating member, and for example a shaft of a propeller 6. A computer or even the automatic piloting computer 41 can be connected to this at least one second torque meter 63 and to this at least one second rotational speed sensor 64 so as to determine an intermediate power per propeller 6. Each intermediate power can be equal to the product of a speed measured by a second speed sensor 64 and a torque measured by a second torque meter 63 and possibly a coefficient of proportionality as a function of the respective locations of the second rotational speed sensor 64 and of the second torque meter 63. The sum of the intermediate powers gives a mechanical power PWlr consumed or produced by all of the propellers 6, this mechanical power PWlr consumed or produced by all of the propellers 6 possibly being transmitted to the automatic piloting computer 41 when calculated by another computer.

(33) In addition, sensors can measure information carrying the pitch of the propeller blades. For example, a first sensor 65 can measure information relative to a position of a first control rod for the pitch of the blades of the first propeller, which is the image of the pitch of the blades of the first propeller. Likewise, a second sensor (not shown) can measure a position of a second control rod for the pitch of the blades of the second propeller, which is the image of the pitch of the blades of the second propeller.

(34) Consequently, the automatic piloting computer 41 is configured to apply the method of the invention. For example, at least one processor executes instructions stored in a memory for this purpose.

(35) According to the method, the automatic piloting computer 41 or another computer can calculate an objective pitch using the following relation:
TCCdesynch=TCCcur−[(PWrp+PWlr+PWinst)/(dPWlr/dTCC),

(36) with “TCCdesynch” which represents the value of the objective pitch, more simply called “objective pitch,” “TCCcur” which represents a current average pitch component of the pitch of the second blades 7, “PWrp” which represents the mechanical power consumed by said lift rotor 2, “PWlr” which represents the mechanical power implemented by said at least one propeller 6, “PWinst” which represents the mechanical power consumed by the power plant 10, “I” represents the division sign, “−” represents the subtraction sign, “+” represents the addition sign, “=” represents the equal sign, “dPWlr/dTCC” represents the derivative of the mechanical power consumed by said at least one propeller 6 with respect to the current average pitch component of the second blade pitch 7.

(37) The objective pitch TCCdesync can be used for information and/or regulation purposes as explained below. The objective pitch may represent an average pitch component of the pitch of the blades of the propeller(s).

(38) With reference to FIG. 2 and whatever the embodiment of the invention, the method comprises a step of determining STP1 the current situation.

(39) This step of determining STP1 the current situation comprises a step STP1.1 for measuring the forward speed TAS of the hybrid helicopter 1 with the speed sensor 15 and a step STP1.2 for verifying the correct operation of each engine 12 for example carried out by each engine computer 13. The automatic piloting computer 41 thus receives information carrying the forward speed TAS and one or more information items carrying the operating state of the engines 12.

(40) The automatic piloting computer 41 then implements a step STP1.3 for choosing the emergency piloting mode to possibly be executed.

(41) Thus, if the engine 12 or, if applicable, all of the engines 12 have failed and the forward speed TAS is greater than a first speed threshold SV1, then the automatic piloting computer 41 implements a first emergency piloting mode MOD1 during a step STP2. On the other hand, if the engine 12 or, if applicable, all of the engines 12 have failed and the forward speed TAS is less than or equal to the first speed threshold SV1, then the automatic piloting computer 41 implements a second emergency piloting mode MOD2 during a step STP3.

(42) When the second emergency piloting mode MOD2 is applied, the method comprises an automatic adjustment step STP3.1 during which the automatic piloting system 40 controls at least one thrust actuator 28 so that the average pitch component TCC of the pitch of the second blades 7 is equal to an average pitch with zero thrust TCC0.

(43) This average pitch with zero thrust TCC0 is possibly calculated by the automatic piloting system 40 so that the propeller(s) 6 only exert an anti-torque function in the presence of a yaw movement order given by the pilot.

(44) For example, the average pitch with zero thrust TCC0 is determined using the following relation:
TCC0=cte+Arctg(TAS/0.75*omega*R)

(45) where “cte” represents a constant, “TAS” represents the true air speed of the hybrid helicopter 1, “omega” represents the speed of rotation of the propellers 6, “R” represents the radius of a circle described by a free end of each second blade 7, “Arctg” represents the arctangent trigonometric function, “I” represents the division sign, “k” represents the multiplication sign, “+” represents the addition sign.

(46) In parallel or even prior to this step, the method can provide for a step of maneuvering by the pilot of the collective pitch control 20 so as to reduce the collective pitch of the first blades 3.

(47) When the first emergency piloting mode MOD1 is applied, during an automatic pitch reduction step STP2.1, the automatic piloting system 40 controls the required actuators to tend to decrease the total pitch or the average pitch component of the total pitch of the second blades 7 making each propeller 6 produce motive power.

(48) The pitch or even the average pitch component reaches an objective pitch which is in fact different from the previous average pitch with zero thrust. The objective pitch may correspond to the value of the total pitch or of the average pitch component to be achieved for each propeller in order to produce power. The objective pitch can correspond to the value of the total pitch or of the average pitch component to be achieved so that the sum of the mechanical power PWlr implemented by the propeller(s) as well as the mechanical power PWrp consumed by the lift rotor 2 and the mechanical power PWinst consumed by the power plant 10 is either substantially zero or: PWlr+PWrp+PWinst=0.

(49) As a reminder, the mechanical power PWinst consumed by the power plant 10 can be equal to the sum of a mechanical power PWprt resulting from installation losses and a mechanical power PWcons consumed by one or more accessories 16, or: PWinst=Pwprt+PWcons.

(50) For example, the automatic piloting system 40 transmits a signal to each thrust actuator 28 to modify the average pitch component of the pitch of the second blades 7.

(51) In addition, neither the pilot nor the automatic piloting system 40 must potentially act on the collective pitch of the first blades 3 in order to keep the lift rotor 2 at a substantially constant speed of rotation.

(52) To establish the signal to be transmitted to each thrust actuator 28 in order to produce the required power with the propeller(s) 6, several embodiments can be envisaged.

(53) According to the first embodiment of FIG. 2, during a step STP2.3, the automatic piloting system 40 slaves the speed of rotation Nr of the lift rotor 2 to a setpoint speed of rotation Nr* by regulating the pitch of the second blades 7 according to a usual regulation loop.

(54) Thus, the automatic piloting computer 41 measures the speed of rotation Nr of the lift rotor 2 during a step STP2.1.1. Then, the automatic piloting computer 41 determines, for example with a comparator 81, an error signal corresponding to the difference between the current rotational speed Nr of the lift rotor 2 and the setpoint rotational speed Nr* aimed at not slowing down the lift rotor 2 following engine failure, if applicable, of the engine(s). The automatic piloting computer 41 can use this error signal in a corrector 82, and for example an integral proportional corrector, to generate an order to reduce the average pitch component of the pitch of the second blades of the propellers 6.

(55) As a result, during the failure, the lift rotor 2 tends to slow down. The regulation described above allows the automatic piloting computer 41 to decrease the pitch of the propellers 6 in order to tend to avoid slowing down the lift rotor 2. The objective pitch is not necessarily calculated according to this embodiment, but can be calculated during a step STP2.2, since the pitch of the second blades is automatically lowered to tend to keep the speed of rotation of the lift rotor constant.

(56) According to the second embodiment of FIG. 3, the automatic piloting system 40 slaves a pitch of the second blades 7 to a calculated objective pitch.

(57) During an intermediate step STP2.2, the automatic piloting computer 41 determines the objective pitch TCCdesynch, for example by applying the formula described above.

(58) Consequently, this calculated objective pitch TCCdesynch becomes, according to the second embodiment, a TCC* setpoint pitch. During the step for automatically reducing STP2.1 a pitch of the second blades 7, the automatic piloting computer 41 can apply a usual regulation loop which takes into account the setpoint pitch and the current average pitch component TCC of the pitch of the second blades 7 to generate a control signal. This control signal is transmitted to each thrust actuator 28 in order to modify, for example, the average pitch component of the pitch of the second blades of the propellers 2 and to make it tend towards the setpoint pitch.

(59) According to the third embodiment of FIG. 4, the automatic piloting computer 41 determines the objective pitch TCCdesynch.

(60) Compared to the second embodiment, the automatic piloting computer 41 also determines an adjustment value ATCC. This adjustment value ATCC is equal to a stored gain K1 multiplied by the integral of a difference between the current speed of rotation Nr of the lift rotor 2 and the setpoint speed of rotation Nr*. Optionally, this adjustment value is clipped by a limiter 84. Consequently, the setpoint pitch TCC* is equal to the sum of the objective pitch TCCdesynch and of the adjustment value ATCC that may be clipped.

(61) Independently of the embodiment and with reference to FIG. 2, the first control mode MOD1 can be disengaged in different ways.

(62) According to a first exit procedure, the method provides for a step for prompting STP4 of the man-machine interface 42 by a pilot. This man-machine interface 42 transmits, for example, an electrical, digital analog or optical mode change signal which is transmitted to the automatic piloting system 40, and for example to the automatic piloting computer 41. Following receipt of this signal, the automatic piloting system 40 applies the second emergency piloting mode MOD2. Consequently, each propeller 6 is controlled so that the average pitch component of the pitch of these second blades 7 is equal to the average pitch with zero thrust TCC0.

(63) According to a second exit procedure STP5, the pilot operates the collective pitch control 20 when said forward speed TAS is less than or equal to the first speed threshold SV1. Due to the regulation performed, the average pitch component of the pitch of the second blades 7 is increased. The second emergency piloting mode MOD2 is then implemented when the average pitch component of the pitch of the second blades 7 is equal to the average pitch with zero thrust TCC0.

(64) According to a third exit procedure STP6, the automatic piloting computer 41 compares the current forward speed TAS with a second speed threshold SV2. When the forward speed TAS is less than the second speed threshold SV2, the automatic piloting computer 41 automatically reduces the pitch of the first blades 3 and applies the second emergency piloting mode MOD2.

(65) According to a fourth exit procedure STP7, at least one engine 12 is restarted. The engine computer 13 of the restarted engine 12 transmits a signal to the automatic piloting computer 41 to inform it. Consequently, the automatic piloting computer 41 detects the restarting of the engine 12 and automatically disengages the first emergency piloting mode MOD1.

(66) FIGS. 5 to 8 illustrate the progress of the method according to the invention implementing the second exit procedure through a display 50 of the hybrid helicopter 1.

(67) This display 50 can include a graduated scale in pitches 54. The display 50 is controlled by a computer or even by the automatic piloting computer 41 so as to present an index 53 for example showing the current average pitch component of the propeller(s) 6. In addition, the display 50 may have a first mark 51 representing the average pitch with zero thrust and a second mark 52 representing the desynchronization pitch.

(68) With reference to FIG. 5 and in the absence of failure of each of the engine(s) 12, the current average pitch component of the pitch of the second blades of the propeller(s) 6 is greater than the average pitch with zero thrust, this average pitch with zero thrust being greater than the desynchronization pitch.

(69) With reference to FIG. 6, following a total failure of each of the engine(s) 12, the automatic piloting system 40 controls the pitches of the second blades of the propeller(s) 6 to make them produce power. The current average pitch component of the pitch of the second blades of the propeller(s) reaches the desynchronization pitch.

(70) With reference to FIG. 7, when the hybrid helicopter 1 reaches a forward speed below the second speed threshold SV2, the pilot reduces the collective pitch of the first blades 3. This results in an increase in the desynchronization pitch to prevent the speed of rotation of the lift rotor 2 from falling, the current average pitch component of the pitch of the second blades of the propeller(s) 6 remaining equal to the desynchronization pitch.

(71) With reference to FIG. 8, the second emergency piloting mode is then engaged, the current average pitch component of the propeller(s) being kept equal to the average pitch with zero thrust.

(72) Of course, the present invention is subject to many variations in its implementation. Although several embodiments have been described, it will be understood that it is not conceivable to exhaustively identify all of the possible modes. It is of course conceivable to replace a described means by an equivalent means without departing from the scope of the present invention.