Method for assisting a single-engine rotorcraft during an engine failure

Abstract

An assistance method for assisting a pilot of a single-engined rotary-wing aircraft during a flight phase in autorotation, the aircraft including a hybrid power plant provided with a main engine, with an electric machine, with a main gearbox, and with an electrical energy storage device. The aircraft also includes a main rotor driven by the hybrid power plant. In the method, during a flight, operation of the main engine is monitored in order to detect a failure, in particular by means of a drop in power on the main rotor, then, when a failure of the main engine is detected, the electric machine is controlled to deliver auxiliary power We to the main rotor, making it possible to assist a pilot of the aircraft in performing the flight phase in autorotation following the failure.

Claims

1. An assistance method for assisting a single-engined rotary-wing aircraft in the event of engine failure, the aircraft including: a hybrid power plant provided with a single main engine, with at least one electric machine, with a main gearbox and with at least one electrical energy storage device; and at least one main rotor driven in rotation by the power plant; the assistance method including the following steps: an acquisition step for acquiring at least one monitoring parameter S for monitoring the aircraft in order to detect any failure of the main engine; at least one monitoring step for activating at least one electric machine when at least one activation relationship containing instantaneous values of at least one monitoring parameter S and of its time derivative is less than a dedicated activation threshold Threshold.sub.i, $S+k_i\frac{dS}{dt}<{\mathrm{Threshold}}_i,$ where i is a positive integer varying from 1 to n, where n is a positive integer greater than or equal to 1, and where Threshold.sub.i is an activation threshold; an assistance step for assisting in flying the aircraft, the at least one electric machine delivering auxiliary mechanical power We to the main rotor, thereby making it possible to assist a pilot of the aircraft, during a flight phase following the failure, in maneuvering the aircraft; and a deactivation step for deactivating the assistance step to stop delivering auxiliary mechanical power We to the main rotor; wherein the method uses a single activation relationship $S+k\frac{dS}{dt}<\mathrm{Threshold},$ where $\frac{\mathrm{dS}}{\mathrm{dt}}$ is the derivative of the parameter S, Threshold is an activation threshold, and k is a positive coefficient that varies as a function of the monitoring parameter S and of the derivative, the activation threshold Threshold and the coefficient k being dependent on the aircraft.

2. The method according to claim 1, wherein a monitoring parameter of the aircraft is a speed of rotation Nr of the main rotor.

3. The method according to claim 2, wherein the speed of rotation Nr is determined following a measurement of a speed of rotation of a gearwheel of main gearbox, the gearwheel being synchronous with the main rotor.

4. The method according to claim 1, wherein the method includes at least one inhibition condition for inhibiting monitoring step, thereby preventing the assistance step from being performed.

5. The method according to claim 4, wherein the at least one inhibition condition is chosen from among the following list: the aircraft is on the ground; aircraft is above a minimum height above the ground; the aircraft is on a training flight; an undercarriage of the aircraft is detected as bearing against the ground; the height of the aircraft relative to the ground is not varying and is less than or equal to a predefined height; a monitoring parameter S is less than a first inhibition threshold; a monitoring parameter S is greater than a second inhibition threshold; and the derivative of a monitoring parameter S lies within a predetermined inhibition range.

6. The method according to claim 1, wherein the deactivation step applies a deactivation relationship that involves at least one monitoring parameter S and optionally its derivative.

7. The method according to claim 1, wherein the method includes at least one inhibition condition for inhibiting the deactivation step, thereby preventing assistance step from being stopped.

8. The method according to claim 1, wherein the method includes at least one pre-activation step for pre-activating the assistance method and at least one inhibition condition for inhibiting the pre-activation step.

9. The method according to claim 8, wherein the method includes a dormant state, a standby state, and an activated state, the method being in dormant state as soon as the aircraft is started, it not being possible for the monitoring step to be performed during the dormant state it not being possible for the assistance step to be activated during the standby state, the assistance step being activated during the activated state, the dormant state including the pre-activation step and a first inhibition step for inhibiting the pre-activation step, the standby state including the monitoring step, a first exit step for coming out of the standby state, and a second inhibition step for inhibiting the monitoring step, the activated state including the assistance step, the deactivation step, a second exit step for coming out of the activated state, and a third inhibition step for inhibiting the deactivation step, the dormant state, the standby state and the activated state including the acquisition step.

10. The method according to claim 1, wherein the method includes a synchronization step for synchronizing the at least one electric machine, during which synchronization step the at least one electric machine is synchronized with the main rotor, without delivering power to it.

11. The method according to claim 1, wherein the at least one activation threshold Threshold.sub.i is replaced with a trigger threshold Threshold′.sub.i taking into account a trigger time Δt for triggering the at least one electrical energy storage device such as: ${\mathrm{Threshold}}_i^{\prime }={\mathrm{Threshold}}_i-\frac{dS}{dt}.Math.\Delta t.$

12. The method according to claim 1, wherein, during the assistance step, the at least one electric machine is regulated so as to drive the main rotor in rotation until the aircraft is landed.

13. The method according to claim 1, wherein, during the assistance step, the at least one electric machine delivers maximum power with a limitation on the electric current consumed by the at least one electric machine.

14. The method according to claim 1, wherein, during the assistance step, the at least one electric machine delivers a first predetermined amount of power to the main rotor during a first predetermined lapse of time and a second predetermined amount of power to the main rotor for landing the aircraft.

15. The method according to claim 1, wherein, during the assistance step, the at least one electric machine firstly delivers constant torque until it delivers auxiliary power We equal to a predetermined nominal amount of power, then the at least one electric machine delivers auxiliary power We equal to a predetermined nominal amount of power until at least one monitoring parameter S reaches a threshold value, the at least one electric machine then being regulated to maintain the at least one monitoring parameter S equal to the threshold value.

16. The method according to claim 1, wherein, during the assistance step, the at least one electric machine firstly delivers constant torque until it delivers auxiliary power We equal to a predetermined nominal amount of power of the at least one electric machine, then the at least one electric machine delivers auxiliary power We equal to the predetermined nominal amount of power until at least one monitoring parameter S reaches a threshold value, the at least one electric machine then being regulated to maintain the at least one monitoring parameter S equal to the threshold value.

17. The method according to claim 1, wherein, during the assistance step, the at least one electric machine is regulated so that the at least one electric machine delivers auxiliary power We equal to a maximum amount of power of the at least one electric machine, while also limiting the auxiliary power We by the maximum torque of the at least one electric machine.

18. The method according to claim 1, wherein, during the assistance step, the at least one electric machine is regulated so as to maintain at least one monitoring parameter S equal to a threshold value.

19. An assistance method for assisting a single-engined rotary-wing aircraft in the event of engine failure, the aircraft including: a hybrid power plant provided with a single main engine, with at least one electric machine, with a main gearbox and with at least one electrical energy storage device; and at least one main rotor driven in rotation by the power plant; the assistance method including the following steps: an acquisition step for acquiring at least one monitoring parameter S for monitoring the aircraft in order to detect any failure of the main engine; at least one monitoring step or activating at least one electric machine when at least one activation relationship containing instantaneous values of at least one monitoring parameter S and of its time derivative is less than a dedicated activation threshold Threshold.sub.i, $S+k_i\frac{dS}{dt}<{\mathrm{Threshold}}_i,$ where i is a positive integer varying from 1 to n, where n is a positive integer greater than or equal to 1, and where Threshold.sub.i is an activation threshold; an assistance step for assisting in flying the aircraft, the at least one electric machine delivering auxiliary mechanical power We to the main rotor, thereby making it possible to assist a pilot of the aircraft, during a flight phase following the failure, in maneuvering the aircraft; and a deactivation step for deactivating the assistance step to stop delivering auxiliary mechanical power We to the main rotor; at least one pre-activation step for pre-activating the assistance method and at least one inhibition condition for inhibiting the pre-activation step wherein the method includes a dormant state, a standby state, a primed state and an activated state, the method being in the dormant state as soon as the aircraft is started, it not being possible for the monitoring step to be performed during the dormant state or during the standby state, it not being possible for the assistance step to be activated during the primed state, the assistance step being activated during the activated state, the dormant state including the pre-activation step and a first inhibition step for inhibiting the pre-activation step, the standby state including a priming step for priming the at least one electric machine, a synchronization step for synchronizing the at least one electric machine, a first exit step for coming out of the standby state, and a fourth inhibition step for inhibiting the priming step and for inhibiting the synchronization step, the primed state including the monitoring step, a third exit step for coming out of the primed state, and a second inhibition step for inhibiting the monitoring step, the activated state including the assistance step, the deactivation step, a second exit step for coming out of the activated state and, a third inhibition step for inhibiting the deactivation step, the dormant state, the standby state, the primed state, and the activated state including the acquisition step.

20. A rotary-wing aircraft including: a hybrid power plant provided with a single main engine, with at least one electric machine, with at least one electrical energy storage device, and with a main gearbox; at least one main rotor driven in rotation by the power plant and turning at a speed of rotation Nr; and an assistance device for assisting in the event of failure of the main engine, which assistance device includes: the hybrid power plant; a control device for controlling the electric machine; a monitoring device; a computer unit; and a memory; wherein the assistance device is configured to assist a pilot of the aircraft during a flight phase in autorotation, the monitoring device acquires at least one monitoring parameter S for monitoring the aircraft in order to detect any failure of the main engine; at least one electric machine is activated when at least one activation relationship containing instantaneous values of at least one monitoring parameter S and of its time derivative is less than a dedicated activation threshold Threshold.sub.i, $S+k_i\frac{dS}{dt}<{\mathrm{Threshold}}_i,$ where i is a positive integer varying from 1 to n, where n is a positive integer greater than or equal to 1, and where Threshold.sub.i is an activation threshold; and the aircraft is assisted in flying with the at least one electric machine delivering auxiliary mechanical power We to the main rotor thereby making it possible to assist a pilot of the aircraft, during a flight phase following the failure, in maneuvering the aircraft, wherein the at least one activation relationship includes a single activation relationship $S+k\frac{dS}{dt}<\mathrm{Threshold},$ where $\frac{\mathrm{dS}}{\mathrm{dt}}$ is the derivative of the parameter S, Threshold is an activation threshold, and k is a positive coefficient that varies as a function of the monitoring parameter S and of the derivative, the activation threshold Threshold and the coefficient k being dependent on the aircraft.

21. The aircraft according to claim 20, wherein the monitoring device includes a measurement device for assessing a drop in power on the main rotor.

22. The aircraft according to claim 20, wherein the monitoring device includes a measurement device for measuring the speed of rotation Nr.

23. A rotary-wing aircraft including: a hybrid power plant provided with a single main engine, with at least one electric machine, with at least one electrical energy storage device, and with a main gearbox; at least one main rotor drivable in rotation by the power plant at a speed of rotation Nr; and an assistance device for assisting in the event of failure of the main engine, which assistance device includes: the hybrid power plant; a control device for controlling the electric machine; a monitoring device; a computer unit; and a memory; wherein the assistance device is configured to assist a pilot of the aircraft during a flight phase in autorotation, the monitoring device is configured to acquire at least one monitoring parameter S for monitoring the aircraft in order to detect any failure of the main engine; at least one electric machine is configured to be activated when at least one activation relationship containing instantaneous values of at least one monitoring parameter S and of its time derivative is less than a dedicated activation threshold Threshold.sub.i, $S+k_i\frac{dS}{dt}<{\mathrm{Threshold}}_i,$ where i is a positive integer varying from 1 to n, where n is a positive integer greater than or equal to 1, and where Threshold.sub.i is an activation threshold; and the aircraft is configured to be assisted in flying by the at least one electric machine delivering auxiliary mechanical power We to the main rotor, thereby making it possible to assist a pilot of the aircraft, during a flight phase following the failure, in maneuvering the aircraft, wherein the aircraft is configured to generate at least one inhibition condition to inhibit pre-activating the aircraft before the monitoring of the aircraft, the aircraft configured to include a dormant state, a standby state, a primed state and an activated state, the aircraft being configured to be in the dormant state as soon as the aircraft is started, it not being possible for the monitoring to be performed during the dormant state or during the standby state, it not being possible for the aircraft to be assisted in flying during the primed state, it being possible for the aircraft to be assisted in flying during the activated state, it being possible for the aircraft to receive the least one inhibition condition to pre-activate the aircraft in the dormant state, it being possible for the at least one electric machine to be primed and synchronized in the standby state, it being possible for the aircraft to come out of the standby state and inhibit the at least one electric machine from being priming and synchronized, it being possible for the aircraft to me monitored in the primed state as well as coming out of the primed state and inhibiting the monitoring of the aircraft in the primed state, it being possible for the aircraft to be assisted in flying in the activated state, it being possible to pre-activate the aircraft in the activated state, and it being possible to come out of the activated state, the monitoring device is configured to monitor the aircraft in the dormant state, the standby state, the primed state, and the activated state.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention and its advantages appear in greater detail from the following description of implementations and embodiments given by way of illustration with reference to the accompanying figures, in which:

(2) FIG. 1 is a block diagram of the method of assisting a single-engined aircraft in the event of engine failure;

(3) FIG. 2 shows a single-engined aircraft as equipped with an assistance device;

(4) FIG. 3 is a graph showing the height-velocity diagram of an aircraft;

(5) FIG. 4 is a flow chart of an assistance method; and

(6) FIG. 5 is a flow chart of an assistance method.

DETAILED DESCRIPTION OF THE INVENTION

(7) Elements that are present in more than one of the figures are given the same references in each of them.

(8) FIG. 1 shows a method of assisting a single-engined rotary-wing aircraft 2 for performing a flight phase in autorotation following engine failure.

(9) FIG. 2 shows a single-engined rotary-wing aircraft 2 equipped with an assistance device 1 configured to implement the method.

(10) In the event of a failure of the main engine of the aircraft 2, the assistance method and the assistance device 1 enable the pilot to reach the flight phase in autorotation safely, and to make a landing in assisted and safe manner.

(11) The single-engined rotary-wing aircraft, shown in FIG. 2, has a hybrid power plant 5 provided with a single main engine 13, with an electric machine 12, with a main gearbox 11, with an electrical energy storage device 14 and with an Electronic Engine Control Unit (EECU) 19 delivering the operating characteristics of the main engine 13. For example, the storage device 14 comprises a battery, a fuel cell, or indeed a supercapacitor.

(12) The aircraft 2 also has a main rotor 3 and a tail rotor 4 that are driven in rotation by the hybrid power plant 5.

(13) The assistance device 1 is formed by the hybrid power plant 5, by a monitoring device 18, by a control device 15 for controlling the electric machine 12, by a computer unit 17 and by a memory 16.

(14) The function of the monitoring device 18 is to measure at least one monitoring parameter S for monitoring operation of the aircraft 2. Each monitoring parameter S makes it possible to detect the presence or indeed the imminence of a failure of the main engine 13. In the example shown in FIG. 2, the monitoring device 18 includes a measurement device 9 for measuring the instantaneous speed of rotation Nr of the main rotor 3. Below, this instantaneous speed of rotation is designated by the term “speed of rotation Nr”.

(15) More precisely, this measurement device 9 measures a speed of rotation of a gearwheel of the main gearbox 11 of the hybrid power plant 5 that drives the main rotor 3 in rotation. The gearwheel is synchronous with the main rotor 3 and has a speed of rotation significantly greater than the speed of rotation of the main rotor 3. The value of the speed of rotation of the main rotor 3 that is obtained by measuring the speed of the gearwheel of the main gearbox 11 is thus more accurate and less noisy than a direct measurement on the hub of the main rotor 3, because this speed can be computed more frequently and therefore the delay in determining the speed is shorter, thereby guaranteeing greater reactivity in detecting the failure.

(16) The assistance device 1 is configured to implement the assistance method shown in FIG. 1. This assistance method comprises four steps 110, 120, 130, and 140.

(17) During an acquisition step 110, at least one monitoring parameter S is monitored for in order to detect a failure of the main engine 13. Then, during a monitoring step 120, the electric machine 12 is activated when a failure of the main engine 13 is detected. Such a failure is detected by making a comparison in which at least one activation relationship containing the instantaneous values of at least one monitored-for monitoring parameter S and of its time derivative is compared with a predetermined dedicated activation threshold Threshold.sub.i. Then during an assistance step 130, the electric machine 12 is controlled to deliver auxiliary mechanical power We in regulated manner to the main rotor 3, thereby making it possible to assist a pilot of the aircraft 2 during the flight phase in autorotation following said failure, for maneuvering the aircraft 2 safely, and during landing of the aircraft. Finally, during a deactivation step 140, the assistance step 130 is deactivated after a deactivation relationship containing the instantaneous values of at least one monitored-for monitoring parameter S and of its time derivative has been compared with a predetermined dedicated deactivation threshold.

(18) During the acquisition step 110, at least one monitoring parameter S is therefore measured. In addition, the time derivative of at least one monitoring parameter S is determined by means of the computer unit 17 and by means of the successive measurements of said monitoring parameter S. This acquisition step 110 is performed throughout the assistance method of the invention, while the aircraft 2 is on the ground and while it is in flight.

(19) A preferred monitoring parameter S is the speed of rotation Nr of the main rotor 3. The computer unit 17 makes it possible to determine the value of the speed of rotation Nr of the main rotor 3 when the measurement device 9 measures the speed of rotation of a gearwheel of the main gearbox 11 that is synchronous with the main rotor 3 using a relationship stored in the memory 16.

(20) In addition, other monitoring parameters S may be substituted for the speed of rotation Nr of the main rotor 3. For example, a monitoring parameter S is torque at the outlet of the main engine 13 as measured by means of a torque meter 6, the value of the torque dropping rapidly when the main engine fails. Similarly, when the main engine 13 of the aircraft 2 is a turboshaft engine having a compressor turbine, a monitoring parameter S may be one of the operating characteristics of the main engine 13, such as the temperatures and the pressures of the fluids in said main engine 13 or indeed the speed of the compressor turbine in order to detect the failure of the turboshaft engine. These operating characteristics of the main engine 13 are delivered by the EECU 19.

(21) Then, during the monitoring step 120, the computer unit 17 executes instructions stored in the memory 16 in order to determine whether there is a failure of the main engine 13 and so that the electric machine 12 is activated as soon as a failure is detected. For this purpose, the memory 16 also stores a plurality of activation relationships involving the instantaneous values of at least one monitored-for monitoring parameter S and of its time derivative, positive coefficients k.sub.i and a plurality of activation thresholds Threshold.sub.i, where i is a positive integer varying in the range 1 to n and n is the number of activation relationships. Each activation relationship is then compared with an activation threshold Thresholds, and a failure of the main engine 13 is considered as being detected when at least one of the activation relationships is less than the activation threshold Threshold.sub.i corresponding to said activation relationship. The coefficients k.sub.i and the activation thresholds Threshold.sub.i are predetermined, dependent on the aircraft, and constant for any given aircraft, the activation thresholds Threshold.sub.i also being dependent on the corresponding coefficient k.sub.i. In addition, these coefficients k.sub.i and these activation thresholds Threshold.sub.i are adapted to the various flight situations that the aircraft 2 can encounter.

(22) In particular, when the monitored-for monitoring parameter S is the speed of rotation Nr of the main rotor 3, the electric machine 12 is activated when at least one inequation [Math 2] is satisfied.

(23) In the event of a failure of the main engine 13, the speed of rotation Nr drops rapidly. In addition, this drop in the speed of rotation Nr takes place under a high deceleration, this deceleration being the time derivative of the speed of rotation Nr. By using this derivative, the deceleration in the speed of rotation Nr can be detected as soon as the instant following the failure, whereas it would be necessary to wait for the speed of rotation Nr to reach a predetermined value in order to be certain there really is a failure if the derivative were not used.

(24) Furthermore, a single activation relationship [Math 5] may be compared with a single activation threshold Threshold during the monitoring step 120, instead of a set of a plurality of activation relationships. The coefficient k is then variable as a function of the monitoring parameter S and of its time derivative in order to cover the various flight situations, and the activation threshold Threshold is predetermined for any given aircraft.

(25) During the assistance step 130, after detecting a failure of the main engine 13, the computer unit 17 executes instructions stored in the memory 16 so that the electric machine 12 delivers auxiliary mechanical power We to the main rotor 3, so as to assist the pilot of the aircraft 2 during the flight phase in autorotation following said failure and during landing of the aircraft 2.

(26) During the assistance step 130, the electric machine 12 is preferably regulated so as to drive the main rotor 3 in rotation until the aircraft 2 has landed.

(27) For example, the electric machine 12 may deliver maximum power with a limitation on the magnitude of the electric current consumed by the electric machine 12.

(28) In another example, the electric machine 12 may deliver a first predetermined amount of auxiliary power We to the main rotor 3 for a first predetermined lapse of time for going into the phase in autorotation, and a second predetermined amount of auxiliary power We to the main rotor 3 at the time landing takes place.

(29) In another example, the electric machine 12 may firstly deliver constant torque until it delivers auxiliary power We equal to a predetermined nominal amount of power, and then the electric machine 12 delivers said auxiliary power We so long as the speed of rotation Nr is less than a threshold value, while also complying with any limits of the electric machine 12 or of the regulation, namely the torque or the power delivered by the electric machine 12. The electric machine 12 is then regulated in order to maintain the speed of rotation Nr substantially equal to the threshold value. The threshold value is a predetermined speed of rotation of the main rotor 3, e.g. equal to the nominal speed of rotation of the main rotor 3 or indeed greater than said nominal speed of rotation.

(30) The electric machine 12 may also be regulated by using a threshold value as the only setpoint for the speed of rotation Nr.

(31) The electric machine 12 may also be regulated by using a setpoint on the power delivered by the electric machine 12, e.g. equal to the maximum power of the electric machine 12, while also being limited by the maximum torque of the electric machine 12.

(32) The electric machine 12 may also be regulated by using a setpoint on the torque delivered by the electric machine 12, optionally as a function of the monitoring parameter S and of its derivative.

(33) Furthermore, when the engine failure occurs in the proximity of the ground, rapidity of reaction is essential for avoiding a crash. In such a situation, the electric machine 12 delivers its maximum power as soon as it is activated and until the aircraft 2 has landed.

(34) Finally, during the deactivation step 140 for deactivating the assistance step 130, the computer unit 17 executes instructions stored in the memory 16 in order to check that the monitoring parameter S used does not exceed a value greater than a maximum limit that could lead to damage to the aircraft 2 and, consequently, to stopping the delivery of the auxiliary mechanical power We by the electric machine 12. To this end, the memory 16 stores a deactivation relationship involving the instantaneous values of at least one monitored-for monitoring parameter S and optionally of its time derivative.

(35) For example, a deactivation relationship is a comparison of a single monitoring parameter S with a deactivation threshold, the assistance step 130 being deactivated as soon as the monitoring parameter S is greater than or equal to said deactivation threshold.

(36) A deactivation relationship may also involve a monitoring parameter S and its derivative, this deactivation relationship being different from each activation relationship used during the monitoring step 120. The deactivation relationship is then compared with a deactivation threshold Threshold.sub.Max, the assistance step being deactivated as soon as the deactivation relationship is greater than or equal to said deactivation threshold. Such a deactivation relationship Threshold.sub.Max may, for example, when the monitoring parameter S is the speed of rotation Nr of the main rotor 3 be written as follows:

(37) $\begin{array}{cc}S+k^{\prime }\frac{dS}{dt}\ge {\mathrm{Threshold}}_{\mathrm{Max}}.& \left[\mathrm{Math}7\right]\end{array}$

(38) The coefficient k′ may be constant or indeed variable, e.g. it may vary as a function of the speed of rotation Nr and of its time derivative. For example, the coefficient k′ may be defined as a function of a reaction time of the deactivation in order to make sure that the delay in deactivating does not lead to exceeding the defined deactivation threshold.

(39) This deactivation step 140 thereby advantageously makes it possible to stop the delivery of the auxiliary mechanical power We and thus to save electrical energy stored in the electrical energy storage device 14 and to avoid any damage to the aircraft 2.

(40) This deactivation step 140 may also use control by the pilot of the aircraft 2, instead of or in addition to an activation relationship in order to deactivate the assistance step 130. Such control is, for example, action on a button or on a touch screen arranged on the instrument panel of the aircraft 2.

(41) In addition, the method of the invention may include inhibition conditions for inhibiting the monitoring step 120, thereby preventing the assistance step 130 from being performed. These inhibition conditions are, for example: the aircraft 2 is on the ground; the aircraft 2 is above a minimum height above the ground; the aircraft 2 is on a training flight; the undercarriage of the aircraft 2 is detected as bearing against the ground; the height of the aircraft 2 relative to the ground as delivered, for example, by a radio altimeter is not varying and is less than or equal to a predefined height; the speed of rotation Nr is greater than a second inhibition threshold; and the derivative of the speed of rotation Nr lies within a predetermined inhibition range.

(42) The information indicating that the aircraft 2 is on the ground may be delivered by an information system 7 with which the aircraft 2 is provided. This information system 7 may be incorporated into the avionics system of the aircraft 2. Similarly, the information indicating an undercarriage of the aircraft 2 is on the ground is delivered by a dedicated sensor, typically connected to the avionics system of the aircraft 2.

(43) Likewise, the method of the invention may include one or more inhibition conditions for inhibiting the deactivation step 140, thereby preventing the assistance step 130 from being stopped.

(44) In addition, when the electrical energy storage device is a fuel cell or indeed any other electrical energy storage device requiring a non-negligible trigger time Δt between being activated and delivery proper of electric current, said trigger time Δt is taken into account in detecting the failure of the main engine of the aircraft 2.

(45) In which case, the activation threshold Threshold.sub.i is, for example, replaced with a trigger threshold Threshold′.sub.i for comparison with each activation relationship [Math 1], such as:

(46) $\begin{array}{cc}{\mathrm{Threshold}}_i^{\prime }={\mathrm{Threshold}}_i-\frac{dS}{dt}.Math.\Delta t.& \left[\mathrm{Math}6\right]\end{array}$

(47) Furthermore, this auxiliary power We being delivered by the electric machine 12 to the main rotor 3 during the failure of the main engine 13 may make it possible firstly to increase the maximum weight on takeoff of a single-engined aircraft 2, and secondly to increase the flight envelope of said aircraft 2.

(48) The maximum weight on takeoff of a single-engined aircraft 2 is limited, in particular, by the performance of said aircraft 2 in a flight phase in autorotation, in particular for taking into account the situation in which the engine fails during the takeoff phase.

(49) Hence, the delivery of the auxiliary power We by the electric machine 12 improving the performance of the aircraft 2 in a flight phase in autorotation can thus make it possible to increase its maximum weight on takeoff.

(50) Furthermore, the flight envelope of the aircraft 2 is, in particular, the height-velocity diagram shown in FIG. 3 and including two curves F and F′. This height-velocity diagram determines the minimum horizontal velocity along the abscissa axis that the aircraft 2 should comply with as a function of its height above the ground up the ordinate axis. The areas G and G′ of this diagram are to be avoided because the flight phase in autorotation may then not be performed safely in said areas G and G′.

(51) Advantageously, the auxiliary power We being delivered by the electric machine 12 improving the performance of the aircraft 2 in the flight phase in autorotation makes it possible to reduce the areas G and G′ to be avoided for the low velocities, which areas are replaced respectively by the areas J and J′, and to achieve the height-velocity diagram formed by the curves I and I′, thereby increasing the flight envelope authorized for the aircraft 2.

(52) FIG. 4 is a flow chart showing an implementation of the assistance method of the invention.

(53) This flow chart shows three states 10, 20, and 30 of the assistance method, namely a dormant state 10, a standby state 20, and an activated state 30. The assistance method of the invention is in the dormant state 10 as soon as the aircraft 2 is started. During this dormant state 10, the monitoring step 120 cannot be activated. In the standby state 20, the monitoring step 120 can be activated when the corresponding criteria are satisfied, but the assistance step 130 cannot be activated, whereas in the activated state 30, the assistance step 130 is activated and the electric machine 12 delivers the auxiliary power We.

(54) The acquisition step 110 is performed throughout the assistance method of the invention, during each of these three states 10, 20, and 30.

(55) The dormant state 10 corresponds mainly to an aircraft 2 on the ground before takeoff or after landing. A pre-activation step 115 is performed in the dormant state 10, making it possible to come out of the dormant state 10 and to go into the standby state 20. A pre-activation criterion applied during this pre-activation step 115 and triggering coming out of the dormant state 10 may be associated with one or more monitoring parameters S of the aircraft 2. A pre-activation criterion is, for example, a speed of rotation Nr of the main rotor 3 greater than a pre-activation speed. The method is therefore in this dormant state 10 when the main rotor 3 is at a standstill or is turning at a speed of rotation Nr less than or equal to the pre-activation speed, provided that the method has not, prior to this, been placed in the standby state or in the activated state.

(56) A pre-activation criterion may also be a first piece of information indicating that the aircraft 2 is no longer on the ground. The pre-activation criterion is, for example, an indication delivered by a sensor present in the undercarriage of the aircraft 2 and not detecting any bearing of the undercarriage against the ground. The pre-activation criterion may also be the value of a radio altimeter that varies, confirming that the aircraft 2 is not stationary or on the ground.

(57) The dormant state 10 also includes a first inhibition step 116 for inhibiting the pre-activation step 115 and that applies one or more inhibition criteria in order to maintain the assistance method in the dormant state 10 and prevent it from going into the standby state 20 even though the pre-activation step 115 is validated. For example, an inhibition criterion applied during the first inhibition step 116 may be apiece of information that a maintenance operation is in progress on the aircraft 2. In such a situation, the main rotor 3 can be turning at a speed of rotation Nr greater than the pre-activation speed even though the aircraft 2 is on the ground and even though no takeoff is planned.

(58) An inhibition criterion may also be that the aircraft has just landed and that its mission is finished. Consequently, no takeoff is planned and the method of the invention can be maintained in the dormant state 10 while the speed of rotation Nr of the main rotor 3 then decreases before the main engine is switched off.

(59) Similarly, an inhibition criterion may be that the aircraft 2 is on a training flight, in particular for training a pilot in flying in autorotation. Such a training flight needs to be performed entirely with the assistance method in the dormant state 10 so that the pilot being trained does not benefit from the assistance of the electric machine 12. In these three situations, the inhibition criterion is, for example, action on a button or on a touch screen arranged on the instrument panel of the aircraft 2.

(60) The standby state 20 corresponds to an aircraft 2 in flight or indeed still on the ground and in a takeoff phase. The monitoring step 120 is performed in this standby state 20 so as then to go over to the activated state 30 and then to perform the assistance step 130 as soon as a failure of the main engine 13 is detected.

(61) The standby state 20 also includes a first exit step 124 for coming out of the standby state 20 and that applies one or more exit criteria so that the assistance method comes out of the standby state 20 and goes back into the dormant state 10. An exit criterion is involved, for example, following landing of the aircraft 2. An exit criterion is, for example the above-described first piece of information indicating that the aircraft 2 is on the ground, or else a control performed by the pilot of the aircraft 2, e.g. via a button or indeed via a touch screen arranged on the instrument panel of the aircraft 2.

(62) The standby state 20 also includes a second inhibition step 126 for inhibiting the monitoring step 120 and that applies one or more inhibition criteria in order to maintain the assistance method in the standby state 20 and to prevent it from going into the activated state 30 even though the monitoring step 120 is validated. For example, an inhibition criterion applied during the second inhibition step 126 may be that the aircraft 2 is above a minimum height above the ground. With the monitoring parameter S used being the speed of rotation Nr of the main rotor 3, an inhibition criterion may also be that the speed of rotation Nr is greater than the second inhibition threshold, or indeed that the derivative of the speed of rotation Nr lies within a predetermined inhibition range.

(63) If, as a result of the monitoring step 120 being performed, engine failure is detected, the method of the invention comes out of the standby state 20 and goes into the activated state 30 and performs the assistance step 130. The deactivation step 140 is also performed during the activated state 30 in order to stop the assistance step 130, if necessary, and go back into the standby state 20.

(64) The activated state 30 also includes a second exit step 144 for coming out of the activated state 30 and that applies one or more exit criteria so that the assistance method comes out of the activated state 30 and goes back into the standby state 20. An exit criterion is, for example, control by the pilot of the aircraft 2 via a button or indeed via a touch screen arranged on the instrument panel of the aircraft 2. This action by the pilot makes it possible, in particular, to save electrical energy stored in the electrical energy storage device 14 when the pilot considers that it is no longer necessary to deliver the auxiliary mechanical power We to the main rotor 3.

(65) The activated state 30 also includes a third inhibition step 146 for inhibiting the deactivation step 140 and that applies one or more inhibition criteria so that the assistance method does not come out of this activated state 30 and does not go back into the standby state 20, the assistance step 130 then not be stopped, even though the deactivation step 140 is validated.

(66) For example, a deactivation relationship satisfied during the deactivation step 140 aims to protect one of the components of the aircraft 2 from damage by comparing the value of a monitoring parameter S or indeed the deactivation relationship containing the monitoring parameter S and its derivative with a deactivation threshold, while, on the contrary, the aim of an inhibition criterion applied during the third inhibition step 146 may be to authorize the monitoring parameter S or the deactivation relationship to exceed the deactivation threshold. This is because it is sometimes preferable to accept damage to one of the components of the aircraft 2 in order to prevent the aircraft 2 from crashing. An inhibition criterion is then the monitoring parameter S or indeed the deactivation relationship. The inhibition criterion may possibly require validation on the part of the pilot of the aircraft 2, e.g. via a button or indeed a touch screen arranged on the instrument panel of the aircraft 2 after emission of an alarm alerting the pilot that the monitoring parameter S or indeed the deactivation relationship was coming close to the deactivation threshold.

(67) Typically, when the monitoring parameter S is the speed of rotation Nr of the main rotor 3, an inhibition criterion applied during the third inhibition step 146 may be authorization of an overspeed of the main rotor 3.

(68) FIG. 5 is another flow chart showing another implementation of the assistance method of the invention.

(69) This flow chart shows four states 10, 20, 25, and 30 of the assistance method, namely a dormant state 10, a standby state 20 with priming of the electric machine 12, a primed state 25, and the activated state 30. The assistance method of the invention is in the dormant state 10 as soon as the aircraft 2 is started, this dormant state 10 being identical to the dormant state 10 of the flow chart of FIG. 4.

(70) The acquisition step 110 is performed throughout the assistance method of the invention, during each of these four states 10, 20, 25, and 30.

(71) As above, the standby state 20 corresponds to an aircraft 2 in flight or indeed still on the ground and in a takeoff phase. The standby state 20 includes a priming step 117 for priming the electric machine 12. This priming step 117 makes it possible, when it is validated, for the electric machine 12 to be started and ready to deliver its mechanical power We to the main rotor 3 and for the assistance method to go into the primed state 25.

(72) To this end, the standby state 20 includes a synchronization step 125 during which the electric machine 12 is synchronized with the main rotor 3 without delivering power to it, a coupling device, typically a freewheel, then being arranged between the electric machine 12 and the main rotor 3. In this way, the electric machine 12 can then advantageously deliver the mechanical power We almost instantaneously if it is needed.

(73) The priming step 117 applies, for example, one or more priming relationships that are equivalent to the activation relationships applied during the monitoring step 120, but with priming thresholds greater than the activation thresholds.

(74) The priming step 117 may also be validated by the pilot of the aircraft 2, e.g. via a button or indeed a touch screen arranged on the instrument panel of the aircraft 2.

(75) The standby state 20 includes the first exit step 124 for coming out of the standby state 20 that is identical to the first exit step of the flow chart in FIG. 4, and that applies the above-mentioned exit criterion or criteria so that the assistance method comes out of the standby state 20 and goes back into the dormant state 10.

(76) The standby state 20 also includes a fourth inhibition step 119 for inhibiting the priming step 117.

(77) The priming step 117 is performed with priming criteria that are more sensitive than the activation criteria, making it possible to go into the activated state 30. Therefore, in the event of turbulence, for example, fluctuations in a monitoring parameter S, in particular in the speed of rotation Nr of the main rotor 3, are possible and might cause untimely triggering of the synchronization step 125. Inhibition criteria may then be applied during the fourth inhibition step 119 in order to inhibit the priming step 117 and the synchronization step 125 so as to place the assistance method in the primed state 25, without starting the electric machine 12, and applying the monitoring step 120 for going into the activated state 30 if necessary.

(78) The primed state 25 includes the monitoring step 120 so that the method goes into the activated state 30 if necessary. The primed state 25 also includes a third exit step 128 for coming out of the primed state 25, and an inhibition step 126 that is identical to the corresponding inhibition step of the flow chart of FIG. 4.

(79) When the third exit step 128 is validated, the assistance method quits the primed state 25 and goes back into the standby state 20. An exit criterion applied by this third outlet step 128 is, for example, a second predetermined lapse of time at the end of which the primed state is deactivated, the assistance method going back into the standby state 20. Another exit criterion may be control by the pilot of the aircraft 2, e.g. via a button or indeed via a touch screen arranged on the instrument panel of the aircraft 2.

(80) If, as a result of the monitoring step 120 being performed, engine failure is detected, the assistance method comes out of the primed state 25 and goes into the activated state 30 and performs the assistance step 130.

(81) The activated state 30 is identical to the activated state in the flow chart of FIG. 4. In particular, the deactivation step 140 is also performed during the activated state 30 in order to stop the assistance step 130 and go back into the standby state 20.

(82) Naturally, the present invention can be the subject of numerous variants as to its implementation. Although several embodiments and implementations are described, it should readily be understood that it is not conceivable to identify exhaustively all possible embodiments and implementations. It is naturally possible to envisage replacing any of the means described by equivalent means without going beyond the ambit of the present invention.