Method of providing torque protection and/or thrust protection for propellers of a hybrid helicopter, and a hybrid helicopter

Abstract

A method of providing torque protection and/or thrust protection for the or each propeller of a hybrid helicopter. The hybrid helicopter includes a control system connected to the blades of each propeller and a thrust control configured to generate an order for modifying a pitch of the blades, which order is transmitted to the control system, the propeller(s) being driven in rotation by a mechanical transmission system of the hybrid helicopter. The method includes a step of having the control system keep the pitch of the blades of a propeller within at least one control envelope relating to a thrust generated by the propeller or to a torque exerted in the mechanical transmission system. In this way, the pitch of the blades of each propeller is kept between a lower limit and an upper limit of the control envelope.

Claims

1. A method of providing torque protection and/or thrust protection for at least one propeller of a hybrid helicopter, the hybrid helicopter including a lift rotor and at least one propeller, the hybrid helicopter including a control system connected to blades of the at least one propeller, the hybrid helicopter having a thrust control configured to generate an order for modifying a pitch of the blades, which order is transmitted to the control system, the blades of the at least one propeller being driven in rotation by a mechanical transmission system of the hybrid helicopter; the method including the following step: having the control system keep the pitch of the blades of the at least one propeller within at least one control envelope relating to a characteristic of the at least one propeller and defined by limits for the characteristic, the characteristic being a thrust generated by the at least one propeller or a torque exerted in the mechanical transmission system; wherein the at least one control envelope is bounded by an upper limit and by a lower limit for the characteristic in a control diagram plotting the pitch of the blades of a propeller along the abscissa axis and the characteristic of the propeller up the ordinate axis, the control diagram including a set of curves for different forward speeds of the hybrid helicopter, and the step of having the control system keep the pitch within the at least one control envelope includes the following steps at each iteration: keeping the pitch of the blades below the upper limit as a function of the forward speed of the hybrid helicopter; and keeping the pitch of the blades above the lower limit as a function of the forward speed of the hybrid helicopter.

2. The method according to claim 1, wherein, when the pitch of the blades reaches the upper limit, keeping the pitch below the upper limit includes the following step at each iteration: reducing the pitch in absolute value when the forward speed of the hybrid helicopter decreases relative to the forward speed reached when the pitch of the blades reaches the upper limit.

3. The method according to claim 1, wherein, when the pitch of the blades reaches the upper limit, keeping the pitch below the upper limit includes the following step at each iteration: preventing any increase in the pitch in absolute value when the forward speed of the hybrid helicopter does not increase above the forward speed reached when the pitch of the blades reaches the upper limit.

4. The method according to claim 1, wherein, when the pitch of the blades reaches the lower limit, keeping the pitch above the lower limit includes the following step at each iteration: increasing the pitch when the forward speed of the hybrid helicopter increases relative to the forward speed reached when the pitch of the blades reaches the lower limit.

5. The method according to claim 1, wherein, when the pitch of the blades of a propeller reaches the lower limit, keeping the pitch above the lower limit includes the following step at each iteration: preventing any reduction in the pitch when the forward speed of the hybrid helicopter does not decrease below the forward speed reached when the pitch of the blades reaches the lower limit.

6. The method according to claim 1, wherein, when the characteristic is the torque exerted in the mechanical transmission system, the step of having the control system keeps the pitch within the at least one control envelope includes the following steps at each iteration: computing a maximum pitch (β.sub.Torque-max) for the blades relating to a maximum allowable torque for a current forward speed of the hybrid helicopter; and computing a minimum pitch (β.sub.Torque-min) for the blades relating to a minimum allowable torque for the current forward speed of the hybrid helicopter.

7. The method according to claim 6, wherein, with the control system including a control computer receiving at least one thrust control signal issued by the thrust control, and with the control computer controlling at least one actuator of the control system, keeping the pitch within the at least one control envelope includes a step of having the control computer control the at least one actuator as a function at least of a law stored in a memory as well as at least of the thrust control signal, of the maximum pitch (β.sub.Torque-max) and of the minimum pitch (β.sub.Torque-min).

8. The method according to claim 1, wherein, when the characteristic is the thrust, the step of having the control system keeps the pitch within the at least one control envelope includes the following steps at each iteration: computing a maximum pitch (β.sub.Thrust-max) for the blades relating to a maximum usable thrust for a current forward speed of the hybrid helicopter; and computing a minimum pitch (β.sub.Thrust-min) for the blades relating to a minimum usable thrust for the current forward speed of the hybrid helicopter.

9. The method according to claim 8, wherein, with the control system including a control computer receiving at least one thrust control signal issued by the thrust control, and with the control computer controlling at least one actuator of the control system, keeping the pitch within the control envelope includes a step of having the control computer control the at least one actuator as a function at least of a law stored in a memory as well as at least of the thrust control signal, of the maximum pitch (β.sub.Thrust-max) and of the minimum pitch (β.sub.Thrust-min).

10. The method according to claim 1, wherein, with the hybrid helicopter including a first propeller and a second propeller performing a forward propulsion function and a yaw control function for the hybrid helicopter, with the control system being connected to first blades of the first propeller and to second blades of the second propeller, with the thrust control being configured to generate an order for modifying a mean pitch of a first pitch of the first blades and of a second pitch of the second blades, which order is transmitted to the control system, with the hybrid helicopter including a yaw control configured to generate an order for modifying a differential pitch component of the first pitch of the first blades and of the second pitch of the second blades, which order is transmitted to the control system, with the at least one control envelope being bounded by an upper limit and by a lower limit for the characteristic in a control diagram, with the control system including a control computer receiving at least one thrust control signal issued by the thrust control and at least one yaw control signal issued by the yaw control, with the pitch of the blades of at least one propeller reaching one of the limits of the at least one control envelope, and with the control computer controlling at least one actuator of the control system, keeping the pitch within the control envelope includes a step of having the control computer control the at least one actuator as a function at least of a law stored in a memory as well as at least of the thrust control signal and of the yaw control signal in order to modify the mean pitch of the pitches of the blades of the propellers in such a manner as to cause the pitch of the blades of at least one propeller having reached one of the limits to move away from the limit, and as to enable the differential pitch of the pitches of the blades of the propellers to be modified in compliance with the yaw control signal without going outside the at least one control envelope.

11. A hybrid helicopter provided with a lift rotor and with at least one propeller, the hybrid helicopter including a control system connected to the blades of the at least one propeller, the hybrid helicopter having a thrust control configured to generate an order for modifying a pitch of the blades, which order is transmitted to the control system, wherein the control system is configured to apply the method according to claim 1.

12. A method of providing torque protection and thrust protection for at least one propeller of a hybrid helicopter, the hybrid helicopter including a lift rotor and at least one propeller, the hybrid helicopter including a control system connected to blades of the at least one propeller, the hybrid helicopter having a thrust control configured to generate an order for modifying a pitch of the blades, which order is transmitted to the control system, the blades of the at least one propeller being driven in rotation by a mechanical transmission system of the hybrid helicopter; the method including the following step: having the control system keep the pitch of the blades of the at least one propeller within at least one control envelope relating to a characteristic of the at least one propeller and defined by limits for the characteristic, the characteristic being a thrust generated by the at least one propeller or a torque exerted in the mechanical transmission system; wherein the at least one control envelope is bounded by an upper limit and by a lower limit for the characteristic in a control diagram plotting the pitch of the blades of a propeller along the abscissa axis and the characteristic of the propeller up the ordinate axis, the control diagram including a set of curves for different forward speeds of the hybrid helicopter, and the step of having the control system keep the pitch within the at least one control envelope includes the following steps at each iteration: keeping the pitch of the blades below the upper limit as a function of the forward speed of the hybrid helicopter; and keeping the pitch of the blades above the lower limit as a function of the forward speed of the hybrid helicopter; wherein, when the characteristic is the thrust, the step of having the control system keeps the pitch within the at least one control envelope includes the following steps at each iteration: computing a maximum pitch (β.sub.Thrust-max) for the blades relating to a maximum usable thrust for a current forward speed of the hybrid helicopter; and computing a minimum pitch (β.sub.Thrust-min) for the relating to a minimum usable thrust for the current forward speed of the hybrid helicopter, and wherein, with the hybrid helicopter including a single propeller, the method of providing torque protection and thrust protection for at least one propeller of a hybrid helicopter includes the following steps at each iteration: computing a maximum overall mean pitch (β.sub.TCC-Complete-max) relating to the maximum usable thrust and to the maximum allowable torque for the propeller for the current forward speed of the hybrid helicopter, the maximum overall mean pitch (β.sub.TCC-complete-max) of the propeller being defined as a function of the maximum pitch (β.sub.Thrust-max) relating to the maximum usable thrust from the propeller and of the maximum pitch (β.sub.Torque-max) of the propeller relating to the maximum allowable torque for the propeller; and computing a minimum overall mean pitch (β.sub.TCC-Complete-min) relating to the minimum usable thrust and to the minimum allowable torque for the propeller for the current forward speed of the hybrid helicopter, the minimum overall mean pitch (β.sub.TCC-complete-min) being defined as a function of the minimum pitch (β.sub.Thrust-min) relating to the minimum usable thrust from the propeller and of the minimum pitch (β.sub.Torque-min) relating to the minimum allowable torque for the propeller.

13. The method according to claim 12, wherein, with the control system including a control computer receiving at least one thrust control signal issued by the thrust control, and with the control computer controlling at least one actuator of the control system, keeping the pitch within the at least one control envelope includes a step of having the control computer control the at least one actuator as a function at least of a law stored in a memory as well as at least of the thrust control signal, of the maximum overall mean pitch (β.sub.TCC-Complete-max) and of the minimum overall mean pitch (β.sub.TCC-Complete-min).

14. The method according to claim 12, wherein the maximum overall mean pitch (β.sub.TCC-Complete-max) is equal to the minimum value from among the maximum pitch (β.sub.Thrust-max) relating to the maximum usable thrust from the at least one propeller and the maximum pitch (β.sub.TCC-Torque-max) relating to the maximum allowable torque for the at least one propeller while the minimum overall mean pitch (β.sub.TCC-complete-min) is equal to the maximum value from among the minimum pitch (β.sub.Thrust-min) relating to the minimum usable thrust from the at least one propeller and the minimum pitch (β.sub.Torque-min) relating to the minimum allowable torque for the at least one propeller.

15. The method according to claim 12, wherein the steps of computing the maximum overall mean pitch (β.sub.TCC-Complete-max) and the minimum overall mean pitch (β.sub.TCC-Complete-min) include a step of correcting the maximum pitch and the minimum pitch relating to the usable thrust, and of correcting the maximum pitch and the minimum pitch relating to the allowable torque, for a rate of yaw of the hybrid helicopter, for each of the first and second propellers, such that: the first and second maximum pitches relating to the maximum usable thrust are replaced respectively by ${\beta }_{L-\mathrm{Thrust}-\max }+\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R}$ and ${\beta }_{R-\mathrm{Thrust}-\max }-\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R};$ the first and second maximum pitches relating to the maximum allowable torque are replaced respectively by ${\beta }_{L-\mathrm{Torque}-\max }+\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R}$ and ${\beta }_{R-\mathrm{Torque}-\max }-\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R};$ the first and second minimum pitches relating to the minimum usable thrust are replaced respectively by ${\beta }_{L-\mathrm{Thrust}-\min }+\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R}$ and ${\beta }_{R-\mathrm{Thrust}-\min }-\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R};$ the first and second minimum pitches relating to the minimum allowable torque are replaced respectively by ${\beta }_{L-\mathrm{Torque}-\min }+\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R}$ and ${\beta }_{R-\mathrm{Torque}-\min }-\frac{r.Math.Y_p.Math.\pi }{180.Math.k.Math.\Omega .Math.R};$ where: r is the yaw rotation rate of the hybrid helicopter, expressed in degrees per second (°/s); Yp is the yaw lever arm of the propellers, expressed in meters (m); π is a trigonometric constant; Ω is the speed of rotation of the propellers, expressed in radians per second (rad/s); R is the radius of the propellers, expressed in meters (m); and k is a positive coefficient less than or equal to 1.

16. A method of providing torque protection and thrust protection for at least one propeller of a hybrid helicopter, the hybrid helicopter including a lift rotor and at least one propeller, the hybrid helicopter including a control system connected to blades of the at least one propeller, the hybrid helicopter having a thrust control configured to generate an order for modifying a pitch of the blades, which order is transmitted to the control system, the blades of the at least one propeller being driven in rotation by a mechanical transmission system of the hybrid helicopter; the method including the following step: having the control system keep the pitch of the blades of the at least one propeller within at least one control envelope relating to a characteristic of the at least one propeller and defined by limits for the characteristic, the characteristic being a thrust generated by the at least one propeller or a torque exerted in the mechanical transmission system; wherein the at least one control envelope is bounded by an upper limit and by a lower limit for the characteristic in a control diagram plotting the pitch of the blades of a propeller along the abscissa axis and the characteristic of the propeller up the ordinate axis, the control diagram including a set of curves for different forward speeds of the hybrid helicopter, and the step of having the control system keep the pitch within the at least one control envelope includes the following steps at each iteration: keeping the pitch of the blades below the upper limit as a function of the forward speed of the hybrid helicopter; and keeping the pitch of the blades above the lower limit as a function of the forward speed of the hybrid helicopter; wherein, when the characteristic is the thrust, the step of having the control system keeps the pitch within the at least one control envelope includes the following steps at each iteration: computing a maximum pitch (β.sub.Thrust-max) for the to a maximum usable thrust for a current forward speed of the hybrid helicopter; and computing a minimum pitch (β.sub.Thrust-min) for the to a minimum usable thrust for the current forward speed of the hybrid helicopter, and wherein, with the hybrid helicopter including at least two propellers, the method of providing torque protection and thrust protection for at least one propeller of a hybrid helicopter includes the following steps at each iteration: computing a maximum overall mean pitch (β.sub.TCC-Complete-max) relating to the maximum usable thrust and to the maximum allowable torque for the propellers for the current forward speed of the hybrid helicopter, the maximum overall mean pitch (β.sub.TCC-complete-max) of the propellers being defined as a function of the maximum pitch (β.sub.Thrust-max) relating to the maximum usable thrust from each of the propellers and of the maximum pitch (β.sub.Torque-max) relating to the maximum allowable torque for each of the propellers; and computing a minimum overall mean pitch (β.sub.TCC-Complete-min) relating to the minimum usable thrust and to the minimum allowable torque for the propellers for the current forward speed of the hybrid helicopter, the minimum overall mean pitch (β.sub.TCC-complete-min) of the propellers being defined as a function of the minimum pitch (β.sub.Thrust-min) relating to the minimum usable thrust from each of the propellers and of the minimum pitch (β.sub.Torque-min) relating to the minimum allowable torque for each of the propellers.

17. The method according to claim 16, wherein, with the hybrid helicopter including a first propeller and a second propeller, and with the control system being connected to first blades of the first propeller and to second blades of the second propeller, the step of computing a maximum overall mean pitch (β.sub.TCC-Complete-max) includes the following substeps: measuring a first current pitch (β.sub.L) of the first blades and a second current pitch (β.sub.R) of the second blades; computing a first maximum computation pitch relating to the maximum usable thrust for the first propeller using the following relationship: ${\beta }_{TCC-L-\mathrm{Thrust}-\max }=\frac{{\beta }_{L-\mathrm{Thrust}-\max }+{\beta }_R}{2},$ where β.sub.L-Thrust-max is the maximum pitch of the first blades relating to the maximum usable thrust for the first propeller; computing a second maximum computation pitch relating to the maximum usable thrust for the second propeller using the following relationship: ${\beta }_{TCC-R-\mathrm{Thrust}-\max }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Thrust}-\max }}{2},$ where β.sub.R-Thrust-max is the maximum pitch of the second blades relating to the maximum usable thrust for the second propeller; computing a maximum computation mean pitch relating to the maximum usable thrust for the two propellers using the following relationship: ${\beta }_{TCC-\mathrm{Thrust}-\max -\mathrm{NODIFF}}=\frac{{\beta }_{TCC-L-\mathrm{Thrust}-\max }+{\beta }_{TCC-R-\mathrm{Thrust}-\max }}{2};$ computing a maximum mean pitch relating to the maximum usable thrust for the two propellers using the following relationship:
β.sub.TCC-Thrust-max=β.sub.TCC-Thrust-max-NODIFF−ABS(β.sub.TCC-L-Thrust-max−β.sub.TCC-R-Thrust-max) where ABS is a function extracting the absolute value; computing a first maximum computation pitch relating to the maximum allowable torque for the first propeller using the following relationship: ${\beta }_{TCC-L-\mathrm{Torque}-\max }=\frac{{\beta }_{L-\mathrm{Torque}-\max }+{\beta }_R}{2},$ where β.sub.L-Torque-max is the maximum pitch of the first blades relating to the maximum allowable torque for the first propeller; computing a second maximum computation pitch relating to the maximum allowable torque for the second propeller using the following relationship: ${\beta }_{TCC-R-\mathrm{Torque}-\max }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Torque}-\max }}{2},$ where β.sub.R-Torque-max is the maximum pitch of the second blades relating to the maximum allowable torque for the second propeller; computing a maximum computation mean pitch relating to the maximum allowable torque for the two propellers using the following relationship: ${\beta }_{TCC-\mathrm{Torque}-\max -\mathrm{NODIFF}}=\frac{{\beta }_{TCC-L-\mathrm{Torque}-\max }+{\beta }_{TCC-R-\mathrm{Torque}-\max }}{2};$ computing a maximum mean pitch relating to the maximum allowable torque for the two propellers using the following relationship:
β.sub.TCC-Torque-max=β.sub.TCC-Torque-max-NODIFF−ABS(⊖.sub.TCC-L-Torque-max−β.sub.TCC-R-Torque-max); computing the maximum overall mean pitch relating to the maximum usable thrust and to the maximum allowable torque for the two propellers using the following relationship:
β.sub.TCC-Thrust-max=MIN(β.sub.TCC-Torque-max) where MIN is a function extracting the minimum value from the two arguments provided as input; and the step of computing a minimum overall mean pitch (β.sub.TCC-Complete-min) includes the following substeps: measuring the first current pitch (β.sub.L) of the first blades and the second current pitch (β.sub.R) of the second blades; computing a first minimum computation pitch relating to the minimum usable thrust for the first propeller using the following relationship: ${\beta }_{TCC-L-\mathrm{Thrust}-\min }=\frac{{\beta }_{L-\mathrm{Thrust}-\min }+{\beta }_R}{2},$ where β.sub.L-Thrust-min is the minimum pitch of the first blades relating to the minimum usable thrust for the first propeller; computing a second minimum computation pitch relating to the minimum usable thrust for the second propeller using the following relationship: ${\beta }_{TCC-R-\mathrm{Thrust}-\min }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Thrust}-\min }}{2},$ where β.sub.R-Thrust-min is the minimum pitch of the second blades relating to the minimum usable thrust for the second propeller; computing a minimum computation mean pitch relating to the minimum usable thrust for the two propellers using the following relationship: ${\beta }_{TCC-\mathrm{Thrust}-\min -\mathrm{NODIFF}}=\frac{{\beta }_{TCC-L-\mathrm{Thrust}-\min }+{\beta }_{TCC-R-\mathrm{Thrust}-\min }}{2};$ computing a minimum mean pitch relating to the minimum usable thrust for the two propellers using the following relationship:
Δ.sub.TCC-Thrust-min=β.sub.TCC-Thrust-min-NODIFF+ABS(β.sub.TCC-L-Thrust-min−β.sub.TCC-R-Thrust-min); computing a first minimum computation pitch relating to the minimum allowable torque for the first propeller using the following relationship: ${\beta }_{TCC-L-\mathrm{Torque}-\min }=\frac{{\beta }_{L-\mathrm{Torque}-\min }+{\beta }_R}{2},$ where β.sub.L-Torque-min is the minimum pitch of the first blades relating to the minimum allowable torque for the first propeller; computing a second minimum computation pitch relating to the minimum allowable torque for the second propeller using the following relationship: ${\beta }_{TCC-R-\mathrm{Torque}-\min }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Torque}-\min }}{2},$ where β.sub.R-Torque-min is the minimum pitch of the second blades relating to the minimum allowable torque for the second propeller; computing a minimum computation mean pitch relating to the minimum allowable torque for the two propellers using the following relationship: ${\beta }_{TCC-\mathrm{Torque}-\min -\mathrm{NODIFF}}=\frac{{\beta }_{TCC-L-\mathrm{Torque}-\min }+{\beta }_{TCC-R-\mathrm{Torque}-\min }}{2};$ computing a minimum mean pitch relating to the minimum allowable torque for the two propellers using the following relationship:
β.sub.TCC-Torque-min=β.sub.TCC-Torque-min-NODIFF+ABS(β.sub.TCC-L-Torque-min−β.sub.TCC-R-Torque-min); computing the minimum overall mean pitch relating to the minimum usable thrust and to the minimum allowable torque for the two propellers Using the following relationship:
β.sub.TCC-Complete-min=MAX(β.sub.TCC-Thrust-min;β.sub.TCC-Torque-min) where MAX is a function extracting the maximum value from the two arguments provided as input.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a diagrammatic view of a hybrid helicopter of the invention;

(3) FIG. 2 is a diagrammatic view of a control system for controlling the propellers of a hybrid helicopter of the invention;

(4) FIG. 3 is a thrust control diagram showing the method of the invention; and

(5) FIG. 4 is a torque control diagram showing the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows a hybrid helicopter 1 of the invention. This hybrid helicopter 1 has a fuselage 4 above which at least one lift rotor 2 is arranged. The lift rotor 2 is provided with a plurality of blades referred to for convenience as “main blades 3”.

(7) In addition, the hybrid helicopter 1 is provided with at least one propeller 10, 15 of the puller type or of the pusher type, having a plurality of blades 11, 16. In FIG. 1, the hybrid helicopter 1 has a first propeller 10 and a second propeller 15 that are disposed laterally relative to the fuselage 4, and in particular on either side of an anteroposterior plane of the hybrid helicopter 1. The sides on which the first and second propellers 10, 15 are arranged may be reversed. The first and second propellers 10, 15 respectively have a plurality of first blades 11 and a plurality of second blades 16. The first and second propellers 10, 15 are optionally carried by a support 5. Such a support 5 may optionally be aerodynamic. For example, the support 5 comprises a wing as shown in FIG. 1. In FIG. 1, the propellers 10, 15 are placed at the leading edge of a wing. In another example, the propellers 10, 15 may be placed at the trailing edge of the wing.

(8) Furthermore, the hybrid helicopter 1 may include surfaces for stabilizing or indeed maneuvering purposes, i.e. stabilizer surfaces and movable control surfaces. For example, for longitudinal (pitch) stability and control, the hybrid helicopter 1 may include at least one substantially horizontal stabilizer 20, optionally provided with movable pitch control surfaces or “elevators” 21. For example, for directional (yaw) stability and control, the hybrid helicopter 1 may include at least one substantially vertical stabilizer 25, optionally provided with movable fins or “rudders” 26. FIG. 1 thus shows a tail assembly that is in the shape of an upside-down U, but the tail assembly may have various shapes without going beyond the ambit of the invention. In another example, the tail assembly may be H-shaped, U-shaped, etc.

(9) Furthermore, the hybrid helicopter 1 includes a power plant 30 for delivering power to the lift rotor 2 and optionally to each propeller 10, 15. For this purpose, the power plant 30 includes at least one engine 31 that is controlled by a usual engine computer 32.

(10) The term “computer” is used below to mean a unit that may, for example, comprise at least one processor and at least one memory, at least one integrated circuit, at least one programmable system, and at least one logic circuit, these examples not limiting the scope given to the expression “computer”. The term. “processor” may be used equally well to mean a central processing unit (CPU), a graphics processing unit (GPU), a digital signal processor (DSP), a microcontroller, etc.

(11) In addition, for example inside an interconnection system, the power plant 30 may further include at least one gearbox, at least one shaft, and/or at least one member for interconnecting two members in rotation, etc. For example, one or more engines 31 are connected mechanically via one or more mechanical connection systems to a main gearbox 33 that drives the lift rotor 2 in rotation. Furthermore, the main gearbox 33 may be connected mechanically via respective shafts to auxiliary gearboxes, one for each of the propellers 10, 15, which auxiliary gearboxes are then in turn connected to the corresponding propellers 10, 15.

(12) The engines 31, the main gearbox 33, each auxiliary gearbox, and the connecting shafts between those elements and each transmission shaft driving the blades 11, 16 of the propellers 10, 15 in rotation form a mechanical power transmission system for the hybrid helicopter 1 that drives the blades 11, 16 of the propellers 10, 15 in rotation.

(13) The speeds of rotation of the outlets of the engine(s) 31, of the propellers 10, 15, of the lift rotor 2, and of the mechanical interconnection system are optionally mutually proportional, with the proportionality ratio optionally being constant regardless of the flight configuration of the hybrid helicopter 1 under normal operating conditions, i.e. except for failure, testing or training situations.

(14) Furthermore, the hybrid helicopter 1 may include various controls for being piloted.

(15) In particular, the hybrid helicopter 1 may include a control system 40 connected to flight controls 45, 47 for collectively and cyclically controlling the pitch of the main blades 3. Such a control system 40 may, for example, include a set of swashplates. Thus, at each instant, the pitch of the main blades 3 may be equal to the sum of a collective pitch that is identical for all of the main blades 3 and of a cyclic pitch that varies as a function of the azimuth position of each main blade 3.

(16) In usual manner, the hybrid helicopter 1 may include controls connected to the control system 40 for controlling the pitch of the first blades 11 of the first propeller 10 and the second blades 16 of the second propeller 15. At each instant, the first pitch of the first blades 11 of the first propeller 10 may be equal to the sum of a mean pitch component and of a differential pitch component, while the second pitch of the second blades 16 of the second propeller 15 is equal to the difference between the mean pitch component and the differential pitch component.

(17) Optionally, the hybrid helicopter 1 has a first measurement sensor 88 for measuring the first value of the first pitch of the first blades 11 and a second measurement sensor 89 for measuring the second value of the second pitch of the second blades 16. For example, the first measurement sensor 88 includes a position sensor that emits an analog or digital, electrical or optical signal that varies as a function of the position of a control shaft for controlling the first pitch of the first blades 11. Similarly, the second sensor 89 may include a position sensor that emits an analog or digital, electrical or optical signal that varies as a function of the position of a control shaft for controlling the second pitch of the second blades 16. Each position sensor may be of a usual type and, for example, comprise a speed sensor making it possible to obtain a position by integration, a potentiometer, etc.

(18) The hybrid helicopter 1 may also include at least one torque sensor 87 making it possible to measure a torque exerted in the mechanical transmission system and driving the blades 11, 16 of each propeller 10, 15 in rotation. Such a torque sensor 87 may be positioned at any point along the mechanical transmission system and is sufficient to determine the torque at any point of said transmission system. For example, two torque sensors 87 are positioned on respective ones of the transmission shafts of the two propellers 10, 15.

(19) Furthermore, the hybrid helicopter 1 may include a measurement device 86 for measuring the current forward speed relative to air or current “airspeed” of the hybrid helicopter 1. For example, the measurement device 89 includes an anemometer, a device provided with a Pitot tube, or any other suitable device. The forward speed may be the “True Air Speed” (TAS) or the “Indicated Air Speed” (IAS).

(20) In usual manner, the hybrid helicopter 1 may include a thrust control 50 that is operable by a pilot and that acts on one or more mechanical and/or electrical control channels of the control system 40 to cause the mean pitch component of the pitch of the first blades 11 and of the pitch of the second blades 16 to vary. FIG. 1 shows a thrust control 50 of the lever type, but the thrust control may also, for example, take the form of a button generating a digital signal, or of a knurled wheel generating an analog signal.

(21) Similarly, the hybrid helicopter 1 may include a yaw control 55 that is operable by a pilot and that acts on one or more mechanical and/or electrical control channels of the control system 40 to cause the differential pitch component of the pitch of the first blades 11 and of the pitch of the second blades 16 to vary. The yaw control 55 may, for example, take the form of pedals, for example.

(22) Furthermore, the control system. 40 may include a control computer 60 that is in communication at least with the thrust control 50 so as to apply the method of the invention and optionally also with the yaw control 55, with the first measurement sensor 88 and/or with the second measurement sensor 89.

(23) FIG. 2 shows an example of a control system 40 for controlling the propellers 10, 15.

(24) In this example, the yaw control 55 and the thrust control 50 communicate with the control computer 60. The control computer 60 is in communication with actuators 76, 77 that are connected to respective hydraulic valves 85.

(25) The control computer 60 then applies one or more laws stored in a memory so as to control the actuators 76, 77 as a function of the signals issued directly or indirectly by the thrust control 50 and optionally by the yaw control 55.

(26) The control system 40 of FIG. 2 is given by way of illustration.

(27) In another aspect, FIGS. 3 and 4 show control diagrams 95 relating to respective ones of two different characteristics of a propeller 10, 15, and plotting the pitch of the blades 11, 16 of said propeller 10, 15 along the abscissa axis 96 and said characteristic of the propeller 10, 15 up the ordinate axis 97. Each control diagram 95 includes a set of curves 101, 102, 103 showing how the characteristic varies as a function of the pitch of the blades 11, 16 of the propeller 10, 15. Each curve 101, 102, 103 is defined for a respective forward speed of the hybrid helicopter 1. For both FIGS. 3 and 4, the left-most curve 102 corresponds to a substantially zero forward speed, and the right-most curve 103 corresponds to a maximum forward speed of the hybrid helicopter 1.

(28) Each control diagram 95 has an upper limit 91 and a lower limit 92 that delimit a control envelope 90.

(29) FIG. 3 shows a control diagram 95 relating to the thrust generated by the propeller 10, 15. The upper limit 91 and the lower limit 92 are associated with the aerodynamic limits of the propeller 10, 15, and in particular with aerodynamic stalling of the blades 11, 16 of the propeller 10, 15. The upper limit 91 and the lower limit 92 are thus defined by pitch values at which stalling occurs as a function of the forward speed of the hybrid helicopter 1, optionally while taking a margin into account.

(30) Furthermore, the thrust taken into account in the control diagram 90 may be the thrust generated by a propeller 10, 15 per se or else a reduced thrust that can be independent of the atmospheric conditions, and in particular of the temperature and of the current atmospheric pressure outside the hybrid helicopter 1.

(31) FIG. 4 shows a control diagram 95 relating to a torque exerted in the mechanical transmission system of the hybrid helicopter 1. The upper limit 91 and the lower limit 92 are associated with mechanical limits relating to the mechanical transmission system that drives the blades 11, 16 of each propeller 10, 15.

(32) The upper limit 91 corresponds to the situation in which the propeller 10, 15 is in drive mode and is therefore participating in propelling the hybrid helicopter 1. This upper limit 91 may be reached when the propeller 10, 15 generates a thrust towards the front of the hybrid helicopter 1, the pitch of the blades 11, 16 then being positive and situated in the region 107 in FIG. 4. This upper limit 91 may also be reached when the propeller 10, 15 generates a thrust towards the rear of the hybrid helicopter 1, the pitch of the blades 11, 16 then being negative and situated in the region 109 in FIG. 4. This particular situation actually occurs during hovering or when flying at a very low forward speed, the yaw control function of the propellers 10, 15 predominating over the forward propulsion function so as to oppose the torque generated by the lift rotor 2 on the fuselage 4.

(33) The lower limit 92 corresponds to the situation in which the propeller 10, 15 is generating mechanical power and is thus not participating in the forward propulsion of the hybrid helicopter 1. This lower limit 92 may be reached transiently following a rapid reduction in the pitch of the blades 11, 16 of the propellers 10, 15 in cruising flight at high forward speeds or indeed in the situation of diving or nose-down flight. In both of these situations, the airflow sweeping over the hybrid helicopter 1 causes the blades 11, 16 of the propellers 10, 15 to rotate. The pitch of the blades 11, 16 remains positive and is situated in the region 108 shown in FIG. 4.

(34) In the method of the invention, at each iteration, the control system 40 keeps the pitch of the blades 11, 16 of each propeller 10, 15 within the respective control envelope 90 for a characteristic of the propeller 10, 15, namely the thrust generated by the propeller 10, 15 and/or the torque exerted in the mechanical transmission system. In this way, at each iteration, the control system 40 bounds the pitches of the blades 11, 16 by keeping them within the authorized control envelopes 90 so as to protect automatically the mechanical transmission system driving the blades 11, 16 of the propellers 10, 15 and/or to protect automatically the advancing of the hybrid helicopter.

(35) Each control envelope 90 is bounded by an upper limit 91 and a lower limit 92 that are specific to the associated characteristic. The above-mentioned keeping step makes it possible, at each iteration and as a function of the forward speed, firstly to keep the pitch of the blades 11, 16 of the propeller 10, 15 below the upper limit 91 and secondly to keep the pitch of the blades 11, 16 of the propeller 10, 16 above the lower limit 92.

(36) In this way, when the pitch of the blades 11, 16 of a propeller 10, 16 reach the upper limit 91, the control system acts, e.g. when the forward speed decreases, to cause a reduction in the pitch of the blades 11, 16 of said propeller 10, 15 in absolute value, or else, when the forward speed does not increase, to prevent an increase in the pitch of the blades 11, 16 of said propeller 10, 15 in absolute value.

(37) When the characteristic of the propeller 10, 15 is the torque exerted in the mechanical transmission system, the use of absolute values makes it possible to take into account the situations of positive pitch and of negative pitch.

(38) Similarly, when the pitch of the blades 11, 16 of a propeller 10, 15 reaches the lower limit 92, the control system acts, e.g. when the forward speed increases, to cause an increase in the pitch of the blades 11, 16 of said propeller 10, 15, or else, when the forward speed does not decrease, to prevent a reduction in the pitch of the blades 11, 16 of said propeller 10, 15.

(39) The position of the current operating point of the hybrid helicopter 1 on the control diagram 90 may be determined via the pitch of the blades 11, 16 of the propeller 10, 15, which pitch is obtained by means of a sensor 88, 89, and via the current forward speed of the hybrid helicopter 1, which speed is obtained by means of the measurement device 86.

(40) When the characteristic of the propeller 10, 15 is the torque exerted in the mechanical transmission system, the position of said current operating point on the control diagram 90 may be determined via the torque exerted in the mechanical transmission system, which torque is obtained by means of a torque sensor 87, and via the current forward speed of the hybrid helicopter 1, which speed is obtained by means of the measurement device 86.

(41) Therefore, when a pilot of the hybrid helicopter 1 acts on the thrust control 50, the control computer 60 receives a thrust control signal directly or indirectly from the thrust control 50 and can then control the actuators 76, 77 as a function at least of a law stored in a memory and making it possible to bound the orders issued via the thrust control signal in order to keep the pitch of the blades 11, 16 of each propeller 10, 15 within the control envelope 90.

(42) Furthermore, during the step of keeping the pitch within the control envelope 90, the control computer 60 makes it possible to compute a maximum pitch and a minimum pitch corresponding respectively to the upper limit 91 and to the lower limit 92 of the control envelope 90 for each propeller and for each characteristic of the propeller 10, 15. For example, the maximum pitch is defined on the control diagram 90 by the intersection where the upper limit 92 intersects a curve 102 corresponding to the current forward speed of the hybrid helicopter 1 and the minimum pitch is defined by the intersection where the lower limit 91 intersects the same curve.

(43) When the characteristic of the propeller 10, 15 is the torque exerted in the mechanical transmission system, during the step of keeping the pitch within the control envelope 90, the control computer 60 makes it possible, at each iteration and for the current forward speed of the hybrid helicopter 1, to compute a maximum pitch β.sub.Torque-max and a minimum pitch β.sub.Torque-min for the blades 11, 16 of the propeller 10, 15 relative respectively to a maximum torque and to a minimum torque allowable by the mechanical transmission system. The minimum allowable torque is actually a negative torque.

(44) Similarly, when the characteristic of the propeller 10, 15 is the thrust from said propeller 10, 15, during the step of keeping the pitch within the control envelope 90, the control computer 60 makes it possible, at each iteration and for the current forward speed of the hybrid helicopter 1, to compute a maximum pitch β.sub.Thrust-max and a minimum pitch β.sub.Thrust-min for the blades 11, 16 of the propeller 10, 15 relative respectively to a maximum usable thrust and to a minimum usable thrust.

(45) The maximum pitches β.sub.Thrust-max and β.sub.Torque-max for the blades 11, 16 may be different for the two propellers, e.g. when the geometrical shapes of the blades 11, 16 of the two propellers 10, 15 are different. The same applies for the minimum pitches β.sub.Thrust-min and β.sub.Torque-min. “β.sub.L-Thrust-max” and “β.sub.L-Torque-max” then identify the maximum pitches for the blades 11 of the first propeller 10, and “β.sub.R-Thrust-max” and “β.sub.R-Torque-max” then identify the maximum pitches for the blades 16 of the second propeller 15 relative respectively to the usable thrust and to the torque allowable by the mechanical transmission system.

(46) The control computer 60 may then control the actuators 76 and 77 as a function at least of a law stored in a memory and of the maximum and minimum pitches relating respectively to maximum and minimum characteristics, after the control computer 60 has received a thrust control signal subsequently to a pilot acting on said thrust control 50. This at least one law makes it possible to bound the orders issued via the thrust control signal in order to keep the pitches of the blades 11, 16 of each propeller 10, 15 within the control envelope 90.

(47) The method of the invention for providing torque protection and thrust protection for at least one propeller 10, 15 of a hybrid helicopter 1 can thus make it possible to provide the (or each) propeller 10, 15 with protection independently for each of its characteristics, namely for its thrust and for the torque exerted in the mechanical transmission system. When the hybrid helicopter 1 includes two or more propellers 10, 15, the method of the invention can thus make it possible to protect each propeller 10, 15 independently from the other(s).

(48) However, it may be advantageous to combine the maximum pitches of a propeller 10, 15 relating to the two characteristics of said propeller 10, 15, namely its thrust and the torque exerted in its mechanical transmission system in order to take into account simultaneously the limits associated with said two characteristics.

(49) Similarly, it may be advantageous to combine the maximum pitches of all of the propellers 10, 15 of the hybrid helicopter 1, when the hybrid helicopter includes two or more propellers 10, 15, in order to take into account simultaneously for each propeller 10, 15 the limits associated with all of the propellers 10, 15.

(50) For this purpose, the method of the invention can make it possible, for the current forward speed of the hybrid helicopter, firstly to compute a maximum overall mean pitch β.sub.TCC-Complete-max relating simultaneously to the maximum usable thrust and to the maximum allowable torque for each propeller 10, 15, and to compute a minimum overall mean pitch β.sub.TCC-Complete-min relating simultaneously to the minimum usable thrust and to the minimum allowable torque for each propeller, for the current forward speed of the hybrid helicopter.

(51) The minimum overall mean pitch β.sub.TCC-Complete-min of a propeller 10, 15 may be defined as a function of the minimum pitch β.sub.Thrust-min and of the minimum pitch β.sub.Torque-min for said propeller 10, 15, and optionally of the minimum pitches for each other propeller 10, 15 of the hybrid helicopter 1.

(52) Similarly, the maximum overall mean pitch β.sub.TCC-Complete-max may be defined as a function of the maximum pitch β.sub.Thrust-max and of the maximum pitch β.sub.Torque-max for said propeller 10, 15, and optionally of the maximum pitches for each other propeller 10, 15 of the hybrid helicopter 1.

(53) The control computer 60 may then control the actuators 76, 77 as a function at least of a law stored in a memory, of the minimum overall mean pitch β.sub.TCC-Complete-min, and of the maximum overall mean pitch β.sub.TCC-Complete-max after the control computer 60 has received a thrust control signal. This at least one law makes it possible to bound the orders issued via the thrust control signal in order to keep the pitches of the blades 11, 16 of each propeller 10, 15 within the control envelope 90.

(54) For a hybrid helicopter 1 including two propellers 10, 15 as shown in FIG. 1, the step of computing a maximum overall mean pitch β.sub.TCC-Complete-max may comprise the following steps: measuring a first current pitch β.sub.L of the first blades 11 and a second current pitch β.sub.R of the second blades 16; computing a first maximum computation pitch relating to the maximum usable thrust for the first propeller 10 such that:

(55) ${\beta }_{\mathrm{TCC}-L-\mathrm{Thrust}-\max }=\frac{{\beta }_{L-\mathrm{Thrust}-\max }+{\beta }_R}{2};$ computing a second maximum computation pitch relating to the maximum usable thrust for the second propeller 15 such that:

(56) ${\beta }_{\mathrm{TCC}-R-\mathrm{Thrust}-\max }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Thrust}-\max }}{2};$ computing a maximum computation mean pitch relating to the maximum usable thrust for the two propellers 10,15 such that:

(57) ${\beta }_{\mathrm{TCC}-\mathrm{Thrust}-\max -\mathrm{NODIFF}}=\frac{{\beta }_{\mathrm{TCC}-L-\mathrm{Thrust}-\max }+{\beta }_{\mathrm{TCC}-R-\mathrm{Thrust}-\max }}{2};$ computing a maximum mean pitch relating to the maximum usable thrust for the two propellers 10,15 such that:
β.sub.TCC-Thrust-max=β.sub.TCC-Thrust-max-NODIFF−ABS(β.sub.TCC-L-Thrust-max−β.sub.TCC-R-Thrust-max); computing a maximum computation pitch relating to the maximum allowable torque for the first propeller 10 such that:

(58) 0${\beta }_{\mathrm{TCC}-L-\mathrm{Torque}-\max }=\frac{{\beta }_{L-\mathrm{Torque}-\max }+{\beta }_R}{2};$ computing a second maximum computation pitch relating to the maximum allowable torque for the second propeller 15 such that:

(59) ${\beta }_{\mathrm{TCC}-R-\mathrm{Torque}-\max }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Torque}-\max }}{2};$ computing a maximum computation mean pitch relating to the maximum allowable torque for the two propellers 10,15 such that:

(60) ${\beta }_{\mathrm{TCC}-\mathrm{Torque}-\max -\mathrm{NODIFF}}=\frac{{\beta }_{\mathrm{TCC}-L-\mathrm{Torque}-\max }+{\beta }_{\mathrm{TCC}-R-\mathrm{torque}-\max }}{2};$ computing a maximum mean pitch relating to the maximum allowable torque for the two propellers 10,15 such that:
β.sub.TCC-Torque-max=β.sub.TCC-Torque-max-NODIFF−ABS(β.sub.TCC-L-Torque-max−β.sub.TCC-R-Torque-max); and computing a maximum overall mean pitch relating to the maximum usable thrust and to the maximum allowable torque for the two propellers 10,15 such that:
β.sub.TCC-Complete-max=MIN(β.sub.TCC-Thrust-max;β.sub.TCC-Torque-max).

(61) Furthermore, during a correction step, for each of the two propellers 10,15, the maximum pitches β.sub.L-Thrust-max, β.sub.R-Thrust-max, β.sub.L-Torque-max and β.sub.R-Torque-max may be corrected for a rate of yaw of the hybrid helicopter 1 and be replaced respectively by

(62) ${\beta }_{L-\mathrm{Thrust}-\max }+\frac{r.Y_p.\pi }{180.k\Omega R},{\beta }_{R-\mathrm{Thrust}-\max }-\frac{r.Y_p.\pi }{180.k\Omega R},{\beta }_{L-\mathrm{Torque}-\max }+\frac{r.Y_p.\pi }{180.k\Omega R}\mathrm{and}{\beta }_{R-\mathrm{Torque}-\max }-\frac{r.Y_p.\pi }{180.k\Omega R},$
where: r is the yaw rotation rate of the hybrid helicopter 1, expressed in degrees per second (°/s); Yp is the yaw lever arm of the propellers 10, 15, expressed in meters (m); π is a trigonometric constant; Ω is the speed of rotation of the propellers 10, 15, expressed in radians per second (rad/s), R is the radius of the propellers 10, 15, expressed in meters (m); and k is a positive coefficient less than or equal to 1.

(63) The step of computing a minimum overall mean pitch β.sub.TCC-Complete-min may also include the following substeps: measuring a first current pitch β.sub.L, of the first blades 11 and a second current pitch β.sub.R of the second blades 16; computing a first minimum computation pitch relating to the minimum usable thrust for the first propeller 10 such that,

(64) ${\beta }_{\mathrm{TCC}-L-\mathrm{Thrust}-\min }=\frac{{\beta }_{L-\mathrm{Thrust}-\min }+{\beta }_R}{2};$ computing a second minimum computation pitch relating to the minimum usable thrust for the second propeller 15 such that,

(65) ${\beta }_{\mathrm{TCC}-R-\mathrm{Thrust}-\min }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Thrust}-\min }}{2};$ computing a minimum computation mean pitch relating to the minimum usable thrust for the two propellers 10,15 such that:

(66) ${\beta }_{\mathrm{TCC}-\mathrm{Thrust}-\min -\mathrm{NODIFF}}=\frac{{\beta }_{\mathrm{TCC}-L-\mathrm{Thrust}-\min }+{\beta }_{\mathrm{TCC}-R-\mathrm{Thrust}-\min }}{2};$ computing a minimum computation mean pitch relating to the minimum usable thrust for the two propellers 10,15 such that:
β.sub.TCC-Thrust-min=β.sub.TCC-Thrust-min-NODIFF+ABS(β.sub.TCC-L-Thrust-min−β.sub.TCC-R-Thrust-min); computing a first minimum computation pitch relating to the minimum allowable torque for the first propeller 10 such that:

(67) ${\beta }_{\mathrm{TCC}-L-\mathrm{Torque}-\min }=\frac{{\beta }_{L-\mathrm{Torque}-\min }+{\beta }_R}{2};$ computing a second minimum computation pitch relating to the minimum allowable torque for the second propeller 15 such that:

(68) ${\beta }_{\mathrm{TCC}-R-\mathrm{Torque}-\min }=\frac{{\beta }_L+{\beta }_{R-\mathrm{Torque}-\min }}{2};$ computing a minimum computation mean pitch relating to the minimum allowable torque for the two propellers 10,15 such that:

(69) ${\beta }_{\mathrm{TCC}-\mathrm{Torque}-\min -\mathrm{NODIFF}}=\frac{{\beta }_{\mathrm{TCC}-L-\mathrm{Torque}-\min }+{\beta }_{\mathrm{TCC}-R-\mathrm{torque}-\min }}{2};$ computing a minimum mean pitch relating to the minimum allowable torque for the two propellers 10,15 such that:
β.sub.TCC-Torque-min=β.sub.TCC-Torque-min-NODIFF+ABS(β.sub.TCC-L-Torque-min−β.sub.TCC-R-Torque-min); and computing a minimum overall mean pitch relating to the minimum usable thrust and to the minimum allowable torque for the two propellers 10,15 such that:
β.sub.TCC-Complete-min=MAX(β.sub.TCC-Thrust-min;β.sub.TCC-Torque-min).

(70) As above, during the correction step, for each of the two propellers 10,15, the minimum pitches β.sub.L-Thrust-min, β.sub.R-Thrust-min, β.sub.L-Torque-min and β.sub.R-Torque-min may be corrected for the rate of yaw of the hybrid helicopter 1 and be replaced respectively by

(71) 0${\beta }_{L-\mathrm{Thrust}-\min }+\frac{r.Y_p.\pi }{180.k\Omega R},{\beta }_{R-\mathrm{Thrust}-\min }-\frac{r.Y_p.\pi }{180.k\Omega R},{\beta }_{L-\mathrm{Torque}-\min }+\frac{r.Y_p.\pi }{180.k\Omega R}\mathrm{and}{\beta }_{R-\mathrm{Torque}-\min }-\frac{r.Y_p.\pi }{180.k\Omega R}.$

(72) Furthermore, when the control system 40 receives a yaw control signal directly or indirectly from the yaw control 55 subsequently to an action from the pilot when the pitch of the blades 11, 16 of at least one propeller reaches one of the limits 91, 92 of the control envelope 90, the control system 40 can then control the actuators 76, 77 as a function at least of a law stored in a memory after the control computer 60 has received the yaw control signal. This at least one law then advantageously makes it possible to modify the mean pitch component of the pitch of the blades 11, 16 of the propellers 10, 15 in such a manner as to make it possible to apply the differential pitch component to the pitch of the blades 11, 16 of the two propellers 10, 15 without going outside the control envelope 40.

(73) Indeed, the mean pitch component of the pitch of the blades 11, 16 of the propellers 10, 15 is modified by a value substantially equal to the absolute value of the modification in the differential pitch component corresponding to the yaw control signal. Therefore, once the pitch of the blades 11, 16 of the propeller 10, 15 has reached one of the limits 91, 92, it moves far enough away from said limit 91, 92 to make it possible to perform the yaw control requested by the pilot without going outside the control envelope 90.

(74) The method of the invention is thus not limited to the actions of the pilot on the yaw control 55 acting on the differential pitch. The maneuvers consequent upon an action on the yaw control 55 such as, for example, a change of direction, may be performed while also advantageously keeping the protection afforded by the method of the invention.

(75) Furthermore, when the hybrid helicopter 1 includes a single propeller 10 or a plurality of propellers 10, 15, the maximum overall mean pitch β.sub.TCC-Complete-max may also be determined such as to be equal to the minimum value of the maximum pitch β.sub.Thrust-max and of the maximum pitch β.sub.Torque-max of each propeller 10, 15 while the minimum overall mean pitch β.sub.TCC-Complete-min may be determined such as to be equal to the maximum value from among the minimum pitch β.sub.Thrust-min relating to the minimum usable thrust and the minimum pitch β.sub.Torque-min relating to the minimum allowable torque for each propeller 10, 15.

(76) Naturally, the present invention may be subjected to numerous variations as to its implementation. Although several implementations are described above, it should readily be understood that it is not conceivable to identify exhaustively all possible 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.