DETECTING THAT A ROTORCRAFT IS APPROACHING A VORTEX DOMAIN, AND SIGNALING THAT DETECTION

Abstract

A method and a device for detecting that a rotorcraft is approaching a vortex domain. After previously determining a limit advance speed threshold and a limit vertical speed threshold defining a limit for said rotorcraft entering into a vortex domain, a predictive advance speed and a predictive vertical speed for said rotorcraft are calculated, said predictive vertical speed being calculated differently depending on the value of said instantaneous advance speed. Thereafter, said predictive advance speed and said predictive vertical speed are compared with said thresholds, which may be thresholds with hysteresis, in order to determine whether said rotorcraft is approaching a vortex domain, and if so to signal this situation to a pilot of said rotorcraft.

Claims

1. A method of detecting that a rotorcraft is approaching a vortex domain, and of signaling that detection, the rotorcraft forming part of a rotorcraft family and having a main rotor with blades, wherein the method comprises the following steps: a) a preliminary step of determining a limit advance speed threshold and a limit vertical speed threshold defining a limit for entering into a vortex domain for the family of rotorcraft; b) a calculation step of calculating in real time a predictive advance speed and a predictive vertical speed of the rotorcraft during a flight of the rotorcraft; the predictive advance speed being calculated as a function of an instantaneous advance speed and of an instantaneous advance acceleration of the rotorcraft over a prediction time interval Δt characterizing the prediction time of the predictive advance speed and of the predictive vertical speed; and the predictive vertical speed being calculated on the basis of the instantaneous advance speed, when the instantaneous advance speed is greater than an upper limit speed, the predictive vertical speed being calculated as a function of the instantaneous vertical speed of the rotorcraft, of the energy balance of the rotorcraft, and of the power variation needed by the main rotor for level flight over the prediction time interval; and when the instantaneous advance speed is less than a lower limit speed, the predictive vertical speed being calculated as a function of the instantaneous vertical speed and of an instantaneous vertical acceleration of the rotorcraft; c) a step of triggering an alarm that a rotorcraft is approaching a vortex domain, the predictive advance speed and the predictive vertical speed being compared respectively with the limit advance speed threshold and with the limit vertical speed threshold, the alarm being triggered when firstly the predictive advance speed has reached the limit advance speed threshold, and secondly the predictive vertical speed has reached the limit vertical speed threshold; and d) a step of signaling the alarm to a pilot of the rotorcraft as a result of the triggering of the alarm.

2. A method according to claim 1, wherein when the instantaneous advance speed is less than or equal to the upper limit speed and greater than or equal to the lower limit speed, the predictive vertical speed is an interpolation between the above two situations, i.e. when the instantaneous advance speed is greater than the upper limit speed or when the instantaneous advance speed is less than the lower limit speed.

3. A method according to claim 1, wherein when the instantaneous advance speed is less than or equal to the upper limit speed and greater than or equal to the lower limit speed, the predictive vertical speed is calculated using logic with hysteresis between the two above situations, i.e. that when the instantaneous advance speed decreases from the upper limit speed and remains greater than or equal to the lower limit speed, the predictive vertical speed is calculated using the first situation, i.e. as a function of the instantaneous vertical speed, of the energy balance of the rotorcraft, and of the variation in the power needed by the main rotor for level flight over the prediction time interval, and when the instantaneous advance speed increases from the lower limit speed and remains less than or equal to the upper limit speed, the predictive vertical speed is calculated using the second situation, i.e. as a function of the instantaneous vertical speed and of the instantaneous vertical acceleration of the rotorcraft.

4. A method according to claim 1, wherein the calculation step comprises substeps: b1) a substep of determining the instantaneous advance speed and the instantaneous vertical speed of the rotorcraft; b2) a first calculation substep of calculating the predictive advance speed using a first relationship: $V_{\mathrm{AP}}=V_A+\Delta .Math..Math.t.Math.\frac{{\mathrm{dV}}_A}{\mathrm{dt}}$ where t and Δt designate respectively time and the prediction time interval, being an instantaneous advance acceleration of the rotorcraft; b3) a second calculation substep of calculating the predictive vertical speed on the basis of the instantaneous forward speed, when the instantaneous advance speed is greater than the upper limit speed, the predictive vertical speed is calculated using a second relationship: $V_{\mathrm{ZP}}=V_Z+A.Math..Math.V_A.Math.\frac{{\mathrm{dV}}_A}{\mathrm{dt}}.Math.+B.Math.\left(V_Z+k\right).Math.\frac{V_{\mathrm{AP}}-V_A}{2.Math.V_Y-V_{\mathrm{AP}}}$ where k is a constant characteristic of the rotorcraft family of the rotorcraft, A is a first weighting coefficient, B is a second weighting coefficient, and V.sub.Y is a predetermined minimum power speed of the family of the rotorcraft, the expression: $B.Math.\left(V_Z+k\right).Math.\frac{V_{\mathrm{AP}}-V_A}{2.Math.V_Y-V_{\mathrm{AP}}}$ being applicable for calculating the predictive vertical speed only when the rotorcraft is flying firstly with a low instantaneous advance speed less than the predetermined minimum power speed of the family of the rotorcraft, but still greater than the upper limit speed, and secondly with an instantaneous advance speed that is decreasing, characterizing deceleration of the rotorcraft; and when the instantaneous advance speed is less than the lower limit speed, the predictive vertical speed is calculated by a third relationship: $V_{\mathrm{ZP}}=V_Z+D.Math.\Delta .Math..Math.t.Math.\frac{{\mathrm{dV}}_Z}{\mathrm{dt}}$ where D is a third weighting coefficient, and is an instantaneous vertical acceleration of the rotorcraft.

5. A method according to claim 4, wherein the characteristic constant k is equal to 4000 ft/min, the first weighting coefficient A is equal to −0.05, the second weighting coefficient B is equal to 1 when V.sub.A≦V.sub.Y and zero when V.sub.A>V.sub.Y, and the third weighting coefficient D is equal to 0.5.

6. A method according to claim 1, wherein the limit advance speed threshold and the limit vertical speed threshold are constituted by a curve for the vortex domain in a diagram where the abscissa and ordinate axes correspond respectively to the instantaneous advance speed and the instantaneous vertical speed of the rotorcraft, the curve for the vortex domain being representative of a vortex state of the family of the rotorcraft and being determined as a result of prior measurements in flight on a reference rotorcraft of the family of rotorcraft, and during the step of triggering an alarm, the alarm is triggered when the predictive advance speed and the predictive vertical speed form a predictive operating point for the rotorcraft that is situated on or below the curve for the vortex domain, the alarm being deactivated as soon as the predictive advance speed and the predictive vertical speed form a predictive operating point that is situated above the curve for the vortex domain.

7. A method according to claim 1, wherein the limit advance speed threshold is constituted by a limit advance speed and the limit vertical speed threshold is constituted by a limit vertical speed, and during the step of triggering an alarm, the alarm is triggered when firstly the predictive advance speed is less than or equal to the limit advance speed and secondly the predictive vertical speed is less than or equal to the limit vertical speed, the alarm being deactivated as soon as the predictive advance speed is greater than the limit advance speed or the predictive vertical speed is greater than the limit vertical speed.

8. A method according to claim 7, wherein the limit vertical speed corresponds to half a mean value of the induced speed of the main rotor when the rotorcraft is hovering outside the ground effect zone.

9. A method according to claim 7, wherein the limit advance speed is equal to 25 kt and the limit vertical speed is equal to −1200 ft/min.

10. A method according to claim 1, wherein the advance speed threshold is a first threshold with hysteresis constituted by a first limit advance speed and a second limit advance speed, the first limit advance speed being less than the second limit advance speed, the limit vertical speed threshold is a second threshold with hysteresis constituted by a first limit vertical speed and by a second limit vertical speed, the first limit vertical speed being less than the second limit vertical speed, and during the step of triggering an alarm of a rotorcraft approaching a vortex domain, the alarm is triggered when firstly the predictive advance speed is less than or equal to the first limit advance speed and secondly the predictive vertical speed is less than or equal to the first limit vertical speed, the alarm being deactivated as soon as the predictive advance speed is greater than the second limit advance speed or the predictive vertical speed is greater than the second limit vertical speed.

11. A method according to claim 10, wherein one of the first and second limit vertical speeds corresponds to half a mean value of the induced speed of the main rotor when the rotorcraft is hovering outside the ground effect zone.

12. A method according to claim 10, wherein the first limit advance speed is equal to 21 kt, the second limit advance speed is equal to 26 kt, the first limit vertical speed is equal to −800 ft/min, and the second limit vertical speed is equal to −1200 ft/min.

13. A method according to claim 1, wherein the lower limit speed is equal to 15 kt and the upper limit speed is equal to 35 kt.

14. A method according to claim 1, wherein the method includes an alarm inhibit step of deactivating the signaling of the alarm to the pilot of the rotorcraft when the rotorcraft is flying at a height relative to the ground that is less than or equal to a limit height or when the rotorcraft have at least two engines and has entered into an emergency mode of operation as a result of a malfunction of one of the engines for a duration less than a predetermined duration.

15. A method according to claim 14, wherein the limit height lies in the range 20 ft to 100 ft and the predetermined duration is equal to 30 s.

16. A method according to claim 1, wherein the during the signaling step time delays are used between the triggering of the alarm and the signaling to the pilot of the rotorcraft that the alarm has been triggered, and also between deactivating the alarm and the signaling to the pilot of the deactivation of the alarm.

17. A method according to claim 1, wherein the rotorcraft is characterized by three leading directions, a longitudinal direction X extending from the rear of the rotorcraft towards the front of the rotorcraft, an elevation direction Z extending upwards perpendicularly to the longitudinal direction X, and a transverse direction Y extending from right to left perpendicularly to the longitudinal and elevation directions X and Z, and when the longitudinal direction X is inclined at an angle θ relative to a horizontal plane, the predictive vertical speed is replaced by a formula (V.sub.ZP.Math.cos θ), which is compared with the limit vertical speed threshold.

18. A method according to claim 1, wherein during the calculation step, the predictive advance speed is equal to the instantaneous advance speed of the rotorcraft and the predictive vertical speed is equal to the instantaneous vertical speed of the rotorcraft in order to determine during the step of triggering an alarm whether the rotorcraft is in a vortex domain at the current instant t.

19. A method according to claim 1, wherein the instantaneous advance speed and the predictive advance speed of the rotorcraft are formed respectively by an instantaneous proper speed and a predictive proper speed of the rotorcraft.

20. A method according to claim 1, wherein the instantaneous advance speed and the predictive advance speed of the rotorcraft are formed respectively by a horizontal component of an instantaneous proper speed and by a horizontal component of a predictive proper speed of the rotorcraft.

21. A device for detecting and signaling that a rotorcraft is approaching a vortex domain, the rotorcraft belonging to a family of rotorcraft, the rotorcraft including a main rotor having blades, the device comprising: first measurement means for measuring an instantaneous vertical speed (V.sub.Z) of the rotorcraft; second measurement means for measuring a calibrated airspeed of the rotorcraft; memory means containing a limit advance speed threshold and a limit vertical speed threshold defining a limit for entering into a vortex domain for the rotorcraft family of the rotorcraft; calculation means connected to the first and second measurement means and to the memory means, the calculation means being for calculating an instantaneous advance speed, an instantaneous vertical acceleration and an instantaneous advance acceleration of the rotorcraft and for detecting that the rotorcraft is approaching a vortex domain; and signaling means for signaling that the rotorcraft is approaching a vortex domain, the signaling means being connected to the calculation means; wherein the device performs the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0126] The invention and its advantages appear in greater detail from the context of the following description showing preferred embodiments that are given without any limiting character and with reference to the accompanying figures, in which:

[0127] FIGS. 1 and 2 are two views of a rotorcraft;

[0128] FIG. 3 shows the flow of air in the presence of a main rotor of a rotorcraft in a vortex domain;

[0129] FIG. 4 shows a device for detecting the approach of a vortex domain by a rotorcraft of the invention;

[0130] FIG. 5 is a block diagram of a method of detecting the approach of a vortex domain being detected by a rotorcraft;

[0131] FIGS. 6 and 7 are two diagrams showing a vortex domain; and

[0132] FIG. 8 is a simplified diagram showing a vortex domain.

[0133] Elements present in more than one of the figures are given the same references in each of them.

DETAILED DESCRIPTION OF THE INVENTION

[0134] In FIGS. 1 and 2, a rotorcraft 20 has a main rotor 21 provided with blades 22 rotating about an axis 25. A rectangular reference frame (X, Y, Z) is associated with the rotorcraft 20. This reference frame (X, Y, Z) is defined by a longitudinal direction X extending from the rear of the rotorcraft 20 towards the front of the rotorcraft 20, a direction in elevation Z extending upwards perpendicularly to the longitudinal direction X, and a transverse direction Y extending from right to left perpendicularly to the longitudinal and elevation directions X and Z. The axis 25 of the main rotor 21 is substantially parallel to the elevation direction Z.

[0135] In FIGS. 1 and 2, a terrestrial reference frame (X.sub.t, Y.sub.t, Z.sub.t) is also shown. This terrestrial reference frame (X.sub.t, Y.sub.t, Z.sub.t) is defined by a direction Z.sub.t parallel to the gravity direction and two directions X.sub.t and Y.sub.t defining a horizontal plane P.sub.h perpendicular to the vertical direction Z.sub.t.

[0136] In FIG. 1, it can be seen that the elevation direction Z associated with the rotorcraft 20 is parallel to the vertical direction Z.sub.t. The plane P.sub.R formed by the main rotor 21 is parallel to the horizontal plane P.sub.h, since the rotorcraft 20 is flying with an attitude angle θ of zero. In FIG. 2, the plane P.sub.R forms an angle θ with the horizontal plane P.sub.h, and the rotorcraft 20 is then flying with an attitude angle θ. The elevation direction Z associated with the rotorcraft 20 also forms an angle equal to the attitude angle θ with the vertical direction Z.sub.t of the terrestrial reference frame (X.sub.t, Y.sub.t, Z.sub.t).

[0137] FIG. 3 shows the main rotor 21 operating in a static vortex domain. The rotor plane P.sub.R formed by the main rotor 21 is parallel to the horizontal plane P.sub.h and thus perpendicular to the gravity direction. The air flow speed directions shown in FIG. 3 correspond to the rotorcraft 20 descending slowly and practically vertically.

[0138] The speed V.sub.V designates the vertical component of the upstream speed of the air flow, normal to the plane P.sub.R formed by the main rotor 21, and in this situation the value V.sub.F, referred to by the person skilled in the art as the “Froude speed”, is greater than the speed V.sub.V. It can be seen that a wake S forms at the bottom portion of the main rotor 21, thereby obliging the top central air streams FCS to create a turbulent zone ZT towards the periphery of the blades 22.

[0139] Under such conditions, a vortex state phenomenon manifested in principle by vibrations perceived by the crew of the rotorcraft 20 arises when the rotorcraft 20 begins descending purely vertically or with a steep descent angle, since the main rotor 21 is then descending through its own downwash and losing lift because it is isolated from the air flow. As a result, the rotorcraft 20 will drop suddenly, unless a correcting maneuver is undertaken by the pilot of the rotorcraft 20.

[0140] In order to remedy this dangerous situation specific to the rotorcraft, the rotorcraft 20 may include a device for detecting and signaling that a rotorcraft 20 is approaching a vortex domain. This device 20 serves to enable the rotorcraft 20 to perform a method of detecting the approach of a vortex domain and of signaling that detection, which method is summarized diagrammatically in FIG. 5.

[0141] The device 10 can then detect the approach of a vortex domain and signal that detection to the pilot of the rotorcraft 20 by way of prediction. Under such circumstances, the pilot can take the necessary measures by taking anticipatory action in the light of this approach, so as to avoid the rotorcraft 20 entering into this vortex domain.

[0142] The device 10 is shown in FIG. 4 and comprises:

[0143] first measurement means 1 for measuring the instantaneous vertical speed V.sub.Z of the rotorcraft 20;

[0144] second measurement means 2 for measuring the calibrated airspeed V.sub.C of the rotorcraft 20;

[0145] memory means 3 for storing a limit advance speed threshold and a limit vertical speed threshold defining a limit for entering into a vortex domain for the rotorcraft family of the rotorcraft 20, and also various relationships needed for performing the method;

[0146] calculation means 4 connected to the first and second measurement means 1 and 2 and to the memory means 3, in order to apply these relationships and detect that the rotorcraft 20 is approaching a vortex domain;

[0147] signaling means 5 for signaling to the pilot of the rotorcraft 20 that the rotorcraft 20 is approaching a vortex domain, the signaling means 5 being connected to the calculation means 4;

[0148] third measurement means 6 connected to the calculation means 4 to measure the longitudinal attitude θ of the rotorcraft 20; and

[0149] fourth measurement means 7 connected to the calculation means 4 for measuring an instantaneous advance acceleration

[00019]$\frac{{\mathrm{dV}}_A}{\mathrm{dt}}$

and an instantaneous vertical acceleration

[00020]$\frac{{\mathrm{dV}}_Z}{\mathrm{dt}}$

of the rotorcraft 20.

[0150] By way of example, the first measurement means 1 is a vertical speed indicator and the second speed measurement means 2 is an airspeed indicator. The memory means 3 may be a memory containing in particular a database with the limit advance speed threshold and the limit vertical speed threshold and also characteristics of the rotorcraft family of the rotorcraft 20. The memory means 3 contain the various relationships used by the calculation means 4. By way of example, the calculation means 4 may be a computer and the signaling means 5 an indicator lamp situated on the instrument panel of the rotorcraft 20. By way of rotorcraft, the third measurement means 6 may be an artificial horizon instrument and the fourth measurement means 7 may be an accelerometer. The third and fourth measurement means 6 and 7 may also be grouped together within a single device known as an attitude and heading reference system (AHRS) that supplies the accelerations and the attitude angles of the rotorcraft relative to the three axes of the reference frame (X, Y, Z).

[0151] Nevertheless, the instantaneous advance acceleration and vertical acceleration may be determined by the calculation means 4 by taking the derivative with respect to time of the instantaneous advance speed and vertical speed V.sub.A and V.sub.Z. The device 10 then does not have the fourth measurement means 7.

[0152] The method of detecting that a rotorcraft 20 is approaching a vortex domain and signaling this detection comprises four main steps, as shown in FIG. 5.

[0153] Firstly, during a preliminary step (a), a series of speed measurements is performed on a reference rotorcraft during preliminary test flights in order to determine a limit advance speed threshold and a limit vertical speed threshold. These preliminary test flights are performed at the limit of vortex domains and until entering into such vortex domains. These preliminary test flights thus make it possible firstly to determine a diagram showing a mean shape of a static vortex for the reference rotorcraft, and consequently for all of the rotorcraft 20 in the family of rotorcraft in which this reference rotorcraft forms a part.

[0154] Such diagrams are shown in FIGS. 6 and 7 in a coordinate system formed by the instantaneous advance speed V.sub.A of the rotorcraft 20 plotted along the abscissa axis and its instantaneous vertical speed V.sub.Z plotted up the ordinate axis. On the basis of each diagram, limit advance speed and limit vertical speed thresholds can be determined, the limit advance speed threshold and the limit vertical speed threshold thus defining a limit for entering into a vortex domain for all of the rotorcraft 20 in the rotorcraft family to which the diagram corresponds.

[0155] In a first variant of the invention corresponding to the diagram of FIG. 6, the limit advance speed threshold and the limit vertical speed threshold are formed respectively by thresholds that are simple and constant, i.e. a limit advance speed V.sub.AL and a limit vertical speed V.sub.ZL.

[0156] The limit advance speed V.sub.AL is less than a first upper limit L.sub.A for advance speed of the vortex domain. Likewise, the limit vertical speed V.sub.ZL, which is equal to half the mean value of the induced speed Vi of the main rotor 21 when hovering outside the ground effect zone, is less than a second upper limit L.sub.Z for vertical speed of the vortex domain.

[0157] In a second variant of the invention, corresponding to the diagram of FIG. 7, the limit advance speed threshold and the limit vertical speed threshold are formed respectively by first and second thresholds having hysteresis. Thus, the limit advance speed threshold is constituted by a first limit advance speed V.sub.AL1 and a second limit advance speed V.sub.AL2. The first limit advance speed V.sub.AL1 is less than the first upper limit L.sub.A and less than the second limit advance speed V.sub.AL2, while the second limit advance speed V.sub.AL2 is greater than the first upper limit L.sub.A.

[0158] Likewise, the limit vertical speed threshold is constituted by a first limit vertical speed V.sub.ZL1 and a second limit vertical speed V.sub.ZL2. The first limit vertical speed V.sub.ZL1, which is equal to half the mean value of the induced speed Vi of the main rotor 21 when hovering outside the ground effect zone, is less than the second upper limit L.sub.Z and less than the second limit vertical speed V.sub.ZL2, whereas the second limit vertical speed V.sub.ZL2 is greater than the second upper limit L.sub.Z.

[0159] These limit advance and vertical speeds are determined as a result of preliminary tests performed on the reference rotorcraft of the rotorcraft family.

[0160] In addition, in a third variant of the invention, the limit advance speed threshold and the limit vertical speed threshold may also be formed by the top curve of the vortex domain as determined as a result of preliminary test flights on the reference rotorcraft of the rotorcraft family. These limit advance speed and limit vertical speed thresholds are thus variable.

[0161] Thereafter, during a calculation step (b), a predictive advance speed V.sub.AP and a predictive vertical speed V.sub.ZP are determined for the rotorcraft 20 in real time during a flight of the rotorcraft 20. This calculation step (b) is made up of substeps.

[0162] During a determination substep (b1), the instantaneous vertical speed V.sub.Z of the rotorcraft 20 is measured by the measurement means 1 in the terrestrial reference frame (X.sub.t, Y.sub.t, Z.sub.t), i.e. relative to the gravity direction.

[0163] During this determination substep (b1), the calibrated airspeed V.sub.C, which corresponds to the speed of the rotorcraft 20 relative to air, is also measured by the second measurement means 2 in the reference frame (X, Y, Z) associated with the invention 20.

[0164] This measurement of the calibrated airspeed V.sub.C may be along the longitudinal direction X of the rotorcraft 20 if the second measurement means 2 is a single-direction airspeed indicator aligned on the longitudinal direction X, for example.

[0165] This measurement of the calibrated airspeed V.sub.C may also be situated in a plane formed by the longitudinal and transverse directions X and Y of the rotorcraft 20 when the second measurement means 2 is a two-directional airspeed indicator, such as an ultrasound airspeed indicator.

[0166] This measurement of the calibrated airspeed V.sub.C may also be represented by a vector relative to all three directions of the reference frame (X, Y, Z) associated with the rotorcraft 20, when the second measurement means 2 is a three-directional airspeed indicator such as a LIDAR airspeed indicator.

[0167] Thereafter, the calculation means 4 determine the instantaneous advance speed V.sub.A on the basis of this measurement of the calibrated airspeed V.sub.C, in particular by applying a barometric correction.

[0168] The instantaneous advance speed V.sub.A may be associated with the calibrated airspeed V.sub.C by the relationship Vc=V.sub.A.Math.√{square root over (σ)}, and may thus be constituted by the instantaneous proper speed V.sub.P of the rotorcraft 20. The predictive advance speed V.sub.AP then constitutes a predictive proper speed V.sub.PP of the rotorcraft 20.

[0169] The instantaneous advance speed V.sub.A may also be approximated by a horizontal component of the instantaneous proper speed V.sub.P of the rotorcraft 20, i.e. a projection onto a horizontal plane of the calibrated airspeed V.sub.C of the rotorcraft as barometrically corrected. The predictive advance speed V.sub.AP then constitutes a horizontal component of the predictive proper speed of the rotorcraft 20.

[0170] Thereafter, during a first calculation substep (b2), the predictive advance speed V.sub.AP is calculated using a first relationship:

[00021]$V_{\mathrm{AP}}=V_A+\Delta .Math..Math.t.Math.\frac{{\mathrm{dV}}_A}{\mathrm{dt}}$

where t and Δt designate respectively time and the prediction time interval,

[00022]$\frac{{\mathrm{dV}}_A}{\mathrm{dt}}$

being the instantaneous advance acceleration of the rotorcraft 20.

[0171] During a second calculation substep (b3), the predictive vertical speed V.sub.ZP is calculated on the basis of knowing the instantaneous advance speed V.sub.A.

[0172] If the instantaneous advance speed V.sub.A is greater than an upper limit speed, then the predictive vertical speed V.sub.ZP is calculated by a second relationship:

[00023]$V_{\mathrm{ZP}}=V_Z+A.Math..Math.V_A.Math.\frac{{\mathrm{dV}}_A}{\mathrm{dt}}.Math.+B.Math.\left(V_Z+k\right).Math.\frac{V_{\mathrm{AP}}-V_A}{2.Math.V_Y-V_{\mathrm{AP}}}$

where:
k is a constant that is characteristic of the rotorcraft family of the rotorcraft 20, A is a first weighting coefficient, B is a second weighting coefficient, and V.sub.Y is a predetermined minimum power speed of the family of the rotorcraft 20. Specifically, the expression

[00024]$B.Math.\left(V_Z+k\right).Math.\frac{V_{\mathrm{AP}}-V_A}{2.Math.V_Y-V_{\mathrm{AP}}}$

is applicable for calculating the predictive vertical speed V.sub.ZP only when the rotorcraft 20 is flying firstly with an instantaneous advance speed V.sub.A less than the predetermined minimum power speed V.sub.Y of the family of the rotorcraft 20, and greater than the upper limit speed, and secondly with an instantaneous advance speed V.sub.A that is decreasing, characteristic of the rotorcraft 20 decelerating.

[0173] If the instantaneous advance speed V.sub.A is less than a lower limit speed, the predictive vertical speed V.sub.ZP is calculated by a third relationship:

[00025]$V_{\mathrm{ZP}}=V_Z+D.Math.\Delta .Math..Math.t.Math.\frac{{\mathrm{dV}}_Z}{\mathrm{dt}}$

where D is a third weighting coefficient, and

[00026]$\frac{{\mathrm{dV}}_Z}{\mathrm{dt}}$

is an instantaneous vertical acceleration of the rotorcraft 20.

[0174] Finally, if the instantaneous advance speed V.sub.A is less than or equal to the upper limit speed and greater than or equal to the lower limit speed, then the predictive vertical speed V.sub.ZP is an interpolation, e.g. a linear interpolation, between the second and third relationships.

[0175] Nevertheless, in another alternative, the predictive vertical speed V.sub.ZP may be calculated using logic with hysteresis. As a result, the second relationship is also applied while the instantaneous advance speed V.sub.A is decreasing from the upper limit speed and is greater than or equal to the lower limit speed. Likewise, the third relationship is applied when the instantaneous advance speed V.sub.A is increasing from the lower limit speed and remains less than or equal to the upper limit speed.

[0176] Thereafter, during a triggering step (c), the predictive advance speed V.sub.AP and the predictive vertical speed V.sub.ZP are compared respectively and simultaneously with the limit advance speed threshold and with the limit vertical speed threshold, and an alarm is triggered to the effect that the rotorcraft 20 is approaching a vortex domain when firstly the predictive advance speed V.sub.AP has reached the limit advance speed threshold and secondly the predictive vertical speed V.sub.ZP has reached the limit vertical speed threshold.

[0177] In the first variant of the invention, the alarm is triggered when firstly the predictive advance speed V.sub.AP is less than or equal to the limit advance speed and secondly the predictive vertical speed V.sub.ZP is less than or equal to the limit vertical speed. By way of example, the alarm is triggered for the first predictive operating point P.sub.1 characterized by a first predictive advance speed V.sub.AP1 less than the limit advance speed V.sub.AL and a first predictive vertical speed V.sub.ZP1 less than the limit vertical speed V.sub.ZL, this first predictive operating point P.sub.1 being situated below the limit advance speed V.sub.AL and of the left of the limit vertical speed V.sub.ZL in FIG. 6.

[0178] The alarm is then deactivated as soon as the predictive advance speed V.sub.AP is greater than the limit advance speed or as soon as the predictive vertical speed V.sub.ZP is greater than the limit vertical speed. By way of example, the alarm is deactivated for the second predictive operating point P.sub.2 characterized by a second predictive advance speed V.sub.AP2 greater than the limit advance speed V.sub.AL and a second predictive vertical speed V.sub.ZP2, still less than the limit vertical speed V.sub.ZL.

[0179] After applying this method of detecting the approach of a vortex domain with a prediction time Δt of 10 seconds, and of signaling such a detection, the diagram shown in FIG. 6 can be simplified as shown in the simplified diagram of FIG. 8. In particular, the bottom zone of the vortex domain in FIG. 6 is not taken into consideration since this region of the flight envelope is not operationally useful, and can therefore be omitted from the simplified diagram. This bottom zone of the vortex domain corresponds in particular to a vertical descent speed of more than 3000 ft/min. Specifically, during rapid descent, the rotorcraft is traveling at high horizontal speeds and in any event at speeds greater than the minimum power speed V.sub.Y.

[0180] Furthermore, the upper zone of the vortex domain can also be simplified by using the limit advance speed V.sub.AL and the limit vertical speed V.sub.ZL directly, as shown in FIG. 8.

[0181] In a second variant of the invention, the alarm is triggered when firstly the predictive advance speed V.sub.AP is less than or equal to the first limit advance speed V.sub.AL1 and secondly the predictive vertical speed V.sub.ZP is less than or equal to the first limit vertical speed V.sub.ZL1. By way of example, the alarm is triggered for the first predictive operating point P.sub.1 characterized by a first predictive advance speed V.sub.AP1 less than the first limit advance speed V.sub.AL1 and a first predictive vertical speed V.sub.ZP1 less than the first limit vertical speed V.sub.ZL1, this first predictive operating point P.sub.1 being situated below the first limit advance speed V.sub.AL1 and to the left of the first limit vertical speed V.sub.ZL1 in FIG. 7.

[0182] The alarm is then deactivated as soon as the predictive advance speed V.sub.AP is greater than the second limit advance speed or as soon as the predictive vertical speed V.sub.ZP is greater than the second limit vertical speed. By way of example, the alarm is deactivated for the third predictive operating point P.sub.3 characterized by a third predictive advance speed V.sub.AP3 greater than the second limit advance speed V.sub.AL2 and a third predictive vertical speed Z.sub.ZP3 still less than the first limit vertical speed V.sub.ZL1.

[0183] In contrast, the alarm is not deactivated for the second predictive operating point P.sub.2 characterized by a second predictive advance speed V.sub.AP2 greater than the first limit advance speed V.sub.AL1 but less than the second limit advance speed V.sub.AL2 and a second predictive vertical speed V.sub.ZP2 less than the first limit vertical speed V.sub.ZL1.

[0184] It should be observed that for both of these variants, a predictive operating point of the rotorcraft 20 may be situated in the vortex domain, even though it is situated below a first limit speed and above a second limit speed, e.g. in the zones B and C in FIGS. 6 and 7. For example, an operating point situated in the zone B is situated below the limit advance speed and above the limit vertical speed. If the pilot of the rotorcraft 20 does not take action, the predictive operating point will continue to lie in the vortex domain and will reach the second limit speed. The alarm that the approach of a vortex domain has been detected is then triggered and corresponds to a new prediction time interval that is less than the prediction time interval Δt. Advantageously, the alarm is nevertheless triggered and corresponds to detecting this approach to the vortex domain with a smaller prediction time interval.

[0185] Likewise, a predictive operating point of the rotorcraft 20 may lie outside the vortex domain, even though it is situated below both limit speeds, e.g. for the zone D in FIGS. 6 and 7. Under such circumstances, the alarm is triggered even before the predictive operating point enters into the vortex domain, but the rotorcraft 20 nevertheless has a flight path that appears to be heading towards the vortex domain. The alarm is thus triggered for detecting an approach to the vortex domain with a prediction time interval that is increased.

[0186] The positions of the limit speed thresholds thus make it possible to find a compromise between the dimensions firstly to the zones B and C and secondly of the zone D, and consequently to limit the variations in the prediction time interval for detecting an approach to the vortex domain. Furthermore, the value of the prediction time interval Δt advantageously enables these variations in the prediction time interval to remain acceptable so that the alarm is triggered soon enough to enable the pilot to carry out the necessary maneuver for avoiding actually entering this vortex domain.

[0187] The invention sets out to detect an approach to a vortex domain over a prediction time interval Δt, and the approximation associated with these positions for the limit speed thresholds thus mainly impacts the prediction time interval, but without generating a risk of not detecting the approach of a vortex domain or the risk of triggering false alarms.

[0188] Finally, during a signaling step (d), the detection of an approach to the vortex domain is signaled to a pilot of the rotorcraft 20 as a result of the alarm being triggered. This alarm may be signaled visually to the pilot, by lightning an indicator lamp 5.

[0189] In addition, during this signaling step, time delays may be used between triggering the alarm and signaling this triggering to the pilot of the rotorcraft 20 and also between deactivating the alarm and signaling this deactivation to the pilot.

[0190] Comparing the predictive advance and vertical speeds V.sub.AP and V.sub.ZP directly and respectively with the limit advance and vertical speeds corresponds to detecting an approach to a static vortex domain, while the plane of the main rotor and the longitudinal attitude θ of the rotorcraft are considered to be substantially zero.

[0191] Nevertheless, a rotorcraft frequently operates in flight with its main rotor occupying a plane that is inclined, in particular while the rotorcraft is decelerating. Consequently, the longitudinal direction X of the rotorcraft is inclined with an attitude angle θ relative to a horizontal plane. Under such circumstances, the rotorcraft may be approaching a dynamic vortex domain.

[0192] Advantageously, the diagrams representing the static vortex domain as shown in FIGS. 6, 7, and 8 are also appropriate for detecting an approach to a dynamic vortex domain. Specifically, a change of reference frame by applying a rotation to the reference frame formed by the instantaneous advance and vertical speeds V.sub.A and V.sub.Z through the attitude angle θ, as shown in FIG. 6, advantageously makes it possible to use the same vortex representation regardless of whether the rotorcraft is approaching a static vortex or a dynamic vortex.

[0193] Thus, when said longitudinal direction X is inclined at an attitude angle θ relative to the horizontal plane, the predictive vertical speed V.sub.ZP that is compared with the limit vertical speed is replaced by the formula (V.sub.ZP.Math.cos θ).

[0194] This change of reference frame is applied solely to the predictive vertical speed V.sub.ZP. Specifically, when the advance speed V.sub.A of the rotorcraft is equal to the proper speed V.sub.P, the predictive advance speed V.sub.AP is situated in the plane of the main rotor 21 both for the static vortex domain and for the dynamic vortex domain.

[0195] The method of the invention thus makes it possible to detect equally well an approach to a static vortex domain and an approach to a dynamic vortex domain depending on the attitude angle θ of the rotorcraft.

[0196] Naturally, the present invention may be subjected to numerous variations as to its implementation. Although several implementations are described, it will 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.