Method and a system for anticipating the entry of a rotorcraft into a vortex domain
A method and a system for anticipating entry of a rotorcraft into a vortex domain, the rotorcraft having a main rotor with blades. After previously determining a specific frequency characterizing the proximity of a vortex domain, measurements are acquired of at least one parameter serving to characterize variation of the flow of air in the environment of the main rotor. Thereafter, the measurements are analyzed in order to isolate frequencies characteristic of the variation of each parameter, and the presence of the specific frequency is detected among the characteristic frequencies. Where appropriate, an alarm can then be issued in order to inform a pilot of the rotorcraft of the proximity of the vortex domain.
1. A method of anticipating the entry of a rotorcraft into a vortex domain, the rotorcraft having a main rotor with blades, wherein the method comprises the following steps:
a) acquiring measurements of at least one parameter suitable for characterizing variation in the flow of air in the environment of the main rotor;
b) analyzing the measurements in order to isolate frequencies that are characteristic of the variation of each parameter; and
c) detecting a specific frequency f characterizing the proximity of a vortex domain, the specific frequency being lower than 1 Hz.
2. The method according to claim 1 , wherein the specific frequency f is estimated using the following relationship:
St
=
f
×
∅
V
where St is the Strouhal number, ø is the diameter of the main rotor, and V is the speed the rotorcraft is moving relative to the air.
3. The method according to claim 2 , wherein the Strouhal number St is considered as being a constant.
4. The method according to claim 2 , wherein the Strouhal number St is determined as a function of the angle between a vector representing the speed with which the rotorcraft is moving relative to the air and a plane formed by the rotating main rotor.
5. The method according to claim 1 , wherein the specific frequency f is detected when its amplitude is greater than a predetermined percentage of the amplitude of a fundamental frequency of the variation of the parameter.
6. The method according to claim 5 , wherein the predetermined percentage is equal to 10%.
7. The method according to claim 1 , wherein the acquisition of measurements of at least one parameter is performed at an acquisition frequency that is greater than or equal to twice the specific frequency f.
8. The method according to claim 7 , wherein the acquisition of measurements of at least one parameter is performed at an acquisition frequency lying in the range twice to ten times the specific frequency f.
9. The method according to claim 1 , wherein the acquisition of measurements of at least one parameter is performed over an acquisition duration that is greater than or equal to twice a period corresponding to the specific frequency f.
10. The method according to claim 9 , wherein the acquisition of measurements of at least one parameter is performed for an acquisition duration lying in the range twice to five times the period corresponding to the characteristic frequency f of a vortex domain.
11. The method according to claim 1 , wherein during the acquisition of measurements, only one parameter is measured.
12. The method according to claim 1 , wherein each parameter measured during the acquisition of measurements is selected from a list comprising a vertical acceleration of the rotorcraft relative to the surface, an air speed in the environment of the main rotor, a pressure of the air in the environment of the main rotor, forces exerted in the flight controls associated with the blades of the main rotor, and lift of the main rotor.
13. The method according to claim 1 , wherein the method includes an additional step of triggering an alarm indicating that a rotorcraft is approaching a vortex domain as soon as the specific frequency f is detected.
14. An anticipating system for anticipating entry of a rotorcraft into a vortex domain, the rotorcraft having a main rotor with blades, and the system comprising:
at least one measurement device for measuring at least one parameter serving to characterize variation in the flow of air in the environment of the main rotor;
a storage device containing calculation instructions and capable of storing the measurements of each parameter;
a calculation device connected to each measurement device and to the storage device, the calculation device being for detecting that the rotorcraft is approaching a vortex domain; and
a signaling device for signaling detection of the rotorcraft being in the proximity of a vortex domain, the signaling device being connected to the calculation device;
wherein the system is configured to perform the following steps:
a) acquiring measurements of the at least one parameter serving to characterize variation in the flow of air in the environment of the main rotor;
b) analyzing the measurements in order to isolate frequencies that are characteristic of the variation of each parameter; and
c) detecting a specific frequency f characterizing the proximity of a vortex domain, the specific frequency being lower than 1 Hz.
15. The system according to claim 14 , wherein the specific frequency f is estimated using the following relationship:
S
t
=
f
×
∅
V
where St is the Strouhal number, ø is the diameter of the main rotor, and V is the speed the rotorcraft is moving relative to the air.
16. The system according to claim 14 , wherein the specific frequency f is detected when its amplitude is greater than a predetermined percentage of the amplitude of a fundamental frequency of the variation of the parameter.
17. The system according to claim 14 , wherein the acquisition of measurements of at least one parameter is performed at an acquisition frequency that is greater than or equal to twice the specific frequency f.
18. The system according to claim 14 , wherein the acquisition of measurements of at least one parameter is performed over an acquisition duration that is greater than or equal to twice a period corresponding to the specific frequency f.
19. The system according to claim 14 , wherein each parameter measured during the acquisition of measurements is selected from a list comprising a vertical acceleration of the rotorcraft relative to the surface, an air speed in the environment of the main rotor, a pressure of the air in the environment of the main rotor, forces exerted in the flight controls associated with the blades of the main rotor, and lift of the main rotor.
20. The system according to claim 14 , wherein the system is configured to perform an additional step of triggering an alarm indicating that a rotorcraft is approaching a vortex domain as soon as the specific frequency f is detected.