Device for indicating a beat level

1. A device for indicating a beat level (L), the device comprising analyzer means (FT) which, in response to a reference frequency (Ω) and to a harmonic coefficient (B) extracts the following from an evaluation signal (E);

a first acceleration signal (γ B ) at a frequency equal to the product of said reference frequency (Ω) multiplied by said harmonic coefficient (B); and

a second acceleration signal (γ B-1 ) at a frequency equal to the product of said reference frequency (Ω) multiplied by said harmonic coefficient minus one;

and the device further comprising a control circuit (CC) which receives said first and second acceleration signals (γ B , γ B-1 ) to produce said beat level (L),

and wherein said beat level is equal to the ratio of the n th power of said second acceleration signal (γ B-1 ) divided by the first acceleration signal (γ B ).

2. A device according to claim 1 , further comprising a comparator (COMP) which produces a warning signal (AL) if said beat level (L) exceeds a predetermined threshold (TH).

3. A device according to claim 1 , wherein the value of n is equal to 2.

4. A device according to claim 1 , wherein said evaluation signal (E) comes from a vibration sensor.

5. A device according to claim 1 , including a concentrator member (MM) for producing said evaluation signal (E) in response to a plurality of measurement signals (C 1 , C 2 , and C 3 ), each coming from a distinct vibration sensor (S 1 , S 2 , S 3 ).

6. A device according to claim 5 , wherein said evaluation signal (E) is equal to the sum of said measurement signals (C 1 , C 2 , C 3 ).

7. A device according to claim 5 , wherein said evaluation signal (E) is the greatest of said measurement signals (C 1 , C 2 , C 3 ).

8. A device according to claim 1 , wherein said reference frequency (Ω) is derived from a tachometer (T).

9. A device for indicating a beat level (L) of a rotary wing aircraft having a rotor with a plurality of blades, the device comprising:

a vibration sensor (S 1 , S 2 , S 3 ) for measuring vibration on board the aircraft;

a speed sensor (T) for measuring the speed of rotation of the rotor, the speed sensor providing a reference signal having a reference frequency (Ω);

a synchronous analysis means (FT) for synchronously analyzing harmonic components of a vibration signal (E) derived from the vibration sensor, the synchronous analysis means (FT) being connected to the vibration sensor and to the speed sensor, the synchronous analysis means (FT) being arranged to extract from the vibration signal (E):

i) a first harmonic component (γ B ) of a frequency (BΩ) equal to the product of said reference frequency (Ω) multiplied by a harmonic coefficient (B) equal to the number of blades of the rotor, and

ii) a second harmonic component (γ B-1 ) of frequency (B-1) (Ω) equal to the product of said reference frequency (Ω) multiplied by said harmonic coefficient minus one (B-1);

a control circuit (DC) connected to the synchronous analysis means (FT), the control circuit receiving said first and second harmonic components and outputting a beat level (L).

10. A device according to claim 9 , further comprising a comparator which produces a warning signal (AL) if said beat level (L) exceeds a predetermined threshold (TH).

11. A device according to claim 9 , wherein said beat level is equal to the ratio of the n th power of said second harmonic component (γ B-1 ) divided by the first harmonic component (γ B ).

12. A device according to claim 11 , wherein the value of n is equal to 2.

13. A device according to claim 9 , including a concentrator member for producing said vibration signal (E) in response to a plurality of measurement signals (C 1 , C 2 , and C 3 ), each coming from a distinct vibration sensor.

14. A device according to claim 13 , wherein said vibration signal (E) is equal to the sum of said measurement signals (C 1 , C 2 , C 3 ).

15. A device according to claim 13 , wherein said vibration signal (E) is the greatest of said measurement signals (C 1 , C 2 , C 3 ).