Hover aircraft rotor comprising a vibration damping device

1. A hover aircraft rotor ( 3 ) comprising:

a hub ( 5 ) which rotates about an axis (A) and comprises a number of blades ( 9 );

a drive shaft ( 6 ) connectable to a drive member of said aircraft ( 1 ) and connected functionally to said hub ( 5 ) to rotate the hub ( 5 ) about said axis (A); and

damping means ( 15 ) for damping vibration transmitted to said shaft ( 6 ), and which comprise a mass ( 17 ) designed to oscillate, in use, to oppose transmission to said shaft ( 6 ) of vibration generated by rotation of said blades ( 9 );

the rotor being characterized in that said mass ( 17 ) is free to oscillate parallel to said axis (A), to oppose transmission to said shaft ( 6 ) of vibration having main components along said axis (A);

said damping means ( 15 ) comprising at least one first actuator ( 30 ) connected functionally to said mass ( 17 ) and controllable to generate on said mass ( 17 ) a first damping force having a main component parallel to said axis (A);

said first actuator ( 30 ) being controllable to generate said first damping force on said mass ( 17 ) with a main component parallel to said axis (A) and at a frequency varying, in use, according to the angular speed of said rotor ( 3 ) about said axis (A), so as to cause said mass ( 17 ) to oscillate parallel to said axis (A);

the rotor further comprising at least one elastic member ( 32 ) interposed between said mass ( 17 ) and said first actuator ( 30 );

said elastic member ( 32 ) having an axis parallel to said axis (A).

2. A rotor as claimed in claim 1 , characterized in that said first actuator ( 30 ) is controllable to generate said first damping force at a first frequency f=N*Ω, where N is the number of said blades, and Ω is the rotation frequency of the rotor ( 3 ).

3. A rotor as claimed in claim 2 , characterized by comprising said elastic member ( 32 ) and said mass ( 17 ) having a natural oscillation frequency, parallel to said axis (A), of N*Ω.

4. A rotor as claimed in claim 1 , characterized in that said first actuator ( 30 ) comprises a guide ( 16 ) coaxial with said axis (A), and on which said mass ( 17 ) is mounted to oscillate with respect to said axis (A).

5. A rotor as claimed in claim 1 , characterized in that said first actuator ( 30 ) comprises a body ( 31 ) connected to said shaft ( 6 ) and which rotates integrally with said shaft about said axis (A);

one ( 17 ) of said body ( 31 ) and said mass ( 17 ) comprising at least one permanent magnet ( 36 ) coupled magnetically to an electric winding ( 35 ) carried by the other ( 31 ) of said body ( 31 ) and said mass ( 17 ).

6. A rotor as claimed in claim 5 , characterized in that said first actuator ( 30 ) also comprises:

a slip-ring ( 61 ) for electrically powering said electric winding ( 35 ); and

an encoder ( 62 ) for determining the angular position of said shaft ( 6 ) about said axis (A).

7. A rotor as claimed in claim 1 , characterized in that said shaft ( 6 ) is hollow, and at least partly houses said first actuator ( 30 ).

8. A rotor as claimed in claim 1 , characterized in that said damping means ( 15 ) comprise a second actuator ( 41 ) and a third actuator ( 42 ), which are connected functionally to said mass ( 17 ) and controllable to generate a second damping force and a third damping force on said mass ( 17 ) respectively; said second damping force and said third damping force each having a main component in a plane perpendicular to said axis (A), so as to prevent transmission to said shaft ( 6 ) of vibration in said plane perpendicular to said axis (A).

9. A rotor as claimed in claim 8 , characterized in that said second actuator ( 41 ) and said third actuator ( 42 ) are rotated about said axis (A) by said shaft ( 6 ), and rotate, in use, with respect to said shaft ( 6 ) and about said axis (A);

said second actuator ( 41 ) being controllable to generate said second damping force at a second frequency of (N−1)*Ω, and by rotating at said second frequency in the same direction as and with respect to said shaft ( 6 );

said third actuator ( 42 ) being controllable to generate said third damping force at a third frequency of (N+1)*Ω, and by rotating at said third frequency in the opposite direction to and with respect to said shaft ( 6 ); wherein N is the number of said blades, and Ω is the rotation frequency of the rotor ( 3 ).

10. A rotor as claimed in claim 8 , characterized by comprising:

a sensor ( 33 ) for generating at least one number of signals associated with the acceleration status of said shaft ( 6 ) along said axis (A) and in a plane perpendicular to said axis (A); and

a control unit ( 34 ) controllable to command said first, second and third actuator ( 30 , 41 , 42 ), on the basis of said quantity detected by said sensor ( 33 ), to generate said first, second and third damping force respectively.

11. A rotor as claimed in claim 10 , characterized in that said sensor ( 33 ) is located along said axis (A).

12. A rotor as claimed in claim 10 , characterized in that said control unit ( 34 ) comprises three stages ( 37 a , 37 b , 37 c ) supplied with respective inputs ( 26 a , 26 b , 26 c ) from said sensor ( 33 ) and designed to generate respective outputs ( 27 a , 27 b , 27 c ) for said first, second, and third actuator ( 30 , 41 , 42 );

each said stage ( 37 a , 37 b , 37 c ) being designed to generate the respective output ( 27 a , 27 b , 27 c ) solely on the basis of a respective input ( 26 a , 26 b , 26 c ).

13. A hover aircraft ( 1 ), characterized by comprising a rotor ( 3 ) as claimed in claim 1 .

14. A rotor as claimed in claim 1 , characterized in that said first actuator ( 30 ) comprises a guide ( 16 ) coaxial with said axis (A), and on which said mass ( 17 ) is mounted to oscillate with respect to said axis (A);

said actuator ( 30 ) comprising a body ( 31 ) housed in said shaft ( 6 ) and defining a cavity for housing said mass ( 17 );

said mass ( 17 ) being a tubular body parallel to said axis (A) and through which said guide ( 16 ) extends coaxially.