Aircraft provided with a silent shrouded rotor

1. An aircraft having a streamlined structure ( 10 ) pierced by an air flow duct ( 20 ) having an axis of symmetry (AX) contained in a plane of symmetry (P) perpendicular to a longitudinal axis (AL) of said aircraft, the aircraft possessing a shrouded rotor ( 1 ) having a plurality of blades ( 2 ) arranged in said air flow duct ( 20 ), wherein the periphery ( 21 ) of said air flow duct ( 20 ) is provided in succession along said axis of symmetry (AX) with a first lip ( 22 ), an intermediate portion ( 23 ), and then a second lip ( 24 ), said first lip ( 22 ) having a first front portion ( 22 ′) situated at the side of the duct that is closer to the front of the aircraft, and having a first rear portion ( 22 ″) situated at the side of the duct that is closer to the rear end ( 50 ) of the aircraft, said second lip ( 24 ) being provided with a second front portion ( 24 ′) situated at the side of the duct that is closer to the front of the aircraft and with a second rear portion ( 24 ″) situated at the side of the duct that is closer to the rear end ( 50 ) of the aircraft, and said aircraft is fitted with blower means ( 40 ) for reducing the noise generated by the shrouded rotor and suitable for propelling compressed air towards a first injection zone ( 44 ) opening out into said first front portion ( 22 ′), and to a second injection zone ( 45 ) opening out to said second front portion ( 24 ′).

2. An aircraft according to claim 1 , wherein said aircraft is a helicopter and said shrouded rotor is a shrouded tail rotor placed in an air flow duct ( 20 ) passing through a streamlined structure ( 10 ) of a vertical fin ( 5 ) of the helicopter.

3. An aircraft according to claim 1 , wherein said blower means ( 40 ) propels compressed air towards at least said first lip ( 22 ) via said first injection zone ( 44 ) when said shrouded rotor generates a flow of air from said first lip ( 22 ) towards said second lip ( 24 ), the blower means ( 40 ) propelling the compressed air towards at least the second lip ( 24 ) via the second injection zone ( 45 ) when the shrouded rotor generates a flow of air going from the second lip ( 24 ) towards the first lip ( 22 ).

4. An aircraft according to claim 1 , wherein when the pitch of the blades ( 2 ) lies between a predetermined maximum negative pitch (PITCH 1 ) and a predetermined minimum negative pitch (PITCH 2 ), said blower means propels compressed air solely towards said first lip ( 22 ) via said first injection zone ( 44 ), whereas said blower means propel compressed solely towards said second lip ( 24 ) via said second injection zone ( 45 ) when the pitch of the blades lies between a predetermined minimum positive pitch (PITCH 3 ) and a predetermined maximum positive pitch (PITCH 4 ).

5. An aircraft according to claim 1 , wherein when the pitch of the blades ( 2 ) is less than a predetermined maximum negative pitch (PITCH 1 ), said blower means does not propel compressed air to either of said first and second lips ( 22 , 24 ).

6. An aircraft according to claim 1 , wherein when the pitch of the blades is greater than a predetermined maximum positive pitch (PITCH 4 ), said blower means ( 40 ) does not propel compressed air towards either the first or the second lips ( 22 , 24 ).

7. An aircraft according to claim 1 , wherein each injection zone ( 44 , 45 ) occupies an angular field lying in the range 0° to 180°, boundary values not included.

8. An aircraft according to claim 1 , wherein at least one injection zone ( 44 , 45 ) comprises a slot.

9. An aircraft according to claim 1 , wherein at least one injection zone ( 44 , 45 ) comprises a succession of orifices.

10. An aircraft according to claim 1 , wherein said blower means comprises an air intake ( 60 ) feeding air for blowing to a valve ( 41 ) suitable for causing said air for blowing to reach the first and second injection zones ( 44 , 45 ) respectively via first and second feed channels ( 42 , 43 ).

11. An aircraft according to claim 10 , wherein said air intake ( 60 ) is a dynamic air intake ( 61 ) picking up said air for blowing by virtue of the movement of said aircraft, said valve ( 41 ) being connected to said dynamic air intake ( 61 ) via a delivery pipe ( 46 ).

12. An aircraft according to claim 11 , wherein a compressor is arranged in said delivery pipe ( 46 ) in order to pressurize the air for blowing.

13. An aircraft according to claim 12 , wherein said compressor ( 47 ) is electrically-driven.

14. An aircraft according to claim 12 , wherein said compressor ( 46 ) is mechanically-driven.

15. An aircraft according to claim 10 , wherein said air intake ( 60 ) is a static air intake ( 62 ), and said valve ( 41 ) is connected to said static air intake ( 62 ) via a delivery pipe ( 46 ) provided with a compressor ( 47 ) enabling said static air intake ( 62 ) to suck in said air for blowing.

16. An aircraft according to claim 1 , wherein the pitch of the blades ( 2 ) of a shrouded rotor ( 1 ) is adjusted by a pilot working pedals connected via transmission means ( 70 ) to a servo-control that acts on a pitch-change spider suitable for modifying said pitch of the blades, the blower means being controlled by said transmission means ( 70 ).

17. An aircraft according to claim 1 , wherein said aircraft is provided firstly with a reference pressure sensor ( 80 ) placed in a side face of said streamlined structure ( 10 ), and secondly with a detection pressure sensor ( 81 ) arranged at the periphery ( 21 ) of said duct ( 20 ) between the first and second air injection zones ( 44 , 45 ), and said aircraft includes control means ( 82 ) receiving first and second signals respectively from the reference and detection pressure sensors ( 80 and 81 ) in order to determine whether the blower means ( 40 ) needs to be activated.