Turboprop having a propeller made up of variable-pitch blades

1. A turboprop including at least one set of variable pitch rotary blades constrained to rotate with a rotary support, wherein, in order to vary its pitch, each blade of said set is coupled to a separate hydraulic rotary actuator carried by said rotary support,

wherein said rotary actuator is of the dual control type, being controlled by two pressurized hydraulic fluid circuits, the pressure of the hydraulic fluid in each circuit being adjustable,

wherein a shaft of said rotary actuator is coupled to a self-blocking locking system, and

wherein said locking system comprises unblocking means controlled by a difference between the hydraulic fluid pressures in the two above-mentioned circuits.

2. A turboprop according to claim 1 , wherein said rotary actuator includes a rotary shaft secured to a swivel pin of said blade.

3. A turboprop according to claim 2 , wherein said rotary actuator has a plurality of adjacent cavities distributed circumferentially around said shaft, and wherein each cavity contains a divider secured to said shaft and subdividing said cavity into two chambers, analogous chambers in all of the cavities being connected to respective ones of the two pressurized hydraulic fluid circuits.

4. A turboprop according to claim 1 , wherein said locking system comprises a dual declutching device with disks interposed between two rectilinear stroke actuators, each actuator comprising a cylinder that is stationary relative to said rotary actuator and two chambers that are connected to the two above-mentioned circuits; the dual declutching device, the rectilinear stroke actuators, and the rotary actuator being arranged on a common axis.

5. A turboprop according to claim 4 , wherein said declutching device comprises a central dual friction disk prevented from moving in translation and two disks movable in translation on either side of said central disk and connected respectively to pistons of the two rectilinear stroke actuators.

6. A turboprop according to claim 5 , wherein each rectilinear stroke actuator piston includes a cavity slidable inside said cylinder and an end wall fastened to the central dual disk and slidably mounted inside said cavity.

7. A turboprop according to claim 6 , wherein a spring is installed in said cylinder of each rectilinear stroke actuator to urge said piston secured to the corresponding moving disk towards said central dual disk.

8. A turboprop according to claim 4 , wherein said dual declutching device is fitted with friction disks.

9. A turboprop according to claim 4 , wherein said dual declutching device is fitted with disks co-operating by inter-engaging, radial ribs.

10. A turboprop including at least one set of variable pitch rotary blades constrained to rotate with a rotary support, wherein each blade of said set is coupled to a separate hydraulic rotary actuator carried by said rotary support,

wherein said rotary actuator is controlled by pressurized hydraulic fluid circuits, the pressure of the hydraulic fluid in each circuit being adjustable,

wherein a shaft of said rotary actuator is coupled to a self-blocking locking system, and

wherein said locking system comprises unblocking mechanism controlled by a difference in fluid pressure in said hydraulic fluid circuits.

11. A turboprop according to claim 10 , wherein said rotary actuator has a plurality of adjacent cavities distributed circumferentially.

12. A turboprop according to claim 11 , wherein each cavity is subdivided into two chambers, each chamber in each cavity being respectively connected to one of the two pressurized hydraulic fluid circuits.

13. A turboprop according to claim 12 , wherein each cavity is subdivided into said two chambers by a divider that slides in a leaktight manner against a wall of said rotary actuator.