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A system of contra-rotating propellers for an aircraft jet engine, including: a free power turbine; a first and second propeller, contra-rotating, the first propeller being placed in a predetermined direction relative to the second propeller, each including a hub, an outer ferrule, and coupling arms

A system of contra-rotating propellers for an aircraft jet engine, including: a free power turbine; a first and second propeller, contra-rotating, the first propeller being placed in a predetermined direction relative to the second propeller, each including a hub, an outer ferrule, and coupling arms linking the two; and a casing inserted between the free turbine and the propellers, the propellers arranged in the predetermined direction relative to the casing and the free turbine arranged in the opposite direction relative to the casing, the casing including a casing extension in the given direction, rotatably supporting the hub of the propeller. The coupling arms extend in the opposite direction, radially towards the outside.

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1. A system of contra-rotating propellers for an aircraft jet engine, comprising: a power free turbine;a first propeller and a second propeller, contra-rotating configured to be rotated around a longitudinal axis of the system of propellers, the first propeller being placed in a downstream direction

1. A system of contra-rotating propellers for an aircraft jet engine, comprising: a power free turbine;a first propeller and a second propeller, contra-rotating configured to be rotated around a longitudinal axis of the system of propellers, the first propeller being placed in a downstream direction relative to the second propeller, each of the first and second propellers comprising a hub centered on the longitudinal axis, an outer ferrule being arranged concentrically thereto and participating in an outward radial definition of a main annular tunnel, and coupling arms linking the outer ferrule to the hub;a mechanical transmission device driven by the power free turbine and driving the first and second propellers; anda casing inserted between the power free turbine and the first and second propellers, the first and second propellers being arranged in the downstream direction relative to the casing and the power free turbine being arranged in an upstream direction relative to the casing, the casing including a casing extension extending in the downstream direction, the casing extension rotatably supporting the hub of at least one of the propellers,wherein the coupling arms of the second propeller extend in the upstream direction and radially outward. 2. The system of contra-rotating propellers according to claim 1, wherein at least one portion of each coupling arm belonging to the second propeller is located in the upstream direction relative to a rolling bearing inserted between the casing extension and the hub of the second propeller. 3. The system of contra-rotating propellers according to claim 1, wherein the second propeller comprises a plurality of blades each mounted so as to be steered in incidence around a pivot axis, the pivot axis to be situated in the upstream direction relative to a rolling bearing inserted between the casing extension and the hub of the second propeller. 4. The system of contra-rotating propellers according to claim 1, wherein the hub of the second propeller rotatably supports the hub of the first propeller. 5. The system of contra-rotating propellers according to claim 1, wherein the coupling arms of the first propeller extend in the downstream direction and radially outward. 6. The system of contra-rotating propellers according to claim 5, wherein the coupling arms of the first propeller extend in the downstream direction and radially outward so as to present an angle with respect to a radial direction of the engine. 7. The system of contra-rotating propellers according to claim 6, wherein the angle is between 20 and 50 degrees. 8. The system of contra-rotating propellers according to claim 1, wherein the coupling arms of the first propeller support a first intermediate ferrule participating in a radial inward definition of the main annular tunnel, and the coupling arms of the second propeller support a second intermediate ferrule participating in the radial inward definition of the main annular tunnel, the first intermediate ferrule being situated in continuity of the second ferrule in the downstream direction. 9. The system of contra-rotating propellers according to claim 8, wherein the coupling arm of the first propeller and its associated outer and intermediate ferrules form a part made in a single piece, and the coupling arms of the second propeller and its associated outer and intermediate ferrules also form a part made in a single piece. 10. The system of contra-rotating propellers according to claim 1, wherein the mechanical transmission device comprises an epicyclic gearing including a sun gear centered on the longitudinal axis and driven by a rotor of the power free turbine, at least one satellite meshing with the sun gear, a planet carrier driving the first propeller, and a crown meshing with each satellite and driving the second propeller. 11. The system of contra-rotating propellers according to claim 10, wherein the planet carrier is secured to the hub of the first propeller, and the crown is secured to the hub of the second propeller. 12. The system of contra-rotating propellers according to claim 1, wherein the mechanical transmission device is housed in a hub of the casing. 13. The system of contra-rotating propellers according to claim 1, wherein each outer ferrule supports a retaining ring of blades of the respective propeller. 14. An aircraft turbomachine comprising a contra-rotating propeller system according to claim 1. 15. The turbomachine according to claim 14, wherein the turbomachine is an open rotor. 16. The system of contra-rotating propellers according to claim 1, wherein the coupling arms of the second propeller extend in the upstream direction and radially outward so as to present an angle with respect to a radial direction of the engine. 17. The system of contra-rotating propellers according to claim 16, wherein the angle is between 20 and 50 degrees. 18. A system of contra-rotating propellers for an aircraft jet engine, comprising: a power free turbine;a first propeller and a second propeller, contra-rotating configured to be rotated around a longitudinal axis of the system of propellers, the first propeller being placed in an upstream direction relative to the second propeller, each of the first and second propellers comprising a hub centered on the longitudinal axis, an outer ferrule being arranged concentrically thereto and participating in an outward radial definition of a main annular tunnel, and coupling arms linking the outer ferrule to the hub;a mechanical transmission device driven by the power free turbine and driving the first and second propellers; anda casing inserted between the power free turbine and the first and second propellers, the first and second propellers being arranged in the upstream direction relative to the casing and the power free turbine being arranged in a downstream direction relative to the casing, the casing including a casing extension extending in the upstream direction, the casing extension rotatably supporting the hub of at least one of the propellers,wherein the coupling arms of the second propeller extend in the downstream direction and radially outward.