Aircraft propulsion system and a method of controlling the same

1. An aircraft propulsion system comprising:

a first propulsive rotor assembly rotatable about an axis of rotation and comprising a plurality of first rotor blades; and

a first vane assembly rotationally fixed about the axis of rotation, located adjacent to the first propulsive rotor assembly and arranged circumferentially around the axis of rotation;

the first vane assembly being configured to direct an airflow entering the first propulsive rotor assembly so as to reduce the velocity of the redirected airflow relative to the first rotor blades, and

wherein the first vane assembly comprises a plurality of vanes and at least one of the plurality of vanes is formed as an aerofoil.

2. The aircraft propulsion system as claimed in claim 1 , further comprising a second propulsive rotor assembly comprising a plurality of second rotor blades and positioned on an opposite side of the first propulsive rotor assembly to the first vane assembly, and rotatable about the axis of rotation.

3. The aircraft propulsion system as claimed in claim 2 wherein the first propulsive rotor assembly and the second propulsive rotor assembly rotate in opposite directions to one another.

4. The aircraft propulsion system as claimed in claim 2 further comprising:

a second vane assembly rotationally fixed about the axis of rotation, arranged circumferentially around the axis of rotation, and positioned between the first and second propulsive rotor assemblies;

the second vane assembly being configured to direct an airflow exiting the first propulsive rotor assembly so as to reduce the velocity of the redirected airflow relative to the second rotor blades.

5. The aircraft propulsion system as claimed in claim 1 wherein the plurality of vanes is arranged asymmetrically around the axis of rotation.

6. The aircraft propulsion system as claimed in claim 1 wherein each of the plurality of vanes has at least one of a span, chord, camber, circumferential lean or sweep which differs from the corresponding span, chord, camber, circumferential lean or sweep of an adjacent vane.

7. The aircraft propulsion system as claimed in claim 1 wherein one or more of the plurality of vanes comprises a winglet formed on a radially outermost portion thereof.

8. The aircraft propulsion system as claimed in claim 1 wherein one or more of the plurality of vanes comprises a respective one or more variable pitch vanes.

9. The aircraft propulsion system as claimed in claim 1 further comprising a housing,

wherein at least one of the plurality of vanes is movable between

a first, stowed position in which the at least one vane is accommodated at least partially within the housing, and

a second, deployed position in which the at least one vane protrudes from the housing and extends into the airflow entering the first rotor assembly.

10. A nacelle comprising an aircraft propulsion system as claimed in claim 1 , wherein an axis of the nacelle is aligned with the axis of rotation.

11. A method of controlling an aircraft propulsion system comprising a first propulsive rotor assembly rotatable about an axis of rotation and comprising a plurality of first rotor blades and a plurality of vanes located adjacent to the first propulsive rotor assembly and arranged circumferentially around the axis of rotation, and the aircraft propulsion system further comprising a housing, the plurality of vanes being accommodated at least partially within the housing, the method comprising the steps of:

moving each of the plurality of vanes from a first, stowed position to a second, deployed position in which each vane extends from the housing into an airflow entering the first rotor assembly; and

collectively changing a pitch of the plurality of vanes so as to direct an airflow entering the first propulsive rotor assembly so as to reduce the velocity of the redirected airflow relative to the first rotor blades.

12. The method as claimed in claim 11 , wherein the step of moving each of the plurality of vanes from a first, stowed position to a second, deployed position in which each vane extends from the housing into an airflow entering the first propulsive rotor assembly comprises the step of:

radially extending each of the plurality of vanes from a first, stowed position to a second, deployed position in which each vane extends from the housing into an airflow entering the first propulsive rotor assembly.

13. The method as claimed in claim 11 , wherein the step of moving each of the plurality of vanes from a first, stowed position to a second, deployed position in which each vane extends from the housing into an airflow entering the first propulsive rotor assembly comprises the step of:

rotating each of the plurality of vanes from a first, stowed position to a second, deployed position in which each vane extends from the housing into an airflow entering the first propulsive rotor assembly.

14. A method of reducing the noise generated by an aircraft propulsion system comprising at least one propulsive rotor assembly having a plurality of rotor blades and rotatable about an axis of rotation, the method comprising the steps of:

installing a plurality of vanes in a circumferential arrangement around the axis of rotation to direct an airflow entering the first propulsive rotor assembly so as to reduce the velocity of the redirected airflow relative to the rotor blades; and

modifying the propulsive rotor blade assembly by adding at least one additional rotor blade to thereby reduce the noise generated by the propulsive rotor assembly when in use.