Annulus filler

1. An annulus filler for mounting to a rotor disc in a gas turbine engine, the annulus filler including:

a body portion having:

a top wall arranged to bridge the gap between two adjacent blades extending from the rotor disc and to define an air flow surface for air passing between the blades; and

one or more support components extending from the top wall and configured to fix the annulus filler to the rotor disc; and

a separately formed rear interface portion, fixed to the body portion adjacent to a down-flow end of the top wall,

wherein the rear interface portion forms a lip projecting below the top wall;

wherein the lip is configured to form an interface with a rear element of or coupled to the rotor disc, such that the top wall and rear element are flush to provide a smooth air flow surface; and

wherein the one or more support components are located underneath the lip in order to support the lip.

2. The annulus filler of claim 1 , wherein the lip includes a secondary surface extending in the same direction as the top wall of the body portion, and wherein the secondary surface extends underneath the rear element.

3. The annulus filler of claim 2 , wherein the secondary surface includes one or more through-holes, such that air can pass from radially above the annulus filler to a volume formed beneath the top wall.

4. The annulus filler of claim 1 , wherein the rear interface portion includes an abutment part, arranged to abut an end of the top wall, such that the lip is wholly formed in the rear interface portion.

5. The annulus filler of claim 1 , wherein the rear interface portion includes a bonding surface extending parallel or substantially parallel to the top wall, and arranged to overlap the top wall.

6. The annulus filler of claim 5 , wherein the bonding surface overlaps the top wall on an underside of the top wall.

7. The annulus filler of claim 5 , wherein the bonding surface overlaps the top wall above the top wall, and wherein the top wall and bonding surface are arranged to provide a smooth air flow surface.

8. The annulus filler of claim 1 , wherein the rear interface portion comprises mounting portions arranged to secure the rear interface portion to the body portion.

9. The annulus filler of claim 8 , wherein a one of the one or more support components includes a pair of sidewalls extending from the top wall, wherein the sidewalls extend beyond a down-flow end of the top wall, and underneath the rear interface portion, and wherein each mounting portion includes a pair of projections arranged to receive one of the sidewalls between them.

10. The annulus filler of claim 1 , wherein the body portion comprises a moulded composite material.

11. The annulus filler of claim 1 , wherein the rear interface portion and body portion are formed of different materials.

12. The annulus filler of claim 1 , wherein the rear interface portion has one or more of the following properties:

the rear interface portion is formed of a material that forms a seal with the rear element;

the rear interface portion is formed of a material that is compatible with a material of the body portion, in order to join the portions using non-mechanical fixing methods; and

the rear interface portion is formed of a material that is at least as resistant to friction wear than a material of the body portion.

13. The annulus filler of claim 1 , wherein the rear interface portion is fixed to the body portion along one or more contact surfaces between the rear interface portion and the body portion.

14. A gas turbine engine having a propulsion fan, one or more compressor stages and one or more turbine stages, wherein the propulsion fan includes a rotor disc, a plurality of blades arranged circumferentially around the rotor disc, and an annulus filler according to claim 1 provided between each of the blades.

15. A method of manufacturing an annulus filler, the method including:

forming a body portion having: a top wall arranged to bridge the gap between two adjacent blades extending from the rotor disc and to define an air flow surface for air passing between the blades; and one or more support components extending from the top wall and configured to fix the annulus filler to the rotor disc;

forming a separate rear interface portion; and

fixing the rear interface portion to the body portion adjacent to a down-flow end of the top wall,

wherein the rear interface portion forms a lip projecting downward from the top wall;

wherein the lip is configured to form an interface with a rear element of or coupled to the rotor disc, such that the top wall and rear element provide a smooth air flow surface; and

wherein the one or more support components are located underneath the lip in order to support the lip.

16. The method of manufacturing an annulus filler as claimed in claim 15 , wherein forming the body portion comprises forming the body portion by a moulding process, using a rigid mandrel in a volume underneath the top wall.

17. The method of claims 16 , wherein the mandrel comprises a single unitary piece, and wherein the method comprises:

after forming the body portion and prior to fixing the rear interface portion to the body portion, withdrawing the mandrel in a down-flow direction, with respect to the top wall.

18. The method of claim 15 , wherein forming the body portion includes:

forming the body portion with the top wall extending to the down-flow end of the annulus filler; and

removing a portion of the top wall, adjacent the down-flow end.

19. The method of claim 15 , wherein fixing the rear interface portion to the body portion comprises fixing the rear interface portion to the body portion along one or more contact surfaces between the rear interface portion and the body portion.