Rotor disc

1. A rotor disc having an enlarged radially outer rim defining a circumferentially-extending dovetail groove for housing the root portion of a rotor blade, the groove having a groove axis wherein the groove is unsymmetrical about a radially-extending plane through the groove axis, wherein

the dovetail groove has a pair of axially opposed curved surfaces on opposite sides of the radially-extending plane,

a first one of the pair of opposing curved surfaces is a multi-radii surface while a second one of the pair of opposing curved surfaces has a single radius, and

the multiple radii of the multi-radii surface vary with respect to a same direction from the multi-radii surface.

2. The rotor disc according to claim 1 wherein the dovetail groove has a restricted radially outer opening extending to an enlarged radially inner bulb profile having the pair of axially opposed curved surfaces.

3. The rotor disc according to claim 2 wherein the radius of the first one of the opposing curved surfaces is greater than the radius (radii) of the second one of the curved surfaces.

4. The rotor disc according to claim 2 wherein each curved surface has a respective radially outer section and radially inner section.

5. The rotor disc according to claim 4 wherein the radially outer sections and the radially inner sections of the curved surface are unsymmetrical about the radially-extending plane through the groove axis.

6. The rotor disc according to claim 4 wherein the radially outer sections or the radially inner sections of the curved surface are unsymmetrical about the radially-extending plane through the groove axis.

7. The rotor disc according to claim 1 wherein the opposing curved surfaces are unsymmetrical about the radially-extending plane through the groove axis.

8. The rotor disc according to claim 1 further comprising a bridging section for connection to an adjacent rotor disc.

9. The rotor disc according to claim 8 wherein the bridging section extends from the radially outer rim such that its radially outer surface is radially outwards of the opening of the groove.

10. The rotor disc according to claim 8 wherein the dovetail groove has a restricted radially outer opening extending to an enlarged radially inner bulb profile having the pair of axially opposed curved surfaces and wherein the curved surface proximal the bridging section has a greater radius of curvature than the curved surface distal the bridging section.

11. The rotor disc according to claim 8 wherein the dovetail groove has a restricted radially outer opening extending to an enlarged radially inner bulb profile having the pair of axially opposed curved surfaces and wherein the curved surface distal the bridging section has a multi-radii profile.

12. The rotor disc according to claim 8 wherein:

the dovetail groove has a restricted radially outer opening extending to an enlarged radially inner bulb profile having the pair of axially opposed curved surfaces;

each curved surface has a respective radially outer section and radially inner section; and

the radially inner sections of the curved surfaces each have a respective radius with the radially inner section of the curved surface proximal the bridging section having a greater radius than the curved surface distal the bridging section.

13. The rotor disc according to claim 8 wherein:

the dovetail groove has a restricted radially outer opening extending to an enlarged radially inner bulb profile having the pair of axially opposed curved surfaces;

each curved surface has a respective radially outer section and radially inner section; and

the radially outer section of the curved surface distal the bridging section has a multi-radii surface.

14. The rotor disc according to claim 8 wherein a distance from the groove to an exterior surface of the rim proximal the bridging section is less than the distance from the groove to an exterior surface of the rim distal the bridging section.

15. A compressor drum comprising at least one rotor disc according to claim 1 .

16. A compressor drum comprising two rotor discs according to claim 1 , each of the two rotor discs including a bridging section for connection to an adjacent rotor disc, the two rotor discs being arranged adjacent one another with the bridging sections joined.

17. A gas turbine engine comprising a rotor disc according to claim 1 .

18. A gas turbine engine comprising a compressor drum, the compressor drum including at least one rotor disc according to claim 1 .