Blade positioning

1. A method of grinding blade tips of blades of a rotor stage of a gas turbine engine, wherein:

the rotor stage comprises:

a rotor disc and a plurality of radially extending blades, each blade comprising a root that cooperates with a slot in the rotor disc to secure the blade to the rotor disc, with a radial gap being formed between a radially inner surface of a respective root and the slot; and

the method comprises:

urging the blades radially outwardly by locating a fluid-tight bag such that the fluid-tight bag is positioned in the radial gap and inflating the fluid-tight bag; and

rotating the rotor stage relative to a grinding surface so as to grind any blade tips that contact the grinding surface during rotation,

wherein the grinding surface relative to which the rotor stage is rotated is a radially inner surface of a casing of the gas turbine engine.

2. A method of grinding blade tips according to claim 1 , wherein:

each root extends substantially in an axial direction of rotor stage;

the slot with which each root cooperates is a respective axially extending slot in the rotor disc, such that the radial gap formed by each root is a respective axially-extending radial gap; and

the method further comprises locating and inflating a separate fluid-tight bag into each respective axially-extending radial gap before rotating the rotor stage to grind the blade tips.

3. A method of grinding blade tips according to claim 2 , further comprising connecting more than one fluid-tight bag to a single fluid supply, the single fluid supply being used to inflate all of the fluid-tight bags to which it is connected.

4. A method of grinding blade tips according to claim 3 , wherein each fluid-tight bag is connected to the single fluid supply.

5. A method of grinding blade tips according to claim 2 , further comprising connecting each fluid-tight bag to a separate respective fluid supply.

6. A method of grinding blade tips according to claim 1 , wherein:

each root extends substantially in a circumferential direction of the rotor stage;

the slot with which each root cooperates is a single slot that extends circumferentially around the rotor disc, such that each root cooperates with a same slot and the radial gap is part of a continuous circumferentially-extending radial gap that extends around an entire circumference of the rotor disc; and

the method further comprises locating and inflating the fluid-tight bag, which is a single circumferentially extending fluid-tight bag, in the continuous circumferentially-extending radial gap before rotating the rotor stage.

7. A method of grinding blade tips according to claim 6 , wherein the circumferentially extending fluid-tight bag is circumferentially split so as to have a first end and a second end that are moveable relative to each other to adjust a circumference of the fluid-tight bag.

8. A method of grinding blade tips according to claim 7 , further comprising:

sliding the first end over the second end and adjusting a position of the first end relative to the second end so as to match the circumference of the fluid-tight bag to a circumference of the circumferentially extending slot.

9. A method of grinding blade tips according to claim 6 , wherein the circumferentially extending fluid-tight bag comprises a circumferential locking feature that extends radially between two blade platforms so as to prevent circumferential rotation of the blades relative to the slot.

10. A method of grinding blade tips according to claim 9 , wherein the circumferential locking feature comprises a one-way valve having a threaded outer portion, with a nut provided to the threaded portion.

11. A method of grinding blade tips according to claim 1 , wherein:

the fluid-tight bag is located into position before blades are assembled to the rotor disc to form the slot.

12. A method of grinding blade tips according to claim 1 , wherein:

the fluid-tight bag comprises a one-way valve; and

the method further comprises:

connecting the fluid-tight bag to a fluid supply via the one-way valve prior to the inflation step; and

disconnecting the fluid-tight bag from the fluid supply after the inflation step but prior to the rotation step.

13. A method of grinding blade tips according to claim 1 , further comprising removing the bag after the step of rotating the rotor stage to grind the blade tips.

14. A method of manufacturing a rotor stage of a gas turbine engine comprising:

providing the rotor disc with the plurality of radially extending blades, each blade extending from the root which is secured into the slot in the rotor disc to the tip at a radially outer extent of the blade; and

grinding the tips of the blades using the method of claim 1 .

15. A method of manufacturing a gas turbine engine comprising grinding the tips of blades of at least one rotor stage of the engine according to the method of claim 1 .