Method of forming an inflated aerofoil

1. A method of forming an inflated aerofoil, the method comprising the steps of: forming a layered, planar pre-form; providing at least one stress-relieving opening through the pre-form; hot creep forming and inflating the pre-form to form an intermediate aerofoil; and subsequently removing material from the intermediate aerofoil, including at least a region containing the or each stress-relieving opening, to form a finished aerofoil.

2. A method according to claim 1 , further including the step of providing at least one datum region on the pre-form, for use in locating the pre-form during said hot creep forming and inflating steps, wherein said step of forming said at least one stress-relieving opening involves forming the or each opening in or adjacent a said datum region, and said step of removing material from the intermediate aerofoil involves removing the or each datum region.

3. A method according to claim 1 , further including a step of defining on the pre-form a nominal profile of the aerofoil to be formed.

4. A method according to claim 3 , wherein said step of defining a nominal profile involves machining the nominal profile on at least one outwardly directed face of the pre-form.

5. A method according to claim 2 , further including a step of defining on the pre-form a nominal profile of the aerofoil to be formed, wherein the or at least one said stress-relieving opening is provided through the pre-form at a position located between a said datum region and a main region of the nominal profile of the aerofoil.

6. A method according to claim 1 , wherein the or at least one said stress-relieving opening is provided in the form of a slot through the pre-form.

7. A method according to claim 6 , wherein the or at least one said slot is located entirely within the periphery of said pre-form.

8. A method according to claim 6 , wherein the or at least one said slot has at least one end which is open to the periphery of the pre-form.

9. A method according to claim 6 , wherein the or at least one said slot is elongate and substantially straight.

10. A method according to claim 6 , wherein the or at least one said slot is furcated.

11. A method according to claim 1 , further comprising a step of profiling the pre-form to define a leading edge and a trailing edge for the aerofoil.

12. A method according to claim 11 , wherein said step of profiling involves cutting the pre-form to define the leading edge and the trailing edge.

13. A method according to claim 2 , further comprising a step of profiling the pre-form to define a leading edge and a trailing edge for the aerofoil, wherein said step of profiling involves cutting the pre-form to define a peripheral edge of the or each datum region.

14. A method according to claim 1 , wherein said step of providing a layered, planar preform involves bonding together a pair of outer skins.

15. A method according to claim 14 , wherein said outer skins comprise titanium or aluminium.

16. A method according to claim 14 , wherein said outer skins are diffusion bonded to one another.

17. A method according to claim 14 , further comprising a step of applying a stop-off material in a predefined region between the outer skins prior to said step of bonding the skins together, to thereby define an inflatable region within the pre-form.

18. A method according to claim 1 , wherein the aerofoil is a blade or a vane for a gas turbine engine.

19. A method according to claim 1 , wherein the aerofoil is an outlet guide vane for a gas turbine engine.

20. A gas turbine engine comprising a blade or vane formed using the method of claim 1 .