Blade clearance control

1. A turbine engine comprising:

a case structure;

a circumferentially segmented shroud within the case structure and comprising a plurality of segments, each segment having an inboard surface portion and mounted for movement between an inboard position and an outboard position;

a stage of circumferentially arrayed blades, each blade having an airfoil having an outboard tip in facing proximity to the shroud;

one or more springs biasing the shroud segments toward their inboard positions; and one or more valves positioned to vent one or more volumes so as to counter the bias of the one or more springs to shift the shroud segments to their outboard positions, the one or more volumes enclosed by the one or more springs and the shroud segments wherein the one or more springs contact the shroud at an upstream end and a downstream end.

2. The engine of claim 1 wherein:

a radial displacement between the inboard position and the outboard position is at least 250 μm.

3. The engine of claim 1 wherein:

deenergizing at least one of the one or more valves shifts the shroud segments to their outboard positions.

4. The engine of claim 1 further comprising:

a stage of circumferentially arrayed vanes, each vane having an airfoil having an outboard portion secured to an associated one of the segments.

5. The engine of claim 1 further comprising:

a bleed port in at least one of the shroud segments, the bleed port extending from a core flowpath surrounded by the shroud;

an outlet duct extending inward from the case structure; and

a collar portion of said at least one of the shroud segments receiving a distal portion of the outlet duct in sliding engagement.

6. The engine of claim 1 further comprising:

a plurality of seals, each seal between an associated pair of the segments.

7. The engine of claim 1 wherein:

each of a group of the plurality of segments has at least an associated first spring of the one or more springs engaged to an outboard surface portion of the segment.

8. The engine of claim 7 wherein:

for each of said group, the first spring consists essentially of sheet metal.

9. The engine of claim 7 wherein:

for a given ring of the segments, an associated group of the one or more springs at least partially enclose an associated volume of the one or more volumes.

10. The engine of claim 1 further comprising:

a control system coupled to the one or more valves and programmed to open the one or more valves during a reburst condition.

11. The engine of claim 1 wherein:

the one or more valves include at least one normally closed solenoid valve.

12. The engine of claim 1 wherein:

the one or more valves are positioned to vent the one or volumes to a fan duct.

13. A turbine engine comprising:

a case structure;

a circumferentially segmented shroud within the case structure and comprising a plurality of segments, each segment having an inboard surface portion and mounted for movement between an inboard position and an outboard position;

a stage of circumferentially arrayed blades, each blade having an airfoil having an outboard tip in facing proximity to the shroud;

a first spring contacting the shroud at an upstream end and a second spring contacting the shroud at a downstream end, the first and second springs biasing the shroud segments toward their inboard positions; and one or more valves positioned to vent one or more volumes so as to counter the bias of the first and second springs to shift the shroud segments to their outboard positions.

14. The engine of claim 13 further comprising:

a control system coupled to the one or more valves and programmed to open the one or more valves during a reburst condition.

15. The engine of claim 13 wherein:

the one or more valves are positioned to vent the one or volumes to a fan duct.

16. A method for operating a gas turbine engine of claim 1 , the method comprising:

determining an impending condition characterized by a loss of blade tip clearance of the blades of the stage; and responsive to the determination, venting the one or more volumes so as to counter a bias of the one or more springs to outwardly shift the segments to maintain the blade tip clearance.

17. The method of claim 16 wherein:

the shift further maintains a vane tip clearance.

18. The method of claim 16 wherein:

the shift is accomplished in 2.0 seconds or less and has a radial amplitude of at least 250 μm.

19. The method of claim 16 wherein:

the venting comprises venting to a fan duct.

20. The method of claim 16 wherein:

the determining comprises monitoring throttle condition so as to identify a reburst situation.

21. The system of claim 1 further comprising:

sealing members sealing between adjacent said springs.

22. The engine of claim 21 wherein:

the sealing members are sleeves spanning junctions between adjacent said springs and bodies of the associated said shroud segments to seal a subchamber, said subchamber being one of said volumes.

23. The engine of claim 1 wherein:

each segment has an associated bleed port feeding from a core flowpath surrounded by the shroud into a bleed plenum surrounded by a wall within one of the volumes, the one or more valves positioned to vent said one of the volumes.

24. The engine of claim 1 wherein:

the one or more springs include a fore portion abutting an outboard surface of the associated segment and an aft portion abutting the outboard surface of the associated segment.

25. The engine of claim 24 wherein:

the fore portion and aft portion are of a single piece bridging the volume.

26. The engine of claim 13 further comprising:

a plurality of seals, each seal between an associated pair of the segments.

27. The engine of claim 13 wherein:

each said segment has a single associated first spring and a single associated said second spring.

28. The engine of claim 13 wherein:

each said first spring and second spring consist essentially of sheet metal.