Blade tip clearance control

The invention is particularly concerned with the control of blade tip clearance in high pressure compressors of gas turbine engines. The compressor comprises a rotor assembly having radially extending rotor blades and a stator assembly. The stator assembly comprises an inner casing and a cylindrical wall member spaced radially from the inner casing to form a chamber. The axial ends of the cylindrical wall member seal with and are moveable radially with respect to the inner casing. The casing has radially inner and radially outer stops to limit radial movement of the cylindrical wall member. The cylindrical wall member carries a ring of shroud segments which are spaced radially from the rotor blades by a clearance. A valve supplies relatively high pressure air from the downstream end of the compressor into the chamber to contract the cylindrical wall member onto the inner stops to give optimum clearance during cruise, or connects the chamber to relatively low pressure air in the fan duct by aperture in outer casing by pipes to allow the cylindrical wall member to expand to the outer stops to prevent rubbing during transients.