Blade containment system for a turbofan engine

Abstract: Casing assembly for a gas turbine engine comprising an annular cross section casing around which a plurality of flexible material are wound. One or more rigid panels are positioned between the wound flexible material and the annular casing. The rigid panel provides enhanced containment of a detached fan blade or part of a fan blade by distributing the load of the detached blade across the wound flexible material.

Description: This invention relates to gas turbine fan duct casings and more particularly to an improved containment ring for use within or forming a part of the fan duct casing. Ducted fan gas turbine engines for powering aircraft conventionally comprise a core engine which drives a propulsive fan. The fan in turn, comprises a number of radially extending aerofoil blades mounted on a common hub and enclosed within a generally cylindrical casing. There is a remote possibility with such engines that part or all of a fan blade could become detached form the remainder of the fan. This may occur as the result of, for example, the engine ingestion of a bird or other foreign body. It is, therefore, extremely important that the blade is retained within the casing and does not pass through and cause damage to the engine. The use of containment rings for gas turbine engine casings is well known. Such rings have previously been manufactured from metal or alternatively glass fibre or carbon fibre etc. They have normally formed an integral part of the compressor casing. More recently the problem of fan containment has been addressed by winding strong fibrous material around a relatively thin fan casing. In the event that a fan blade becomes detached, it passes through the casing and is contained by the fibrous material. The problem associated with such fibre wrap is that there is a danger that a blade could in certain circumstance cut though the fibre wrap and thereby pass straight through. This problem is addressed by GB 2159886B by the provision of fibrous patches positioned between the layers of material. The patches wrap around the blade during it's passage through some of the material thus effectively blunting it's leading edge and impeding it's progress through the remaining layers. An additional difficulty with fan casing constructions is that in the interest of lightness the fan casing is made as thin as possible which leads to a lack of stiffness in the casing as a whole. This problem is particularly severe in large diameter fan casings. EP 0626 502 A1 discloses continuous lengths of material wound around rails which are mounted on the outer surface of the fan casing. The space between the rails is filled with discrete pieces of flexible material. Therefore, a detached blade initially breaks through the thin alloy casing, becomes 'blunted' by the discrete pieces which attached thereto and is then retained by the material wound around the rails. However retaining the blade within the fibrous wrap can in some circumstances be difficult to achieve. Another design solution is given in document US-A-5 447 411. It is therefore an aim of the present invention to alleviate the aforementioned problems and to provide improved fan blade containment apparatus. According to the present invention there is provided a gas turbine engine casing assembly comprising an annular cross-section casing configured to surround an annular array of rotary aerofoil blades, said casing defining a radially outer surface and positioned therewith a plurality of layers of flexible material wound as continuous lengths around said casing, at least one substantially rigid panel is interposed between said flexible material and said annular cross-section casing; characterised in that said rigid panel is frangible and is detachably mounted to said casing at spaced apart points on said panel. Advantageously the rigid panel serves to distribute the load of the detached blade, along the length of the carbon panel. This helps prevent the detached blade form cutting through the wound Kevlar® and enables the number of layers of flexible material to be reduced. Additionally the provision of the rigid panel also provides additional support in the event that the casing should develop a circumferential crack. In such circumstances the rigid panel would act as a secondary load path for the aerodynamic and inertia forces within the engine mountings. Preferably the rigid panel is moulded from carbon fibre or steel. The present invention will now be described by way of example, with reference to the accompanying drawing which is a sectioned side view of part of the fan casing of the ducted fan gas turbine engine in accordance with the present invention. A ducted gas turbine engine shown at is of generally conventional configuration. It comprises a core engine which drives a propulsive fan enclosed within a fan casing assembly. The exhaust from the fan is divided into two flows. The first and largest flow is directed to the exterior of the engine over an annular array of outlet guides located at the downstream end of the fan casing 13. The outlet guide vanes are generally radially extending and interconnect the fan casing with the core engine. The remainder of the air flow from the fan is directed into the core engine where it is compressed and mixed with fuel before being combusted to drive the core engine by conventional turbines. The fan comprises an annular array of radially extending aerofoil cross section blades 15 mounted on a common hub. During the operation of the ducted fan gas turbine engine, the core engine drives the fan at high speed. There is a remote chance that as a result of mechanical failure, all or part of one or more of the fan blades 15 could become detached from the remainder of the fan. Such mechanical failure could arise, for example, as the result of a foreign body, such as a bird, impacting the fan. The high rotational speed of the fan ensures that any such detached fan blade 15 is flung radially outwards with great force towards the fan casing assembly. It is extremely important from a safety point of view that the detached fan blade 15 should be contained within the fan casing 13. Thus it should not pass through the fan casing assembly 13 and cause damage to the aircraft upon which the engine is mounted. The fan casing 13 comprises an annular cross-section casing 17 which is supported from the core engine by means of outlet guide vanes. Flange 18 is provided at the upstream end of the casing to facilitate attachment of the casing to the engine intake and outlet guide vanes and to provide stiffening of the casing. The radially inner surface of the fan casing 17 supports an annular liner 22 which surrounds the radially outer extents of the fan blades 15. The liner 22 protrudes a significant distance radially inwardly so that it terminates immediately adjacent the radially outer tips 23 of the fan blades 15. The liner 22 also supports an annular flow defining structure 32. The majority of the liner 22 is formed from a metallic honeycomb material 24, part of which is axially inclined to follow the profile of the fan blade tips 23. The radially inner surface of the fan casing is, however, provided with a coating of a suitable abradable material. As the fan blades rotate during normal engine operation their tips 23 cut a path through the abradable coating. This ensures that the radial clearance between the liner 22 and the fan blade tips 23 is as small as possible, thereby minimising efficiency damaging air leakage across the blade tips 23. As well as minimising air leakage across the blade tips 23, the liner 22 performs two further important functions. Firstly it assists in the stiffening of fan casing 17. Clearly any lack of stiffness in the fan casing 17 could result in flexing of the liner 22 and the fan blade tips 23. Secondly, in the event that the whole or part of one of the fan blades 15 should become detached, the honeycomb construction of the liner 22 defines a region which the fan blade 15 or fan blade 15 portion can move into. This tends to minimise the possibly damaging interaction between the detached fan blade and the remaining fan blades 15 thereby causing additional engine damage. The fan casing 17 is of such a thickness that in the event of a detached blade 15 or fan blade 15 portion coming into contact with it, it is pierced. Thus although the fan casing 17 alone is not capable of containing a detached fan blade 15 or fan blade portion 15 it does absorb some of the kinetic energy of the blade 15. Containment of a detached fan blade 15 or fan blade portion 15 is provided by containment material which is provided around the radially inner surface of the fan casing 17. More specifically the portion of the radially outer surface of the fan casing 17 which is radially outwardly of the fan blade tips 23 and slightly upstream thereof, is provided with two annular axially spaced apart frangible rail members 26. The rail members 26 are attached to the fan casing 17 thereby providing additional stiffness of the casing 17. The axial space between the rails 26 is filled with discrete pieces of flexible material 27 woven from aromatic polyamide fibres known as KEVLAR (KEVLAR is a registered trademark of Dupont Ltd). The pieces 27 are held loosely together by cotton stitching. A number of continuous layers 28,29,31 of KEVLAR are wound around the fan casing 17 and the rails 26 on a region of the fan casing downstream of flange 18. These layers 28,29,31 provide blade containment. A number of carbon fibre panels 30 are interposed between the fibrous patches and the wound layers of KEVLAR. The panels are positioned over the rails 26 around which the layers 29,31 of KEVLAR are wound. In the event that a fan blade 15 or portion becomes detached it pierces the liner 22 and the fan casing 17, before encountering the discrete pieces 27. The pieces 27 which are impacted by the detached fan blade 15 or fan-blade portion 15 effectively blunt the sharp edges of the blade 15 by wrapping themselves around the blade 15. The blunted detached blade 15 then encounters the rigid panel 30 and detaches the rigid panel 30 from its fixed points 33. The detached panel 30, under the force of the blade 15, moves into the area of wound KEVLAR®. The impact of the moving blade on the KEVLAR® is spread over a larger area which helps to minimise the cutting forces of the blade. This also has the advantage that less KEVLAR® is required. Another advantage of the provision of a rigid panel 30 is that if the fan casing 17 itself should develop a circumferential crack due to the force from the impact of a detached blade 15, the panel 30 or panels will act as a secondary load path. Thus the carbon fibre panel 30 would accommodate the subsequent aerodynamic and inertia forces within the fan and engine mountings.

Claims: A gas turbine engine casing assembly (13) comprising an annular cross section casing (17) configured to surround an annular array of rotary aerofoil blades (15), said casing (17) defining a radially outer surface and positioned therewith a plurality of layers of flexible material (29,31) wound as continuous lengths around said casing (17), at least one substantially rigid panel (30) is interposed between said flexible material (29,31) and said annular cross section casing (17); characterised in that said rigid panel (30) is frangible and is detachably mounted to said casing (17) at spaced apart points (33) on said panel (30). A gas turbine engine casing assembly (13) as claimed in claim 1 characterised in that said rigid panel (30) is a carbon fibre panel. A gas turbine engine casing assembly (13) as claimed in claim 1 or 2 characterised in that said rigid panel (30) is fastened to the casing (17) at each end of said panel (30). A gas turbine engine casing assembly (13) as claimed in any preceding claim characterised in that said layers of flexible material (29,31) comprise woven aromatic polyamide fibres. A fan blade containment structure for a gas turbine engine comprising an annular cross section fan casing (17) configured to surround an annular array of rotary aerofoil blades (15), said casing (17) defining a radially outer surface and positioned therewith a plurality of layers of flexible material wound as continuous lengths around said casing (17), at least one substantially rigid panel (30) is interposed between said flexible material (29,31) and said annular cross section casing; characterised in that said rigid panel (30) is frangible and is detachably mounted to said casing (17) at spaced apart points (33) on said panel (30) .

Inventors: GOODWIN JEREMY PAUL, GOODWIN, JEREMY PAUL
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1997
Main classification: F01D21
IPC codes: F01D21