Method of securing a liner panel to a casing

A liner panel, such as a cassette 120 of a fan track liner of a gas turbine engine, is secured to a casing of the engine at flanges 124 and 126. Foam pads 42 are disposed between the casing 40 and the cassette 120 to support the cassette 120 from the casing 40. The pads 42 may be made from a reticulated polyurethane foam which is sufficiently resilient to be compressed and to expand to remain in contact with the cassette 120 and the casing 40 as the cassette 120 moves towards and away from the casing 40 during the assembly process. The foam is impregnated with a hardener composition which is uncured during installation of the cassette 120, but subsequently cures to increase the rigidity of the pads 42.

This invention relates to a method of securing a liner panel to a casing and is particularly, although not exclusively, concerned with securing a fan track liner to a fan casing of a turbomachine such as a gas turbine engine.

It is usual for the blades of the fan of a turbo fan engine to run along a fan track liner supported by a fan casing. The fan track liner is designed to be abradable by the blade tips, and is consequently susceptible to damage. It is desirable for the fan track liner to be easily removable from the fan casing without damage to the casing that can occur if the liners have been securely bonded to the casing so that a damaged liner, or a section of a damaged liner, can be replaced.

Fan track liners typically comprise a panel in the form of a cassette or tray which supports the abradable liner structure and may also include portions having noise attenuation properties. In the interests of weight saving, the liner panel may be relatively thin, and therefore flexible, and so requires support from the fan casing to resist the loads applied to it in operation of the engine, for example loads generated by the fan blades rubbing against the fan track liner. A stack up of tolerances means that it is not possible to ensure intimate contact with the casing over the full extent of the cassette, and so it is known to apply a layer of an compressible material between the cassette and the fan casing. The compressible material needs to be relatively stiff to provide the required support. There is a tendency for the compressible material to be over-compressed during installation of the fan track liner so that the thickness does not recover sufficiently to fill the space between the cassette and the casing in the final assembled position. It is consequently difficult to achieve the required support over the desired extent of the liner panel because gaps are present between the compressible material and the casing. The gaps may be filled with a filler adhesive but the quantity used can vary between engine installations and between operatives, giving the final product weight variation.

According to the present invention there is provided a method of securing a liner panel to a casing of a turbomachine with a foam material disposed between the liner panel and the casing, in which method the foam material is impregnated with a curable hardener composition, and the liner panel is secured to the casing to compress the foam material between the liner panel and the casing while the hardener composition is in an uncured condition, the hardener composition subsequently being cured to increase the rigidity of the foam material.

A method in accordance with the present invention thus enables the use of a foam material which is sufficiently resilient to recover after over-compression during installation of the liner panel, but is sufficiently rigid, after curing of the hardener composition, to provide support for the liner panel from the casing. The relatively high resilience can provide additional manufacturing tolerance which assists the locating of the panel before it is hardened and secured for normal use.

The foam material may be an open cell foam material, such as a reticulated foam. In the absence of the hardener composition, the foam may be a relatively high flexibility, low density foam. For example, the foam material may have a density not greater than 100 kg/m3, for example less than 10 kg/m3. The flexibility of the foam, as indicated by its compression force deflection (CFD—the force per unit area required to compress a sample of the foam to 25% of its original thickness), which may be less than 30 kPa, for example less than 20 kPa, less than 10 kPa, or less than 5 kPa.

The foam material may be a polyurethane based foam material. The hardener may be an epoxy adhesive, for example a syntactic adhesive. The hardener composition may be in the form of a viscous liquid or paste when uncured. The hardener may be curable by heating, for example to a relatively low temperature such as a temperature below 100° C., such as 50° C. The hardener composition may include filler or spacer materials, such as small hollow glass beads in order to reduce the density of the hardener composition.

A release coating may be applied to the surface of the casing contacted by the foam material, so that, upon removal of the liner panel, the foam material releases easily from the casing and remains attached to the liner panel.

During the securing of the liner panel to the casing, the liner panel may be displaced successively towards and away from the casing, respectively to compress the foam material and subsequently to allow the foam material to expand while the hardener composition is uncured, curing of the hardener material occurring while the foam material is in the expanded condition, and the liner in its installed position. Since the foam material is resilient while the hardener material remains uncured, it can be compressed, and subsequently expanded, to maintain contact with both the liner panel and the casing so that, following curing of the hardener material, the foam material forms a relatively rigid “bridge” between the casing and the liner panel, so that the liner panel is adequately supported by the casing.

The foam material may be in the form of at least one pad to support the liner panel at one or more desired locations. There may be four or more pads. There may be a pad located at multiple corners of the liner panel with one or more centrally located pads. The or each pad may have a diameter, or equivalent maximum transverse dimension, which is not less than 1 cm and not more than 5 cm, for example not less than 2 cm and not more than 3 cm. More than one pad size or shape may be used. On completion of the method, when the liner panel is fully secured to the casing and the hardener composition has cured, the thickness of the or each pad may be not less than 0.2 cm and not more than 2 cm, for example not less than 0.3 cm and not more than 1 cm.

The liner or casing may be provided with markings or features to assist in the correct location, orientation and number of pads applied.

A turbofan gas turbine engine10, as shown inFIG. 1, comprises in flow series an intake12, a fan section14, a compressor section16, a combustor section18, a turbine section20and an exhaust22. The turbine section20comprises one or more turbines arranged to drive one or more compressors in the compressor section16via shafts. The turbine section20also comprises a turbine to drive the fan blades34within the fan section14via a shaft. The fan section14comprises a fan duct24defined partially by a fan casing26. The fan duct24has an outlet at its axially downstream end. The fan casing26is secured to the core engine casing36by a plurality of radially extending fan outlet guide vanes30. The fan casing26surrounds a fan disc32, which carries a plurality of circumferentially spaced radially extending fan blades34. The fan disc32and fan blades34rotate about the axis X of the gas turbine engine10, substantially in a plane Y perpendicular to the axis X. The fan casing26also comprises a fan blade containment assembly37, which is arranged substantially in the plane Y of the fan blades34. The fan blades34each have a leading edge33, a trailing edge35and a tip38. The fan casing26and fan blade containment assembly37is shown more clearly inFIG. 2. The fan blade containment assembly37comprises a metal cylindrical, or frustoconical, casing40. The metal casing40is connected by means which are not shown in the Figures to the rest of the fan casing26. The metal casing40comprises an annular hook54which is positioned axially upstream of the leading edge33of the tip38of the fan blade34. The annular hook54extends in an axially downstream direction towards the fan blade34.

A fan blade track panel71is installed in the metal casing40. The panel71is one of a plurality of fan blade track panels71which are arranged circumferentially and axially along the inner surface of the metal casing40to form an annular fan blade track liner. Each fan blade track panel71comprises a skin74and a structure76to form an abradable structure. The skin74of the abradable lining defines the flow path through the fan duct24. The skin74of each fan blade track panel71comprises a composite material, for example fibre reinforced plastic e.g. glass fibre reinforced epoxy resin, forming a layer between the honeycomb76and the abradable layer on the skin74. The structure76comprises a honeycomb structure, for example an aluminium honeycomb, a titanium honeycomb, a composite material honeycomb, a resin impregnated paper honeycomb or other suitable honeycomb. The composite material honeycomb may comprise fibre reinforced plastic e.g. glass fibre reinforced epoxy resin. There may be an abradable material on the skin74. The structure76of each fan blade track panel71is secured to a liner panel or cassette120by adhesive bonding.

The cassette120is secured to the casing40at a forward axially extending flange124on the axially upstream end of the cassette120and at an aft axially extending flange126on the axially downstream end of the cassette120. The forward flange124engages the hook to secure the upstream end of the cassette120to the casing40. An annular resilient sealing member132acts between the hook54and the flange124to bias the cassette120radially outwardly into contact with the inner surface of the casing40.

The casing40has a plurality of circumferentially spaced inserts136extending radially inwardly from the inner surface of the casing40. The inserts136have threaded apertures. The aft flange126is secured by one or more countersunk bolts or screws140to the inserts136to secure the downstream end of the cassette120to the casing40.

The cassette120comprises a composite material, for example graphite reinforced plastics. Although not apparent inFIG. 2, the cassette120is not in direct contact with the inner surface of the casing40. Instead, as shown inFIG. 3, pads42are disposed between the cassette120and the casing40.

Each cassette120is installed by moving it in an axially upstream direction so that the forward flange124locates on the hook54. Then the aft flange126is located on the inserts136and secured by the bolts or screws.

Three pads42are shown inFIG. 3. The pads42may be arranged in any desired disposition that provides adequate support for the cassette120on the casing40. For example, each cassette120(representing a segment of the fan blade track liner) may be provided with five of the pads42, one disposed generally towards each corner of the portion of the cassette120which lies close to the casing40, and one disposed generally centrally of that portion. The use of individual pads42of defined size and weight enables precise positioning of the pads42, to achieve quality control and engine build uniformity. Locating features may be provided on the radially outer side of the cassette tray120to aid in the positioning of the pads42. Each pad42comprises an open-cell, reticulated, polyurethane foam material having a relatively low density and relatively high flexibility. By way of example, the foam material may have a density less than 10 kg/m3, and a CFD which is less than 10 kPa and possibly less than 5 kPa. In the completed structure as shown inFIGS. 2 and 3, the foam material is impregnated with a cured hardener composition which may, for example, be a viscous hard setting adhesive such as a two-part epoxy syntactic adhesive which is in the form of a paste when uncured. A suitable adhesive is available from Hexcel Corporation under the name Hexcel Redux 830. When cured, the adhesive becomes rigid, having a modulus in excess of 1000 MPa, for example 1500 MPa and a Shore D hardness in excess of 70. The cured adhesive thus serves to reduce or eliminate the resilience of the foam material so that it provides substantially rigid support for the cassette120.

During the assembly process as described above, the pads42, with the hardener composition in an uncured condition, are applied to the back (ie radially outer) surface of the cassette120before the cassette120is offered to the casing40. A release coating, for example a mould release agent such as Chemlease41available from Chem-Trend LP is applied to the inner surface of the casing40at least at the locations which will be engaged by the pads42. As an alternative, the release film or coating may be applied to the pads42, with or without an additional adhesive. The forward flange124is then engaged with the hook54and the cassette120is manipulated to engage the aft flange126with the insert136so that the bolts or screws140can be inserted. During this manipulation, the cassette120may be moved towards and away from the casing40by a small distance one or more times before it reaches its final position. During this process the pads42may be compressed between the cassette120and the casing40to different extents, as shown inFIGS. 4A,4B and4C.FIG. 4Arepresents the condition before the pad40first contacts the casing42. The pad42is thus in its initial, uncompressed state. As shown inFIG. 4B, manipulation of the cassette20with respect to the casing40during the installation procedure may cause the pad42to be substantially compressed before it is allowed to expand again as the cassette120moves away from the casing40to its final position as shown inFIG. 4C.

Because the syntactic adhesive with which the foam material of the pad42is impregnated is uncured during the installation of the cassette120, it retains the resilience of the basic polyurethane foam material. Consequently, as the cassette120moves away from the casing40(ie from the position shown inFIG. 4Bto the position shown inFIG. 4C) it can expand again so as to remain in contact with both the cassette120and the casing40.

When the cassette120is fully secured to the casing40at the flanges124and126, the adhesive within the foam material of the pads42is caused or allowed to cure. For example, this can be achieved by heating the assembly to a temperature of 50° C., and maintaining that temperature for a period of 5 hours. Because the curing temperature is relatively low, the heating of the assembly can be performed by heater mats applied to the casing40. Once the adhesive has cured, the pads42provide a rigid interface between the cassette120and the casing40.

The pads42may be supplied pre-impregnated with the hardener composition (ie the adhesive) under conditions in which curing is prevented, for example at low temperature or hermetically sealed. The pads42may, for example, be pre-prepared on a backing sheet and stored in a freezer until required. When required, they may be removed from the backing sheet by an operator and applied to the cassette120.

Although measures may need to be taken, such as increasing the temperature, in order to achieve rapid cure of the adhesive, curing may begin as soon as the pads are at room temperature, or exposed to the atmosphere. Consequently, it is desirable to use an adhesive, or other hardener composition, which cures only slowly under atmospheric conditions. For example, Hexcel Redux 830 allows approximately 70 minutes, while the adhesive remains sufficiently fluid to undergo compression and recovery as described above with reference toFIGS. 4A,4B and4C. In the context of the present specification, the hardener composition is considered to be uncured, even if the curing process has begun, while the hardener composition remains resiliently compressible to the extent required for the assembly process.

If a cassette120needs to be replaced, it can simply be detached from the insert136by releasing the bolt or screw140, and pivoted away from the casing40until the forward flange124can be disengaged from the hook54. The release agent applied to the casing40prevents bonding of the pads42to the casing40, so that little and preferably no trace of the pad42is left on the casing40. Instead, the pads42are removed with the cassette120.

Although the present invention has been described with reference to the attachment of a fan track liner panel71to a fan casing40, it is equally applicable to the attachment of panels to other casings of the engine, for example a compressor casing or a turbine casing. The present invention may be employed in the assembly of front acoustic panels, thrust reversers and outlet guide vane (OGV) infill panels. More generally, the present invention is also applicable to the attachment of panels in machinery other than a gas turbine engine. It will be appreciated that the size and location of the adhesive pads described in the above description are exemplary and may vary depending on the size and application of each part being secured.