Patent Description:
A fairing may be disposed in a bypass duct of a turbofan gas turbine engine to provide an outer aerodynamic shell around a structural strut extending across the bypass duct or around service connections routed through the bypass duct. Fairings are typically designed to have a stiffness sufficiently high to resists static and dynamic loads during operation of the gas turbine engine. As a result, fairings typically have a relatively stiff and heavy construction. Improvement is desirable.

<CIT> discloses a prior art shroud for a gas turbine engine anticipating the technical features of the preamble of claim <NUM>.

According to a first aspect of the present invention, there is provided a bypass duct as set forth in claim <NUM>.

In one embodiment of the foregoing, the damper interconnects the second fairing portion with the second shroud.

In one embodiment of any of the foregoing, the damper is configured to damp movement of the second fairing portion in a lateral direction relative to the axis.

In one embodiment of any of the foregoing, the damper permits relative movement between the second fairing portion and the second shroud along an axial direction relative to the axis.

In one embodiment of any of the foregoing, the damper permits relative movement between the second fairing portion and the second shroud along a radial direction relative to the axis.

In one embodiment of any of the foregoing, a compressive preload is applied to the compressible member.

In one embodiment of any of the foregoing, the interface includes a sliding interface permitting relative translation between the second fairing portion and the second shroud axially and radially relative to the axis.

In one embodiment of any of the foregoing, the bypass duct comprises a spring in an interconnection between the second fairing portion and the second shroud.

In one embodiment of any of the foregoing, the fairing defines a through internal fairing passage; and one or more service connections are routed through the fairing passage.

In one embodiment of any of the foregoing, the frame includes an arm defining a or the spring, the spring is a cantilever spring; and.

the compressive member includes a polymeric pad operatively disposed to be compressed between the arm and the bracket secured to the second shroud.

In one embodiment of any of the foregoing, the arm and the bracket exert a compressive preload on the polymeric pad.

In one embodiment of any of the foregoing, the first shroud is disposed radially outwardly of the second shroud.

According to a further aspect of the present invention, there is provided a gas turbine engine as set forth in claim <NUM>.

In one embodiment of the foregoing, the damper is configured to damp movement of the second fairing portion in a tangential direction relative to the secured end of the fairing; and the damper defines a sliding interface permitting axial and radial movement of the free end of the fairing.

According to a further aspect of the present invention, there is provided a method for mitigating a vibration of a cantilered service fairing as set forth in claim <NUM>.

In one embodiment of the foregoing, the method comprises damping movement of the free end of the cantilevered service fairing in a tangential direction relative to the secured end of the fairing; and when damping the vibration of the free end of the cantilevered service fairing, permitting axial and radial translation of the free end of the cantilevered service fairing relative to a central axis of the bypass duct.

The following disclosure describes fairing installations inside bypass ducts of turbofan gas turbines, and associated methods. In some embodiments, the fairing installations described herein may provide improvements to cantilevered service fairings. In some embodiments, a damper may be engaged with a free end of a cantilevered fairing so that vibrations (e.g., fluttering) of the free end of the fairing potentially induced by a flow of fluid (e.g., bypass air) may be damped. In some embodiments, this may reduce vibration amplitudes and permit the use of a smaller gap between the free end of the fairing and a shroud of the bypass duct. In some embodiments, as a result of the damping of the free end, a fairing having a lower stiffness and a lighter construction may be permitted compared to other cantilevered fairing installations devoid of such damping. In some embodiments, the damper may be tuned to provide a desired dynamic response in view of the specific fairing construction and operating conditions/loading to mitigate vibration and reduce the risk of flutter of the free end of the fairing.

The terms "secured to" and "engaged with" may include both direct securement and engagement (in which two elements contact each other) and indirect securement and engagement (in which at least one additional element is located between the two elements). The term "substantially" as used herein may be applied to modify any quantitative representation which could permissibly vary without resulting in a change in the basic function to which it is related.

<FIG> shows a schematic axial cross-section view of turbofan gas turbine engine <NUM> (referred hereinafter as "engine <NUM>") including one or more fairings <NUM> (referred hereinafter in the singular) as described herein. Engine <NUM> may be of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication, fan <NUM> through which ambient air is propelled, multistage compressor <NUM> for pressurizing the air, combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and turbine section <NUM> for extracting energy from the combustion gases. <FIG> also shows FORWARD and AFT directions relative to engine <NUM>. The FORWARD direction may correspond to a forward direction of travel of engine <NUM> when engine <NUM> is propelling a (e.g., fixed-wing) aircraft.

Compressor <NUM>, combustor <NUM> and turbine section <NUM> may be considered part of the core of engine <NUM>. The core may receive core air corresponding to a portion of the ambient air propelled by fan <NUM>. Bypass air corresponding to another portion of the air propelled by fan <NUM> may be received into bypass duct <NUM> and conveyed along a passage defined by bypass duct <NUM>. Bypass duct <NUM> may extend at least partially around the core of engine <NUM>. Bypass duct <NUM> may be substantially annular about axis A, which may correspond to a centerline of engine <NUM>. For example, axis A may correspond to an axis of rotation of fan <NUM>. Axis A may correspond to an axis of rotation of one or more spools including compressor stage(s) and/or turbine stage(s) of the core of engine <NUM>.

Bypass duct <NUM> is defined by radially-outer shroud <NUM> at least partially extending around axis A, and radially-inner shroud <NUM> at least partially extending around axis A. Shrouds <NUM>, <NUM> are radially spaced apart from each other to define the passage of bypass duct <NUM> therebetween, through which the bypass air may be conveyed. The flow of bypass air being conveyed through bypass duct <NUM> is illustrated by arrows F in <FIG>.

Fairing <NUM> is disposed in bypass duct <NUM>. Two or more fairings <NUM> may be disposed at different angular and/or axial positions inside bypass duct <NUM>. Fairing <NUM> extends between radially-outer shroud <NUM> and radially-inner shroud <NUM>. For example, fairing <NUM> may extend radially from radially-outer shroud <NUM> to radially-inner shroud <NUM> to cover an entire radial distance between radially-outer shroud <NUM> and radially-inner shroud <NUM>. The term "radially" is intended to encompass orientations that have non-zero radial and axial vector components in relation to axis A and that are not perfectly radial. For example, it is understood that fairing <NUM> may extend perfectly radially or mainly radially (e.g., inclined relative to the radial direction).

In some embodiments, fairing <NUM> may be a type of service fairing, sometimes called "service tube fairing" extending radially across bypass duct <NUM>. Accordingly, fairing <NUM> may permit the routing of one or more service connections <NUM> therethrough and radially across bypass duct <NUM>. Service connections <NUM> may include one or more electric connections (e.g., cables, wires), hydraulic lines and/or fuel lines for example. Even though fairing <NUM> is illustrated herein as a service fairing as a non-limiting example, it is understood that aspects of the present disclosure are also applicable to other types of fairings including strut fairings that may surround a structural strut extending across bypass duct <NUM>.

<FIG> is a schematic representation of fairing installation <NUM> including fairing <NUM> installed in bypass duct <NUM> of engine <NUM>. The vantage point in <FIG> is substantially along axis A looking in the AFT direction of <FIG> so that axis A is substantially perpendicular to the page of <FIG>. In some embodiments, fairing <NUM> may be cantilevered from radially-outer shroud <NUM> as illustrated in <FIG> but is it understood that fairing <NUM> could instead be cantilevered from radially-inner shroud <NUM>. In the non-limiting example shown in <FIG>, fairing <NUM> may have secured portion 12A (e.g., secured end) secured to radially-outer shroud <NUM>, and radially-opposite free portion 12B (e.g., free end) proximate radially-inner shroud <NUM> and/or extending through aperture <NUM> formed in radially-inner shroud <NUM>.

Secured portion 12A may be rigidly secured to radially-outer shroud <NUM> and/or to other structure in a cantilevered manner so that at least a majority, or substantially all of the structural support for fairing <NUM> is provided via the securement of secured portion 12A to radially-outer shroud <NUM> and/or to the other structure. In other words, a principal load path between fairing <NUM> and outer shroud <NUM> and/or other structure may include the interface of secured portion 12A with outer shroud <NUM> and/or the other structure. In some embodiments, the principal load path may be the sole load path supporting fairing <NUM>.

In some embodiments, damper <NUM> may provide a physical interconnection between free portion 12B of fairing <NUM> and radially-inner shroud <NUM> and/or to other structure. However, such interconnection may provide either no significant structural support, or significantly less structural support for fairing <NUM> than the securement of secured portion 12A. In other words, free portion 12B may be relatively unsecured compared to secured portion 12A. Accordingly, fairing <NUM> may be considered to be cantilevered from radially-outer shroud <NUM> and/or other structure at secured portion 12A.

In some embodiments, secured portion 12A of fairing <NUM> may include one or more braces <NUM> that are (e.g., directly) secured (e.g., fastened) to radially-outer shroud <NUM> and/or other structure via one or more fasteners <NUM> (e.g., bolts, machine screws) for example. Braces <NUM> may be secured to or part of an internal framework supporting shell elements of fairing <NUM> as explained below. Other securing means including rivets and/or welds may be used to secure secured portion 12A to radially-outer shroud <NUM> and/or to the other structure. In various embodiments, secured portion 12A may be secured directly to radially-outer shroud <NUM>. Alternatively or in addition, secured portion 12A may extend through an aperture formed through radially-outer shroud <NUM> and be secure to a structure other than radially-outer shroud <NUM> and disposed radially outwardly of radially-outer shroud <NUM>.

Free portion 12B of fairing <NUM> may be disposed proximate to radially-inner shroud <NUM>. Radially-inner shroud <NUM> may be a relatively thin and lightweight shell structure. Free portion 12B may extend into aperture <NUM> formed in radially-inner shroud <NUM>. Aperture <NUM> may be larger than free portion 12B to provide gap G between free portion 12B and radially-inner shroud <NUM>. Gap G may be empty space providing positional clearance to accommodate assembly tolerances and relative movement between free portion 12B and radially-inner shroud <NUM>. Gap G may extend partially or completely around free portion 12B of fairing <NUM>. Gap G may provide a clearance to accommodate relative movement between free portion 12B and radially-inner shroud <NUM> in the axial direction relative to axis A, in the radial direction relative to axis A, and/or in the tangential direction T relative to secured portion (end) 12A of fairing <NUM>. Gap G may also allow for some leakage of bypass air out of bypass duct <NUM> and it may be desirable to have a smaller gap G to avoid excessive leakage of bypass air. In some embodiments, aspects of the present disclosure may facilitate the use of a smaller gap G.

One or more dampers <NUM> (referred hereinafter in the singular) may be engaged with free portion 12B of fairing <NUM> to damp movement (e.g., vibration) of free portion 12B of fairing <NUM>. In some embodiments, the use of damper <NUM> may reduce the risk of flutter of fairing <NUM>. Fairing <NUM> may have longitudinal axis L that extends between secured portion 12A and free portion 12B of fairing <NUM>. In some embodiments, longitudinal axis L may extend radially relative to axis A. During operation, vibratory motion of free portion 12B may be induced by the flow F of bypass air. In some embodiments, at least some (e.g., a majority) of the vibratory motion may be generally tangential to secured portion 12A of fairing <NUM> as indicated by arrow T in <FIG>. For example, the vibratory motion may be rotational motion about a point within secured portion 12A. In some situations, the tangential direction T may be generally lateral relative to axis A. In some embodiments, the tangential direction T may be generally lateral relative to longitudinal axis L of fairing <NUM>. The magnitude and frequency of the vibratory motion may be dependent upon the stiffness of fairing <NUM>.

In some embodiments, the use of damper <NUM> may reduce movement amplitudes of free portion 12B and thereby facilitate the use of a relatively small gap G. In some embodiments, the use of damper <NUM> may also permit lower stiffness requirements for the construction of fairing <NUM> and consequently permit a more weight-efficient construction of fairing <NUM>. As explained below, structural parameters of damper <NUM> may be selected (i.e., tuned) to provide a desired dynamic response based on the structural construction of fairing <NUM> and the operating conditions.

Damper <NUM> may be configured to define a (e.g., tunable) mass-spring-damper system between two masses including fairing <NUM> or part thereof, and radially-inner shroud <NUM> or part thereof. In some embodiments, radially-inner shroud <NUM> may be considered a fixed structure and free portion 12B of fairing <NUM> may be considered a mass movable relative to radially-inner shroud <NUM>. Alternatively, radially-inner shroud <NUM> and fairing <NUM> may be considered two masses movable relative to each other.

In some embodiments, damper <NUM> may include one or more spring elements and one or more damping elements. The spring and damping elements may be connected in series as shown in <FIG>. Alternatively, the spring and damping elements may be disposed in parallel. Damper <NUM> may interconnect free portion 12B of fairing <NUM> with radially-inner shroud <NUM> and/or interconnect free portion 12B with structure other than radially-inner shroud <NUM> within engine <NUM>. Damper <NUM> may be configured to damp movement of free portion 12B generally along the tangential direction T. For example, spring and damping elements of damper <NUM> may be tuned to mitigate potential mechanical and/or aerodynamic excitations of fairing <NUM>. In some embodiments, damper <NUM> may leave axial and/or radial movement of free portion 12B substantially unconstrained.

<FIG> is a tridimensional view of another exemplary fairing installation <NUM> including fairing <NUM> installed in bypass duct <NUM> of engine <NUM>. Fairing installation <NUM> may include elements described above in relation to fairing installation <NUM>. Like elements are identified using like reference numerals. Fairing <NUM> may defines a through internal passage <NUM> that permits service connections <NUM> to be routed longitudinally through fairing <NUM> and radially across bypass duct <NUM>. Fairing <NUM> may have leading (i.e., upstream) end LE relative to the flow F of bypass air, and trailing (i.e., downstream) end TE relative to the flow F of bypass air.

Fairing <NUM> may include an assembly of parts. For example, fairing <NUM> includes one or more outer shell elements 40A, 40B supported by internal frame <NUM>. Shell elements 40A, 40B may define an aerodynamic outer skin of fairing <NUM>. In some embodiments, shell elements 40A, 40B may, for example, be made (e.g., stamped) from a metallic material, or may be molded from a fibre-reinforce composite material. Shell elements 40A, 40B may be laterally opposed relative to axis A. Shell elements 40A, 40B may be (e.g., releasably) secured to frame <NUM>. For example, shell elements 40A, 40B may be fastened to frame <NUM> using fasteners <NUM>, rivets and/or secured to frame <NUM> by welding in various embodiments.

In some embodiments, fairing installation <NUM> may include two laterally- opposed dampers <NUM>. In some embodiments, fairing installation <NUM> may include two pairs of laterally-opposed dampers <NUM>. In some embodiments, one or more (e.g., forward) dampers <NUM> may be axially spaced apart from one or more (e.g., aft) dampers <NUM>.

<FIG> is a partial cross-sectional view of fairing <NUM> taken along line <NUM>-<NUM> in <FIG>. Damper <NUM> may be defined by any suitable components providing the desired damping and spring properties. In some embodiments, damper <NUM> may include one or more compressible (e.g., polymeric, elastomeric) member <NUM> operatively disposed between frame <NUM> of fairing <NUM> and bracket <NUM> secured to radially-inner shroud <NUM> via fasteners <NUM>. For example, compressible member <NUM> may be made of an elastomer such as (e.g., silicone) rubber. Geometric parameters (e.g., size, shape, thickness) and mechanical properties (e.g., Young's modulus, damping coefficient) of compressible member <NUM> may be selected to provide a desired dynamic response.

Frame <NUM> may include laterally-opposed arms <NUM> each defining a cantilevered spring for interfacing with compressible member <NUM>. Geometric parameters (e.g., size, shape, thickness) and mechanical properties (e.g., Young's modulus, damping coefficient) of arms <NUM> may be selected to provide a desired dynamic response in cooperation with compressible member <NUM>.

In some embodiments, L-shaped brackets <NUM> may be also each define a cantilevered spring for interfacing with a side of compressible member <NUM> opposite of arms <NUM>. Geometric parameters (e.g., size, shape, thickness) and mechanical properties (e.g., Young's modulus, damping coefficient) of brackets <NUM> may be selected to provide a desired dynamic response in cooperation with compressible member <NUM> and arms <NUM>. In some embodiments, compressible member <NUM> may be secured to either one of bracket <NUM> or arm <NUM> using a suitable adhesive or fastener, and a sliding interface/joint may be provided between compressible member <NUM> and the other one of bracket <NUM> or arm <NUM>.

In some embodiments, a compressive preload may be applied to compressible member <NUM> during assembly of fairing installation <NUM>. Such preload may be applied by having arm <NUM> and bracket <NUM> have respective resting shapes that result in arm <NUM> and bracket <NUM> being resiliently biased toward each other after assembly of compressible member <NUM> therebetween. Frame <NUM> and bracket <NUM> may be made from a metallic or a fiber-reinforced composite material. In some embodiments, frame <NUM> and bracket <NUM> may be made using suitable sheet metal forming technique(s) such as stamping, bending and cutting for example.

<FIG> is an enlarged tridimensional exploded view of damper <NUM> for engagement with fairing <NUM>. Bracket <NUM> may be secured to radially-inner shroud <NUM> via one or more fasteners <NUM> which may be bolts or machine screws. Fasteners <NUM> may be inserted from inside of bypass duct <NUM> through holes formed in radially-inner shroud <NUM>. Fasteners <NUM> may also extend through holes <NUM> formed in bracket <NUM> and be threadably engaged with respective (e.g., lock) nuts <NUM> disposed radially outside of radially-inner shroud <NUM>. Holes <NUM> may be slotted (elongated) to permit (e.g., lateral, tangential) positional adjustment of bracket <NUM> relative to radially-inner shroud <NUM> along arrow T. Such positional adjustment may be used to locate (e.g., center) free portion 12B of fairing <NUM> in aperture <NUM> formed in radially-inner shroud <NUM> when free portion 12B extends through aperture <NUM>. The positional adjustment may also be used to adjust the compressive preload on compressible member <NUM> by adjusting a flexure of arm <NUM>.

Compressible member <NUM> may be secured to bracket <NUM>. Arm <NUM> may be pressed against and in sliding engagement with compressible member <NUM> so that the damping function of damper <NUM> may be mainly exerted along arrow T and substantially perpendicular to the surface of compressible member <NUM> facing arm <NUM>. In some embodiments, compressible member <NUM> may have the shape of substantially flat block, pad or sheet. In various embodiments, compressible member <NUM> may have a uniform or non-uniform thickness. Interfacing surfaces of bracket <NUM> and compressible member <NUM> may be substantially planar. Interfacing surfaces of arm <NUM> and compressible member <NUM> may be substantially planar. The configuration of damper <NUM> may provide a sliding interface permitting sliding of arm <NUM> relative to compressible member <NUM> substantially axially (along axis A) and also radially along arrow R. Accordingly, the configuration of damper <NUM> may permit relative axial and/or radial movement between radially-inner shroud <NUM> and free portion 12B of fairing <NUM>. Such configuration of damper <NUM> may permit radial and axial movement to accommodate assembly-related and/or thermal-related displacement of fairing <NUM> while providing the tangential support required for damping.

The engagement between arm <NUM> and compressible member <NUM> may provide frictional resistance to sliding movement in the axial direction and radial direction R. Such frictional resistance may provide some frictional damping of motion of free end 12B in the axial direction and radial direction R. In other words, damper <NUM> may function as a frictional damper in the axial direction and radial direction R.

Alternatively or in addition to the configuration shown in <FIG>, one or more dampers <NUM> may be arranged so that the sliding interface between member <NUM> and arm <NUM> is oriented to be generally parallel to tangential direction T (e.g., generally perpendicular to the orientation shown in <FIG>). In such embodiments, damper <NUM> may function mainly as a friction damper where the frictional resistance at the sliding interface provides damping of relative movement in direction T. In such embodiments, materials and surface finishes of the interfacing components in damper <NUM> may be selected to provide a desired coefficient of friction at the sliding interface, and the preload may be selected in combination with the coefficient of friction to provide a desired resistance to sliding movement. In such embodiments of damper <NUM>, the damping function may be provided by way of frictional resistance at the sliding interface instead of, or in addition to, the compression of member <NUM>.

Tuning of the properties (e.g., damping coefficient, spring stiffness, resonant frequency, damping ratio) of damper <NUM> may be performed by numerical simulation/modelling, and/or may be performed empirically. The specific configuration of damper <NUM> illustrated herein is provided as a non-limiting example. It is understood that other types of damping and spring elements may be used.

<FIG> is a flowchart of a method <NUM> for mitigating vibration of fairing <NUM> disposed in bypass duct <NUM> of engine <NUM>. Method <NUM> may be performed using fairing installations <NUM> or <NUM> described herein. During operation of engine <NUM>, the flow of bypass air in bypass duct <NUM> may interact with fairing <NUM> and could potentially induced an aerodynamic excitation of fairing <NUM> in the tangential direction T. Other sources of vibration within engine <NUM> could also induce vibration of fairing <NUM>. In cases where fairing <NUM> has a cantilevered installation, it may be desirable to damp such vibration(s) of fairing <NUM> using one or more dampers <NUM> engaged with free portion 12B of fairing <NUM>.

It is understood that aspects of method <NUM> may be combined with aspects of other methods/steps and/or aspects of fairing installations <NUM>, <NUM> described herein. In various embodiments, method <NUM> may include: receiving a flow of bypass air in bypass duct <NUM> (block <NUM>); and using one or more dampers <NUM> engaged with free portion 12B of fairing <NUM>, damping the vibration of free portion 12B of fairing <NUM> induced by the flow of bypass air.

Method <NUM> may include damping movement of free portion 12B of fairing <NUM> in a tangential direction (see arrow T in <FIG>) relative to secured portion 12A. When damping the vibration of free portion 12B of fairing <NUM>, method <NUM> may permit axial and radial movement of free portion 12B of fairing <NUM> relative to central axis A via the sliding interface of damper <NUM> illustrated in <FIG>.

Claim 1:
A bypass duct (<NUM>) of a turbofan gas turbine engine (<NUM>), the bypass duct (<NUM>) comprising:
a first shroud (<NUM>) extending at least partially around an axis (A);
a second shroud (<NUM>) extending at least partially around the axis (A), the second shroud (<NUM>) being radially spaced apart from the first shroud (<NUM>) to define a bypass passage between the first and second shrouds (<NUM>, <NUM>);
a fairing (<NUM>) having a first fairing portion (12A) secured to the first shroud (<NUM>) and a second fairing portion (12B) radially opposite the first fairing portion (12A), a gap (G) defined between the second fairing portion (12B) and the second shroud (<NUM>) accommodates relative movement between the second fairing portion (12B) and the second shroud (<NUM>); and
a damper (<NUM>) engaged with the second fairing portion (12B) to damp movement of the second fairing portion (12B);
wherein:
the damper (<NUM>) includes a compressible member (<NUM>) disposed at an interface between the second fairing portion (12B) and the second shroud (<NUM>);
characterized in that
the fairing (<NUM>) is disposed in the bypass passage and extends between the first and second shrouds (<NUM>, <NUM>);
the fairing (<NUM>) includes one or more outer shell elements (40A, 40B) supported by a frame (<NUM>); and
the compressible member (<NUM>) is disposed between the frame (<NUM>) of the fairing (<NUM>) and a bracket (<NUM>) secured to the second shroud (<NUM>).