Patent Description:
It is relatively common for gas turbine engines to have one or more relatively large and heavy components secured externally to the casing. The nature of such components varies from one engine model to another, and can include an accessory gearbox (AGB), an oil tank, a starter/generator, an electronics housing, and an oil cooler to name a few examples. Various motivations may exist for mounting such a component externally to the casing, remotely from the engine's main rotation axis, such as seeking an environment which is cooler than the engine core, for instance, or connection convenience with another component. In some engine designs, it can be considered non-feasible to mount such components rigidly to the casing, and this can be due, for instance, to the requirement of allowing for different degrees of thermal expansion between the component and the casing, which can be particularly true for larger components. Accordingly, mounts, typically consisting of a number of discrete structures, are used between the component and the casing. Such structures can allow for varying degrees of freedom of movement of their attachment point on the component and the casing. Design consideration in mount configurations include the weight and static structural resistance of the mount, allowance for varying degrees of relative thermal expansion over the operating envelope. While existing mounts were satisfactory to a certain degree there remained room for improvement, particularly in terms of taking into consideration dynamic effects such as vibrational resonance.

<CIT> discloses a mid-turbine frame with oil system mounts, and <CIT> discloses an accessory drive arrangement for a ducted fan type aircraft engine.

In one aspect, there is provided a gas turbine engine as set forth in claim <NUM>.

In another aspect, there is provided a method as set forth in claim <NUM>.

<FIG> illustrates an example of a turbine engine. In this example, the turbine engine <NUM> is a turboshaft engine generally comprising in serial flow communication, a multistage compressor <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases around the engine rotation axis <NUM>, and a turbine section <NUM> for extracting energy from the combustion gases. The turbine engine terminates in an exhaust section.

The fluid path extending sequentially across the compressor <NUM>, the combustor <NUM> and the turbine <NUM> can be referred to as the core gas path <NUM>. In practice, the combustor <NUM> can include a plurality of identical, circumferentially interspaced, combustor units. The exact design varies from one engine type to another, but it is common for turboshaft engines, for instance, to have two rotors, including one high pressure shaft <NUM>, connecting a high pressure turbine section to the compressor <NUM>, and a low pressure shaft <NUM>, sometimes referred to as a power shaft, which is used as a power source during use. Turbofan, electric, and hybrid aircraft engines can also have one or more shaft associated to corresponding rotors.

Turboshaft engines, similarly to turboprop engines, typically have some form of gearing by which the power of the low pressure shaft <NUM> is transferred to an external shaft <NUM> bearing the blades (or propeller). This gearing, which can be referred to as a gearbox <NUM> for the sake of simplicity, typically reduces the rotation speed to reach an external rotation speed which is better adapted to rotate the blades or propeller for instance.

Aircraft engines have casings which can include a plurality of non-rotary components assembled to one another, and the exact design thereof can vary from one aircraft engine type to another. In the case of gas turbine engines or hybrid engines, the casing <NUM> typically has a radially-inner wall, relative the rotation axis <NUM>, forming a radially outer delimitation to the core gas path <NUM>, and can also have a radially outer wall <NUM>. The casing <NUM> can be housed within a nacelle having an aerodynamic configuration, or otherwise within the aerodynamic external skin of the aircraft.

Aircraft engines typically have an accessory gearbox (AGB) <NUM> which can serve for exchanging power from the aircraft engine's core and "accessories". Accessory gearboxes <NUM> typically provide for the connection to more than one accessory. The exact selection of accessories can vary from one engine to another, but it is relatively common for the accessories to include one or more fuel pump, oil pump, engine starter, etc. The accessory gearbox <NUM> can be located externally to the casing <NUM>, and somewhat remotely from the engine's core to avoid the harsh temperatures which can be sustained during operation of the higher pressure compressors, combustor <NUM>, and turbine sections <NUM>. The accessory gearbox <NUM> can be connected to transfer power to an engine shaft (e.g. shaft <NUM>) by a radially-extending driveshaft <NUM>.

Given the presence of internal gearing, and especially in the case of AGB's having multiple ports, AGBs <NUM> can be relatively heavy components. Their placement, especially when remote to the engine's rotation axis <NUM>, can make them prone to dynamic effects, such as resonant vibration frequency response, in addition to more typical structural considerations. This can also be true for other relatively large and heavy components secured externally to the casing, such as an oil tank, a starter/generator, an electronics housing, and an oil cooler, to name a few examples.

Various motivations may exist for mounting such relatively large and heavy components <NUM> externally to the casing <NUM>, remotely from the engine's rotation axis <NUM>, such as seeking an environment which is cooler than the engine core, for instance, or connection convenience with another component. In some engine designs, it can be considered non-feasible to mount such components <NUM> rigidly to the casing <NUM>, and this can be due, for instance, to the requirement of allowing for different degrees of thermal expansion between the component <NUM> and the casing <NUM>, which can be particularly true for larger components. Accordingly, mounts <NUM>, typically consisting of a number of discrete structures 40a, 40b, 40c, can be used between the component <NUM> and the casing <NUM>. Such structures 40a, 40b, 40c can allow for varying degrees of freedom of displacement at their attachment point on the component <NUM> or the casing <NUM>. Design consideration in mount configurations include the weight and static structural resistance of the mount, allowance for varying degrees of relative thermal expansion over the operating envelope. The structures 40a, 40b, 40c can be brackets, for instance.

An example mount configuration which can be used to mount a relatively large and heavy component <NUM> externally to a casing <NUM> of an engine <NUM> is presented in <FIG>. In this example, the mount <NUM> has three structures 40a, 40b, 40c and is arranged in a so-called <NUM> - <NUM> - <NUM> configuration. More specifically, a first structure 40b is fixed in displacement in <NUM> orthogonal orientations, a second structure 40c is fixed in displacement in <NUM> orthogonal orientations, and free to displacement in the third orthogonal orientation to accommodate, say, thermal expansion in that orientation relative the first structure 40b, and a third structure 40a fixed in <NUM> orthogonal orientation, and free to displacement in the second and third orthogonal orientations. The second structure 40c can have a sliding engagement in the third orthogonal orientation, and the third structure 40a can have a double spherical joint allowing movement in the second and third orthogonal orientations, for instance. Such a mount configuration can allow mounting a relatively large and heavy component externally to the casing in a relatively low weight manner, and can be suitable to a certain degree in many embodiments. However, it may not be suitable for all embodiments.

In particular, if the component <NUM> in question is relatively long/tall in an orientation normal to a major vibration orientation, one of its ends, typically the one associated with the structure 40a offering the greatest degree of freedom, may be prone to resonant vibration due to the alternating excitation forces operating in the major vibration orientation, and it may be challenging or impossible to limit the amplitude of the resonant vibration of that end within a specified range without an unsatisfactory penalty in terms of weight.

Such a scenario can occur, for instance, in a context such as presented in <FIG> where a tall component <NUM>, such as an oil tank, starter/generator, electronics housing, or oil cooler to name a few examples, is mounted externally to a casing <NUM> in a manner that its height extends normal to the engine's rotation axis <NUM>, and where the engine <NUM> has a strong excitation vibration in the orientation of its rotation axis <NUM>, which may particularly be the case in many turboshaft engines which rotate helicopter blades. Indeed, the helicopter blades can be a significant source of "chucking" vibration. The chucking load can be transferred to the engine through a front mount of the engine (not shown), which is attached to the case <NUM>, and the load can therefore be carried by the case <NUM>, which can translate in strong vibrations in the orientation of the rotation axis <NUM>. In a context where a <NUM> structure 40b and a <NUM> structure 40c are used to support the bottom <NUM> of such a tall component <NUM> onto the casing <NUM>, and where a <NUM> structure 40a is used to support the top <NUM> of such a tall component <NUM>, the top <NUM> of the component <NUM> can experience vibration in the form of alternating forward <NUM> and backward <NUM> movement parallel to the rotation axis <NUM>, such as schematized in <FIG>, for instance.

For convenience, a first side <NUM> and a second side <NUM> is defined relative the torsion axis <NUM>, in opposite directions along the horizontal radial orientation <NUM>.

In the scenario presented in <FIG>, another relatively large and heavy component <NUM> is mounted to the engine casing <NUM>, circumferentially adjacent the first component <NUM>. This second component <NUM> is mounted to the engine <NUM> here with a corresponding mount <NUM>, which can include a plurality of structures, such as three or four, for instance, and the mount <NUM> is subject to torsion around a torsion axis <NUM> oriented parallel to a height of the first component <NUM>. In this scenario, the second component <NUM> has a center of gravity <NUM> which is offset from the torsion axis <NUM>, as perhaps best seen in <FIG>, such that the axial vibration generates alternating forward and backward movement on the center of gravity <NUM>, and the center of gravity <NUM> being laterally offset from the torsion axis <NUM>, the forward and backward movement translates into a moment around the torsion axis <NUM>, and thereby torsion around the torsion axis <NUM>. The torsion movement leads to an opposite forward and backward movement on the other side <NUM> of the second component <NUM>. This scenario can occur when the second component <NUM> is an AGB <NUM> for instance which has an relatively heavy overhang <NUM> protruding laterally on a side opposite the first component <NUM>, for instance, relative to the torsion axis <NUM> of the mount <NUM>. The overhang <NUM> can be considered relatively heavy when it laterally offsets the center of gravity by more than <NUM>%, more than <NUM>%, or more than <NUM>% of the width of the second component <NUM>, for instance, depending on the embodiment. In such as case, the torsion axis <NUM> can coincide with the rotation axis of the driveshaft <NUM>, for instance, and the overhang <NUM> can house a relatively heavy bearing oil pump for instance. It will be noted here that in this scenario, the side <NUM> of the second component <NUM> and the top <NUM> of the first component <NUM> experience alternating axial movements in opposite directions, at the same frequency. As will now be detailed, this phenomena can be harnessed to dynamically stiffen the overall assembly.

More specifically, a structure <NUM> can be used to connect the top <NUM> of the first component <NUM> to the adjacent side <NUM> of the second component <NUM> in a manner to stiffen the overall assembly via axially oriented flexing stresses occurring in the structure <NUM> and communicated between the two components via the structure. The connection between the structure <NUM> and the side <NUM> of the second component can be made by directly, such as by connecting the structure <NUM> to a point or area forming part of the second component <NUM> itself (e.g. a housing of the second component <NUM>), or indirectly, by connecting the structure to a point or area of an adjacent component which moves dynamically together with the second component <NUM>. In particular, the mount <NUM> by which the second component <NUM> is mounted to the casing can by dynamically integral with the second component <NUM>, and move with it, especially the portions of the mount <NUM> which are the closest to the second component <NUM>. Accordingly, the structure <NUM> can be connected to the second component <NUM> indirectly, via a corresponding portion of its mount <NUM>.

It is not necessary for the structure <NUM> to be stiff in the axial orientation, and in fact, in some embodiments, it can be preferred for the structure to exhibit some degree flexibility in the axial orientation. More specifically, the structure <NUM> can be stiff in the horizontal radial orientation <NUM>, free in the vertical radial orientation <NUM>, and flexible in the axial orientation <NUM>. More specifically, in the horizontal radial orientation <NUM>, the bracket can be pushed or pulled, which leads to compression or tension in the bracket instead of bending. The flexibility in the axial orientation <NUM> is designed in a manner to lead to bending within the elastic domain. The structure <NUM> can be designed in a manner to exhibit a lower degree of bending, or no bending at all, from stress exhibited in the width orientation. Depending of the embodiment, the structure can have a length extending mainly in the width orientation, or mainly in the height orientation, as both cases can be designed in a manner to exhibit flexibility in the axial orientation, rigidity in the horizontal radial orientation, and freedom of movement in the vertical radial orientation, for instance. The degree of flexibility can be tuned via the choice of material and shape/configuration, and the dimensions, particularly in the axial orientation <NUM>. More specifically, the spring rate of the structure <NUM> can be adjusted by changing the length, thickness, height and material type, which in turn can adjust the system dynamic response. More precisely, the stiffness of the structure <NUM> can be tuned precisely to couple the torsional mode of the second component <NUM> to the axial mode of the first component <NUM>. To achieve this, the modes can be simulated and observed using computer assisted modelization using the components <NUM>, <NUM> individually mounted with respective mounts <NUM>, <NUM>, but without the interconnecting structure <NUM>. The presence of the structure <NUM> can increase the natural frequency of the components to a much higher frequency for a same mounting stiffness.

The mounting configuration for the component <NUM> can be as schematized in <FIG>. For ease of reference, three orthogonal orientations can be defined as the axial orientation <NUM>, the horizontal radial orientation <NUM>, and the vertical radial orientation <NUM>, but it will be understood that these expressions are attributed arbitrarily and imply no particular reference to the orientation of the gas turbine engine relative to the ground, or relative to the aircraft. In alternate embodiments, for instance, the two components <NUM>, <NUM> can be in different relative orientations relative the main orientation of vibration, the torsion axis can be defined as being normal to the main orientation of vibration, for instance. Similarly, the relative position and orientation of the components can be alternately presented in a frame of reference of the torsion axis <NUM> instead of the frame of reference of the engine rotation axis <NUM>, for instance. We will continue the explanation on the basis of the three orthogonal orientations as defined previously for simplicity and convenience. Here, the second mount <NUM> includes a <NUM> structure 40c and a <NUM> structure 40b securing a bottom <NUM> of the component <NUM> to the casing <NUM>, as in the mount <NUM>. However, a different structure <NUM> is used to secure the top <NUM> of the component <NUM> to the second component <NUM>. The structure <NUM> still fixes displacement in the horizontal radial orientation <NUM> and is still free to displacement along the vertical radial orientation <NUM>, but is elastically flexible in the axial orientation <NUM>, and thus offers only a partial resistance to displacement along the axial orientation <NUM>.

<FIG> shows one possible example embodiment of a component <NUM>, more precisely an oil tank <NUM>. Indeed, it is common in gas turbine engines to make the oil tank <NUM> tall in a manner to stabilize the level of oil within the tank <NUM>, and given the tallness, and weight of that component, and the context of a helicopter, the oil tank <NUM> can be particularly sensitive to axially oriented resonant frequency modes in the top portion <NUM> thereof. The tank can be outfitted with a <NUM>-structure 140c and a <NUM>-structure 140b at the bottom thereof, which directly connect to the engine casing. Another structure <NUM> can be used to connect the top <NUM> directly to the AGB's housing laterally opposite the overhang portion.

<FIG> are top and transverse cross-section views, respectively, showing the example structure <NUM> in further detail. The structure <NUM> can include a beam <NUM> extending between a first end <NUM> connecting the tank, and a second end <NUM> connecting the AGB <NUM>. The beam <NUM> can be taller in the vertical radial orientation <NUM> than thick in the axial orientation <NUM>, to be elastically bendable in the axial orientation <NUM>. In other words, it can be more elastically flexible in the axial orientation <NUM> than in the horizontal radial orientation <NUM>, where it is fixed. It can be at least three times narrower in the axial orientation <NUM> than in the vertical radial orientation <NUM>, and in this specific embodiment, it is roughly <NUM> times narrower in the axial orientation <NUM> than in the vertical radial orientation <NUM>. One of the two ends, the one <NUM> connecting the tank <NUM> in this embodiment, can have a sliding mount <NUM> providing sliding-ability, and thus a degree of freedom, along the vertical radial orientation <NUM>. One of the two ends, or both, and more specifically the end <NUM> connecting the AGB <NUM> housing in this embodiment, can be provided with one or more flanges <NUM> having one or more clearance holes <NUM> through which one or more fasteners <NUM> are to be introduced to secure the end <NUM> to the component. As perhaps best seen in <FIG>, clearance holes <NUM> have a cross-sectional shape which is larger than a cross section of the corresponding fastener <NUM>, leaving a clearance c, such that the two components can remain loosely connected to accommodate assembly tolerance stackups, until the final assembly step where the fastener sandwiches the flange between a head or nut and a corresponding surface of the component. The use of clearance holes can avoid pre-stressing the structure <NUM> during assembly.

In accordance with one embodiment, the following process can occur during the operation of the gas turbine engine. Generating axial vibrations along the axis <NUM>, these axial vibrations can be generated, for instance, by the rotation of helicopter blades as a function of the speed of rotation and of the number of blades during operating conditions such as cruising. The axial vibrations then entrain two effects simultaneously : a torsional vibration mode <NUM> in a first component mounted externally to the casing, the torsional vibration mode <NUM> being defined around a torsion axis normal to the rotation axis, and an axial vibration mode <NUM> in a portion of a second component mounted externally to the casing, the portion of the second component extending adjacent a side of the first component, said torsional vibration mode <NUM> entraining an axial movement of the side of the first component opposite an axial movement of the portion of the second component entrained by the axial vibration mode. Moreover, energy is transferred <NUM> from the torsional vibration mode to oppose energy from the axial vibration mode via a structure connecting the portion of the second component to the side of the first component.

Claim 1:
A gas turbine engine comprising :
a casing (<NUM>);
a rotor rotatable around a rotation axis (<NUM>) relative the casing (<NUM>), the casing (<NUM>) extending along and around the rotation axis (<NUM>), an axial direction (<NUM>) defined parallel to the rotation axis (<NUM>), a vertical radial direction (<NUM>) defined normal to the axial direction (<NUM>), the vertical radial direction (<NUM>) extending from a bottom to a top, and a horizontal radial direction (<NUM>) defined normal to both the axial direction (<NUM>) and to the vertical radial direction (<NUM>);
a first component (<NUM>) mounted externally to the casing (<NUM>) by a first mount (<NUM>), the first mount (<NUM>) defining a torsion axis (<NUM>) extending along the vertical radial direction (<NUM>), a first side (<NUM>) and a second side (<NUM>) defined relative the torsion axis (<NUM>) in opposite directions along the horizontal radial direction (<NUM>), the first component (<NUM>) having a center of gravity located on the first side (<NUM>) of the torsion axis (<NUM>);
a second component (<NUM>) mounted externally to the casing (<NUM>) on the second side (<NUM>), extending along the vertical radial direction (<NUM>) from a bottom portion (<NUM>) of the second component (<NUM>) to a top portion (<NUM>) of the second component (<NUM>), a second mount (<NUM>) connecting the bottom portion (<NUM>) of the second component (<NUM>) to the casing (<NUM>); characterised by
a structure (<NUM>) connecting the top portion (<NUM>) of the second component (<NUM>) to at least one of the first component (<NUM>) and the first mount (<NUM>), on the second side (<NUM>) of the torsion axis (<NUM>), wherein the structure (<NUM>) is more elastically flexible in the axial direction (<NUM>) than in the horizontal radial direction (<NUM>), and has an end freely slidable along the vertical radial direction (<NUM>).