HIGH POWER EPICYCLIC GEARBOX AND OPERATION THEREOF

Abstract

A gas turbine engine for an aircraft including: an engine core including a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan including a plurality of fan blades; a gearbox that can receive an input from the core shaft, and can output drive to a fan shaft via an output of the gearbox so as to drive the fan at a lower rotational speed than the core shaft; and a fan shaft mounting structure arranged to mount the fan shaft within the engine, the fan shaft mounting structure including at least two supporting bearings connected to the fan shaft. A fan-gearbox axial distance is defined as the axial distance between the output of the gearbox and the fan axial centreline, the fan-gearbox axial distance being greater than or equal to 0.35 m.

Claims

1. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades, the fan having a fan axial centreline; a gearbox that is configured to: receive an input from the core shaft, and output drive to a fan shaft via an output of the gearbox so as to drive the fan at a lower rotational speed than the core shaft; and a fan shaft mounting structure arranged to mount the fan shaft within the engine, the fan shaft mounting structure comprising at least two supporting bearings connected to the fan shaft, wherein: the fan comprises 22, 24 or 26 fan blades; a system radial bending stiffness is defined as: $\frac{1}{\begin{matrix} (\frac{1}{\begin{matrix} a radial bending stiffness of \\ the fan shaft mounting structure \end{matrix}}) + \\ (\frac{1}{\begin{matrix} a radial bending stiffness of \\ the fan shaft at the output of the gearbox \end{matrix}}) \end{matrix}};$ a system tilt stiffness is defined as: $\frac{1}{\begin{matrix} (\frac{1}{\begin{matrix} a tilt stiffness of \\ the fan shaft mounting structure \end{matrix}}) + \\ (\frac{1}{\begin{matrix} a tilt stiffness of \\ the fan shaft at the output of the gearbox \end{matrix}}) \end{matrix}};$ and either: (i) the system radial bending stiffness is greater than or equal to 3.90×10.sup.6 N/m; or (ii) the system tilt stiffness is greater than or equal to 1.10×10.sup.5 Nm/rad.

2. The gas turbine engine according to claim 1, wherein a fan shaft mounting radial bending stiffness ratio of: $\frac{the system radial bending stiffness}{\begin{matrix} the radial bending stiffness of the \\ fan shaft mounting structure \end{matrix}}$ is in the range from 1.0×10.sup.−3 to 4.0×10.sup.−1.

3. The gas turbine engine according to claim 1, wherein a fan shaft mounting tilt stiffness ratio of: $\frac{the system tilt stiffness}{\begin{matrix} the tilt stiffness of the \\ fan shaft mourting structure \end{matrix}}$ is in the range from 1.5×10.sup.−3 to 5.0×10.sup.−1.

4. The gas turbine engine according to claim 1, wherein at least one of the following is satisfied: a) the system radial bending stiffness is in a range from 3.90×10.sup.6 N/m to 1.40×10.sup.9 N/m; b) the radial bending stiffness of the fan shaft mounting structure is greater than or equal to 7.00×10.sup.8 N/m; and c) the radial bending stiffness of the fan shaft at the output of the gearbox is greater than or equal to 4.00×10.sup.6 N/m.

5. The gas turbine engine according to claim 1, wherein at least one of the following is satisfied: a) the gearbox has a gear ratio of between 3 and 3.1; b) an axial distance between an input to the fan and a closest bearing of the at least two supporting bearings in a rearward direction from the fan is in a range from 0.12 m to 0.40 m; c) a fan tip loading is configured to be in a range from 0.30 to 0.34 at cruise conditions, where the fan tip loading is defined as dH/U.sub.tip.sup.2, where dH is an enthalpy rise across the fan and U.sub.tip is a translational velocity of a fan tip of the fan; and d) the at least two supporting bearings comprise a first supporting bearing and second supporting bearing, wherein both of the first and the second supporting bearings are located at positions forward of the gearbox.

6. The gas turbine engine according to claim 1, wherein at least one of the following is satisfied: a) the fan has a diameter in a range of 220 cm to 240 cm; b) a first bearing separation product defined as:
a first bearing separation distance×an axial distance between a fan input position and a gearbox output position is in a range from 5.2×10.sup.−2 m.sup.2 to 2.6×10.sup.−1 m.sup.2; c) the fan blades are manufactured using a titanium based metal; and d) a temperature of a flow at an exit to the combustor is configured to be greater than 1600K at cruise conditions.

7. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades, the fan having a fan axial centreline; a gearbox that is configured to: receive an input from the core shaft, and output drive to a fan shaft via an output of the gearbox so as to drive the fan at a lower rotational speed than the core shaft; and a fan shaft mounting structure arranged to mount the fan shaft within the engine, the fan shaft mounting structure comprising at least two supporting bearings connected to the fan shaft, wherein: the fan blades are manufactured using a titanium based metal; a system radial bending stiffness is defined as: $\frac{1}{\begin{matrix} (\frac{1}{\begin{matrix} a radial bending stiffness of \\ the fan shaft mounting structure \end{matrix}}) + \\ (\frac{1}{\begin{matrix} a radial bending stiffness of \\ the fan shaft at the output of the gearbox \end{matrix}}) \end{matrix}};$ a system tilt stiffness is defined as: $\frac{1}{\begin{matrix} (\frac{1}{\begin{matrix} a tilt stiffness of \\ the fan shaft mounting structure \end{matrix}}) + \\ (\frac{1}{\begin{matrix} a tilt stiffness of \\ the fan shaft at the output of the gearbox \end{matrix}}) \end{matrix}};$ and either: (i) the system radial bending stiffness is greater than or equal to 3.90×10.sup.6 N/m; or (ii) the system tilt stiffness is greater than or equal to 1.10×10.sup.5 Nm/rad.

8. The gas turbine engine according to claim 7, wherein a fan shaft mounting radial bending stiffness ratio of: $\frac{the system radial bending stiffness}{\begin{matrix} the radial bending stiffness of the \\ fan shaft mounting structure \end{matrix}}$ is in a range from 1.0×10.sup.−3 to 4.0×10.sup.−1.

9. The gas turbine engine according to claim 7, wherein a fan shaft mounting tilt stiffness ratio of: $\frac{the system tilt stiffness}{\begin{matrix} the tilt stiffness of the \\ fan shaft mounting structure \end{matrix}}$ is in a range from 1.5×10.sup.−3 to 5.0×10.sup.−1.

10. The gas turbine engine according to claim 7, wherein at least one of the following is satisfied: a) the system radial bending stiffness is in a range from 3.90×10.sup.6 N/m to 1.40×10.sup.9 N/m; b) the radial bending stiffness of the fan shaft mounting structure is greater than or equal to 7.00×10.sup.8 N/m; and c) the radial bending stiffness of the fan shaft at the output of the gearbox is greater than or equal to 4.00×10.sup.6 N/m.

11. The gas turbine engine according to claim 7, wherein at least one of the following is satisfied: a) the gearbox has a gear ratio of between 3 and 3.1; b) an axial distance between an input to the fan and a closest bearing of the at least two supporting bearings in a rearward direction from the fan is in a range from 0.12 m to 0.40 m; c) a fan tip loading is configured to be in a range from 0.30 to 0.34 at cruise conditions, where the fan tip loading is defined as dH/U.sub.tip.sup.2, where dH is an enthalpy rise across the fan and U.sub.tip is a translational velocity of a fan tip of the fan; and d) the at least two supporting bearings comprise a first supporting bearing and second supporting bearing, wherein both of the first and the second supporting bearings are located at positions forward of the gearbox.

12. The gas turbine engine according to claim 7, wherein at least one of the following is satisfied: a) the fan has a diameter in a range of 220 cm to 240 cm; and b) a first bearing separation product defined as:
a first bearing separation distance×an axial distance between a fan input position and a gearbox output position is in a range from 5.2×10.sup.−2 m.sup.2 to 2.6×10.sup.−1 m.sup.2; c) the fan comprises 22, 24 or 26 fan blades; and d) a temperature of a flow at an exit to the combustor is configured to be greater than 1600K at cruise conditions.

13. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades, the fan having a fan axial centreline; a gearbox that is configured to: receive an input from the core shaft, and output drive to a fan shaft via an output of the gearbox so as to drive the fan at a lower rotational speed than the core shaft; and a fan shaft mounting structure arranged to mount the fan shaft within the engine, the fan shaft mounting structure comprising at least two supporting bearings connected to the fan shaft, wherein: a temperature of a flow at an exit to the combustor is configured to be greater than 1600K at cruise conditions; a system radial bending stiffness is defined as: $\frac{1}{\begin{matrix} (\frac{1}{\begin{matrix} a radial bending stiffness of \\ the fan shaft mounting structure \end{matrix}}) + \\ (\frac{1}{\begin{matrix} a radial bending stiffness of \\ the fan shaft at the output of the gearbox \end{matrix}}) \end{matrix}};$ a system tilt stiffness is defined as: $\frac{1}{\begin{matrix} (\frac{1}{\begin{matrix} a tilt stiffness of \\ the fan shaft mounting structure \end{matrix}}) + \\ (\frac{1}{\begin{matrix} a tilt stiffness of \\ the fan shaft at the output of the gearbox \end{matrix}}) \end{matrix}};$ and either: (i) the system radial bending stiffness is greater than or equal to 3.90×10.sup.6 N/m; or (ii) the system tilt stiffness is greater than or equal to 1.10×10.sup.5 Nm/rad.

14. The gas turbine engine according to claim 13, wherein a fan shaft mounting radial bending stiffness ratio of: $\frac{the system radial bending stiffness}{\begin{matrix} the radial bending stiffness of the \\ fan shaft mounting structure \end{matrix}}$ is in a range from 1.0×10.sup.−3 to 4.0×10.sup.−1.

15. The gas turbine engine according to claim 13, wherein a fan shaft mounting tilt stiffness ratio of: $\frac{the system tilt stiffness}{\begin{matrix} the tilt stiffness of the \\ fan shaft mounting structure \end{matrix}}$ is in a range from 1.5×10.sup.−3 to 5.0×10.sup.−1.

16. The gas turbine engine according to claim 13, wherein at least one of the following is satisfied: a) the system radial bending stiffness is in a range from 3.90×10.sup.6 N/m to 1.40×10.sup.9 N/m; b) the radial bending stiffness of the fan shaft mounting structure is greater than or equal to 7.00×10.sup.8 N/m; and c) the radial bending stiffness of the fan shaft at the output of the gearbox is greater than or equal to 4.00×10.sup.6 N/m.

17. The gas turbine engine according to claim 13, wherein at least one of the following is satisfied: a) the gearbox has a gear ratio of between 3 and 3.1; b) an axial distance between an input to the fan and a closest bearing of the at least two supporting bearings in a rearward direction from the fan is in a range from 0.12 m to 0.40 m; c) a fan tip loading is configured to be in a range from 0.30 to 0.34 at cruise conditions, where the fan tip loading defined as dH/U.sub.tip.sup.2, where dH is an enthalpy rise across the fan and U.sub.tip is a translational velocity of a fan tip of the fan; d) the at least two supporting bearings comprise a first supporting bearing and second supporting bearing, wherein both of the first and the second supporting bearings are located at positions forward of the gearbox.

18. The gas turbine engine according to claim 13, wherein at least one of the following is satisfied: a) the fan has a diameter in a range of 220 cm to 240 cm; b) a first bearing separation product defined as:
a first bearing separation distance×an axial distance between a fan input position and a gearbox output position is in a range from 5.2×10.sup.−2 m.sup.2 to 2.6×10.sup.−1 m.sup.2; c) the fan comprises 22, 24 or 26 fan blades; and d) the fan blades are manufactured using a titanium based metal.

Description

[0327] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0328] FIG. 1 is a sectional side view of a gas turbine engine;

[0329] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0330] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0331] FIG. 4 is a schematic diagram illustrating the radial bending stiffness of a cantilevered beam;

[0332] FIG. 5 is a schematic diagram illustrating the tilt stiffness of a cantilevered beam;

[0333] FIG. 6 is a schematic diagram illustrating the torsional stiffness of a shaft;

[0334] FIG. 7 is a schematic close up sectional side view of a portion of a gas turbine engine around its gearbox;

[0335] FIGS. 8 and 9 are schematic diagrams illustrating the radial bending stiffness a fan shaft mounting structure;

[0336] FIG. 10 is a close-up view showing only the bearings supporting the fan shaft to illustrate the measurement of the radial bending stiffness;

[0337] FIGS. 11 and 12 are schematic diagrams illustrating the tilt stiffness the fan shaft mounting structure;

[0338] FIG. 13 is a close-up view showing only the bearings supporting the fan shaft to illustrate the measurement of the tilt stiffness;

[0339] FIGS. 14 to 19 are schematic diagrams illustrating various embodiments of the fan shaft mounting structure;

[0340] FIG. 20 is a schematic diagram illustrating the gearbox output position for a gearbox in a star configuration;

[0341] FIG. 21 is a schematic diagram illustrating the gearbox output position for a gearbox in a planetary configuration;

[0342] FIGS. 22 and 23 are schematic diagrams illustrating fan shaft end radial bending stiffnesses;

[0343] FIGS. 24 and 25 are schematic diagrams illustrating fan shaft end tilt stiffnesses;

[0344] FIG. 26 is a schematic diagram illustrating an alternative interface between a fan shaft and fan;

[0345] FIG. 27 is a schematic diagram illustrating a planet carrier;

[0346] FIG. 28 is a schematic diagram illustrating the torsional stiffness of the carrier in side view;

[0347] FIG. 29 is a schematic diagram illustrating torsional stiffness of an alternative carrier in front view;

[0348] FIG. 30 is a schematic diagram illustrating torsional stiffness of the carrier of FIG. 29;

[0349] FIG. 31 is a schematic diagram illustrating a front view of a carrier comprising lugs;

[0350] FIG. 32 shows an aircraft having a gas turbine engine attached to each wing;

[0351] FIG. 33 shows a method of operating a gas turbine engine on an aircraft; and

[0352] FIG. 34 shows a graph of applied load against displacement to illustrate measurement of the stiffness of a component.

[0353] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0354] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0355] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0356] The linkages 36 may be referred to as a fan shaft 36, the fan shaft 36 optionally comprising two or more shaft portions coupled together. For example, the fan shaft 36 may comprise a gearbox output shaft portion 36a extending from the gearbox 30 and a fan portion 36b extending between the gearbox output shaft portion and the fan 23. In the embodiment shown in FIGS. 1 and 2, the gearbox 30 is a planetary gearbox and the gearbox output shaft portion 36a is connected to the planet carrier 34—it may therefore be referred to as a carrier output shaft 36a. In star gearboxes 30, the gearbox output shaft portion 36a may be connected to the ring gear 38—it may therefore be referred to as a ring output shaft 36a. In the embodiment shown in FIGS. 1 and 2, the fan portion 36b of the fan shaft 36 connects the gearbox output shaft portion 36a to the fan 23. The output of the gearbox 30 is therefore transferred to the fan 23, to rotate the fan, via the fan shaft 36. In alternative embodiments, the fan shaft 36 may comprise a single component, or more than two components. Unless otherwise indicated or apparent to the skilled person, anything described with respect to an engine 10 with a star gearbox 30 may equally be applied to an engine with a planetary gearbox 30, and vice versa.

[0357] Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0358] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0359] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate. In various other exemplary embodiments the gearbox may be any other type of gearbox, and so may not be an epicyclic gearbox.

[0360] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility as defined or claimed herein. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2 (for example as described in connection with other embodiments disclosed herein which have a star gearbox arrangement).

[0361] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0362] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0363] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example.

[0364] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0365] The following general definitions of stiffnesses may be used herein:

Radial Bending Stiffness

[0366] A radial bending stiffness is a measure of deformation for a given force applied in any one selected radial direction (i.e. any direction perpendicular to and passing through the engine axis). The radial bending stiffness is defined with reference to FIG. 4 in terms of the deformation of a cantilevered beam 401. As illustrated in FIG. 4, a force, F, applied at the free end of the beam in a direction perpendicular to the longitudinal axis of the beam causes a linear perpendicular deformation, δ. The radial bending stiffness is the force applied for a given linear deformation i.e. F/δ. In the present application, the radial bending stiffness is taken relative to the rotational axis of the engine 9, and so relates to the resistance to linear deformation in a radial direction of the engine caused by a radial force. The beam, or equivalent cantilevered component, extends along the axis of rotation of the engine, the force, F, is applied perpendicular to the axis of rotation of the engine, along any radial direction, and the displacement 6 is measured perpendicular to the axis of rotation, along the line of action of the force. The radial bending stiffness as defined herein has SI units of N/m. In the present application, unless otherwise stated, the radial bending stiffness is taken to be a free-body stiffness i.e. stiffness measured for a component in isolation in a cantilever configuration, without other components present which may affect its stiffness.

[0367] When the force is applied perpendicular to the cantilevered beam, and at the free end of the beam, the resultant curvature is not constant but rather increases towards the fixed end of the beam.

Tilt Stiffness

[0368] A tilt stiffness is defined with reference to FIG. 5, which shows the resulting deformation of a cantilevered beam 401 under a moment M applied at its free end. The tilt stiffness is a measure of the resistance to rotation of a point on the component at which a moment is applied. As can be seen in FIG. 5, an applied moment at the free end of the cantilevered beam induces a constant curvature along the length of the beam between its free and fixed ends. The applied moment M causes a rotation θ of the point at which it is applied. For any component of constant section (like the beam), the angle θ is constant along the length of the component. The tilt stiffness as defined herein therefore has SI units of Nm/rad.

[0369] The tilt stiffness may be expressed as an effective linear tilt stiffness for a component having a given radius by expressing the tilt stiffness in terms of a pair of equal and opposite forces, F, acting at either end of that radius (rather than the moment) and the arc displacement at that radius (i.e. displacement measured along a circumference of a circle having that radius). An approximate or overall tilt angle, a, may be defined for the purposes of calculating the effective linear stiffness. The arc displacement may be referred to as rα. The effective linear tilt stiffness is given by the ratio of effective force divided by the displacement, F/rα and has the units N/m.

Torsional Stiffness

[0370] Torsional stiffness is a measure of deformation for a given torque. FIG. 6 illustrates the definition of the torsional stiffness of a shaft 401 or other body. A torque, τ, applied to the free end of the beam causes a rotational deformation, θ (e.g. twist) along the length of the beam. The torsional stiffness is the torque applied for a given angle of twist i.e. τ/θ. The torsional stiffness has SI units of Nm/rad.

[0371] An effective linear torsional stiffness may be determined for a component having a given radius. The effective linear torsional stiffness is defined in terms of an equivalent tangential force applied at a point on that radius (with magnitude of torque divided by the radius) and the distance δ (with magnitude of the radius multiplied by θ) moved by a point corresponding to the rotational deformation θ of the component.

[0372] The following general definitions of other parameters may also be used herein:

Torque

[0373] Torque, which may also be referred to as moment, is the rotational equivalent of linear force, and can be thought of as a twist to an object.

[0374] The magnitude, τ, of torque, τ, of a body depends on three quantities: the force applied (F), the lever arm vector connecting the origin to the point of force application (r), and the angle (A) between the force and lever arm vectors:

τ=r×F

τ=|τ|=r×F|=|r∥F|sin A

where [0375] τ is the torque vector and τ is the magnitude of the torque; [0376] r is the position vector or “lever arm” vector (a vector from the selected point on the body to the point where the force is applied); [0377] F is the force vector; [0378] × denotes the cross product; and [0379] A is the angle between the force vector and the lever arm vector (sin(A) is therefore one when the force vector is perpendicular to the position vector, such that τ=rF, i.e. magnitude of the force multiplied by distance between the selected point on the body and the point of application of the force).

[0380] Torque has dimensions of [force]×[distance] and may be expressed in units of Newton metres (N.Math.m).

[0381] The net torque on a body determines the rate of change of the body's angular momentum.

Moment of Inertia

[0382] Moment of inertia, otherwise known as angular mass or rotational inertia, is a quantity that determines the torque needed for a desired angular acceleration of a body about a rotational axis—this is substantially equivalent to how mass determines the force needed for a particular acceleration.

[0383] Moment of inertia depends on the body's mass distribution and the axis chosen, with larger moments requiring more torque to change the body's rotation rate. Moment of inertia has dimensions of [mass]×[distance].sup.2 and may be expressed in units of kilogram meter squared (kg. m.sup.2).

[0384] Moment of inertia I is defined as the ratio of the net angular momentum L of a body to its angular velocity ω around a principal axis:

[00047] $I = \frac{L}{ω}$

[0385] Provided that the shape of the body does not change, its moment of inertia appears in Newton's law of motion as the ratio of an applied torque τ on a body to the angular acceleration α around the principal axis:

τ=Iα

[0386] For bodies constrained to rotate in a plane, only the moment of inertia about an axis perpendicular to the plane matters, and I can therefore be represented as a scalar value. The skilled person would appreciate that a fan of a gas turbine engine (and more generally a fan rotor of the gas turbine engine comprising the fan disc and blades, and optionally also the fan shaft and/or other related components) is constrained to rotate in only one plane—a plane perpendicular to the engine axis—and that the fan's moment of inertia can therefore be defined by a single, scaler, value.

[0387] The fan's moment of inertia about the engine axis can therefore be measured or defined using any standard methodology.

[0388] More specific definitions of stiffnesses and other parameters relating to embodiments described herein are provided below for ease of understanding.

Fan Shaft Mounting Structure Stiffness

[0389] An embodiment of the gas turbine engine having a star configuration gearbox is shown in FIG. 7. Corresponding reference numbers to those used when describing the embodiment of FIGS. 1 and 2 have been used. FIG. 7 illustrates a close up of the engine core that shows the mounting of the fan shaft 36. The fan shaft 36 is mounted within the engine by a fan shaft mounting structure 503. The fan shaft mounting structure 503 comprises at least two bearings connected to or otherwise in engagement with the fan shaft at points spaced apart axially along the length of the engine. The fan shaft mounting structure 503 may take a number of different forms, and may comprise one or more separate supporting structures provided to support the fan shaft. It may also include other structures provided to support the fan shaft such as inter-shaft bearings. It therefore includes any supporting structure that extends between a bearing in contact with the fan shaft and a stationary structure of the engine (e.g. of the engine core).

[0390] In the arrangement shown in FIG. 7, the fan shaft mounting structure 503 comprises two bearings, a first supporting bearing 506a and a second supporting bearing 506b, via which it is coupled to the fan shaft 36. The supporting bearings 506a, 506a are spaced apart along the axial length of the fan shaft 36. In the described arrangement, both supporting bearings 506a, 506b are provided at positions that are forward of the gearbox 30. In other arrangements, one of the two supporting bearing 506a, 506b used to support the fan shaft 36 may be located at a position rearward of the gearbox 30, as will be described later. In yet other arrangements, more than two supporting bearings may be provided as part of the fan shaft mounting structure or fan shaft mounting structure.

[0391] In addition to the supporting bearings 506a, 506b described above the fan shaft mounting structure may also comprise additional non-supporting bearings. These may be additional softly mounted bearing or provided as part of a gearbox output shaft supporting structure as described in more detail later. The supporting bearings may be defined as those having a primary function of locating the fan shaft within the engine, rather than having a primary function of aligning other components such as the gearbox components.

[0392] The supporting bearings may be considered to be those transmitting an equal share of the total load that is an order of magnitude greater than any non-supporting bearing. More specifically, a supporting bearing may be defined as any bearing that transmits a load greater than 1/(10n) of the total load transmitted by the mounting structure of which it is a part, where n is the total number of bearings provided in that mounting structure. For example, for a mounting structure having three bearings, any that contribute less than 1/30th (i.e. (⅓)10) would be considered insignificant and so not considered to be supporting bearings within the meaning of this application.

[0393] FIG. 7 shows a schematic example of a suitable fan shaft mounting structure 503. Other forms of mounting structure may however be used to support the fan shaft. The fan shaft mounting structure 503 is coupled to the stationary support structure 24 of the engine so as to provide a stationary mounting for the fan shaft within the engine (with rotation relative to the static structure of the engine core provided by the bearings 506a, 506b). In the presently described arrangement, the stationary support structure 24 is an engine section stator (ESS) that acts as both a structural component to provide a stationary mounting for components such as the fan shaft 36, and as a guide vane provided to direct airflow from the fan 23. In other embodiments, the stationary supporting structure 24 may comprise a strut extending across the core gas flow path and a separate stator vane provided to direct airflow, or any other suitable stationary structure relative to which the fan shaft may be mounted.

[0394] The fan shaft mounting structure 503 is considered to comprise the component or components extending between the point of contact between each of the bearings 506a, 506b and the fan shaft 36 and the stationary support structure 24. Any number of separate components may be provided between these points in order to provide a coupling between the fan shaft 36 and the stationary support structure 24. The fan shaft mounting structure 503 is shown schematically in FIG. 7 for illustration only and other shapes and arrangements may be provided. For example, as discussed above additional bearings may be provided. These may be non-supporting softly mounted bearings, and/or additional bearings provided for redundancy in cases of high load transmission by the support structure. These additional bearings may be provided forward or rearward of each of the first and second bearings 506a, 560b. Any additional bearings provided for redundancy may protect against failure modes of the primary components, either in response to normal operating loads (e.g. fatigue) or higher loads from failure cases (e.g. Fan Blade Off). In some embodiments, any of the bearings (e.g. the first and/or second bearings 506a, 506b) may be double bearings formed by providing a pair of bearings in the same bearing housing. This arrangement may be used, for example, when the magnitude of the normal operating load would exceed that which would give reliable service with the use of single bearings.

[0395] The fan shaft mounting structure 503 has a degree of flexibility characterized by its radial stiffness and its tilt stiffness.

Fan Shaft Mounting Structure Radial Bending Stiffness:

[0396] The radial bending stiffness of the fan shaft mounting structure 503 is defined with reference to FIGS. 8 and 9. The radial bending stiffness represents the resistance of the fan shaft mounting structure 503 to a radial force. The radial bending stiffness is determined by treating the components forming the fan shaft mounting structure 503 as a free body that is fixed at its point (or points) of connection with the stationary support structure (or structures) of the engine core (e.g. ESS 24). In order to evaluate the radial bending stiffness of the fan shaft mounting structure 503 a radial force F is applied to a point on the fan (e.g. at the interface between the fan blades and the fan hub) along its axial centreline Z (i.e. along an axial centreline of the blades forming the fan). Specifically, the radial force F is applied along the fan axial centreline (wherein the fan axial centreline is defined as the axial midpoint of the fan blades, e.g. the midpoint of the fan blades in an axial plane of the engine).

[0397] Application of this force causes a radial displacement 6 of the point of contact between the supporting bearings 506a, 506b and the fan shaft. Deformation of the fan shaft mounting structure 503 caused by the applied force is illustrated in FIG. 9, with the shape when no force is applied shown in broken lines for comparison. The force, F, is shown radially away from the engine axis 9, but could equivalently be a force in a radial direction toward the centreline 9.

[0398] The radial bending stiffness of the fan shaft mounting structure 503 is defined as the force F divided by the average displacement at the supporting bearings provided as part of the fan shaft mounting structure forward of the gearbox 30. These displacements are illustrated in the close-up view of FIG. 10 in which only the bearings are shown for ease of explanation. For the arrangement shown in FIGS. 9 and 10, the radial bending stiffness is therefore given by:

[00048] $\frac{F}{1 / 2 (δ_{a} + δ_{b})}$

[0399] The radial bending stiffness of the fan shaft mounting structure has units of N/m.

[0400] In various embodiments, the radial bending stiffness of the fan shaft mounting structure may be greater than or equal to 7.00×10.sup.8 N/m and optionally greater than or equal to 1.25×10.sup.9 N/m.

[0401] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the radial bending stiffness of the fan shaft mounting structure may be greater than or equal to 7.0×10.sup.8 N/m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the radial bending stiffness of the fan shaft mounting structure may be greater than or equal to 1.4×10.sup.9 N/m.

[0402] In various embodiments, the radial bending stiffness of the fan shaft mounting structure may be in the range from 7.00×10.sup.8 N/m to 6.00×10.sup.11 N/m, and optionally in the range from 1.25×10.sup.9 N/m to 2.0×10.sup.11 N/m.

[0403] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the radial bending stiffness of the fan shaft mounting structure may be in the range from 7.0×10.sup.8 N/m to 5.0×10.sup.11 N/m and optionally in the range from 7.0×10.sup.8 N/m to 2.3×10.sup.9 N/m (and may be equal to 1.5×10.sup.9 N/m).

[0404] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the radial bending stiffness of the fan shaft mounting structure may be in the range from 1.4×10.sup.9 N/m to 6.0×10.sup.11 N/m, and optionally in the range from 1.4×10.sup.9 N/m to 3.0×10.sup.9 N/m (and may be equal to 2.2×10.sup.9 N/m).

Fan Shaft Mounting Structure Tilt Stiffness:

[0405] The tilt stiffness of the fan shaft mounting structure 503 is determined in a similar way to the radial bending stiffness except that a moment, M, is applied at a point on the fan blade axial centre line in place of the Force, F. An example of how the tilt stiffness can be determined is illustrated in FIGS. 11 and 12. The fan shaft mounting structure tilt stiffness represents its resistance to an applied moment. Referring to FIGS. 11 and 12, moment M is applied to the axial centre line of the fan blades (i.e. the same place as the force, F, in FIG. 8 described above). The tilt stiffness is determined by calculating a straight line shaft angle between a pair of bearing positions at which the fan shaft is supported. The application of the moment M causes displacements in opposite directions at two bearing positions as illustrated in FIG. 13, which shows a close-up view including only the bearings for ease of explanation. The straight line angle θ is defined as the change in angle of an axis R extending through the point of contact between the bearings 506a, 506b and the fan shaft when the moment M is applied. As can be seen in FIG. 13, the angle θ extended between axis R when no moment is applied and R′ when moment M is applied. The tilt stiffness is defined as M/θ, and has units of Nm/rad.

[0406] The tilt stiffness of the fan shaft mounting structure is defined by measuring the angle θ between the first two supporting bearings in a direction moving reward from the fan. In the arrangement shown in FIGS. 11, 12 and 13 this corresponds to supporting bearings 506a, 506b provided forward of the gearbox. In other arrangements the tilt stiffness may be defined using the displacement at a supporting bearing rearward of the gearbox (e.g. if only one supporting bearing is provided forward of the gearbox).

[0407] In various embodiments, the tilt stiffness of the fan shaft mounting structure may be greater than or equal to 1.50×10.sup.7 Nm/rad, and optionally greater than or equal to 2.1×10.sup.7 Nm/rad.

[0408] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the tilt stiffness of the fan shaft mounting structure may be greater than or equal to 2.1×10.sup.7 Nm/rad or greater than or equal to 2.3×10.sup.7 Nm/rad. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the tilt stiffness of the fan shaft mounting structure may be greater than or equal to 3.8×10.sup.7 Nm/rad or greater than or equal to 7.3×10.sup.7 Nm/rad.

[0409] In various embodiments, the tilt stiffness of the fan shaft mounting structure may be in the range from 1.5×10.sup.7 Nm/rad to 2.70×10.sup.10 Nm/rad, and optionally in the range from 2.1×10.sup.7 Nm/rad to 1×10.sup.10 Nm/rad.

[0410] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the tilt stiffness of the fan shaft mounting structure may be in the range from 2.1×10.sup.7 Nm/rad to 1.9×10.sup.10 Nm/rad and optionally in the range from 2.3×10.sup.7 Nm/rad to 4.3×10.sup.7 Nm/rad (and may be equal to 3.3×10.sup.7 Nm/rad).

[0411] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the tilt stiffness of the fan shaft mounting structure may be in the range from 3.8×10.sup.7 Nm/rad to 2.7×10.sup.10 Nm/rad, and optionally in the range from 7.1×10.sup.7 Nm/rad to 9.1×10.sup.7 Nm/rad (and may be equal to 8.1×10.sup.7 Nm/rad).

[0412] Equivalent radial and tilt stiffness may be defined for other arrangements of fan shaft mounting structures. Various other arrangements of fan shaft mounting structure 503 are shown in FIGS. 14 to 19. These embodiments are provided as examples. The skilled person will understand that others arrangements are possible and are considered to fall within the scope of the present disclosure.

[0413] FIG. 14 shows a schematic representation of an arrangement in which the fan shaft 36 is supported by the fan shaft mounting structure already described having first and second supporting bearings 506a, 506b forward of the gearbox 30. In all of FIGS. 14 to 19 the core shaft 26 is supported by two core shaft bearings 507a, 507b. These bearing are connected to a stationary structure 24a of the engine via a core shaft support structure. In the arrangement of FIG. 14, the fan shaft mounting structure 503 comprises both the first and second bearings 506a, 506b and the structure linking them to the stationary support structure 24 (i.e. the fan shaft support structure 504). Any additional bearing or bearings provided as part of this structure is included in the fan shaft mounting structure 503. In the arrangement of FIG. 14, a non-supporting softly mounted bearing 506a′ is also provided. As discussed above, the radial bending or tilt stiffness is determined without considering displacements at this non-supporting bearing 506a′.

[0414] FIG. 15 shows an arrangement in which the fan shaft is supported by a first supporting bearing 506a forward of the gearbox 30 and a second supporting bearing 506b located at a position rearward of the gearbox 30. In this embodiment, the fan shaft 36 extends through the gearbox 30 and is mounted by supporting bearings 506a, 506b either side of the gearbox. The first supporting bearing 506a is therefore located forward of the gearbox 30 and the second supporting bearing 506b is located rearward of the gearbox 30. In this embodiment, the second supporting bearing 506b is an inter-shaft bearing between the fan shaft 36 and the core shaft 26. For this arrangement the radial bending stiffness is determined by measuring the displacement at the first supporting bearing 506a only. The tilt stiffness is however determined by measuring the change in angle of an axis linking the first and second supporting bearings 506a, 506b.

[0415] FIG. 16 shows an arrangement in which the fan shaft is supported by the first and second supporting bearings 506a, 506b either side of the gearbox. In this embodiment, the fan shaft mounting structure 503 further comprises a third supporting bearing 506c that is located between the first supporting bearing 506a and the gearbox 30. In this arrangement the radial bending stiffness of the fan shaft mounting structure is determined by measuring an average displacement at the first and third supporting bearings 506a, 506c. Any displacement at of the second supporting bearing 506b is not included as it is not forward of the gearbox 30. The tilt stiffness is determined by measuring the change in angle of an axis linking the first and third supporting bearings 506a, 506c as they are the first two bearings rearward of the fan.

[0416] FIG. 17 shows a modification to the arrangement of FIG. 14 in which the fan shaft 36 comprises a gearbox output shaft 36a. The gearbox output shaft 36a forms a flexible portion of the fan shaft 36 (i.e. more flexible relative to an intermediate portion, e.g. the fan portion 36a, of the fan shaft to which it is connected) at the end at which it connects to the gearbox 30. The gearbox output shaft 36a is supported by a bearing 508a forming part of a gearbox output shaft support structure 41. In this embodiment, the fan shaft mounting structure 503 therefore comprises both the first and second bearings 506a, 506b and the structure linking them to the stationary support structure 24 (i.e. the fan shaft support structure 504) and the gearbox output shaft support structure 41 including its bearing 508a. As the bearing 508a of the gearbox output shaft support structure is a non-supporting bearing it is not taken into account when measuring the radial bending stiffness. For this arrangement the radial bending stiffness is again determined by measuring the average of the displacement at the first and second supporting bearings 506a, 506b. The tilt stiffness is also measured at the first and second bearings 506a, 506b.

[0417] FIG. 18 shows an arrangement in which the gearbox output shaft 36a extends in an axial direction forward and rearwards of the gearbox 30, and is supported either side of the gearbox 30 by a first gearbox output shaft bearing 508a, and a second gearbox output shaft bearing 508b. Both the first and second gearbox output shaft bearings 508a, 508b form part of the gearbox output shaft supporting structure 41 and so are connected to a stationary structure of the engine. In this arrangement, the fan shaft mounting structure 503 comprises both the first and second bearings 506a, 506b and the structure linking them to the stationary support structure 24 (i.e. the fan shaft support structure 504) and the gearbox output shaft support structure 41 including its two bearings. Again the output shaft bearings 508a, 508b are not included in the measurement of the radial bending stiffness of the fan shaft mounting structure as they are non-supporting bearings. The tilt stiffness is also measured at the first and second supporting bearings.

[0418] FIG. 19 shows an arrangement having the same gearbox output shaft support as that of FIG. 16, but in which the second bearing 506b provided to support the fan shaft 36 is provided rearward of the gearbox 30 as described in connection with FIG. 16. In this arrangement, the fan shaft mounting structure includes: the fan shaft support structure 504; the gearbox support structure 41 and its bearings; and the inter-shaft bearing 506b and the components linking it to the stationary structure of the engine 24a. The radial bending stiffness is determined by measuring a displacement of the first supporting bearing 506a. Any displacement at the second supporting bearing 506b is not included as it is not forward of the gearbox. Any displacement at the gearbox output shaft bearings 508a, 508b is also not included as they are non-supporting bearings. The tilt stiffness is determined by measuring the angle of an axis linking the first supporting bearing 506a and the second supporting bearing 506b.

Fan Shaft Stiffness

[0419] The stiffness of the fan shaft is defined with reference to FIGS. 20 to 26. The fan shaft 36 is defined as the torque transfer component that extends from the output of the gearbox to the fan input. It therefore includes part or all of any gearbox output shaft and fan input shaft that may be provided between those points. For the purposes of defining the stiffness of the fan shaft 36 it is considered to extend between a fan input position and a gearbox output position, and includes all of the torque transfer components between those points. It does not therefore include any components of the gearbox itself (e.g. the planet carrier or connecting plate coupled to it) which transmit discrete forces, rather than the fan shaft torque. The gearbox output position therefore may be defined as the point of connection between the fan shaft 36 and the gearbox 30. The fan input position may be defined as the point of connection between the fan shaft 36 and the fan.

[0420] Referring to FIG. 20, where the gearbox is in a star configuration, the gearbox output position is defined as the point of connection 702 between the ring gear 38 and the fan shaft 36. More specifically, it is the point of connection to the annulus of the ring gear 38 (with any connection component extending from the outer surface of the annulus being considered to be part of the ring gear). Where the point of connection is formed by an interface extending in a direction having an axial component, the point of connection is considered to be the axial centreline of that interface as illustrated in FIG. 22.

[0421] The fan shaft 36 includes all torque transmitting components up to the point of connection 702 with the ring gear 38. It therefore includes any flexible portions or linkages 704 making up the fan shaft 36 that may be provided, and any connection(s) 706 (e.g. spline connections) between them.

[0422] Where the gearbox 30 is in a planetary configuration, the gearbox output position is again defined as the point of connection between the fan shaft 36 and the gearbox 30. An example of this is illustrated in FIG. 21, which shows a carrier comprising a forward plate 34a and rearward plate 34b, with a plurality of pins 33 extending between them and on which the planet gears are mounted. The fan shaft 36 is connected to the forward plate 34a via a connection 708 (e.g. a spline connection). In an embodiment such as this, the gearbox output position is taken as any point on the interface between the fan shaft 36 and the forward plate 34a. The forward plate 34a is considered to transmit discrete forces, rather than a single torque, and so is taken to be part of the gearbox 30 rather than the fan shaft. FIG. 21 shows only one example of a type of connection between the fan shaft and planet carrier 34. In embodiments having different connection arrangements, the gearbox output position is still taken to be at the interface between components transmitting a torque (i.e. that are part of the fan shaft) and those transmitting discrete forces (e.g. that are part of the gearbox). The spline connection 708 is only one example of a connection that may be formed between the fan shaft and gearbox (i.e. between the fan shaft and the forward plate 34b in the presently described arrangement). In other embodiments, the interface which forms the gearbox output position may be formed by, for example, a curvic connection, a bolted joint or other toothed or mechanically dogged arrangement.

[0423] Referring to FIG. 22, the fan input position is defined as a point on the fan shaft at the axial midpoint of the interface between the fan and the fan shaft. In the presently described arrangement, the fan 23 comprises a support arm 23a arranged to connect the fan 23 to the fan shaft 36. The support arm 23a is connected to the fan shaft by a spline coupling 36c (shown in FIG. 22) that extends along the length of a portion of the fan shaft 36. The fan input position is defined as the axial midpoint of the spline coupling as indicated by axis Y in FIG. 22. The spline coupling shown in FIG. 22 is only one example of a coupling that may form the interface between the fan and fan shaft. In other embodiments, for example, a curvic connection, a bolted joint or other toothed or mechanically dogged arrangement may be used.

[0424] FIG. 26 illustrates an arrangement in which an alternative coupling is provided between the fan 23 and the fan shaft 36. Similarly to FIG. 22, the fan 23 is coupled to the fan shaft 36 via a support arm 23a. In this arrangement however, a flange coupling 36d is provided between the support arm 23a and the fan shaft 36. In this embodiment, the support arm 23a is connected at the rear of the fan hub. The flange coupling 36d may be a curvic coupling. In other embodiments, other forms of flange coupling may be provided. In the embodiment of FIG. 26, the fan input position is the axial midpoint of the flange coupling.

[0425] The fan shaft 36 has a degree of flexibility characterized by its radial bending stiffness and tilt stiffness.

Fan Shaft End Stiffness at Fan Input and Gearbox Output:

[0426] The stiffness of each end of the fan shaft where it couples to the fan 23 and the gearbox 30 is defined with reference to FIGS. 22 to 25.

[0427] The radial bending stiffness of the fan shaft 36 at the input to the fan 23 is measured by applying a force F.sub.1 to the fan shaft at the fan shaft input position defined above (illustrated in FIG. 22). The fan shaft 36 is treated as a free body, and is held fixed at the position of all of bearing positions at which it is supported i.e. the first and second supporting bearings 506a,506b in the embodiment of FIG. 22. As a result of the application of the force F.sub.1 the fan shaft 36 deforms so that the fan shaft input position is displaced by a distance of δ.sub.1 (see FIG. 23). The radial bending stiffness of the fan shaft 36 at the input to the fan 23 is then given by F.sub.1/δ.sub.1.

[0428] In various embodiments, the radial bending stiffness of the fan shaft at the input to the fan may be greater than or equal to 3.00×10.sup.6 N/m, and optionally greater than or equal to 6.3×10.sup.6 N/m.

[0429] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the radial bending stiffness of the fan shaft at the input to the fan may be greater than or equal to 6.4×10.sup.6 N/m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the radial bending stiffness of the fan shaft at the input to the fan may be greater than or equal to 6.9×10.sup.6 N/m or greater than or equal to 8.9×10.sup.6 N/m.

[0430] In various embodiments, the radial bending stiffness of the fan shaft at the input to the fan may be in the range from 3.00×10.sup.6 N/m to 2.00×10.sup.9 N/m, and optionally in the range from 6.3×10.sup.6 N/m to 1.0×10.sup.9 N/m.

[0431] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the radial bending stiffness of the fan shaft at the input to the fan may be in the range from 6.4×10.sup.6 N/m to 1.0×10.sup.9 N/m and optionally in the range from 6.4×10.sup.6 N/m to 7.6×10.sup.6 N/m (and may be equal to 7.0×10.sup.6 N/m).

[0432] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the radial bending stiffness of the fan shaft at the input to the fan may be in the range from 6.9×10.sup.6 N/m to 2.0×10.sup.9 N/m, and optionally in the range from 8.9×10.sup.6 N/m to 1.1×10.sup.7 N/m (and may be equal to 9.9×10.sup.6 N/m).

[0433] The tilt stiffness of the fan shaft 36 at the input to the fan 23 is measured by applying a moment M.sub.1 to the fan shaft at the fan shaft input position defined above (illustrated in FIG. 24). The fan shaft 36 is again treated as a free body, and is held fixed at the position of all of the bearing positions at which it is supported i.e. the first and second supporting bearings 506a,506b in the arrangement of FIG. 24. As a result of moment M.sub.1 the fan shaft 36 deforms so that the fan shaft input position is displaced by an angular displacement of θ.sub.1 as illustrated in FIG. 25. The tilt stiffness of the fan shaft 36 at the input to the fan 23 is then given by M.sub.1/θ.sub.1.

[0434] In various embodiments, the tilt stiffness of the fan shaft at the input to the fan may be greater than or equal to 5.00×10.sup.5 Nm/rad, and optionally greater than or equal to 9.0×10.sup.5 Nm/rad.

[0435] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the tilt stiffness of the fan shaft at the input to the fan may be greater than or equal to 9.5×10.sup.5 Nm/rad. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the tilt stiffness of the fan shaft at the input to the fan may be greater than or equal to 1.5×10.sup.6 Nm/rad or greater than or equal to 2.5×10.sup.6 Nm/rad.

[0436] In various embodiments, the tilt stiffness of the fan shaft at the input to the fan may be in the range from 5.00×10.sup.5 Nm/rad to 7.00×10.sup.8 Nm/rad, and optionally in the range from 9.0×10.sup.5 Nm/rad to 3.5×10.sup.8 Nm/rad.

[0437] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the tilt stiffness of the fan shaft at the input to the fan may be in the range from 9.5×10.sup.5 Nm/rad to 3.5×10.sup.8 Nm/rad and optionally in the range from 9.5×10.sup.5 Nm/rad to 1.9×10.sup.6 Nm/rad (and may be equal to 1.2×10.sup.6 Nm/rad).

[0438] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the tilt stiffness of the fan shaft at the input to the fan may be in the range from 1.5×10.sup.6 Nm/rad to 7.0×10.sup.8 Nm/rad, and optionally in the range from 2.5×10.sup.6 Nm/rad to 4.5×10.sup.6 Nm/rad (and may be equal to 3.5×10.sup.6 Nm/rad).

[0439] The radial bending stiffness of the fan shaft 36 at the output of the gearbox 30 is measured by applying a force F.sub.2 to the fan shaft at the gearbox output position defined above (illustrated in FIG. 22). The fan shaft 36 is treated as a free body, and is held fixed at the position of all of bearing positions at which it is supported i.e. the first and second supporting bearings 506a,506b in the arrangement of FIG. 22. As a result of force F.sub.2 the fan shaft 36 deforms so that the gearbox output position is displaced by a distance of 62 (as illustrated in FIG. 23). The radial bending stiffness of the fan shaft 36 at the output of the gearbox is then given by F.sub.2/δ.sub.2.

[0440] In various embodiments, the radial bending stiffness of the fan shaft at the output of the gearbox may be greater than or equal to 4.00×10.sup.6 N/m, and optionally greater than or equal to 3.7×10.sup.7 N/m.

[0441] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the radial bending stiffness of the fan shaft at the output of the gearbox may be greater than or equal to 3.7×10.sup.7 N/m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the radial bending stiffness of the fan shaft at the output of the gearbox may be greater than or equal to 3.9×10.sup.7 N/m or greater than or equal to 5.0×10.sup.7 N/m.

[0442] In various embodiments, the radial bending stiffness of the fan shaft at the output of the gearbox may be in the range from 4.00×10.sup.6 N/m to 1.5×10.sup.9 N/m, and optionally in the range from 3.7×10.sup.7 N/m to 1.0×10.sup.9 N/m.

[0443] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the radial bending stiffness of the fan shaft at the output of the gearbox may be in the range from 3.7×10.sup.7 N/m to 5.0×10.sup.8 N/m and optionally in the range from 3.7×10.sup.7 N/m to 5.0×10.sup.7 N/m (and may be equal to 4.0×10.sup.7 N/m).

[0444] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the radial bending stiffness of the fan shaft at the output of the gearbox may be in the range from 3.9×10.sup.7 N/m to 1.5×10.sup.9 N/m, and optionally in the range from 5.0×10.sup.7 N/m to 9.0×10.sup.7 N/m (and may be equal to 7.0×10.sup.7 N/m).

[0445] The tilt stiffness of the fan shaft 36 at the output of the gearbox 30 is measured by applying a moment M.sub.2 to the fan shaft at the gearbox output position defined above (illustrated in FIG. 24). The fan shaft 36 is again treated as a free body, and is held fixed at the position of all of the bearing positions at which it is supported i.e. the first and second supporting bearings 506a,506b in the embodiment of FIG. 24. As a result of moment M.sub.2 the fan shaft 36 deforms so that the gearbox output position is displaced by an angular displacement of θ.sub.2 as illustrated in FIG. 25. The tilt stiffness of the fan shaft 36 at the output of the gearbox is then given by M.sub.2/θ.sub.2.

[0446] In various embodiments, the tilt stiffness of the fan shaft at the output of the gearbox may be greater than or equal to 7.00×10.sup.4 Nm/rad, and optionally greater than or equal to 9.5×10.sup.5 Nm/rad.

[0447] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the tilt stiffness of the fan shaft at the output of the gearbox may be greater than or equal to 9.5×10.sup.5. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the tilt stiffness of the fan shaft at the output of the gearbox may be greater than or equal to 1.1×10.sup.6 Nm/rad and optionally may be greater than or equal to 2.6×10.sup.6 Nm/rad.

[0448] In various embodiments, the tilt stiffness of the fan shaft at the output of the gearbox may be in the range from 7.00×10.sup.4 Nm/rad to 7.00×10.sup.7 Nm/rad, and optionally in the range from 9.5×10.sup.5 Nm/rad to 3.5×10.sup.7 Nm/rad.

[0449] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the tilt stiffness of the fan shaft at the output of the gearbox may be in the range from 9.5×10.sup.5 Nm/rad to 2.0×10.sup.7 Nm/rad and optionally in the range from 9.5×10.sup.5 Nm/rad to 2.2×10.sup.6 Nm/rad (and may be equal to 1.2×10.sup.6 Nm/rad).

[0450] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the tilt stiffness of the fan shaft at the output of the gearbox may be in the range from 1.1×10.sup.6 Nm/rad to 7.0×10.sup.7 Nm/rad, and optionally in the range from 2.6×10.sup.6 Nm/rad to 4.6×10.sup.6 Nm/rad (and may be equal to 3.6×10.sup.6 Nm/rad).

Relative Distance to and Between Fan Shaft Supporting Bearings

[0451] Referring to FIG. 22, the fan shaft end stiffness is at least partly determined according to the respective distance between the fan input position, the gearbox output position and the bearings by which the fan shaft is supported. For example, the stiffness of the fan shaft at the fan input may depend on the axial distance d.sub.1 between the fan input position and the closest supporting bearing rearward of the fan i.e. the first supporting bearing 506a in the arrangement of FIG. 22. The axial position of the first supporting bearing 506a is taken to be its axial centreline. The stiffness of the fan shaft at the gearbox output may depend on the axial distance d.sub.2 between the gearbox output position and the closest supporting bearing forward of the gearbox i.e. the second supporting bearing 506b in the arrangement of FIG. 22. The axial position of the second supporting bearing 506b is taken to be its axial centreline. The closest supporting bearing forward of the gearbox output position does not include any non-supporting flexible or softly mounted bearings that also may be coupled to the fan shaft as described above in connection with the measurement of the stiffness of the fan shaft mounting structure. It does not, for example, include the gearbox output shaft bearing 508a included in arrangements such as those shown in FIGS. 17, 18 and 19. By ‘supporting bearing’ we therefore again mean a bearing transmitting a significant amount of the load transmitted by the fan shaft supporting structure during normal operation as defined elsewhere herein.

[0452] The gas turbine engine 10 may be configured such that the relative positions of the bearings (e.g. axial distances d.sub.1 and d.sub.2) provide a fan shaft end stiffness within the desired range.

[0453] In various embodiments, distance d.sub.1 may be greater than or equal to 0.12 m, and optionally greater than or equal to 0.13 m.

[0454] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.1 may be greater than or equal to 0.12 m or greater than or equal to 0.13 m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.1 may be greater than or equal to 0.13 m or greater than or equal to 0.15 m.

[0455] In various embodiments, distance d.sub.1 may be in the range from 0.12 m to 0.40 m, and optionally in the range from 0.13 m to 0.30 m.

[0456] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.1 may be in the range from 0.12 m to 0.30 m and optionally in the range from 0.13 m to 0.15 m (and may be equal to 0.14 m).

[0457] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.1 may be in the range from 0.13 m to 0.40 m and optionally in the range from 0.15 m to 0.25 m (and may be equal to 0.20 m).

[0458] In various embodiments, distance d.sub.2 may be greater than or equal to 0.15 m, and optionally greater than or equal to 0.16 m.

[0459] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.2 may be greater than or equal to 0.15 m or greater than or equal to 0.16 m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.2 may be greater than or equal to 0.16 m or greater than or equal to 0.20 m.

[0460] In various embodiments, distance d.sub.2 may be in the range from 0.15 m to 0.45 m, and optionally in the range from 0.16 m to 0.40 m.

[0461] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.2 may be in the range from 0.15 m to 0.35 m and optionally in the range from 0.16 m to 0.18 m (and may be equal to 0.17 m).

[0462] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.2 may be in the range from 0.16 m to 0.45 m and optionally in the range from 0.20 m to 0.28 m (and may be equal to 0.24 m).

[0463] In the arrangement of FIG. 15, the distance d.sub.1 corresponds to the distance between the first supporting bearing 506a and the fan input position. The distance d.sub.2 corresponds to the distance between the first supporting bearing 506a and the gearbox input position.

[0464] A bearing axial separation d.sub.3 is defined as the axial distance between the first supporting bearing 506a and the second supporting bearing 506b as shown in FIG. 22. This distance therefore corresponds to the axial separation between the closest supporting bearing rearward of the fan (i.e. the first supporting bearing 506a) and the closest supporting bearing forward of the gearbox (i.e. the second supporting bearing 506b). Again, the axial position of the first and second supporting bearings 506a,b is taken to be their respective axial centreline.

[0465] In various embodiments, distance d.sub.3 may be in the range from 0.1 m to 0.4 m, and optionally in the range from 0.18 m to 0.32 mm and further optionally in the range from 0.20 m to 0.30 m.

[0466] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.3 may be in the range from 0.19 m to 0.23 m (and may be equal to 0.21 m).

[0467] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.3 may be in the range from 0.26 m to 0.30 m (and may be equal to 0.28 m).

[0468] An axial distance d.sub.4 is defined as the axial distance between the fan input position and the gearbox output position as can be seen in FIG. 22 (e.g. between Y and X). The axial distance d.sub.4 is equal to the sum of distances d.sub.1, d.sub.2 and d.sub.3 defined above.

[0469] The axial distance d.sub.3 may only be defined for arrangements in which the first and second supporting bearings 506a, 506b are located forward of the gearbox. The axial distance d.sub.4 is however defined for all embodiments. Where only one supporting bearing is located forward of the gearbox distance d.sub.3 is zero. An example of how d.sub.1, d.sub.2 and d.sub.4 may be defined for an arrangement in which a single supporting bearing is provided forward of the gearbox is shown in FIG. 15.

[0470] In various embodiments, distance d.sub.4 may be greater than or equal to 0.43 m, and optionally greater than or equal to 0.46 m.

[0471] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.4 may be greater than or equal to 0.43 m or greater than or equal to 0.48 m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.4 may be greater than or equal to 0.56 m or greater than or equal to 0.65 m.

[0472] In various embodiments, distance d.sub.4 may be in the range from 0.43 m to 0.95 m, and optionally in the range from 0.46 m to 0.85 m.

[0473] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, distance d.sub.4 may be in the range from 0.43 m to 0.62 m and optionally in the range from 0.48 m to 0.56 m (and may be equal to 0.52 m).

[0474] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, distance d.sub.4 may be in the range from 0.56 m to 0.95 m and optionally in the range from 0.65 m to 0.76 m (and may be equal to 0.71 m).

Relative Fan and Gearbox Positions

[0475] Referring again to FIG. 7, a fan-gearbox axial distance 110 is defined as the axial distance between the output of the gearbox (i.e. the axial position P of the gearbox output position) and the fan axial centreline Q. The fan axial centreline is defined as the axial midpoint of the fan blades forming the fan (and may correspond to axis Z). The gearbox output position is defined as the point of connection between the fan shaft 36 and the gearbox as defined elsewhere herein. This may be defined differently for different types of gearbox as described elsewhere herein.

[0476] In various embodiments, the fan-gearbox axial distance may be greater than or equal to 0.35 m, and optionally greater than or equal to 0.37 m.

[0477] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan-gearbox axial distance may be greater than or equal to 0.38 m or greater than or equal to 0.40 m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan-gearbox axial distance may be greater than or equal to 0.48 m or greater than or equal to 0.50 m.

[0478] In various embodiments, the fan-gearbox axial distance may be in the range from 0.35 m to 0.8 m, and optionally in the range from 0.37 m to 0.75 m.

[0479] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan-gearbox axial distance may be in the range from 0.38 m to 0.65 m and optionally in the range from 0.40 m to 0.44 m (and may be equal to 0.42 m).

[0480] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan-gearbox axial distance may be in the range from 0.48 m to 0.8 m and optionally in the range from 0.50 m to 0.68 m (and may be equal to 0.58 m).

Fan Moment of Inertia

[0481] The fan 23 has a moment of inertial IF. The moment of inertia of the fan is measured based on the total mass of the rotor forming the fan, i.e. including the total mass of the plurality of fan blades, the fan hub and any support arm or other linkages provided to connect the fan to the fan shaft. The moment of inertia therefore includes all rotating components apart from the fan shaft. The moment of inertia is the mass moment of inertia or rotational inertia of the fan with respect to rotation around the principal rotational axis 9 of the engine. Rotation of the fan will cause a gyroscopic effect meaning that the fan shaft will tend to maintain a steady direction of its axis of rotation. During maneuvering of the aircraft to which the gas turbine engine is mounted the orientation of the axis of rotation of the fan shaft will however change. The gyroscopic effect will result in a reaction force at the fan shaft mounting structure to resist the tendency of the fan shaft to maintain its orientation. The moment of inertia of the fan will have an effect on the magnitude of the gyroscopic effect produced, and so has an impact on the design of the fan shaft mounting structure as is discussed elsewhere herein.

[0482] In various embodiments, the moment of inertia of the fan may be greater than or equal to 7.40×10.sup.7 kgm.sup.2, and optionally greater than or equal to 8.3×10.sup.7 kgm.sup.2.

[0483] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the moment of inertia of the fan may be greater than or equal to 7.4×10.sup.7 kgm.sup.2 or 8.6×10.sup.7 kgm.sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the moment of inertia of the fan may be greater than or equal to 3.0×10.sup.8 kgm.sup.2 or 4.0×10.sup.8 kgm.sup.2.

[0484] In various embodiments, the moment of inertia of the fan may be in the range from 7.40×10.sup.7 kgm.sup.2 to 9.00×10.sup.8 kgm.sup.2, and optionally in the range from 8.3×10.sup.7 kgm.sup.2 to 6.5×10.sup.8 kgm.sup.2.

[0485] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the moment of inertia of the fan may be in the range from 7.4×10.sup.7 kgm.sup.2 to 1.5×10.sup.8 kgm.sup.2 and optionally in the range from 8.6×10.sup.7 kgm.sup.2 to 9.6×10.sup.7 kgm.sup.2 (and may be equal to 9.1×10.sup.7 kgm.sup.2).

[0486] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the moment of inertia of the fan may be in the range from 3.0×10.sup.8 kgm.sup.2 to 9.0×10.sup.8 kgm.sup.2 and optionally in the range from 4.0×10.sup.8 kgm.sup.2 to 5.0×10.sup.8 kgm.sup.2 (and may be equal to 4.5×10.sup.8 kgm.sup.2).

Power Transmitted by the Gearbox

[0487] Power is transmitted by the gearbox during operation of the engine. The power values given herein for the power transmitted by the gearbox are the power transmitted by the gearbox at maximum take-off conditions. The maximum take-off conditions are as defined elsewhere herein. The power transmitted by the gearbox is defined as the power at the gearbox output position defined elsewhere herein.

[0488] In various embodiments, the power transmitted by the gearbox may be greater than or equal to 2.25×10.sup.7 W, and optionally greater than or equal to 2.5×10.sup.7 W.

[0489] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, power transmitted by the gearbox may be greater than or equal to 2.25×10.sup.7 W or greater than or equal to 2.7×10.sup.7 W. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the power transmitted by the gearbox may be greater than or equal to 4.0×10.sup.7 W or greater than or equal to 5.0×10.sup.7 W.

[0490] In various embodiments, the power transmitted by the gearbox may be in the range from 2.25×10.sup.7 W to 1.00×10.sup.8 W, and optionally in the range from 2.5×10.sup.7 W to 8.0×10.sup.7 W.

[0491] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the power transmitted by the gearbox may be in the range from 2.25×10.sup.7 W to 3.6×10.sup.7 W and optionally in the range from 2.7×10.sup.7 W to 3.3×10.sup.7 W (and may be equal to 3.0×10.sup.7 W).

[0492] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the power transmitted by the gearbox may be in the range from 4.0×10.sup.7 W to 1.0×10.sup.8 W and optionally in the range from 5.0×10.sup.7 W to 6.0×10.sup.7 W (and may be equal to 5.5×10.sup.7 W).

Carrier Torsional Stiffness

[0493] The torsional stiffness of the gearbox carrier is defined with reference to FIGS. 27 to 31. FIG. 27 illustrates a close up view of the planet carrier provided in the gearbox of any arrangement of the engine described herein. As already described, the planet carrier 34 holds the planet gears 32 in place. In planetary gearboxes 30, there may be relatively large centrifugal forces to be reacted by the carrier 34. In star gearboxes, the centrifugal forces to react may be negligible or zero as there is no carrier rotation.

[0494] In the described arrangement, the planet carrier 34 comprises a forward plate 34a and a rearward plate 34b, and pins 33 extending between the plates, as illustrated in FIG. 27. The pins 33 are arranged to be parallel to the engine axis 9. In alternative embodiments, a plate 34b may be provided on only one side—no plate or only a partial plate may be provided on the other side of the carrier 34.

[0495] In the arrangement shown in FIG. 31, the carrier 34 additionally comprises lugs 34c, which may also be referred to as wedges or a web, between the forward and rearward plates 34a, 34b. The lugs 34c may increase the overall stiffness of the carrier 34.

[0496] The stiffness of the carrier 34 is selected to be relatively high to react centrifugal forces and/or to maintain gear alignment. The skilled person would appreciate that stiffness is a measure of the displacement that results from any applied forces or moments, and may not relate to strength of the component. Hence to react a high load, any stiffness is acceptable so long as the resulting displacement is tolerable. How high a stiffness is desired to keep a displacement within acceptable limits therefore depends on position and orientation of the gears, which is generally referred to as gear alignment (or mis-alignment).

[0497] Carrier torsional stiffness is a measure of the resistance of the carrier 34 to an applied torque, τ, as illustrated in FIG. 28 (axial cross-section) and FIGS. 29 to 31 (a radial cross-section). The axis of the torque is parallel to the engine axis 9.

[0498] The diagonal lining of the plate 34b at the rearward end of the carrier 30 indicates that plate 34b being treated as rigid and non-rotating (as for a cantilever beam mounting). In embodiments with only one plate 34a, the ends of the pins 33 (and of the lugs 34c if present) further from the single plate 34a are held in place instead.

[0499] A torque, τ, is applied to the carrier 34 (at the position of the axial mid-point of the forward plate 34a) and causes a rotational deformation, θ (e.g. twist) along the length of the carrier 34. The twist causes the carrier 34 to “wind up” as the ends of the pins 33 (and of the lugs 34c if present) are held at a fixed radius on the carrier plates 34a, 34b.

[0500] The angle through which a point on an imaginary circle 902 on the forward plate 34a passing through the rotation axis of each pin 33 moves is θ where θ is the angle measured in radians. The imaginary circle 902 may be referred to as the pin pitch circle diameter (pin PCD). The pin PCD may be in the range from 0.38 m to 0.65 m, for example being equal to 0.4 m or 0.55 m. An effective linear torsional stiffness can therefore be defined for the carrier 34 as described above, using the radius r of the imaginary circle 902 (e.g. as illustrated in FIG. 30).

[0501] In various embodiments, the torsional stiffness of the carrier 34 is greater than or equal to 1.60×10.sup.8 Nm/rad, and optionally greater than or equal to 2.7×10.sup.8 Nm/rad.

[0502] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the torsional stiffness of the carrier 34 may be greater than or equal to 1.8×10.sup.8 Nm/rad, and optionally may be greater than or equal to 2.5×10.sup.8 Nm/rad (and optionally may be equal to 4.83×10.sup.8 Nm/rad). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the torsional stiffness of the carrier 34 may greater than or equal to 6.0×10.sup.8 Nm/rad and optionally may be greater than or equal to 1.1×10.sup.9 Nm/rad (and optionally may be equal to 2.17×10.sup.9 Nm/rad).

[0503] In various embodiments, the torsional stiffness of the carrier 34 is in the range from 1.60×10.sup.8 Nm/rad to 1.00×10.sup.11 Nm/rad, and optionally in the range from 2.7×10.sup.8 Nm/rad to 1×10.sup.10 Nm/rad.

[0504] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the torsional stiffness of the carrier 34 may be in the range from 1.8×10.sup.8 Nm/rad to 4.8×10.sup.9 Nm/rad, and optionally may be in the range from 2.5×10.sup.8 Nm/rad to 6.5×10.sup.8 Nm/rad (and optionally may be equal to 4.83×10.sup.8 Nm/rad). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the torsional stiffness of the carrier 34 may be in the range from 6.0×10.sup.8 Nm/rad to 2.2×10.sup.10 Nm/rad and optionally may be in the range from 1.1×10.sup.9 to 3.0×10.sup.9 Nm/rad (and optionally may be equal to 2.17×10.sup.9 Nm/rad).

[0505] In various embodiments, the effective linear torsional stiffness of the carrier 34 may be greater than or equal to 7.00×10.sup.9 N/m, and optionally greater than or equal to 9.1×10.sup.9 N/m.

[0506] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the effective linear torsional stiffness of the carrier 34 may be greater than or equal to 7.70×10.sup.9 N/m. In other such embodiments, the effective linear torsional stiffness of the carrier 34 may be greater than or equal to 9.1×10.sup.9 N/m, optionally greater than or equal to 1.1×10.sup.10 N/m (and optionally may be equal to 1.26×10.sup.10 N/m). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the effective linear torsional stiffness of the carrier 34 may be greater than or equal to 1.2×10.sup.10 N/m and optionally may be greater than or equal to 2.1×10.sup.10 N/m (and optionally may be equal to 2.88×10.sup.10 N/m).

[0507] In various embodiments, the effective linear torsional stiffness of the carrier 34 may be in the range from 7.00×10.sup.9 to 1.20×10.sup.11 N/m, and optionally in the range from 9.1×10.sup.9 N/m to 8.0×10.sup.10 N/m.

[0508] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the effective linear torsional stiffness of the carrier 34 may be in the range from 9.1×10.sup.9 to 6.0×10.sup.10 N/m, and optionally may be in the range from 7×10.sup.9 N/m to 2×10.sup.10 N/m, or from 8.5×10.sup.9 N/m to 2×10.sup.10 N/m (and optionally may be equal to 1.26×10.sup.10 N/m). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the effective linear torsional stiffness of the carrier 34 may be in the range from 1.2×10.sup.10 N/m to 1.2×10.sup.11 N/m and optionally may be in the range from 1.0×10.sup.10 N/m to 5.0×10.sup.10 N/m (and optionally may be equal to 2.88×10.sup.10 N/m).

[0509] The torsional stiffness of the carrier 34 may be controlled so as to be within a desired range by adjusting one or more parameters, including carrier material(s), carrier geometry, and the presence or absence of lugs.

Parameter Ratios

[0510] The inventor has discovered that the ratios (and/or products) of some properties have a considerable impact on the operation of the gearbox and its inputs/outputs/support structure. Some or all of the below may apply to any embodiment:

[0511] A system radial bending stiffness is defined by combining the radial bending stiffness of the fan shaft mounting structure 503 and the radial bending stiffness of the fan shaft 36 at the output of the gearbox in series. The system radial bending stiffness is defined as:

[00049] $\frac{1}{(1 / K 1) + (1 / K 2)}$

[0512] Where K1 is the radial bending stiffness of the fan shaft mounting structure, and K2 is radial bending stiffness of the fan shaft 36 at the output of the gearbox as defined elsewhere herein.

[0513] In various embodiments, the system radial bending stiffness may be greater than or equal to 3.90×10.sup.6 N/m and optionally greater than or equal to 3.6×10.sup.7 N/m.

[0514] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the system radial bending stiffness may be greater than or equal to 3.6×10.sup.7 N/m. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the system radial bending stiffness may be greater than or equal to 4.1×10.sup.7 N/m or greater than or equal to 5.8×10.sup.7 N/m.

[0515] In various embodiments, the system radial bending stiffness may be in the range from 3.90×10.sup.6 N/m to 1.40×10.sup.9 N/m, and optionally in the range from 3.6×10.sup.7 N/m to 6.8×10.sup.8 N/m.

[0516] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the system radial bending stiffness may be in the range from 3.6×10.sup.7 N/m to 4.0×10.sup.8 N/m and optionally in the range from 3.6×10.sup.7 N/m to 4.9×10.sup.7 N/m (and may be equal to 3.9×10.sup.7 N/m).

[0517] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the system radial bending stiffness may be in the range from 4.1×10.sup.7 N/m to 1.4×10.sup.9 N/m, and optionally in the range from 5.8×10.sup.7 N/m to 7.8×10.sup.7 N/m (and may be equal to 6.8×10.sup.7 N/m).

[0518] A fan shaft mounting radial bending stiffness ratio can be defined as:

[00050] $\frac{the system radial bending stiffness}{\begin{matrix} the radial bending stiffness \\ of the fan shaft mounting structure (503) \end{matrix}}$

[0519] In various embodiments, the fan shaft mounting radial bending stiffness ratio may be greater than or equal to 1.0×10.sup.−3 optionally greater than or equal to 5.0×10.sup.−3, or further optionally greater than or equal to 2.0×10.sup.−2.

[0520] In various embodiments, the fan shaft mounting radial bending stiffness ratio may be in the range from 1.0×10.sup.−3 to 4.0×10.sup.−1, and optionally in the range from 5.0×10.sup.−3 to 1.5×10.sup.−1, in the range from 5.0×10.sup.−3 to 2.0×10.sup.−2 or in the range from 2.0×10.sup.−2 to 1.5×10.sup.−1.

[0521] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft mounting radial bending stiffness ratio may be in the range from 2.2×10.sup.−2 to 3.2×10.sup.−2 (and may be equal to 2.7×10.sup.−2).

[0522] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft mounting radial bending stiffness ratio may be in the range from 2.6×10.sup.−2 to 3.6×10.sup.−2 (and may be equal to 3.1×10.sup.−2).

[0523] In various embodiments, in addition to or alternatively to the fan shaft mounting radial bending stiffness ratio, a product of the parameters making up the fan shaft mounting radial bending stiffness ratio may be defined. This product (referred to as the fan shaft mounting radial bending stiffness product) may be defined as:

the system radial bending stiffness×the radial bending stiffness of the fan shaft mounting structure (503)

[0524] In various embodiments, the fan shaft mounting radial bending stiffness product may be greater than or equal to 2.7×10.sup.5 (N/m).sup.2, and optionally greater than or equal to 4.0×10.sup.15 (N/m).sup.2.

[0525] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft mounting radial bending stiffness product may be greater than or equal to 4.3×10.sup.16 (N/m).sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft mounting radial bending stiffness product may be greater than or equal to 4.3×10×10.sup.16 (N/m).sup.2.

[0526] In various embodiments, the fan shaft mounting radial bending stiffness product may be in the range from 2.7×10.sup.15 (N/m).sup.2 to 9.0×10.sup.19 (N/m).sup.2, and optionally in the range from 4.0×10.sup.15 (N/m).sup.2 to 1.5×10.sup.19 (N/m).sup.2.

[0527] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft mounting radial bending stiffness product may be in the range from 4.3×10.sup.16 (N/m) to 3.0×10.sup.18 (N/m).sup.2.

[0528] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft mounting radial bending stiffness product may be in the range from 4.3×10.sup.16 (N/m).sup.2 to 9.0×10.sup.19 (N/m).sup.2.

[0529] A system tilt stiffness is defined by combining the tilt stiffness of the fan shaft mounting structure 503 and the tilt stiffness of the fan shaft 36 at the output of the gearbox in series. The system tilt stiffness is defined as:

[00051] $\frac{1}{(1 / K 3) + (1 / K 4)}$

[0530] Where K3 is the tilt stiffness of the fan shaft mounting structure, and K4 is the tilt stiffness of the fan shaft 36 at the output of the gearbox as defined elsewhere herein.

[0531] In various embodiments, the system tilt stiffness may be greater than or equal to 1.10×10.sup.5 Nm/rad and optionally greater than or equal to 8.5×10.sup.5 Nm/rad.

[0532] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the system tilt stiffness may be greater than or equal to 8.5×10.sup.5 Nm/rad. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the system tilt stiffness may be greater than or equal to 1.5×10.sup.6 Nm/rad or greater than or equal to 2.9×10.sup.6 Nm/rad.

[0533] In various embodiments, the system tilt stiffness may be in the range from 1.10×10.sup.5 Nm/rad to 6.80×10.sup.7 Nm/rad, and optionally in the range from 8.5×10.sup.5 Nm/rad to 3.4×10.sup.7 Nm/rad.

[0534] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the system tilt stiffness may be in the range from 8.5×10.sup.5 Nm/rad to 1.2×10.sup.7 Nm/rad and optionally in the range from 8.5×10.sup.5 Nm/rad to 1.7×10.sup.6 Nm/rad (and may be equal to 1.2×10.sup.6 Nm/rad).

[0535] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the system tilt stiffness may be in the range from 1.5×10.sup.6 Nm/rad to 6.8×10.sup.7 Nm/rad, and optionally in the range from 2.9×10.sup.6 Nm/rad to 3.9×10.sup.6 Nm/rad (and may be equal to 3.4×10.sup.6 Nm/rad).

[0536] In various embodiments, a fan shaft mounting tilt stiffness ratio is defined as:

[00052] $\frac{the system tilt stiffness}{tilt stiffness of the fan shaft mounting structure (503)}$

[0537] In various embodiments, the fan shaft mounting tilt stiffness ratio may be greater than or equal to 1.5×10.sup.−3, and optionally greater than or equal to 6.0×10.sup.−3 and further optionally greater than or equal to 2.5×10.sup.−2.

[0538] In various embodiments, the fan shaft mounting tilt stiffness ratio may be in the range from 1.5×10.sup.−3 to 5.0×10.sup.−1, and optionally in the range from 6.0×10.sup.−3 to 2.0×10.sup.−1, in the range from 6.0×10.sup.−3 to 2.5×10.sup.−2 or in the range from 2.5×10.sup.−2 to 2.0×10.sup.−1.

[0539] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft mounting tilt stiffness ratio may be in the range from 3.0×10.sup.−2 to 4.0×10.sup.−2 (and may be equal to 3.5×10.sup.−2).

[0540] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft mounting tilt stiffness ratio may be in the range from 3.7×10.sup.−2 to 4.7×10.sup.−2 (and may be equal to 4.2×10.sup.−2).

[0541] In various embodiments, in addition to or alternatively to the fan shaft mounting tilt stiffness ratio, a product of the parameters making up that ratio may be defined. This product (referred to as the fan shaft mounting tilt stiffness product) may be defined as:

the system tilt stiffness×the tilt stiffness of the fan shaft mounting structure (503)

[0542] In various embodiments, the fan shaft mounting tilt stiffness product may be greater than or equal to 1.7×10.sup.12 (Nm/rad).sup.2, and optionally greater than or equal to 1.6×10.sup.13 (Nm/rad).sup.2.

[0543] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft mounting tilt stiffness product may be greater than or equal to 1.9×10.sup.13 (Nm/rad).sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft mounting tilt stiffness product may be greater than or equal to 3.0×10.sup.1 (Nm/rad).sup.2.

[0544] In various embodiments, the fan shaft mounting tilt stiffness product may be in the range from 1.7×10.sup.12 (Nm/rad).sup.2 to 3.0×10.sup.17 (Nm/rad).sup.2, and optionally in the range from 1.6×10.sup.13 (Nm/rad).sup.2 to 3.0×10.sup.16 (Nm/rad).sup.2.

[0545] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft mounting tilt stiffness product may be in the range from 1.9×10.sup.13 (Nm/rad).sup.2 to 1.5×10.sup.16 (Nm/rad).sup.2.

[0546] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft mounting tilt stiffness product may be in the range from 3.0×10.sup.13 (Nm/rad).sup.2 to 3.0×10.sup.17 (Nm/rad).sup.2.

[0547] A fan shaft radial bending stiffness ratio is defined as:

[00053] $\frac{\begin{matrix} the radial bending stiffness of \\ the fan shaft (36) at the input to the fan (23) \end{matrix}}{\begin{matrix} the radial bending stiffness of the fan \\ shaft (36) at the output of the gearbox (30) \end{matrix}}$

[0548] In various embodiments, the fan shaft radial bending stiffness ratio may be greater than or equal to 6.0×10.sup.−3, and optionally greater than or equal to 0.015.

[0549] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft radial bending stiffness ratio may be greater than or equal to 0.03 or greater than or equal to 0.07. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft radial bending stiffness ratio may be greater than or equal to 0.02 or greater than or equal to 0.04.

[0550] In various embodiments, the fan shaft radial bending stiffness ratio may be in the range from 6.0×10.sup.−3 to 2.5×10.sup.1, and optionally in the range from 0.015 to 2.5.

[0551] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft radial bending stiffness ratio may be in the range from 0.03 to 0.85 and optionally in the range from 0.07 to 0.27 (and may be equal to 0.17).

[0552] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft radial bending stiffness ratio may be in the range from 0.02 to 1.5, and optionally in the range from 0.04 to 0.24 (and may be equal to 0.14).

[0553] In various embodiments, in addition to or alternatively to the fan shaft radial bending stiffness ratio, a product of the parameters making up that ratio may be defined. This product (referred to as the fan shaft radial bending stiffness product) may be defined as:

the radial bending stiffness of the fan shaft (36) at the input to the fan (23)×the radial bending stiffness of the fan shaft (36) at the output of the gearbox (30)

[0554] In various embodiments, the fan shaft radial bending stiffness product may be greater than or equal to 1.2×10.sup.13 (N/m).sup.2, and optionally greater than or equal to 2.4×10.sup.14 (N/m).sup.2.

[0555] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft radial bending stiffness product may be greater than or equal to 2.4×10.sup.14 (N/m).sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft radial bending stiffness product may be greater than or equal to 5.0×10.sup.13 (N/m).sup.2.

[0556] In various embodiments, the fan shaft radial bending stiffness product may be in the range from 1.2×10.sup.13 (N/m).sup.2 to 3.0×10.sup.18 (N/m).sup.2, and optionally in the range from 2.4×10.sup.14 (N/m).sup.2 to 3.0×10.sup.17 (N/m).sup.2.

[0557] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft radial bending stiffness product may be in the range from 2.4×10.sup.14 (N/m).sup.2 to 2.7×10.sup.15 (N/m).sup.2.

[0558] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft radial bending stiffness product may be in the range from 5.0×10.sup.13 (N/m).sup.2 to 3.0×10.sup.18 (N/m).sup.2.

[0559] A fan shaft tilt stiffness ratio is defined as:

[00054] $\frac{\begin{matrix} the tilt stiffness of the fan shaft (36) \\ at the input to the fan (23) \end{matrix}}{\begin{matrix} the tilt stiffness of the fan shaft (36) \\ at the output of the gearbox (23) \end{matrix}}$

[0560] In various embodiments, the fan shaft tilt stiffness ratio may be greater than or equal to 2.5×10.sup.−2, and optionally greater than or equal to 0.05.

[0561] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft tilt stiffness ratio may be greater than or equal to 0.2 or greater than or equal to 0.5. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft tilt stiffness ratio may be greater than or equal to 0.1 or greater than or equal to 0.2.

[0562] In various embodiments, the fan shaft tilt stiffness ratio may be in the range from 2.5×10.sup.−2 to 3.7×10.sup.2, and optionally in the range from 0.05 to 4.0×10.sup.1.

[0563] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft tilt stiffness ratio may be in the range from 0.2 to 5.0 and optionally in the range from 0.5 to 1.5 (and may be equal to 1.00).

[0564] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft tilt stiffness ratio may be in the range from 0.1 to 1.0×10.sup.1, and optionally in the range from 0.2 to 1.4 (and may be equal to 0.98).

[0565] In various embodiments, in addition to or alternatively to the fan shaft tilt stiffness ratio, a product of the parameters making up that ratio may be defined. This product (referred to as the fan shaft tilt stiffness product) may be defined as:

the tilt stiffness of the fan shaft (36) at the input to the fan (23)×the tilt stiffness of the fan shaft (36) at the output of the gearbox (30)

[0566] In various embodiments, the fan shaft tilt stiffness product may be greater than or equal to 3.5×10.sup.10 (Nm/rad).sup.2, and optionally greater than or equal to 7.2×10.sup.11 (Nm/rad).sup.2.

[0567] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft tilt stiffness product may be greater than or equal to 7.2×10.sup.11 (Nm/rad).sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft tilt stiffness product may be greater than or equal to 3.5×10.sup.11 (Nm/rad).sup.2.

[0568] In various embodiments, the fan shaft tilt stiffness product may be in the range from 3.5×10.sup.10 (Nm/rad).sup.2 to 5.0×10.sup.16 (Nm/rad).sup.2, and optionally in the range from 7.2×10.sup.11 (Nm/rad).sup.2 to 5.0×10.sup.15 (Nm/rad).sup.2.

[0569] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the fan shaft tilt stiffness product may be in the range from 7.2×10.sup.11 (Nm/rad).sup.2 to 1.5×10.sup.13 (Nm/rad).sup.2.

[0570] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the fan shaft tilt stiffness product may be in the range from 3.5×10.sup.11 (Nm/rad).sup.2 to 5.0×10.sup.16 (Nm/rad).sup.2.

[0571] In various embodiments, a first bearing separation ratio is defined as:

[00055] $\frac{the first bearing separation distance (d_{1})}{\begin{matrix} the axial distance between the fan input \\ position and the gearbox output position (d_{4}) \end{matrix}}$

[0572] In various embodiments, the first bearing separation ratio may be greater than or equal to 1.6×10.sup.−1, and optionally greater than or equal to 1.8×10.sup.−1, greater than or equal to 1.6×10.sup.−1 or greater than or equal to 2.2×10.sup.−1.

[0573] In various embodiments, the first bearing separation ratio may be in the range from 1.6×10.sup.−1 to 3.3×10.sup.−1, and optionally in the range from 1.8×10.sup.−1 to 3.0×10.sup.−1, in the range from 1.6×10.sup.−1 to 2.2×10.sup.−1, or in the range from 2.2×10.sup.−1 to 3.3×10.sup.−1. The values in this and the previous paragraph may, for example, apply to embodiments in which the fan diameter is in the range from 240 to 280 cm or in the range from 330 cm to 380 cm.

[0574] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm or in the range from 330 cm to 380 cm, the bearing separation ratio may be in the range from 2.5×10.sup.−1 to 2.9×10.sup.−1 (and may be equal to 2.7×10.sup.−1).

[0575] In various embodiments, in addition to or alternatively to the first bearing separation ratio, a product of the parameters making up that ratio may be defined. This product (referred to as the first bearing separation product) may be defined as:

the first bearing separation distance (d.sub.1)×the axial distance between the fan input position and the gearbox output position (d.sub.4)

[0576] In various embodiments, the first bearing separation product may be greater than or equal to 5.2×10.sup.−2 m.sup.2, and optionally greater than or equal to 5.7×10.sup.−2 m.sup.2, or optionally greater than or equal to 7.5×10.sup.−2

[0577] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first bearing separation product may be greater than or equal to 5.2×10.sup.−2 m.sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first bearing separation product may be greater than or equal to 7.5×10.sup.−2 m.sup.2.

[0578] In various embodiments, the first bearing separation product may be in the range from 5.2×10.sup.−2 m.sup.2 to 2.6×10.sup.−1 m.sup.2, and optionally in the range from 5.7×10.sup.−2 m.sup.2 to 2.4×10.sup.−1 m.sup.2, and optionally in the range from 7.5×10.sup.−2 m.sup.2 to 2.6×10.sup.−1 m.sup.2

[0579] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first bearing separation product may be in the range from 5.2×10.sup.−2 m.sup.2 to 1.4×10.sup.−1 m.sup.2.

[0580] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first bearing separation product may be in the range from 7.5×10.sup.−2 m.sup.2 to 2.6×10.sup.−1 m.sup.2.

[0581] For embodiments in which there is at least a first and second supporting bearing forward of the gearbox (and rearward of the fan input position) a second bearing separation ratio is defined as:

[00056] $\frac{the first bearing seperation distance (d_{1})}{the bearing axial seperation (d_{3})}$

[0582] In various embodiments, the second bearing separation ratio may be greater than or equal to 4.1×10.sup.−1, and optionally greater than or equal to 4.5×10.sup.−1, and further optionally greater than or equal to 6.0×10.sup.−1.

[0583] In various embodiments, the second bearing separation ratio may be in the range from 4.1×10.sup.−1 to 8.3×10.sup.−1, and optionally in the range from 4.5×10.sup.−1 to 7.7×10.sup.−1, or in the range from 4.1×10.sup.−1 to 6.0×10.sup.−1, or in the range 6.0×10.sup.−1 to 8.3×10.sup.−1. The values in this and the previous paragraph may, for example, apply to embodiments in which the fan diameter is in the range from 240 to 280 cm or in the range from 330 cm to 380 cm.

[0584] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm or in the range from 330 cm to 380 cm, or in other various embodiments, the second bearing separation ratio may be in the range from 6.4×10.sup.−1 to 7.0×10.sup.−1 (and may be equal to 6.7×10.sup.−1).

[0585] In various embodiments, in addition to or alternatively to the second bearing separation ratio, a product of the parameters making up that ratio may be defined. This product (referred to as the second bearing separation product) may be defined as:

the first bearing separation distance (d.sub.1)×the bearing axial separation (d.sub.3)

[0586] In various embodiments, the second bearing separation product may be greater than or equal to 2.0×10.sup.−2 m.sup.2, and optionally greater than or equal to 2.3×10.sup.−2 m.sup.2 and further optionally greater than or equal to 3.5×10.sup.−2 m.sup.2.

[0587] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second bearing separation product may be greater than or equal to 2.0×10.sup.−2 m.sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second bearing separation product may be greater than or equal to 3.5×10.sup.−2 m.

[0588] In various embodiments, the second bearing separation product may be in the range from 2.0×10.sup.−2 m.sup.2 to 1.1×10.sup.−1 m.sup.2, and optionally in the range from 2.3×10.sup.−2 m.sup.2 to 8.5×10.sup.−2 m.sup.2, or may be in the range from 3.5×10.sup.−2 m.sup.2 to 1.1×10.sup.−1 m.

[0589] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second bearing separation product may be in the range from 2.0×10.sup.−2 m.sup.2 to 5.6×10.sup.−2 m.

[0590] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second bearing separation product may be in the range from 3.5×10.sup.−2 m.sup.2 to 1.1×10.sup.−1 m.

[0591] In various embodiments, a first planet carrier stiffness ratio may be defined as:

[00057] $\frac{\begin{matrix} the effective linear torsional \\ stiffness of the planet carrier (34) \end{matrix}}{\begin{matrix} radial bending stiffness of \\ the fan shaft mounting structure (503) \end{matrix}}$

[0592] In various embodiments, the first planet carrier stiffness ratio may be greater than or equal to 7.0×10.sup.−3, and optionally greater than or equal to 7.0×10.sup.−2.

[0593] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first planet carrier stiffness ratio may be greater than or equal to 6.9 or may be greater than or equal to 7.0 or 4.0. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first planet carrier stiffness ratio may be greater than or equal to 7.0 or greater than or equal to 1.0×10.sup.1 or 5.0

[0594] In various embodiments, the first planet carrier stiffness ratio may be in the range from 7.0×10.sup.−3 to 1.9×10.sup.3, and optionally in the range from 7.0×10.sup.−2 to 9.0×10.sup.1.

[0595] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first planet carrier stiffness ratio may be in the range from 6.9 to 1.2×10.sup.2 and optionally in the range from 7.0 to 11.0 (and may be equal to 8.5). In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first planet carrier stiffness ratio may be in the range from in the range from 4.0 to 6.0 (and may be equal to 5.2)

[0596] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first planet carrier stiffness ratio may be in the range from 7.0 to 1.9×10.sup.3 and optionally in the range from 1.0×10.sup.1 to 2.0×10.sup.1 (and may be equal to 1.33×10.sup.1). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first planet carrier stiffness ratio may be in the range from 5.0 to 9.0 (and may be equal to 7.1).

[0597] In various embodiments, a first planet carrier stiffness product may be defined as:

(the effective linear torsional stiffness of the planet carrier (34))×(radial bending stiffness of the fan shaft mounting structure (503))

[0598] In various embodiments, the first planet carrier stiffness product may be greater than or equal to 2.9×10.sup.18 (N/m).sup.2, and optionally greater than or equal to 5.0×10.sup.18 (N/m).sup.2.

[0599] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first planet carrier stiffness product may be greater than or equal to 8.0×10.sup.18 (N/m).sup.2 or greater than or equal to 9.0×10.sup.18 (N/m).sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first planet carrier stiffness product may be greater than or equal to 1.0×10.sup.19 (N/m).sup.2 or 2.0×10.sup.19 (N/m).sup.2 or 5.0×10.sup.19 (N/m).sup.2.

[0600] In various embodiments, the first planet carrier stiffness product may be in the range from 2.9×10.sup.18 (N/m).sup.2 to 8.0×10.sup.22 (N/m).sup.2, and optionally in the range from 5.0×10.sup.18 (N/m).sup.2 to 8.0×10.sup.21 (N/m).sup.2.

[0601] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the first planet carrier stiffness product may be in the range from 8.0×10.sup.18 (N/m).sup.2 to 8.0×10.sup.21 (N/m).sup.2 and optionally in the range from 9.0×10.sup.18 (N/m).sup.2 to 3.0×10.sup.19 (N/m).sup.2 (and may be equal to 1.1×10.sup.19 (N/m).sup.2 or may be equal to 1.9×10.sup.19 (N/m).sup.2).

[0602] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the first planet carrier stiffness product may be in the range from 1.0×10.sup.19 (N/m).sup.2 to 8.0×10.sup.22 (N/m).sup.2 and optionally in the range from 2.0×10.sup.19 (N/m).sup.2 to 4.0×10.sup.19 (N/m).sup.2 (and may be equal to 3.3×10.sup.19 (N/m).sup.2), or optionally in the range from 5.0×10.sup.19 (N/m).sup.2 to 8.0×10.sup.19 (N/m.sup.2) (and may be equal to 6.2×10.sup.19 (N/m).sup.2).

[0603] In various embodiments, a second planet carrier stiffness ratio may be defined as:

[00058] $\frac{\begin{matrix} the torsional \\ stiffness of the planet carrier (34) \end{matrix}}{\begin{matrix} the stiffness of \\ the fan shaft mounting structure (503) \end{matrix}}$

[0604] In various embodiments, the second planet carrier stiffness ratio may be greater than or equal to 6.0×10.sup.−3, and optionally greater than or equal to 6.0×10.sup.−2.

[0605] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second planet carrier stiffness ratio may be greater than or equal to 1.36×10.sup.1 or 7.9. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second planet carrier stiffness ratio may be greater than or equal to 1.5×10.sup.1 or greater than or equal to 1.7×10.sup.1 or 0.4×10.sup.1.

[0606] In various embodiments, the second planet carrier stiffness ratio may be in the range from 6.0×10.sup.−3 to 7.0×10.sup.3, and optionally in the range from 6.0×10.sup.−2 to 7.0×10.sup.2.

[0607] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second planet carrier stiffness ratio may be in the range from 1.36×10.sup.1 to 7.0×10.sup.2 and optionally in the range from 1.36×10.sup.1 to 2.4×10.sup.1 (and may be equal to 1.4×10.sup.1). In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second planet carrier stiffness ratio may be in the range from 7.9 to 9.9 (and may be equal to 8.9)

[0608] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second planet carrier stiffness ratio may be in the range from 1.5×10.sup.1 to 7.0×10.sup.3 and optionally in the range from 1.7×10.sup.1 to 3.7×10.sup.1 (and may be equal to 2.7×10.sup.1).

[0609] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second planet carrier stiffness ratio may in the range from 0.4×10.sup.1 to 2.4×10.sup.1 (and may be equal to 1.4×10.sup.1).

[0610] In various embodiments, a second planet carrier stiffness product may be defined as:

(the torsional stiffness of the planet carrier (34))×(tilt stiffness of the fan shaft mounting structure (503))

[0611] In various embodiments, the second planet carrier stiffness product may be greater than or equal to 2.4×10.sup.5 (Nm/rad).sup.2, and optionally greater than or equal to 4.9×10.sup.5 (Nm/rad).sup.2 In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second planet carrier stiffness product may be greater than or equal to 4.9×10.sup.15 (Nm/rad).sup.2 or 7.9×10.sup.15 (Nm/rad).sup.2 or 1.0×10.sup.16 (Nm/rad).sup.2. In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second planet carrier stiffness product may be greater than or equal to 9.0×10.sup.15 (Nm/rad).sup.2 or 7.4×10.sup.16 (Nm/rad).sup.2 or 1.0×10.sup.17 (Nm/rad).sup.2.

[0612] In various embodiments, the second planet carrier stiffness product may be in the range from 2.4×10.sup.15 (Nm/rad).sup.2 to 2.7×10.sup.21 (Nm/rad).sup.2, and optionally in the range from 4.9×10.sup.15 (Nm/rad).sup.2 to 2.0×10.sup.20 (Nm/rad).sup.2.

[0613] In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the second planet carrier stiffness product may be in the range from 4.9×10.sup.15 (Nm/rad).sup.2 to 9.0×10.sup.19 (Nm/rad).sup.2 and optionally in the range from 7.9×10.sup.15 (Nm/rad).sup.2 to 1.2×10.sup.16 (Nm/rad).sup.2 (and may be equal to 9.9×10.sup.15 (Nm/rad).sup.2), or optionally in the range from 1.0×10.sup.16 (Nm/rad).sup.2 to 1.2×10.sup.16 (Nm/rad).sup.2 (and may be equal to 1.6×10.sup.16 (Nm/rad).sup.2).

[0614] In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the second planet carrier stiffness product may be in the range from 9.0×10.sup.15 (Nm/rad).sup.2 to 2.7×10.sup.21 (Nm/rad).sup.2 and optionally in the range from 7.4×10.sup.16 (Nm/rad).sup.2 to 1.1×10.sup.17 (Nm/rad).sup.2 (and may be equal to 9.4×10.sup.16 (Nm/rad).sup.2), or optionally in the range from 1.0×10.sup.17 (Nm/rad).sup.2 to 2.6×10.sup.17 (Nm/rad).sup.2 (and may be equal to 1.8×10.sup.17 (Nm/rad).sup.2).

[0615] FIG. 32 illustrates an example aircraft 1000 having a gas turbine engine 10 attached to each wing 1002a, 1002b thereof. Each gas turbine engine 10 is attached via a respective pylon 1004a, 1004b. The gas turbines 10 may be that of any embodiment described herein. The aircraft shown in FIG. 32 is to be understood as the aircraft for which the gas turbine engine 10 of any embodiment or aspect disclosed herein has been designed to be attached. The aircraft 1000 has a cruise condition corresponding to the cruise conditions defined elsewhere herein and a max take-off condition corresponding to the MTO conditions defined elsewhere herein.

[0616] The present disclosure also relates to a method 2000 of operating a gas turbine engine on an aircraft (e.g. the aircraft of FIG. 32). The method 2000 is illustrated in FIG. 33. The method 2000 comprises operating 2010 the gas turbine engine 10 described elsewhere herein to provide propulsion for the aircraft to which it is mounted under maximum take-off conditions. The method further comprises operating 2020 the gas turbine engine to provide propulsion during cruise conditions. The gas turbine engine is operated such that any of the parameters or ratios defined herein are within the specified ranges. Cruise conditions and max-take off conditions are as defined elsewhere herein.

[0617] The torque on the core shaft 26 may be referred to as the input torque, as this is the torque which is input to the gearbox 30. The torque supplied by the turbine 19 to the core shaft (i.e. the torque on the core shaft) at cruise conditions may be greater than or equal to 10,000 Nm, and optionally greater than or equal to 11,000 Nm. In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the torque on the core shaft 26 at cruise conditions may be greater than or equal to 10,000 or 11,000 Nm (and optionally may be equal to 12,760 Nm). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the torque on the core shaft 26 at cruise conditions may be greater than or equal to 25,000 Nm, and optionally greater than or equal to 30,000 Nm (and optionally may be equal to 34,000 Nm).

[0618] The torque on the core shaft at cruise conditions may be in the range from 10,000 to 50,000 Nm, and optionally from 11,000 to 45,000 Nm. In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the torque on the core shaft 26 at cruise conditions may be in the range from 10,000 to 15,000 Nm, and optionally from 11,000 to 14,000 Nm (and optionally may be equal to 12,760 Nm). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the torque on the core shaft 26 at cruise conditions may be in the range from 25,000 Nm to 50,000 Nm, and optionally from 30,000 to 40,000 Nm (and optionally may be equal to 34,000 Nm).

[0619] Under maximum take-off (MTO) conditions, the torque on the core shaft 26 may be greater than or equal to 28,000 Nm, and optionally greater than or equal to 30,000 Nm. In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the torque on the core shaft 26 under MTO conditions may be greater than or equal to 28,000, and optionally greater than or equal to 35,000 Nm (and optionally may be equal to 36,300 Nm). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the torque on the core shaft 26 under MTO conditions may greater than or equal to 70,000 Nm, and optionally greater than or equal to 80,000 or 82,000 Nm (and optionally may be equal to 87,000 Nm).

[0620] Under maximum take-off (MTO) conditions, the torque on the core shaft 26 may be in the range from 28,000 Nm to 135,000 Nm, and optionally in the range from 30,000 to 110,000 Nm. In some embodiments, for example in embodiments in which the fan diameter is in the range from 240 to 280 cm, the torque on the core shaft 26 under MTO conditions may be in the range from 28,000 to 50,000 Nm, and optionally from 35,000 to 38,000 Nm (and optionally may be equal to 36,300 Nm). In some embodiments, for example in embodiments in which the fan diameter is in the range from 330 to 380 cm, the torque on the core shaft 26 under MTO conditions may be in the range from 70,000 Nm to 135,000 Nm, and optionally from 80,000 to 90,000 Nm or 82,000 to 92,000 Nm (and optionally may be equal to 87,000 Nm).

[0621] Torque has units of [force]×[distance] and may be expressed in units of Newton metres (N.Math.m), and is defined in the usual way as would be understood by the skilled person.

[0622] FIG. 34 illustrates how the stiffnesses defined herein may be measured. FIG. 34 shows a plot of the displacement 6 resulting from the application of a load L (e.g. a force, moment or torque) applied to a component for which the stiffness is being measured. At levels of load from zero to L.sub.G there is a non-linear region in which displacement is caused by motion of the component (or relative motion of separate parts of the component) as it is loaded, rather than deformation of the component; for example moving within clearance between parts. At levels of load above L.sub.H the elastic limit of the component has been exceeded and the applied load no longer causes elastic deformation—plastic deformation or failure of the component may occur instead. Between points G and H the applied load and resulting displacement have a linear relationship. The stiffnesses defined herein may be determined by measuring the gradient of the linear region between points G and H (with the stiffness being the inverse of that gradient). The gradient may be found for as large a region of the linear region as possible to increase the accuracy of the measurement by providing a larger displacement to measure. For example, the gradient may be found by applying a load equal to or just greater than L.sub.G and equal to or just less than L.sub.H. Although the displacement is referred to as δ in this description, the skilled person would appreciate that equivalent principles would apply to a linear or angular displacement.

[0623] The stiffnesses defined herein, unless otherwise stated, are for the corresponding component(s) when the engine is off (i.e. at zero speed/on the bench) The stiffnesses generally do not vary over the operating range of the engine; the stiffness at cruise conditions of the aircraft to which the engine is used (those cruise conditions being as defined elsewhere herein) may therefore be the same as for when the engine is not in use. However, where the stiffness varies over the operating range of the engine, the stiffnesses defined herein are to be understood as being values for when the engine is at room temperature and unmoving.

[0624] Anything described herein with reference to a planetary type gearbox can apply equally to a star type gearbox unless otherwise stated or where it is apparent that a feature is specific to a particular gearbox type.

[0625] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

HIGH POWER EPICYCLIC GEARBOX AND OPERATION THEREOF

Assignee

Inventors

Cpc classification

Classification Explorer

F02C7/36

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F05D2220/36

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F02K3/06

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F05D2200/14

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F05D2260/96

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F05D2300/501

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F05D2260/40311

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

Y02T50/60

GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

Classification Explorer

F05D2220/323

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

International classification

Classification Explorer

F02C7/36

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Classification Explorer

F02K3/06

MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING

Abstract

Claims

Description