High power epicyclic gearbox and operation thereof
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 receives an input from the core shaft and outputs 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.
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, and wherein:
a fan-gearbox axial distance is defined as an 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;
a system radial bending stiffness is defined as:
1
(
1
a
radial
bending
stiffness
of
the
fan
shaft
mounting
structure
)
+
(
1
a
radial
bending
stiffness
of
the
fan
shaft
at
the
output
of
the
gearbox
)
and
a fan shaft mounting radial bending stiffness ratio of:
the
system
radial
bending
stiffness
the
radial
bending
stiffness
of
the
fan
shaft
mounting
structure
is greater than or equal to 1.0×10 −3 .
2. The gas turbine engine according to claim 1 , wherein the fan shaft mounting radial bending stiffness ratio is greater than or equal to 5.0×10 −3 .
3. The gas turbine engine according to claim 1 , wherein at least one of the following is satisfied:
a) the system radial bending stiffness is greater than or equal to 3.90×10 6 N/m;
b) the radial bending stiffness of the fan shaft mounting structure is greater than or equal to 7.00×10 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 6 N/m.
4. The gas turbine engine according to claim 1 , wherein a product of the system radial bending stiffness and the radial bending stiffness of the fan shaft mounting structure is greater than or equal to 2.7×10 15 (N/m) 2 .
5. The gas turbine engine according to claim 1 , wherein:
a system tilt stiffness is defined as:
1
(
1
a
tilt
stiffness
of
the
fan
shaft
mounting
structure
)
+
(
1
a
tilt
stiffness
of
the
fan
shaft
at
the
output
of
the
gearbox
)
;
and
a fan shaft mounting tilt stiffness ratio of:
the
system
tilt
stiffness
the
tilt
stiffness
of
the
fan
shaft
mounting
structure
is greater than or equal to 1.5×10 −3 .
6. The gas turbine engine according to claim 1 , wherein the at least two supporting bearings comprise a first supporting bearing and second supporting bearing, and wherein:
a) both of the first and the second supporting bearings are located at positions forward of the gearbox; or
b) the first supporting bearing is located at a position forward of the gearbox and the second supporting bearing is located at a position rearward of the gearbox.
7. The gas turbine engine according to claim 6 , wherein at least one of the following is satisfied:
a) the fan shaft mounting structure further comprises a third supporting bearing, the third supporting bearing being optionally located between the fan and the gearbox;
b) the fan shaft comprises a gearbox output shaft forming a relatively flexible portion of the fan shaft that is more flexible than an intermediate portion of the fan shaft to which the relatively flexible portion is connected, and the fan shaft mounting structure comprises a gearbox output shaft support structure having at least one gearbox output shaft bearing arranged to support the gearbox output shaft;
c) the fan shaft mounting structure further comprises one or more non-supporting softly mounted bearings; and
d) one or more bearings provided as part of the fan shaft mounting structure are double bearings.
8. The gas turbine engine according to claim 1 , wherein:
a) 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 greater than or equal to 0.12 m; and/or
b) an axial distance between the output of the gearbox and a closest bearing of the at least two supporting bearings in a forward direction from the gearbox is greater than or equal to 0.15 m.
9. The gas turbine engine according to claim 1 , wherein the fan-gearbox axial distance is greater than or equal to 0.37 m.
10. The gas turbine engine according to claim 1 , wherein at least one of the following is satisfied:
i) the gearbox has a gear ratio in a range of from 3.2 to 4.5;
ii) the gas turbine engine is configured to have a specific thrust in a range of from 70 to 90 NKg −1 ;
iii) the gas turbine engine is configured to have a bypass ratio at cruise conditions in a range of from 12.5 to 18; and
iv) the fan has a fan diameter greater than 240 cm and less than or equal to 380 cm.
11. The gas turbine engine or propulsor of claim 1 , wherein the gearbox is an epicyclic gearbox comprising a sun gear, a plurality of planet gears, a ring gear, and a planet carrier arranged to have the plurality of planet gears mounted thereon.
12. A method of operating the gas turbine engine of claim 1 , wherein the fan shaft mounting radial bending stiffness ratio of:
the
system
radial
bending
stiffness
the
radial
bending
stiffness
of
the
fan
shaft
mounting
structure
is greater than or equal to 3.9×10 6 ,
the method comprising operating the gas turbine engine to provide propulsion for the aircraft under cruise conditions.
13. The method of claim 12 , further comprising driving the gearbox with an input torque that is greater than or equal to 10,000 Nm.
14. 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, and wherein:
a fan-gearbox axial distance is defined as an 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;
a system tilt stiffness is defined as:
1
(
1
a
tilt
stiffness
of
the
fan
shaft
mounting
structure
)
+
(
1
a
tilt
stiffness
of
the
fan
shaft
at
the
output
of
the
gearbox
)
;
and
a fan shaft mounting tilt stiffness ratio of:
the
system
tilt
stiffness
the
tilt
stiffness
of
the
fan
shaft
mounting
structure
is greater than or equal to 1.5×10 −3 .
15. The gas turbine engine according to claim 14 , wherein the fan shaft mounting tilt stiffness ratio is greater than or equal to 6.0×10 −3 .
16. The gas turbine engine according to claim 14 , wherein at least one of the following is satisfied:
a) the system tilt stiffness is greater than or equal to 1.10×10 5 Nm/rad;
b) the tilt stiffness of the fan shaft mounting structure is greater than or equal to 1.50×10 7 Nm/rad; and
c) the tilt stiffness of the fan shaft at the output of the gearbox is greater than or equal to 7.00×10 4 Nm/rad.
17. The gas turbine engine according to claim 14 , wherein a product of the system tilt stiffness and the tilt stiffness of the fan shaft mounting structure is greater than or equal to 1.7×10 12 (Nm/rad) 2 .
18. A method of operating the gas turbine engine of claim 14 , the method comprising operating the gas turbine engine to provide propulsion for the aircraft under cruise conditions.
19. A propulsor for an aircraft, comprising:
a fan comprising a plurality of fan blades, the fan having a fan axial centreline;
a gearbox;
a power unit for driving the fan via the gearbox, the gearbox being arranged to:
receive an input from the power unit via a 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 propulsor, the fan shaft mounting structure comprising at least two supporting bearings connected to the fan shaft, and wherein:
a fan-gearbox axial distance is defined as an 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;
a system radial bending stiffness is defined as:
1
(
1
a
radial
bending
stiffness
of
the
fan
shaft
mounting
structure
)
+
(
1
a
radial
bending
stiffness
of
the
fan
shaft
at
the
output
of
the
gearbox
)
;
and
a fan shaft mounting radial bending stiffness ratio of:
the
system
radial
bending
stiffness
the
radial
bending
stiffness
of
the
fan
shaft
mounting
structure
is greater than or equal to 1.0×10 −3 .
20. A propulsor for an aircraft, comprising:
a fan comprising a plurality of fan blades, the fan having a fan axial centreline;
a gearbox;
a power unit for driving the fan via the gearbox, the gearbox being arranged to:
receive an input from the power unit via 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 propulsor, the fan shaft mounting structure comprising at least two supporting bearings connected to the fan shaft, and wherein:
a fan-gearbox axial distance is defined as an 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;
a system tilt stiffness is defined as
1
(
1
a
tilt
stiffness
of
the
fan
shaft
mounting
structure
)
+
(
1
a
tilt
stiffness
of
the
fan
shaft
at
the
output
of
the
gearbox
)
;
and
a fan shaft mounting tilt stiffness ratio of:
the
system
tilt
stiffness
the
tilt
stiffness
of
the
fan
shaft
mounting
structure
is greater than or equal to 1.5×10 −3 .