Note: Descriptions are shown in the official language in which they were submitted.
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VARIABLE-PITCH ROTOR WITH REMOTE COUNTERWEI GHTS
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to variable-pitch rotors and more
particularly
to control mechanisms for such rotors.
[0002] Aircraft powerplants are typically used to drive thrust-generating
airfoil
elements such as propellers or fan blades. It is known to vary the angle of
incidence (i.e.
"pitch angle") of the airfoil elements relative to the rotating hub carrying
them, in order to
provide the maximum possible propulsive efficiency at various flight
conditions.
[0003] A common method of pitch control employs a hydraulic actuator which
changes
the blade pitch angle in response to pressurized fluid flow. The actuator may
move the
blade through pitch angles from "coarse" to "fine" and may also provide pitch
angles
suitable for ground operation.
[0004] For safety reasons, it is important to limit the blade pitch angle
during flight.
This avoids overspeeding the powerplant, or imposing excessive structural
loads or
unexpected yawing moments to the aircraft. A typical prior art variable-pitch
rotor includes
a mechanical pitch lock which limits the blade pitch angle in the case of
actuator failure.
Pitch locks can be complicated and themselves subject to failure.
[0005] It is also known to provide variable-pitch rotors with
counterweights. The
counterweights provide a countervailing force that drives the blades to a safe
pitch angle
in case of actuator failure. However, these are typically mounted to the
individual blades
and therefore limit design flexibility.
[0006] Accordingly, there remains a need for a pitch control mechanism
incorporating
counterweights not directly mounted to the blades.
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BRIEF SUMMARY OF THE INVENTION
[0007] This need is addressed by the present invention, which provides a
pitch control
mechanism having counterweights which are mounted remotely from the blades and
which
are mechanically interconnected to the blades. The pitch control mechanism
allows the
design of the counterweights (including for example, their number, size, and
position) to
be determined independently from the design of the blades and trunnions.
[0008] According to one aspect of the invention, a pitch control mechanism
includes:
a rotor structure configured for rotation about a longitudinal axis; a row of
blades carried
by the rotor structure, each blade having an airfoil and a trunnion mounted
for pivoting
movement relative to the rotor structure, about a trunnion axis which is
perpendicular to
the longitudinal axis; a unison ring interconnecting the blades; an actuator
connected to the
unison ring and the rotor structure, operable to move the unison ring relative
to the rotor
structure; at least one moveable counterweight carried by the rotor structure,
remote from
the blades; and an interconnection between the blades and the counterweight,
such that
movement of the counterweight causes a change in the pitch angle of the
blades.
[0009] According to another aspect of the invention, the actuator is
configured to
produce rotary movement between the rotor structure and the unison ring.
[0010] According to another aspect of the invention, the unison ring and
counterweights are interconnected by gears.
[0011] According to another aspect of the invention, the rotor structure
carries an array
of counterweight assemblies each including: a pinion shaft, a pinion gear, and
a
counterweight with an offset mass.
[0012] According to another aspect of the invention, all of the pinion
gears are engaged
with a ring gear that is part of the unison ring, and with a sun gear that is
stationary relative
to the rotor structure.
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[0013] According
to another aspect of the invention, the pinion gears are meshed with
a ring gear that is part of the unison ring.
[0014] According
to another aspect of the invention, each counterweight includes a
hollow shell with a slug of high-density material inside.
[0015] According
to another aspect of the invention, each trunnion is connected to the
unison ring with a yoke.
[0016] According
to another aspect of the invention, each yoke includes a pin that
engages a pivot hole in a slider that is mounted for longitudinal sliding
movement in the
unison ring.
[0017] According
to another aspect of the invention, the trunnions are connected to the
unison ring by a geared connection.
[0018] According
to another aspect of the invention, the counterweights are mounted
to a pinion shaft that rotates about a radial axis.
[0019] According
to another aspect of the invention, the trunnions are connected to the
unison ring by a geared connection.
[0020] According
to another aspect of the invention, the actuator is configured to
produce linear movement between the rotor structure and the unison ring.
[0021] According
to another aspect of the invention, the actuator is configured to
produce linear movement between the rotor structure and the unison ring; and
the trunnions
are connected to the counterweights by a geared connection.
[0022] According
to another aspect of the invention, the actuator is configured to
produce linear movement between the rotor structure and the unison ring; and
the
counterweights are connected to the actuator by a geared connection.
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[0023] According to another aspect of the invention, the pitch angle is
variable between
a fine pitch angle and a coarse pitch angle, and the counterweights are
configured to drive
the pitch angle towards the coarse pitch angle.
[0024] According to another aspect of the invention, a gas turbine engine
includes:
turbomachinery core operable to produce a core gas flow; a low pressure
turbine positioned
downstream of the turbomachinery core; an inner shaft coupled to the low
pressure turbine;
and the pitch control mechanism described above, wherein the rotor structure
is coupled to
the inner shaft.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The invention may be best understood by reference to the following
description
taken in conjunction with the accompanying drawing figures in which:
[0026] FIG. 1 is a half-sectional, schematic view of a gas turbine engine
incorporating
variable-pitch fan blades;
[0027] FIG. 2 is a schematic dingram illustrating different pitch positions
of a blade of
the pitch control mechanism;
[0028] FIG. 3 is sectioned, schematic, perspective view of a pitch control
mechanism
constructed according to an aspect of the present invention;
[0029] FIG. 4 is a cross-sectional view of the mechanism of FIG. 3;
[0030] FIG. 5 is a functional diagram of the mechanism of FIG. 3;
[0031] FIG. 6 is a functional diagram of an alternative pitch control
mechanism;
[0032] FIG. 7 is a functional diagram of an alternative pitch control
mechanism;
[0033] FIG. 8 is a functional diagram of an alternative pitch control
mechanism;
[0034] FIG. 9 is a functional diagram of an alternative pitch control
mechanism; and
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[0035] FIG. 10 is a functional diagram of an alternative pitch control
mechanism.
DETAILED DESCRIPTION OF THE INVENTION
[0036] Referring to the drawings wherein identical reference numerals
denote the same
elements throughout the various views, Figure 1 depicts a gas turbine engine
10. The engine
has a longitudinal axis 11 and includes a fan 12 and a low pressure turbine
("LPT") 16
collectively referred to as a "low pressure system". The LPT 16 drives the fan
12 through
an inner shaft 18, also referred to as an "LP shaft". The engine 10 also
includes a high
pressure compressor ("HPC") 20, a combustor 22, and a high pressure turbine
("HPT") 24,
collectively referred to as a "gas generator" or "core". The HPT 24 drives the
HPC 20
through an outer shaft 26, also referred to as an "HP shaft". Together, the
high and low
pressure systems are operable in a known manner to generate a primary or core
flow as
well as a fan flow or bypass flow. While the illustrated engine 10 is a high-
bypass turbofan
engine, the principles described herein are equally applicable to any other
type of engine
requiring variable-pitch blades, including turboprop engines and piston
aircraft engines.
[0037] The fan 12 includes an annular array of blades 28. Each blade 28
includes an
airfoil 30 mounted to that it can pivot about a trunnion axis "T" which
extends radially
from the longitudinal axis 11. Pivoting motion of the blade 28 about this axis
changes its
pitch angle 0. As seen in FIG. 2 the pitch angle 0 is defined as the angle
between a zero-
lift line of the airfoil 30 and a plane perpendicular to the longitudinal axis
11. A blade is
shown at an intermediate pitch angle at "I", while a blade is shown at a
maximum high (or
coarse) pitch angle at "II", corresponding to a feathered condition, and a low
(or fine) pitch
angle at "III".
[0038] It is noted that, as used herein, the term "axial" or "longitudinal"
refers to a
direction parallel to an axis of rotation of a gas turbine engine, while
"radial" refers to a
direction perpendicular to the axial direction, and "tangential" or
"circumferential" refers
to a direction mutually perpendicular to the axial and tangential directions.
(See arrows
"L", "R", and "C" in FIG. 1). As used herein, the terms "forward" or "front"
refer to a
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location relatively upstream in an air flow passing through or around a
component, and the
terms "aft" or "rear" refer to a location relatively downstream in an air flow
passing through
or around a component. The direction of this flow is shown by the arrow "F" in
FIG. 1.
These directional terms are used merely for convenience in description and do
not require
a particular orientation of the structures described thereby.
[0039] FIGS. 3 and 4 illustrate pictorially an exemplary pitch control
mechanism 100
constructed according to an aspect of the present invention, while FIG. 5 is a
functional
diagram showing the pitch control mechanism 100 in half-section. The pitch
control
mechanism 100 is one of several mechanisms that may be used to control the
pitch angle 0
of the blades 28 shown in FIG. 1. The pitch control mechanism 100 includes a
centrally-
mounted rotor shaft 102 which rotates about the longitudinal axis 11. In
operation it would
be coupled to and rotated by the engine 10, for example by the inner shaft 18
shown in
FIG. 1. A drum 104 surrounds the rotor shaft 102 and is functionally coupled
to the rotor
shaft by an actuator 106.
[0040] The actuator 106 is shown schematically in FIGS. 3 and 4. The
actuator 106
may be any mechanism which is effective to selectively rotate the drum 104
about the
longitudinal axis 11, and thereby change the relative angular orientation of
the drum 104
and the rotor shaft 102. Known ty pes of actuators include electrical,
mechanical, and
hydraulic devices. The actuator 106 may operate to provide rotary motion
directly, or a
linear actuator may be used with an appropriate mechanism to covert its motion
to a rotary
output, so long as the ultimate movement of the drum 104 is rotary.
[0041] An annular unison ring 108 with forward and aft ends 110 and 112,
respectively,
surrounds the drum 104 and is coupled to the drum 104 so as to rotate in
unison therewith.
A plurality of axially-oriented slots 114 are formed around the periphery of
the unison ring
108, adjacent the forward end 110. Optionally, a slider 116 is disposed in
each slot 114 and
is free to move longitudinally forward or aft therein. Each slider 116 has a
pivot hole 118
passing therethrough.
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[0042] Blades 28 are arrayed around the unison ring 108. The airfoil 30 of
each blade
28 is attached to a trunnion 120 carried in suitable trunnion bearings 122, so
that the blade
28 can pivot about the trunnion axis "T" as shown in FIG. 1. An inner end of
each trunnion
120 is connected to the aft end of a yoke 124. The forward end of each yoke
124 includes
a pin 126 that extends radially inward and passes through the pivot hole 118
in one of the
sliders 116. Thus connected, rotary motion of the unison ring 108 causes a
simultaneous
change in the pitch angle 0 of all the blades 28.
[0043] A carrier 128 shaped like a shallow cylinder with a forward disk 130
and a
peripheral wall 132 is disposed aft of the unison ring 108, and is mounted for
rotation in
unison with the rotor shaft 102. The carrier 128 includes a plurality of
counterweight
assemblies. Each counterweight assembly comprises a pinion shaft 134 aligned
parallel to
the longitudinal axis 11 and passing through the forward disk 130, with a
pinion gear 136
mounted at its forward end and a counterweight 138 at its aft end. The
counterweight 138
comprises an offset mass. In other words, the center of mass of the
counterweight 138 is
not coaxial with the pinion shaft axis 144. In the illustrated example, each
counterweight
138 is constructed from a hollow housing 140 with a slug 142 of dense material
inside.
Each assembly of pinion gear 136, pinion shaft 134, and counterweight 138 is
rotatable as
a unit relative to the carrier 128, about that respective assembly's pinion
shaft axis 144.
[0044] An internal ring gear 146 is carried at the aft end 112 of the
unison ring 108,
and all of the pinion gears 136 are meshed with the ring gear 146. Thus
connected, the
movement of the blades 28, unison ring 108, and counterweights 138 are linked
together
such that rotary motion of the unison ring 108 (for example, caused by the
actuator 106)
will cause a simultaneous change in the pitch angle 0 of all of the blades 28,
and of the
angular orientation of all of the counterweights 138. Furthermore, the unison
ring 108 and
gear train transmits forces between the blades 28 and the counterweights 138.
[0045] During engine operation, the rotor shaft 102 and the carrier 128,
along with the
pinion gears 136, pinion shafts 124, and counterweights 138, rotate about the
longitudinal
axis 11. In FIG. 5, a rotor structure 148 is shown which functionally
represents the rotor
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shaft 102, carrier 128, and a structure carrying the trunnion bearings 122,
all of which rotate
in unison about the longitudinal axis 11. Typically, a selected torque would
be input to the
rotor structure 102 through the LP shaft 18 (see FIG. 1). At the same time,
the actuator 106
is used to move the unison ring 108 and the blades to a selected pitch angle
O. In accordance
with known principles, coarse pitch angles 0 increase the aerodynamic drag on
the blades
28 and result in a lower rotor rotational speed (designated "Nl"), and finer
pitch angles
result in a higher rotational speed Nl.
[0046] During normal operation, the actuator 106 is effective to move both
the blades
28 and the counterweights 138, so that the blades 28 take up the desired pitch
angle 0.
During actuator failure, the sum of aerodynamic and mass forces acting on the
blades 28
tend to drive them to a fine pitch angle O. Therefore, a failure of the
actuator 106 could
result in N1 increasing to an unacceptably high speed. However, the
counterweights 138
provide a countervailing force to drive the blades to a safe pitch angle (i.e.
a feathered
position).
[0047] More specifically, each counterweight 138 is subject to a reactive
centrifugal
force acting radially outward, computed as F=mco2/r, where m is the mass of
the
counterweight 138, co is the rotational velocity (i.e. 27r/60 x N1), and r is
the distance of the
center of mass of the counterweight 138 from the longitudinal axis 11. Because
the
counterweights 138 are offset from the axes 144, the counterweights 138 apply
a torque to
the pinion shafts 134, thereby rotating the pinion gears 136. Ultimately, the
pitch angle 0
of the blades 28 is determined by the dynamic balance of the blade forces and
the
counterweight forces. When the mechanism 100 is assembled, the angular
orientation of
the counterweight assemblies about their axes 144 are set relative to the
blades 28 such that
the counterweight torque tends to move the blades towards a full coarse or
feathered
position. The individual counterweight mass, number of counterweights 138,
lever arm
dimension, and the mechanical advantage between the counterweights 138 and the
blades
28 is selected to achieve the desired pitch angle 0 during periods of actuator
failure.
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[0048] The functional principles of remotely-mounted counterweights
described above
may be implemented using various physical configurations, several examples of
which are
described below.
[0049] FIG. 6 illustrates an alternative pitch control mechanism 200. The
pitch control
mechanism 200 includes a rotor structure 248 which rotates about the
longitudinal axis I 1,
an annular unison ring 208 with forward and aft ends 210 and 212,
respectively, and an
actuator 206 effective to rotate the unison ring 208 about the longitudinal
axis 11, and
thereby change the relative angular orientation of the unison ring 208 and the
rotor structure
248.
[0050] A plurality of blades 28 are arrayed around the unison ring 208.
Each blade 28
includes an airfoil 30 attached to a trunnion 120 carried in suitable bearings
222, such that
the blade 28 can pivot about a trunnion axis "T". The trunnions 120 are
coupled to the
forward end 210 of the unison ring 208 by yokes 224, such that rotary motion
of the unison
ring 208 causes a simultaneous change in the pitch angle 0 of all the blades
28.
[0051] A plurality of counterweight assemblies are carried by the rotor
structure 248.
Each counterweight assembly comprises a pinion shaft 234 aligned along a
radial axis, with
a pinion bevel gear 236 mounted at one end and a counterweight 238 at the
other end. The
counterweight 238 comprises an offset mass, and is moveable in a plane
tangential to the
longitudinal axis 11. The entire assembly of pinion bevel gear 236, pinion
shaft 234, and
counterweight 238 is rotatable as unit relative to the rotor structure 248,
about that
respective assembly's pinion shaft axis 244.
[0052] A ring bevel gear 246 is carried at the aft end 212 of the unison
ring 208, and
all of the pinion bevel gears 236 are meshed with the ring bevel gear 246.
Thus connected,
the movement ofthe blades 28, unison ring 208, and counterweights 238 are
linked together
such that rotary motion of the unison ring 208 (for example, caused by the
actuator 206)
will cause a simultaneous change in the pitch angle 0 of all of the blades 28,
and of the
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angular orientation of all of the counterweights 238. Furthermore, the unison
ring 208
transmits forces between the blades 28 and the counterweights 238.
[0053] The overall
function of the mechanism 200 is the same as the mechanism 100
described above, with the counterweights 238 providing a countervailing force
through the
gear train and unison ring 208, to drive the blades 28 to a safe, preselected
pitch angle (i.e.
a feathered position) in the case of actuator failure.
[0054] FIG.7
illustrates an alternative pitch control mechanism 300. The mechanism
300 includes a rotor structure 348 which rotates about the longitudinal axis
11, an annular
unison ring 308 with forward and aft ends 310 and 312, respectively, and an
actuator 306
effective to rotate the unison ring 308 about the longitudinal axis 11, and
thereby change
the relative angular orientation of the unison ring 308 and the rotor
structure 348.
[0055] Blades 28
are arrayed around the unison ring 308. Each blade 28 includes an
airfoil 30 attached to a trunnion 120 carried in suitable bearings 322, such
that the blade 28
can pivot about a trunnion axis "T". The trunnions 120 are coupled to the
forward end 310
of the unison ring 308 by yokes 324, such that rotary motion of the unison
ring 308 causes
a simultaneous change in the pitch angle 0 of all the blades 28.
[0056] A plurality
of counterweight assemblies are carried by an annular carrier 328
which is free to rotate relative to the rotor structure 348. Each
counterweight assembly
comprises a pinion shaft 334 aligned along an axis parallel to the
longitudinal axis, with a
pinion gear 336 mounted at one end and a counterweight 338 at the other end.
The
counterweight 338 comprises an offset mass. The entire assembly of pinion gear
336,
pinion shaft 334, and counterweight 338 is rotatable as a unit relative to the
carrier 328,
about that respective assembly's pinion shaft axis 344.
[0057] An internal
ring gear 346 is carried at the aft end 312 of the unison ring 308,
and all of the pinion gears 336 are meshed with the internal ring gear 346, as
well as a
central sun gear 350 that is fixed to the rotor structure 348. Thus connected,
the movement
of the blades 28, unison ring 308, and counterweights 338 are linked together
such that
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rotary motion of the unison ring 308 (for example, caused by the actuator 306)
will cause
a simultaneous change in the pitch angle 0 of all of the blades 28, and of the
angular
orientation of all of the counterweights 338. Furthermore, the unison ring 308
transmits
forces between the blades 28 and the counterweights 338.
[0058] The overall function of the mechanism 300 is the same as the
mechanism above,
with the counterweights 338 providing a countervailing force through the gear
train and
unison ring 308, to drive the blades 28 to a safe pitch angle (i.e. a
feathered position) in the
case of actuator failure.
[0059] FIG.8 illustrates an alternative pitch control mechanism 400. The
mechanism
400 includes a rotor structure 448 which rotates about the longitudinal axis
11, an annular
unison ring 408 with forward and aft ends 410 and 412, respectively, and an
actuator 406
effective to rotate the unison ring 408 about the longitudinal axis 11, and
thereby change
the relative angular orientation of the unison ring 408 and the rotor
structure 448.
[0060]. A plurality of blades 28 are arrayed around the unison ring 408.
Each blade 28
includes an airfoil 30 attached to a trunnion 120 carried in suitable bearings
422, such that
the blade 28 can pivot about a trunnion axis "T". Each trunnion 120 has a
trunnion bevel
gear 452 mounted at its inner end. A ring bevel gear 454 is disposed at the
forward end 410
of the unison ring 408, and all of the trunnion bevel gears 452 are meshed
with the ring
bevel gear 454. Thus arranged, rotary motion of the unison ring 408 causes a
simultaneous
change in the pitch angle 0 of all the blades 28.
[0061] A plurality of counterweight assemblies are carried by an annular
carrier which
is free to rotate relative to the rotor structure 448. Each counterweight
assembly comprises
a pinion shaft 434 aligned along an axis parallel to the longitudinal axis 11,
with a pinion
gear 436 mounted at one end and a counterweight 438 at the other end. The
counterweight
438 comprises an offset mass. The entire assembly of pinion gear 436, pinion
shaft 434,
and counterweight 438 is rotatable as a unit relative to the rotor structure
448, about that
respective assembly's pinion shaft axis 444.
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[0062] An internal ring gear 446 is carried at the aft end 412 of the
unison ring 408,
and all of the pinion gears 436 are meshed with the internal ring gear 446.
Thus connected,
the movement of the blades 28, unison ring 408, and counterweights 438 are
linked together
such that rotary motion of the unison ring 408 (for example, caused by the
actuator 406)
will cause a simultaneous change in the pitch angle 0 of all of the blades 28,
and of the
angular orientation of all of the counterweights 438. Furthermore, the unison
ring 408
transmits forces between the blades 28 and the counterweights 438.
[0063] The overall function of the mechanism 400 is the same as the
mechanism above,
with the counterweights 438 providing a countervailing force through the gear
train and
unison ring 408, to drive the blades 28 to a safe pitch angle (i.e. a
feathered position) in the
case of actuator failure.
[0064] FIG.9 illustrates an alternative pitch control mechanism 500. The
mechanism
500 includes a rotor structure 548 which rotates about the longitudinal axis
11, an annular
unison ring 508 with forward and aft ends 510 and 512, respectively. An
actuator 506 is
mounted between the unison ring 508 and the rotor structure and is effective
to move the
unison ring 508 relative to the rotor structure. The motion may be either
linear or rotary.
[0065] A plurality of blades 28 are arrayed around the unison ring 508.
Each blade 28
includes an airfoil 30 attached to a trunnion 120 carried in suitable bearings
522, such that
the blade 28 can pivot about a trunnion axis "T". The trunnions 120 are
coupled to the aft
end 512 of the unison ring 508 by yokes 524, such that linear or rotary motion
of the unison
ring 508 causes a simultaneous change in the pitch angle 0 of all the blades
28. Each
trunnion 120 has a trunnion gear 552 mounted adjacent the yoke 524. All of the
trunnion
gears 552 are meshed with a ring gear 554 of an annular coupler 556.
[0066] A plurality of counterweight assemblies are carried by an annular
carrier which
is free to rotate relative to the rotor structure 548. Each counterweight
assembly comprises
a pinion shaft 534 aligned along an axis parallel to the longitudinal axis,
with a pinion gear
536 mounted at one end and a counterweight 538 at the other end. The
counterweight 538
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comprises an offset mass. The entire assembly of pinion gear 536, pinion shaft
534, and
counterweight 538 is rotatable as a unit relative to the rotor structure 548,
about that
respective assembly's pinion shaft axis 544.
[0067] The coupler 556 also includes an internal ring gear 558, and all of
the pinion
gears 536 are meshed with the internal ring gear 558. Thus connected, the
movement of
the blades 28, unison ring 508, and counterweights 538 are linked together
such that rotary
motion of the unison ring 508 (for example, caused by the actuator 506) will
cause a
simultaneous change in the pitch angle 0 of all of the blades 28, and of the
angular
orientation of all of the counterweights 538. Furthermore, the unison ring 508
transmits
forces between the blades 28 and the counterweights 538.
[0068] The overall function of the mechanism 500 is the same as the
mechanism above,
with the counterweights 538 providing a countervailing force through the gear
train and
unison ring 508, to drive the blades 28 to a safe pitch angle (i.e. a
feathered position) in the
case of actuator failure.
[0069] FIG.10 illustrates an alternative pitch control mechanism 600 The
mechanism
600 includes a rotor structure 648 which rotates about the longitudinal axis
11, an annular
unison ring 608 with forward and aft ends 610 and 612 , respectively. An
actuator 606 is
mounted between the unison ring 608 and the rotor structure and is effective
to move the
unison ring 608 in a linear motion relative to the rotor structure.
[0070] A plurality of blades 28 are arrayed around the unison ring 608.
Each blade 28
includes an airfoil 30 attached to a trunnion 120 carried in suitable bearings
622, such that
the blade 28 can pivot about a trunnion axis "T". The trunnions 120 are
coupled to the aft
end 612 of the unison ring 608 by iokes 624, such that linear motion of the
unison ring
608 causes a simultaneous change in the pitch angle 0 of all the blades 28.
[0071] A plurality of counterweight assemblies are arrayed around the
actuator 606.
Each counterweight assembly comprises a pinion shaft 634 aligned along an axis
644
tangential to the longitudinal axis 11, with a pinion gear 636 mounted at one
end and a
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counterweight 638 at the other end. The counterweight 638 comprises an offset
mass. The
entire assembly of pinion gear 636, pinion shaft 634, and counterweight 638 is
rotatable as
a unit, about that respective assembly's pinion shaft axis 644.
[0072] The unison ring 608 also includes one or more axially-extending rack
gears 658,
and all of the pinion gears 636 are meshed with the rack gears 658. Thus
connected, the
movement of the blades 28, unison ring 608, and counterweights 638 are linked
together
such that rotary motion of the unison ring 608 (for example, caused by the
actuator 606)
will cause a simultaneous change in the pitch angle O of all of the blades 28,
and of the
angular orientation of all of the counterweights 638. Furthermore, the unison
ring 608
transmits forces between the blades 28 and the counterweights 638.
[0073] The overall function of the mechanism 600 is the same as the
mechanism above,
with the counterweights 638 providing a countervailing force through the gear
train and
unison ring 608, to drive the blades 28 to a safe pitch angle (i.e. a
feathered position) in the
case of actuator failure.
[0074] The pitch control mechanisms described herein permit the safe
control of blade
pitch angle in the event of actuator failure, while permitting design
flexibility in the
number, size, and location of the counterweights. Among other advantages is
the ability to
reduce the size of the hub. Referring to FIG. 1, the fan hub radius ratio is
defined as the fan
blade leading edge hub diameter "r 1 " divided by the overall fan blade tip
radius "r2", or
r 1 /r2. Because of the need to incorporate counterweights attached to the
blades within the
hub, prior art pitch control mechanisms often have a radius ratio
significantly greater than
0.5. In contrast, the mechanism described herein, where the counterweights are
moved
away from the fan blades, permit ratios significantly less than 0.5,
potentially less than
0.35, and further potentially less than 0.25. This will increase the
aerodynamic efficiency
of the fan.
[0075] The foregoing has described a variable-pitch rotor with remote
counterweights.
All of the features disclosed in this specification (including any
accompanying claims,
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abstract and drawings), and/or all of the steps of any method or process so
disclosed, may
be combined in any combination, except combinations where at least some of
such features
and/or steps are mutually exclusive.
[0076] Each feature disclosed in this specification (including any
accompanying
claims, abstract and drawings) may be replaced by alternative features serving
the same,
equivalent or similar purpose, unless expressly stated otherwise. Thus, unless
expressly
stated otherwise, each feature disclosed is one example only of a generic
series of
equivalent or similar features.
[0077] The invention is not restricted to the details of the foregoing
embodiment(s).
The invention extends any novel one, or any novel combination, of the features
disclosed
in this specification (including any accompanying claims, abstract and
drawings), or to any
novel one, or any novel combination, of the steps of any method or process so
disclosed.
