Rotor assembly apparatus

Rotor assembly apparatus are disclosed. An example rotor assembly includes a twist actuator to drive a first rotation of a first shaft about a first axis, the twist actuator positioned at a center of rotation of the rotor assembly; and a first gear assembly to convert the first rotation into a plurality of second rotations of a plurality of second shafts, each of the second shafts to provide torque to a respective blade coupled to the rotor assembly.

FIELD

The present disclosure relates generally to aircraft and, more particularly, to rotor assembly apparatus.

BACKGROUND

Rotorcraft employ one or more blades coupled to a rotor. The rotor drives a rotation of the blades. An ability to manipulate certain characteristics of the blades, such as collective pitch and cyclic pitch, enables an operator to control movement of the rotorcraft.

SUMMARY

An example disclosed rotor assembly includes a twist actuator to drive a first rotation of a first shaft about a first axis, the twist actuator positioned at a center of rotation of the rotor assembly; and a first gear assembly to convert the first rotation into a plurality of second rotations of a plurality of second shafts, each of the second shafts to provide torque to a respective blade coupled to the rotor assembly.

An example disclosed apparatus includes a carrier coupled to a drive shaft, the drive shaft extending through a pitch shaft of a rotor assembly, the drive shaft providing a first amount of torque to the carrier; a first stage planetary gear system disposed in the carrier, the carrier to drive rotation of the first stage planetary gear system; a second stage planetary gear system disposed in the carrier, the first stage planetary gear system to drive rotation of the second stage planetary gear system; and an output of the second stage planetary gear system to provide a second amount of torque to a blade.

An example disclosed apparatus includes a first gear assembly to convert torque about a first axis provided by a central actuator of a rotor assembly into second torques about second axes, the second axes being different than the first axis, the first gear assembly comprising a first angled gear concentric with a first drive shaft of the central actuator; and second angled gears, each of the second angled gears to mesh with the first angled gear; and a second gear assembly to receive one of the second torques about one of the second axes via a second drive shaft, the second gear assembly comprising a first planetary gear system driven by the second drive shaft; and a second planetary gear system driven by the first planetary gear system, the second planetary gear system comprising a sun gear coupled to an output shaft.

Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. As used in this patent, stating that any part (e.g., a layer, film, area, or plate) is in any way positioned on (e.g., positioned on, located on, disposed on, or formed on, etc.) another part, means that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween. As used herein, the terms “coupled” and “operatively coupled” are defined as connected directly or indirectly (e.g., through one or more intervening structures and/or layers). Stating that any part is in contact with another part means that there is no intermediate part between the two parts.

DESCRIPTION

FIG. 1shows an example machine in which example methods and apparatus disclosed herein may be utilized. While example methods and apparatus disclosed herein are described in connection withFIG. 1, examples disclosed herein may be implemented in connection with any type of machine or device having rotor blades, such as aircraft, watercraft, hovercraft, wind turbines, etc.FIG. 1shows a helicopter100having a rotor system102that drives a plurality of rotor blades104. The rotor system102spins the blades104to provide the helicopter100with lift and thrust. As the blades104spin through air, each of the blades104rotates along a tracking path or plane of rotation. The amount of lift and/or thrust provided by each of the blades104and the tracking path traveled by each of the blades104depends on aerodynamic characteristics of the respective blade. For example, each of the blades104has a particular airfoil, a pitching moment, a weight distribution, a twist, a chord length, etc.

An operator of the helicopter100manipulates the blades104via controls in communication with the rotor system102. For example, the operator uses a collective input to control an altitude of the helicopter100. The collective input simultaneously changes an angle of attack or pitch of each blade a same or equal amount. Additionally, the operator uses a cyclic input to control lateral movement (e.g., left, right, forward, aft) of the helicopter100. The cyclic input changes the angle of attack or pitch of each blade as a function of position during a revolution relative to, for example, an airframe. The example rotor system102ofFIG. 1includes a swashplate in communication with the collective and cyclic inputs to implement the changes in angle of attack or pitch. The example rotor system ofFIG. 1includes links (e.g., rods and/or pins) extending from the swashplate to the blades104to implement the controls. For collective inputs, the swashplate is raised or lowered such that the each of the links alters the pitch of the corresponding blade104a same amount. For cyclic inputs, the swashplate is tilted such that the links alter the pitch of the corresponding blades104as a function of a position in a rotation. That is, as the blades104rotate, the cyclic input alters respective ones of the blades104based on where the blades104are in the rotation.

Additionally, twisting the blades104(e.g., statically during a certain type of flight such as take-off, landing, or cruise or multiple times per revolution) can reduce vibration, reduce noise and/or, more generally, increase performance. In the illustrated example, a torsion shaft is positioned in the blade. A first end of the blade104is fixed and another end of the blade104is allowed to twist. Applying a torque to the torsion shaft of the blade104causes the blade104to twist at the flexible tip of the blade104. Thus, twisting the blade104alters the blade104from a baseline airfoil to a modified airfoil. The operator of the helicopter100may desire the modified airfoil of the blade104to, for example, more evenly distribute lift across the corresponding blade104during certain conditions and/or operating modes.

Known rotor systems that utilize such blade twisting techniques include multiple twist actuators, one for each blade to be twisted. As such, these known systems require duplicate position sensors, motors, motor brakes, reduction gearboxes, etc. Further, the individual actuators employed by known systems are each located at a distance (e.g., 6.5% of a rotor radius) away from a center of rotation of the rotor assembly. As such, the twist actuators of these known systems experience G-loading associated with positions located away from the center of rotation. Further, because blade pitch and blade twist involve rotational changes in similar directions (e.g., about a same axis or substantially similar axes), changes in blade pitch may undesirably affect blade twist. That is, alterations in blade pitch may introduce a certain measure of error in the corresponding blade twist. For example, when the blade104in known systems is forwardly pitched about a pitching moment, the twist of the blade104and the corresponding modified airfoil of the blade104are not preserved as the blade104pitches forward. Thus, in known systems, the twist of the blade104may be undesirably influenced when the blade104is pitched.

Examples disclosed herein include resolve or improve these and other problems of known systems that twist rotor blades. As described in detail below, examples disclosed herein include a single twist actuator to control the twist of any number of blades. For example, the single twist actuator of disclosed examples can control the twist of all four (4) of the example blades104ofFIG. 1. Thus, examples disclosed herein reduce the weight, complexity, cost, and reliability issues associated with the duplicative parts involved in known systems having individual actuators for each blade. Further, the single twist actuator of examples disclosed herein is located at a center of rotation of the blades. Thus, examples disclosed herein eliminate or at least reduce the G-loading induced by the known systems that locate the individual twist actuators at distances away from the center of rotation. Additionally, examples disclosed herein include a gear assembly having first and second stages that operatively interact to reduce the amount of required actuator torque to twist the blades104. Details and advantages of examples disclosed herein are provided below in connection withFIGS. 2-12.

FIG. 2depicts an example rotor assembly200constructed in accordance with teachings of this disclosure. The example rotor assembly200ofFIG. 2receives commands such as, for example, collective control signals and cyclic control signals from an operator of the helicopter100ofFIG. 1. As described above, the collective control signals adjust a pitch of each blade104similarly, while the cyclic control signals adjust pitch of the different blades104differently depending on the blade position in the rotation. To implement the collective controls and the cyclic controls, the example rotor assembly200includes a swashplate202that moves up and down (e.g., away from and towards a frame of the helicopter100ofFIG. 1) via, for example, one or more hydraulic or other type of actuator (not shown) controlled by the control signals received from the operator of the helicopter100. As the swashplate202moves up and down, pitch links204coupled to the swashplate202move up and down, collectively or cyclically according to the received control signals. Each of the pitch links204is coupled to a respective one of a plurality of pitch arms206. Each of the pitch arms206converts the up and down movement of the corresponding pitch link204into rotational movement of a pitch shaft (not shown inFIG. 2) disposed in a hub barrel208. For example, as one of the pitch links204moves upward, the corresponding one of the pitch arms206rotates the corresponding one of the pitch shafts upward (e.g., toward the top of the helicopter100). Conversely, as one of the pitch links204moves downward, the corresponding one of the pitch arms206rotates the corresponding one of the pitch shafts downward (e.g., toward the bottom of the helicopter100). Different ones of the pitch shafts may be pitched in a same manner (e.g., upwardly) simultaneously (e.g., in response to collective inputs) or differently (e.g., some upwardly and some downwardly) simultaneously (e.g., in response to cyclic inputs). Each of the pitch shafts of the hub barrels208is coupled to a blade root210via a finger joint212and a blade grip214. As such, the pitch of the pitch shafts is applied to the blades104. Accordingly, the swashplate202and the corresponding actuators that receive commands from the operator enable the helicopter100to pitch, roll, and/or translate.

The example rotor assembly200ofFIG. 2includes a twist actuator216to control a twist of each blade of the example rotor assembly200. Notably, the example twist actuator216ofFIG. 2is positioned or located at a center of rotation of the example rotor assembly200ofFIG. 2. This location of the twist actuator216corresponds to a low-G position, especially relative to locations inside the example hub barrels208, as in known systems. Notably, the example twist actuator216ofFIG. 2controls twist for all of the blades, rather than implementing separate actuators for each blade104, as in known systems. In the illustrated example ofFIG. 2, the twist actuator216includes an electric motor, a motor brake, a position feedback sensor, and motor speed-reduction gears. Additionally or alternatively, the example twist actuator216may include one or more hydraulic and/or any other suitable type of driving component(s) and/or sources of torque. The example rotor assembly200ofFIG. 2includes a twist actuator stabilizer218through which the example twist actuator216is mounted. As such, the example twist actuator216is mounted inside a rotor head mast, which reduces an aerodynamic drag associated with the twist actuator216and protects the components of the twist actuator216from external factors.

As described in detail below in connection withFIGS. 5-12, the example twist actuator216drives a rotation of an actuator drive shaft about a first axis220. While not always corresponding to a strictly vertical axis due to, for example, movement of the helicopter100, the first axis220is referred to herein as the vertical axis220. Further, as described in detail below in connection withFIGS. 5-12, the example rotor assembly200ofFIG. 2includes a bevel gear assembly to convert the rotation of the actuator drive shaft about the vertical axis220into a plurality of rotations about second axes, of a plurality of planet carrier drive shafts each disposed in one of the hub barrels208. One of the second axes222is shown inFIG. 2While not always corresponding to a strictly horizontal axis due to, for example, movement of the helicopter100, the second axes222are referred to herein as the horizontal axes222. Further, as described in detail below in connection withFIGS. 5-12, each of the individual planet carrier drive shafts is coupled to a gear assembly disposed in a respective one of the hub barrels208and the corresponding blade grip214. The gear assemblies provide a gear reduction for an output shaft that is coupled to a torsion shaft in the blades104. As the output shaft rotates in accordance with the input provided by the twist actuator216, the torsion shaft twists the blades104. Accordingly, the twist actuator216drives the twisting of each of the blades104.

FIG. 3depicts the example rotor assembly200ofFIG. 2from a different perspective. As shown inFIG. 3, the twist actuator216is centrally located in the rotor assembly200, thereby positioning the twist actuator216at a low-G location in the rotor assembly200. The twist actuator216provides torque to each of the torsion shafts of the blades104from its central location in the rotor assembly200. Although not shown inFIG. 3, the distal ends of the blades104twist in response to the torque provided by the twist actuator216due to a torque tube inside the blade root210being rotated at the blade grip214. Therefore, as the torque tube inside the blade root210is rotated, the blade104is twisted near the tip to form a modified airfoil. The example ofFIG. 3illustrates a first rotational direction300in which the torque is applied to the torque tube inside the blade root210. In some examples, the torque is applied to the torque tube inside the blade root210in a second rotational direction opposite the first rotational direction300ofFIG. 3.

FIG. 4is an isometric view of the example rotor assembly200FIGS. 2 and 3. In the example ofFIG. 4, the blade roots210are not shown in the blade grips214. As such, an opening400in the blade grip214is visible inFIG. 4. As disclosed in detail below, the opening400receives a twist output shaft (not shown inFIG. 4), which engages a torsion shaft disposed in the blade root210.

FIG. 5is a partial cross-sectional view of the example rotor assembly200that illustrates an example manner in which the twist actuator216drives the twisting of the blade104. In the example ofFIG. 5, an actuator input shaft500is driven by a motor of the twist actuator216(not shown). The motor of the twist actuator216is, for example, an example motor coupled to the actuator input shaft500. In the example ofFIG. 5, the actuator input shaft500is coupled to an actuator drive shaft502. Thus, the motor of the twist actuator216drives the rotation of the actuator drive shaft502about the vertical axis220.

In the example ofFIG. 5, the actuator drive shaft502is coupled to a bevel gear assembly504via a diaphragm coupling506. The example diaphragm coupling506ofFIG. 5enables the motor of the twist actuator216to be independent of the gear assemblies described below, which improves maintainability and protects the twist actuator216and the gear assemblies from misalignment loads. Further, the diaphragm coupling506ofFIG. 5is flexible and internally mounted within a rotor head mast, thereby physically protecting components of the twist actuator216and reducing the aerodynamic drag profile of the rotor assembly200. Further, the example ofFIG. 5includes a coupling preload stud508to provide preload compression to the diaphragm coupling506to extend its lifetime and protect the twist actuator216from overload conditions.

In the example ofFIG. 5, the bevel gear assembly504converts rotation of the actuator drive shaft502about the vertical axis220into rotation of a planet carrier drive shaft510about the horizontal axis222. In the example ofFIG. 5, the bevel gear assembly504includes a bull gear512and a bull gear ball-bearing514. The bull gear512is concentric with the actuator drive shaft502and rotates in conjunction with the actuator drive shaft502about the vertical axis220. The bull gear512has a first toothed face516that is angled (e.g., relative to the vertical axis220). The example bevel gear assembly504includes a plurality of pinion gears518, one for each of the hub barrels208. Each of the pinion gears518corresponds to and travels about the rotor assembly200inside a hub cartridge519. Each of the pinion gears518is centrally retained inside a hub cartridge519. Each of the planet carrier drive shafts510is retained in the respective hub barrels208via an elastomeric bearing520that requires the planet carrier drive shaft510to slide inside the pinion gear518while experiencing the centrifugal forces associated with the rotor assembly200. In particular, bearing deflections from high rotor blade centrifugal loads may cause movement in the planet carrier drive shafts510during, for example, startup and shutdown. In the illustrated example ofFIG. 5, the retention of the pinion gears518via the hub cartridge519prevents such movement from affecting the pinion gear mesh with the bull gear512. In particular, the elastomeric bearing520receives the centrifugal forces and maintains the position of the planet carrier shafts510, as well as the blades104, in the hub barrels208.

The pinion gears518each have a second toothed face522to mesh with the first toothed face516of the bull gear512. The second toothed face522of the pinion gears518is angled (e.g., relative to the vertical axis220). That is, the teeth of the bull gear512mesh with the teeth of the pinion gears518as the actuator drive shaft502rotates about the vertical axis220. Accordingly, in the example ofFIG. 5, the rotation of the bull gear512, via its coupling with the actuator drive shaft502, drives a rotation of the pinion gears518about the respective horizontal axes222. It is noted that the horizontal axis222for each of the pinion gears518does not move with a pitch rotation but does move with the blades104about the rotor assembly200. However, the rotation of the pinion gears518is consistently substantially (e.g., within a threshold) perpendicular to the vertical axis220.

For each of the pinion gears518, the corresponding planet carrier shaft510extends from the pinion gear518such that the rotation of the planet carrier shaft510about the horizontal axis222corresponds to the rotation of the pinion gear518. In the example ofFIG. 5, the planet carrier shaft510extends through a pitch shaft523that is coupled to the corresponding one of the pitch arms206(not shown inFIG. 5). Notably, the planet carrier shaft510rotates independently of the pitch shaft523, thereby separating the twist driving rotation of the planet carrier shaft510from the pitch driving rotation of the pitch shaft523. Thus, the bull gear512rotates in response to input from the twist actuator216, the planet carrier shaft510rotate about the horizontal axis222. Thus, as the actuator drive shaft502rotates about the vertical axis220, the first toothed face516of the bull gear512meshes with each of the second toothed faces522of the pinion gears518, thereby causing the rotation of the planet carrier drive shaft510about the horizontal axis222. Thus, the twist actuator216, via the actuator drive shaft502and the bevel gear assembly504, causes the plant carrier drive shaft510to rotate about the horizontal axis222.

The example bevel gear assembly504is a straight tooth design that generates similar loads in different rotational directions about the horizontal axis222. In the illustrated example ofFIG. 5, the gear ratio of the bevel gear assembly504generates an increase in speed of the planet carrier drive shaft510of approximately fifty (50) percent to allow the use of the multiple pinion gears518(e.g., one for each rotor blade104being driven by the twist actuator216) mated to the single bull gear512. Accordingly, the example rotor assembly200ofFIG. 5distributes the power provided by the twist actuator216to each rotor blade104from the central location of the twist actuator216.

Additionally, the example rotor assembly200ofFIG. 5includes a gear reduction assembly524. The example gear reduction assembly524ofFIG. 5is described in detail below in connection withFIGS. 6-10. The view of the gear reduction assembly524ofFIG. 5is reproduced inFIG. 10for clarity. Generally, the example gear reduction assembly524receives a first amount of torque about the horizontal axis222from the planet carrier shaft510and outputs a second amount of torque about the horizontal axis222on an output shaft526. The output shaft526is connected to a torsion shaft located in the blade root210. Thus, the gear reduction assembly524facilitates delivery of torque from the pinion gear518, which is driven by the example twist actuator216, to the torsion shaft located in the blade root210. As described in detail below, in delivering the torque to the output shaft526, the example gear reduction assembly524achieves a gear reduction ratio that amplifies torque provided by the example twist actuator216to reduce the torque requirement and/or size of the twist actuator216.

FIG. 6is an isometric view of an implementation of the gear reduction assembly524ofFIG. 5constructed in accordance with teachings of this disclosure.FIG. 7is an exploded view of the example gear reduction assembly524ofFIG. 6. The example gear reduction assembly524ofFIG. 6includes a planet carrier600that carries a first stage planetary gear system602and a second stage planetary gear system604. The first stage planetary gear system602is housed (at least partially) in the pitch shaft523. The second stage planetary gear system604is housed (at least partially) in the blade grip214outboard of the pitch shaft523. The first stage planetary gear system602has a first stage sun gear700(FIG. 7) and first stage planet gears702(FIG. 7). In the illustrated example, the first stage planetary gear system602includes five (5) first stage planet gears702. However, the first stage planetary gear system602may include an alternative number of first stage planet gears702. The second stage planetary gear system604has a second stage sun gear704(FIG. 7) and second stage planet gears706(FIG. 7). In the illustrated example, the second stage planetary gear system604includes five (5) second stage planet gears706. However, the second stage planetary gear system604may include an alternative number of second stage planet gears704.

As described above, the twist actuator216drives a rotation of the planet carrier drive shaft510when the blade is to be twisted. The example planet carrier600is keyed to the planet carrier drive shaft510such that as the planet carrier drive shaft510rotates, so does the planet carrier600. In the example ofFIGS. 6 and 7, the planet carrier600is coupled to a first cap assembly610that is coupled to the planet carrier drive shaft510. As shown inFIG. 7, the first cap assembly610includes a plurality of bores708(FIG. 7), each to receive an end710(FIG. 7) of one of a plurality of planet shafts712(FIG. 7). That is, the ends710of the planet shafts712are positioned in the bores708of the first cap assembly610, which is coupled to the planet carrier600. As such, rotation of the planet carrier drive shaft510about the horizontal axis222causes the first cap assembly610and the planet carrier600to rotate about the horizontal axis222. Further, rotation of the planet carrier600and the first cap assembly610cause the first stage planet gears702to traverse, as a set, about the horizontal axis222.

The first stage planet gears702are meshed with the first stage sun gear700. The first sun gear700is fixed relative to the first stage planet gears702. In particular, the first stage sun gear700is fixed to the pitch shaft523of the rotor assembly200via a mounting shaft612. As described above in connection withFIG. 5, the planet carrier drive shaft510extends through the pitch shaft523and through the mounting shaft612, which is fixed to the pitch shaft523. Thus, the planet carrier drive shaft510rotates within the mounting shaft612, which is fixed to the first stage sun gear700. As such, when the planet carrier drive shaft510rotates the planet carrier600, the first stage planet gears702mesh with and rotate about (as a set) the first stage sun gear700.

In the illustrated example, the ratio between the first stage sun gear700and the first stage planet gears702is one (1) to one (1). However, alternative ratios are possible to achieve different reduction(s). Rotation of the first stage planet gears702causes rotation of the planet shafts712, to which the first stage planet gears702are mounted. The second stage planet gears706are also mounted to the planet shafts712. Thus, rotation of the first stage planet gears702drives a rotation of the planet shafts712, which causes the second stage planet gears706to rotate, as a set, about the horizontal axis222. The second stage planet gears706are meshed with the second stage sun gear704. The rotation of the second stage planet gears706, as meshed with the second stage sun gear704, cause the second stage sun gear704to rotate about its axis (i.e., the horizontal axis222). In the illustrated example, the ratio between the second stage planet gears706and the second stage sun gear704is seven (7) to eight (8). However, alternative ratios are possible to achieve different reduction(s).

The second stage sun gear704is coupled to the output shaft526such that the rotation of the second stage sun gear704causes the output shaft526to rotate about the horizontal axis222. Thus, the output shaft526rotates in accordance with the rotation of the second stage sun gear704. The output shaft526extends through a second cap assembly614. As described above, the output shaft526is coupled to the blade twist torque tube (not shown) via the opening400(FIG. 4) in the blade grip214. As such, rotation of the output shaft526applies torque to the blade twist torque tube (not shown).

Thus, the example gear reduction assembly524facilitates delivery of torque from the planet carrier drive shaft510, which is driven by the example twist actuator216, to the blade twist torque tube. In particular, the planet carrier drive shaft510drives rotation of the planet carrier600, which drives the first and second planetary gear systems602,604, which cooperate to rotate the output shaft526while providing a gear reduction. In the illustrated example, the operative interaction of the planet carrier600and the planetary gear systems602,604results in a gear reduction ratio of eight (8) to (1). As described above, such a gear reduction ratio is achieved via a one (1) to one (1) ratio between the first stage sun gear700and the first stage planet gears702, and a seven (7) to eight (8) ratio between the second stage planet gears706and the second stage sun gear704. Alternative gear reduction ratios are possible with alternative ratios between the first stage planet gears702and the first stage sun gear700and/or between the second stage planet gears706and the second stage sun gear704. The gear reduction provided by the example gear assembly524amplifies torque provided by the example twist actuator216to reduce the torque requirement and/or size of the twist actuator216. In some examples, the gear reduction ratio includes a speed up contribution from the bevel gear assembly504.

FIG. 8depicts an example implementation of the example first stage planetary gear system602and the example planet carrier600. In the example ofFIG. 8, the first stage planet gears702each have fifteen (15) teeth. In the example ofFIG. 8, the first stage sun gear700has fifteen (15) teeth. As such, the ratio provided by the example first stage planetary gear system602is one (1) to (1). As described above, the planet carrier drive shaft510(which is driven by the example twist actuator216) drives the rotation of the planet carrier600and the first stage planet gears702about the first stage sun gear700. As a result, the first stage planet gears702drive rotation of the planet shafts712to which the first stage planet gears702are mounted.

FIG. 9depicts an example implementation of the example second stage planetary gear system604and the example planet carrier600. In the example ofFIG. 9, the planet shafts712, as driven by the rotation of the first stage planet gears702, cause the second stage planet gears706to mesh with the second stage sun gear704. In the example ofFIG. 9, the second stage planet gears706have fourteen (14) teeth. In the example ofFIG. 9, the second stage sun gear704has sixteen (16) teeth. As such, the ratio provided by the example first stage planetary system530is seven (7) to eight (8). The meshing of the second stage planet gears706with the second stage sun gear704cause the second stage sun gear704to rotate about its axis, thereby providing torque to the output shaft526to which the second stage sun gear704is coupled. When the example reduction provided by the first stage planetary gear system602and the reduction provided by the second stage planetary gear system604are combined, the achieved reduction ratio is eight (8) to (1). Accordingly, the example gear reduction assembly524reduces the torque required of the twist actuator216to provide a suitable amount of torque to the output shaft526.

FIG. 10is an enlarged version of the cross-sectional view of the example gear reduction assembly524ofFIG. 5. As described above, the planet carrier drive shaft510extends through the mounting shaft612, which is fixed to the pitch shaft523. The mounting shaft612is coupled to the first stage sun gear700, thereby fixing the first stage sun gear700to the pitch shaft523. As described above, the planet carrier drive shaft510is coupled to the planet carrier600such that rotation of the planet carrier drive shaft510drives rotation of the planet carrier600. As described above, the planet carrier600houses the first stage sun gear700and the first stage planet gears702. Additionally, as described above, the planet carrier600houses the second stage sun gear704and the second stage planet gears706. Further, as described above, the first stage planet gears702and the second stage planet gears706are mounted to the planet shafts712. The first and second planetary gear systems602,604amplify and deliver the torque provided by the planet carrier drive shaft510to the output shaft526via a coupling between the second stage sun gear704and the output shaft526.

FIG. 11is a partial plan view of the example rotor assembly200. As shown inFIG. 11, a plurality of roller bearings1100are disposed between the pitch shaft523and the hub barrel208. The roller bearings1100enable the pitch shaft523, which is pitched upward and downward by the pitch arm206, to rotate within the hub barrel208. However, because the first stage sun gear700is fixed to the pitch shaft523and the pitch shaft523is coupled to the pitch arm206, pitch changes of the rotor blades104may cause minor changes in the twist of the blades104. In some examples, these minor changes in twist may be considered errors. For example, in some systems, an error of one (1) degree at the second stage sun gear704may result from eight (8) degrees of blade pitch angle of the pitch shaft1000. However, the reduction provided by the example gear reduction assembly524coupled to the blade twist torque tube (not shown) significantly reduces the potential error in blade twist in such scenarios. In particular, the reduction provided by the gear reduction assembly524coupled to the blade twist torque tube enables ten (10) degrees of rotation of the second stage sun gear704to result in one (1) degree of blade twist due to torque tube windup, thereby also reducing any potential errors in twist. In particular, the illustrated example including the torque tube reduces the above example error of one (1) to eight (8) down to 0.1 (or one tenth) of blade twist to every eight degrees of blade pitch. For rotor blade steady pitch, the error can be eliminated by jogging the twist actuator216to achieve a proper twist setting. For rotor blade oscillatory or cyclic pitch, the error can be further reduced with sun and planet gears that have a greater number of teeth. For example, if the number of gear teeth is doubled, the error will be cut in half.

FIG. 12is an isometric view of the example rotor assembly200as described above in connection withFIGS. 2-11.