Swash plate assembly

Abstract

An inner swashplate 204, suitable for a helicopter rotor head, has an inner surface (fig.3,211) in contact with a split part-spherical bearing element 250 having a first part (fig.5,252) and a second

Description

Technical Field The present invention relates to helicopter swashplate assemblies and more particularly to the arrangement of bearings in such assemblies. Background Unmanned aerial vehicles (UAVs) are increasingly being used for a variety of different functions. For example, UAVs may be used for reconnaissance purposes, or the delivery of cargo or a payload to a desired location. It is often desirable for such vehicles to have good manoeuvrability, and therefore to be as small and lightweight as possible. Some UAVs can take the form of an unmanned helicopter. Helicopters are typically controlled through the use of a swashplate assembly. In particular, rotor blades are typically coupled to a swashplate, the movement and position of which can be adjusted to change the pitch of the blades and thereby control movement of the helicopter. In more detail, swashplate assemblies are configured to convert inputs from the flight controls into motion of the helicopter rotor blades, such as adjustments to their angle of attack, whilst the blades are spinning. Swashplate assemblies therefore typically include two swashplates, one that is stationary with respect to the helicopter and coupled to the flight controls via control rods, and another which is coupled to and rotates with the blades. The two swashplates are coupled together in the assembly, so that they can tilt together about a central element, thereby enabling motion from the stationary flight control mechanism to be transferred to the spinning blades. Some helicopters have coaxial counterrotating rotor heads, which drive a pair of rotor blade sets in opposite directions, thereby requiring more complex swashplate arrangements. Existing swashplate assemblies typically require a separate bearing socket component that receives the central element about which the swashplates pivot. This component needs to be provided in addition to the swashplates, and to be secured to them using retention components such as screws and rings. To be secured in this way the socket and the central element itself also needs to be made of materials such as steel, which add weight to the helicopter. These materials are also prone to corrosion, particularly in marine environments. The existing bearing and socket arrangements therefore adds to the weight, bulk and complexity of the helicopter. In addition, in order to remove or replace the central element in the assembly, the entire swashplate assembly often needs to be disassembled. Summary of Invention Embodiments of the present invention aim to address the above problems and others by providing a swashplate assembly in which the inner swashplate of the assembly acts as a bearing race for a part-spherical bearing element about which the swashplates tilt. This may provide a more compact assembly with a reduced longitudinal length in comparison to existing swashplate assemblies. This reduced height of the swashplate assembly may allow the helicopter rotor hubs to be provided closer to the drive components (e.g. gearbox) associated bearings of the helicopter, thereby providing more support to the rotor system. The assembly may operate without the need for additional spherical bearing retention components, thereby also reducing complexity in comparison to existing swashplate assemblies. Aspects of the disclosure are set out in the independent claims and optional features are set out in the dependent claims. Aspects of the disclosure may be provided in conjunction with each other, and features of one aspect may be applied to other aspects. An aspect of the disclosure provides a swashplate assembly for a coaxial counterrotating helicopter rotor head, the swash pate assembly comprising: a first bearing, comprising: a part-spherical element, and an inner swashplate arranged to house the part-spherical element, wherein the inner swashplate is tiltable on the part-spherical element; wherein the inner swashplate comprises an opening in which the part-spherical element is seated in contact with an inner surface of the inner swashplate; an outer swashplate rotatably coupled to the inner swashplate, wherein the inner swashplate is configured to rotate relative to the outer swashplate about a fixed axis; and wherein the inner swashplate is tiltable together with the outer swashplate on the part-spherical element. The inner swashplate may be unitary, e.g. provided by or formed from a single piece. The inner swashplate may be a body of material. The opening for seating the part-spherical element may be machined into and / or formed in the inner swashplate, e.g. into the body of material. The swashplate assembly may further comprise a second bearing arranged between the inner swashplate and the outer swashplate, wherein the inner swashplate is rotatably coupled to the outer swashplate via the second bearing. The second bearing may be a ball bearing. The second bearing may comprise an inner bearing race and an outer bearing race. The second bearing may further comprise a plurality of bearing balls arranged between the inner bearing race and the outer bearing race. The inner bearing race may be in engagement, e.g. fixed contact, with the inner swashplate, e.g. with an exterior surface of the inner swashplate. The outer bearing race may be in engagement, e.g. fixed contact, with the outer swashplate, e.g. with an interior surface of the inner swashplate. The outer swashplate may be configured to rotate / spin relative to the inner swashplate, e.g. about a fixed axis of rotation. The inner swashplate may extend axially from a first end to a second end. The outer swashplate may be arranged around the inner swashplate and may be arranged between the first longitudinal end and the second end, for example entirely between the first and second ends. An exterior surface of the outer swashplate may be axially aligned with an exterior surface of the inner swashplate. The part-spherical element may have a axial length, e.g. diameter, that is greater than the height / longitudinal extension of the of inner swashplate, and / or the outer swashplate, for example the part-spherical element may extend longitudinally beyond the first end and beyond the second end. The inner swashplate and the outer swashplate may comprise complementary surface features, such as notches, grooves, and protrusions. The inner swashplate may comprise a first set of connection points for coupling to at least one of a control mechanism of the helicopter, (control arms or control rods) and a set of rotor blades of the helicopter, e.g. via rigid connectors. The inner swashplate may comprise a first surface arranged to provide a bearing race for the part-spherical element, and may also provide a second surface in engagement with the second bearing. The part-spherical element may comprise a lumen or opening for receiving a drive shaft of the helicopter, the lumen having a first end and a second end opposite the first end. The lumen may be cylindrical or substantially cylindrically shaped. The part-spherical element may be a spherical ring. The part-spherical element may be a split element comprising a first part and a second part (e.g. a first unitary piece and a second unitary piece), wherein the first part is couplable to the second part to form the part-spherical element. The second part may be separable from the first part for removal from the opening. For example, the first part and the second part may be twistable, e.g. by hand, relative to each other and / or relative to the inner swashplate, for removal from the inner swashplate. This may allow for fast replacement of the part-spherical element. The part-spherical element may be arranged for attaching onto a drive shaft of the helicopter. The swashplate assembly may be retained on the drive shaft by the part-spherical element, e.g. without the use of screws or other retention components. The first part and the second part may be shaped for slidable engagement with the inner surface of the inner swashplate upon coupling together. The part-spherical element may comprise a self-lubricating material. The part-spherical element may comprise a fibrous material. The fibrous material may be impregnated within the self-lubricating material. The part-spherical element may have an anodised, e.g. a hard-anodised surface. The self-lubricating material may be bound in resin. The self-lubricating material may comprise a polymer, for example a low friction polymer such as PTFE. Such materials may help to minimise wear and corrosion, and may aid in marinization due to saltwater compatibility. The inner swashplate may have an inner surface suitable for engagement, e.g. slidable engagement, with the part-spherical element. The inner swashplate may have a surface, e.g. at least the inner surface, that is anodised, e.g. hard-anodised. The inner surface may provide a mating surface for the part-spherical element. The part-spherical element may be slidable relative to a driveshaft of the helicopter in the direction of the longitudinal axis of the driveshaft. The part-spherical element, the inner swashplate and the outer swashplate may be slidable together in the direction of the longitudinal axis of the drive shaft. The part spherical element may be arranged on a drive shaft of the helicopter, for example surrounding a portion (e.g. a longitudinal length) of the drive shaft. The part-spherical element may be arranged for insertion onto the drive shaft. The part spherical element may be self-retaining on the drive shaft. One of the outer swashplate and the inner swashplate may be arranged for coupling to a control mechanism for controlling the tilt of the outer swashplate and the inner swashplate relative to the part-spherical element. For example the swashplate may be coupled to said control mechanism, e.g. via control rods. The other of the outer swashplate and the inner swashplate may arranged for rigidly coupling to rotor blades of the helicopter. For example the inner or the outer swashplate may be rigidly coupled to said rotor blades. The second bearing may be arranged between the axial / longitudinal position of the first end and the axial / longitudinal position of second end. Another aspect of the disclosure provides a helicopter comprising the swashplate assembly. The helicopter may comprise a first set of rotor blades arranged to rotate in a first direction, and a first of the swashplate assemblies, arranged to control the first set of rotor blades. The helicopter may further comprise a second set of rotor blades arranged to rotate in a second direction opposite the first direction and having the same axis of rotation as the first set of rotor blades. The helicopter may further comprise a second of the swashplate assemblies arranged to control the second set of rotor blades. The helicopter may comprise a coaxial counter-rotating rotor head, which may comprise the first set of rotor blades, the second set of rotor blades, the first swashplate assembly and the second swashplate assembly. Another aspect of the disclosure provides a swashplate for a helicopter, the swashplate having an internal surface bounding an opening extending through the swashplate; wherein the internal surface is configured to provide a bearing face for contacting a part-spherical bearing element and for slidable movement on the part-spherical bearing element; and wherein the swashplate further comprises an external surface that is configured to fit a bearing race of a second bearing for a second swashplate. The external surface may be fixedly coupled to the bearing race, for rotational movement relative to a second swashplate coupled to a second bearing race of the second bearing element. Another aspect of the disclosure provides a method of assembling a helicopter rotor head, the method comprising: providing an inner swashplate and a part-spherical bearing element onto a drive shaft for a helicopter rotor head, wherein the inner swashplate is provided by a body of material having an inner surface of part-spherical form for contacting a part-spherical bearing element; and wherein the part-spherical bearing element is in contact with the inner surface of the inner swashplate. The method may further comprise inserting the part-spherical element into an opening in the inner swashplate so that the part-spherical element is in contact with the inner surface of the inner swashplate. Providing an inner swashplate and a part-spherical bearing element onto the drive shaft comprises sliding the inner swashplate and the part-spherical element together onto or over the drive shaft. This may secure, e.g. lock, the part-spherical bearing element between the drive shaft and the inner swashplate. In other examples, the method may comprise first providing the inner swashplate onto the drive shaft, and then providing the part-spherical bearing element onto the drive shaft. Alternatively, the method may comprise first providing the part-spherical bearing element onto the drive shaft, and then providing the inner swashplate onto the drive shaft. The part-spherical bearing element is a split element comprising a first part and a second part. Providing the part-spherical bearing element onto the drive shaft may comprise providing the first part onto a first portion of the drive shaft, and providing the second part onto a second portion of the drive shaft. The first and second portions of the drive shaft may be axially aligned, for example the first portion may correspond to a half of a circumference of a cylindrical section of the drive shaft, and the second portion may correspond to a half of the circumference of that section of the drive shaft. The method may further comprise providing a second bearing so that it is contact with the inner swashplate, for example with an external surface of the inner swashplate. The method may further comprise providing an outer swashplate in contact with the second bearing. It will be appreciated that terms ‘axial’ and ‘longitudinal’, when used above and in the description below, refers to the direction along the axis of rotation of the drive shaft of the helicopter, and that the term ‘radial’ refers to the plane that is perpendicular to that axis. Another aspect of the disclosure provides a swashplate assembly for a helicopter rotor head, the swash pate assembly comprising: a first bearing, comprising: a part-spherical element, and an inner swashplate arranged to house the part-spherical element, wherein the inner swashplate is tiltable on the part-spherical element; wherein the inner swashplate comprises an opening in which the part-spherical element is seated in contact with an inner surface of the inner swashplate; an outer swashplate rotatably coupled to the inner swashplate, wherein the inner swashplate is configured to rotate relative to the outer swashplate about a fixed axis; and wherein the inner swashplate is tiltable together with the outer swashplate on the part-spherical element. Brief Description of Figures Some examples of the present disclosure will now be described with reference to the figures, in which: Figure 1 shows a highly schematic view of an example helicopter with coaxial counterrotating rotor head; Figure 2a shows a cross-sectional perspective view of an example swashplate assembly; Figure 2b shows an exterior perspective view of the example swashplate assembly; Figure 3 shows a cross-sectional view of an example swashplate. Figure 4 shows an example bearing element; Figure 5 illustrates the removal of an example bearing element from a swashplate assembly. In the drawings like reference numerals are used to indicate like elements. Specific Description Described below with reference to the figures are examples of swashplate assemblies for helicopters, and in particular the bearings in such assemblies. The present disclosure provides a swashplate assembly with an inner swashplate and an outer swashplate, one of which spins with the rotor blades of the helicopter relative to the other. The assembly also includes a part-spherical bearing element about which the swashplates pivots, which is housed by and is in direct contact with an interior surface of the inner swashplate. The inner swashplate thus acts as an outer bearing race for the part-spherical element, which may result in a more compact assembly in comparison to existing mechanisms. Also described below is a part-spherical central bearing element which is a split element provided by two (or more) component parts. When assembled within the swashplate, these parts together provide a part-spherical element for the swashplates to pivot about. The split nature of this element may allow for easier removal and replacement. Figure 1 is a highly schematic view of an exemplary helicopter 100. In particular, the helicopter 100 shown in Figure 1 is a coaxial counter-rotating helicopter, comprising a lower first set of rotor blades 102, and an upper second set of rotor blades 104. The first set of blades 102 and the second set of blades 104 are configured to rotate in opposite directions (e.g. clockwise and anticlockwise respectively), thereby balancing the generated torque and avoiding the need for an additional tail rotor. The helicopter 100 comprises a coaxial drive shaft assembly comprising a first drive shaft 106 connected to drive the first set of blades 102 and a second drive shaft 108 connected to drive the second set of blades 104. The first drive shaft 106 and the second drive shaft 108 share a common axis of rotation, for example, the first drive shaft 106 may be arranged within the second drive shaft 108. The first drive shaft 106 and the second drive shaft are connected at a first end to the respective first and second sets of rotor blades 102, and at a second end to a drive mechanism 120. The drive mechanism 120 may comprise a motor or engine, and a gearbox. The drive mechanism 120 is configured to drive the first drive shaft 106 in a first direction (e.g. clockwise) and the second drive shaft 108 in a second direction opposite the first (e.g. anticlockwise), thereby to drive the first set of rotor blades 102 and the second set of rotor blades 104 in opposite directions. The helicopter 100 further comprises a first swashplate assembly 110 associated with the first set of rotor blades 102, and a second swashplate assembly 112 associated with the second set of rotor blades 104. Each of the swashplate assemblies 110, 112 are configured to translate inputs from the flight controls into tilting (i.e. pivoting) or translating motion of the spinning rotor blades 102, 104. Each of the swashplate assemblies 110, 112 comprise a first swashplate 111 and a second swashplate 113. Each second swashplate 113a, 113b may be configured to spin with their associated rotor blades 102, 104 and drive shafts 106, 108, relative to their associated first swashplate 111a, 111b. The first swashplate part 111a of the first swashplate assembly 110 is stationary with respect to the flight controls 114 and the helicopter 100 as a whole, and so may be considered as a “stationary swashplate” The second swashplate part 113a of the assembly 110 is configured to spin with the rotor blades 102 and so may conversely be considered a “rotating swashplate”. The lower first swashplate part 111b of the second swashplate assembly 112 coupled to and configured to rotate with the rotating part of the lower rotor hub, e.g. the swashplate 113a and / or the rotor blades 102, whilst the second swashplate 113b of the second assembly 112 is coupled to spin with the second set of rotor blades 104. Thus, the swashplates 111b and 113b are arranged to spin in opposite directions to one another in the assembly 112, when the helicopter is in use. The stationary swashplates 111a, 111b are coupled to an actuator 114, which for example may include one or more servo motors, via control rods 115. The rotating swashplates 113a, 113b are each rigidly connected to their associated rotor blades 103, 104, via rigid connectors 116. In operation, in response to commands from a user, the actuator 114 may actuate one or more of the control rods 115, thereby to tilt one or both of the stationary swashplates 111a, 11b relative to the longitudinal axis of the drive shafts 106, 108, or to translate said swashplate along the direction of the longitudinal axis (i.e. to move the swashplate up or down). The tilting or translating action is transferred to the rotating swashplate 113a, 113b in the swashplate assembly 110, 112 and thereby onto the spinning rotor blades 102, 104, to adjust the angle of the rotor blades 102, 104. The structure and function of example swashplate assemblies are discussed in more detail below. Figure 2a is a cross sectional view of an exemplary swashplate assembly 200, whilst figure 2b is an external view of the same swashplate assembly 200. The swashplate assembly 200, may for example be used as the swashplate assemblies 110, 112, in the helicopter 100 with a coaxial counter rotating rotor head shown in Figure 1. However, it will be appreciated that the swashplate assembly 200 may also be used in other types of aircraft, e.g. single rotor helicopters and other helicopters without coaxial rotor heads. The swashplate assembly 200 comprises an outer swashplate 202 and an inner swashplate 204. The swashplate assembly further comprises a part-spherical element 250, in particular a bearing element, arranged in a central opening in the inner swashplate 204, such that the inner swashplate 204 houses the part spherical element 250. The part-spherical element 250 is in particular shaped as a spherical ring. It is substantially hollow and defines a substantially cylindrical inner lumen 252 such that the part-spherical element 250 can be mounted on a portion of the drive shaft of the helicopter (such as driveshafts 106 or 108 shown in Fig. 1). The inner surface of the part-spherical element 250 is configured such that the element 250, and the entire swashplate assembly 200, can slide longitudinally along the drive shaft (e.g. up and down) within certain limits. The outer surface of the part-spherical element 250 is in slidable engagement with an inner surface of the inner swashplate 204, such that the inner swashplate 204 can pivot (tilt) about the drive shaft. The inner swashplate 204 and the part-spherical element 250 together provide a first bearing, where the inner swashplate 204 houses the part-spherical element 250, and wherein the inner swashplate 204 is tiltable on the part-spherical element 250. The inner surface of the inner swashplate 204 thus acts as a bearing race for the part-spherical element 250. The outer swashplate 202 is coupled to the inner swashplate 204 such that the outer swashplate is configured to pivot / tilt together with the inner swashplate 204 on the part-spherical element 250. In particular, the outer swashplate 202 is rotatably coupled to the inner swashplate 204 about a second bearing 260, which in this example is a ball bearing. The second bearing 260 is annular in shape and extends around a circumference of the inner swashplate 204. The second bearing 260 comprises an inner race 262 in engagement with an exterior surface of the inner swashplate 204, and an outer race 264 in engagement with an interior surface of the outer swashplate 202. A plurality of bearing balls (not shown) are provided in the space between the inner race 262 and the outer race 264. The outer swashplate 202 is thus configured to spin relative to the inner swashplate 204, e.g. with the spinning of the rotor blades. In this example, the outer swashplate 202 thus acts as the “rotating swashplate” and the inner swashplate 204 as the “stationary swashplate” discussed above. The second bearing 260 is configured to constrain the relative motion of the inner swashplate 204 and outer swashplate 202 to this spinning, i.e. so that this is the only degree of freedom for the relative movement between the two swashplates. The inner 204 and outer 202 swashplates are arranged to otherwise move together e.g. when tilting relative to the part-spherical element upon action of the actuator. The structure of the inner swashplate 204 will now be described in more detail with reference to Figure 3. The inner swashplate 204 is a unitary piece comprising a sidewall portion 206 and a lip potion 208, such that the inner swashplate 204 has a substantially T-shaped cross-section. The sidewall portion 206 is substantially cylindrical, and has a part-spherical inner surface 211 defining an opening 215 for receiving a spherical or part- spherical element, for example the part-spherical element 250. The lip portion 208 extends radially outwards from one end of the sidewall portion 206. The lip portion 208 provides a flat exterior surface 210 of the inner swashplate 204, and an interior surface (e.g. an undersurface) 212. The lip portion 208 extends outwards substantially perpendicular to an exterior surface 214 of the sidewall portion 206, such that the undersurface 212 of the lip portion 208 and the exterior surface 214 of the sidewall portion 206 together partially define a space 216 for receiving the second bearing 260 and the outer swashplate 202. The undersurface 212 itself comprises a protruding lip 218 which protrudes opposite and parallel to the sidewall surface 214, and extends around the entire circumference of the inner swashplate 204. The lip 218 is arranged for radially restraining the outer swashplate 202 as shown in Fig. 2a. A right-angled protrusion 220 is also provided on the surface of the inner swashplate 204 and extends around the entire circumference of the inner swashplate 204 for engagement with the inner race 262 of the second bearing 260. As such, the second bearing 260 is held in the space 216 in engagement with the sidewall exterior surface 214 and the protrusion 220, as shown in Figure 2a. A substantially rectangular shaped groove 222 is also provided in the sidewall surface 214 and extends around the entire circumference of the inner swashplate 204. The groove 222 is spaced from the protrusion 220 by a distance substantially equal to the width of the second bearing 260. The inner swashplate 202 may be made of a lightweight material such as aluminium, and may have an-anodised e.g. hard-anodised surface. This may enable the part-spherical element 250 to slide on the inner surface 211 with minimal friction. It will be appreciated however that the inner swashplate 202 may be made of other materials, e.g. any materials which are suitable for slidable engagement with the part-spherical bearing element. The inner swashplate 204 further comprises a plurality of connectors 230 arranged around the circumference of the inner swashplate 204 (see also Figs 2a, 2b). In particular, each connector 230 comprises a pair of protrusions 232 that are substantially L-shaped and extend radially outward from the outer edge of the lip portion 208 of the inner swashplate 202 and then longitudinally (in the direction of the axis of the drive shaft) substantially opposite and parallel to the sidewall portion 206 towards a curved end 234. A hole 236 is provided through each of the protrusions 232 proximal to the curved end. The inner swashplate 204 comprises three such connection points 230 that are spaced evenly around the circumference of the inner swashplate 204. Returning to Figs 2a, 2b, the outer swashplate 202 also comprises a plurality of connectors 240 (three in this example) that are arranged evenly around the circumference of the outer swashplate 202. Each connector 240 also comprises a pair of protrusions 242 that are substantially L-shaped and extend radially outward from the outer edge of the outer swashplate 202, further than the protrusions 232 of the inner swashplate 204. The protrusions 242 then extend longitudinally (in the direction of the axis of the drive shaft) towards the lip portion 208 of the inner swashplate 204 to a curved end 244. A hole 246 is provided through each of the protrusions 242 proximal to the curved end 244. As shown in Figures 2a and 2b, when the assembly 200 is assembled, the holes 246 of the outer swashplate 202 and the holes 236 of the inner swashplate 204 are longitudinally aligned. The L-shaped protrusions 232, 242 provide both inner and outer swash plate holes 236, 246 that are aligned with each other and with the centre of the spherical bearing axis of rotation on the same plane. This may nullify any geometric differences between the input / output movement transfer paths of the inner 204 and outer 202 swash plates. It will be appreciated however that this alignment is optional. In other examples, the difference between the input and output geometry of the two swashplates may be used as part of the control mechanism. It will also be appreciated that the shapes of the protrusions 232 and 242 that are shown in the figures are merely exemplary, and that other shapes are envisaged. For example, each connection point could be provided by a single protrusion or arm, rather than a pair of protrusions as shown. The outer swashplate 202 also comprises a notch 224 at one end on its outer surface extending around the entire circumference of the outer swashplate 202, for receiving the protruding lip 218 of the inner swashplate. The notch 224 is complementarily shaped to the shape of the protruding lip 218 of the inner swashplate 204. The outer swashplate 202 further comprises a protrusion 226 extending radially inwards from its inner surface. The protrusion 226 extends around the entire circumference of the outer swashplate 202 and is substantially longitudinally aligned with the notch 224. The protrusion 226 is arranged to engage with a surface of the second bearing 260, in particular with the outer race 264 of the second bearing 260. A substantially rectangular shaped groove 228 is also provided in the interior surface of the outer swashplate 202 and extends around the entire circumference. The assembly 200 further comprises first and second retaining elements 266, 268 for retaining and / or securing the second bearing 260 in position between the inner swashplate 204 and the outer swashplate 202. The first retaining element 266 is provided in the groove 222 of the inner swashplate 204, and the second retaining element 268 is provided in the groove 228 of the outer swashplate 202. The retaining elements in this example are circlips. However, it will be appreciated that the use of circlips as retaining elements is entirely optional. The inner swashplate 204 and the outer swashplate 202 thus comprise a plurality of complementary surface features (e.g. notches, grooves, protrusions) and are shaped so that the inner swashplate 204 sits securely around the outer swashplate with the second bearing 260 held between them, with the inner and outer races of the bearing 260 in fixed engagement with the inner 202 and outer 204 swashplates respectively. As shown in Fig. 2a, the longitudinal extent A of the outer swashplate 202 is less than the longitudinal extent B of the inner swashplate 204, and substantially equal to the longitudinal extent of the sidewall portion 202. The outer swashplate 202 thus fits within the space 216 bounded by the interior surface 212 of the lip portion 208 and the exterior surface 214 of the sidewall portion 206, and does not longitudinally extend beyond the extent of the inner swashplate 204. The part-spherical element 250 has a longitudinal extent, e.g. diameter of the spherical ring, that is greater than that of the inner swashplate 204, such that the part-spherical element 250 partially protrudes beyond each end face of the inner swashplate 204. The connectors 230 of the inner swashplate 204 are arranged for connection to the control mechanism of the helicopter. For example the connectors 230 may be configured for connection to control rods 115 which are coupled to the actuator 114 shown in Figure 1, such that upon operation of the actuator the swashplate assembly 200 tilts about the part-spherical element and / or translates along the drive shaft as discussed above. The connectors 240 of the outer swashplate 204 are arranged for connection to the rotor blades of the helicopter. In particular, the connectors 240 may be configured for connection to rigid connectors 116 coupled to rotor blades 102 or 104 as shown in Figure 1. The outer swashplate 202 is thus configured to spin with the driven rotor blades 102 / 104, and is also arranged to transfer the tilting or translating movement of the assembly 200 discussed above to the blades, for example to adjust their pitch and thereby control the helicopter. It will be appreciated, however, that this arrangement is merely exemplary. In other examples, the entire swashplate assembly 200 may be flipped vertically relative to the example shown in Figures 2a &2b. In such examples, the inner swashplate is instead coupled to the rotor blades of the helicopter (and thus acts as the rotating swashplate), whilst the inner swashplate is instead coupled to the actuator and control mechanism of the helicopter (and thus acts as the stationary swashplate). The relative spinning movement between the swashplates and the joint tilting about the part-spherical element 250 is otherwise provided in the same way as described above with reference to Figures 2a, 2b and 3. Figure 4 illustrates an example of a part-spherical element 250, when not incorporated into the swashplate assembly 200. As shown, the part-spherical element 250 is a split spherical ring, comprising a first part 252 and a second part 254. The first part 252 and second part 254 are substantially identical, and each provide half of the overall spherical ring shape of the part-spherical element 250. In particular, each part 252, 254 has an inner surface 256 that is shaped as a half-cylinder, such that when assembled a cylindrical lumen is provided between the two parts 252, 254, e.g. for receiving the drive shaft. Each part 252, 254 has an outer surface 258 that is shaped as a section of a surface of a sphere. Once assembled the parts 252, 254 clip orclamp onto either side of a portion of the drive shaft, such that the inner surface of the lumen now provided by the assembled part-spherical element is in slidable engagement with the drive shaft. The assembled part-spherical element 250 has a part-spherical outer surface, thereby enabling the inner swashplate 202 to pivot / tilt over the surface. The element parts 252, 254, and the opening 215 of the inner swashplate 204 are sized so that the parts 252, 254 are secured together tightly to form a continuous part-spherical surface for slidable engagement with the inner surface 211 of the inner swashplate 202. Figure 5 illustrates the removal of one of the element parts 254 from the swashplate assembly 200. As shown, the part 254 can be removed by hand by twisting it, e.g. through 90°, relative to the assembly, and removed from the opening in the inner swashplate 202. The part-spherical element 250 can thus be removed from the assembly 200 without disassembling or splitting the assembly 200 itself or the swashplates 202, 204. A replacement part can similarly be introduced into the assembly 200 using the reverse process. The part-spherical element 250 (e.g. each part 252, 254) may comprise a self-lubricating material. For example, the element 250 may be made from a fibrous material that is impregnated with a self-lubricating substance, such as PTFE or graphite, which may be bound in resin. Such materials may enable the element 250 to slide easily with minimal friction on the interior surface of the inner swashplate (e.g. the hard-anodised surface) in operation, whilst also having the properties necessary to enable assembly, removal and replacement of the parts of the element as described above with reference to Figures 4 and 5. It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any features of any aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example, method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination with, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention. Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

1. A swashplate assembly for a coaxial counter-rotating helicopter rotor head, the swash plate assembly comprising:a first bearing, comprising:a part-spherical element, andan inner swashplate arranged to house the part-spherical element, wherein the inner swashplate is tiltable on the part-spherical element;wherein the inner swashplate comprises an opening in which the part-spherical element is seated in contact with an inner surface of the inner swashplate;an outer swashplate rotatably coupled to the inner swashplate, wherein the inner swashplate is configured to rotate relative to the outer swashplate about a fixed axis; andwherein the inner swashplate is tiltable together with the outer swashplate on the part-spherical element.

2. The swashplate assembly of claim 1, further comprising a second bearing arranged between the inner swashplate and the outer swashplate, wherein the inner swashplate is rotatably coupled to the outer swashplate via the second bearing.

3. The swashplate assembly of claim 2, wherein the inner swashplate comprises a first surface arranged to provide a bearing race for the part-spherical element, and a second surface in engagement with the second bearing.

4. The swashplate assembly of claim 2 or 3, wherein the part-spherical element comprises a lumen for receiving a drive shaft of the helicopter, the lumen having a first end and a second end opposite the first end, wherein the second bearing is arranged between the axial position of the first end and the axial position of second end5. The swashplate assembly of any preceding claim, wherein the part-spherical element is a split element comprising a first part and a second part, wherein the first part is couplable to the second part to form the part-spherical element.

6. The swashplate assembly of claim 5, wherein the second part is separable from the first part for removal from the opening.

7. The swashplate assembly of claim 5 or 6, wherein the first part and the second part are shaped for slidable engagement with the inner surface of the inner swashplate upon coupling together.

8. The swashplate assembly of any preceding claim, wherein the part-spherical element comprises a self-lubricating material.

9. The swashplate assembly of any preceding claim, wherein the part-spherical element comprises a fibrous material.

10. The swashplate assembly of claim 9 as dependent on claim 8, wherein the fibrous material is impregnated with the self-lubricating material.

11. The swashplate assembly of any preceding claim, wherein at least one of the part-spherical element and the inner swashplate has a hard-anodised surface.

12. The swashplate assembly of claim 8 or any claim dependent thereon, wherein the selflubricating material is bound in resin.

13. The swashplate assembly of claim 8 or any claim dependent thereon, wherein the selflubricating material comprises a polymer such as PTFE.

14. The swashplate assembly of any preceding claim, wherein the part-spherical element is slidable relative to a driveshaft of the helicopter in the direction of the longitudinal axis of the driveshaft.

15. The swashplate assembly of claim 14, wherein the part-spherical element, the inner swashplate and the outer swashplate are slidable together in the direction of the longitudinal axis of the drive shaft.

16. The swashplate assembly of any preceding claim, wherein one of the outer swashplate and the inner swashplate is coupled to a control mechanism for controlling the tilt of the outer swashplate and the inner swashplate relative to the part-spherical element.

17. A helicopter comprising the swashplate assembly of any preceding claim.

18. The helicopter of claim 17, wherein the helicopter comprises:a first set of rotor blades arranged to rotate in a first direction;a first swashplate assembly according to any of claims 1 to 16, arranged to control the first set of rotor blades.

19. The helicopter of claim 18, further comprising:a second set of rotor blades arranged to rotate in a second direction opposite the first direction and having the same axis of rotation as the first set of rotor blades;a second swashplate assembly according to any of claims 1 to 16, arranged to control the second set of rotor blades.

20. A swashplate for a helicopter, the swashplate having an internal surface bounding an opening extending through the swashplate;wherein the internal surface is configured to provide a bearing face for contacting a part-spherical bearing element and for slidable movement on the part-spherical bearing element;and wherein the swashplate further comprises an external surface that is configured to fit a bearing race of a second bearing for a second swashplate.

21. The swashplate of claim 20, wherein the external surface is fixedly coupled to the bearing race, for rotational movement relative to a second swashplate coupled to a second bearing race of the second bearing element.

22. A method of assembling a helicopter rotor head, the method comprising:providing an inner swashplate and a part-spherical bearing element onto a drive shaft for a helicopter rotor head, wherein the inner swashplate is provided by a body of material having an inner surface of part-spherical form for contacting a part-spherical bearing element; and wherein the part-spherical bearing element is in contact with the inner surface of the inner swash plate.

23. The method of claim 22, further comprising inserting the part-spherical element into an opening in the inner swashplate so that the part-spherical element is in contact with the inner surface of the inner swashplate, first providing the inner swashplate onto the drive shaft, and then providing the part-spherical bearing element onto the drive shaft.

24. The method of claim 22 or23, wherein providing an inner swashplate and a part-spherical bearing element onto the drive shaft comprises sliding the inner swashplate and the part-spherical element together onto the drive shaft.

25. The method of any of claims 22 to 24, wherein the part-spherical bearing element is a split element comprising a first part and a second part, wherein providing the part-spherical bearing element onto the drive shaft comprises providing the first part onto a first portion of the drive shaft, and providing the second part onto a second portion of the drive shaft.26 11 24Amendments to the Claims have been filed as follows:CLAIMS:

1. A swashplate assembly for a coaxial counter-rotating helicopter rotor head, the swash plate assembly comprising:a first bearing, comprising:a part-spherical element, andan inner swashplate arranged to house the part-spherical element, wherein the inner swashplate is tiltable on the part-spherical element;wherein the inner swashplate comprises an opening in which the part-spherical element is seated in contact with an inner surface of the inner swashplate;an outer swashplate rotatably coupled to the inner swashplate, wherein the inner swashplate is configured to rotate relative to the outer swashplate about a fixed axis; andwherein the inner swashplate is tiltable together with the outer swashplate on the part-spherical element; andwherein the part-spherical element is a split element comprising a first part and a second part, wherein the first part is couplable to the second part to form the part-spherical element.

2. The swashplate assembly of claim 1, further comprising a second bearing arranged between the inner swashplate and the outer swashplate, wherein the inner swashplate is rotatably coupled to the outer swashplate via the second bearing.

3. The swashplate assembly of claim 2, wherein the inner swashplate comprises a first surface arranged to provide a bearing race for the part-spherical element, and a second surface in engagement with the second bearing.

4. The swashplate assembly of claim 2 or 3, wherein the part-spherical element comprises a lumen for receiving a drive shaft of the helicopter, the lumen having a first end and a second end opposite the first end, wherein the second bearing is arranged between the axial position of the first end and the axial position of second end5. The swashplate assembly of any preceding claim, wherein the second part is separable from the first part for removal from the opening.26 11 246. The swashplate assembly of any preceding claim, wherein the first part and the second part are shaped for slidable engagement with the inner surface of the inner swashplate upon coupling together.

7. The swashplate assembly of any preceding claim, wherein the part-spherical element comprises a self-lubricating material.

8. The swashplate assembly of any preceding claim, wherein the part-spherical element comprises a fibrous material.

9. The swashplate assembly of claim 8 as dependent on claim 7, wherein the fibrous material is impregnated with the self-lubricating material.

10. The swashplate assembly of any preceding claim, wherein at least one of the part-spherical element and the inner swashplate has a hard-anodised surface.

11. The swashplate assembly of claim 7 or any claim dependent thereon, wherein the selflubricating material is bound in resin.

12. The swashplate assembly of claim 7 or any claim dependent thereon, wherein the selflubricating material comprises a polymer such as PTFE.

13. The swashplate assembly of any preceding claim, wherein the part-spherical element is slidable relative to a driveshaft of the helicopter in the direction of the longitudinal axis of the driveshaft.

14. The swashplate assembly of claim 13, wherein the part-spherical element, the inner swashplate and the outer swashplate are slidable together in the direction of the longitudinal axis of the drive shaft.

15. The swashplate assembly of any preceding claim, wherein one of the outer swashplate and the inner swashplate is coupled to a control mechanism for controlling the tilt of the outer swashplate and the inner swashplate relative to the part-spherical element.

16. A helicopter comprising the swashplate assembly of any preceding claim.26 11 2417. The helicopter of claim 16, wherein the helicopter comprises:a first set of rotor blades arranged to rotate in a first direction;a first swash plate assembly according to any of claims 1 to 16, arranged to control the first set of rotor blades.

18. The helicopter of claim 17, further comprising:a second set of rotor blades arranged to rotate in a second direction opposite the first direction and having the same axis of rotation as the first set of rotor blades;a second swashplate assembly according to any of claims 1 to 16, arranged to control the second set of rotor blades.

19. A method of assembling a helicopter rotor head, the method comprising:providing an inner swashplate and a part-spherical bearing element onto a drive shaft for a helicopter rotor head, wherein the inner swashplate is provided by a body of material having an inner surface of part-spherical form for contacting a part-spherical bearing element; and wherein the part-spherical bearing element is in contact with the inner surface of the inner swashplate;wherein the part-spherical bearing element is a split element comprising a first part and a second part, wherein providing the part-spherical bearing element onto the drive shaft comprises providing the first part onto a first portion of the drive shaft, and providing the second part onto a second portion of the drive shaft.

20. The method of claim 19, further comprising inserting the part-spherical element into an opening in the inner swashplate so that the part-spherical element is in contact with the inner surface of the inner swashplate.

21. The method of claim 19 or 20, the method comprising first providing the inner swashplate onto the drive shaft, and then providing the part-spherical bearing element onto the drive shaft.

22. The method of any of claims 19 to 21, wherein providing an inner swashplate and a part-spherical bearing element onto the drive shaft comprises sliding the inner swashplate and the part-spherical element together onto the drive shaft.

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