FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Cooperative Agreement #W911W6-05-2-0006, awarded by the United States Army. The government has certain rights in this invention. 
    
    
     BACKGROUND 
     This disclosure relates to vertical takeoff and landing (VTOL) rotorcraft, in general, and in particular, to a high speed, low drag, low maintenance, stiff in-plane, gimbaled rotor head for a helicopter that enables three or more rotor blades to be used per rotor, while enabling a more compact tandem rotor helicopter by allowing greater blade intermesh through the elimination of the lead-lag motions and dampers associated with the fully articulated rotor heads of the prior art. 
     A fully articulated type of rotor head  100  of the prior art, such as used on the Boeing CH-47 “Chinook” tandem rotor helicopter, is illustrated in  FIG. 9 . As illustrated therein, the fully articulated rotor head comprises a rotor shaft  102 , a hub  104  disposed at the end of the shaft and three blades  106  radiating outward from it. Each of the rotor blades is pinioned to the hub by a pair of hinges, viz., a “flap” hinge  108  and a “lag” hinge  110  that respectively enable the associated blade to pivot both up and down, and fore and aft, relative to the hub. Additionally, the blades and lag hinges are rotatably coupled to respective flap hinges by respective pitch shafts  112  rotatably disposed within respective pitch shaft housings  113  for conjoint rotation with the respective blades about the respective long axes thereof. Thus, in addition to being capable of up and down and pitching movements, the blades are also capable of lead-lag movement in the plane of rotation of the blades, i.e., they are “flexible in-plane.” 
     A disadvantage of flexible in-plane rotor head designs when used in tandem rotor aircraft is that it is difficult to get the respective blades of the two rotors to intermesh with each other when rotating in the same plane due to the range of angular displacement that each blade may undergo within its respective plane of rotation. As a result, the two rotors must be spaced apart from each other, either horizontally or vertically, such that the respective blades do not overlap, or their respective planes of rotation are not coplanar. 
     In order to overcome this drawback, efforts have been made to develop “stiff in-plane” rotor hubs, i.e., hubs with blades that are incapable of pivotal movement in the plane of rotation of the blades. The existing solutions for stiff in-plane hubs are the so-called two-bladed “teeter” hubs, such as used on many light rotorcraft, and three-bladed gimbaled hubs, such as are used on the Bell-Boeing V-22 “Osprey” hybrid tilt-rotorcraft. 
     Studies have shown that rotors having greater solidity are required for next-generation, high speed, heavy lift, tandem rotor helicopter designs. This greater rotor solidity is most efficiently delivered with a large number of blades (as many as 6 blades per head). Teeter rotor heads inherently can employ only two blades per hub, and are therefore unsuitable for high speed, heavy lift helicopter configurations. The use of stiff in-plane hubs enables a larger number of blades (more than three) of the two rotors to intermesh tightly when rotating in the same plane so as to keep the configuration compact and performance high, while at the same time avoiding the limitations of the flexible in-plane hubs of the prior art. 
     Accordingly, there is a need in the rotorcraft field for a high speed, low drag, low maintenance, stiff in-plane, gimbaled rotor head for a high-speed, heavy lift helicopter that achieves a more compact tandem rotor blade intermesh by eliminating the lead-lag motions and dampers used in the fully articulated rotor heads of the prior art, and that also enables more than three rotor blades to be used per rotor. 
     SUMMARY 
     In accordance with the present disclosure, high speed, low drag, low maintenance, stiff in-plane, gimbaled rotor heads for helicopters are provided that enable three or more rotor blades to be used per rotor, and that also enable a compact tandem rotor blade intermesh to be achieved by eliminating the lead-lag motions and dampers of fully articulated rotor heads. 
     In one exemplary embodiment, a stiff in-plane, gimbaled rotor head for a rotorcraft comprises an elongated, vertically extending rotor shaft having an axis of rotation. A center hub is disposed at an upper end of and is rotationally driven by the rotor shaft. A split outer hub surrounds and is coupled to the center hub through a spherical main gimbal bearing such that the outer hub is capable of an angular range of gimbaling movement relative to the center hub. An elongated, radially extending blade has an airfoil cross-section and an inboard end rigidly coupled to the outer hub, and a constant velocity joint couples driving torque from the rotor shaft to the blade through the center and outer hubs such that, during rotation of the blade about the axis of rotation of the rotor shaft, the rotational velocity of the blade about the tilted axis of rotation remains substantially constant during gimbaling movements of the outer hub relative to the inner hub. 
     In another exemplary embodiment, a method for rotating each of a plurality of rotorcraft blades in a common plane and about an axis of rotation while controlling the respective pitches of the blades and substantially preventing any other relative in-plane and out-of-plane movements of the blades during the rotating comprises providing a rotating rotor shaft concentric to the axis of rotation; fixing a central hub to the rotor shaft for conjoint rotation therewith; coupling an outer hub to the center hub for conjoint rotation therewith and such that the outer hub is also capable of an angular range of gimbaling movement relative to the center hub; and, coupling an inboard end of the blades to the outer hub such that each blade is capable of pitching movement relative to the outer hub and is substantially incapable of any other movements relative thereto. 
     A better understanding of the above and many other features and advantages of the novel rotor heads of the present disclosure can be obtained from a consideration of the detailed description of an exemplary embodiment thereof below, particular if such consideration is made in conjunction with the appended drawings, wherein like reference numbers are used to refer to like elements in the respective figures thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional plan view of an exemplary embodiment of a stiff in-plane, gimbaled rotor head in accordance with the present disclosure; 
         FIG. 2  is a partial cross-sectional elevation view of the exemplary rotor head of  FIG. 1 , as seen along the lines of the section  2 - 2  taken therein; 
         FIG. 3  is an enlarged partial cross-sectional elevation view of a central hub portion of the exemplary rotor head; 
         FIG. 4  is an enlarged partial cross-sectional elevation view of a blade retention and pitch control portion of the rotor head; 
         FIG. 5  is partial cross-sectional plan view of a blade and associated pitch control shaft of the rotor head, showing two cross-sectional detail views through the blade at two stations along a pitch axis thereof; 
         FIG. 6  is an enlarged partial cross-sectional elevation view of the blade retention mechanism of the rotor head of  FIG. 4 , showing details of a retention pin and a dry-lube ball-and-socket joint thereof; 
         FIG. 7  is an enlarged partial cross-sectional elevation view of a constant velocity joint of the rotor head, showing details of the bearings thereof; 
         FIG. 8  is an enlarged partial cross-sectional plan view of the constant velocity joint of  FIG. 7 ; and, 
         FIG. 9  is a partial perspective view of an exemplary embodiment of a fully articulated type of rotor head in accordance with the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     The novel rotor head or hub disclosed herein includes some elements that are similar to those used in the V-22 tilt-rotor, but is otherwise mechanically quite different from the latter. The hub disclosed herein uses a low maintenance, all-elastomer-and-metal laminate (i.e., completely oil-less) bearing system. The exemplary rotor head also provides a high speed, low drag design for a helicopter hub having three or more rotor blades, while enabling a more compact intermeshing blade tandem rotorcraft configuration by eliminating the lead-lag blade motions and dampers associated with conventional fully articulated rotor heads  100 , such as that illustrated in  FIG. 9 . 
     One of the major problems of the prior art rotor heads that is overcome by the rotor head of the present disclosure is the provision of a low maintenance hub design that uses all lubrication-free elastomeric-metal laminated bearings of a type referred to as “high capacity laminate” (HCL) bearings, available from, e.g., Lord Aerospace Corp., Cary, N.C., and described in, e.g., U.S. Pat. Nos. 4,105,266 to R. Finney and 4,913,411 to F. Collins et al. These types of bearings not only provide superior vibration control, but also require no lubrication, thereby substantially lowering operating and maintenance costs, and are available in a variety of configurations, including cylindrical, conical, spherical and disc-shaped sections, and various combinations of the foregoing. 
     Another problem solved by the novel stiff in-plane rotor head disclosed herein is that it enables a greater number of rotor blades to be used on the hub than does the prior art, viz., greater than three blades per rotor head. In the particular exemplary embodiment illustrated in the figures, the hub herein incorporates six blades (# 1 -# 6 ), but can incorporate either more or less blades, as may be indicated by the particular design constraints at hand. 
     The novel rotor head also enables a more compact intermeshing tandem rotor configuration to be achieved than the fully articulated rotor heads of the prior art, which require substantial clearance between the two rotors due to in-plane leading and lagging motions and out-of-plane “flapping” of the blades. This combination of features of this disclosure results in a rotor head that virtually eliminates large hub moments generated by thrust offset in high speed flight of a type that occurs if a rigid (e.g., a “propeller” type) hub is used. They also result in rotor head assemblies that are relatively light in weight, due to the low hub moments that are generated only by the respective spring rates of the hub bearings themselves. The constant velocity gimbal system provided by the rotor head is thus well suited for high power and high torque applications. It includes a “paddle bearing” arrangement that results in a much larger bearing area than can be achieved with the rod ends of a three-drive link installation, such as that used in the prior art. 
     When compared to a prior art “teeter” rotor head (not illustrated), the primary advantage provided the hub of the present disclosure is that it can handle a significantly larger number of rotor blades. A teeter hub pivots like a teeter-totter, and as a consequence, can incorporate only 2 rotor blades, which makes such a hub arrangement completely unsuitable for high speed, heavy lift rotorcraft. By contrast, the exemplary rotor hub described herein can incorporate six or more rotor blades. 
     The main differences between the rotor head  10  of the present disclosure and those of the prior art are as follows: 
     1) High capacity, pivoting paddle bearings are used to transmit torque across the gimbal joint instead of drive links; 
     2) Vertical Pitch Arms internal to the hub are used to minimize the “Δ3” pitch-flap coupling effect that occurs when more than three blades are used; 
     3) A hub assembly of 6 or more blades is made possible; and, 
     4) The instant rotor head is configured with stationary hub spindle housings for each blade, situated external to the blade&#39;s movable pitch control shaft. 
     The rotor head of the present disclosure is thus superior to the existing solutions because it can be designed to handle the very high torque demands of a large, high speed, heavy lift helicopter. It can be configured for “high solidity” rotors using a large number of rotor blades, e.g., six or more. It also incorporates a low drag hub fairing that enhances high speed performance. 
       FIG. 1  is a partial cross-sectional top plan view of an exemplary embodiment of a stiff in-plane, gimbaled rotor head  10  in accordance with the present disclosure, and  FIG. 2  is a partial cross-sectional elevation view of the exemplary rotor head  10  of  FIG. 1 , as seen along the lines of the section  2 - 2  taken therein. 
     The exemplary rotor head  10  illustrated in  FIGS. 1 and 2  comprises a split outer hub  12 , a splined center hub  14  and a main rotor shaft  16  having splines  18  at an upper end that are drivingly engaged with the corresponding splines of the center hub  14  in the manner of a spline gear. The center hub  14 , in turn, is coupled to the outer hub  12  through upper and lower spherical, high capacity laminated metal-and-elastomeric (HCL) bearings  20  and  22  described in more detail below. A plurality of blades  24 , each having an airfoil cross-section, are rigidly coupled to an outboard end of a respective pitch control shaft  26 , each of which, in turn, has an inboard end pivotally coupled to the outer hub  12  through a respective dry-lube ball-and-socket joint  28  to enable the respective pitch control shafts and blades associated therewith to rotate about the long axis  30 , i.e., the “pitch” axes, of the respective shafts and blades. An outboard end portion of each pitch control shaft  26  is concentrically supported in a pitch bearing housing  32  having an inboard end coupled to the inboard end of a corresponding one of the pitch control shafts  26  through a spherical bearing  46 . The hub  10  also incorporates a novel constant velocity joint, described in more detail below, that enables the hub to gimbal about ±12 degrees in any direction relative to a vertical axis extending through the main rotor shaft  16  while maintaining a constant rotational velocity in each of the blades  24 . 
       FIG. 3  is an enlarged partial cross-sectional elevation view of the center hub  12  portion of the exemplary rotor head  10 . As illustrated in  FIG. 3 , the splined center hub  14  sits atop a splined drive collar  32 , which may be made of steel or titanium, located at the bottom center of the hub assembly, and which, together with the center hub, is driven rotationally by the rotor shaft  16 . The drive collar  32  is used as a hub spacer and to drive a pitch control swashplate (not illustrated) disposed below the hub. A hub nut and washer  34  are used to retain the hub assembly to the rotor shaft  16 . 
     The splined center hub  14  may also be made of steel or titanium, and is used to transmit torque from the rotor shaft  16  through the paddle bearings  40 ,  42 ,  44  and the paddle shaft  38  to the split outer hub  12 . The center hub contains features adapted to provide limit stops for the hub&#39;s gimbal joint, described below. The upper and lower spherical elastomeric set of bearings  20  and  22  are integral to the center hub and are used to support rotor thrust. This set of bearings has the capability of pivoting about a spherical center point that helps to create the constant velocity joint of the hub. The spherical elastomeric bearing set  20  and  22  comprises a main contributor to the hub&#39;s gimbal spring stiffness. 
     As illustrated in  FIG. 3 , the split outer hub  12 , referred to as such because it comprises upper and lower halves that mate with each other across a horizontal plane in the manner of a clam shell, may be made of, e.g., aluminum or titanium, and surrounds the center hub  14 . It performs the following functions, among others: 1) It retains the gimbaled hub&#39;s paddle bearing assemblies described below, which are used for torque drive; 2) it connects the gimbaling split outer hub to the non-gimbaling inner hub through the upper and lower spherical bearing set  20  and  22 ; 3) it is used to connect to each rotor blade&#39;s pitch bearing housing  32  and pitch shaft  26  spindle; 4) it supports the centrifugal forces of each blade  24  through a corresponding pitch bearing housing  32 ; and, 5) it is used to attach a streamlined composite hub-fairing  36  to the rotor head  10 . 
     As illustrated in  FIGS. 1-3 , a plurality of paddle bearing shafts  38  are respectively located about the circumference of the outer hub  12  and between each pair of adjacent rotor blade  24  installations thereon. Each paddle bearing shaft  38  is a subassembly comprising a titanium or steel center shaft integrated with three elastomeric bearing installations, described below in connection with  FIGS. 7 and 8 . The center shaft and elastomeric bearings comprise an important innovation of the gimbal mechanism of the rotor head  10  described below. As illustrated in  FIG. 3 , the inboard end of each center-shaft includes a paddle bearing  40  that, acting in combination with the center hub  14 , provide limit stops of about ±12° for the gimbal joint of the rotor head  10 . Following is a description of the three elastomeric bearing assemblies disposed on the paddle bearing shaft  38 . 
     As those of skill in the art will appreciate, in order to provide stiff in-plane movement of the rotating blades  24 , i.e., to eliminate in-plane pivoting of the rotating blades during gimbaling movement of the hub  10 , it is necessary to maintain a substantially constant angular velocity, or rotational speed, of each radial point in each of the blades during such motion. In order to achieve this, it is necessary to provide a constant velocity joint between the blades  24  and the rotor shaft  16  that applies the torque used to drive the blades. 
       FIG. 7  is an enlarged partial cross-sectional elevation view of the constant velocity joint of the rotor head  10 , showing details of the bearings thereof, and  FIG. 8  is an enlarged partial cross-sectional top plan view of the constant velocity joint of  FIG. 7 . As illustrated in these figures, a tapered stack, flat pack, elastomeric paddle bearing  40  shaped like a hollow disc sector is located at the inboard end of each paddle shaft  38 . The disc-sector or paddle bearing  40  is used to transmit rotor shaft  16  torque from the non-gimbaled inner or center hub  14  to the gimbaled split outer hub  12 . Its disc shape follows the gimbaling motion of the set of upper and lower spherical bearings  20  and  22  described above that is part of the inner hub  14 . The combination of all of the paddle bearings  40  and the center hub&#39;s spherical bearing set  20  and  22  serves as the main contributor to the ability of the rotor head  10  to engage in gimbaling movement in any direction relative to a vertical axis through the main rotor  16  and to the spring stiffness of the gimbal joint defined thereby. 
     Referring to  FIGS. 7 and 8 , each paddle bearing shaft  38  is disposed above a web  39  (shown in dashed outline in  FIG. 8 ) of the split outer hub  12 , and a main radial support bearing  42  is located on each paddle shaft  38  outboard of the paddle bearing  40 . The main radial support bearing serves as one of the pivot bearings for the paddle bearing shaft  38 . The paddle bearing shaft transmits rotor torque loads into the main radial support bearing  42 . This radial bearing also incorporates a small conical section that provides the ability to carry the centrifugal loads of the paddle bearing shaft  38 . The combination of all of the main radial support bearings  42  adds to the total spring rate of the gimbal joint. 
     As also illustrated in  FIGS. 7 and 8 , a radial tail support bearing  44  is located at the outboard end of the paddle bearing shaft  38 . This second radial bearing serves as a second pivot bearing for the paddle bearing shaft  38 . Acting in cooperation with the main radial support bearing  42 , it reacts paddle shaft  38  moments generated by rotor torque. The combination of all the radial tail support bearings  44  further adds to the total spring rate of the gimbal joint. As illustrated in the top plan view  FIG. 8 , each of the main radial support and radial tail support bearings  42  and  44  may be coupled to the respective paddle bearing shaft  38  through respective anti-rotation tabs  43 . 
     Turning to  FIG. 4 , which is an enlarged partial cross-sectional elevation view of a rotor blade  24  retention and pitch control portion of the rotor head  10 , the pitch bearing housing  32 , which may be made of aluminum or titanium, is connected to the split outer hub  12  and to an inboard end of a corresponding one of the pitch control shafts  26  through a corresponding spherical elastomeric bearing  46 , and is used to retain spherical and conical elastomeric bearings  46  and  48  utilized for retention and pitch control of the rotor blades  24 . As illustrated in  FIG. 4 , the pitch bearing housing  32  also incorporates a support lug  74  on its outer diameter to mount a pitch control bell-crank  52  of another blade, as described in more detail below. 
     The outboard end of each pitch control shaft  26  incorporates a concentric integral conical elastomeric bearing  48  required for rotor blade pitch control. The conical bearing is used to bear the very high shear loads transmitted into it from the rotor blade assembly. The taper angle of the conical bearing is arranged to provide a preload capability, together with the spherical blade retention bearing  46  located at the opposite end of the pitch control shaft  26 . The bearing&#39;s taper angle also allows for a large outboard cross section on the pitch control shaft  26  where blade-induced moments are highest. 
     A two-pin clevis joint  54 , which is used to rigidly attach the inboard end of each rotor blade  24  to the outboard end of the corresponding pitch control shaft  26 , is disposed adjacent to the conical bearing  48  at the very outboard end of the pitch control shaft  26 . The inboard end of the pitch control shaft  26  is connected with a main retention pin  56  to the spherical elastomeric bearing  46  used for blade  24  pitch control and retention. Disposed adjacent to the spherical elastomeric bearing  46  at the inboard end of the shaft  26  is a spherical ball  28  that is machined, or otherwise formed, on the inboard end of the shaft, and which is used as an inboard support within the hub. The ball  28  picks up the pitch control shaft&#39;s inboard shear loads and prevents that load from being transmitted into the spherical elastomeric bearing  46 . The ball  28 , which may be made of steel, also serves as a positive center pivot for the spherical elastomeric bearing  46 . The outer race of the ball is preferably lined with a dry-film bearing material and is mounted into a corresponding socket formed in the split outer hub  12 . The combination of the outboard conical bearing  48  and the inboard spherical bearing  46  provides a mechanism to preload the bearing elastomers so as to improve bearing service life. 
     As illustrated in  FIG. 4 , the spherical blade retention bearing  46  is located at the inboard end of the pitch control shaft  26 , and is pinned to the pitch control shaft with the main retention pin  56 . The blade retention bearing  46  is also an elastomeric laminate bearing assembly that is used to transmit the very high centrifugal loading of the rotor blade  24  into the inboard end of the corresponding pitch bearing housing  32 , and thence, into the split outer hub  12 . The inboard end plate of the blade retention bearing  46  includes a pitch arm  58  that is used for controlling the pitch of the associated rotor blade  24 . The outboard end of the blade retention bearing rests on a shoulder  60  in the associated pitch bearing housing and is keyed into the shoulder with shear tabs  62  disposed on the endplate of the bearing. 
       FIG. 6  is an enlarged partial cross-sectional elevation view of the blade retention mechanism of the exemplary rotor head  10  illustrated in  FIG. 4 , and shows details of the main retention pin  56  and the dry-lube ball-and-socket joint  28  thereof. The main retention pin  56 , which may be made of steel, is similar to a tie-bar pin of a type used on a conventional hub, such as that used on the prior art rotor of  FIG. 9 . The annular main retention pin  56  is used to couple the spherical blade retention bearing  46  to the pitch control shaft  26 . It also functions to transmit retention loads and pitch control loads from the associated blade  24  into the spherical blade retention bearing  46  and pitch arm  58 . Due to its critical function within the rotor head  10  assembly, it is configured with a fail-safe capability described below. 
     The annular main retention pin  56  is held into the assembly with a high tensile bolt  64  extending through its center. A small amount of clearance is provided between the inside diameter of the retention pin and the bolt  64  so as to define a sealed annular chamber  66  into which a crack detection dye may be injected. In the event of a crack in the main retention pin  56 , the high tensile retention bolt  64  has the capability of carrying the full centrifugal and pitch loads. Any leakage of dye from the chamber  66  serves to alert ground personnel that the main retention pin  56  has been compromised. If desired, an optional short spline (not illustrated) can be added to the pitch control shaft  26  and spherical retention bearing  46  joint as a secondary load path for coupling pitch control loads. 
       FIG. 5  is partial cross-sectional top plan view of a blade  24  and associated pitch control shaft  26  of the rotor head  10 , showing two cross-sectional detail views through the blade at two stations along the pitch, or long axis  30  thereof. As discussed above, each blade  24  is rigidly fixed to the outboard end of a corresponding one of the pitch control shafts  26  with a two-pin clevis joint  54 . The clevis pins and bolts  54 , which may be made of steel, are located at the interface of the rotor blade  24  and the outboard end of the pitch control shaft  26 . Two pins are used to attach the rotor blade  24  to the pitch control shaft rigidly so as to prevent any leading/lagging movements of the blade in the plane of rotation relative to the shaft, in contradistinction to the in-plane movement of the blades in the flexible in-plane rotor hub  100  discussed above. 
     One of the principal innovations of the exemplary gimbaled rotor head  10  disclosed herein and illustrated in  FIGS. 1-4  and  6 - 8  is the pitch control cross links  68  used to control the pitch of the respective blades  24 . The pitch control cross links, each of which may be made of titanium, comprise a control rod assembly with spherical rod end beatings  70  located at each end thereof. By utilizing a vertical pitch arm  58  (see  FIG. 4 ) located internal to the rotor head  10 , the pitch control cross links respectively connect to the pitch arms  58  and pass horizontally below an adjacent blade installation and over to a respective associated pitch control bell-crank  52 . The pitch control bell-cranks are then located in the hub assembly at a strategic point that (to an acceptable level) minimizes the pitch—flap coupling, i.e., the “Δ3 angle,” of the vertical pitch link  72  in the rotor&#39;s upper controls, which are located below the gimbaled rotor head assembly  10 . 
     As illustrated in  FIG. 4 , the pitch control bell-cranks  52  (shown by dotted outline), which may be made of aluminum or titanium, are approximately 80° bell-cranks that respectively convert vertical motion from the respective vertical pitch links  72  to nearly horizontal movement of the respective pitch control cross links  68 . In a six-bladed rotor assembly, such as the exemplary embodiment illustrated in the figures, the bell-cranks may be mounted into a machined clevis  74  that is a part of the pitch bearing housing  32  of an adjacent blade installation. 
     As illustrated in the figures, the streamlined hub fairing assembly  36  incorporates a split fiberglass or carbon fiber honeycomb composite construction. It is a light weight assembly that comprises upper and lower clam shell portions, as well as a removable access cover for the main rotor hub nut  34 . The fairing incorporates a streamlined shape that covers the rotor hub  10  assembly and its appendages that extend out to the roots of the airfoil rotor blades  24 . The fairing enhances the performance of the host rotorcraft in high speed flight by reducing hub drag, which is a major contributor to the overall drag of such aircraft. 
     The novel rotor hub  10  disclosed herein provides an advance in the ‘state of the art’ in rotor head design that enables helicopters to operate at higher speeds, higher gross weights, and higher power levels than conventional rotorcraft, such as the CH-47 or CH-53 rotorcraft, can operate. 
     The stiff in-plane feature of the rotor head  10  is particularly suited for tandem helicopters with overlapping rotors. Because there is no lead-lag hinge, it eliminates the lag damper, adds simplicity, allows for the installation of up to six or more rotor blades for higher speeds and gross weights, and provides good rotor-to-rotor clearance, even when the respective rotor centers are placed relatively close to each other. For both single and tandem rotor designs, the stiff in-plane gimbaling hub  10  in high speed flight reduces large pitch link loads generated by the large lead-lag excursions of advancing and retreating blades of the prior art. 
     The novel gimbal joint of the hub  10  is also well suited for both single rotor and tandem rotor aircraft. When compared to a rigid rotor, it relieves large hub moments in high speed flight generated by the lateral thrust differential of advancing and retreating rotor blades. This overall reduction in moment and force in the rotor head thereby substantially simplifies rotor head parts and reduces part weight. 
     One of the reasons that the rotor head  10  herein is well suited for high power and high torque applications is the novel gimbal system provided thereby. The paddle shaft and bearing arrangement of the hub thus results in a much larger bearing area then can be achieved with the rod ends of a three-drive-link installation, such as used on prior art rotor heads. In a six-bladed installation, the rotor head  10  can incorporate up to six paddle bearing assemblies, thereby providing a very high torque capability. 
     The horizontal pitch control cross links  68  also provide an advantage over the prior art. By virtue of their passing below adjacent blade installations, the horizontal pitch control link  68  enable the use of an acceptable pitch-flap Δ3 angle at the vertical pitch links  72 , even when six or more blades are used. 
     The rotor head  10  also makes wide use of elastomeric rotor bearing technology that results in fewer parts and lower production, maintenance and life cycle costs, in that at least one of the spherical main gimbal bearing, the pitch bearing housing conical bearing, the spherical blade retention bearing, the paddle bearing, the radial tail support bearing and the main radial support bearing comprises a lubrication-free elastomeric-metal laminated bearing. Indeed, most of the parts of the rotor heads of the forward and aft rotors of a tandem rotor installation can be identical, thereby providing further production cost effectiveness. 
     In accordance with the exemplary embodiments described herein, high speed, low drag, low maintenance, stiff in-plane, gimbaled rotor heads are provided for helicopters that enable three or more rotor blades to be used per rotor, and that also enable a compact tandem rotor blade intermesh to be achieved by eliminating the lead-lag motions and dampers of fully articulated rotor heads. 
     As those of skill in this art will appreciate, many modifications, substitutions and variations can be made in the applications and methods of implementation of the stiff in-plane, gimbaled rotor heads of the present disclosure without departing from its spirit and scope. In light of this, the scope of the present disclosure should not be limited to that of the particular embodiments illustrated and described herein, as they are only by way of some examples thereof, but instead, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.
A stiff in-plane gimbaled rotor head for a rotorcraft, such as a helicopter, includes a vertically extending rotor shaft, a center hub disposed on the rotor shaft for conjoint rotation therewith, and an outer hub surrounding the center hub and coupled thereto through a spherical gimbal bearing for conjoint rotation therewith, and such that the outer hub is also capable of an angular range of gimbaling movement relative to the center hub. A plurality of rotor blades, which may include three or more blades, is coupled to the rotor shaft through the inner and outer hubs by a constant velocity joint that enables the blades to be rotated in a common plane about the axis of the rotor shaft while controlling the respective pitches of the blades and such that any other relative in-plane and out-of-plane movements of the blades during rotation is prevented.