Abstract:
A rotor-hub for a rotary-wing aircraft is disclosed. The rotor-hub comprises a yoke comprising a plurality of yoke arms and a plurality of yoke straps, wherein the yoke arms are joined together by the yoke straps, and wherein a plurality of inner walls of the yoke define a central void space. A pitch horn is movably connected to the yoke and a portion of the pitch horn is located within the central void space. A connecting shell is fixedly attached to the yoke.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of rotary-wing aircraft rotor-hubs. In particular, the present invention relates to a stiff-in-plane, gimbaled tiltrotor hub. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Rotor-hubs have been in use for many years. There are numerous successful designs of rotor-hubs for various types of rotary-wing aircraft. Rotor-hubs are typically designed for, and therefore particularly well suited as, a means of connecting rotor-blades to a rotating shaft or mast. 
         [0003]    It is common for those of ordinary skill in the art of rotary-wing aircraft design to classify rotor-hubs into two major categories, “stiff-in-plane” and “soft-in-plane.” A stiff-in-plane rotor-hub is used in rotary-wing aircraft wherein the natural frequency of in-plane/lead-lag vibration of the rotor-blades is higher than both the rotor rotational frequency and the natural frequency of out-of-plane/flapping vibration of the rotor-blades. A soft-in-plane rotor-hub is used where the natural frequency of in-plane/lead-lag vibration of the rotor-blades is lower than both the rotor rotational frequency and the natural frequency of out-of-plane/flapping vibration of the rotor blades. It is well known that the rotor-blades and associated rotor-hub of a rotary-wing aircraft become more dynamically unstable as the natural frequencies of out-of-plane/flapping vibration of the rotor-blades and in-plane/lead-lag vibration of the rotor-blades converge toward equal values. Accordingly, it is not uncommon for a rotary-wing aircraft to be designed such that the natural frequencies of out-of-plane/flapping vibration of the rotor-blades and in-plane/lead-lag vibration of the rotor-blades maintain a minimum separation of about 25% of the rotor rotational frequency. 
         [0004]    In choosing between stiff-in-plane and soft-in-plane systems, several high level generalizations are often considered while designing a rotary-wing aircraft. The combined weight of the rotor-hub and rotor-blades of a stiff-in-plane, rotary-wing aircraft are typically heavier than the combined weight of the rotor-hub and rotor-blades of a soft-in-plane, rotary-wing aircraft. However, stiff-in-plane componentry is currently thought to be a better solution for traveling at higher speeds and/or producing greater thrust, while more readily maintaining dynamic vibratory stability. 
         [0005]    One of the numerous variables in achieving desired dynamic vibratory stability of the rotor-hub and rotor-blades of a stiff-in-plane rotary-wing craft is the δ 3  angle. Prior Art FIG. 1 shows a schematic of a rotor-hub which illustrates the δ 3  angle in relation to a rotor system. Because one end of the pitch horn is restrained by the pitch link and the other end of the pitch horn is attached to the blade, a pitch change will occur as the blade flaps. Hence, the δ 3  angle represents a correlation between the rotor flapping and rotor-blade pitch. As the rotor-blade flaps upward, a rotor system with a positive δ 3  angle will experience a nose-down pitch, while a rotor system with a negative δ 3  angle will experience a nose-up pitch. The δ 3  angle is manipulated to provide dynamic stability as well as to reduce rotor flapping amplitudes during gust disturbances and/or pilot maneuvers. As an example, the δ 3  angle on a three-bladed, tilt-rotor aircraft is typically set to values near −15 degrees, which provides an adequate level of stability and flapping attenuation. 
         [0006]    Demand is increasing for rotary-wing aircraft to achieve more thrust, higher speeds, and carry heavier loads. For example, there is a demand for more powerful tilt-rotor aircraft. One way of producing more thrust is to increase the number of rotor-blades. Current tilt-rotor aircraft typically utilize three-bladed rotor systems. In three-bladed rotor systems, the pitch horn and pitch link (see Prior Art FIG. 1) are usually located generally in-plane with the rotor-hub and outside of the rotor-hub. However, achieving small δ 3  angles (e.g., δ 3  angles near −15 degrees) for a multi-bladed rotor having four or more blades, while locating the pitch horn and pitch link generally in-plane with the hub and outside of the hub, presents a serious design challenge. The rotor-hub configuration, as described above for multi-bladed rotor systems, does not allow the pitch horns to be located at the proper positions due to structural interferences. Further, it is widely accepted as desirable by those of ordinary skill in the art of rotary-wing aircraft design to configure rotating componentry of rotor systems to remain as close to the axis of rotation as possible to minimize undesirable resultant forces that lead to early component failure. 
         [0007]    While the above described rotor-hub advancements represent significant developments in rotor-hub design, considerable shortcomings remain. 
       SUMMARY OF THE INVENTION 
       [0008]    There is a need for an improved rotor-hub. 
         [0009]    Therefore, it is an object of the present invention to provide an improved rotor-hub which allows connection to four or more rotor-blades while maintaining optimal δ 3  angles. 
         [0010]    This object is achieved by providing a rotor-hub in which both the pitch links and pitch horns are located within an interior void of the rotor-hub. For example, the rotor-hub may be configured: (1) with a connecting shell located above the yoke; (2) with a connecting shell located below the yoke; and (3) with two connecting shells, one connecting shell being located above the yoke and one connecting shell being located below the yoke. 
         [0011]    The present invention provides significant advantages, including: (1) allowing the use of more than three blades in a rotor system of a tilt-rotor aircraft; (2) reducing opportunity for pitch horn damage due to debris or ballistic attack; (3) reducing opportunity for drive link damage due to debris or ballistic attack; (4) offering hub-spring redundancy; and (5) improving force transfer between hub-springs and the yoke. 
         [0012]    Additional objectives, features, and advantages will be apparent in the written description that follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as, a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
           [0014]    Prior Art  FIG. 1  is a simplified schematic representation of the effect of δ 3  angle in a rotor system; 
           [0015]      FIG. 2  is an elevational view of a tilt-rotor aircraft having a rotor-hub according to the preferred embodiment of the present invention; 
           [0016]      FIG. 3A  is a perspective view of the rotor-hub used in the tilt-rotor aircraft of  FIG. 2 ; 
           [0017]      FIG. 3B  is a perspective view of the yoke of the rotor-hub of  FIG. 3A ; 
           [0018]      FIG. 4  is a perspective view of the rotor-hub of  FIG. 3A  with the connecting shell removed; 
           [0019]      FIG. 5  is a top view of the rotor-hub of  FIG. 3A  with the connecting shell removed; 
           [0020]      FIG. 6  is a top view of the rotor-hub of  FIG. 3A ; 
           [0021]      FIG. 7  is cross-sectional view of the rotor-hub of  FIG. 3A  taken along the line  7 - 7  in  FIG. 3A ; 
           [0022]      FIG. 8  is a partial perspective view of a rotor-hub having a connecting shell located below the yoke according to an alternate embodiment of the present invention; 
           [0023]      FIG. 9  is a cross-sectional view of the rotor-hub of  FIG. 8  taken along the line  9 - 9  in  FIG. 8 ; 
           [0024]      FIG. 10  is a perspective view of a rotor-hub having two connecting shells according to an alternate embodiment of the present invention; and 
           [0025]      FIG. 11  is a cross-sectional view of the rotor-hub of  FIG. 10  taken along the line  11 - 11  in  FIG. 10 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    The present invention is an improved rotor-hub which allows connection to four or more rotor-blades while maintaining optimal δ 3  angles. There are three main embodiments of the invention: (1) with a connecting shell located above the yoke; (2) with a connecting shell located below the yoke; and (3) with two connecting shells, one connecting shell located above the yoke and one connecting shell located below the yoke. The scope of the present invention, however, is not limited to the particular embodiments disclosed herein and depicted in the drawings. The rotor-hub of the present invention allows for the incorporation of four-bladed rotor systems on tilt-rotor rotary-wing aircraft. However, while specific reference is made to using the present invention with tilt-rotor rotary-wing aircraft, the present invention may alternatively be used with any other rotary-wing vehicle/craft. Further, the rotor-hub of the present invention may alternatively be used with rotary system having more or fewer than four rotor-blades. 
         [0027]      FIG. 2  depicts a tilt-rotor, rotary-wing aircraft incorporating a rotor-hub of the present invention.  FIG. 2  illustrates a tilt-rotor aircraft  11  in an airplane mode of flight operation. Wings  15 ,  17  are utilized to lift craft body  13  in response to the action of rotor systems  19 ,  21 . Each rotor system  19 ,  21  is illustrated as having four rotor-blades  23 . Nacelles  25 ,  27  substantially enclose rotor-hubs  29 , obscuring rotor-hubs  29  from view in  FIG. 2 . Of course, each rotor system  19 ,  21  is driven by an engine (not shown) substantially housed within each nacelle  25 ,  27 , respectively. 
         [0028]      FIG. 3A  illustrates a perspective view of the preferred embodiment of the rotor-hub  29  of the present invention. Rotor-hub  29  is illustrated as comprising a yoke  31  having yoke arms  33  and yoke straps  35 . Yoke arms  33  are integrally connected to yoke straps  35 . In one embodiment, yoke  31  is constructed of composite materials. More specifically, yoke  31  is constructed of a multiplicity of discrete bonded layers of directional fiber material. However, yoke  31  may alternatively be constructed of any other suitable material in any other suitable fashion. Further, while yoke  31  is illustrated as having four yoke arms  33 , other rotor-hub configurations according to the present invention may comprise more or fewer than four yoke arms  33  for connection with more or fewer than four rotor-blades  23 , respectively. 
         [0029]    Rotor-hub  29  is further illustrated with representative pitch change axes  37 A,  37 B, about which the pitch of rotor-blades  23  (see  FIG. 2 ) is altered. Additionally, rotor-hub  29  is illustrated with a representative mast rotation axis  39 , about which a mast (not shown) is rotated when driven by an operably associated transmission (not shown). 
         [0030]    Outboard feathering bearings  41  are attached to the outermost portions of yoke arms  33 . Outboard feathering bearings  41  allow at least some degree of rotation of rotor-blades  23  about pitch change axes  37 A,  37 B. Centrifugal force (CF) bearings  43  are attached to outboard feathering bearings  41 . CF bearings  43  are the primary intermediary connective devices between rotor-blades  23  and rotor-hub  29 . CF bearings  43  withstand the often enormous centrifugal force generated by rotating rotor-blades  23  about mast rotation axis  39 . 
         [0031]      FIG. 3B  illustrates a simplified view of yoke  31  of rotor-hub  29 . A central void space  30  is defined by inner walls  32  of yoke  31 . 
         [0032]    As illustrated in  FIG. 7 , hub-spring  45  includes an inner-core  47  comprising a first series of various alternatingly stacked rubber elements and metal shim elements (neither shown in detail) sandwiched between an upper, outer connecting shell  49  and an inner shell  51  and a second series of various alternatingly stacked rubber elements and metal shim elements sandwiched between a lower, outer shell  50  and another inner shell  51 . Shells  49 - 51  are illustrated as being constructed of metal. Hub-spring  45  allows gimbaling of yoke  31  with respect to the mast and mast rotation axis  39 . Hub-spring  45  also accommodates flapping of rotor-blades  23  and transfers thrust. 
         [0033]    As can more clearly be seen in  FIG. 4 , wherein rotor-hub  29  is illustrated without connecting shell  49 , rotor-hub  29  further comprises four pitch horns  53 . Pitch horns  53  comprise pitch horn arms  55  and pitch horn inboard beams  57 . Pitch horns  53  are rotatably connected to crotches  59  through inboard feathering bearings  61 . Inboard feathering bearings  61  are substantially centered along corresponding pitch change axes  37 A,  37 B. Inboard feathering bearings  61  are operably associated with like-sized apertures in pitch horns  53  located substantially at the intersection of pitch horn arms  55  and pitch horn inboard beams  57 . Grips (not shown) are connected to pitch horn inboard beams  57  such that, when pitch horns  53  are rotated about their corresponding pitch change axes  37 A,  37 B, grips cause rotor-blades  23  (shown in  FIG. 2 ), which are attached to grips, to correspondingly rotate about the pitch change axes  37 A,  37 B. Ends  63  of pitch horns  53  are illustrated as being located in a neutral/nominal position when ends  63  are substantially centered about the plane created by pitch change axes  37 A,  37 B. Ends  63  of pitch horns  53  are connected to upper ends of pitch links  65 . Pitch links  65  are rod-like elements oriented substantially parallel to mast rotation axis  39 . Movement of pitch links  65  in either direction along a path parallel to mast rotation axis  39  will either raise or lower ends  63 , thereby rotating pitch horn arms  55  and pitch horn inboard beams  57  about their pitch change axes  37 A,  37 B, ultimately changing the pitch of rotor-blades  23 . Pitch horns  53  are located substantially within central void space  30 . A central void column is defined by extending the vertical boundaries of central void space  30  both upward and downward and represents the vertical footprint of central void space  30 . For example, the central void column occupies at least the space between upper footprint  34 A and lower footprint  34 B as illustrated in  FIG. 3B . In this embodiment, arms  55  extend outside of the central void column. However, in other embodiments of the present invention, arms  55  may alternatively remain within the central void column. 
         [0034]    As can more clearly be seen in  FIG. 5 , wherein a top view of rotor-hub  29  is illustrated without connecting shell  49  and lower, outer shell  50 , rotor-hub  29  further comprises a constant velocity/homokinetic joint (not fully shown) which comprises drive links  67 . Drive links  67  are oriented substantially parallel to the plane created by pitch change axes  37 A,  37 B. One end of each drive link  67  is adapted for connection to a trunnion (not shown) splined to the mast/drive shaft (not shown). The trunnion transfers rotational force from the mast to drive links  67 . The other end of each drive link  67  is adapted for attachment to drive legs  68  of connecting shell  49  (see  FIGS. 10 and 11 ) which transfers the rotational force from drive links  67  to connecting shell  49 . Connecting shell  49  is connected to yoke  31  along yoke straps  35  such that rotational force is transferred from connecting shell  49  to yoke  31 .  FIG. 6  illustrates a top view of rotor-hub  29 , while  FIG. 7  illustrates a cross-sectional view of rotor-hub  29  taken along a line  7 - 7  of  FIG. 3A  corresponding to the pitch change axis  37 A,  37 B. 
         [0035]    Referring now to  FIGS. 8-9  in the drawings, a rotor-hub embodiment according to the present invention also incorporates a hub-spring  71  similar to hub-spring  45 . However, connecting shell  72  of hub-spring  45  is located below a yoke  73 . As illustrated in  FIG. 8 , rotor-hub  69  is substantially similar to rotor-hub  29  and comprises substantially similar components with three main differences: (1) connecting shell  72  is located on the underside of yoke  73  rather than on the upper side of yoke  73 ; (2) pitch horns  75  are curved, rod-like structures, portions of which are located slightly above the plane created by pitch change axes  77 A,  77 B but still within a central void space defined by the inner walls  32  of yoke  73 ; and (3) drive links  81  are illustrated as being located slightly below the plane created by pitch change axes  77 A,  77 B but still substantially within central void column. It will be appreciated that rotor-hub  69  may alternatively comprise pitch horns  53  which would lie substantially within the plane created by pitch change axes  77 A,  77 B. Similar to the embodiment of  FIGS. 3-7 , hub-spring  71  allows gimbaling of yoke  73  with respect to the mast and mast rotation axis  39  (shown in  FIG. 3A ). Hub-spring  71  also accommodates flapping of rotor-blades  23  (shown in  FIG. 2 ) and transfers thrust. 
         [0036]    Referring now to  FIGS. 10-11  in the drawings, a rotor-hub embodiment according to the present invention having a hub-spring  85  comprising two connecting shells  86  is illustrated. As illustrated in  FIG. 10 , rotor-hub  83  is substantially similar to rotor-hub  29  and comprises substantially similar components except that two connecting shells  86  are present within rotor-hub  83 . One connecting shell  86  is mounted on the underside of yoke  87  while another connecting shell  86  is mounted on the upper side of yoke  87 . One important advantage of rotor-hub  83  is connecting shell  86  redundancy. For example, if one of connecting shells  86  is damaged by a ballistic round or fail for any other reason, the remaining connecting shell  86  can continue to function normally. Another important advantage of an embodiment having two connecting shells  86  is the resulting improved distribution of forces being transferred from hub-springs  85  to yoke  87 . Similar to the embodiment of  FIGS. 3-7 , hub-springs  85  allows gimbaling of yoke  87  with respect to the mast and mast rotation axis  39  (shown in  FIG. 3A ). Hub-springs  85  also accommodate flapping of rotor-blades  23  (shown in  FIG. 2 ) and transfers thrust. 
         [0037]    An important advantage of the present invention is that, while providing for use of four or more rotor-blades per rotor-hub, the majority of components are compactly packaged substantially within an interior void space between the yoke straps. This arrangement makes the rotor-hubs of the present invention a tougher target for enemy combatants and a less likely target for unintentional debris. Further, the present invention allows for several variations in pitch horn travel. For example, where a connecting shell is only located on the top of a yoke, more space is available for downward pitch horn travel. Similarly, where a connecting shell is only located on the underside of a yoke more space is available for upward pitch horn travel. Also, where connecting shells are located both on the top and underside of a yoke, pitch horn travel may be more evenly divided between upward travel and downward travel. Finally, for each of the embodiments described above, a CF bearing failure would generally not result in losing a rotor-blade. Rather, the pitch horn associated with the failed CF bearing would be drawn toward the associated crotch of the yoke, such that, at least temporarily, safe operation of the aircraft could occur. 
         [0038]    It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.