Abstract:
A fairing system according to an exemplary aspect of the present disclosure includes, among other things, a shaft fairing mounted for rotation about an axis of rotation and a planetary gear set configured to control a position of the shaft fairing about the axis of rotation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/810,186 filed Jun. 23, 2010, which is the national stage application of PCT/US2008/050010 filed Jan. 2, 2008. 
    
    
     BACKGROUND 
     The present invention is directed to a de-rotation system for a shaft fairing mounted between an upper hub fairing and a lower hub fairing. 
     The aerodynamic drag associated with a rotor hub of a rotary-wing aircraft is a significant portion of the overall aircraft drag, typically 25 percent to 30 percent for conventional single-rotor helicopters. The rotor system drag increases for a rotary-wing aircraft having a counter-rotating, coaxial rotor system primarily due to the dual rotor hubs and the interconnecting main rotor shaft assembly. For high-speed rotary wing aircraft, the increased drag resulting from the counter-rotating, coaxial rotor system may result in a relatively significant power penalty. 
     The aerodynamic drag of the dual counter-rotating, coaxial rotor system is generated by three main components—the upper rotor hub assembly, the lower rotor hub assembly, and the interconnecting main rotor shaft assembly. The drag contribution may be approximately 40 percent for each of the hubs, and 20 percent for the interconnecting main rotor shaft assembly. Typically, a rotor hub fairing arrangement is mounted to each of the upper rotor hub and the lower rotor hub such that overall drag on the rotorcraft is reduced. The interconnecting main rotor shaft between the upper rotor hub assembly and the lower rotor hub assembly, however, is typically exposed. 
     For a variety of reasons including, but not limited to, reduced drag and low observability, a shaft fairing has been developed to streamline the exposed interconnecting main rotor shaft. The shaft fairing is mounted to the counter-rotating, coaxial rotor system within a rotational environment between the upper hub fairing and the lower hub fairing through a bearing arrangement such that the shaft fairing is aligned with the fuselage in forward flight but is free to align with the relative wind during low speed maneuvering. 
     During some flight conditions, the shaft fairing may undesirably rotate relative the airframe. Rotation of the shaft fairing may increase drag and reduce the low-observability benefits of the shaft fairing. 
     SUMMARY 
     A fairing system according to an exemplary aspect of the present disclosure includes, among other things, a shaft fairing mounted for rotation about an axis of rotation and a planetary gear set configured to control a position of the shaft fairing about the axis of rotation. 
     A coaxial rotor system according to an exemplary aspect of the present disclosure includes, among other things, a lower rotor hub mounted to a lower rotor shaft which is configured to rotate about an axis of rotation. An upper rotor hub is mounted to an upper rotor shaft which is configured to rotate about the axis of rotation, the upper rotor shaft mounted through the lower rotor shaft and rotating in a direction opposite a direction of rotation of the lower rotor shaft. An upper hub fairing is mounted at least partially about the upper rotor hub and a lower hub fairing is mounted at least partially about the lower rotor hub. A shaft fairing is mounted between the upper hub fairing and the lower hub fairing for rotation about the axis of rotation. A planetary gear set is configured to control a position of the shaft fairing about the axis of rotation. 
     An aircraft according to an exemplary aspect of the present disclosure includes, among other things, a lower rotor hub mounted to a lower rotor shaft and configured to rotate about an axis of rotation. An upper rotor hub is mounted to an upper rotor shaft and configured to rotate about the axis of rotation, the upper rotor shaft mounted through the lower rotor shaft and rotating in a direction opposite a direction of rotation of the lower rotor shaft. An upper hub fairing is mounted at least partially about the upper rotor hub and a lower hub fairing is mounted at least partially about the lower rotor hub. A shaft fairing is mounted between the upper hub fairing and the lower hub fairing for rotation about the axis of rotation and a planetary gear set is a configured to control a position of the shaft fairing about the axis of rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG. 1A  is a general schematic view of an exemplary rotary wing aircraft embodiment for use with exemplary embodiments of the present invention; 
         FIG. 1B  is a general perspective of a counter-rotating coaxial rotor system mounting a rotor hub fairing system; 
         FIG. 1C  is an expanded partial phantom view of a counter-rotating coaxial rotor system mounting a rotor hub fairing system according to an exemplary embodiment of the present invention; 
         FIG. 2A  is a perspective view of a counter-rotating coaxial rotor system illustrating a de-rotation system; 
         FIG. 2B  is an expanded perspective view of the de-rotation system illustrated in  FIG. 2A ; 
         FIG. 3A  is a top schematic view of a lower ring gear of an exemplary de-rotation system; 
         FIG. 3B  is a top schematic view of an upper ring gear of the de-rotation system of  FIG. 3A ; and 
         FIG. 4  is a perspective view of another exemplary de-rotation system with an active control. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1A  illustrates an exemplary vertical takeoff and landing (VTOL) rotary-wing aircraft  10  having a dual, counter-rotating, coaxial rotor system  12  which rotates about an axis of rotation A. The aircraft  10  includes an airframe  14  which supports the dual, counter rotating, coaxial rotor system  12  as well as an optional translational thrust system  30  which provides translational thrust generally parallel to an aircraft longitudinal axis L. Although a particular aircraft configuration is illustrated in this non-limiting embodiment, other counter-rotating, coaxial rotor systems will also benefit from the present invention. 
     The dual, counter-rotating, coaxial rotor system  12  includes an upper rotor system  16  and a lower rotor system  18 . Each rotor system  16 ,  18  includes a plurality of rotor blades  20  mounted to a rotor hub  22 ,  24  for rotation about a rotor axis of rotation A. A plurality of the main rotor blades  20  project substantially radially outward from the hub assemblies  22 ,  24 . Any number of blades  20  may be used with the rotor system  12 . 
     A main gearbox  26  which may be located above the aircraft cabin  28  drives the rotor system  12 . The translational thrust system  30  may be driven by the same main gearbox  26  which drives the rotor system  12 . The main gearbox  26  is driven by one or more engines (illustrated schematically at E). The gearbox  26  may be interposed between the gas turbine engines E, the rotor system  12  and the translational thrust system  30 . 
     The translational thrust system  30  may be mounted to the rear of the airframe  14  with a rotational axis T oriented substantially horizontal and parallel to the aircraft longitudinal axis L to provide thrust for high-speed flight. The translational thrust system  30  includes a pusher propeller  32  mounted within an aerodynamic cowling  34 . Although a tail mounted translational thrust system  30  is disclosed in this illustrated non-limiting embodiment, it should be understood that any such system or other translational thrust systems including tractor and pod mounted systems may alternatively or additionally be utilized. 
     The rotor system  12  includes a rotor hub fairing system  36  generally located between and around the upper and lower rotor systems  16 ,  18  such that the rotor hubs  22 ,  24  are at least partially contained therein. The rotor hub fairing system  36  provides significant drag reduction in which large-scale flow separation is greatly reduced. 
     The rotor hub fairing system  36  generally includes an upper hub fairing  38 , a lower hub fairing  40  and a shaft fairing  42  therebetween (also illustrated in  FIG. 1B ). The rotor hub fairing system  36  is integrated such that the shaft fairing  42  generally follows the contours of the upper hub fairing  38  and the lower hub fairing  40  at the rotational interfaces therebetween to reduce interference effects between the separate fairings  38 ,  40 ,  42  and minimize flow separation in the junction areas. Furthermore, the lower hub fairing  40  is integrated with the airframe  14  in an area typically referred to on a rotorcraft as a pylon  14 D (see  FIG. 1C ). It should be understood that fairing systems of various configurations will be usable with the exemplary embodiments of the present invention presented herein. 
     Referring to  FIG. 1C , the shaft fairing  42  may be mounted to the counter-rotating, coaxial rotor system  12  through a bearing arrangement  43 U,  43 L (illustrated schematically) such that the shaft fairing  42  may be positioned at a relative angular position about the axis of rotation A relative the airframe  14  by a de-rotation system  44 . The upper bearing arrangement  43 U and the lower bearing arrangement  43 L may be respectively located adjacent an upper portion and a lower portion of the shaft fairing  42 . The upper bearing arrangement  43 U may be attached to one rotor shaft  12 U while the lower bearing arrangement  43 L attached to the other rotor shaft  12 L such that bearings in the arrangements  43 U,  43 L are counter rotating and the net bearing drag is relatively low. 
     The de-rotation system  44  controls the position of the shaft fairing  42  about the axis of rotation A such that the shaft fairing  42  remains in a desired azimuthal position relative the airframe  14 . Although exemplary embodiments of the present invention are described in connection with a particular non-limiting aircraft embodiment, it should be readily appreciated that other systems which require a stationary fairing in a rotational environment will also benefit herefrom. 
     Referring to  FIG. 2A , the de-rotation system  44  generally includes a planetary gear system  46  to control a rotational position of the shaft fairing  42  (see e.g.,  FIG. 1C ). The planetary gear system  46  generally includes a first ring gear  48 , a second ring gear  50 , a planetary gear set  52  a cage assembly  54  and a fairing support structure  56 . 
     The second ring gear  50  is rotationally fixed to the airframe  14  though attachments  14 A or such like. The first ring gear  48  is mounted to the inter-rotor fairing support structure  56  which is mounted to the shaft fairing  42 . 
     The planetary gear system  46  generally includes a multitude of planet gear assemblies  58 . Each planet gear assembly  58  includes an upper planet gear  60 , a lower planet gear  62  and an interconnect shaft  64  that rotationally connects the upper planet gear  60  and the lower planet gear  62 . The upper planet gear  60  is in meshing engagement with the inner diameter of the first ring gear  48  and the lower planet gear  62  is in meshing engagement with the inner diameter of the second ring gear  50 . Although four planet gear assemblies  58  are illustrated in the non-limiting embodiment shown in  FIGS. 2A and 2B , it should be understood that other numbers of assemblies may alternatively be provided—typically one planet gear assembly  58  would be located between each pair of main rotor blades. 
     The multitude of planet gear assemblies  58  are supported by the cage assembly  54 . The cage assembly  54  includes an upper interface  54 U and lower interface  54 L (also illustrated in  FIG. 2B ) which support the planetary gear set  52 . The upper interface  54 U and the lower interface  54 L are mounted to the main rotor system  12  for rotation therewith. The upper interface  54 U may be mounted to the lower bearing  43 L or other rotational support. That is, the upper interface  54 U is axially retrieved and rotationally supported by the lower bearing  43 L. 
     In operation, with reference to  FIG. 3A , as the cage assembly  54  is rotated by the main rotor system  12 , the lower planet gears  62  react with the fixed second ring gear  50  to rotate each planet gear assembly  58  about each of their respective planet axes P. The upper planet gear  60  of each planet gear assembly  58  is thereby rotated by the interconnect shaft  64 . The upper planet gear  60  rotates the first ring gear  48  in an equal but opposite direction of the cage assembly  54  ( FIG. 3B ). Rotation of the first ring gear  48  rotates the inter-rotor fairing support structure  56  to rotate the shaft fairing  42  such that the shaft fairing  42  maintains a stable azimuthal position relative the airframe  14 . That is, the first ring gear  48  and the attached shaft fairing  43  appear stationary to the fixed airframe  14   
     The de-rotation system  44  is a passive system that derives mechanical input from the main rotor system  12 . The power required is minimal as friction is the only opposing force and gear meshes are noted as efficient power transfer mechanisms. Since the fixed and rotating ring gears are rigidly connected via a gear and shaft arrangement, the de-rotation system  44  will maintain alignment, regardless of main rotor RPM variations. 
     Referring to  FIG. 4 , another de-rotation system  44 ′ provides an active, in-flight adjustable position capability. That is, the second ring gear  50 ′ is azimuthally positionable relative the airframe  14 . A drive system  70  controls the rotational position of a second ring gear  50 ′ relative the airframe  14  ( FIG. 1C ) in response to a control system  72 . The control system  72  may be in communication with a shaft fairing position sensor  74  and a flight control system  76  to azimuthally position the second ring gear  50 ′ and thus the shaft fairing  42  relative the airframe  14  throughout all flight regimes to, for example, actively align the shaft fairing  42  with prevailing wind conditions during particular flight regimes. 
     It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to an illustrated attitude of the structure and should not be considered otherwise limiting. 
     Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the exemplary embodiments of the present invention. 
     The foregoing description is exemplary rather than defined by the subject matter within. Many modifications and variations of the present invention are possible in light of the above teachings. Although certain embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.