Abstract:
An aircraft landing gear steering system comprising a turning member that is operably coupled to a landing gear leg through a harmonic drive mechanism. The landing gear steering system may be electrically driven. Additionally, the landing gear system has various advantages, including the provision of a safe failure mode should the harmonic drive mechanism fail.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(a)-(d) or (f) to prior-filed, co-pending British patent application serial number 0808087.1, filed on May 2, 2008, which is hereby incorporated by reference in its entirety. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not Applicable 
       REFERENCE TO A SEQUENCE LISTING, A TABLE, OR COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON COMPACT DISC 
       [0004]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    The field of the invention relates generally to aircraft landing gear, and more particularly to improved aircraft landing gear steering systems. 
         [0007]    2. Description of Related Art 
         [0008]    Most aircraft, and particularly those that are heavier than air, are provided with some type of landing gear. Often such landing gear takes the form of one or more sets of freely rotateable wheels provided on respective wheel train units, which may each be independently retractable into a respective landing gear bay within the fuselage of the aircraft so as to reduce induced drag during flight. Usually, one or more of the wheel train units will also be steerable in order that a pilot can direct the aircraft on the ground. For example, it is fairly common with a tricycle-type undercarriage arrangement that the nose-wheel wheel train unit can be independently rotated relative to the aircraft fuselage in order to steer the aircraft. 
         [0009]    Various steering arrangements for aircraft landing gear are known. One common type uses a hydraulically driven mechanical arrangement to steer aircraft [ 1 - 7 ]. However, the hydraulic pipes and actuators needed for such steering arrangements tend to be heavy and bulky, thereby taking up a relatively large amount of space in a landing gear bay. Additionally, hydraulic fluid is used and any leaks from such systems are problematic in the aircraft. 
         [0010]    As an alternative to hydraulically driven arrangements, various electrically driven aircraft steering systems have also been developed [ 8 - 11 ]. However, these tend to be used mainly on various light aircraft because they generally have poor mechanical failure characteristics which can, for example, lead to a locking of the steering mechanism should the steering system fail. Such failure modes can be dangerous, and this is one reason why electrical steering systems have not generally been adopted for larger aircraft, such as, for example, those used for commercial air transport applications. 
         [0011]    Additionally, certain conventional aircraft landing gear steering systems may also have poor mechanical vibration characteristics. For example, nose wheels of conventional systems can have a tendency to shimmy, or oscillate uncontrollably from side-to-side, when the aircraft touches down. This characteristic is undesirable, and may be reduced for hydraulically driven systems, for example, by providing a hydraulic accumulator and valves to increase hydraulic fluid pressure in the steering system during touch-down. Such a hydraulic accumulator and valves, however, not only add to the weight and complexity of certain conventional aircraft steering systems, but also increase the likelihood that hydraulic fluid leaks will occur. 
         [0012]    Various aspects and embodiments of the present invention have therefore been developed whilst bearing the aforementioned disadvantages of conventional aircraft landing gear steering systems in mind. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    According to a first aspect of the present invention, there is provided an aircraft landing gear steering system. The aircraft landing gear steering system comprises a turning member that is operably coupled to a landing gear leg through a harmonic drive mechanism. 
         [0014]    According to a second aspect of the present invention, there is provided a method of providing an aircraft steering system. The method comprises coupling a turning member to a landing gear leg through a harmonic drive mechanism. The method may also, advantageously, be used to retrofit various embodiments of the present invention to conventional aircraft landing gear mechanisms. 
         [0015]    By using a harmonic drive mechanism to couple a landing gear leg to a turning member, a lightweight compact steering system can be provided. Moreover, use of a harmonic drive mechanism also enables a high gearing ratio to be provided between an actuator and the turning member, thus enabling relatively low torque actuators to be used. 
         [0016]    For example, in various embodiments of the present invention an electric motor module may be used, which in turn enables an even more compact steering device to be provided with the added benefit of being able to reduce or even eliminate the need for the provision of hydraulic actuation systems in a landing gear bay. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Various aspects and embodiments of the present invention will now be described in connection with the accompanying drawings, in which: 
           [0018]      FIG. 1  shows a schematic diagram of aircraft landing gear according to an embodiment of the present invention; 
           [0019]      FIG. 2  shows a cross-section through an aircraft landing gear steering system according to an embodiment of the present invention; 
           [0020]      FIG. 3  shows a cross-section through an aircraft landing gear steering system according to a further embodiment of the present invention; and 
           [0021]      FIG. 4  shows a method of retrofitting an aircraft landing gear steering system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  shows schematically a diagram of aircraft landing gear  100  according to an embodiment of the present invention. The aircraft landing gear  100  is retractable into a landing gear bay (not shown) provided in the fuselage  10  of an aircraft. A retraction/extension mechanism  104  is provided to move the landing gear  100  into and out of the landing gear bay. 
         [0023]    The landing gear  100  comprises a landing gear yoke  102  that is pivotally mounted at a first end  114  within the landing gear bay. The landing gear  100  also comprises a wheel train unit  108  connected to a shock absorbing oleo  112  and a torque link  106 . The oleo  112  and the torque link  106  are connected to a leg portion  116  provided at a second end of the landing gear yoke  102 , distal the first end  114  thereof, via a landing gear steering system  110 . 
         [0024]    The landing gear steering system  110  comprises a turning member  118  coupled to an internal harmonic drive mechanism (for examples, see  FIGS. 2 and 3 , below) that is additionally coupled to the leg portion  116 . The torque link  106 , in this example, is connected to the turning member  118 , and the oleo  112  passes concentrically through the turning member  118 . 
         [0025]      FIG. 2  shows a cross-section through part of an aircraft landing gear steering system  210  according to an embodiment of the present invention. The steering system  210  includes a tube-like turning member  218  having an oleo  212  mounted concentrically therein in alignment with a central axis  250  by way of bearings  238 . The turning member  218  projects through the centre of a harmonic drive mechanism  260 , described in more detail below. Additionally, the turning member  218  comprises a projecting lug  219  for coupling to a torque link  206  that is attachable to a wheel train unit (not shown). 
         [0026]    A concentrically mounted casing  240  surrounds and is connected to the turning member  218  by way of a connector mechanism  234 . An electric motor module  226  is mounted to the casing  240 , aligned by shape and fit and retained in place by four screws. The motor provided within the electric motor module  226  is a brushless DC motor, and the electric motor module  226  additionally has a tooth clutch provided at the output. 
         [0027]    The casing  240  additionally retains a bearing  232  that supports a wave generator  220  in concentric relationship with the turning member  218 . The wave generator  220  is thus able to rotate with respect to the turning member  218 . The wave generator  220  has an elliptical profile when viewed in a plane perpendicular to the central axis  250 , and is made of a metal material, such as steel. The electric motor module  226  is also coupled to the wave generator  220  by way of an intermeshing reciprocal toothed arrangement, such that activation of the electric motor module  226  causes the wave generator  220  to rotate about the central axis  250  and the turning member  218 . 
         [0028]    The steering system  210  includes a landing gear leg  216 . This leg may be connected to or formed as an integral part of a landing gear yoke, for example. Differential rotational movement between the landing gear leg  216  and the turning member  218  enables controlled steering of a wheel train unit coupled to the turning member, and thus of an aircraft. 
         [0029]    The landing gear leg  216  comprises a flex spline housing  228  and supports (distal) bearings  236  that in turn support the turning member  218  therein, thereby maintaining the turning member  218  and the landing gear leg  216  in concentric relationship. A flex spline  222  is provided within the flex spline housing  228  and is able to rotate therein relative to the flex spline housing  228  and landing gear leg  216 . Additionally, a drive spline  239  is also provided to support the flex spline  222  and to maintain it in concentric relationship with respect to the landing gear leg  216 . 
         [0030]    The flex spline  222  is cup-shaped and can be made of relatively thin walled material, such as, for example, steel material. Additionally, the flex spline  222  includes a set of N teeth  223  provided externally about its periphery. 
         [0031]    A fixed spline  224  is fixed to a first end of the landing gear leg  216 . The fixed spline  224  is ring-shaped and comprises a set of N+Δ internal teeth provided on an internal circumferential surface thereof. The fixed spline  224  is concentric with the wave generator  220  and the flex spline  222 . Additionally, the internal teeth of the fixed spline  224  intermesh with the external teeth  223  of the flex spline  222 . 
         [0032]    Together the wave generator  220 , the flex spline  222  and the fixed spline  224  form the harmonic drive mechanism  260 . The harmonic drive mechanism  260 , sometimes referred to as a strain wave gear, provides a gearing ratio from the output of the electric motor module  226  to the landing gear leg  216 . This ratio can be made high; see, for example, the explanation of Musser [12] for more detail. 
         [0033]    In one example embodiment of the invention the harmonic drive mechanism  260  provides a gearing ratio of 160:1. In this embodiment, an additional gear mechanism  280  is provided at the output of the electric motor module  226  and its output used for driving the harmonic drive mechanism  260 . For example, the gear mechanism  280  can have a gearing ratio of 13:1 which when combined with a harmonic drive gear ratio of 160:1 can be used to provide a 75 kNm torque at the turning member  218 , for rotating the turning member  218  and the wheel train unit connected thereto, using an electric motor having a relatively low 4 Nm output torque. 
         [0034]    One further advantage of these, and various related embodiments of the present invention, lies in the failure mode should the harmonic drive mechanism break. The Applicant has found that one common failure mode for such harmonic drive mechanisms, should they fail at all, is with splitting of the relatively thin walled cup-shaped flex spline, which breaks in an analogous manner to a plastic drinks cup when crushed. However, should this happen for various embodiments of the present invention, the landing gear steering system fails in a free-castor mode in which the turning member is free to rotate relative to the landing gear leg. This, however, is an inherently safe failure mode in which an aircraft can be safely landed and still be steered on the ground by use of differential braking and/or asymmetric power application (for multi-engine aircraft). 
         [0035]      FIG. 3  shows a cross-section through an aircraft landing gear steering system  310  according to a further embodiment of the present invention. The aircraft landing gear steering system  310  is packaged as an assembly that is easily removable from a landing gear leg  316 . 
         [0036]    The steering system  310  includes a tube-like turning member  318  having an oleo  312  mounted concentrically therein, in alignment with a central axis  350 , by way of bearings  338  and  344 . The turning member  318  projects through the centre of a harmonic drive mechanism  360 , that is described in more detail below. The turning member  318  can be attached to a wheel train unit (not shown) and/or support a projecting lug (not shown), as desired. 
         [0037]    A concentrically mounted support  340  surrounds and is connected to the turning member  318  by way of the bearing  344  that abuts both a shoulder portion provided circumferentially about the turning member  318  and a flex spline bearing  342 . The bearing  344  is fixed to the support  340 . The support  340  is also connected to a fixed spline  324  that forms part of the harmonic drive mechanism  360 . The fixed spline  324  is ring-shaped and comprises a set of M+A internal teeth provided on an internal circumferential surface thereof. 
         [0038]    The steering system  310  also comprises a casing  390  that can be releasably coupled to the landing gear leg  316  at joint  399 . The joint  399  may, for example, include one or more of bolts, rivets, screws or the like, permitting the steering system  310  to be quickly and easily released from the landing gear leg  316 . 
         [0039]    An electric motor module  326  is coupled to the casing  390 . The casing  390  also retains a bearing  332  that supports a wave generator  320  disposed in concentric relationship with the turning member  318 . The wave generator  320  is thus able to rotate with respect to the turning member  318 . The wave generator  320  has an elliptical profile when viewed in a plane perpendicular to the central axis  350 . The electric motor module  326  is also coupled to the wave generator  320  by way of an intermeshing reciprocal toothed arrangement, such that activation of the electric motor module  326  is able to cause the wave generator  320  to rotate about the central axis  350  and the turning member  318 . 
         [0040]    The support  340  additionally provides a housing for a flex spline  322  that is retained therein by the flex spline bearing  342 . The flex spline bearing  342  ensures that the flex spline  322  maintains a concentric relationship with the principal central axis  350 . The flex spline  322  is also able to rotate relative to both the support  340  and the turning member  318 . 
         [0041]    The flex spline  322  is cup-shaped and can be made of relatively thin walled material, such as, for example, a stainless steel material. Additionally, the flex spline  322  includes a set of M teeth  323  provided externally about its periphery. 
         [0042]    The fixed spline  324  is concentrically mounted with respect to the wave generator  320  and the flex spline  322 . Further, the internal teeth of the fixed spline  324  intermesh with the external teeth  323  of the flex spline  322 . Together the wave generator  320 , the flex spline  322  and the fixed spline  324  form the harmonic drive mechanism  360  that is operable to rotate the landing gear leg  316  with respect to the turning member  318  when the electric motor module  326  is driven with its clutch mechanism engaged. 
         [0043]    Again, this embodiment provides the substantial advantage that should the harmonic drive mechanism fail, the inherently safe free-castor mode is the likely outcome. 
         [0044]      FIG. 4  shows a method  400  of retrofitting an aircraft landing gear steering system according to an embodiment of the present invention. The method  400  may be used, for example, to replace conventional aircraft steering mechanisms with various of the embodiments described herein, and thereby to enable the removal of certain hydraulic systems from the landing gear bay of those aircraft as well as providing for improved safety in the event of any steering system failures that might occur. 
         [0045]    The method  400  comprises the step of removing  402  the existing steering mechanism from a conventional aircraft, for example, by cutting, unbolting, it etc., from a landing gear yoke. Removal of conventional hydraulic actuators, piping, accumulators, etc. from the landing gear bay may also be performed at this time. 
         [0046]    Next a step of connecting  404  a landing gear steering system according to an embodiment of the present invention is performed. The landing gear steering system is connected in place of the conventional steering system and may be coupled to a landing gear yoke. For example, the landing gear steering system may be connected by welding, bolting, brazing, riveting, etc., various parts of an existing landing gear yoke to a portion of the landing gear steering system, such as a landing gear leg thereof. 
         [0047]    In various embodiments, a casing housing a harmonic drive mechanism is connected to either a turning member or the landing gear leg. A fixed spline of the harmonic drive mechanism may then be physically secured to the landing gear leg, an intermediate support, or the turning member, as appropriate. 
         [0048]    Additionally during this phase, various electrical connections may be made, for example, to an electric motor module of the landing gear steering system in order that it can be supplied with power. Other connections can be made, such as those for providing control signals for controlling a motor and/or various clutch mechanisms or the like provided within the landing gear steering system. 
         [0049]    Finally, the step of connecting  406  the torque link, oleo and turning member of the landing gear steering system to a wheel train unit is performed. The torque link may be connected in a conventional manner, as may be the oleo once it has been guided through the turning member into a landing gear leg of the new landing gear steering system. 
         [0050]    Whilst various embodiments of the present invention have been described in connection with a concentrically mounted harmonic drive mechanism having a radially innermost wave generator coupled to a radially outermost fixed spline by way of an intervening flex spline, those skilled in the art will realise that other configurations for harmonic drive mechanisms are possible, and accordingly that the present invention is not limited solely to the embodiments described herein in detail. For example, a harmonic drive mechanism may be configured such that a wave generator is provided as a radially outermost component. 
         [0051]    Moreover, although certain embodiments of the present invention are configured such that a turning member extends through the centre of a harmonic drive mechanism, those skilled in the art will realise that various alternative configurations would also be possible. 
         [0052]    Additionally, or alternatively, one or more electric motors or electric motor modules may be used to drive a harmonic drive mechanism. For example, two independently operable electric motor modules may be provided to further improve operational safety and reliability by providing equipment redundancy. 
         [0053]    In certain embodiments, a wave generator, a flex spline and a fixed spline may form a harmonic drive mechanism that is operable to drive a landing gear leg directly (e.g. without the essential need to provide additional intervening gear/transmission/clutch/etc. mechanisms, although these can be provided optionally in various alternative embodiments). 
         [0054]    Various embodiments of the present invention may also be designed such that the centre of gravity of a wheel train unit lies aftwards of a landing gear steering system, such that should the latter fail the wheel train unit will naturally align with the direction in which the aircraft is travelling so that, for example, upon touch-down the wheels are substantially in an optimal landing configuration. 
         [0055]    Furthermore, various embodiments of the present invention may be made by customising commercially available harmonic drive components, such as, for example, certain of those commercially available from Harmonic Drive AG of Limburg, Germany (http://www.harmonicdrive.de). 
         [0056]    One or more harmonic drive components may, for example, be concentrically mounted and/or circumferentially configured about the principal axis of rotation of a landing gear steering system. Certain embodiments may also, or alternatively, be provided with a clutch mechanism for disengaging the turning member from the landing gear leg, so as to enable the harmonic drive mechanism to be disconnected in order that an aircraft may be towed, pushed-back, etc. 
         [0057]    Therefore, whilst the present invention has been described in accordance with various aspects and preferred embodiments, it is to be understood that the scope of the invention is not considered to be limited solely thereto and that it is the Applicant&#39;s intention that all variants and equivalents thereof also fall within the scope of the appended claims.