Abstract:
A mechanical flight control system for a rotary-wing aircraft is disclosed. The flight control system comprises an upstream portion, a downstream portion, and a booster means for connecting the upstream portion to the downstream portion. The booster means may comprise dual concentric valve actuators and/or a variety of system load limiting features.

Description:
GOVERNMENT LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00019-96-C-0128 awarded by NAVAIR. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the field of mechanical flight control systems. 
     DESCRIPTION OF THE PRIOR ART 
     Mechanical flight control systems (MFCSs) have been in use for many years for aiding in the control of various types of aircraft. A MFCS typically used in helicopters is a cyclic control system (CCS). A CCS commonly includes a pilot input device, usually a stick controlled by the right hand of a pilot, connected to hydraulic actuators by various mechanical linkages. The hydraulic actuators are often arranged to connect to and cause changes in the physical orientation of a swash plate. Lateral, forward, and aft movement control of the helicopter is primarily controlled by the physical orientation of the swash plate. A CCS is normally designed such that when a pilot displaces a cyclic stick from a centered position, the attached mechanical linkages cause the actuators to adjust the physical orientation of the swash plate such that the helicopter tends to move in the direction of the stick movement. 
     A CCS is often described as having particular mechanical characteristics. The mechanical characteristic of a CCS are typically summarized as the effective forces perceived by the pilot through the cyclic stick as the pilot manipulates the cyclic stick. The CCS is normally designed to be balanced such that such that without pilot intervention, the cyclic stick centers to a position called “trim position”. When the cyclic stick is centered or at trim position, no lateral, forward, or aft movement of the helicopter occurs due to the CCS. The major contributing forces which combine to establish the mechanical characteristic of a CCS include: (1) a “breakout force” or “return-to-center force” which is a constant force applied toward centering the cyclic stick to trim position despite how far the cyclic stick is displaced and despite at what velocity the cyclic stick is moved, (2) a “gradient force” or “spring force” that also returns the cyclic stick to a centered position but varies with how far the cyclic stick is displaced from trim position such that the farther the cyclic stick is moved, the stronger the force applied toward centering the cyclic stick to trim position, (3) a constant “friction force” that is opposite to the direction of cyclic stick movement, (4) a “damping force” opposite to the direction of cyclic stick movement and which varies with the velocity at which the cyclic stick is moved, and (5) a “hard stop force” which simulates a mechanical limit of travel of the cyclic stick. 
     The sources of the above described forces vary. Breakout force often emanates from the combination of mechanical balancing of a CCS, the breakout friction force associated with the joints connecting the various mechanical linkages, and the spring preload force associated with the force-gradient cartridges. Gradient force and spring preload both typically primarily emanate from the inclusion of “force-gradient cartridges” situated along a force path between the cyclic stick and the connection to swash plate actuators. Force-gradient cartridges are typically canisters comprising bi-directional spring elements. Hard stop forces are normally forces transmitted to the cyclic stick for purposes of informing the pilot that the CCS is at its control limit for the current directional command. 
     Automatic flight control systems (AFCSs) are often incorporated into CCSs such that motors or other devices provide mechanical input to the CCS resulting in automated holding of the cyclic stick and/or automated adjustment of the “trim position”. It is common to incorporate a “trim release button” on the cyclic stick which allows the pilot to move the cyclic to any desired position and then release the trim release button to command the AFCS to hold the current cyclic stick position. Often, the “trim position” or “attitude” can be adjusted by moving a four-way thumb switch on the cyclic stick. If a CCS has good mechanical characteristics, it is easy for the pilot to “push through” the cyclic stick position held by the AFCS by applying force to the cyclic stick without disengaging the AFCS. 
     If the friction forces of a CCS are too high and/or the mechanical leverage offered by the cyclic stick design is too low, significant negative impacts on the mechanical characteristics of the CCS may exists. For example, a cyclic stick offering a lowered mechanical leverage results in higher breakout forces and amplifies CCS mechanical imbalance resulting in poor control harmony. Where frictional forces cannot otherwise be reduced adequately to accommodate the low leverage cyclic stick, force-gradient cartridges fail to provide proper levels of spring force. With low spring force levels, poor cyclic stick centering occurs during manual operation of CCS and the AFCS is prevented from “back-driving” the CCS. While the above described MFCS advancements represent significant developments in MFCS design, considerable shortcomings remain. 
     SUMMARY OF THE INVENTION 
     There is a need for an improved mechanical flight control system. 
     Therefore, it is an object of the present invention to provide an improved mechanical flight control system which provides a lower perceived system friction. 
     This object is achieved by providing a CCS in which a cyclic secondary boost actuator is connected in a parallel load path between an upstream portion of the CCS and a downstream portion of the CCS. 
     The present invention provides significant advantages, including: (1) improved cyclic stick centering to the trim position; (2) masking from the pilot all friction and mass imbalances associated with the downstream portion of the CCS; (3) allowing pilot to perceive the friction associated with only the upstream portion of the CCS, and (4) providing back-driving or push through capability during use of an AFCS of a CCS with unequal friction forces in the upstream portion of a CCS and the downstream portion of the same CCS. 
     Additional objectives, features, and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a perspective view of the preferred embodiment of a helicopter according to the present invention; 
         FIGS. 2 and 3  are perspective views of the preferred embodiment of a CSS according to the present invention; 
         FIGS. 4-7  are perspective and side views of a longitudinal boost assembly of the CSS of  FIGS. 2 and 3 ; and 
         FIGS. 8-11  are perspective and side views of a lateral boost assembly of the CSS of  FIGS. 2 and 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is an improved mechanical flight control system (MFCS) which allows an upstream portion of the MFCS to operate with lower friction and lower preloads than a downstream portion of the MFCS. While specific reference is made to a cyclic control system CCS for a helicopter, the present invention may alternatively be incorporated with any other mechanical control system where operating an upstream input control portion having lower friction than a downstream output control portion is desired and/or is beneficial. 
       FIG. 1  depicts a helicopter  101  incorporating a CCS (not shown) according to the present invention. Helicopter  101  has a fuselage  103 , a crew compartment  105 , and rotor blades  107  powered by a power plant (not shown) and driven by a mast  109 . Cyclic sticks (not shown) of CCS and other portions (not shown) of CCS are located within crew compartment  105  where a pilot and copilot are seated during operation of helicopter  101 . Helicopter  101  also has a swash plate (not shown) which is physically manipulated in part by CCS. Physical manipulations of the swash plate results in altered cyclic control outputs. Of course CCS may optionally include an auto-pilot feature for controlling a cyclic input. 
     Referring now to  FIGS. 2 and 3  in the drawings, perspective views of the preferred embodiment of a CCS according to the present invention are illustrated. CCS  111  comprises an upstream portion  113 , a downstream portion  115 , and a boosting means  117  connected to both upstream portion  113  and downstream portion  115 . CCS  111  also comprises a lateral load path  119  and a longitudinal load path  121 . In this embodiment of the present invention, boosting means  117  comprises a lateral boost assembly  123  and a longitudinal boost assembly  125 . Generally, boost assemblies  123 ,  125  are installed parallel to the convention direct load path rather than in series with the conventional direct load path. Forces are transferred from upstream portion  113  of lateral load path  119  to downstream portion  115  of lateral load path  119  through lateral boost assembly  123 . Forces are transferred from upstream portion  113  of longitudinal load path  119  to downstream portion  115  of longitudinal load path  119  through longitudinal boost assembly  125 . Both lateral boost assembly  123  and longitudinal boost assembly  125  may be shaped, sized, and otherwise adapted to achieve a particular input/output leverage ratio between various system elements. Upstream portion  113  has lower inherent friction than downstream portion  115 . 
     Upstream portion  113  and downstream portion  115  of CCS further comprise cyclic sticks  127  and associated buttons (not labeled) for inputting pilot commands by moving sticks  127  and pressing buttons, force-gradient cartridges  129  for introducing spring force to CCS  111  mechanical characteristics, trim motor assemblies  131  for actuating CCS  111  elements during autopilot use, and various fixed mounts  133  (all not labeled) for attaching stationary portions of CCS  111  to stationary features (not shown) of interior portions of a helicopter fuselage (not shown) such that movable interlinked elements such as tubular control linkages  135  (not all labeled), mechanical idlers  137  (not all labeled), and mechanical bellcrancks  139  (not all labeled) are movable with relation to the stationary features of interior portions of the helicopter fuselage. While bearings are typically used to connect discreet linking elements, bearings are not labeled. A lateral output linkage  141  and a longitudinal output linkage  143  transmit forces from lateral boost assembly  123  and longitudinal boost assembly  125 , respectively, to other structures (not shown) which ultimately control swash plate actuators (not shown). The swash plate actuators are hydraulic actuators controlled and activated by movements of lateral output linkage  141  and a longitudinal output linkage  143 . 
     Referring now to  FIGS. 4-7 , the preferred embodiment of longitudinal boost assembly  125  is illustrated. Assembly  125  is a unity feedback, moving body hydro-mechanical device. Longitudinal boost assembly  125  comprises a longitudinal boost assembly mount  145 , longitudinal boost assembly input lever  147  hingedly attached to mount  145 , longitudinal boost assembly output lever  149  also hingedly attached to mount  145 , longitudinal boost assembly adjustable hard stops  151 , longitudinal boost assembly hydraulic unit  153 , and longitudinal direct link  155 . Hard stops  151  are adjusted to contact input lever  147  and output lever  149  before over-travel of CCS  111  components occurs. Hydraulic unit  153  comprises a hinged portion  157  hingedly attached to mount  145  and a translating portion  159  attached to hinged portion  157  such that translating portion  159  may translate along hinged portion  157 . Translating portion  159  is also hingedly attached to output lever  149 . Hinged portion  157  is connected to input lever  147  with direct link  155  which is connected to a piston locking bar  181  (discussed infra) for actuating a control piston  179  (discussed infra) such that if input lever  147  is moved toward hydraulic unit  153 , direct link  155  moves locking bar  181  to actuate hydraulic unit  153  in a manner causing translating portion  159  to translate along hinged portion  157  in the direction of movement supplied by input lever  147 . Similarly if input lever  147  is moved away from hydraulic unit  153 , direct link  155  moves locking bar  181  to actuate hydraulic unit  153  in a manner causing translating portion  159  to translate along hinged portion  157  in the direction of movement supplied by input lever  147 . Of course as translating portion  159  moves, output lever  149  also moves in a manner dictated by the geometry of interconnection of the two elements. 
     Referring now to  FIGS. 8-11 , the preferred embodiment of lateral boost assembly  123  is illustrated. Assembly  123  is a unity feedback, moving body hydro-mechanical device. Lateral boost assembly  123  comprises a lateral boost assembly mount  161 , lateral boost assembly input lever  163  hingedly attached to mount  161 , lateral boost assembly output lever  165  also hingedly attached to mount  161 , lateral boost assembly adjustable hard stops  167 , lateral boost assembly hydraulic unit  169 , and lateral direct link  171 . Hard stops  167  are adjusted to contact input lever  163  and output lever  165  before over-travel of CCS  111  components occurs. Hydraulic unit  169  comprises a hinged portion  173  hingedly attached to mount  161  and a translating portion  175  attached to hinged portion  173  such that translating portion  175  may translate along hinged portion  173 . Translating portion  175  is also hingedly attached to output lever  165 . Hinged portion  173  is connected to input lever  163  with direct link  171  which is connected to a piston locking bar  181  (discussed infra) for actuating a control piston  179  (discussed infra) of such that if input lever  163  is moved toward hydraulic unit  169 , direct link  171  moves locking bar  181  to actuate hydraulic unit  169  in a manner causing translating portion  175  to translate along hinged portion  173  in the direction of movement supplied by input lever  163 . Similarly if input lever  163  is moved away from hydraulic unit  169 , direct link  171  moves locking bar  181  to actuate hydraulic unit  169  in a manner causing translating portion  175  to translate along hinged portion  173  in the direction of movement supplied by input lever  163 . Of course as translating portion  175  moves, output lever  165  also moves in a manner dictated by the geometry of interconnection of the two elements. 
     Both hydraulic units  153 ,  167  are powered by a single hydraulic system (not shown). Assemblies  123 ,  125  integrate features which minimize impacts to CCS mechanical characteristics even in the event of loss of hydraulic supply pressure failure. For example, to maintain aircraft control when supply pressure is lost, pressure-operated bypass locking valves (not shown) release internal actuator pins  179  to a non-pressure assisted position which subsequently allows control pistons  179  to extend from translating portions  159 ,  175 . When extended from translating portions  159 ,  175 , pistons  179  are engaged with locking bars  181 , thereby precluding freeplay movement of system elements due to internal valve travel. Also, fluid flow between multiple internal cylinders is allowed while the input levers  147 ,  163  are fixed to translating portions  159 ,  175 , respectively, such that instead of introducing freeplay into CCS  111 , hydraulic units  153 ,  169  merely act as viscous dampers. Further, to prevent overloading of the elements of downstream portion  115 , hydraulic units  153 ,  169  incorporate dual concentric main control valves that port hydraulic pressure to return channels before stops  151 ,  167  contact the respective input and output levers. This function disables the hydraulic unit  153 ,  169  output just prior to the pilot being able to transmit more load to the elements of CSS  111  than the elements are structurally designed to withstand. 
     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.