Abstract:
A flight control system includes carbon fiber pull rods to substitute in whole or in part for conventional steel cables. The stretch and thermal expansion issues associated with composite cables are minimal, making them much better for the very long flight-control runs on large aircraft and spacecraft, and also on aircraft and spacecraft operating though a wide range of temperatures during flight. In a preferred embodiment, the carbon fiber pull rods are manufactured by winding a resin-impregnated carbon tow around a pair of bobbins, one of which is fixed and one of which is rotatably mounted. When the second bobbin is rotated, the windings are twisted to form the carbon fiber pull rod, which is then cured to complete the manufacturing process.

Description:
[0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/083,795, entitled “Composite Flight Control Cables,” filed Jul. 25, 2008, which is hereby incorporated by reference in its entirety for each of its embodiments and teachings. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Steel cables have traditionally been used in aircraft control systems to connect the cockpit to control surfaces of the aircraft. But issues arise when using traditional steel cables for long flight-control runs on large aircraft and spacecraft, particularly where those large aircraft and spacecraft must operate through a range of temperatures during flight. Steel cables have a tendency to stretch, which can reduce effectiveness of the control system. Moreover, the properties of steel change significantly with temperature due to its Coefficient of Thermal Expansion (CTE). 
         [0003]    For example, in aircraft with cable runs that approach or exceed 1400 inches, a 5/32 inch diameter steel cable subjected to a Federal Aviation Regulation (“FAR”) allowable maximum pilot effort load would exhibit 12.1 inches of cable stretch over the 1400 inch run. Moreover, steel cable preloaded to 150 pounds tension would contract by 2.6 inches over a 1400 inch run due to temperature changes between sea level and a design service ceiling. That thermal contraction increases the steel cable load from 150 pounds to 500 pounds. 
       SUMMARY OF THE INVENTION 
       [0004]    Improved flight control cables are disclosed that do not exhibit the above problems of cable stretch and thermal expansion. In a preferred embodiment, the flight control cables comprise one or more carbon fiber pull rods. The carbon fiber pull rods are manufactured by winding a resin-impregnated carbon tow around a pair of bobbins, one of which is fixed and one of which is rotatably mounted. When the second bobbin is rotated, the windings are twisted to form the carbon fiber pull rod, which is then cured to complete the manufacturing process. 
         [0005]    The finished carbon fiber pull rods may be used to substitute in whole or in part for conventional steel cables in the flight control system. Where a control cable in the flight control system passes around a pulley or sector, that portion of the cable that passes around the pulley or sector is preferably constructed of conventional stainless steel and a transition or interface between the steel portions of the cable and the carbon fiber portions of the cable is made. 
         [0006]    The carbon fiber pull rods of the present invention provide excellent stretch and thermal expansion properties making them much better for very long flight control runs on large aircraft and spacecraft, and also on aircraft and spacecraft operating though a wide range of temperatures during flight. In addition, the thermal expansion properties of the carbon fiber pull rods of the present invention will typically be more similar to those of other composite components of the aircraft, which may include all or part of the fuselage, wings, or tail structure. Thus, the carbon fiber pull rods of the present invention and these other aircraft components will expand and contract in a similar fashion in response to temperature changes. 
         [0007]    The present disclosure describes the use of control cables and carbon fiber pull rods primarily in connection with aircraft control systems intended to be compliant with FAR Parts 23 and 25 pertaining to airplane airworthiness standards. The cables and carbon fiber pull rods of the present invention may also find use in other applications, particularly in applications where long cables are required such as control systems on yachts and other large boats. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other features, benefits, and advantages of the present disclosure will become apparent from the following detailed description, which refers to the accompanying drawings, wherein like reference numerals refer to like features across the several views, and wherein: 
           [0009]      FIG. 1  illustrates an aircraft in which use of the cables of the present invention is contemplated; 
           [0010]      FIG. 2  illustrates a preferred embodiment of a carbon fiber pull rod of the present invention; 
           [0011]      FIG. 3  is a flow chart illustrating a preferred embodiment for manufacturing carbon fiber pull rods in accordance with the principles of the present invention; 
           [0012]      FIG. 4  illustrates the bonding surfaces of a bobbin that may be used in constructing a carbon fiber pull rod of the present invention; 
           [0013]      FIGS. 5-12  illustrate aspects of the process for manufacturing carbon fiber pull rods in accordance with the principles of the present invention; and 
           [0014]      FIG. 13  illustrates a carbon fiber pull rod connected to a steel cable in accordance with the principles of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring now to  FIG. 1 , an aircraft titled White Knight 2 is illustrated. The White Knight 2 is a twin-fuselage, twin-empennage design, currently contemplated to have a wingspan of approximately 140 feet, and the length of approximately 80 feet. The distances involved in the control system of this aircraft are significant, particularly the link between the control surfaces of the port empennage and the cockpit located in the starboard fuselage, and there is a need to ensure that the control surfaces of both empennage work in coordination with each other.  FIG. 1  illustrates the longest of cable runs in the control system, stretching from the cockpit in the fore section of the starboard fuselage to the starboard empennage, and from the cockpit through the wing section spanning the two fuselage to the port empennage. 
         [0016]    A preferred embodiment of a carbon fiber pull rod manufactured in accordance with the principles of the present invention is shown in  FIG. 2 . The finished rod in this preferred embodiment comprises 20 complete wraps of carbon tow wound around a pair of aluminum bobbins  202 ,  204  that are twisted to create the cable, as described in more detail below. In a preferred embodiment, the carbon fiber used to manufacture the cable may be 12K carbon tow, standard modulus (AS4/T300 equivalent) impregnated with a resin such as Magnolia Plastics MB7500 Adhesive. 
         [0017]    A preferred embodiment of a manufacturing process for fabricating the carbon fiber pull rod shown in  FIG. 2  will now be described in connection with  FIG. 3  and  FIGS. 4-12 . As shown in  FIG. 3 , the first step  302  in the manufacturing process is to prepare the bonding surfaces of the bobbins  202 ,  204 , indicated by arrows in  FIG. 4 , by for example alodining and applying epoxy or self-etching bonding primer to the bonding surfaces. 
         [0018]    The next step  304  in the process is to position bobbins  202 ,  204  on a workstation  502  as shown in  FIG. 5  and  FIG. 9 . As shown in those figures, workstation  502  preferably comprises a fixed support  504  to which bobbin  202  is secured by a nut and inverted bolt. Workstation  502  further comprises a twistable support  506  to which bobbin  204  is secured. As best shown in  FIG. 10  and  FIG. 12 , twistable support  506  comprises a handle  1008  secured to a rotatable shaft  1010  oriented parallel to the length of workstation  502 . The distal end of shaft  1010  is provided with a means for securing bobbin  204 , such as the nut and bent bolt shown in  FIG. 10  and  FIG. 12 . A spring  1012  is provided to permit tensioning of the cable during the manufacturing process, as described below. As illustrated by the dotted line connecting the midpoints of bobbins  202 ,  204  in  FIG. 5 , it is important in this preferred embodiment of workstation  502  to ensure that the two bobbins are level with each other in order to provide for a properly formed cable. Spacers  508 ,  510  may be used as necessary to ensure that the two bobbins are level with each other. 
         [0019]    The next step  306  of the process is to wet out the carbon tow prior to winding in accordance with standard aerospace procedures to achieve desired fiber volume. In a preferred embodiment, this desired fiber volume may be approximately 70%. The tow may be wet out by running it through a resin bath  702  placed between a spool of carbon fiber  704  and fixed bobbin  202  as shown in  FIG. 7 . 
         [0020]    The next step  308  of the process is to wrap one loop of carbon fiber drawn off spool  704  around fixed bobbin  202 , and clamp the free end of the carbon fiber with a spring clamp  602  as shown, for example, in  FIG. 6 , or otherwise secure the free end such as by tieing it off to some part of workstation  502 . 
         [0021]    The next step  310  of the process is to wind the cable around bobbins  202 ,  204 . In the preferred embodiment illustrated here, 20 complete circuits of wet tow are wound between fixed bobbin  202  and twistable bobbin  204 , resulting in a total of 40 strands of tow between the two bobbins. 
         [0022]    The next step  312  of the process is to terminate the tow by wrapping a final loop around fixed bobbin  202  and clamping the end of the tow or tying it off to some part of workstation  502 , as illustrated in  FIG. 8 . 
         [0023]    The next step  314  of the process is to fill the crotch at each bobbin  202 ,  204  with paste adhesive. In a preferred embodiment, the paste adhesive may be a thixotropic mixture of MB7500 and 4% by weight Wacker Silicones HDK N20 Pyrogenic Silica-Fumed Silica. 
         [0024]    The next step  316  of the process is to twist the wound fiber by turning handle  1008 . In a preferred embodiment, the cable is twisted six turns per foot of carbon fiber pull rod to be fabricated. 
         [0025]    The next step  318  of the process is to wipe away any resin drips from the carbon fiber pull rod. Then, in step  320 , spring  412  is set to tension the carbon fiber pull rod with 50 pounds to ensure proper squeeze out of the resin and to remove sag as the carbon tow settles in. Finally, in step  322 , the resin is cured in accordance with the manufacturer&#39;s instructions. Cables of any desired length may be constructed using the manufacturing process described above by positioning bobbins  202 ,  204  at an appropriate distance from each other. 
         [0026]    In a preferred embodiment, the control system of the aircraft shown in  FIG. 1  is implemented using carbon fiber pull rods of approximately 0.20 inches diameter exhibiting a test break strength of approximately 6000 pounds (RT-dry) manufactured in accordance with the process described above. 
         [0027]    It should be recognized that although the carbon fiber pull rods of the present invention exhibit superior characteristics to traditional steel with respect to use in aircraft control cables, the carbon fiber pull rods of the present invention are not sufficiently flexible to permit them to be used around pulleys or sectors. Accordingly, in a preferred embodiment of the present invention, an aircraft control cable may be constructed of two or more segments, wherein one or more of the segments are carbon fiber and one or more of the segments are stainless steel. In particular, for lengths of the cable where no change of direction is required, the cable is preferably constructed from carbon fiber pull rods, while conventional stainless steel cable is used for portions of the control cable that pass around pulleys and sectors. In a preferred embodiment, a cable primarily constructed of carbon fiber may also be provided with one or more steel segments where it is desired to provide the cable with stretching capacity to allow for changes in cable length during operation of the aircraft. For example, where the desired length of a control cable to be run along the length of a wing may be expected to change as a result of changes in curvature of the wing during flight, a steel segment in an otherwise carbon fiber cable may be used to provide adequate stretching capacity to avoid undesirable stress on the cable. Preferably, the system is preloaded to 150 pounds tension on all cables. 
         [0028]    The transition or interface between the carbon composite pull rods and steel cable may be achieved, for example, using standard connecting hardware such as a fork or clevis. As shown, for example, in  FIG. 13 , a standard steel cable  1302  terminating with a spade  1304  may be connected via a fork  1306 , turnbuckle  1308 , and second fork  1312  to a carbon fiber pull rod  1310 . 
         [0029]    In a preferred embodiment, the stainless steel cable used to go around pulleys or sectors may be 5/32 stainless steel cable (7×19) having a breaking strength of 2400 pounds. Additionally, all sectors, bell cranks, and similar actuating structures in the control force path are preferably mounted on ball bearings for low friction and high reliability. 
         [0030]    In a preferred embodiment, additional protection of carbon fiber pull rods against abrasion or other damage, including heat or water damage, in highly susceptible areas is provided. This may be an important aspect of the use of carbon fiber pull rods of the present invention because, although these carbon fiber pull rods exhibit superior performance characteristics to those of steel in control system applications, they may also be more susceptible to abrasion or other damage than steel rods if not properly protected. 
         [0031]    In addition to meeting the requirements of FAR Parts 23 and 25, the control system of the present disclosure is preferably designed with the following factors of safety in order to balance robustness under load with the need for low weight. When subjected to maximum FAR permissible pilot effort, i.e., dual pilot control, push rods in buckling and carbon fiber pull rods in tension preferably exhibit a 3.0 safety factor. General stresses are preferably designed to a safety factor between 1.5 and 2.0. Bearing stresses are designed to a 2.0 safety factor. Tension loads on steel cable are designed to a 1.5 safety factor based upon the minimum rated break strength. 
         [0032]    The present disclosure presents many advantages over a conventional steel cable control system. In addition to those already mentioned, in an aircraft such as the White Knight 2, which is designed and constructed of a composite material in the fuselage and internal structure, composite control cables will exhibit similar thermal expansion patterns, therefore reducing stress on the control cables due to any thermal expansion, which itself is reduced as compared to conventional steel cables. The carbon fiber pull rod cables of the present disclosure exhibit a higher stiffness than equivalent metal cables of the same weight. The potential for weight saving is particularly important in the presently contemplated application of aircraft or spacecraft. 
         [0033]    Although the present disclosure has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Accordingly, the scope of the present invention should be limited not by the specific disclosure herein, but only by the appended claims.