Abstract:
A draw spring linkage includes a hollow rod formed with a shoulder structure at the first end of the rod and with a slot that extends longitudinally of the rod and is located between the shoulder structure and the second end of the hollow rod. A tongue member includes an attachment portion and a stem that projects from the attachment portion. The stem fits slidingly in the interior space of the hollow rod and is formed with a slot that is aligned with the slot in the hollow rod. A key extends through the slot in the tongue member and the slot in the hollow rod and projects laterally from the hollow rod. A hollow cylindrical spring is threaded by the hollow rod. The first end of the spring engages the shoulder structure, the second end of the spring engages the key, and the spring is held under compression between the shoulder structure and the key.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims benefit of U.S. Provisional application No. 60/493,438 filed Aug. 8, 2003. 

   BACKGROUND OF THE INVENTION 
   This invention relates to a draw spring linkage, particularly a tail wheel draw spring linkage for a light aircraft. 
   A conventional light aircraft has a vertical stabilizer and a rudder that is attached to the vertical stabilizer and is pivotable relative to the vertical stabilizer about an approximately vertical axis. Rudder control cables are connected to a rudder control horn (a generally horizontal bar that is attached to the rudder at its lower end and extends to each side of the rudder) to allow the pilot to adjust the angular position of the rudder in flight. 
   Light aircraft with conventional landing gear are commonly equipped with spring-loaded tail wheel steering linkages that connect the rudder control horn to the tail wheel for positive steering control while on the ground. Typically, the tail wheel is mounted for pivotal movement about an axis that is approximately parallel to the pivotal axis of the rudder and is coupled to a tail wheel horn (a generally horizontal bar that is attached to the tail wheel and extends to each side of the tail wheel). The traditional tail wheel steering linkages comprise draw springs that are connected between the control horns of the rudder and the tail wheel. The draw springs provide shock absorption to protect the rudder control system in the event that the wheel strikes an irregularity during taxiing, take-off or landing. The draw spring linkage is constructed so that the two ends of the linkage remain connected even if the spring fails. The draw spring that is used in the traditional tail wheel steering linkage has some disadvantages, including the fact that it is bulky and not aerodynamic, it is prone to corrosion, and the clips that attach the linkage to the tail wheel control horn may open up and detach in use unless they are wired closed. 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the invention there is provided a draw spring linkage comprising an elongate hollow rod defining an interior space extending longitudinally of the rod, the hollow rod having first and second opposite ends and being formed with a shoulder structure at the first end of the rod and with a slot that extends longitudinally of the rod and is located between the shoulder structure and the second end of the hollow rod, a tongue member comprising an attachment portion and a stem that projects from the attachment portion, wherein the stem fits slidingly in the interior space of the hollow rod and is formed with a slot that is aligned with the slot in the hollow rod, a key member that extends through the slot in the tongue member and the slot in the hollow rod and projects laterally from the hollow rod, and a hollow cylindrical spring member having first and second ends, the hollow spring member being threaded by the hollow rod, the first end of the spring member engaging the shoulder structure, the second end of the spring member engaging the key member, and the spring member being held under compression between the shoulder structure and the key member. 
   In accordance with a second aspect of the invention there is provided an aircraft comprising a rudder, a rudder control horn secured to the rudder for transmitting pilot effort to the rudder, a tail wheel, a tail wheel control horn secured to the tail wheel and having two opposite ends, a first tail wheel steering linkage attaching the first end of the rudder control horn to the first end of the tail wheel control horn and a second tail wheel steering linkage attaching the second end of the rudder control horn to the second end of the tail wheel control horn, for transmitting effort from the rudder control horn to the tail wheel, wherein each tail wheel steering linkage comprises an elongate hollow rod defining an interior space extending longitudinally of the rod, the hollow rod having first and second opposite ends and being formed with a shoulder structure at the first end of the rod and with a slot that extends longitudinally of the rod and is located between the shoulder structure and the second end of the hollow rod, a tongue member comprising an attachment portion and a stem that projects from the attachment portion, wherein the stem fits slidingly in the interior space of the hollow rod and is formed with a slot that is aligned with the slot in the hollow rod, a key member that extends through the slot in the tongue member and the slot in the hollow rod and projects laterally from the hollow rod, and a hollow cylindrical spring member having first and second ends, the hollow spring member being threaded by the hollow rod, the first end of the spring member engaging the shoulder structure, the second end of the spring member engaging the key member, and the spring member being held under compression between the shoulder structure and the key member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which 
       FIG. 1  is a schematic side elevation of a portion of the tail structure of a light aircraft equipped with draw spring tail wheel linkages embodying the present invention, 
       FIG. 2  is a schematic top plan view of the portion of the tail structure shown in  FIG. 1 , 
       FIG. 3  is a top plan view of a partially assembled draw spring linkage embodying the present invention, 
       FIG. 4  is a side elevation of the linkage shown in  FIG. 3 , 
       FIG. 5  is a plan view of the tongue element, 
       FIG. 6  is a perspective view of the key element, 
       FIG. 7  is a vertical sectional view of the linkage when fully assembled, and 
       FIG. 8  is a top plan of the linkage shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows schematically in broken lines the skin  8  and a bulkhead  12  of a light aircraft  10 . The aircraft has a vertical stabilizer  14 . A rudder  16  is attached to the vertical stabilizer  14  by rudder control hinge bearings  18 , which are typically rod end bearings, for rotation about an approximately vertical axis that lies in the central longitudinal plane of the aircraft. The pilot of the aircraft exercises pivotal control over the rudder  16  by means of rudder control cables  20  that are attached to the rudder structure through a rudder control horn  24 . As shown in  FIG. 2 , the rudder control horn  24  comprises a bar that extends generally horizontally and transversely of the aircraft and has forward and aft holes at each end. The left and right rudder control cables are coupled to the rudder control horn by fasteners (not shown) that pass through the forward holes in the rudder control horn at the two opposite ends thereof.  FIG. 1  also shows a tail wheel support strut  26  which is attached at its forward end to the tail structure of the aircraft and is provided at its aft end with a bearing  30  in which a tail wheel control shaft  34  is journalled for rotation about an axis approximately perpendicular to the longitudinal axis of the strut  26 . The tail wheel control shaft  34  is disposed in the central longitudinal plane of the aircraft and is provided at its lower end with a bearing structure in which the tail wheel  38  is mounted.  FIG. 1  illustrates the forward end of the strut  26  attached to the bulkhead  12 , but it will be appreciated that this is simply for ease of illustration and that it may be attached to any other suitable part of the tail structure of the aircraft. The tail wheel control shaft  34  is provided with a tail wheel control horn  40  having left and right ends. The left end of the rudder control horn is connected to the left end of the tail wheel control horn by a left draw spring linkage  42  and similarly the right end of the rudder control horn is connected to the right end of the tail wheel control horn by a right draw spring linkage. 
   The draw spring linkage  42  is shown in  FIGS. 3–8  and comprises a cable subassembly  50 , a spring retainer body  54 , a helical compression spring  58  ( FIGS. 7 and 8 ), a spring retainer tongue  62  ( FIG. 5 ) and a key  66  ( FIG. 6 ). The cable subassembly  50  comprises a flexible cable  70 , an eye fitting  74  that is attached to the cable at one end by swaging, and a ball fitting  78  that is attached to the cable at its opposite end by swaging. 
   The spring retainer body  54  is configured as a cylindrical rod that has been drilled out from one end  80  to form a passage  82  that is large enough to receive the ball fitting  78  with a loose sliding fit. The passage  82  ends just short of the end  84  of the spring retainer body and at that end the spring retainer body is provided with a hole  86  that is large enough to allow the shank  88  of the ball fitting to enter but is not large enough to allow the ball part of the ball fitting to pass. The hole is also too small to allow the shank  90  of the eye fitting  74  to enter and accordingly the spring retainer body  54  is captive on the cable subassembly  50 . 
   The spring retainer body has an annular shoulder  92  at its end  80  and is formed with a longitudinal slot  94 . The helical compression spring  58  is threaded by the cylindrical portion of the spring retainer body and abuts the shoulder  92 . The eye portion of the eye fitting  74  is too large pass through the helical spring and accordingly the spring  58  is captive on the assembly of the cable subassembly  50  and the spring retainer body  54 . 
   Referring to  FIG. 5 , the spring retainer tongue  62  has an eye portion  96  and a stem  100 . The eye portion  96  is provided with an attachment hole  98  and has shoulders  102  from which the stem  100  projects. The stem  100  is formed with a slot  104  near the end that is distal from the eye portion  96 . The spring retainer tongue is dimensioned so that the stem can be inserted into the interior space of the spring retainer body with a loose sliding fit and the shoulders of the eye portion  96  limit penetration of the stem  100  by engaging the end  80  of the spring retainer body. When the stem is fully inserted in the spring retainer body and the cable subassembly is positioned with the shank of the ball fitting  78  located in the hole  86 , the distal end of the stem is spaced from the ball fitting. The slot  104  in the stem is of substantially the same width as the slot in the spring retainer body. 
   The key  66  is generally C-shaped in configuration and has a forward segment  110 , a middle segment  112  and a rear segment  114 . The middle segment  112  has two parallel sides, the forward segment  110  has two sides that converge towards the forward end of the key, and the rear segment  114  of the key has two lugs  118  that project laterally from the parallel sides of the middle segment. A notch  120  is formed at the rear end of the key. 
   The thickness of the key is such that the key will fit easily in the slot  94  in the spring retainer body and the slot  104  in the stem of the spring retainer tongue. 
   The draw spring linkage  42  is assembled by using a spring compression tool to compress the spring  58  against the shoulder  92  of the spring retainer body while the shoulders  102  of the spring retainer tongue are held firmly against the end  80  of the spring retainer body and the slot  104  in the stem of the spring retainer tongue is aligned with the slot  94  in the spring retainer body. One suitable tool comprises a short metal tube formed with a pair of diametrically opposed longitudinal notches at each end. The eye portion  96  of the tongue  62  is held in a vise attached to the bed of a drill press with the spring retainer body  54  extending vertically upwards from the vise. The eye fitting  74  of the cable subassembly is threaded upwards through the metal tube and the cable  70  is placed in one of the notches at the upper end of the tube. The lower end of the tube is fitted over the upper end of the spring retainer body with the notches at the lower end of the tube aligned with the slot  94  in the spring retainer body. The arbor of the drill press is forced downwards onto the upper end of the metal tube and the spring is compressed until the upper end of the spring is slightly below the lower end of the slot  104  in the stem of the spring retainer tongue. The aligned slots in the spring retainer body and spring retainer tongue are accessible through the notches at the lower end of the spring compression tool, and the key  66  is then inserted in the aligned slots with the forward end of the key towards the shoulder  92  of the spring retainer body. The arbor is then raised and the spring  58  is permitted to expand. The spring engages the abutment lugs  118  of the key and forces the key towards the end  84  of the spring retainer body until the distal end of the stem is firmly seated in the notch  120  of the key and is restrained against movement farther away from the end  80  of the spring retainer body. The key is held firmly in position by the force exerted by the spring and the spring compression tool is removed. The completed draw spring linkage  42  is attached to the rudder control horn using a fastener that extends through the hole  98  in the eye portion of the spring retainer tongue and the rear hole at the left end of the rudder control horn and is attached to the tail wheel control horn using a fastener that extends through the hole in the eye fitting  74  and the hole at the left end of the tail wheel control horn. Preferably, the fastener that is used to attach the tongue to the rudder control horn is provided with a sacrificial spacer sleeve, so that wear takes place in the spacer sleeve, which is readily replaceable, rather than in the tongue or the rudder control horn. 
   In use of the draw spring linkage, the spring provides shock absorption to protect the rudder control system from impact in the event that the tail wheel strikes an irregularity during taxiing, takeoff or landing. 
   The draw spring linkage may be made of corrosion resistant materials. The illustrated draw spring linkage is more compact and streamlined in shape than the conventional draw spring linkages that are currently employed. In addition, the illustrated draw spring linkage does not employ clips for attaching the linkage to the tail wheel control horn and accordingly the danger of a clip opening and becoming detached during use does not arise. 
   It will be appreciated that in the event of failure of the spring  58 , the spring remains captive on the spring retainer body, between the key  66  and the annular shoulder  92 . Consequently, the tail wheel control horn remains attached to the rudder control horn. 
   In a preferred embodiment of the invention, the eye portion of the spring retainer tongue is not drilled with the attachment hole  98  until installation. This allows for some variation in the distance between the rudder control shaft  16  and the tail wheel control shaft  34 . Each linkage is adapted to its particular installation by cutting the eye portion of the tongue to the appropriate length and drilling the eye portion at the appropriate location. 
   It is desirable that the shank of the eye fitting and the shank of the ball fitting be coaxially aligned in order to avoid bending the cable. This might necessitate bending the eye portion of the eye fitting and/or the eye portion of the tongue, for example as shown at the locations  124  illustrated in the drawings. In some applications it might be necessary or desirable to use different mechanisms from those described above in order to attach the ends of the linkage to the control horns, particularly in order to avoid excessive bending of the eye portion of the eye fitting and/or the eye portion of the spring retainer tongue. 
   It will be appreciated that although the draw spring linkage has been described for use in a light aircraft, the invention is not restricted to this specific application and the linkage may be used in other applications also. 
   It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated.