Abstract:
A towing apparatus capable of moving aircraft and other workpieces about an airfield or other workspace. The present invention includes a first frame which is rotatable relative to a second frame about a common substantially vertical axis. A wheel assembly is positioned within the first frame and driven by a motor which is also mounted on the first frame. A gripper assembly for engaging the aircraft or other workpiece is attached to the second frame. In this manner, the present invention provides for the towing of aircraft or other workpieces along an axis which is proximate the axis of the wheel assembly. Additionally, it provides for a low center of gravity significantly enhancing the stability and maneuverability of the present invention in a towing attitude or a non-towing attitude.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 08/865,202, now U.S. Pat. No. 6,112,838, filed May 29, 1997, and hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for towing a workpiece. More particularly, the present invention relates to a motorized tractor-towing apparatus of enhanced stability capable of towing an airplane, trailer, or other workpiece. 
     2. Description of the Related Art 
     It is frequently necessary to move closely-parked private aircraft, for example, around an airfield or within a hanger. State of the art devices disclose tractors having some capability of moving such aircraft but are unstable, particularly when pulling a heavy workpiece, such as an aircraft. See, for example, U.S. Pat. Nos. 3,819,001 and 3,861,483. Because of their unstable nature, such prior art devices require a stabilizer bar or other mechanism to stabilize it and make it more maneuverable particularly in a towing attitude. However, such additional structure makes the prior art devices particularly difficult to operate in congested conditions such as those frequently found at private airfields. 
     Other prior art aircraft handling and towing devices are cumbersome and do not provide for sharp turning radii required in maneuvering aircraft about a congested airfield, for example. See U.S. Pat. Nos. 3,049,253; 2,732,088; 2,734,716; 3,038,550; 3,662,911; 4,318,448; and 4,576,245. 
     Additionally, there exists the need for an enhanced gripper mechanism to attach to the nosewheel or tailwheel of an aircraft in a secure manner and yet operate with a small turning radius. Prior art disclosures, such as U.S. Pat. Nos. 2,874,861 and 2,877,911, do not permit the type of gripping necessary for a tight turning radius operation with enhanced locking capability. 
     Accordingly, the need exists for an improved towing apparatus for aircraft and other workpieces which is stable even when not towing a workpiece. However, once engaged to either a nosewheel or a tailwheel of an aircraft for example, the device provides for a tight turning radius and is easy to maneuver in forward or reverse. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus for towing a workpiece, such as an aircraft. The apparatus comprises first and second frames which are in spaced vertical relationship to one another. The frames are attached in such a manner that the first frame rotates relative to the second frame about a common substantially vertical axis. A wheel assembly is supported by the first frame and includes a wheel for rotational movement about a first axis. A motor is used to power the wheel assembly. A handle assembly is attached to the first frame to enable the operator to rotate the first frame relative to the second frame. A gripper assembly is supported by the second frame and offset from the vertical axis. The gripper assembly is positioned on the second frame to engage a workpiece for towing along a second axis which is positioned above or below the first axis by no more than about 30% of the radius of the wheel. In this manner, the present invention provides a low center of gravity and permits the towing axis, or the working axis, to be at or near the rotating axis of the wheel. Thus, a stable towing apparatus is provided which permits the workpiece, such as the nosewheel or tailwheel of an aircraft, to be in close proximity to the wheel of the present invention. Consequently, the turning radius is small. 
     The present invention may also include a transmission, such as a hydrostatic transmission, positioned between the motor and the wheel assembly to enable the transmission of power from the motor to the wheel on a smooth basis and permit motorized forward and rearward motion of the towing apparatus and the workpiece. 
     The gripper assembly of the present invention may be used to tow the nosewheel of an aircraft or another wheel of a workpiece. Alternatively, the gripper assembly may be used to tow the tailwheel of an aircraft. If the nosewheel of an aircraft is being pulled, it may be preferable to permit the nosewheel to remain in contact with the ground while the present invention tows the aircraft. In this event, the gripper assembly is supported by the second frame and includes a stationary arm and a pivotal arm. The pivotal arm is rotated relative to the second frame. Once engaged the ends of both the first and second arms compress the axis of a nosewheel to permit a pulling or pushing towing operation by the present invention. The gripper assembly may also include a gas cylinder, for example, positioned between the first and second arms to dampen any movement between the ends of either arm and the nosewheel thereby further securing the first arm relative to the second arm. 
     In the event the present invention is intended to engage the tailwheel of an aircraft, the gripper assembly may include a tow frame supporting a cradle. The gripper assembly may include means to tilt one end of the tow frame toward the ground enabling the cradle to engage and support the tailwheel. The gripper assembly may also include means for activating the tilt means to lower one end of the tow frame and then raise that same end thereby elevating the tailwheel off the ground and permitting the present invention to tow the aircraft along a towing axis positioned between about 60% of the radius of the wheel of the present invention above the axis of that wheel to about 60% of the radius of that wheel below the rotational axis of the wheel. 
     Thus, the present invention provides a stable pushing and pulling towing apparatus for a workpiece such as an aircraft that provides for a sharp turning radius, forward and rear motion, and the introduction of substantial power in a forward or rearward direction on a smooth and continuous basis. 
     The more important features of this invention have been summarized rather broadly in order that the detailed description may be better understood. There are, of course, additional features of the present invention which will be described hereinafter and which will also form the subject of the claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to more fully describe the drawings used in the detailed description of the present invention, a brief description of each drawing is provided. 
     FIG. 1 is an elevation view of the present invention. 
     FIG. 2 is an elevation view of the present invention from the opposite side shown in FIG.  1 . 
     FIG. 3 is a cross-sectional view of the present invention taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a detailed cross-sectional view taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a bottom view of a portion of the present invention. 
     FIG. 6 is a view similar to FIG. 5 but in a different operational sequence from that shown in FIG.  5 . 
     FIG. 7 is a top view of an alternate embodiment of a portion of the present invention. 
     FIG. 8 is an elevation view of the alternate embodiment shown in FIG.  7 . 
     FIG. 9 is a different operational sequence of the alternate embodiment shown in FIG.  8 . 
     FIG. 10 is a top view of another alternate embodiment of a portion of the present invention. 
     FIG. 11 is a partial top view of a portion of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1-4, the present invention is a towing apparatus  20  having a first frame  22  and a second frame  24 . Each frame  22  and  24  is generally rectangular in configuration. In the case of frame  22 , it is shown to include longitudinal members  26  and transverse members  28 . Second frame  24  includes longitudinal members  30  and a transverse member  32 . The present invention also includes a handle assembly  38  having a member  39  which is attached to first frame  22  at pin connection  40 . Member  42  serves to brace member  39  to first frame  22 . 
     Referring still to FIGS. 1-4, a motor  44  is attached to, and supported by, first frame  22 . Motor  44  may be a conventional internal combustion motor such as a lawnmower motor. In the prototype of the present invention, the motor is a 3.5 horsepower Briggs &amp; Stratton engine, readily commercially available. Alternatively, motor  44  may be another type of motor such as an electric motor or an air motor. Obviously, in such cases the user would need to provide a power source such as electricity or compressed air through a power cord or air hose up to motor  44 . In the case of an internal combustion engine as shown, a throttle  46 /cable  48  is attached to handle  38  and connects with the throttle setting of motor  44  to power up or down motor  44 . The throttle  46 /cable  48  assembly is well known to those skilled in the art and is similar to that found on commercially-available lawnmowers. 
     The present invention also includes a wheel assembly  50  which is rotatably supported within first frame  22 . Referring to FIGS. 3 and 4, wheel assembly  50  includes a wheel/tire  52 . Axle  54  passes through wheel  52  and is rotatably supported in the preferred embodiment by flanges  56  which are attached to longitudinal members  26  of first frame  22 . Each end of axle  54  is held in place by a hub  58  having a lynch pin  60  or other fastener such as a set screw. A sprocket  62  is attached to axle  54  and fixed relative to axle  54  and wheel  52 . In this manner, wheel  52  rotates about axle  54 , but within first frame  22 . 
     Referring back to FIGS. 1-2, the present invention may include a transmission  64 , preferably a hydrostatic transmission, such as that manufactured by the Eaton Corporation, model no. C-250-801. Such a hydrostatic transmission is well known to those skilled in the art and commonly used on riding lawnmowers, garden tractors and off-road vehicles. Such a transmission serves to provide a gradual increase and decrease in power from motor  44  and transfer that power to a wheel assembly  50  in accordance with the present invention as described below. In addition, such a transmission provides for motorized forward and rearward motion. 
     Power is transferred from motor  44  to transmission  64  by means of a belt  66 . As shown in FIG. 2, belt  66  passes around drive pulley  68  of motor  44  and pulley  70  of transmission  64 . In accordance with the operation of the present invention as will be described in more detail below, the power output side of transmission  64  is shown in FIG. 1 as drive shaft  72 . Shaft  72  is connected to a sprocket  74 . A chain  76  is used to drivably engage sprocket  74  with sprocket  62  thereby rotating wheel  52  and driving the present invention. 
     Referring still to FIGS. 1-4, but in particular FIGS. 1 and 4, second frame  24  is rotatably supported relative to first frame  22  by concentric drums  78  and  80 . As shown in FIG.  2  and as noted above, second frame  24  comprises longitudinal members  30 . Each longitudinal member  30  is fixedly attached to outer drum  80 . Outer drum  80  is a cylindrical member which is vertically supported by, and rotates within, inner drum  78 . Drum  78  is securely attached to members  26  of first frame  22 . Referring still to FIGS. 1 and 4, it can be seen that outer drum  80  is supported vertically by shoulder  82  of inner drum  78 . Thus, second frame  24  can rotate relative to first frame  22  since outer drum  80  can rotate about inner drum  78  about a full 360°. If desirable, second frame  24  may be locked to first frame  22  by a bracket  84  which is pivotably attached at connection  86  to first frame  22 . When it is desirable to permit the rotation of the first frame  22  relative to the second frame  24 , in accordance with the operation of the present invention as described below, the operator pulls knob  88  upwardly displacing cable  91  and thereby pivoting bracket  84  about connection  86 . Thus, bracket  84  releases first frame  22  relative to second frame  24  enabling the rotational movement of first frame  22  relative to second frame  24 . 
     Referring still to FIGS. 1 and 2 and now FIGS. 5 and 6, the present invention also includes a gripper assembly  90  which is used to engage a workpiece. In the case of FIGS. 1-6, the workpiece as shown in phantom lines is a wheel  92 , such as the nosewheel of an aircraft. Obviously, it will be apparent to one skilled in the art that wheel  92  may be the wheel of a workpiece other than an aircraft, Gripper assembly  90  comprises a first arm  94  and second arm  96 . Referring to FIG. 5 (which is a bottom view looking upwardly), first arm  94  is fixedly attached to second frame  24 . First arm  94  includes a longitudinal member  97  which is shown bolted to longitudinal member  30  of second frame  24 . A sleeve  100  is attached at one end to member  97  and a pin  98  is adapted to pass through sleeve  100 . Pin  98  is bolted by screw  102  to sleeve  100 . A hub  104  is attached at one end of pin  98 . Hub  104  includes a recess and is selected in size to pass over the axle or wheel hub  106  of nosewheel  92 . 
     Referring still to FIG. 5, second arm  96  of gripper assembly  90  is pivotably connected to frame  24  at pin connection  108 . The second arm  96  also includes a hub  126 /pin  128  arrangement similar to that described earlier with respect to hub  104 /pin  98  of first arm  94 . Again, hub  126  engages wheel hub  130  of nosewheel  92 . A linkage assembly  110  supported by second frame  24  is used to pivotably rotate second arm  96  relative to second frame  24 . Linkage assembly  110  includes a lever arm  112  attached at one end to pin connection  108  and at its other end  114  to rod  116 . Rod  116  is in turn pivotably connected to handle  118  at connection  120 . Handle  118  is pivotably connected and supported by bracket  124  at pin connection  122 . Bracket  124  is welded directly to drum  80  as are longitudinal members  30  of second frame  24 . The selection of the length of bracket  124  from drum  80  to pin connection  122  is such that when handle  118  is rotated about pin connection  122  to the closed position as shown in FIG. 6, an over-the-center locking action occurs which prevents handle  118  from being prematurely released except by the operator physically moving handle  118  back to the position shown in FIG.  5 . This results in the locking of nosewheel  92  relative to said second frame  24 . The gripper assembly  90  may also include a dual-acting gas cylinder  132  which serves to provide compressive resistance in either direction. It is attached at one end  134  to second frame  24  and at its other end  136  to lever arm  112 . When handle  118  is rotated to the position shown in FIG. 6 enabling the engagement of second arm  96  against hub  130  of wheel  92 , shaft  138  of gas cylinder  132  is extended thereby resisting any movement of second arm  96  relative to second frame  24  as may be caused by any jarring or bouncing movement of nosewheel  92 . Yet cylinder  132  serves to permit extreme movement of the end of second arm  96  at hub  126  which may occur if nosewheel  92  hits a pothole or other obstruction on the airfield while being towed. This permits the emergency release of the nosewheel  92  without damaging the nosewheel axle or nosewheel undercarriage assembly. 
     Referring back to FIG. 2, as discussed above transmission  64  receives its input power from belt  66 . Transmission  64  provides forward or rearward direction of the present invention by placement of lever  142  in a forward or rearward direction as discussed herein. This is the mechanism most commercially available hydrostatic transmissions use to shift the direction of rotation of its drive shaft. The Eaton model employed in the prototype of the present invention provides for forward or rearward motion by shifting the rotation of drive shaft  72  from a clockwise revolution to a counterclockwise revolution, depending on the orientation of lever  142 . 
     Referring to FIGS. 2 and 11, handle linkage assembly  140  includes a handle  144  which enables the operator to pull up on either side of handle  144  activating the forward and rearward direction of the present invention in accordance with the foregoing description. Handle  144  is pivotably bolted at connection  146  to member  39  and to a transverse member  147 . Member  147  is pivotally connected to link  148 . Link  148  is in turn connected to a link  150  through a triangular plate  152 . Link  150  is pivotably connected to rod  153 , and rod  153  connects to lever  142 . 
     Referring to FIG. 2, the present invention also includes a mechanism to return handle  144  to a centered position as shown in FIGS. 2 and 11. That mechanism is shown in FIG. 2 as centralizer system  151 . Link  150  is also pivotally attached at connection  154  to member  155 . Member  155  is pivotally connected to a vertical member  157  (FIG. 3) at pin connection  159 . Vertical member  157  is fixed to first frame  22 . In this manner, member  155  pivots about connection  159  as handle  144  is pulled. Centralizing system  151  also includes a camming member  162  which is fixedly attached at connection  164  to vertical member  157 . Camming member  160  pivots at connection  164  and is restrained at its other end by spring  170 . In this manner, when either end of handle  144  is pulled and released, the notched portion  161  of camming member  162  serves to return member  155  to the vertical position as shown in FIG.  2 . This then serves to return link  150  to its neutral position and also handle  144  to the neutral position as shown in FIG.  11 . 
     In addition to the centralizer system  151  shown in FIG. 2, a tensioner system  163  is also shown in FIG. 2 which serves to ensure adequate tension is maintained on belt  66 . This is achieved through a pulley  168  which is supported by member  166 . Member  166  is in turn pivotally connected to vertical member  157 , and the other end of member  166  is forced in a downward position by compression spring  172 . Pulley  168  is urged against the top of belt  66  to ensure that belt  66  remains tight against both pulleys  68  and  70 . 
     The operation of transmission  64  to drive the present invention is as follows. When the operator pulls up or squeezes the right hand portion of handle  144  as shown in FIG. 11, links  148  and  150  are advanced forwardly. This in turn advances rod  153  and lever  142  forward. Advancement of lever  142  forward causes the rotation of drive shaft  72  (see FIG. 1) of the Eaton transmission selected to rotate in a clockwise direction. This in turn causes the clockwise rotation of sprocket  62  which advances the present invention forward. Similarly, when the operator pulls up on the left hand portion of handle  144  as shown in FIG. 11, links  148  and  150  and rod  153  are advanced rearwardly which in turn causes lever  142  to move to the right as shown in FIG.  2 . This causes the Eaton transmission to rotate drive shaft  72  in a counterclockwise direction providing for rearward motion of the present invention. Thus, whether the operator is pulling up on the left or right hand portion of handle  144  will determine whether the present invention moves in a forward or rearward direction. If the operator is not pulling up on either portion of handle  144 , the lever  142  remains in a neutral position as shown in FIG. 2 due to the centralized system  151  and sprocket  74  of transmission  64  does not rotate. 
     Referring now to FIGS. 7-10, alternate embodiments of the gripper assembly are shown. Gripper assembly  290  is intended to be used on a workpiece such as the tailwheel of an aircraft. In the case of an aircraft which has a tailwheel (also known as a “tailtragger”), there must be sufficient horizontal distance from the tailwheel of the aircraft to the end of its rudder to clear the towing apparatus. Thus, gripper assembly  290  includes longitudinal members  294  and  296 . Unlike the preferred embodiment of gripper assembly  90 , longitudinal members  294  and  296  do not pivot relative to one another. Rather, they are bolted to longitudinal members  30  of second frame  24  by bolts  295 . While lever arm  112  and rod  116  of locking assembly  110  are shown in FIG. 7, they are not used. Longitudinal members  294  and  296  are held fixed relative to one another by cross members  298 . A cradle  300  is provided having members  301 ,  302 ,  303 . Cradle  300  is used to support the tailwheel  292  of the aircraft, or similar workpiece. Members  294  and  296  include apertures  305 . Pins are provided at each end of member  302  and are adapted to fit within corresponding apertures  305  enabling the operator to select the size of opening  310  so as to accommodate a particular size tailwheel  292 . 
     Referring still to FIGS. 7 and 8, assembly  290  includes sled  312  having longitudinal members  314  adapted to slide relative to members  294  and  296 . Sled  312  supports a hydraulic jack  316  which is in fluid communication by hose  315  to a hydraulic ram  318 . Members  320  are provided which connect at one end  321  to a flange  322  of each member  314 . The other end of each member  320  is pivotally connected to a rotating arm  324 . A wheel  326  is attached to one end of each arm  324 . Each arm  324  is pivotally supported by a member  294  or  296  at pin connection  327 . 
     In the operation of this alternate embodiment, the operator releases all pressure from hydraulic jack  316  which permits sled  312  to slide to the left as shown in FIG.  7 . This results in the pivotal movement of wheel  326  about pin connection  327 , thereby lowering end  400  of gripper assembly  290  as seen in FIG.  9 . In this lowered position, the operator may advance the present invention under a tailwheel  292  into space  310  defined by cradle  300 . The operator then pumps handle  317  of jack  316  introducing hydraulic pressure into ram  318  and advancing piston  319  to the right as shown in FIG.  7 . Such movement of piston  319  to the right causes sled  312  also to move to the right. This causes the pivotal movement of wheels  326  in a clockwise direction about connection  327  until an elevated position is achieved as shown in FIG.  8 . The present invention may then be used to tow the aircraft as described below in more detail below. 
     Referring to FIG. 10, an alternate embodiment of gripper assembly  292  as seen in FIG. 7 is depicted. In this alternate embodiment, gripper assembly  292 ′ includes a mechanical linkage assembly  401  in place of hydraulic jack  316 /hydraulic ram  318  as shown in FIG.  7 . Linkage assembly  401  includes a handle  402  pivotally attached to sled  312  at pin connection  404 . Handle  402  is connected to member  406  at connection  407 . The other end of member  406  is attached to a cross member  298 . Thus, rather than pumping a handle  317  to displace a piston  319  and move sled  312  as discussed above with respect to FIGS. 7 and 8, the operator rotates handle  402  about pivot connection  404 . If handle  402  is in the position shown by solid lines in FIG. 10, the end  400 ′ of gripper assembly  292 ′ is in the position shown in FIG.  9 . Once the operator has positioned the present invention under a tailwheel within cradle opening  310 ′ as discussed above, the operator rotates handle  402  to the position shown by phantom lines in FIG.  10 . This causes the advancement of sled  312 ′ to the right as shown in FIG.  10  and elevates wheels  326 ′ to the position shown in FIG.  8 . The operator would then be in the position of moving the aircraft in accordance with the present invention as described below. 
     OPERATION OF THE PRESENT INVENTION 
     In the operation of the present invention, the operator starts motor  44 . The throttle would be placed initially in an idle position and handle  144  would be in a neutral position as seen in FIG.  11 . As noted above, the present invention provides that wheel assembly  50  is fixed relative to first frame  22 , but can rotate about a first axis  501  defined by axle  54 . Similarly, outer drum  80  rotates relative to inner drum  78  about a common substantially vertical axis  502  which, for purposes of FIG. 1, is shown as passing through the center of axle  54  of wheel  52  because wheel assembly  50  is positioned in the center of drums  78 / 80 . However, it is not essential that axis  502  intersect axle  54  for the successful operation of the present invention. 
     If gripper assembly  90  is used, the operator throttles up motor  44  using throttle  46 . If the operator wished to move the present invention in a forward direction, the operator would squeeze the right hand portion of handle  144  thereby advancing lever  142  forward. Since lever  142  is moved in a forward direction, drive shaft  72  rotates in a clockwise direction which therefore rotates sprocket  62  in a clockwise direction and moves the present invention forward. To move the present invention in reverse, the operator squeezes the left-hand portion of handle  144  which moves lever  142  rearward and causes the counterclockwise rotation of drive shaft  72 . 
     Preferably, bracket  84  remains in the locked position as shown in FIG. 1 while the present invention is being maneuvered about in a non-towing mode. However, when it is time to attach gripper assembly  90  to a workpiece, knob  88  is rotated upwardly pivoting bracket  84  about bolt connection  86  thereby permitting relative movement of the first frame  22  relative to the second frame  24 . 
     Referring to FIG. 5, if the preferred embodiment of gripper assembly  90  is used, the operator advances gripper assembly  90  to the position shown in FIG.  5  and engages hub  104  against axle hub  106  of nosewheel  92 , for example. At that point, the operator rotates lever  118  into a locked position as shown in FIG. 6 advancing hub  126  of second arm  96  against hub  130  of the nosewheel. In this position, the gripper is fully engaged and gas cylinder  132  provides additional load further securing arm  96  against hub  130 . With bracket  84  disengaged, the operator may easily rotate the first frame relative to second frame as shown in FIG.  6 . Since wheel  52  has a single point of contact against the ground, it is very easy to rotate the present invention about vertical axis  502 . In FIG. 6, first frame  22  has only been rotated about 45° relative to its original position; however, it will be understood by one skilled in the art that first frame  22  may be rotated virtually 360° relative to its original position or with reference to second frame  24 . The only obstruction that may limit its rotation is that handle assembly  140  may contact the workpiece. However, except for this limitation, first frame  22  may rotate 360° relative to second frame  24 . 
     Referring back to FIG. 6, once the operator has moved first frame  22  to a particular angle relative to second frame  24 , the left portion of handle  144  is pulled upwardly thereby advancing lever  142  to the right as shown in FIG. 2 which is the reverse mode. This means that drive shaft  72  would rotate in a counter clockwise direction which in turn rotates sprocket  62  in a counterclockwise direction and causes the present invention to move in a reverse direction, i.e., it is pulling the aircraft. Moving in a reverse direction enables the operator to advance the workpiece rearwardly and easily maneuver it. 
     Since the present invention is balanced, it is very easy to operate. As can be seen, the weight of transmission  64  largely balances the weight of the motor  44  about vertical axis  502 . In this manner, the present invention is very easy to handle. Additionally, since transmission  64  and motor  44  are close to the ground, the present invention is very stable. 
     It will be apparent to one skilled in the art that the present invention provides means to simultaneously power and rotate first frame  22  relative to second frame  24  without the need to lift any portion of wheel  52  off the ground. Additionally, the present invention provides a very stable design since it has a low center of gravity. Furthermore, the present invention provides for the towing of a workpiece along an axis  503  (occasionally referred to as a second axis) which is generally co-linear with the point-of-contact of the nosewheel of the workpiece, for example, and the point-of-contact of second frame  24  with drum  80 . The present invention provides for the placement of axis  503  proximate axis  501  of wheel  52 . By positioning the tow axis  503  proximate axis  501  of wheel  52  enhanced stability is achieved not found in the prior art. For example, in both U.S. Pat. Nos. 3,819,001 and 3,861,483, the prior art devices are unstable due to the significant vertical distance between the towing axis from the rotational axis of the wheel. This creates a large moment which therefore requires a stabilizing system as noted therein. 
     In the present invention, it has been found preferable to position axis  503  no more than between about 60% of the radius of wheel  52  above to about 60% of the radius of wheel  52  below axis  501 . This relationship is shown in FIG. 4 wherein “r” represents the radius of wheel  52  and α represents about 60% of “r”. As used herein, the term “wheel” includes the tire. More preferably, α is about 40% of “r”, and most preferably, α is about 20% of “r”. 
     Additionally, the present invention provides for a very short turning radius—the distance from the contact point of wheel  52  with the ground and the axle of nosewheel  92 . This is also a significant advantage because it permits very sharp turns. In fact, the operator could move first frame  22  90° to 160° off center and then move the aircraft very sharply. 
     In the use of the alternate gripper assemblies  290  and  290 ′, arms  94  and  96  of gripper assembly  90  are disconnected at bolts  99  as shown in FIG. 5, and arms  294  and  296  are connected to members  30  with bolts  295 . If the embodiment shown in FIG. 7 is used, the operator would release pressure within jack  316  allowing sled  312  to slide to the left as shown in FIG. 7 permitting wheels  326  to pivot about connection  327  as seen in FIG.  9 . This lowers end  400  of gripper assembly  290 . The operator would then advance the present invention by pulling on handle  144  to move the present invention in a forward direction. This would advance end  400  under a tailwheel until it rests within cradle opening  310 . At that point, the operator pumps handle  317  moving piston  319  to the right which causes sled  312  to move to the right. Wheels  326  then rotate about connection  327  until in an upright position as shown in FIG.  8 . At that point, the operator may move the present invention in a rearward direction as described above and easily maneuver the aircraft about the airfield or within a hanger or other confined space. 
     If gripper assembly  290 ′ is used as shown in FIG. 10, the operator would simply place handle  402  in the position shown by, solid lines. This lowers gripper assembly  290 ′ to the position shown in FIG.  9 . Once the tailwheel is positioned within cradle opening  310 ′, the operator moves handle  402  to the position shown by phantom lines in FIG.  10 . This elevates gripper assembly  290 ′ to the position shown in FIG.  8 . Once again, the operator is then free to operate the present invention as described above. 
     The foregoing invention has been described in terms of various embodiments. Modifications and alterations to these embodiments will be apparent to those skilled in the art in view of this disclosure. It is, therefore, intended that all such equivalent modifications and variations fall within the spirit and scope of invention as claimed.