Abstract:
A method of applying a test load to a landing gear mounted on an aircraft, the method comprising parking the aircraft with at least one tire of the landing gear on a platform; and moving the platform so as to apply the test load to the landing gear via the tire. An array of six platforms is mounted on a sliding chassis in a recessed oil bath. The platforms can be independently rotated to apply torque. The spacing between the platforms can be adjusted to adapt for different landing gear configurations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and apparatus for applying a test load to a landing gear mounted on an aircraft. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional methods of applying a test load to a landing gear of an aircraft involve placing the whole landing gear, or a component thereof, in a test rig. 
         [0003]    Such test rigs can be complex and can fail to accurately re-create the conditions of a landing gear in service on an aircraft. 
       SUMMARY OF THE INVENTION 
       [0004]    A first aspect of the invention provides a method of applying a test load to a landing gear mounted on an aircraft, the method comprising parking the aircraft with at least one tire of the landing gear on a platform; and moving the platform so as to apply the test load to the landing gear via the tire. 
         [0005]    A second aspect of the invention provides apparatus for applying a test load to a landing gear mounted on an aircraft, the apparatus comprising one or more platforms for engaging one or more tires of the landing gear, and one or more platform actuators for moving the platform(s) so as to apply the test load to the landing gear via the tire. 
         [0006]    The invention provides a method (and associated apparatus) for applying a test load to a landing gear which is mounted on an aircraft, thus re-creating the service conditions of the landing gear, including the tire. 
         [0007]    The platform is typically held substantially stationary after it has been moved, so as to apply a substantially static load to the landing gear. 
         [0008]    Preferably the movement of the platform includes at least a component of rotation about an axis substantially normal to the platform (typically a vertical axis), so as to apply an element of torque to the tire. This enables the platform to simulate certain loading situations such as self-aligning torque. A component of linear load may also be applied to the tire, in horizontal and/or vertical directions. 
         [0009]    All tires of the landing gear may be parked on a single platform, but preferably the method comprises parking two or more tires of the landing gear on different platforms; and moving the platforms relative to each other. For instance the platforms may be moved in different directions, or may be moved in the same direction but at different rates. 
         [0010]    Where more than one platform is provided, then a gap between the platforms is typically filled with a bridge which can be removed after the tires have been rolled onto the platforms. 
         [0011]    Load in the platform (or platforms) and in the landing gear is typically measured and recorded during the application of load to the landing gear. 
         [0012]    Typically the platform (or platforms) is moved in accordance with a pre-stored loading sequence. 
         [0013]    Typically the platform is driven by a platform actuator which is housed in a recess below a support surface. 
         [0014]    In a preferred embodiment the platforms can be moved relative to each other when no aircraft is parked on the platforms, so as to vary the spacing between the platforms. Also one or more of the platforms can be removed so as to vary the number of platforms. This modular arrangement enables the pitch between platforms and/or the number of platforms to be adjusted to accommodate different landing gears. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a plan view of a calibration fixture with the covers closed; 
           [0017]      FIG. 2  is a plan view of the calibration fixture with the covers open; 
           [0018]      FIG. 3  is an isometric view of the calibration fixture with two of the covers open, and two of the covers not shown; 
           [0019]      FIG. 4   a  is a schematic cross-section through the chassis  FIG. 4   b  is a view of the underside of the chassis; 
           [0020]      FIG. 5  is a cross-sectional view taken from a side of the calibration fixture, showing a landing gear being parked onto the fixture; 
           [0021]      FIG. 6  is a schematic plan view of one of the pads showing the pad actuators; 
           [0022]      FIG. 7  is a cross-sectional view taken from the front of the calibration fixture, with a landing gear parked on the fixture; 
           [0023]      FIG. 8  is a schematic view of the electronic control system of the fixture; and 
           [0024]      FIG. 9  shows a loading example simulating a left hand turn. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]    The calibration fixture shown in the figures comprises an oil bath  1  which is recessed below a support surface  2  shown in  FIGS. 5 and 7 , and has front and rear walls  3 , 4  and left and right side walls  5 , 6  shown most clearly in  FIG. 2 . The oil bath  1  communicates with a sump  7  via a channel  8 . The fixture may be installed inside a hangar (in which case the support surface  2  is the floor of the hangar) or may be installed in an external testing area of an airfield. 
         [0026]    Housed in the oil bath is a chassis  10  (shown in dotted lines in  FIGS. 2 and 3 ) and an array of six pads  11  mounted on the chassis. The pads  11  are modular items which can be moved in relation to the chassis, in order to vary the spacing between the pads and adapt the fixture for landing gears with different spacing between the wheels. The pads can also be removed in order to adapt the fixture for landing gears with different numbers of wheels. This modular construction can be achieved in a number of ways. In this case the upper surface of the chassis has a square array of closely spaced holes  26  shown in  FIG. 4   a , and the lower surface of the pads has pegs (not shown) which push-fit into the holes  26 . 
         [0027]    The chassis  10  has three lugs  15  arranged along its right edge, and two lugs  16  along its lower edge. The lugs  15  are attached to piston rods  17  of Y-load application actuators  18 , and the lugs  16  are attached to piston rods  19  of X-load application actuators  20 . The X and Y-load application actuators can each be driven independently to generate a desired translational and/or rotational movement of the chassis  10 . 
         [0028]    The chassis  10  is mounted on a hydrostatic film bearing shown in  FIGS. 4   a  and  4   b . Oil is pumped from the sump  7  by a pump (not shown) through an umbilical  27  (not shown in  FIGS. 1-3 ). As shown in  FIG. 4   a , the chassis  10  has an upper plate carrying the holes  26 , and a lower plate carrying an array of equally spaced channels  25 , each channel  25  being indicated by a cross in the underside view of  FIG. 4   b . Oil from the umbilical  27  is distributed to an array of hoses  28  by a manifold  29 , each hose  28  communicating with a respective one of the channels  25  in the lower plate Thus oil is continuously pumped through the channels  27  to form a low friction hydrostatic film bearing. 
         [0029]    Referring to  FIG. 5 , each pad  11  comprises a wheel platform  30  mounted on a pedestal  31  via a thrust bearing (not shown). The platform  30  is rotated about a vertical axis  32  through the centre of the pad by a torque actuation mechanism shown schematically in  FIG. 6 . The torque actuation mechanism comprises a pair of linear actuators  33  which are located on opposite sides of the axis  32  and configured to apply load to the platform in opposite directions. Each actuator  33  comprises a hydraulic cylinder  34  with a pivot  35  attached to the pedestal  31 , and a piston rod  36  with a pivot  37  at its end attached to the underside of the wheel platform  30 . These actuators provide plus or minus 15 degree movement, as shown in  FIG. 6 . 
         [0030]    Referring back to  FIGS. 1 and 2 , the calibration fixture comprises front and rear covers  40 , 41  and left and right covers  42 , 43 . Each cover is attached to a pair of cover rods  45  which are driven by cover actuator pistons  46 , shown most clearly in  FIG. 1 . Note that the cover rods and pistons for the front, right and rear covers are omitted from  FIG. 1  for clarity, and the rear and right covers are omitted from  FIG. 3  for clarity. 
         [0031]      FIG. 5  shows the rear cover  41  in detail. The front cover  40  is identical. The rear cover  41  comprises a plate  80  carrying a set of wheels  81 . The chassis  10  has a support wall  82 , and when the cover is in its closed position (shown in solid lines in  FIG. 5 ) the underside of the plate  80  is supported by the wall  82 , and the front edge of the plate abuts the pad  30 . Thus the cover  41  can support the weight of the wheels rolling over it during parking. When the aircraft has been parked, the cover  41  is retracted to its open position shown in dashed lines in  FIG. 5 . The wheels  81  run on tracks (not shown) which are recessed below the support surface  2  and have an angled ramp so that as the cover rods  45  are retracted, the wheels  81  roll up the ramp to the open position. 
         [0032]      FIG. 7  shows the left cover  42  in detail. The right cover  43  is identical. The left cover  42  comprises a pair of plates  90 , 91  carrying a set of wheels  92 , 93 . The chassis  10  has a support wall  94 , and when the cover is in its closed position the underside of the plate  91  is supported by the wall  94 , and the front edge of the plate abuts the pad  30 . Thus the cover  42  can support the weight of a left-hand wheel if the wheel is misaligned and rolls over it during parking. When the aircraft has been parked, the cover  42  is retracted to its open position shown in dashed lines in  FIG. 7  in a similar manner to the front and rear covers. That is, the wheels  92 , 93  run on tracks (not shown) which are recessed below the support surface  22  and have an angled ramp so that as the cover rods  45  are retracted, the wheels roll up the ramp to the open position. 
         [0033]    If clearance is required between the cover  42  and an adjacent landing gear (not shown) of the aircraft, then the cover  42  can be folded as shown in  FIG. 7 . That is, the plates  90 , 91  are joined by a hinge  95 , and before the cover is retracted to its open position, the plate  90  is rotated about the hinge to the folded position before the cover  42  is retracted to its folded open position (shown in solid lines in  FIG. 7 ). 
         [0034]    The gaps between the pads  11  are filled by removable axial blocks  50  and removable transverse blocks  51  shown in  FIG. 1 . Before an aircraft is parked on a fixture, the covers are closed and the blocks are inserted as shown in  FIG. 1 . The landing gear is then parked on the fixture as shown in  FIG. 5 .  FIG. 5  is a view from a side of the fixture and shows three wheels  60 - 62  (with tires) of a six wheel landing gear mounted on an aircraft (not shown). The front wheel  60  rolls over the rear cover  41 , two pads  11  and two transverse blocks  51  before parking on the front pad. Similarly, the wheel  61  rolls over the rear cover  41 , the rear pad and one transverse block  51  before parking on the middle pad. Similarly, the rear wheel  62  rolls over the rear cover  41  before parking on the rear pad. The axial blocks  50  shown in  FIG. 7  will not be used if the landing gear is correctly aligned: they are merely provided to bridge the gap between the pads in the event that the landing gear is not lined up correctly as it is parked on the fixture. After the landing gear has been parked as shown, the covers are retracted to their open positions shown in  FIG. 2 , and the blocks  50 , 51  are removed. 
         [0035]    In an alternative embodiment (not shown) the two transverse blocks  51  can be replaced by four smaller transverse blocks, each lying in the transverse gap between a pair of pads and having a width equal to a width of one of the pads. The three axial blocks  51  are replaced by a single axial block running along the full length of the axial gap between the pads. 
         [0036]      FIG. 8  is a schematic view of the electronic system of the fixture coupled with a flight test installation (FTI) system on the aircraft. After the landing gear has been parked on the fixture, the electronic system of the fixture is coupled to the FTI bus shown in  FIG. 8 , and a processor  70  onboard the aircraft sends an electronic signal to the cover actuators  46  to retract the covers. On request from a keyboard  71 , the processor  70  retrieves a loading sequence from a memory  72 , which may be housed on the ground or on the aircraft. The processor  70  then sends electronic control signals to the X and Y-load actuators  18 ,  20  and the pad torque actuators  33  in accordance with the loading sequence. The various actuators can all be driven independently to enable any chosen combination of loads to be applied. The X and Y-load actuators  18 , 20  each have strain gauges  53  (shown in  FIG. 3 ) and each linear pad actuator  33  has a strain gauge (not shown), the collection of strain gauges being indicated schematically at  73  in  FIG. 8 . Instead of using strain gauges, the load applied by the fixture may alternatively be measured by measuring pressure in the various hydraulic actuators. Similarly, the landing gear has a set of strain gauges indicated schematically at  74  in  FIG. 8 . The readings from the strain gauges  73 , 74  are stored in the memory  72  during the loading sequence. The strain gauge readings can then be used by the processor  70  to calculate the coefficients of a transfer function linking the load applied by the fixture with the load measured by the landing gear strain gauges  74 , and to verify a mathematical model used to design the landing gear. Appropriate reports can be generated and output on a display  75 . These reports form part of the aircraft certification requirements of the aircraft manufacturer. 
         [0037]    A first loading example is illustrated in  FIG. 9  The pads and chassis are shown in their unloaded state in dashed lines. From the unloaded state, the X and Y-load actuators  18 ,  20  and the pad torque actuators  33  gradually extend or contract to move the chassis and pads towards the loaded position shown in dashed lines. This is achieved by applying preset loads to the Y-load actuators, whilst extending the X-load actuators to the position shown without applying any load. The preset loads are measured by strain gauges  53  on the end of the piston rods  17 , 19  shown in  FIG. 3 . Note that the amount of translation and rotation are exaggerated in  FIG. 9  for illustrative purposes. 
         [0038]    The loading example of  FIG. 9  simulates a slow left-hand turn. As a result the chassis translates to the right and rotates about its centre (illustrated by a circle  28 ) in a clockwise direction. Self-aligning torque is simulated by applying clockwise torque to the front four wheels, and anticlockwise torque to the pair of rear wheels. Thus for each pad there is a component of horizontal (Y) translation so as to apply an element of linear horizontal load to the tire, and a component of rotation about a vertical (Z) axis so as to apply an element of torque to the tire. In the loading example of  FIG. 9  the pads are all moved together by the chassis, as well as being moved relative to each other by their respective independently controlled torque actuation mechanisms. 
         [0039]    In a second loading example (not shown) simulating a high speed turn, the Y-load actuators are all compressed together to apply equal amounts of linear load to the right, and an equal amount of clockwise torque is applied by each torque actuator. 
         [0040]    In a third loading example (not shown) simulating a braking load, the X-load actuators are all expanded together to apply equal amounts of linear load towards the rear. 
         [0041]    In a fourth loading example (not shown) simulating a slow left hand turn with pivot braking, the actuators are controlled as in  FIG. 9  but with an additional component of horizontal (X) translation in the aft direction applied by the X-load actuators. 
         [0042]    A loading sequence comprises a series of applications of a number of different loading examples, of the type described above. The load sequences apply static loads: that is, for each loading example in the loading sequence the load is gradually applied, held at a preset level for some preset time, then gradually released. 
         [0043]    After the loading sequence, the blocks are placed between the pads, and the covers closed. The aircraft can then be driven off the fixture (either forwards or backwards) and manoeuvred to park another one of the landing gears onto the same fixture. The compact form of the fixture, with all of the components recessed below the support surface  2 , enables the aircraft to be manoeuvred without being obstructed by actuation cables or other equipment lying on the support surface  2 . 
         [0044]    In the embodiment described above, the left and right covers  42 , 43  are designed with load-bearing capability so that they act as bridges if the landing gear is misaligned during parking. In an alternative embodiment (not shown) the covers may be designed without such load-bearing capability, for instance they may be formed from flexible material. In this case, the covers merely act to prevent the ingress of dust and other debris into the oil bath. Alternatively the left and right covers may be omitted entirely. 
         [0045]    In the embodiments described above, the covers  40 , 41 , 42 , 43  are opened and closed by electrically controlled hydraulic actuators, and slide in and out on tracks. In an alternative embodiment the covers may be replaced by blocks (similar to the blocks  50 , 51  between the pads) which are lifted out by a small crane after the aircraft has been parked on the fixture. Blocks of different sizes can be used to accommodate changes in the pitch between the pads and/or changes in the number of pads. 
         [0046]    The embodiments described above can translate and/or rotate the chassis whilst independently applying torque to selected pads. In an alternative embodiment (not shown) a single screw jack, scissor actuator or hydraulic actuator could be fitted between the lower plate of the chassis and the upper plate of the chassis, in order to move the pads up and down and thus apply a linear vertical (Z) load to the pads. Alternatively each pad may be fitted with a respective Z-load actuator so that the wheel platforms  30  can be driven up and down independently. That is, a Z-load actuator (for instance a screw jack, scissor actuator or hydraulic actuator) is fitted between each pedestal  31  and each thrust bearing, or between each thrust bearing and each platform  30 . 
         [0047]    The embodiments described above have a single pad per wheel. In an alternative embodiment (not shown) each pad may be sized and positioned to support more than one wheel. 
         [0048]    The embodiments described above employ hydraulic actuators to drive the covers, chassis and pads. However in alternative embodiments (not shown) some or all of the actuators may be replaced by pneumatic, electric or magnetic actuators, or any other type of actuator which can be controlled remotely by an electronic control signal from a computer. 
         [0049]    Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.