Patent Publication Number: US-2012043419-A1

Title: Aircraft landing gear assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to an aircraft landing gear assembly and a method of protecting an aircraft landing gear. 
     BACKGROUND OF THE INVENTION 
     Aircraft landing gear can suffer impact damage and corrosion due to the presence of foreign objects on the runway such as, for example, runway de-icing materials. Airports use a variety of products for runway de-icing, but there are specific airports that use grit or sand in addition to chemical de-icers. Impact and corrosion damage can be particularly severe in these cases. 
     One known method of protecting the landing gear is to apply strips of impact resistant, durable, self-adhesive polyurethane film that affords a level of impact protection. This method is limited in that the adhesion between the polyurethane film and the structure is insufficient to resist the air drag forces when the gear is deployed. The film becomes detached and is lost and then has to be replaced/repaired. 
     Another known method of protecting the landing gear is to apply a sprayable elastomeric sealant that is subsequently protected with a layer of polyurethane topcoat spray applied over the sealant. The elastomeric sealant provides impact protection. However, because of the differences in the elastic properties of the sealant, and the coating, impacts on the landing gear can cause the protective topcoat to become detached, exposing the underlying sealant layer. As the sealant is not fully resistant to some of the fluids used on commercial aircraft (such as phosphate ester hydraulic fluids), the loss of the topcoat and exposure to these fluids can lead to a deterioration in sealant properties. In addition, the erosion resistance of the sealant is limited and therefore the protective sealant layer can be progressively lost, under adverse conditions, simply due to erosion. Replacement of this protection layer is also time consuming and expensive due to its complexity and the nature of the materials. 
     U.S. Pat. No. 2,378,046 describes a protection boot for a landing gear with a main shock absorber comprising a cylinder and a piston. The boot is fitted to the piston in order to prevent the accumulation of sand, dust and dirt from jamming the piston when it is retracted into the cylinder. The boot is formed from a pair of ring segments which are stitched together at one end and secured to each other at the other end by a single length of cord. Optionally clamps may be used to secure the upper and lower ends of the boot to the cylinder and piston respectively. One problem with this design is that the mounting and removal of the boot to and from the piston (for example, during maintenance) is difficult and time consuming. Another problem is that it can provide insufficient protection to the landing gear from impact and corrosion damage, particularly from the runway de-icing materials described above. Another problem is that the cylinder of the main shock absorber is not protected. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides an aircraft landing gear assembly comprising an impact protection boot mounted to a landing gear component, the impact protection boot comprising: a resilient inner impact energy absorption layer which engages the component; and an outer protective layer which substantially covers the impact energy absorption layer, wherein the inner layer is more compressible than the outer layer, and wherein the boot has first and second ends which are tied to each other by a connector which exerts oppositely directed forces on the first and second ends respectively, the forces being directed around the circumference of the boot so as to tighten the boot against the landing gear component. 
     A second aspect of the invention provides an impact protection boot comprising: a resilient inner impact energy absorption layer; an outer protective layer attached to the inner layer, the inner layer being more compressible than the outer layer; and a connector which is adapted to exert oppositely directed forces on first and second ends of the boot in order to tie them together, the forces being directed around the circumference of the boot so as to tighten the boot. 
     A third aspect of the invention provides a kit of parts comprising: an impact protection boot having a resilient inner impact energy absorption layer and an outer protective layer attached to the inner layer, the inner layer being more compressible than the outer layer; and one or more connectors, each connector comprising a strap which is adapted to exert oppositely directed forces on first and second ends of the boot, the forces being directed around the circumference of the boot so as to tighten the boot. 
     A further aspect of the invention provides a method of protecting an aircraft landing gear component from an impact with an impact protection boot comprising: a resilient inner impact energy absorption layer which engages the component; and an outer protective layer which substantially covers the impact energy absorption layer, the inner layer being more compressible than the outer layer, the method comprising: a) wrapping the boot around the landing gear component; b) securing first and second ends of the boot to each other with a connector which exerts oppositely directed forces on the first and second ends respectively, the forces being directed around the circumference of the boot so as to tighten the boot against the landing gear component; c) resiliently compressing the inner layer to absorb impact energy; and d) relaxing the inner layer so that it expands after step c). 
     Typically the connector comprises a strap and the oppositely directed forces are tension forces within the strap. The strap may comprises a first strap part which is coupled to the first end of the boot; and a second strap part which is coupled at one end to the second end of the boot and tied at the other end to the first strap part. Alternatively the strap may form a loop with its ends tied together. 
     Preferably the outer and/or inner layer of the boot is moulded to fit the shape of the component, in order to ensure an intimate fit. 
     The inner layer of the boot may be formed from a resilient foam comprising an array of closed cells. The foam may be formed from ethylene diene M-class (EPDM) rubber or any other suitable resilient material, typically an elastomer. 
     The outer layer may be formed from a thermoplastic or elastomeric material such as polytetrafluoroethylene (PTFE) or polyurethane (PU). 
     Typically the inner and outer layers are either bonded together by an adhesive layer, or co-cured. 
     The landing gear component may be the cylinder of a landing gear shock absorber, or any other landing gear component requiring protection. 
     Typically the first and second ends of the boot do not overlap. 
     Preferably the boot is not bonded to the landing gear component, although optionally an assembly aid such as a spot adhesive may be used to locate and secure the boot during installation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic front view of the lower part of a fully deployed aircraft landing gear, including a close up view of part of the impact protection boot; 
         FIG. 2  is a rear view of the cylinder and impact protection boot of  FIG. 1 ; 
         FIGS. 3 and 4  are plan and top elevation views respectively of a single-part strap attaching the two free ends of the impact protection boot together; 
         FIGS. 5 and 6  are plan and top elevation views respectively of an alternative two-part strap attaching the two free ends of the impact protection boot together; 
         FIG. 7  is a section view of a moulding apparatus moulding the outer layer of the impact protection boot; 
         FIG. 8  is a sectional view across the line A-A indicated on  FIG. 7 ; 
         FIG. 9  is a section view of the moulding apparatus adding the inner layer of the impact protection boot; and 
         FIG. 10  is a top elevation view of an alternative aircraft landing gear fitted with an alternative impact protection boot. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
       FIG. 1  is a schematic front elevation view of a fully deployed aircraft landing gear  1  comprising a shock absorber  2  which extends vertically between the underside of an aircraft fuselage and a wheel assembly  6 . The shock absorber  2  comprises a piston  8  and a cylinder  10 . The cylinder  10  extends vertically downwards from the fuselage when the landing gear is deployed, while the piston  8  extends vertically downwards from the cylinder  10  to the wheel assembly  6 . The wheel assembly  6  comprises a bogey  12  which extends horizontally between a pair of wheels  14 ,  16 . The piston  8  has a distal end which is attached to the bogey  12  at a central position along its length and a proximal end which is housed within the cylinder  10 . An impact protection boot  17  is mounted to the cylinder  10 . 
     When the aircraft lands, the piston  8  is retracted upwards into the cylinder  10  in order to absorb the landing forces exerted on the wheel assembly  6 . 
     The impact protection boot  17  is designed to protect the cylinder  10  from impact damage (from, for example, runway de-icing grit or sand thrown up by the wheels  14 ,  16 ) and to prevent it from coming into contact with corrosive substances (such as runway de-icing fluid). As shown most clearly in the close up view in  FIG. 1 , the boot  17  comprises an inner layer  20  which engages the cylinder  10  and an outer layer  22  which (at least substantially) covers the inner layer  20 . The inner layer  20  is formed from a resilient foam comprising an array of cells, which may be made from, for example, an elastomer such as ethylene diene M-class (EPDM) rubber or ethylene-propylene rubber. Preferably, the foam cells are closed in order to prevent the absorption of water, hydraulic fluids or other liquids by the foam. Closed cell foam is also more resilient than open cell foam. The outer layer  22  is formed from a hard wearing thermoplastic or elastomeric material such as PTFE or polyurethane. 
     When a foreign object impacts the outer layer  22  of the boot  17 , the resilient inner layer  20  is compressed under the force of the impact, thus absorbing the impact energy, before relaxing so that it expands again after the impact. The outer layer provides both the inner layer  20  and the cylinder  10  with chemical and environmental protection. As the purpose of the inner layer  20  is to absorb impact energy from foreign objects, it is intentionally designed to be more compressible than the outer layer  22 . 
       FIGS. 2-4  illustrate how the boot  17  is mounted to the cylinder  10 . Before being mounted to the cylinder  10 , the boot  17  has two free ends  30 ,  32 . A series of belt-loops  34 ,  36  is provided down the length of each end  30 ,  32 . To mount the boot  17  to the cylinder, it is wrapped around the cylinder  10  so that the two free ends  30 ,  32  oppose each other with a gap  33  between them as shown in  FIG. 2 . Alternatively, the free ends  30 ,  32  may overlap or abut each other exactly so that there is no gap  33  between them. The belt-loops  34 ,  36  are positioned so that, when the boot is wrapped around the cylinder  10 , the belt loops  34  on one end of the boot line up with the belt-loops  36  on the other end of the boot. A series of connectors is then fitted through opposing pairs of belt loops  34 ,  36  in order to secure the free ends  30 ,  32  of the boot together. Optionally, an assembly aid such as spot adhesive may be used to temporarily secure the boot directly to the cylinder at this point in the installation. 
     Any suitable connectors may be used to secure the free ends  30 ,  32  of the boot together. For example, as illustrated in  FIGS. 3 and 4 , a looped zip-tie  40  may be used. The zip-tie  40  comprises an elongate flexible strap  42  with first and second free ends  44 ,  46 . On the first end  44 , the zip-tie  40  has a series of resilient teeth (not shown) extending from its surface, while on the second end  46 , the zip-tie has a block  48  with a passageway (now shown). The zip-tie  40  is passed through an opposing pair of belt-loops  34 ,  36  and the first end  44  of the zip-tie is passed through the passageway in the block  48  to form a loop. Each of the resilient teeth has a chamfered leading edge and a trailing edge which is wider than the passageway in the block  48 . The interaction between the chamfered edges and the internal surface of the passageway causes the teeth to deform in order to pass through the passageway before expanding again. The perimeter of the passageway acts as a locking surface which engages the trailing edge of one of the teeth to prevent the withdrawal of the first end of the zip-tie from the passageway. The first end  44  of the zip-tie is pulled through the passageway and tightened so that it exerts oppositely directed tension forces on the first and second ends  30 ,  32  of the boot  17  via the belt-loops  34 ,  36 , the forces being directed towards each other around the circumference of the boot  17 . These tension forces tighten the boot against the cylinder  10  and ensure that a snug fit is achieved between the boot  17  and the cylinder  10 . 
     An alternative connector  60  is shown in  FIGS. 5 and 6 . In this case, the connector  60  comprises separate first and second strap parts  62 ,  64 . In a similar way to the zip-tie  40 , the first strap part  62  comprises an elongate strap with a distal end with a series of resilient teeth  65  extending from its surface, while the second part  64  comprises an elongate strap with a block  66  at its distal end. The teeth  65  on the first part  62  and the block  66  on the second part are identical to the teeth on the first end of the zip-tie  40  and the block  48  on the second end of the zip-tie respectively. Therefore, in order to secure the two parts  62 ,  64  of the connector  60  together, the first part  62  is passed through the passageway in the block  66  and tightened so that it exerts oppositely directed tension forces on the first and second ends  30 ,  32  of the boot as before. In this case, both straps are in tension. However, the separate first and second strap parts  62 ,  64  are only coupled together by their distal ends. The other ends of the strap parts  62 ,  64  have “crow&#39;s feet” stoppers  68  which engage the belt-loops  34 ,  36  to prevent the first and second strap parts  62 ,  64  of the connector  60  from being pulled through the belt-loops  34 ,  36 . 
     It will also be understood that any other interlocking mechanism may be employed between the first and second strap parts (or the first and second ends of the connector) such as laces or bindings, while hooks or eyelets may be employed in place of the belt-loops  34 ,  36 . It will be further understood that, rather than having straps which are formed separately from the boot  17 , the straps and the boot  17  may be integrally formed. For example, each of the two strap parts  62 ,  64  of connector  60  may be integrally formed with a respective free end  30 ,  32  of the boot  17 . In this case belt loops will not be required. 
     By attaching the boot  17  to the cylinder  10  by connectors rather than by, for example, adhesive, the boot can be fitted and removed much more easily. For example, the fit between the boot  17  and the cylinder  10  can be tightened by tightening the connectors rather than by first stretching the boot around the landing gear and using the connector to hold it in place, thus making installation easier. Moreover, the boot can be removed from the landing gear by cutting the connectors with a knife (or by disconnecting the connectors in any other way) and unwrapping the boot  17  from the cylinder. This is particularly advantageous, for example, when ultrasonic testing is required to be performed on the landing gear structure as the time taken by maintenance engineers to remove and re-mount (or replace) the boot is greatly reduced. 
       FIGS. 7-9  illustrate a method of manufacturing the boot  17 .  FIG. 7  is a transverse section view through a first substantially cylindrical inner mould tool  70  and a pair of arcuate outer mould tools  72 ,  74 . The inner mould tool  70  has a ridge  80  which projects from its outer surface  71  and extends along its length. The outer surface of the inner mould tool  70  is also provided with a series of bosses (two of which  76 ,  78  are shown in  FIG. 7 ) which are positioned at regular intervals along the length of the mould tool  70  on either side of the ridge  80 . The outer mould tools  72 ,  74  are provided with a corresponding series of recesses (two of which  82 ,  84  are shown in  FIG. 7 ) in their inner surfaces  73 ,  75 . The recesses  82 ,  84  are aligned with the bosses  76 ,  78  such that the bosses  76 ,  78  partially fill the recesses  82 ,  84 . This is illustrated most clearly in  FIG. 8 , which is a cross section along line A-A indicated in  FIG. 7 . As shown in  FIG. 8 , the recesses  82 ,  84  are wider than the bosses  76 ,  78 . Moreover, the outer surfaces of the bosses  76 ,  78  are prevented from engaging with the inner surfaces of the recesses  82 ,  84  by the interaction between the outer surface of the ridge  80  and the inner surfaces  73 ,  75  of the outer mould tools  70 ,  72 . 
     To form the outer protective layer  22  of the boot  17 , a sheet of partially cured polyurethane  90  is wrapped most of the way around the inner mould tool  70 , terminating either side of the ridge  80 . The outer mould tools  72 ,  74  are then positioned around the polyurethane layer such that the recesses  82 ,  84  are aligned with the bosses  76 ,  78  on the outer surface of the inner mould tool  70  as described above. A force is applied to the outer mould tools  72 ,  74  towards the inner mould tool  70  so that the bosses  76 ,  78  press the polyurethane into the recesses  82 ,  84  in order to form the belt-loops  34 ,  36 . The polyurethane sheet is thus shaped by the outer surface of the cylindrical inner mould tool  70  and the inner surfaces  73 ,  75  of the arcuate outer mould tools  72 ,  74 . 
     A thin gap  77  is present between the sides of the bosses  76 ,  78  and the sides of the recesses  82 ,  84 . A thin layer of polyurethane fills this gap  77  and is punched out after the moulding process with a tool. 
     To create the inner impact protection layer  20  of the boot  17 , the inner mould tool  70  is removed from the apparatus illustrated in  FIG. 7 . The inner mould tool  70  is preferably made from wax, or a similar material with a relatively low melting point, so that it can be removed simply by heating it so that it melts away. As shown in  FIG. 9 , the first inner mould tool  70  is replaced with a second inner mould tool  92 , which is also provided with a ridge  94  projecting from its outer surface and extending along its length. The second inner mould tool  92  has a smaller outer diameter than the first inner mould tool  70  and is aligned with the polyurethane layer  90  such that the ridge  94  extends through the gap between the free ends of the polyurethane layer  90  and contacts the inner surfaces of the outer mould tools  72 ,  74 . Liquid EPDM  96  is then injected into the region between the outer surface of the second inner mould tool  92  and the inner surface of the polyurethane layer  90 . The EPDM  96  and the polyurethane layer  90  are then co-cured to create an integrally formed two-layer boot  17 . During the cure process the liquid EPDM becomes a foam, and so called “skinning” will occur which will cause the foam to form inner and outer skin layers, making the foam completely closed-cell. No external forces are applied between the outer mould tools  72 ,  74  and the second inner mould tool  92  so that the foam is not compressed during cure. This maximises the foam&#39;s compressibility after manufacture. 
     It will be understood that the inner foam layer  20  may be formed separately from the outer polyurethane layer  22 . In this case, the two separately formed layers  20 ,  22  may be adhesively bonded to each other before being mounted to the landing gear cylinder  10 . 
     As illustrated in  FIG. 10 , the outer surface of the cylinder  10  may be provided with a series of reinforcing webs  100 - 104 . In this case, at least the outer layer  22  of the boot  17  is moulded to the shape of the outer surface of the cylinder  10  (including the webs  100 - 104 ) in order to achieve an intimate fit between them. This may be done by using the same manufacturing process as that described above, but replacing the first inner mould tool  70  with a mould tool which conforms to the shape of the cylinder  10  including the webs  100 - 104 . Optionally the inner layer  20  may also be moulded to fit the shape of the cylinder  10 , including the webs  100 - 104 , in a similar way (i.e. by substituting the second inner mould tool  92  with a mould tool which conforms to the shape of the cylinder  10  including the webs  100 - 104 ). 
     As well as being provided with reinforcing webs  100 - 104 , the cylinder  10  may also have other objects such as cables and other systems equipment on its outer surface. In this case, the shape of the outer layer  20  (and optionally the inner layer  22 ) of the boot  17  may be moulded to take account of these other objects. 
     Any of the embodiments described may be provided as a kit of parts comprising the boot and the connector straps, which can then be assembled in situ. 
     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.