Abstract:
A support structure for an aircraft seat includes load bearing spaced side members, each having elements for securing a seat for continuous movement between first and second positions, and a ground engaging portion. The structure further includes a stiffening web extending between the side members. The side members and the stiffening web substantially define a hollow box section of at least four sides to stiffen the side members, at least in the regions beneath the element securing the seat. The support structure allows other material than the conventional steel or aluminum constructions to be used for the support frame. For example, substantially all off the structural strength of the support may be provided by fiber reinforced composite materials.

Description:
BACKGROUND 
     This invention relates to aircraft seating and seating arrangements. The invention is particularly, but not exclusively, applicable to seating for commercial aircraft. 
     It is becoming increasingly necessary for airlines to install a seat in a commercial aircraft that converts into a bed, at least in first class on long haul flights. The conflicting commercial considerations are the provision of a good service, on the one hand, and the pressure to maintain cabin seating density and weight considerations, on the other. Thus, it has become the goal of the seat designer to make as much use of as little space and weight as possible while providing the necessary level of space and comfort expected in first and business class. 
     WO 03/013903 discloses a seat unit for an aircraft that can be converted into a bed. The seat unit comprises a supporting structure for attaching a seat to the floor of the aircraft. The supporting structure comprises a load bearing aerospace-grade steel subframe which is clad with one or more shaped composite panels. The steel subframe provides a structurally strong point of attachment for the passenger seat and other components of the seat unit. 
     The use of a steel subframe in constructing sleeper/seat units for aircraft can be understood from the need to comply with stringent safety regulations aimed at ensuring that a seat unit can withstand the foreseeable loads and stresses which may arise during a crash of the aircraft. Due to these stringent safety regulations, there is thus an apparent prejudice in the field of aircraft seat construction against using other materials for the structural component of a seat. 
     BRIEF SUMMARY 
     Embodiments of the invention are based on the realisation that structural frames having sufficient structural performance to comply with the stringent aviation safety regulations can be obtained by careful design of a structure. Using composite materials for the structural, load bearing, frame or support structure of the aircraft seat results in significant weight savings as compared with prior metal subframe designs. Any reference to composite or fibre-reinforced composite materials herein is understood to refer to any material comprising a reinforcing material such as carbon or glass fibres embedded in a matrix of a secondary material, usually a polymer. The reference to composite or fibre-reinforced composite materials also includes materials which are laminates of layers of composite materials and/or core materials such as a honeycomb material, for example, an aluminium or plastics honeycomb core. 
     In one embodiment the structural frame comprises side members spaced apart by a cross member which is designed to distribute any load or shear forces over a relatively large area at the interface between the side members and the cross members. Additional structural stability is achieved by integrally including additional elements forming a backshell member within the structural frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be put into practice in various ways, some of which will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of an embodiment of a seat for an aircraft; 
         FIG. 2  is an exploded perspective view of a structural frame of the seat in  FIG. 1 ; and 
         FIGS. 3A  and B are perspective views of the assembled structural frame of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a seat unit for an aircraft cabin comprises a back shell  10  and load bearing side frame members  12  and  14 , each defining an integral arm  16 . In this embodiment, the side members and backshell each comprise decorative cladding  18  that covers the rigid structural frame members  12  and  14  that support the constituent parts of a seat  20 . The parts of the seat are a seat base  22 , a leg rest  24  and a back rest  26 . The seat base  22 , in this embodiment, is arranged to slide longitudinally and/or to be adjustable for seat base angle, according to passenger preference, by means of slots formed in each of the frame members  12 / 14  in which the parts of the seat ride. Likewise, the seat back rest  26  is adjustable as part of a seat configuration. 
     In this case, conventional manual or electrically motorised slide mechanisms can be used, as are known from conventional seats for this type of application. The leg rest  24  is essentially conventional in function, being either manually, pneumatically or electrically deployable. It is pivotably mounted at a point  28  at or near the front of the seat base  20 . 
     It will be appreciated that the components of the seat unit are covered in suitable cushioning and material in appropriate areas, such as the arms, seat base and seat back. These are omitted from the drawings showing the component parts for the sake of clarity. 
     The seat back  26  is mounted to pivot at a point  29  about or near its junction with the seat base  22  to fold forward on top of the seat base  22 . As with the seat base, the seat back  26  is mounted for slidable movement between the two side frame members  12  and  14 . The leg rest  24  is mounted on the front seat base  22 , as described. The side frame members  12  and  14  are spaced by one, or more than one, cross member both to provide rigidity for the structure and to mount components such as the actuating mechanisms for movement of the seat components that have to be supported between the side frames  12  and  14 . 
     The structural frame that supports the seat of  FIG. 1  is now described with reference to  FIGS. 2 and 3A  and B, whereby the direction which is substantially perpendicular to the aircraft floor on which the seat rests is referred to as vertical, the direction along arm rest  16  is referred to as the longitudinal direction and the direction across seat base  20 , perpendicular to the vertical and lateral direction is referred to as longitudinal. The notional plane of the aircraft floor or ground is referred to as horizontal. 
     The structural frame comprises load bearing side members  30  and  40 , each having a slot  32 ,  42  which is situated approximately halfway between the bottom  34 ,  44  and the top  36 ,  46  (acting as a base for arm rest  16 ) of each side member. Slots  32  and  42  are arranged, when the structural frame is assembled, to act as a guide and locator in which the parts of the seat ride. It is understood that slots  32  and  42  can be replaced by any other suitable means for securing a seat for continuous movement between a first and second position, for example a ledge. Side members  30 ,  40  further comprise protrusions forming horizontal ledges  38 , 48  extending transversally which provide a connecting surface for bonding to a cross member  50 , which spaces side members  30  and  40 . 
     The cross member  50  comprises a plate extending transversely between side member  30  and  40  and defines an S-shaped profile in the longitudinal/vertical plane. The cross member  50  thus comprises two surfaces  51  and  52  respectively extending vertically in the downwards direction and upwardly at an intermediate angle, and a horizontal surface  53  extending laterally and longitudinally and generally parallel beneath the seat base  20 . 
     The side and cross members ( 30 ,  40 ,  50 ) hence define a hollow box section of four sides between them. It is understood that the box section may have more than four sides (e.g. being closed at the bottom) and that the angles between the sides are not limited to be rectangular and may be rounded over a region of the box section. 
     Cross member  50  further comprises vertical surfaces  54  situated at the lateral edges. The vertical surfaces  54  provide a further surface for bonding cross member  50  to the ledges  38 ,  48  on the side members  30  and  40 . A further front lip  58  is formed at the lower end of cross member  50  which, in-cooperation with lower ends  34  and  44  of side members  30  and  40 , forms part of the overall surface of the structural frame that rests on the aircraft floor when the seat is installed. The lip  58  comprises a downwardly facing recess  59  for accepting a track fixing interface or ground engaging member  90  for fixing the seat to the aircraft floor. Member  90  comprises a first set of holes  92  for bolting it to corresponding holes  96  in side members  30  and  40  and a second set of holes  94  for bolting it to the aircraft floor. A further track fixing member  100  is situated towards the rear of the seat assembly. It also comprises a first set of holes  102  for bolting member  100  to corresponding holes  104  towards the rear in side members  30  and  40  and a second set of holes for fixing to the aircraft floor (not shown). 
     Side members  30  and  40  comprise a further rear horizontal surface  39 / 49  at the rear end of each of the ledges  30 ,  40 , which act as an interface for bonding backshell  10  to the structural frame assembly. It will be seen from  FIG. 2  that the backshell  10  is made up of a back part  60  and two sides  70  and  80 . 
     The corresponding bonding surfaces  72  and  82  on the backshell  10  are formed on the respective sides  70  and  80 . Sides  70  and  80  are bonded to back wall  60  along their lateral edges  74 ,  84  and  62 , respectively. In addition, the front edge  64  of the back wall  60  is bonded to the rear edge  57  of cross member  50 . Integrally bonding the backshell to the structural frame further contributes to the structural stability of the frame. 
     Considering side members  30  and  40  and cross member  50  in isolation, a structural frame or support structure for supporting the passenger seat is formed, which supports the passenger seat above a substantially cuboidal structural base defined on four sides by the side members and the cross member and by the floor to which the structure is secured. In this arrangement, the bonding surfaces  54 ,  38  and  48  form an interface with projections covering substantially all of the longitudinal and vertical extent of side members  30  and  40  underneath the passenger seat (apart from cutout  56  in horizontal surface  53 ). This extended, elongate interface between side members  30  and  40  and cross member  50  distributes any load onto a much larger surface than, for example, the relatively small interaction surfaces of conventional bolted cross members. Such a structure which is designed to distribute forces allows the use of a much larger range of materials in the construction of the structural frame than previously possible. In particular, fibre-reinforced composite materials can be employed rather than the conventional aerospace grade steel or aluminium machined, pressed or cast members. 
     The fact that the nature of the interface between the members of the structural frame, described above, distributes load and stress forces evenly across the members compensates for the relatively lower yield stress of, for example, fibre-reinforced composite laminates perpendicular to the plane of the fibres in order to exploit the relatively high yield stress these materials exhibit within this plane. A much lighter structural frame than conventional steel or aluminium frames can thus be constructed by using materials which have high yield stress in the dimensions where it is crucial by compensating for the relatively lower yield stress in other dimensions through design of the frame structure to produce a rigid shape. 
     The structural frame of the specific embodiment described above with reference to  FIGS. 2 and 3A  and B is constructed from shaped reinforced fibre composite and honeycomb laminates. Any reinforced fibre composite material may be used, but carbon fibre based composites with a Phenolic Resin matrix have been found to be advantageous in terms of weight saving. Other fibres such as glass, polyamide (e.g. Kevlar, registered trade mark) may also be advantageous. The use of epoxy or polyester matrices is equally envisaged. Any suitable honeycomb may be used to form the laminate, but the specific embodiment uses an aluminium honeycomb core (Aeroweb produced by Cyba-Geigy, registered trade marks). 
     The laminate parts are bonded together using methacrylate. 
     In the specific embodiment described above, the backshell  10  is constructed from a laminate of aluminium honeycomb sandwiched between three layers of biaxial carbon fibre composites with a resulting thickness of under 14 mm in this embodiment. The structural frame comprising the side members  30 ,  40  and cross member  50  is constructed from a laminate of aluminium honeycomb sandwiched between 5 layers of biaxial carbon fibre composite on each side, resulting in a thickness of less than 15 mms in this embodiment. The thicker of the two materials has a density of just under 400 kg/m 3 , which is substantially less than the density of corresponding aluminium parts which would be in the region of 3000 kg/m 2  or even higher for steel parts. Track fixing members  90  and  100  are manufactured as aluminium extrusions having a density of 2800 kg/m 2 , but do not contribute significantly to the overall weight due to the small volumes involved. The structure described above thus results in significant weight savings as compared with traditional steel or aluminium subframe constructions. 
     The decorative cladding  18  shown in  FIG. 1  may also contribute to the structural stability of the frame. To exploit this, the cladding may be made from structurally sufficiently strong material to add to the overall stability. For example, the cladding may be of the same material as the other parts and may be bonded thereto using a structural adhesive (although any other means of securing the cladding is also envisaged). Thus, a double-skinned construction with added stability is achieved. 
       FIG. 3B  shows the surfaces of the structural frame to which the cladding may be attached, denoted by  41  for the side members and  71  for the backshell. A cavity  43  is thus defined between the respective cladding and the side member  40  and a cavity  73  is defined between the respective cladding and the backshell  70 . 
     The cavities  73  and  43  can be used to stow and conceal auxiliary equipment and/or accessories of the seat. For example, a suitable video screen such as an LED screen, and/or reading or ambient lights may be installed in cavity  73  (in combination with suitable cut-outs in the backshell  10  or cladding). 
     Similarly, any electrical circuitry or circuit board can be stowed in the cavity so that it is concealed, and is not exposed to damage. 
     It is understood that features of the specific embodiment described above may be altered, omitted or juxtaposed without departing from the scope of the intention. For example, the structural frame may be formed as a single laminate structure or the adhesive bonds between laminate parts may be replaced by other securing means. 
     The embodiment discussed above describes a frame structure for an aircraft seat. It will be apparent to the skilled person that such a frame structure can be employed to construct an aircraft seating unit by providing a passenger seat and other fittings. The specific embodiment described above is meant to illustrate, by way of example only, the invention, which is defined by the wording of the claims set out below.