Abstract:
A self-transporting support arrangement particularly useful as a helicopter undercarriage is disclosed. Ground-engaging members, e.g. skids, have associated shoes, and horizontal and vertical force is applied simultaneously between them by a plurality of inflatable bags located between opposed inclined planar surfaces. When friction is overcome, the ground-engaging member slides across the ground, without lifting.

Description:
BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     This invention relates to self-transporting support arrangements and is particularly, though not exclusively, applicable to helicopter undercarriages. The invention extends to a helicopter having such an undercarriage. 
     2. DESCRIPTION OF THE PRIOR ART 
     A skid undercarriage is widely used in helicopters, since it permits both normal and emergency landings, while keeping weight, cost and aerodynamic drag to a minimum. Once landed, however the aircraft requires additional equipment to be moved. On firm ground, wheels can be attached, and small helicopters can then be manhandled, while larger ones can be towed. On unprepared ground, however, typically either very large wheels have to be fitted or the aircraft has to be towed on its skids by a powerful tractor. Neither of these alternatives is convenient. 
     Self-transporting support arrangements, which work in a manner analogous to walking, are known in connection with large apparatus such as cranes and earth-working machines. These arrangements have large ground-contacting and load-supporting &#34;feet&#34; which can be moved horizontally and vertically relative to each other. Motion over the ground is achieved by transferring load onto a first &#34;foot&#34;  (or set of &#34;feet&#34;), raising a second &#34;foot&#34; and moving it horizontally before putting it down again, whereupon the load is transferred to the second &#34;foot&#34; so that the first &#34;foot&#34; can be raised and moved horizontally. In one such proposal (British patent specification No. 1,215,456) a force acting obliquely to the vertical is applied to the &#34;feet&#34; by a pressure cylinder through a plurality of levers which are pivotally interconnected, so that the &#34;foot&#34; off which weight is being transferred slides over the ground surface as soon as the horizontal component of the applied force overcomes the friction between it and the ground. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a self-transporting support arrangement which can be light and simple in construction, yet robust, and which applies both horizontal and vertical forces simultaneously so that movement occurs as soon as friction is overcome without lifting of the ground-contacting members. It is another object to provide such an arrangement which is suitable for use on uneven and unprepared ground while providing good stability, so that it is especially suitable for use as a helicopter undercarriage. 
     According to this invention, the self-transporting support arrangement has force applying means in the form of at least one inflatable flexible bag located between opposed surfaces respectively of the support member and the shoe, said opposed surfaces being generally inclined to the horizontal and the bag being arranged to roll on the said opposed surfaces when inflated. Thus when the bag is inflated, the support member slides over the base surface, e.g. the ground, when friction is overcome, while the bag rolls on the said opposed surfaces. Because the support member does not leave the ground, good stability is maintained. 
     Preferably the said opposed surfaces are parallel and planar. Suitably at least two bags are employed to move each slidable support member in each cirection, to provide forces acting at spaced apart points. 
     It is particularly preferred if the shoe is integrated with the support member in plan, i.e. is located between longitudinal ends of the support member and lies on this axis of the support member. 
     It has been found that a particularly useful form of bag is one having an inner inflatable chamber and an outer chamber in the form of a flexible inflatable tube extending around the inner chamber and lying between the inner chamber and the said opposed surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of parts of a helicopter skid undercarriage embodying the invention; 
     FIG. 2 is a longitudinal cross-section through one skid, and a shoe integrated therewith, of the undercarriage of FIG. 1; 
     FIG. 3 is a partial cross-section on the line III--III of FIG. 2 showing an alternative form of the inflatable bag; 
     FIG. 4 is a partial cross-section on the line IV--IV of FIG. 3; and 
     FIGS. 5a to 5e are diagrammatic side views of the bag of FIGS. 3 and 4 and the surfaces it engages, in various stages of operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The helicopter undercarriage of FIG. 1 has a pair of skids 2 having their longitudinal axes extending parallel to each other and to the longitudinal axis of the helicopter (not shown). The skids 2 are jointed by two transverse frames 4 to which the body structure of the helicopter is attached. In this embodiment a transport shoe 6 is integrated into each of the skids 2, i.e. the shoe 6 lies on the longitudinal axis of the skid 2 and between the ends of the skid 2. Preferably as here the skid has a larger ground contact area than the shoe 6. Alternatively, the transport shoes may be mounted alongside the skid, either inboard or outboard of the skid, or both, so that the skid is not interrupted. 
     The shoes 6 in FIG. 1 are adapted to propel the helicopter in either direction. Referring now to FIG. 2, which shows the skid 2 in detail, the propulsive force is provided by two pairs of inflatable flexible bags 8 and 10. Each bag lies between an inclined planar surface on the shoe 6 and an inclined planar surface on the skid 2, so that on inflation the bag urges these surfaces apart. The bags 8 which urge the skid 2 relatively to the left in FIG. 2 lie between the parallel opposed surfaces 12,14, which are inclined downwardly towards the left hand end of the skid 2. The bags 10 which urge the skid 2 relatively to the right lie between the parallel opposed surfaces 16,18 which are inclined downwardly towards the right. The bags are inflated and vented via pipes 20, and the bags 10 via pipes 22. A tension spring 24 secured at one end to the shoe 6 and at its other end to the skid 2 urges the shoe and skid towards each other. In the rest position, shown in FIG. 2, the ground-contacting surface of the shoe 6 is above the ground-contacting surface of the skid 2. The skid and shoe should be designed to minimize aerodynamic drag in flight, while providing adequate ground-contacting area. The skid 2 and shoe 6 are suitably formed as hollow tubes fabricated from sheet metal. 
     In the alternative embodiment illustrated in FIGS. 3 to 5, the skid 2 and shoe 6 are the same as in FIGS. 1 and 2, but the simple, single-chamber bags 8,10 of FIGS. 1 and 2 are replaced by two-chamber bags of a more complex shape, one such bag 9 being shown. FIG. 3 shows the surfaces 12,14 at maximum separation, while FIG. 4 shows the bag 9 in a partially inflated state. The aim of the design of the bag 9 is to maintain a large area in contact with the opposed inclined planar surfaces 12,14 or 16,18 even with a wide variation of separation between the opposed surfaces, while at the same time the bag should be able to roll a large distance as the surfaces 12,14 etc. move relatively in the horizontal direction. For instance the bag may be required to roll by a distance nearly equal to half the length of its perimeter. The flexible, inflatable bag 9 has an inner portion 9a which has a shape such that it can be inflated substantially into a sphere (if unhindered), and an outer portion 9b in the form of a tube extending circumferentially around the inner portion 9a so as always to lie between the inner portion 9a and the surfaces 12,14 in a manner similar to that of a pneumatic tire around a wheel. The two portions 9a, 9b are sealed together continuously, or as here at a number of spaced points 26, along an equator of the portion 9a. Additionally, in order to increase the stability of the inner portion 9a within the outer portion 9b when inflated, opposed areas of the interior surface of the outer portion 9b are secured together at a plurality of points 29 on the same equator line 26 (see FIGS. 3 and 4). 
     Apertures 28 intercommunicate the air volumes within them, and the outer portion 9b is connected to the feed and vent pipe 20. In use, the outer portion 9b as seen in transverse section (FIG. 3) forms a lobe 30 on either side of the equator line 26. 
     The two illustrated embodiments operate similarly, as shown in FIGS. 5a to 5d, where the skid 2 is shown resting on a ground surface 32. When the helicopter has landed on the surface 32, the skids 2 of the undercarriage support it on the ground and the spring 24 holds the bags 8,10 deflated and the shoe 6 off the ground (FIG. 5a). To move the helicopter to the left as seen in FIG. 5, the bags 8 are inflated so that the shoe 6 is moved into contact with the ground (FIG.5b). Further increase in pressure causes a force to be applied to the skid 2 at an angle to the vertical determined by the slope of the surfaces 12,14. When this force reaches the level such that its horizontal component overcomes the frictional force resisting movement of the skid 2, the skid slides along the ground (without losing contact) to the left (FIG. 5c). The bag 9, which is fixed to the surface 12 in the neighbourhood of the pipe 20, rolls on the surfaces 12,14. Further inflation causes further movement. The approximate useful limit of travel is shown in FIG. 5d. The helicopter has now been moved to the left, and to complete the cycle of movement the bags 9 are vented so that the load is transferred from the shoes 6 back onto the skids 2 and the shoes 6 are brought back to their initial position relative to the skids 2 by the spring 24. 
     The separation between the surfaces 12,14 when the bags are inflated depends on the extent of yielding of the ground under the shoe 6, as well as on the relative horizontal displacement of the shoe 6 and the skid 2. On soft and yielding ground the system will continue to work so long as enough of the bag remains in contact with the surfaces 12,14 to provide the required force within the limits of available air pressure. FIG. 5e illustrates a situation in which the ground has yielded and insufficient force is developed to provide motion. 
     The compressed air supply for the inflatable bags may be located on or off the helicopter, or may be derived from the air bleed from the drive turbine of the aircraft. The inflation and venting of the bags may be controlled by simple two-way valves, and automatic repetition of the cycle may be achieved using limit switches, e.g. electrical or pneumatic, which detect the relative movement of the skid and the shoe. 
     To move the helicopter to the right (in FIG. 2) the bags 10 are used in the same way. By suitable control of the inflation of the various bags, it is possible to turn and steer the aircraft. This may be done for example by throttling the air supply to bags on one side of the undercarriage to restrict the speed of movement of that side, and arranging for both shoes 6 to return to their starting positions when either of them reaches the limit of its travel. Thus one shoe moves a shorter distance than the other in each cycle of operation. 
     The controls can be either inside the helicopter, or outside it for operation by a man walking beside the aircraft where he may have the best possible view of obstacles. 
     The asymmetry of the arrangement as seen in FIG. 2 about a vertical plane perpendicular to the paper should be particularly noted; if the construction were in the form of a mirror image about the vertical pane at the position of the return spring 24, and the positions of the bags 8 and 10 on for instance the right of the spring 24 were interchanged, the resultant lift force would not act through the centre of the skid, and this would increase the tendency of the skids to lift at one end when the centre of gravity of the helicopter was near an extreme position. It in some cases is desirable to make provision for cutting off (or reducing the pressure in) bags at one end or the other of the skid, in order to bring the effective lift more nearly into coincidence with the centre of gravity of the helicopter. 
     The weight of the helicopter may vary typically by a factor of about two between the laden and the unladen conditions, and the force required to slide it on its skids by a factor of, for example, about six, depending on both the weight and the ground conditions. The arrangement of the invention applies an inclined force with a constant ratio of horizontal and vertical components, so that it can operate over a wide variety of loads, with high stability in all cases since the skids 2 are not lifted off the ground.