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Aviation History
1964
1964 - 0814.PDF
FLIGHT International supplement, 26 March 1964 Air-Cushion Vehicles P and PAYLOADS Cushion Pressure and Economy: a recent paper by Vickers' ACV Manager VICKERS-ARMSTRONGS (ENGINEERS) LTD, of South Marston, Swindon, Wilts, are one of the world's leading ACV com- panies. Manager of the Vickers Hover- craft Division is Mr S. R. Hughes, AFRAes, and what follows is Improved Economics through Increased Cushion Pressures, part of a lecture delivered by him at the symposium held in Stock- holm last November and previously reported in this journal in December. By virtue of its amphibious nature [states Mr Hughes], the hovercraft can open routes that are closed to other forms of transport. Varying tidal regions, shallows, sandbanks and solid steps are surmountable; and the ability to load and unload cargo by beaching on roughly prepared surfaces obviates the need for expensive terminal or port facilities, and minimizes the turn-round time. However, to establish itself fully as an attractive transport vehicle it should also be capable of providing a practical alternative for existing traffic. This means offering an improved service on routes already covered by conven- tional services. In short, it will have to compete with ships, aircraft and hydro- foils; and competition implies an eco- nomic assessment. There are several factors which affect the economic operation of hovercraft: payload capacity, first cost, choice of route and available traffic are promi- nent contributing factors. Two of these factors, payload capacity and first cost, are directly related to the engineei ing of the craft and can be controlled in the design stages. For a given craft the gross weight may be broken down into five major elements:— (1) Total disposable load (payload, fuel and crew). (2) Total structure (less equipment and flexible skirts). (3) Equipment and flexible skirts. (4) Machinery (fans, transmissions and propellers). (5) Powerplant. To achieve a good payload capacity for a given range, items 2 to 5 must be minimized. The performance require- ments will chiefly control items 2, 4 and 5- However, item 2, total structure weight, can contribute up to half of the weight, and can be optimized by l structural design. essential requirement of the structure is that it should have high strength but minimum weight consis- tent with the loads and conditions occur- ring within the specified performance boundary. On this basis, a stressed-skin type of construction, manufactured from high-strength light alloy, has been chosen for the majority of present-day craft. This technique is backed by con- siderable flying-boat and seaplane ex- perience, and has been found to be satisfactory in operation; but it is ex- pensive to manufacture, and also requires considerable design time to achieve the high strength properties available from the light-alloy materials. The stressing cases used in the design of the main load-carrying, or primary, structure are derived from wave-impact forces acting on the bottom surface of the buoyant hull. These forces arise from the relative vertical and horizontal velocities between the craft and the water when alighting at high speed or when encountering waves in adverse conditions. Mean distributed water pressures of up to 251b/sq in are generated by the impacts, and transient local pressures of up to 601b/sq in can occur. Stresses are induced in the structural members by the applied bending moments, the magnitude of which is a function of the craft size and weight. If the dimensions for a given weight can be reduced to a minimum, the loads within the members will be alleviated, allowing a lighter structure (Fig 1). This implies that there is an optimum structural density, and hence craft density, for a given gross weight; and that the design cushion pressure, Pc, O5r O4 - 03- O-2 Fig 1 Plot of structure weight as proportion of gross weight for varying cushion pressure directly related to density, should be as high as is practical from performance considerations. Thus, by designing a high-density craft operating at high cushion pressure, the weight and first cost of the structure are reduced (Fig 2). At this stage it will be advisable to consider the nature, or density, of the payload. The following table lists a number of representative cargoes, the unit used being the inverse of density, i.e., volume for a given weight. TABLE I: PAYLOAD SPECIFIC BULK Passengers: In Atlantic liners In other liners Ist-class aircraft High-density aircraft Suburban trains Private cars Crowded tube train Vehicles: Private cars Bulk freight: Wheat Oil ron ore Cu ft per pas* 2,000 1,000 65 45 30 20 12 — — — — Cu ft per ton 20,000 10,000 650 450 300 200 120 400 4040 15 In the case of bulk freight, which is inherently of high density, economics dictate a high cushion pressure. How- ever, a more immediate conception of the hovercraft is as a passenger or passenger/car ferry, for this role offers the most acceptable returns to the operator. It is an economic proposition to carry a high-value payload at high speeds; and this form of traffic is con- tinually increasing, especially on the short Continental routes. Optimum cushion pressure will vary with the type of cargo and the limits set on performance. Craft built so far have W«IOO TONS GROSS WEIGHT _ CU5H|ON PRESSUBE CUSHION AREA 35
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