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Aviation History
1963
1963 - 2308.PDF
JGHT International supplement, 24 October 1963 Air-Cushion Vehicles this important aspect of hovercraft development. But before discussing that, it should be noted that another programme has been running in parallel, the development of skirts for overland air-cushion vehicles. Early in 1961 a simple model was built, consisting of a platform bearing the source of pressurized air, around which was fitted a convoluted skirt of Vickers design. Despite the obstacles, the platform itself was quite steady. In a later stage of development, a similar 1 8 6 WAVE HEIGHT (ft) 4- 2 --^ \ 0 20 40 60 80 100 SPEED (knots) * Fig 3 Effect of flexible skirts on wave- riding (50-ton ACV, 7,000 h.p., 6in hover- height, zero headwind) skirt was fitted to a Land-Rover. The principle of this operation was that, while the road wheels still provided traction, steering and stability, the use of the air cushion enabled the driver to vary the proportion of the total weight bearing on the wheels. Thus, optimum traction on a variety of soils could be achieved. The principal features of the con voluted skirt are the differential stiffness and the considerable peripheral stretch. Thus, while its maximum stiffness is employed in opposing the air cushion, it can pass over obstacles with little resistance and with negligible additional escape of air. It is of small overall thickness, and so is readily mounted on a retractable frame. Another type of skirt which has been fully developed by Vickers is suitable for a vehicle operating over smooth undulating ground. This type achieves '• air gap of as little as jfein, resulting in very low power requirements to maintain the air cushion. Thus, large cushion pressures are possible. Perhaps the best application of this skirt is with multi-wheeled transporters carrying v cry heavy equipment over smooth but unmetalled surfaces at modest speeds. Turning now to high-speed marine hovercraft, significant results have been achieved in the development of the •nflated flexible skirt. Taking VA-2 as a « example, originally the design hover height was 7in, which meant it could clear obstacles of a similar height. The fitting of a 20in flexible skirt now enables the VA-2 to cross discrete obstacles of 18in, and the improvement in its wave-riding is of a similar order. The inflated skirt of the type now fitted to VA-2 incorporates features which represent a technical advance over all other skirts, giving a better wave-riding performance as a result of an air-flow characteristic which is unaffected by skirt distortion. The test programme took VA-2 to an airfield on the south coast of England. On the beach near the airfield we found a 4ft shingle bank which VA-2 surmounted quite easily at 20 m.p.h. Much of the area around the airfield consisted of saltings—soft mud and vegetation interlaced with deep gullies, and at times almost impassable to other forms of transport, except at very slow speeds. VA-2 overcame these obstacles, including saltings 2ft 3in deep and 3 to 4ft wide with the ease that would be the envy of, say, a conventional airfield crash/rescue vehicle. The facility with which VA-2 moved across the saltings can be attributed to the employment of the inflated flexible skirt. Its features include the use of material of minimum thickness and therefore of minimum weight. This represents a saving in capital and re placement costs, and, together with the property of the skirt stretching along its length, combines to reduce main tenance costs as well as skirt drag. A further stage of the test programme took VA-2 to an area which presented a different problem—the problem of dust. In the past, the problem of operating in desert areas has often been discussed, and this was a small but useful step towards the objective. The experience of this dust-laden atmosphere gave renewed confidence for the future. Other obstacles included a ditch 14ft wide and light scrub (vegetation) 5ft high. Artist's impression of VA-3B 10 MAXIMUM WAVE HEIGHT g fEET 4 2- HOUBORM n*. F0ILB0RNE *fe %. xfc 0 10 10 30 40 SO 60 70 (0 COMFORTABLE CRUISE SPEED-KNOTS Fig 4 Wave-riding comparison. Hydrofoil: displacement, 60 tons; capacity, 100-140 seats, no cars; engines, 2 x 1,350 s.h.p. Hovercraft: laden weight, 50 tons; capacity, 150 seats or four cars; engines, 2 X 3,500 h.p. Although there appears to be a limit to the length of flexible skirt used, this limit being related to the beam of the hovercraft, the maximum has not yet been reached with VA-2. However, in order to preserve the use of the under carriage without necessitating major structural alterations, the skirt length has been kept at 20in. Vickers believe that there is still scope for further development of flexible ACVs, and present indications are that there are more technical breakthroughs to come. Bigger hovercraft of the future will employ skirts which will ride the waves, permitting a negligible leakage of air. We have, for example, developed a simple type of flexible skirt that senses the approach of waves and land obstacles, and moves over them without contact. There is no doubt that the future of the various applications of the air- cushion principle is dependent on flexible skirts. We are confident that our present programme will give the hovercraft high operating efficiency, and will enable it to take a very im portant place in the transport systems of the world. 59
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