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Aviation History
1963
1963 - 2307.PDF
Air-Cushion Vehicles FLIGHT International supplement, 24 October 1963 The Development of Vickers Flexible Skirts BY S. R. HUGHES* THE FOLLOWING is the gist of a paper by Mr Hughes, released at Malmo, Sweden, last month. Like all surface vehicles, the ACV is most happy when operating over a smooth surface. Its operation is not restricted to smooth surfaces, however. By passing more power through the lift fans the hover height can be increased and the vehicle can negotiate larger waves and obstacles. To negotiate a dis crete obstacle like a boulder or a post the hover height must be greater than the height of the obstacle. If one doubles the power in the lift system, the hover height is almost doubled and the vehicle can negotiate obstacles and waves of almost twice the height that it could before. A small air-cushion vehicle could be given a capability of negotiating high increased loads incurred by the struc ture. By attaching flexible members to the bottom of the craft's periphery the main structure can be raised relative to the surface for a given installed power. The flexible members are designed such that they can contain the air cushion but can readily be deflected by waves and obstacles without incurring large loads or decelerations. Fig 1 illustrates the principle of the flexible skirt. The left diagram shows how large waves impact a rigid ACV, but the same craft with flexible skirts can ride over the waves as shown in the right-hand figure. The advantages of flexible skirts have been well known for a number of years, although it is only in recent months that their development has been advanced sufficiently to be emphasized publicly by the manufacturing companies. wake of displacement vessels and when making the transition between land and water. The difference in shape of the two curves is also significant. The flat curve of the rigid craft shows that there is a critical height of waves above which a craft cannot operate. There is a small margin between wave conditions in which high-speed operation is possible and wave conditions in which a craft cannot operate at any speed. For the craft with flexible skirts the slope of the curve is such that, when an increase in wave height is encountered, the craft continues to operate satisfactorily at a slightly reduced speed. A given wave capability and operating cost per seat mile can be achieved with a 50-ton craft having flexible skirts, which otherwise would require a craft of 150 tons. The flexible skirt increases the margin Fig J Action without left) and with flexible skirt waves and obstacles, but in order for it to do so its power would be dispropor tionate to its size. Such a vehicle would be extremely uneconomic and of no practical use to a civil operator. Keep ing the relationship between installed power and vehicle weight constant, larger ACVs can tackle larger waves and obstacles than smaller ones. For example, a 50-ton craft could operate economically in waves of only 2ft in height, whereas a 500-ton craft could achieve comparable economics in waves up to 7ft high. All that has been said so far applies to the original conception of the hover craft, but more recent developments have brought about improvements. The limiting wave-height is associated with water impacting the craft's struc ture, resulting in high resistance to motion, discomfort to occupants and * Manager, Hovercraft lDivision, Vickers-Armstrongs Engineers) Ltd. An example of advantages of fitting a flexible skirt to an ACV is given in Fig 2. For given values of loaded weight, speed and operating range, a comparison is made between rigid and skirted craft. The skirt, even with a modest length of 3ft, gives an improved wave capability in addition to the con siderable reduction in installed power. The capital cost is reduced, the payload increased by nearly three times and the operating cost is consequently reduced to less than one-quarter of that of a rigid ACV. The advantage of fitting a flexible skirt to a 50-ton vehicle having a given installed power is shown in Fig 3. This figure illustrates the wave height that can be tolerated at each speed. There is, of course, no advantage over calm water when the skirt does not come into action but incurs an additional resist ance to motion. But the maximum benefit is obtained when the waves are larger. The skirt shows to advantage particularly when encountering the of wave-riding capability that the hover craft enjoys at a given speed over other craft of similar size. Fig 4 illustrates that a hovercraft offers a far more com fortable ride at high speed than does a hydrofoil. During the past two years Vickers have carried out an intensive pro gramme on flexible skirts for marine hovercraft, in continuation of Mr Christopher Cockerell's early work on FIG_2: INFLUENCE_OF FLEXIBLE SKIRTS "ON PERFORMANCE'AND ECONOMICS Loaded weight Speed Range Wave height Lift power Propulsive power Total power Mean air gap Payload/loaded weight Capital cost Operating cost (seat-mile) 50 tons 60kt 100 n.m. Without skirt 3ft . ~70% 30% I8in 10.4% 100% 100% With 3ft ikirt 4ft ^0% 35% 4in 28.6% 78% 24% 58
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