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Aviation History
1963
1963 - 2333.PDF
Air-Cushion Vehicles FLIGHT International supplement, 26 December LIGHT HOVERCRAFT . . . diameter Heba* fan, and that provision should be made for external engines. Failure of the original fans to achieve design efficiency meant that not only was the hoverheight less than predicted (about 12in instead of 20in), but the maximum speed was also 20kt below the design figure of 45kt. The Army decided to purchase the second machine for evaluation and test at the Fighting Vehicles Research and Development Establishment. CC-2/002 was delivered in March 1963, after a short period of tests at Bembridge when it hovered at 16in and achieved a speed of about 30kt without auxiliary propul sion. It was subsequently fitted with two 30 b.h.p. Volkswagen external engines. A useful increase in control was obtained with the external engines, an increase in speed of about lOkt. The extra propulsive power was particularly useful in overcoming hump drag over water. CC-2/003 In December 1962, before tests had started with CC-2/002, an order for a commercial machine was received from a newly formed company called Seair- glide. This order was later cancelled by mutual agreement after it was estab lished that the CC-2 would not be able to carry the specified 14 passengers satisfactorily without either an increase in power or the addition of some form of flexible skirt. 003 is now being used as the company's research vehicle, and has recently been fitted with two 90 h.p. Continental engines. A skirt is being made to the design of Hover- * High-efficiency backwards aerofoil. craft Development Ltd, which will be fitted by the end of 1963. Subject to satisfactory progress with these tests, plans have been made to charter CC-2/003 to an oil company for use both by their exploration and sales-promotion departments. Flexible Structures In common with all other current ACVs, the Cushioncraft series of vehicles was designed with a hard bot tom structure. Some form of flexible skirting has since been added to almost every current hovercraft, but the original fan duct systems are not designed to take full advantage of flexible nozzles or skirts. In light hovercraft design, where considerations of cost really require that a single system provide both lift and propulsion, the advent of flexible nozzles is of great significance. As originally conceived the CC-2 craft were intended to have a sufficient ground clearance to permit running over reasonably dis turbed water and rough ground. Even with the 16in clearances attained with the latest machine, over-water perform ance has proved disappointing because of wave impact. Forward speed using thrust from the lift system alone has also been disappointing, because in practice it was found unsafe to tilt the machine bow-down to enable the design propulsion to be realized. For this reason the later craft were fitted with the auxiliary propulsion engines to extend their speed range. Flexible nozzles and inflated flexible lower structures will permit operation with a very much smaller air gap beneath the craft. The horsepower required for lift will be substantial reduced, and the fan system can J optimized for propulsion. It is reaB at this point that the design and techrl logy of light hovercraft branches awl from that of the heavier commerca machines, where the design is nf disciplined by the desirability ofl single lift/propulsion system. Propulsion The most efficient means of propu sion for an ACV travelling in excess i 50kt appears to be an air propelle However, propellers must have variabl pitch and should be capable of providiiJ reverse thrust. Preferably there shoul be two propellers to give yaw contra at low forward speeds, otherwise a approach downwind on to a lee beac is almost impossible. The propellei must be so mounted that they are clea of the spray (to avoid erosion) and o large diameter. They should be gear© into the lift system so that the drive can interchange lift and propulsio: power according to the sea state. At present the only practical alterna tive to propellers is integrated propul sion, but the system must be mos carefully designed. Losses can aris< through the following causes:— Poor ram recovery: This is principally a function of the ratio between intak< velocity and forward speed. Fan efficiency: The fan should be designed to work at maximum efficiencj when the craft is moving at its design cruising speed. Duct losses: These are best avoided by letting propulsion air out as soon as possible after passing through the fan. It can be shown that this is more efficient than directing the air into the Continued on page 96 Fig I Fan diameter for maximum efficiency plotted against fan delivery pressure Fig 2 Total propulsive efficiency plotted against speed for two sizes of fan p = 250 T|R= 90 7. 40 60 SPEED (kt) 82
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