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Aviation History
1963
1963 - 1070.PDF
• FLIGHT International supplement, 24 January 1963 Air-Cushion Vehicles and, using full lift power, could probably nex to lower the effective height of an obstacle below the main body of the craft considerably. Research will deter mine what proportion of ground clear ance can be met by a skirt and how much needs to be "daylight"'; for the purpose of this article it is assumed that the skirt can account for one-third of the total clearance. For travel over flat surfaces the craft need be only a few inches above that surface, thereby saving lift power and fuel (in this case the skirt would not deflect at all). From the army requirement of maximum clear ance, therefore, one-third can perhaps be subtracted as being covered by the flexible skirts fitted. However, the air cushion has a limit to its height, and the centre of gravity of the craft itself can not be raised too high above the surface. Sizejhoverheight Accepted craft equivalent diameter/true hoverheight ratios could perhaps be taken within a bracket of - = 0.07 to 0.10. Most craft, however, do not have a circular plan- form, as this—although efficient for pure lift—is inefficient at speed. Planforms The most convenient are probably either basically rectangular, with the corners considerably rounded off, or roughly oval. The ratio of length/ breadth is basic, and study of existing vehicles suggest this can vary consider ably. However, apart from the purely overwater craft, an — ratio between 1.5 Jo and 2.3 is probably reasonably efficient for both lift and propulsion. Ideally, the optimum planform for static maximum hoverheight would be circular ( — = 1 but over-wide craft would be unwieldy and inefficient at speed. Pay load/weight This, it would be thought, is bound to vary from small craft, tending to use very light structures (perhaps based on airframe practice), to structures of heavy construction based more on shipbuilding or general engin eering practice. Study of existing craft suggests that the ratio tends to lie between 20 and 40 per cent. Power/weight This is not easily arrived at, since ACVs designed to attain significant hoverheights inevitably re quire much more power than those which are designed to operate near the ground. Power requirements also rise with cushion pressure and speed. For overland operation requiring good maxi mum lift power and propulsion power to surmount gradients, a bracket of between 10 and 181b/s.h.p. is currently being achieved. Power can be provided separately for lift and propulsion, or integrated systems can be used. The former is more likely to be used over land, where propulsion power will be required to surmount gradients and assist in steering and control. Never theless, the total installed power should be considered in relation to the size of the craft. These rules of thumb cannot take account of every factor, and they ignore the fuel carried to achieve long ranges. This is partly compensated for by the fact that all vehicles increase their range with size. This is because the spare carrying capacity available for fuel increases more rapidly than the rate of fuel consumption as the vehicle gets bigger. Another important para meter—hoverheight/annular -jet thick ness (h/t0)—is also ignored at this stage. However, those outlined so far should allow some idea to be gathered as to the eventual feasibility or otherwise of ACVs for each role. Light Vehicles Motorcycles and motorcycle com binations are used for despatch-rider duties, and occasionally the carriage of machine guns or other light weapons. Basically the payload may vary from one man to two men, their equipment and a weapon. The total payload W,, could be 6001b. Applying the rules of thumb: Gross weight, with WP = 32%, 1,8501b: cushion pressure Pc for light craft could be 151b/sq ft: this gives a planform area A of about 125 sq ft; or an equivalent diameter D of some 12ft; which gives a ^h true hoverheight h (=-= 0.085 j of, say, lft (max); which, with a "skirt," gives a surface clearance at maximum lift of 16in. At 121b/s.h.p., power is 150 h.p. (small vehicles need extra power for high lift); giving maximum speed of about 60kt. Probable = ratio not ex-b ceeding 1.5 to achieve best possible lift, so overall L would be 14ft, and B 9ft. The normal operating height would be a few inches off the ground, and the craft may not even be able to ride over small hummocks. Such a vehicle would be too large for its payload. and thus unsuitable for the duties normally per formed by the more compact motor cycles—which already possess a good cross country performance. Replacement of all motorcycles would involve too great an expense, and fuel consumption might well be greatly increased. Nevertheless, small numbers of such craft might be used for the carriage of personnel or anti-tank missiles in re connaissance units, where rapid trans port over open country plus an amphi bious capability would be welcome. The craft does not possess the ideal obstacle clearance of more than 4ft. Its payloads are suitable for light helicopters, which seem at present more suitable to fulfil this role. Light liaison, reconnaissance and com mand vehicles Soft-skinned vehicles of the "jeep" or Land-Rover type are in heavy demand in all units and head quarters. These provide fast transport on and off roads for small groups of men or between one-quarter and one-half ton of important stores; they are also used as ambulances in forward areas. A typical payload might be three men and something over one-quarter ton of stores. Here payload Wp could be, say, 1,2001b: so the gross weight (at 33 per cent) becomes 3,6001b. Use a cushion pressure Pc for this slightly "Not suitable for ACVs
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