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Aviation History
1963
1963 - 1071.PDF
Air-Cushion Vehicles FLIGHT International supplement, 24 January 196. ACVs FOR THE ARMY . . . heavier vehicle of 171b/sq ft, giving a planform area A of some 212 sq ft, or a craft equivalent diameter D of about 16ft 6in, which (assuming—= 0.08) gives h, the true hoverheight, of lft 4in (max); the maximum surface clearance with skirt enables obstacles to be crossed up to, say, lft 9in. Assum ing an installed power/weight ratio of 131b/s.h.p., probable power becomes 280 s.h.p.; this would give a maximum speed, at reduced hoverheight, of about 60kt. Possible dimensions, at an - B ratio of about 1.7, are length L of 19ft and breadth (beam) B of lift. Normal operating height would be kept to the minimum which the surface allowed; perhaps 9in would be the average over flat country. Reductions in payload and fuel would increase the hoverheight significantly—possibly to 2ft 6in—but hoverheights above normal require increased lift power, resulting in greater fuel consumption per mile. As with all ACVs, a large overload potential is available at the expense of reduced hoverheight, so over really flat surfaces it may be possible tem porarily to double the payload. Such a high-speed vehicle could have acceptable overall dimensions and a hoverheight capability to give good performance over bad ground. Remem bering that roads and tracks are best avoided by ACVs (and remembering the lift width), the obstacle problem presented by hedges, walls and fences persists. Although the amphibious capability would be useful, these over land restrictions would not appear to justify use in this role as the payload thus transported is so small. Again, helicopters may prove cheaper and more effective; alternatively, some kind of jeep-type wheeled vehicle might be equipped with a ducted fan or other system to give it a short free-flight capability enabling it to cross rivers and "hop" over woods and other major obstacles. Such a vehicle would be able to leave the ground only for short periods (heavy fuel consumption), and its payload would be severely limited due to the proportion of the gross weight taken by the lift system. Other light load-carriers The two craft outlined so far have demonstrated that they can probably offer higher speeds than ordinary land vehicles, an amphibious capability and attractive performance over rough ground, bog, desert and so on; but their inability to cross the larger hedge, fence and wall- type obstacles is probably a deciding factor against them. Two alternatives have been suggested in this payload class: helicopters and some form of vehicle with an aerial ability for hopping over rivers and major obstacles. Most heli copters cannot operate in bad weather, nor move along the ground freely. The other alternative, obtaining lift from some type of ducted-fan principle, is bound to be relatively inefficient in sustained flight—even assuming this to be possible—as the airflow through a fan within the vehicle's dimensions must be moved at a much higher velo city than that handled by the very much larger "fan" formed by the blades of an equivalent-payload helicopter. Its "flights" would therefore involve the "Remembering that roads are best avoided by ACVs" use of a large amount of power, and fuel consumption would be excessive. These first two examples suggest that, to produce an ACV with unim peded travel over most land obstacles, the basic hoverheight should be some 4ft. Heavier-payload ACVs must there fore be by-passed, the basic calculations being determined from this starting point. This appears to be the way tc decide the smallest worthwhile ACV for an army. Smallest Practicable ACV for Overland Limitations Minimum average ob stacle clearance, 4ft; deducting one- third of this accounted for by the "skirt," leaves 2ft 8in; but, for rapid travel over such obstacles, too close a "fit" is inadvisable, so allow an extra lft 4in to let h (true) be 4ft. If — = 0.085. D will be some 48ft; giving planform A of some 1,800 sq ft; and assuming a cushion pressure Pc of, say, 601b/sq ft gives a gross weight of about 50 tons. Of this the payload may be held at some 35 per cent, to enable sufficient lift power to be installed (even if struc ture is lightweight), giving payload Wp of 17£ tons. Dimensions should be considered with maximum lift in mind: say, - = 1.6, so L (min) becomes J3 55ft, and B (min), 33ft. Installed power, to include boost power lor worst ob stacles, say at 151b/s.h.p., comes to 7,400 s.h.p., giving a probable maxi mum speed of 75kt. This is a craft of some size, clearly unsuitable for roads or narrow routes. However it does carry a payload ap proaching 20 long tons, and may have attractive military applications. Before considering these, the normal army load- carriers between the light Land-Rover type and the 20-ton-payload vehicles should be noted and briefly discussed. Load carriers up to 3 tons soft-skinned) The present British range includes large quantities of soft-skinned 1-ton and 3-ton cargo trucks. At present these are used not only within field and L of C units but also as supply vehicles (mostly 3-tonners) for bringing ammu nition, fuel and rations to the fighting formations. Units will need to retain such vehicles for their own internal use, and any replacement by ACVs—even if financially acceptable—might not offer very great advantages. This may not be the case with supply vehicles. The unit vehicles have all-wheel drive, a good cross-country performance and are easily dispersed into sub-units where their compact dimensions make them easy to camouflage and park. Further more, they are ideal for relatively eco nomic use by an army in peace-time. 8
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