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Aviation History
1963
1963 - 1069.PDF
ACVs FOR THE ARMY by Major A. J. D. Hughes, RE SKETCHES BY GORDON HORNER PART TWO-LIKELY ROLES BEFORE COMPARING the existing range of land vehicles with possible ACV equival ents, it would be as well at this stage to envisage the basic technical parameters which would determine the approxi mate size, gross weight, payload, ground clearance and power required for such craft. The reasonably large ground clear ance clearly indicated as a basic army requirement in Part 1 is best met by the simple momentum-jet curtain type of ACV. Shortly, developments on labyrinth-seal systems may lead to their suitability for such craft, and this more refined system may well save power and prove to be more efficient. The attractive French development by Bertin of a plat form supported by several independent "bells" or "multiple petticoats" (flexible skirts) could give a reasonable clearance over low rocks and uneven ground; but walls, hedges and fences might well lead to damaged "petticoats." Under load the latter are always within inches of the ground—or, if unduly deflected, might render the craft unstable. For the purpose of this article, there fore, a few "rules of thumb" must be used, and these are taken to refer to the already proven momentum-jet curtain type of craft. In practice each design parameter will vary with every craft configuration, and the number of permutations is legion. Here only the broad characteristics of each craft need be outlined. Rules of Thumb Craft dimensions are dependent upon the payload (which affects the gross weight) and the cushion pressure. An increase in cushion pressure means a decrease in size, but to achieve this in creased cushion pressure extra power (and therefore extra weight) is required. Additionally, the air curtain is more difficult to seal. Cushion pressures for craft requiring significant maximum hoverheights must therefore be restricted, and such pres sures may vary from 10 to 201b/sq ft in small craft to 60 to 901b/sq ft in large craft. In addition, to retain stability at maximum hoverheight (even assuming the cushion to be segmented by vertical stability jets, or stabilized by concentric annular jets or by some other system) there is a limit to the ratio of craft size/ hoverheight. Flexible skirts, as used on SR.N1 and the Ford ACV, will naturally give obstacle clearance in addition to the true hoverheight of the lowest part of the craft from the surface. It is diff cult to say how much true clearance would be required to cross a wall or hedge, and how much skirt could reasonably be de flected by the obstacle without materially affecting the cushion or damaging the skirt. Flexible skirts could be assumed to compensate for the worst obstacles, 6
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