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Aviation History
1963
1963 - 2290.PDF
FLIGHT International supplement, 26 September 1963 Air-Cushion Vehicles What Price Part 2 TRENDS IN FIRST COST OF OVERWATER PASSENGER TRANSPORT By R. L. TRILLO, AMIMMkE, AFRAaS, MAIAA, MCASI* IN THE FIRST PART of this article the first cost (purchase price) of various types of overwater passenger-carrying craft was examined, and numerous factors affect ing this cost considered. A series of eight illustrations showed various plots of these parameters, singly and in com bination, to emphasize the importance of the product of payload times cruising speed (ton-kt) and of installed power divided by payload times cruising speed. Over the past 25 years the installed power per payload ton-kt required by transport aircraft has been approxim ately halved (Fig 9). For both peripheral and sidewall types of Hovercraft, increase in size, the use of flexible structures and sealing elements and aero dynamic improvements will in general bring about reduced specific power requirements, reflecting increases in lift/drag and payload-to-gross-weight ratio. Considering the size effect alone, this arises from the air-curtaiti horse power being proportional to cushion perimeter, and the resultant lift being proportional to cushion area. For instance, for a nominal circular craft of 25ft diameter and weight of 10 tons (cushion pressure, 451b/sq ft) the air- curtain horsepower required per ton of lift per foot of hoverheight would be about 64, while a 100ft diameter craft at 160 tons (cushion pressure as before) w>uld have a corresponding value of tolly 16. This 4 to 1 reduction would, of bourse, be felt on a practical craft only f hoverheight remained unchanged, i.e. f hoverheight-to-diameter ratio was illowed to decrease. In practice, an ncrease in hard-structure cleaiance height with increase in size is likely, but fas will most probably be accom- 'lished by the use of flexible skirts or funks. For the same reason that the elative power penalty of the peripheral urtain decreases with increase in craft •ze, so will the relative skirt-drag enalty decrease. The net result will be reduction in specific curtain horse- 'Ower requirement. Hence we may °nfidently expect that as Hovercraft r e developed in the 100- to 200-ton a nge, horsepower per ton-knot values "" fall. And as horsepower per ton- f JJ (right): Trends of ACV £ EAS?*. (Per'Pheral-jet and TON-KNOT ^all) i„ relation to similar trends oiner forms of overwater pas-m Ser transport knot appears to have a direct bearing on first cost we may expect a corres ponding decrease in specific first cost. Studies for a 150-ton peripheral-jet ACVf show that we may expect the cost per ton-knot to be on a level with ferry boats and aircraft. An interesting feature of Fig 8 J is that although the £/ton-knot values are similar for ferry boats and aircraft, the mean h.p./ton-knot values for aircraft are about double those of the ferry boats. This tends to suggest that the cost of the boats is kept up by the sheer bulk size of the craft in relation to its payload, and that the influence of the powerplant is not so great on first cost as it is for aircraft. By building in heavier structure, equipment and fur nishings, the useful life of the vessel is, of course, extended; and, presumably, the correct compromise between useful life and first cost has long ago been established. As world prices rise, the values in Fig 8 will also rise; some of the scatter in that illustration may arise from the fact that the quoted prices which have been used span the period 1959-1963, and some escalation is inevitably involved. Plenty has been heard of the rise in aircraft prices over the past few years, but it is interesting to take a further look at this factor in the context of this article. Fig 10 shows how the specific first cost of aircraft per ton-knot has risen since 1936. Against this rise there has been the previously discussed fall in the power requirements per ton-knot (Fig 9), which factor produces some of the scatter in Fig 10 and reduces the slope of the curve. Fig 11 brings out the " size " effect, in terms of payload x cruising speed, which benefits the ACV; and, of course, increases in size naturally lead to general development in many direc- * Assistant to the technical director, Hover craft Development Ltd. t The recently announced SR. N4 proposal confirms this argument. % Reproduced in Pt 1. HP TON-KNOT 7n 6- 4- 3- 2- 1- 19: OC-3- 0 19 cot O 0C-4 "Q DC-S ^ JSTEILATION C-7 3 BRI1 REF DATA 40 IS YE; FROM .IGHT INT! NOV 29, 1 I 50 1 ANNIA 100 flF 27 RNAT 362 960 ONAL 19 0 * TON-KNOT 300 200 1930 Fig 9: Decrease in horsepower per payload ton- kt of passenger-carrying aircraft over the past 25 years Fig 10: Rise in specific first cost of passenger-carrying aircraft over the past 25 years 1,000 v////// KEV O AIRCRAFT O HELICOPTERS • HOVERCRAFT •W HYDROFOIL BOATS V FERRY BOATS
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