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Aviation History
1962
1962 - 0479.PDF
FLIGHT International, 29 March 1962 SR.N2 . . . Allowing for 20 per cent of the thrust to be provided by the angled peripheral jet, 80 per cent must be imparted by a separate system. The speed range of ground-effect vehicles is such that the most efficient mechanism is an aircraft-type propeller, and it is fortunate that the optimum thrust-line to hold the craft in a level attitude throughout the speed range is high enough for propellers to clear the superstructure. At the same time, this high thrust-line can be combined with a pivoting or deflection system to impart a powerful rolling moment in the correct direction for properly banked turns to be made. If the lateral force required to effect a turn had to be imparted by the jets the vehicle would roll in the direction opposite to that desired, and in any case the turning power would be quite inadequate. In a transport Hovercraft the fundamentally poor lift/drag ratio is counteracted by the fact that relatively large loads can be carried, and it is largely this fact which enables ground-effect vehicles to compete with existing forms of transport (Fig 2). Transport per formance depends upon payload density, usable floor area and cushion area (which are normally directly related), cushion pres sure, hover height and cruising height, cruising speed and range. Optimum cushion pressure varies with the density of the payload, but for passengers and cars (which are of comparable density) one obtains a curve similar to that of Fig 6. Mean cushion pressure rises with design range, for proportionately more fuel is carried and fuel is dense. When optimum cushion pressure is plotted against gross weight the result has the surprising form of Fig 6. This shows that at a range of 200 n.m., minimum cushion pressure is reached at a gross weight of 200 tons or less (and with an even smaller vehicle over a shorter stage). It can thus be seen that there is no economic advantage in building ground-effect vehicles much larger than this —although giant specimens may prove to be desirable on grounds of traffic, over-wave performance or other considerations. In this context it.is worth noting that most present ground-effect vehicles are designed, stressed and constructed in traditional aircraft ways, using standard aircraft materials. For gross weights much above 200 tons these techniques would become impracticable, and a switch to large-ship methods would result in a rise in percentage structure weight. The latter is a critical factor in determining the economic characteristics of such vehicles. The SR.N1 Detailed design of this pioneer craft began under contract to the National Research Development Corporation in October 1958. The original idea was to use an Alvis Leonides piston engine bevel- geared to a vertical fan discharging through a single nozzle around a hull of relatively slender elliptical form. The nozzle was to be 3in wide and pointing radially inwards at 45°, and propulsion was to be provided by bleeding air from the plenum chamber and discharging it through an aft-facing duct on either side. Fig 7 A comparative portrait of the two Westland (Saunders-Roe Division) Hovercraft so far built, at rest on the slipway at East Cowes. The difference between the two craft in weight and performance is much greater than their relative sizes suggest lOO 200 300 400 GROSS WEIGHT ( LONG TONS) Fig 6 A plot of optimum cushion pressure against gross weight, showing that for short ranges the most efficient Hovercraft willl weigh less than 300 tons It was appreciated that a separate propulsion-engine would have been preferable, but this was ruled out on grounds of cost. A more serious cause for concern was the fact that it was by no means certain that a single-nozzle curtain system could be made stable. After extensive model tests it was decided to extend the machine's plan area and add an additional nozzle, separated from the original (inner) nozzle by a distance equal to twice the design hover height. By this time the SR.N1 had grown to a total cushion area of 535 sq ft, and the estimated weight had risen from 4,0001b to about 6.6001b. Even so, considerable '*beefing-up" was then necessary in order to meet certain design cases which became apparent during detailed stressing. The worst cases were: engine failure over waves of critical length and 2ft vertically from trough to crest, so that the stern hits one wave and tips the bow on to the next; under the same conditions, when the machine just clears the first wave and ploughs into the next; and, worst of all, the crash case in which the machine impacts a large log or other object. The first two cases produce maximum acceleration at the bow in excess of 1g; the log impact of some 4g could be allowed to buckle the bow, but it was essential that the engine and surrounding structure should remain intact. Moreover, the upper decking had to withstand the movements of heavy men with large boots, and the whole craft had to be con structed from standard material in the company's stores. When the SR.N1 was finally rolled out on its beaching chassis in the spring of 1959 it weighed 8,5001b. Free hovering trials began at East Cowes on June 1, 1959, and the machine was soon operating well out in the Solent. The most serious snag was that the valves in the propulsion ducts had to be redesigned; these ducts now faced both fore and aft, as shown in the cutaway drawing in Flight for September 11, 1959. Another major problem was the severity of the cloud of dust generated over land and spray over water; 2hr at sea left 0.25in of salt on the Leonides cylinders. This was particularly perturbing in view of the fact that future Hovercraft would have a higher cushion loading, and give rise to conditions then thought to be much worse. As installed in the SR.N1 the Leonides gives a maximum of some 435 b.h.p., and it drives a four-bladed axial-flow fan. At the design Continved on page 480, after double-page drawing of the SR.N2
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