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Aviation History
1962
1962 - 0477.PDF
Wrlill^ STEPPING-STO By the Technical Editor ON September 11, 1959, this journal published an analysis of the original SR.N1 Hovercraft, produced by what is now the Saunders-Roe Division of Westland Aircraft Ltd, at East Cowes, Isle of Wight. Here we continue the narrative up to the SR.N2, which has now started its operational trials. The new machine should prove the vital link between the rudimentary SR.N1 and the wide variety of commercial and military vehicles currently being studied. Before describing the SR.N2 it is desirable to fill in the back ground created by its predecessor, and to outline the theory of air-cushion vehicles in general. The theory has now been fairly completely determined; and it is gratifying to note that the measure ments obtained with most of the ground-effect craft so far con structed tally very closely with predicted values. It is easy for the lay Press and public to hail an invention simply because it is novel and makes a good news-story. Air-cushion vehicles could have fallen into this category; but their potential worth is now known quite accurately, and Figs 1, 2 and 3 overleaf indicate that they will occupy a secure place in the spectrum of transport vehicles. It was in 1953 that Mr Christopher Cockerel! first put forward the idea of supporting a vehicle on a cushion of air at a pressure slightly above atmospheric, contained within a surrounding curtain produced by the ejection of a jet of air from a nozzle around the periphery of the flat underside of the hull. Such a scheme enables the craft to operate over any reasonably fiat and level surface, including water, marshland and most types of terrain. There are as many basic variables in the design of a ground-effect vehicle as there are in an aeroplane. Fundamental factors include the pressure in the jet which forms the curtain, the jet angle (in any vertical plane through the curtain), the jet sweepback (along the sides of the vehicle, to give propulsion), the cushion area, cushion pressure, empty and gross weights, hover height, cruising speed, propulsion and control systems, maximum bow impact forces (from waves, for example), and the behaviour of the craft about all three axes and particularly in pitch and roll. E TO THE ECONOMICAL HOVERCRAFT This is a largely new field, which the Saunders-Roe Division learned to explore with the SR.N1. Anybody can build a working ground-effect machine, as the current profusion of small projects (particularly in the US) demonstrates. It is much more difficult to produce an optimized transport vehicle, which will be commercially successful not on account of its novelty but owing to the fact that it fulfils a genuine need better than any other vehicle. Westland's Saunders-Roe Division are one of four groups licensed to work on Hovercraft (a registered name) based on Mr Cockerelfs idea, by Hovercraft Development Limited, a subsidiary of the National Research Development Corporation, the Govern ment agency responsible for exploiting promising inventions. Saunders-Roe have from the beginning concentrated upon the logical evolution of economic Hovercraft, and it is tremendously encouraging to record that they can see their way with reasonable clarity to the early production of such vehicles. So closely integrated are the factors affecting Hovercraft design that it is difficult to know where to start a discussion; but the basic air-cushion principle varies but little. It might be thought that maximum cushion pressure is achieved by directing the jet to blow horizontally inwards, but in practice this so reduces the effective plan-area of the cushion that the lift produced is not a maximum for any given expenditure of power. As Fig 4 shows, changing the jet angle from 30° (to the horizontal) to 45° typically increases cushion area by some 10 per cent. This means a reduced cushion pressure is used on a larger area, with beneficial side-effects: duct losses are reduced, the air has to be turned through a smaller angle, payload-carrying area is increased, the structure is simplified and less spray and dust is created. One of the smaller curves of Fig 4 indicates that, over a wide range, lift is almost independent of jet thickness, i.e., the width of the nozzle slit through which it issues. This is fortunate, for it makes vehicle design more flexible. The third curve shows that it is desirable to deflect the jet slightly backwards as it leaves the nozzle in order to contribute to the propulsion of the vehicle. A first
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