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Aviation History
1962
1962 - 0482.PDF
480 FLIGHT International, 29 March 1962 SR.N2 . .. Fig 8 A "Flight International'" general- arrangement drawing cf the SR.N2 in its present form. Major contributors to Westland Aircraft (Saunders-Roe Division) are: Bristol Siddeley Engines (Blackburn Engines), powerplants; Dowty-Rotol, propellers; The Airscrew Co and Jicwood, fans; Sperry Gyroscope, steering control system; ENV, spiral bevel gears; SPE, fuel and water pumps; Lelaney Gallay, oil coolers; Saunders Valve, motorized and manual spherical plug valves; Dunlop, flexible skirting and beach ing chassis; RFD, life-saving equipment; Fire proof Tanks, fuel cells; Automotive Products, filters; Plessey, Rotax and Hellerman, electrics speed of 25kt the total fan flow is 4,500 cu ft/sec, and the total head in the plenum chamber is 43 lb/sq ft. Jet velocities are 100 ft/sec (inner) and 135 ft/sec, the cushion pressure at maximum weight is 19 Ib/sq ft (inside the inner jet) and 11 lb/sq ft, and one- third of the total airflow can be discharged through the propulsion nozzles to give an aggregate thrust of 4501b. Drag curves plotted for the SR.N1 in its original configuration showed that the momentum drag due to the air drawn in at the chimney-like intake rapidly became dominant, and amounted to 4001b at the original maximum speed of 25kt. At lower speeds severe wave-drag was experienced when operating over water, manifesting itself as a nose-up trim change. This effect was worst at a little over lOkt, and it created a marked "hump" which could be easily surmounted only on a perfectly calm day. Nevertheless, during the summer of 1959 test pilot Peter Lamb accomplished what has been called "a rather marginal crossing of the Channel." This exploit took place in July 1959, and it was then that Saunders-Roe began serious consideration of what their chief designer, Mr R. Stanton Jones, has called "a real Hovercraft." In his words: "We knew even at that time that the minimum size for a really efficient vehicle was at least 50 tons all-up weight and probably more, but to jump from a five-ton experimental craft like the SR.N1, with only 25kt to its credit, to a properly engineered Hovercraft of 50 tons with a speed of 70kt would have been tech- cal folly, particularly when we were not too certain exactly what we were up against. For this reason we decided to concentrate on the smallest machine which could carry the fan, transmission and control systems that we would like to use in a really large Hover craft of the order of 125 tons or more. This proposal had the great advantage of minimizing both the financial and technical risk." While the configuration of the SR.N2 crystallized, a series of modifications to the original SR.N1 permitted startling increases in weight and speed to be achieved, and thus greatly extended its use ful life. The first major addition was a Blackburn-Turbomeca Marbore turbojet, which added some 7001b to the available thrust. Theoretically a turboprop would have been better, but the Marbore was readily available and easy to instal. To minimize the ingress of spray an aft-facing intake duct was employed, which was later extended rearwards to a large filter box mounted high above the stern. The tiny Marbore made all the difference to the perform ance of the SR.N1; maximum airspeed rose to 45kt, and operation in the Solent became virtually unrestricted. By the end of 1960 Westland were inquisitive enough to add still more thrust in an endeavour to push the SR.N1 so fast that the cushion would begin to break down. Means were available in the form of a Bristol Siddeley Viper rescued from the company's SR.53 intercepter programme. Even when installed in an engine compart ment flanked by large curved ducts from intakes facing inwards at the rear, the effective thrust was some 1,5001b, giving an aggregate push double that previously available. It was found that, although the lift intake became almost completely stalled above 45kt, the cushion continued to support the craft and the maximum true airspeed reached was 62kt; maximum water speed was 68kt. This is still not the end of the story, for late last year the SR.N1 emerged in Mk 4 form, with a pointed bow and stern, a Doppler water-speed measurer and other modifications. Originally schemed as a quick lash-up to prove a principle, the SR.N1 has now logged well over 300hr in over-sea operation, and achieved speeds and weights never dreamed of when it was designed. The SR.N2 The basic philosophy behind the new Hovercraft is explained by Mr Stanton Jones in the passage quoted earlier. Again quoting his company, this may be elaborated upon as follows:— "The machine is to be a prototype fundamentally, but designed for operational research rather than purely engineering research. "It should be designed to have a fairly wide application for use over land and water, and to obtain operational experience as a military vehicle, and more especially as a naval craft. In addition, it should have a civil application in specialized roles. "It should be able to gain operational experience of ground-effect vehicle problems over sea routes used by conventional ferries. "The fan, transmission and control system should be so designed that they can be used directly on economical 70- to 125-ton craft."' By Christmas 1959 it had been decided that the SR.N2 should have a cushion area of 810 sq ft. This is only a 55 per cent increase over the SR.N1: but, by adopting a cushion pressure more akin to that of later machines (see data table), the initial gross weight was fixed at 27 long tons, or five times greater. Negotiations with the National Research Development Corporation were completed in June 1960, whereupon design work began in earnest. Compared WESTLAND (SAUNDERS-ROE) HOVERCRAFT SR.NI Mkl SR.NI Mk2 SR.NI Mk3 SR.NI Mk4 SR.N2 SR.N3 Length (ft) 30 30 30 41 64.5 75 Beam (ft) 25 25 25 25 29.5 — Cushion area (sq ft) 535 535 535 572 810 — Gross weight (long tons) 4 5.5 6.1 7 27 40 Mean cushion pressure f/b/sqft) 16.75 23 25.5 27 75 — Rise height (in) 13 10 8 6 t2-l8 — Max com Max IK> (kt) power b.h.p. 25 435 45 J 435 65 1 435 62 | 435 85 3.260 — 4,000 Ibthrust nil 700 1,500 1,500 nil nil
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