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Aviation History
1963
1963 - 2261.PDF
Much of the accompanying report deals with driving positions, or flight decks, or bridges. This is the Vickers VA-3 Air-Cushion Vehicles FLIGHT Internationa! supplement, 25 July 1963 Above is the cockpit of the Westland SR.N2, showing Decca Flight Log Navigator equipment marine radar, and the straightforward aircraft-type (aileron)elevator sense) control without pedals "Bridge" is obviously the correct term for the driving station of the Denny D.2 marine ACV, which is frequently controlled from a standing position Below, Figs 22 and 23 from Hovercraft Report No 11158 relevant text in Col 3) '•'A. THE COCKERELL PAPERS . . . be to give it controls more usually associated with aircraft or helicopters. Unlike other surface craft, a Hovercraft possesses no resistance to sideways motion, and, therefore, unless some device is let down from the craft to connect with the surface over which it is operating, the only means of bringing about move ment and control must be by direct thrust or incidence thrust. It is interesting to note here that the Hovercraft produced by aircraft firms tend to have aircraft-type controls and are driven by pilots, whilst the craft produced by the Hovercraft subsidiary of the shipbuilders William Denny and Bros Ltd, have simple wheel steering and may be driven by people without any specialized experience in piloting or driving transport craft. It is, of course, only fair to point out that the design speed of any craft has a great bearing on the requirements for operating crew, and that the speed of the Denny D.2 craft is only one-third to one-quarter of that of, say, the SR.N2. However, in the relatively high-speed SR.N2 a simplification in controls over those of the SR.Nl has been achieved by the removal of all foot controls. After discussing various ducting ar rangements to enable air from the fan duct to be bled out fore and aft for propulsion, and sideways for manoeuv ring, Cockerell goes on to deal with the problem of directional instability:— The system so far described is incom plete, because a Hovercraft is unstable directionally, whereas other surface craft are stable directionally. In order, there fore, to make a Hovercraft feel and behave like other surface craft, it is necessary to introduce a system of directional stability, and this is so easy to achieve that it may be made automatic when the craft is in motion, and fitted with a manual over-ride for use when hovering.* When the craft is cruising along, a weathercock in the form of a vertical symmetrical aerofoil which is free to pivot—or alternatively a similar device pivoted or trailing on the surface of the water (as suggested by Mr Stanton Jones)—may be used to control small vertical "rudders" placed in the way of the issuing thrust air, as shown in Fig 22. This will hold the axis of the craft either along its direction of motion through the air, or along its direction of motion with respect to the water. It is suggested that it would be con venient to be able to disconnect or over ride or put a bias on this automatic directional system when hovering, in order to be able to swing the heading of the craft, and also to be able to give the craft motion sideways with respect to the air, while holding the craft still with respect to the water or land surface beneath it. Relatively small forces are required to propel ACVs, and so direct thrust forces required for manoeuvring are correspondingly small. If Hovercraft are incapable of turning quickly to avoid danger, then they must be given an efficient braking system, and it therefore seems inevitable that water-brakes will be used. It also seems possible that the turning characteristics could be improved by the use of drop keels—see Fig 23—but at the speeds (lOOkt) and weights (5,000 tons) involved, the stresses would be j enormous. Fig 23, extracted from Cockerells' report, shows a sketch of the drop keel idea, looking along the direction of motion. The drop keel consists simply of a freely rotating disc partially immersed in the water, offering the possibility of a low-drag control surface. It follows also that this lack of available jet thrust means that the basic stability must be arranged to be inherent in thep configuration adopted, and that the fort® required for static and dynamic trinwiM must also come from control of '»e relative cushion pressure distribution- * This, and the previous reference to auto matic trim control, is probably the fir mention of the idea of applying automat' control systems to Hovercraft.
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