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Aviation History
1964
1964 - 1941.PDF
THE TRUE ORIGINATOR?... the rear part of the hull. The other re- sembled the Lightning plenum-chamber hull with which we are more immediately concerned. And now, to supplement the evidence of the patent Specification and the physical appearance of the Lightning model in a photograph, we are able to give an account of the tank tests in Sir John's (for such he was when the account was written) own words. He describes how he "tried models intended to carry air under them in a hollow," continuing: "This construction is favourable to high speed, for the angle of the surface rubbing on the water can be considerably reduced by the adoption of this plan, so reducing the resistance due to gravity on the incline. At the same time the friction due to air on the supporting water is much less than that due to water on a solid; and if the air could be carried with the model, the power required to force it under would be small and would only amount to that necessary to make up waste. It may be safely taken that when air goes under a vessel without forcing, it will fail to give any support. It is evident that the air Air-Cushion Vehicles The model in the fore- ground is of HMS "Light- ning," and that immediately behind is of the same vessel with a hollow bottom in order to be "lifted by air". The circular model is of a form shown in Sir John Thorny croft's 1877 Patent Specification, and the most distant one appears to have a very deep plenum chamber under a vessel to serve this purpose must have a pressure above that of the atmosphere." Later he added: ". . . in a vessel in which the support is due to air in a hollow, the centre of pres- sure must coincide with the centre of gravity of the area of the hollow. It is much more advantageous to have such a definite centre of support than the ever-shifting position of the centre of the lifting force due to contact with the water surface. The practical difficulties, however, to be overcome in producing a type embodying this principle are many. Care must be taken, for instance, to avoid disturbing the water surface by the surfaces which must surround the hollow containing the air. If this be not done, waves will form and increase the rubbing surface, besides taking up energy in their formation. With a model the author designed to overcome these difficulties the resistance has not proved very small, but it was observed that the wake of the model indicated very little disturbance of the water surface. This model was very wide and nearly circular in plan, and it skimmed at quite a low velocity. It may be interesting to state that this form of model was made in 1873. A similar model was towed from a launch at about 8 knots; this speed FLIGHT International supplement, 25 June I96i was more than sufficient to make it rise to the surface. When air was forced under it, the resistance was greatly reduced, but it was never very low. At the best it L 11 to a value of about one- quarter the wujht of the model. At a lower speed—about 5 knots—a greater resistance was recorded." Sir John concluded: "It has often been proposed to fcrce air under a vessel of ordinary form, and Ericson is sdd to have tried this, but found it was not a success. In a skimmer, however, if the air can be carried with the model for the most part, there must be an advantage in this system, and perhaps it might also be used with advantage in very shallow vessels, not intended to skim, but ha> inj a very large surface of bcttom, and not rcqu ring the air to be pumped against much head of water." And here, for the present, we are content to leave the story, although the nams of Thcrnycroft was thereafter to be app.ied more than once in connection with air lubrication or air cushioning. One is eft surmising that, had more efficient pjwerplants and pumps been available during the latter half of the last century, the construction of air- cushion craft—if only for limited applications—might well have come to pass many decades ago. Meanwhile, we in Bniain can be thankful that when the new form of transport does eventually come into its own it will have done so through the ingenuity and the persever- ance of Christopher Cockerell. Postscript: Only after the above had been set in type did I come to realize that this is the centenary year of the firm of John I. Thornycroft & Co Ltd. If Sir John's pioneering air-cushion work is, in fact, to be accorded recogni- tion, then surely there could ^e no more appropriate time than the present. HMSissue 0,"Lightning" for which an air-cushion hull reached the tank-test stage. From a contemporary engraving in if "The Illustrated London News") an 1877 90
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