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Aviation History
1959
1959 - 2208.PDF
FLIGHT, 11 September 1959 •4 . . . RELEASE FROM THE WHEEL" THE HOVERCRAFT — by its inventor 195 THERE seem to be only two basic methods of moving a bodyalong a surface: either it is slid along, or the parts of itwhich are in contact with the surface are arranged to roll along. Nearly all land vehicles in existence use the wheel, androll along; and all existing water vehicles use the alternative, and slide along, in or through the water. The sleigh (or the skate) is an interesting exception to normalland vehicles, for here the friction melts a minute quantity of ice and there is a very efficient water-lubricated sliding motion. Fora sliding motion to compete with the rolling motion of a wheel, the key factor is lubrication, and the only suitable substanceuniversally to hand is air. A universal sliding-surface vehicle must, therefore, be air-lubricated, by being separated from thesupporting surface by a layer of air. Early experiments with air lubrication soon showed that thebasic problem is to find an effective method of containing the pressurized air under the vehicle. Assuming that the methodmust be capable of exerting lift at zero speed, perhaps the simplest approach consists of a fan positioned in the top of a box which isopen at the bottom, the air leaking out round the bottom edges. The efficiency of such a system depends upon attaining a lowcoefficient of flow through the gap between the bottom of the vehicle and the ground surface, and upon the extent to whichthe velocity head through the fan can be converted back into pressure head within the box. Such an air-bearing system, although simple, and thereforecheap and attractive on that score, is not very efficient for large clearances, because it requires rather large powers and mass flows,which result in the fan areas being an inordinately large part of the plan area. An analysis of the rim configurations which yielda low coefficient of flow shows that they possess some component of flow normal to the gap; and this leads to the idea of an annularjet discharging inwards, the pressure being contained by the bend- ing of the airstream, as in the SR-N1. However, closed or semi-closed curtain systems are possible, and yield power savings of the order of 100 per cent on the SR-N1, and such improvements byno means exhaust the possibilities. Now this "release from the wheel," with its contact pressuresof a ton or more per square foot, and the substitution of an air- lubricated sliding surface, brings in its train the chance of beingable for the first time to design the pressures so that they are suitable for the surface over which the vehicle has to operate; andit is possible to conceive a whole host of different families of sliding vehicles, each of which requires different treatment, thegoverning factors being the hardness of the surface which supports them, the smoothness of that surface, and the speed of operation.The fact that a moving air curtain rather than the rubber wall of a tyre is used to contain the pressurized air on which they ride is C. S. COCKERELL,M.A., is the inventor of the Hovercraft and adirector of Hovercraft Developments Ltd., thecompany controlling the project on behalf of theNational Research and Development Corpora-tion. Readers will, we think, agree that Mr.Cockerell's article, speci- ally written for "Flight,"is a model of lucidity. purely fortuitous and a matter of convenience, and does not turnsuch a vehicle (which would fall without the surface beneath it) into an aeroplane. Water curtains could be used for slow-speedwater hovercraft. If the vehicle is to run on a prepared surface, then the hardestand smoothest prepared surface is a steel rail, and the vehicle might rest on air-bearing pads, or be hung from a Mono-rail asproposed by Fords. Very small air clearances would be used, so that only a small fraction of the total power would be used in theair-bearings, the majority going to overcome the form drag of the body itself. A suitable yardstick by which to judge the efficiencyof a surface vehicle is the power to overcome form-drag divided by the total power.A review of wheeled vehicles shows that in almost all other applications the wheel is called upon not only to achieve rollingmotion but also to be able to deal with surface irregularities; and it therefore seems logical that the air layer of the sliding vehicleshould also be asked to fulfil this dual role. But in this regard it is perhaps significant that it has been found expedient for most landvehicles to use a two-stage shock-absorber between the surface and the body of the vehicle, i.e., pneumatic tyres and springs ratherthan pneumatic tyres alone. In all air-lubricated systems, lift power is proportional to theclearance, and doubling the clearance doubles the power required —or, due to an area effect, rather more than doubles the powerrequired; and therefore for a land vehicle there is a nice balance between the cost of preparing the track and the cost of runningthe vehicle. The more perfect the track, then the lower the permissible clearance, and the lower the lift power required.Again, with all air-lubricated systems, doubling the linear dimen- sions doubles the lift power but quadruples the gross weight andpayload. Alternatively, instead of taking the gain with size in the form of improved lift efficiency, it can be taken in the form of (Continued on page 198, after double-page drawing o] the SR-N1) History in the making: these cine-type pictures record the successful con- clusion of the Channel crossing by the Sounders- Roe SR-N1 Hovercraft on July 25 last, fiftieth anniversary of Bleriot's pioneer air crossing. The pilot was Peter Lamb, with Mr. Cockerell and Mr. J. B. Chaplin as crew
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