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Aviation History
1963
1963 - 1092.PDF
FLIGHT International supplement, 28 February 1963 Air-Cushion Vehicles to have an increasing number of amphib ious vehicles, whilst use is likely to be made of special-purpose types of aircraft such as the crane helicopter. It may also be unwise to employ ACVs on any form of statically located ferry or pontoon bridge, as these highly expen sive devices would be operating in locations which were priority enemy targets. Hover Sledges These require power for their air cushion from the towing vehicle, or, if manually propelled, from elsewhere. It seems unlikely that they would show any significant gain over much cheaper wheeled trailers, which only badly affect towing-vehicle per formance in soft sand or deep mud. Again, any such sledges would require a good ground clearance over rough ground, and this would need plenty of power. Hover Stretchers These only just clear fiat ground, throw up dust, and become an extra burden to the stretcher bearers when crossing rough country. ACV-mounted Mine-detectors and Ex plosive-hose Layers Against existing types of mine these could be technically successful initially, either by hovering with detectors or by towing or paying- out an explosive hose over the mine field (the hose could then be detonated to clear a lane). However, many modern mines are of wooden or plastic construc tion and the only really safe (if slow) method of detection is by "prodding." Furthermore, detection in the form of minefield reconnaissance must be unob trusive and is generally done at night. If ACVs were used in these roles they could be initially successful, but it would not seem difficult to fit mines with a proximity switch that would be oper ated either magnetically or by the relatively low cushion pressure. From the foregoing it follows that a large number of "gadgets" might employ the ACV principle, but these do not affect the primary function of an ACV, which is to provide a method of transport. It is with this function that this article is mainly concerned. Configurations and Characteristics The basic outlines of possible craft were discussed in Part 2, but to obtain a better idea of the configurations which may be suitable for Army ACVs moving overland and for the Assault Craft (if it is to have a reasonable over land capability) several other aspects have to be considered. Structure All three of the ACVs proposed will be required to carry pay- loads which can include different vehicles with varying concentrations of loads. Loads must be quickly mounted and disgorged, so a through passage for vehicles on to and off the craft would be ideal. It is important to be able to place any combination of loads in such a way that the trim of the craft is easily adjusted. Additionally, it is necessary for the structure to be able to sustain point loads when resting on the ground and to support any weight such as lift fans or engines on its periphery. The most suitable answer to these problems would appear to be to build the craft round a central "bridge," which would act as a backbone for the craft, and to use subsidiary cross members to spread the load over the base. The Base A glance at the miniature graphs in Part 2 shows that the most economical planform shape is that of two semicircles joined by a straight portion. Alternatively the oval shape of the SR.N2 planform could be even more efficient for lift. However, for overland use it would be advisable to keep the width to a minimum, so the planform shown in the accompanying drawing is probably a reasonable com promise. The front section would be required lar ladder" directly above the annular jet, and virtually no ducting was neces sary. The best alternative may be to locate multiple horizontally mounted fans round the craft's periphery. Each fan would feed a section of the annular jet, and these could be made to overlap if necessary. On a large craft such as the Tank Carrier it may pay to have a separate engine for each fan and to locate these engines with them. On smaller craft it may be possible to drive more than one fan from engines located alongside the cargo "bridge." It is appreciated that such systems will involve more engine controls, but the sections of the annular jet could be arranged to compensate for one of the engines failing. Several ACV designs show forward- facing air intakes to the lift fans. Pre sumably this is done with a view to assisting propulsion. It is felt, however, that to turn the lift air (a large mass flow) through 90° is probably as wasteful in energy as any propulsion gain. Furthermore, water, spray and dust are more likely to enter the fans. Separately Suggested planform, showing arrangement of load accommod ations and ramps RAMP DOWN CENTRAL BRIDGE FOR PAYLOAD INCL. VEHICLES. RAMP ] RAMP DOOR DOWN to be extended to form a bow, and this should be sufficiently strong to deflect scrub and other light obstacles beneath the craft. Rear and front loading doors will be required, and these could conveniently be combined as front and rear ramps. The results of using this planform may raise the overall length and/or width beyond that given as L and B for the basically rectangular planform cal culated for the Logistic Supply ACV, but it may be that a slightly higher cushion pressure could be used to compensate for this. Lift The use of one or two large, centrally mounted lift fans is not thought to be the most efficient means of pro viding the cushion from an annular jet, as the large amount of ducting needed will involve considerable losses. The most efficient ACV from this point of view is probably the Britten-Norman Cushioncraft CC-1. The air in this craft was sucked into a rotating "annu- mounted propulsion units have there fore been chosen affording an unrestric ted air intake to the horizontally mount ed lift fans. Propulsion For overland ACVs the problems of control will be more severe than over water, and propulsion power must be made available to counter inertial forces and the effects of gradients. Two systems may be suitable. Reversible-pitch propellers can be driven by separate engines, two mounted almost the width of the craft apart at the rear and two mounted centrally on pylons. In this system the craft would be turned by means of rudders on the rear fins and by varying the thrust on the rear propellers, whilst the centrally mounted propellers would swivel to oppose inertial forces and the effects of gradients. Another possibly suitable system would be to mount the two central engines forward and to enable all four engines to swivel. Separately mounted 27
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