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Aviation History
1964
1964 - 0543.PDF
Air-Cushion Vehicles Classification of Hovercraft ... Hovercraft is therefore a surface vehicle, and we will pursue the various possible methods of engineering surface vehicles, from the point of view of surface friction. There seem to be only two basic methods of moving a body along a sur- face; either it is slid along, or the parts of it which are in contact with the surface are arranged to roll along. Nearly all land vehicles in existence use the wheel, and roll along; and all existing water vehicles use the alternative, and slide along, in or through the water. The sleigh (or the skate) is an interest- ing exception to normal land vehicles, for here the friction melts a minute quantity of ice, and there is a very efficient water-lubricated sliding motion. For a sliding motion to compete with the rolling motion of a wheel, the key factor is lubrication, and the only suit- able substance universally to hand is air. A universal sliding-surface vehicle must, therefore, be air-lubricated, by being separated from the supporting surface by a layer of air. Now this may be turned into ordinary engineering language by saying that a planing craft is equivalent to a water- lubricated plain bearing; a Hovercraft is equivalent to an air-lubricated plain bearing; and a wheeled land vehicle is equivalent to a roller bearing. Again, I think it is significant that for very heavy loads we use plain bearings; and for very high rotational speeds we again revert to plain bearings; and if we require bearings of very low friction, we use air-lubricated plain bearings. For medium loads and medium speeds, we use roller bearings. So over land, the Hovercraft is cer- tainly "the other thing." We have plenty of examples of land vehicles which roll along, and this includes tracked vehicles3. Over ice and snow we already have both wheeled vehicles and water- lubricated sleighs, and a Hovercraft adds air-lubrication. Over water, we have water-lubricated planing craft, and Hovercraft add the air-lubricated version. It will be seen that for completeness all that is needed is an over-water craft on wheels or tracks, and this would appear to be very difficult to do for small over-water craft, and, it is suggested, just about impossible to do for large over- water craft, i.e., one reverts to plain bearings for really heavy loads. See, however, Note 3. Now the other factor which is as basic as surface friction to the engineering of surface vehicles is the smoothness or roughness of the surface over which they have to pass. All road vehicles operated at speeds above about 15 m.p.h. have suspension systems of pneumatic tyres and springs, and all rail vehicles are 20 fLIGHT International supplement. 27 February CLASSIFICATION OF HOVERCRAFT ii. iii. iv. Vehicles which, while cruising, are supported by self-generated thrust greater than their weizht .-Jcarry all their own fuel. ">*"', and Environment (a) In vacuum (b) In air (c) In water Example: Rocket Rocket Rocket Vehicles which, while cruising, are supported without expending power, by buoyancy. (a) In air (b) At air-water boundary (c) In water Airships Displacement ships Submarines Vehicles which, while cruising, are supported by the supporting members doing work on the supporting (a) In air (b) At air-water boundary (c) In water Aircraft, helicopters Planing craft, Hovercraft (I) Hydrofoils Vehicla which, while cruising are supported by a hard surface, without expending power on that surfacei.e., np wave drag. ' (a) At air-land boundary (b) At water-seabed boundary "All land vehicles. Hovercraft (2)None Note I The cushion pressure sets up a wave motion. Note 2 The air supply to the cushion is there firstly to air-lubricate the bottom and, secondly, is given thickness to act as a suspension system. The imperfections of the engineering system, i.e., the leaks, necessitate a continuous supply, which may be likened to a puncture in a sprung. So, if there is to be a land vehicle for use at more than about lOkt over unprepared tracks, or prepared unsurfaced tracks, not only must it have a very low bearing pressure (Hover- craft?), but it must have a very effective suspension system (flexible Hovercraft ?). Again, just as the roller bearing is discarded for a plain bearing at very high rotational speeds, so there is a maximum speed for a wheel (about lOOkt). If, therefore, there is to be a fast surface vehicle (lOOkt to, say, 300kt), it will have to revert to a lubricated sliding motion (tracked Hovercar?), with an effective dynamically controlled suspen- sion system (tracked flexible Hovercar ?). The sea is the worst of all "unprepared tracks," and for surface craft of above 30kt, say, the choice seems to be between planing craft and Hovercraft, as in Cate- gory iii (b), and the hydrofoil of Category iii (c). The planing craft can be ruled out, both on the score of effic- iency and of ride. This leaves the choice between the flexible Hovercraft and the hydrofoil. Note 1 In the early days, of course, the reverse was true, and nobody would own it. The naval architects could see quite clearly that it wasn't a ship, and therefore none of their business; the aircraft manufacturers felt that it wasn't their business either. Note 2 Perhaps a Hovercraft is something in its own right after all. Note 3 Roll along—that is, not skid. Nature uses reciprocating arrangements such as a man walking, and both lateral and longitudinal wave motions, implying struc- tural flexibility and elasticity, as in the various snakes. All these fall into the category of "roll along," and the engineer could copy these ideas if he wished. BP Exhibits AGVs An ambitious display of air cushion vehicle models was recently arranged by BP at Britannic House, Finsbury Circus, in connection with the showing of four of the company's hover- craft films. In addi- tion to the working exhibition models of current and projected machines seen here, the display included an early test model built by Christopher Cockerell
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