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Aviation History
1964
1964 - 0542.PDF
ft/CHT International supplement, 27 February 1964 Air-Cuthion Vehicle* The Classification of Hovercraft BY C. S. COCKERELL WHEN THE NEWSPAPERS first heard about the Hovercraft, they classified it as an aircraft, and, indeed, most members of the aircraft industry see it in that light, and it is so licensed. I attended a lecture of the Helicopter Society, and the eminent chairman took it for granted that it was of the genus "helicopter"— a bad helicopter, of course, but since it could hover, a helicopter. Articles in the motoring press look on it as a new- fangled motor car—after all, cars have been riding on four air cushions for years. The Navy naturally classifies it as a ship—it goes over the sea, doesn't it?1 Apart from all this being rather amus- ing, is it not, when one comes to think of it, also very significant? A 20kt Hover- craft has the characteristics of a ship, an air-lubricated ship maybe, but still a ship. As one goes up the Hovercraft speed range, 5Okt, lOOkt, and later to partially cushion-supported, partially aerodynamic-lift-supported craft of 15Okt or so, the thing looks more and more like an aircraft—structures, engines, controls, the lot—so it is an aircraft, after all. On land, well, on land it obviously isn't a ship or an aircraft, so it must be a motor car. But surely, the significant thing is that it lies in the no- man's-land between all these arts, and links them, and equally significantly, borrows techniques from each of them2. In the light of all this, it is worth trying to place Hovercraft in the genus "Vehicle," see accompanying graph, and to see whether anything of value can be got out of some rather dry arguments. A vehicle may be denned as a device which is capable of supporting a load and translating it from one place to another. This definition implies two ideas, lift and drag; and of the two, lift is the more fundamental, because if one cannot support the load, it is not much use trying to propel it. Vehicles may be classified, therefore, according to their method of supporting a load and according to the environment in which they are designed to operate in cruising conditions as shown in the table. It is necessary to insist on "in cruising con- ditions," because, for example, a flying- boat when stationary is a displacements «ip; and in the process of taking off, a Planing craft; and "in cruising con- ditions," an aircraft. Category iv (a), vehicles on a hard surface boundary, is really the limiting c of Category iii (b). In practice, a microscopic-to-small amount of "land wave drag" exists in the case of land vehicles on roads and rails. One can feel the shake of a heavy lorry passing, and feel the earth wavetrain set up by and travelling with an express train if one is sitting in a station. These vehicles must be experiencing a small attitude drag due to the finite stiffness and finite inertia of their supporting surface. I think that three things can be got out of the classification of the table. The first is that there cannot be any other categories as defined, based on the known laws of Nature. The second is that we already possess examples of vehicles operating in all the possible environments, i.e., in space, in air, at the air-water boundary, at the air-land boundary, and in water, with the one exception of the water-sealed bound- Principcri types of vehicle ary, which does not look very im- portant at present. The third thing is that, if the above is true, then future developments can only lie in the realm of improvements in the engineering of the vehicles for the various environ- ments; for example, air-lubricated dis- placement ships, laminar-flow wing, over-water craft on wheels, etc, or alternatively by a mixture of supporting systems. A hovercraft is therefore not a new category of vehicle (except in so far as it is amphibious), but rather its position may be described exactly by the title of the first British patent: "Improvements in or relating to vehicles for travelling over land or water." In other words, it represents an alternative engineering approach for vehicles operating at the air-water or air-land boundaries. A SURFACE SUPPORTED \ |\ ! 20 30 40 50 60 70 80 100 200 300 400 500 700 1000 SPEED - KNOTS 2000 19
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