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HOVERING • on the Brink

16th September 1960
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Which of the following most accurately describes the problem?

THROUGHOUT the history of transport no major innovation

can ever have got away to such a good start as has the air-cushion vehicle, or Hovercraft. It is likely that within the next five years groundcushion-effect vehicles capable of competing successfully with established forms of transport will be constructed in this country.

Meanwhile in the U.S.A., where parallel development is taking place, the Curtiss-Wright Corporation already offers air-cushion vehicles on the open market. Among them is a freight carrier for 6-ton payloads that is capable of operating over . semiprepared hard surfaces or water. This craft, known as the Air Boat and apparently existing only in model form at the moment, is .promised for delivery eight months from receipt of order at 5330,000 .(£117,437) subject to adjustment for individual, .acces

sories required. .

If -overland air-cushion ,vehicles are forthcoming they may present a small threat to wheeled vehicles, but will be more likely to replace aircraft and helieopters Which, • particularly over short distances, are Uneconomic to use.

Central Direction

Five concerns are actively engaged on the production of air-cushion vehicles in this country. Four of them—Saunders-Roe, Ltd., VickersArmstrongs (Marston), Ltd.,. William Denny and Sons, Ltd., and Folland Aircraft, Ltd.—are working under the central direction of Hovercraft Development, Ltd., the organization set up by the Government-sponsored National Research Development Corporation to exploit this new field. The chairman of 'Hovercraft Development is Mr. G. S. Cockerell, who invented the lift principle employed.

Britten-Norman, Ltd., are outside the group and have on their own initiative (but sponsored by Elders and Fyffes, Ltd.) produced a prototype air-cushion vehicle for transporting bananas from Elders' plantations in the Southern Cameroons to deep-water

shipping. If the experiment is successful, more such craft will be used. In this role the air-cushion machines will replace lorries and will, presumably, operate over land and normally unnavigable rivers, and swamps. This is, of course, the sort of job for which the air-cushion machine is particularly suited.

Are they a potential competitor to lorries? I do not think so. Because although development of commercial air-cushion vehicles has proceeded so rapidly, they have substantial limitations for use over land. In fact the majority of development work now under way is directed towards the production of craft to replace or augment shipping. The air-cushion machine proposed by William Denny is, in effect, an air-lubricated ship whilst both Saunders-Roe and VickersArmstrongs appear at present to be more concerned with Hovercraft for use over water, employing their amphibious capabilities merely to simplify docking facilities.

However, the SRN.2 vehicle which Saunders-Roe are building as their second experimental type will be capable of overland operation although, in appearance, it owes more to ship than wheeled-vehicle design.

Of the Hovercraft Developmentaided concerns, Folland Aircraft, the last of the four to make known its interest, is the only one which appears to be going to build craft primarily for overland use. Indeed, it was only in August that they announced (The Commercial Motor, August 19) that a demonstration ground-effect research machine was to be constructed as the precursor of a range of Hovertrucks. The first of these will carry a payload of 5 tons and will have simple controls which should make it almost as easy to drive as a heavy vehicle. It will be no more difficult to maintain. The system of control and propulsion developed for it is claimed to provide better manceuvrability than has been achieved to date by any other groundeffect vehicle.

Controllability is one of the major snags of the Hovercraft as an overland vehicle, at least in congested areas. The well-known SRN.1 research craft employed a fairly rudimentary system of forwardand rearward

facing air ducts for propulsion and braking, with rudders in these ducts for directional control.

This system appeared to demand considerable anticipation from the pilot if any sort of planned course were to be followed. Such controls can and have been much improved, but it is obviously unreasonable to expect manceuvrability from Hovercraft to the fine limits possessed by wheeled vehicles. They should be compared with ships in the matter of control.

This consideration applies only to free air-cushion craft such as those projected in Britain, which have no connection other than ground effect with the surface over which they travel. It does not affect rail-guided vehicles of the type planned by the Ford Company of America.

These rail-guided vehicles are intended for use as high-speed Passenger transport between urban centres. The vehicle travels on a single rail from which it is separated by a very thin layer of air, being, indeed, an airlubricated machine, It would employ air lubrication merely to gain speed and silence; exact directional control would be obtained from the rail, on which the vehicle could presumably exert a braking force. Rail-guided vehicles are much more expensive to put into operation thanfree craft.

Another characteristic of the present air-cushion vehicle which restricts its performance over land is its inability to climb gradients much more than three times as severe as those which can be tackled by non-funicular railways.

Obviously, as the machine is inclined, lift is progressively transferred from the low-pressure air cushion beneath the vehicle to the propulsion medium, whether this is an air duct or a propeller. As the angle increases the vehicle assumes the

characteristics of a rocket and would require immense power to maintain forward progress on really steep gradients. Retardation when travelling down severe inclines presents the same problem.

So the Hovercraft would have no place on the road network in this country, and no one has yet suggested that it should. Indeed, if it were used at all in a country such as Britain the free air-cushion vehicle, because of its inflexibility at terminals, would take the place of railways and never that of flexible road transport.

Use in Scotland?

The only section of the British Isles which could usefully employ Hovercraft might be the islands around Scotland, where its amphibious capability and speed would be invaluable for the inter-island haulier. Otherwise its immediate uses in this country will be for still-water and, later, for cross-Channel ferries.

It is overseas that the air-cushion vehicle will find an enthusiastic reception for use over land. Whereas Britain has 21 miles of road for every square mile of country, there are only 2+ miles for every 25 sq. miles in Canada, 2+ miles for every 100 sq. miles in Africa, and practically no roads in Brazil.

All these countries and many others could benefit from the availability of a really robust high-payload-capacity air-cushion craft capable of traversing all types of terrain. For example, had large air-cushion vehicles been available 10 years ago, exploitation of oil deposits in the Sahara Desert by the French (eventually made possible by the development of very large wheeled vehicles) might have been accomplished much sooner. But I wonder what the effect would be on these vehicles of the sand cloud in which they would inevitably operate under these conditions?

Although the pressure Of air beneath air-cushion machines is quite low, ranging from about 25 to 150 p.s.i., the annular air jets which contain the cushion cannot be prevented from disturbing dust and this, too, weighs against the Hovercraft for use in populated areas.

It could be overcome by operating the machines only over fully prepared, sealed surfaces but this robs them of their main advantage, which is that they require only semi-levelled ground over which to travel. Semi-level " refers to ground free ,from any substantial projections, such as could be prepared by a bulldozer, track pressure from the bulldozer tending to compress the surface to prevent excessive erosion by the air flow.

It is the degree of surface preparation which governs the efficiency of air-cushion craft. If a very smooth surface is provided, the machine needs to be lifted only a small distance from the ground. Thus the power requirement for maintaining the cushion is low, whilst the cushion itself has a low form drag effect to resist propulsion. The rougher the surface the higher must be the craft, so increases in height sharply raise the power requirement for the cushion.

Mr. Cockerell has suggested that a 100-ton Hovercraft capable of speeds up to 60 m.p.h., and provided with a track at least as smooth as a ploughed field, would require about 30-40 b.h.p. per ton. This, in his words, is: " Very much the same power-to-weight ratio as a conventional lorry.

That is not strictly accurate, the average British maximum-capacity vehicle having a power-to-weight ratio of about 6 b.h.p. per ton. It is only among much lighter vehicles that ratios of the order Mr. Cockerell quotes are usual.

Small air-cushion vehicles are difficult to make efficient in terms of payload to unladen weight. For instance, using current lift devices a 5-ton Hovercraft with a hover height of 1 ft. would be able to carry 1 ton at a cushion loading of 25 p.s.i., whilst a 100-ton vehicle would operate 3 ft. above the ground with a 50 p.s.i. cushion loading to carry 50 tons.

Sophisticated Frames

There is also the fact that Hovercraft below about 100 tons require light, sophisticated frames similar to those developed for aircraft, whilst heavier air-cushion vehicles could have a cruder structure similar to ships' superstructure, made from 14 in. thick light-alloy plating. So the larger Hovercraft would be very much cheaper in cost per lb. weight.

Britten-Norman, however, who are not so closely tied to aircraft principles as some of the other firms engaged on this work, suggest that small aircushion vehicles could be produced at no greater cost per lb. than that of light aircraft and, if they were produced in sufficient quantity, this cost might be brought down to 10s.-£1 per lb. to approach that of heavy commercial vehicles. This concern is of considerable interest in that it has set out to establish a market for its projected vehicles by building a cheap, small air-cushion craft which could be used for route-proving trials by prospective purchasers.

The prototype Britten-Norman .Cushioneraft, as it is known, is very simple. It is circular with a diameter of 18 ft. 10 in. and weighs 1 ton unladen. Lift is produced by an annular compressor running round the periphery of the vehicle, the output from which is fed to the flush undersurface of the craft by a continuous, inward-facing duct.

Friction Drive

The compressor is driven by a Coventry Climax 170 b.h.p. petrol engine through a gearbox and a rubber-tyred wheel. Propulsion and control are achieved by two variable and reversible-pitch propellers. The Cushioncraft is designed to operate 1 ft. 3 in. above water or hard surfaces and is controlled by a combined throttle and clutch, which governs the hover height, and a steering wheel which adjusts propeller pitch for forward and reverse movement and changes of direction.

The lift principle of the Cushioncraft is basically similar to that employed for other machines under development in this country. The air cushion is maintained and replenished by a continuous curtain of air around the circumference of the craft. The air ducts for this normally face inwards to present greater resistance to the trapped cnshion of air, which has to bend the curtain to escape.

On over-water craft the efficiency of this system can be improved by solid side 'walls which extend downwards into the 'water to reinforce a large part of the air curtain. This, of course, is not practicable on, land vehicles. However, multi-curtain configurations now under development promise much higher efficiencies than could be obtained with the earlier single-duct tYpes, In the U.S.A. much effort has been concentrated on the plenum chamber principle, whereby air is fed to the underside of the vehicle through a central orifice and is prevented from escaping merely by the restricted space between the edges of the chamber and the surface over which it is travelling. This layout is simple and less expen sive than curtain designs, but gives rise to a serious loss of efficiency as the hover height is increased. Thus, vehicles using this type of lift are normally suitable for use only over still water or well-prepared hard surfaces.

The Curtiss-Wright Air Boat employs this principle. It has a maxi mum hover height of 10 in. over hard surfaces. It appears that this is obtainable only while the vehicle is stationary. When air is bled off from the cushion for propulsion and con trol, achieved by louvres at the body sides and rear, ground clearance drops until the machine is only 3+ in. above the ground at its maximum speed of 47 m.p.h.

Annular Curtain Potential

For true cross-country operation over easily prepared tracks it is the annular curtain type of vehicle, which can quite easily hover at a height approximating to one-tenth of its diameter, that appears most promising. Development of machines using this principle is now well advanced in this country and the first commercially operable vehicles could well come from Britain.

Saunders-Roe consider that the first large-scale 'application of Hovercraft may be for cross-Channel vehicle ferries. The speed of such machines, coupled with their' independence of normal harbours, could greatly improve the flexibility and capacity of ferry services Without the expense of extensive modifications to existing terminal facilities. Loading and unloading of heavy vehicles would be much simplified as the air-cushion ferries could run up ramps on to a hard standing at each end of the journey.

Tags

Organisations: US Federal Reserve
People: G. S. Cockerell

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