Air Cushion Vehicles (ACV)
Fluid cushioned vehicles which are suspended a slight distance above an underlying ground or water surface by pressurized fluid flow output beneath such vehicles are known as an air-cushion vehicle, also known as a ground-effect machine (GEM) or a Hovercraft. These devices are also known as surface effect machines, ground effect vehicles and airborne surface vehicles.
Since they move on a cushion of air, they can maneuver over water and most terrains. Air cushion vehicles can easily traverse terrains from asphalt to quicksand and can be used for tasks ranging from their well-known use as high-speed water transportation for people, vehicles and freight to ice or flood rescue and harvesting cranberries. Since an air cushion vehicle travels on a cushion of air it reduces or eliminates damage to ground surfaces. This makes it the transport of choice when avoidance of environmental damage is of concern. An air cushion vehicle also can travel on surfaces which are not otherwise easily traversable such as thin ice, swamps and marshes.
Air cushion vehicles are generally of two basic types, one type having a flexible or partially flexible skirt to contain lifting air, which leaks out under the skirt and can be controlled to some extent for balance and steering. Usually the propulsion and directional control are obtained by separate means, such as propellers and rudders, on top of the vehicle. The other basic type has rigid side walls surrounding an air chamber, the walls being immersed in or in sliding contact with the supporting surface to minimize air leakage. The supporting air can be used for propulsion, since the energy is well contained, but additional propulsion means is often used.
The rigid sidewall hovercraft have lift engines powered fans that created a pressurized air cushion under the cutter, thereby lifting the craft, thus reducing drag and draft. The solid sidewalls pierced the water, creating a catamaran hull, and the air cushion was sealed by flexible rubberized skirts at the bow and stern. This allowed these craft to operate at high speeds in waters both shallow and deep, making them ideal patrol craft. Their wide beam and the catamaran hull also made them extremely stable craft, even in high seas.
In a sidewall air cushion vehicle the forward end of each sidewall is generally shaped to an edge in order to reduce its drag through the water. Since the inner surface of each sidewall has to co-operate with the front cushion seal to prevent escape of air from the cushion it is usual for this inner surface to be straight and for the outer surface at the forward end of each sidewall to taper or curve inwardly to meet with the inner surface at the leading edge of the sidewall.
A curved or angled surface having movement relative to a fluid produces a force normal to the direction of movement, and because the shape of the forward end of each sidewall is asymmetric a net differential force is produced in each sidewall substantially at right angles to the normal direction of travel of the vehicle. This net differential force, which has to be absorbed by the structure of the sidewall and the vehicle body, can become significant in the large, high speed, type of sidewall air cushion vehicles presently being proposed known as surface effect ships.
Air-cushion vehicles comprise a hull and one or several propulsion units. These units are located either close to the longitudinal axis of the vehicle or symmetrically at both sides thereof in the front or the rear of the vehicle. Turning of the vehicle is obtained, for example, by means of one or several side rudders. When the propulsion units are located symmetrically at both sides of the longitudinal axis of the vehicle, steering can also be carried out by varying the output power of one or several of the propulsion units.
Technically, an inflatable skirt formed of a flexible material, such as rubber, plastic, etc., is mounted beneath the vehicle about its periphery and directs fluid flow from a motive source, such as a fan mounted on the vehicle, through the inflatable skirt against the underlying ground or water surface to both raise the vehicle a short distance, such as nine to twelve inches and no more than a few feet, above the underlying surface as well as propelling and providing thrust rearward of the vehicle for forward or sideway movements as controlled by a steering mechanism.
Large-sized, air cushions vehicles have been devised for multiple passenger and freight use, such as in passenger ferry applications, as well as a small sized versions for use by one or two people. However, in the smaller sized vehicles designed for one or two people, lift becomes a major design factor due to the lack of a large size surface on the vehicle against which the fluid flow force may be directed against the underlying surface of the vehicle and the ground or water surface. Previously devised individual or small sized air cushion vehicles have utilized a sharply angled upward surface approaching 90 degrees on the sides of flanges adjacent the periphery of the vehicle. This has resulted in a low amount of lift, requires greater motive source and air flow and necessitates a larger sized vehicle due to the weight required by the larger sized motor and air source.
When using air cushion vehicles on water, and particularly with smaller craft when moving at speed across water they will operate satisfactorily if the flow of air into the cushion is maintained. If the vehicle moves downwind, the drag, and therefore the thrust required to maintain speed, decreases. Under certain conditions the volume of air fed into the cushion may decrease. If this reduction allows the cushion to decay the craft will "plough in", and cause the occupants to be thrown in the direction of the "plough-in" with possible resultant injury to the vehicle and its occupants.
A primary problem with all air cushion vehicles is stabilization and directional control, since there is no appreciable contact with the supporting surface. The air cushion provides a very low friction support and the vehicle is easily displaced by small disturbances. Small air cushion vehicles, in particular, are affected by movement of the occupants and can be steered, although unreliably, by offsetting the load or balance. For effective control it is necessary to have complete control over the pressure distribution and direction of exit of the air flow from the air cushion supporting the vehicle.
Aerodynamic control surfaces have been used in air cushion vehicles primarily for providing directional control by creating yaw moments. They have not been widely used because they are rather inefficient at the low speeds and large yaw angles which are often encountered in operation. This has led to placement of control surfaces in the slipstream of the propeller or fan used for propulsion, where their effectiveness is considerably improved. With a single propeller the rudder is commonly at the rear. With propellers and their respective engines mounted fore and aft on the vehicle, individual rudders may be located behind each propeller for steering.
Because of the location of their propulsion units, some art air-cushion vehicles had relatively poor steering characteristics as well as inefficient cargo space. The steering response of the vehicle at low speeds is usually poor, when the propulsion units are located close to the longitudinal axis of the vehicle. At high speeds the response is better, but there is then considerable sideway sliding in curves. If the propulsion units are located symmetrically on both sides of the longitudinal axis in the front as well as in the rear of the vehicle, a good steering response can be obtained also at low speed. However, then at least four propulsion units are needed. This is expensive, and furthermore, the load capacity and the free deck area of the vehicle are considerably reduced.
Prior to the 1980s British Hovercraft Corporation Ltd ACV's had been equipped with a number of landing pads built onto the underside of the A.C.V. rigid base structure. The number and location of the landing pads was, in general, dictated by the supporting structure, the arrangement of skirt sub-dividers, and the need to be able to land on the crest of a one in ten slipway without damaging the bottom of the A.C.V. The number of landing pads involved varied from four on an A.C.V. of the Winchester Class (SR.N6) having an all-up weight in the order of 10 tons, and an A.C.V. of the Wellington Class (BH.7) having an all-up weight in the order of 45 tons, up to seven on an A.C.V. of the Mountbatten Class (SR.N4) having an all-up weight in the order of 200 tons.
In addition to the landing pads, A.C.V.'s of the Winchester and Wellington Classes have attachment points which allow a separate external jacking system to be fitted to lift the A.C.V. for skirt inspection and repair, and they also have lifting points to allow the A.C.V. to be slung from a crane or a straddle carrier. On the other hand, a large A.C.V. of the Mountbatten Class was generally lifted by hydraulic jacks provided in hard-standing service areas at shore bases, the jacks lifting the A.C.V. via landing pads, although there is provision for a secondary portable hydraulic system to be fitted for emergency lifts when quickness is not of major concern.
In order to effect directional and/or trim control of an A.C.V., it may be necessary to move one or more of the flexible wall structures forming the vehicle cushion sealing means either towards and away from or laterally relative to the surface over which the vehicle travels. Thus, for example, a fully flexible skirted A.C.V. tends to roll outwardly under the action of centrifugal force when turning about a remote point. Among the various means that may be used to counteract this tendency and to assist in directional control of the A.C.V. are lift vectoring devices. These devices can be divided into two categories: those which physically move the cushion and hence the center of pressure; and those which modify the cushion pressure distribution thus changing the center of pressure position and hence lift moment about the center of gravity. Both of these types may be used to provide rolling and/or pitching moments and hence side and thrust forces, and should surface contact occur at forward speed, some yawing moment.
By effecting inward movement of the skirt hemline along that side of the A.C.V. which is on the inside of a turn, the position of the center of pressure of the cushion can be shifted with respect to the center of gravity to produce a lift moment that counteracts the tendency of the vehicle to roll outwardly. Alternatively, or additionally, the hemline of the skirt may be lifted along that side of the A.C.V. which is on the inside of the turn, so as to allow cushion air to escape, thereby modifying the pressure distribution so that the position of the center of pressure is shifted.
Movement of the flexible skirt for such purposes has generally been effected by mechanical means such as cables and pulleys, or levers and rod arms, operated by hydraulic jacks. Such systems introduce considerable complexity at the underside of the vehicle, which is exposed to a very corrosive environment when the vehicle is operating over a sea surface. This has given rise to problems such as failure of the hydraulic jacks, due to sticking or leakage of hydraulic fluid. Another disadvantage with such systems is that unless a longitudinally extending rigid rod is interposed in the system between the hydraulic jacks and the connection to the flexible skirt, the hydraulic jacks will not give equal movement of the hemline of the skirt over that length of the skirt on which they are acting.
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