19 July 2009

Hovercraft

The invention: A vehicle requiring no surface contact for traction that moves freely over a variety of surfaces—particularly water—while supported on a self-generated cushion of air. The people behind the invention: Christopher Sydney Cockerell (1910- ), a British engineer who built the first hovercraft Ronald A. Shaw (1910- ), an early pioneer in aerodynamics who experimented with hovercraft Sir John Isaac Thornycroft (1843-1928), a Royal Navy architect who was the first to experiment with air-cushion theory Air-Cushion Travel The air-cushion vehicle was first conceived by Sir John Isaac Thornycroft of Great Britain in the 1870’s. He theorized that if a ship had a plenum chamber (a box open at the bottom) for a hull and it were pumped full of air, the ship would rise out of the water and move faster, because there would be less drag. The main problem was keeping the air from escaping from under the craft. In the early 1950’s, Christopher Sydney Cockerell was experimenting with ways to reduce both the wave-making and frictional resistance that craft had to water. In 1953, he constructed a punt with a fan that supplied air to the bottom of the craft, which could thus glide over the surface with very little friction. The air was contained under the craft by specially constructed side walls. In 1955, the first true “hovercraft,” as Cockerell called it, was constructed of balsa wood. It weighed only 127 grams and traveled over water at a speed of 13 kilometers per hour. On November 16, 1956, Cockerell successfully demonstrated his model hovercraft at the patent agent’s office in London. It was immediately placed on the “secret” list, and Saunders-Roe Ltd. was given the first contract to build hovercraft in 1957. The first experimental piloted hovercraft, the SR.N1, which had a weight of 3,400 kilograms and could carry three people at the speed of 25 knots, was completed on May 28, 1959, and publicly demonstrated on June 11, 1959. Ground Effect Phenomenon In a hovercraft, a jet airstream is directed downward through a hole in a metal disk, which forces the disk to rise. The jet of air has a reverse effect of its own that forces the disk away from the surface. Some of the air hitting the ground bounces back against the disk to add further lift. This is called the “ground effect.” The ground effect is such that the greater the under-surface area of the hovercraft, the greater the reverse thrust of the air that bounces back. This makes the hovercraft a mechanically efficient machine because it provides three functions. First, the ground effect reduces friction between the craft and the earth’s surface. Second, it acts as a spring suspension to reduce some of the vertical acceleration effects that arise from travel over an uneven surface. Third, it provides a safe and comfortable ride at high speed, whatever the operating environment. The air cushion can distribute the weight of the hovercraft over almost its entire area so that the cushion pressure is low. The basic elements of the air-cushion vehicle are a hull, a propulsion system, and a lift system. The hull, which accommodates the crew, passengers, and freight, contains both the propulsion and lift systems. The propulsion and lift systems can be driven by the same power plant or by separate power plants. Early designs used only one unit, but this proved to be a problem when adequate power was not achieved for movement and lift. Better results are achieved when two units are used, since far more power is used to lift the vehicle than to propel it. For lift, high-speed centrifugal fans are used to drive the air through jets that are located under the craft. A redesigned aircraft propeller is used for propulsion. Rudderlike fins and an air fan that can be swiveled to provide direction are placed at the rear of the craft. Several different air systems can be used, depending on whether a skirt system is used in the lift process. The plenum chamber system, the peripheral jet system, and several types of recirculating air systems have all been successfully tried without skirting. Avariety of rigid and flexible skirts have also proved to be satisfactory, depending on the use of the vehicle. Skirts are used to hold the air for lift. Skirts were once hung like curtains around hovercraft. Instead of simple curtains to contain the air, there are now complicated designs that contain the cushion, duct the air, and even provide a secondary suspension. The materials used in the skirting have also changed from a rubberized fabric to pure rubber and nylon and, finally, to neoprene, a lamination of nylon and plastic. The three basic types of hovercraft are the amphibious, nonamphibious, and semiamphibious models. The amphibious type can travel over water and land, whereas the nonamphibious type is restricted to water travel. The semiamphibious model is also restricted to water travel but may terminate travel by nosing up on a prepared ramp or beach. All hovercraft contain built-in buoyancy tanks in the side skirting as a safety measure in the event that a hovercraft must settle on the water. Most hovercraft are equipped with gas turbines and use either propellers or water-jet propulsion. Impact Hovercraft are used primarily for short passenger ferry services. Great Britain was the only nation to produce a large number of hovercraft. The British built larger and faster craft and pioneered their successful use as ferries across the English Channel, where they could reach speeds of 111 kilometers per hour (160 knots) and carry more than four hundred passengers and almost one hundred vehicles. France and the former Soviet Union have also effectively demonstrated hovercraft river travel, and the Soviets have experimented with military applications as well. The military adaptations of hovercraft have been more diversified. Beach landings have been performed effectively, and the United States used hovercraft for river patrols during the Vietnam War. Other uses also exist for hovercraft. They can be used as harbor pilot vessels and for patrolling shores in a variety of police-and customs- related duties. Hovercraft can also serve as flood-rescue craft and fire-fighting vehicles. Even a hoverfreighter is being considered. The air-cushion theory in transport systems is rapidly developing. It has spread to trains and smaller people movers in many countries. Their smooth, rapid, clean, and efficient operation makes hovercraft attractive to transportation designers around the world.

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