04-12-2012, 11:40 AM
Hovercraft
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A hovercraft, also known as an air-cushion vehicle or ACV, is a craft capable of travelling over land, water, mud or ice and other surfaces both at speed and when stationary. Hovercraft are hybrid vessels operated by a pilot as an aircraft rather than a captain as a marine vessel.
They operate by creating a cushion of high-pressure air between the hull of the vessel and the surface below. Typically this cushion is contained within a flexible "skirt". They typically hover at heights between 200mm and 600mm above any surface and operate above 20 knots and can clear gradients up to 20 degrees.
The first practical design for hovercraft derived from a British invention in the 1950s to 1960s. They are now used throughout the world as specialised transports in disaster relief, coastguard, military and survey applications as well as for sport or passenger service. Very large versions have been used to transport hundreds of people and vehicles across the English Channel whilst others have military applications used to transport tanks, soldiers and large equipment in hostile environments and terrain.
History
Early efforts
There have been many attempts to understand the principles of high air pressure below hulls and wings. To a great extent the majority of these can be termed "ground effect" or "water effect" vehicles rather than hovercraft. The principle difference being that a hovercraft can lift itself while still, whereas the majority of other designs require forward motion to create lift. These active-motion "surface effect vehicles" are known in specifically cases as ekranoplan and hydrofoils.
The first mention in the historical record of the concepts behind surface effect vehicles that used the term hovering was by Swedish scientist Emanuel Swedenborg in 1716. [1]
In 1915 Austrian Dagobert Müller built the world's first "water effect" vehicle. Shaped like a section of a large aerofoil (this creates a low pressure area above the wing much like an aircraft), the craft was propelled by four aero engines driving two submerged marine propellers, with a fifth engine that blew air under the front of the craft to increase the air pressure under it. Only when in motion could the craft trap air under the front, increasing lift. The vessel also required a depth of water to operate and could not transition to land or other surfaces. Designed as a fast torpedo boat, the Versuchsgleitboot had a top speed over 32 knots (59 km/h). It was thoroughly tested and even armed with torpedoes and machine guns for operation in the Adriatic. It never saw actual combat, however, and as the war progressed it was eventually scrapped due to lack of interest and perceived need, and its engines returned to the Air Force.[2]
Post-war progress
John Frost at work in his Avro Canada laboratory. The silver disk is being lifted into the air by a jet of air from the red pipe, blowing down on top. The curved shape directs the air around the outside to produce stable lift.
When the war ended a number of groups took up development of low friction water vessels including hydrofoil and water effects. The Soviets, specifically Rostislav Alexeyev and his Central Hydrofoil Design Bureau, returned to the ground effect design pioneered by Levkov, and produced a wide variety of such craft over the next 30 years. However, Alexeyev's systems were always experimental, and never entered production. Famous among these is the Lun-class ekranoplan, a massive missile-firing boat powered by eight jet engines. Unlike Levkov's design, however, these boats generally lacked a lift engine, using power and short wings to create lift at speed. This represented a unique evolution of craft but were also not classed as true hovercraft.
In the 1950s and 1960s in Canada, John Carver Meadows Frost at Avro Canada started experimenting with the Coandă effect and noticed that he could produce a ring of airflow by blowing the air down over a convex surface. He originally intended this to produce VTOL lift, and then use the same basic effect to provide for high-speed, high-altitude flight. This work led to the development of the Avrocar with the US Army, a design of much more modest performance more typical of a modern helicopter. In testing it proved incapable of flying more than a few feet off the ground and at speeds greater than about 45 km/h, and after a lengthy period of testing the program was abandoned in 1961.
Christopher Cockerell
The idea of the modern hovercraft is most often associated with Sir Christopher Cockerell. Cockerell came across the key concept in his design when studying the ring of airflow when high-pressure air was blown into the annular area between two concentric tin cans, one coffee and the other from cat food. This produced a ring of airflow, as expected, but he noticed an unexpected benefit as well; the sheet of fast moving air presented a sort of physical barrier to the air on either side of it. This effect, which he called the "momentum curtain", could be used to trap high-pressure air in the area inside the curtain, providing lift based on pressure, not airflow. In theory, only a small amount of active airflow would be needed to create lift and much less than a design that relied only on the momentum of the air to provide lift, like a helicopter. In terms of power, a hovercraft would only need between one quarter to half of the power required by a helicopter.
Skirts and other improvements
Testing quickly demonstrated that the idea of using a single engine to provide air for both the lift curtain and forward flight required too many trade-offs. A Blackburn Marboré for forward thrust and two large vertical rudders for directional control were added, producing the SR.N1 Mk II. A further upgrade with the Armstrong Siddeley Viper produced the Mk III. Further modifications, especially the addition of pointed nose and stern areas, produced the Mk IV.
Although the SR.N1 was successful as a testbed, the design hovered too close to the surface to be practical; at 23 cm[citation needed] even small waves would hit the bow. The solution was offered by Cecil Latimer-Needham. In 1958 he suggested the use of two rings of rubber to produce a double-walled extension of the vents in the lower fuselage[citation needed]. When air was blown into the space between the sheets it exited the bottom of the skirt in the same way it formerly exited the bottom of the fuselage, re-creating the same momentum curtain, but this time at some distance from the bottom of the craft.
Latimer-Needham and Cockerell devised a 4 foot (1.22 m) high skirt design which was fitted to the SR.N1 to produce the Mk V,[9] displaying hugely improved performance, with the ability to climb over obstacles almost as high as the skirt[citation needed]. In October 1961, Latimer-Needham sold his skirt patents to Westland, who had recently taken over Saunders Roe's interest in the hovercraft. Experiments with the skirt design demonstrated a problem; it was originally expected that pressure applied to the outside of the skirt would bend it inward, and the now-displaced airflow would cause it to pop back out. What actually happened is that the slight narrowing of the distance between the walls resulted in less airflow, which in turn led to more air loss under that section of the skirt. The fuselage above this area would drop due to the loss of lift at that point, and this led to further pressure on the skirt.
Commercialisation
With these improvements the hovercraft became an effective transport system for high-speed service on water and shallow land, leading to widespread developments for military vehicles, search and rescue, and commercial operations. By 1962 many UK aviation and ship building firms were working on hovercraft designs, including Saunders Roe/Westland, Vickers-Armstrong, William Denny, Britten-Norman and Folland.[10] Small-scale ferry service started as early as 1962 with the launch of the Vickers-Armstrong VA-3. With the introduction of the SR.N4 in 1968, hovercraft had developed into useful commercial craft.
Design
Hovercraft can be powered by one or more engines. Small craft, such as the SR.N6, usually have one engine with the drive split through a gearbox. On vehicles with several engines, one usually drives the fan (or impeller), which is responsible for lifting the vehicle by forcing high pressure air under the craft. The air inflates the "skirt" under the vehicle, causing it to rise above the surface. Additional engines provide thrust in order to propel the craft. Some hovercraft use ducting to allow one engine to perform both tasks by directing some of the air to the skirt, the rest of the air passing out of the back to push the craft forward.
Uses
Commercial
The British aircraft manufacturer Saunders-Roe developed the first practical man-carrying hovercraft, the SR.N1, which carried out several test programmes in 1959 to 1961 (the first public demonstration in 1959), including a cross-channel test run in July 1959 piloted by Peter ("Sheepy") Lamb, an ex-naval test pilot and the Chief Test Pilot at Saunders Roe. Christopher Cockerell was on board, and the flight took place on the 50th anniversary of Louis Blériot's first aerial crossing.[12]