Veleučilište u Šibeniku
Ivan Stojanovski / Branimir Maretić
HOVERCRAFT
Seminarski rad
Šibenik, travanj, 2012.
Veleučilište u Šibeniku
HOVERCRAFT
Seminarski rad
Kolegij: Engleski jezik II
Mentor: prof. Željka Periša Perkov
Studenti: Ivan Stojanovski / Branimir Maretić
Matični broj: 13083 1131/ 13057 1131
Šibenik, travanj, 2012.
CONTENT:
1. INTRODUCTION 1
2. HOVERCRAFT 1
2.1 History 2
2.2 Creation of Hovercrafts 2
2.2.1 Thrust Propellers 3
2.2.2 Air Box 3
2.2.3 Lifting Fan 4
2.2.4 Hovercraft skirt 4
3. CONCLUSION 5
4. LITERATURE 6
1. INTRODUCTION
Ahovercraft(air-cushion vehicle,ACV) is acraftcapable of traveling over land, water, mud or ice and other surfaces both at speed and when stationary. 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". Hovercraft are hybrid vessels operated by a pilot as an aircraft rather than a captain as a marine vessel. 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.
Picture 1: Passenger-carrying hovercraft[1]
2. HOVERCRAFT
2.1 History
The first recorded design for a hovercraft was in 1716 put forward by Emmanual Swedenborg, a Swedish designer and philosopher. The project was short-lived and a craft was never built. Swedenborg realized that to operate such a machine required a source of energy far greater than any available at that time. In the mid-1870s, the British engineer Sir John Thornycroft built a number of model craft to check the air-cushion effects and even filed patents involving air-lubricated hulls, although the technology required to implement the concept did not yet exist. From this time both American and European engineers continued work on the problems of designing a practical craft. In the early 1950s the British inventor Christopher Cockerell began to experiment with such vehicles, and in 1955 he obtained a patent for a vehicle that was "neither an airplane, nor a boat, nor a wheeled land craft." He had a boat builder produce a two-foot prototype, which he demonstrated to the military in 1956 without arousing interest. Cockerell persevered, and in 1959 a commercially built one-person Hovercraft crossed the English Channel. In 1962 a British vehicle became the first to go into active service on a 19-mi (31-km) ferry run.
2.2. Creation of Hovercrafts
When building a hovercraft it is imperative that you are sure you have a firm grasp of the important concepts and principles involved. An elementary knowledge of physics is required. Ease of use, cost, availability and safety are all significant considerations when building a hovercraft. Care must be taken in selecting a motor and propeller for the proper function and stability of the hovercraft and to meet your needs for thrust and lift. A good skirt design is essential for stability and of course, body designs must be well thought-out in order to meet your needs for speed and stability. Finally, the rudders must be well weighed out in order to avoid weighing down your hovercraft and also well shaped in order to move air as efficiently as possible.
Picture 2: Design (1. Propellers, 2. Air, 3. Fan, 4. Flexible skirt)[2]
2.2.1. Thrust Propellers
The propeller used to drive the hovercraft along is usually an aircraft type with variable pitch blades. Its speed of rotation must remain fixed to that of the engine and the lift fan. This is because the amount of lift air required dictates the engine speed to drives the lift fan. In turn the amount of propulsion, which the propellers provide, must be obtained by varying the propeller pitch and not its rate of rotation. This system is termed 'integrated lift/propulsion'. A Hovercraft having more than one lift fan and propeller generally has a separate engine for each fan-and propeller unit.
The propellers used on hovercraft can vary from four-bladed versions and about nine feet in diameter on the smaller craft to the four propellers on the SRN4 cross-Channel hovercraft. These are four-bladed and nineteen feet in diameter! On the SRN 4 the pylons on which they are mounted can be rotated to change the direction of thrust. On smaller craft, rudders like on aircraft, are used for direction control.
2.2.2. Air box
The box-like structure at the rear of the hovercraft, right behind the propeller, the box-like structure is called an air box. The air box takes about 10% of the air being pushed backward by the propeller and forces it downward, underneath the hovercraft. There are three small ducts cut into the base of the hovercraft, underneath the air box. Two of these ducts lead into the skirt, which is basically a bag that goes all the way around the perimeter of the craft, while the third duct leads directly underneath the hovercraft.
2.2.3. Lifting Fan
Firstly the volume of air needed is very large and a propeller is designed to be most efficient in open air like on an aircraft. Also the fan needs to force air into the chamber below the craft so creating a specific pressure under the craft. Propellers again are not efficient in applications when an air backpressure will be applied to the propeller blades as they rotate. Because of this the lifting fan on most Hovercraft uses what is known as a centrifugal fan. This is a fan in which two discs and fitted together and looks rather like a doughnut with angled slats at their edges.
When the assembly is rotated at high-speed air is sucked into the center hole in the fan and the slats force it out at the edges. The advantages of the fan are two fold. They operate efficiently in an environment when backpressure is high and they will move larger volumes of air for a given rotation speed than a propeller with the same speed and power input. The lifting fan is coupled via a gearbox to the engine. The engine also drives the propeller on the craft, which provides thrust for forward motion of the Hovercraft.
2.2.4. Hovercraft Skirt
Despite the momentum curtain being very effective the hover height was still too low unless great, and uneconomical, power was used. Simple obstacles such as small waves, or tide-formed ridges of shingle on a beach, could prove to be too much for the hover height of the craft. These problems led to the development of the 'skirt'.
The skirt is a shaped, flexible strip fitted below the bottom edges of the plenum chamber slot. As the hovercraft lifts, the skirt extends below it to retain a much deeper cushion of air. The development of the skirt enables a hovercraft to maintain its normal operating speed through large waves and also allows it to pass over rocks, ridges and gullies.
The skirt of a hovercraft is one of its most design sensitive parts. The design must be just right or an uncomfortable ride for passengers or damage to the craft and the skirts results. Also, excessive wear of the skirt can occur if its edges are flapping up and down on the surface of the water. The skirt material has to be light flexible and durable all at the same time.
3. CONCLUSION
Hovercrafts are generally simple mechanisms in theory. Yet the process from theory to manifestation is not as easy as it may seem. A plethora of problems exist and must be faced in order to attain a well functioning hovercraft. The plans and designs must be flawless. One must take under consideration the weight and the shape of each component in order to avoid problems such as instability and dysfunction. This is a marvelous machine which greatly cuts down the friction which in turn helps it to attain greater speed and more stability.
Varieties of problems and factors have to be taken into account in designing and constructing a hovercraft. The difficulties involved in maintaining stability and functional competency has limited the application to only transportation or for military purpose. The cost involved in the developing of a hovercraft is also another impediment to the widespread use of this machine.
Picture 3. Hovercraft[3]
4. LITERATURE
1. http://en.wikipedia.org/wiki/Hovercraft
2. www.hovertechnics.com
3. www.rescuehovercraft.com
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[1] http://upload.wikimedia.org/wikipedia/commons/d/d3/Hovercraft-MVPP10.jpg
[2] http://upload.wikimedia.org/wikipedia/commons/3/31/Hovercraft_-_scheme.svg
[3] http://www.anything-rc.com/images/hovercraft.jpg