[attachment=2222]

HOVERCRAFT
1. INRODUCTION
Vehicles designed to travel close to but above ground or water. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect). Hovercrafts are usually supported by fans that force air down under the vehicle to create lift, Air propellers, water propellers, or water jets usually provide forward propulsion. Air-cushion vehicles can attain higher speeds than can either ships or most land vehicles and use much less power than helicopters of the same weight. Air-cushion suspension has also been applied to other forms of transportation, in particular trains, such as the French Aero train and the British hover train.
Hovercraft is a transportation vehicle that rides slightly above the earthâ„¢s surface. The air is continuously forced under the vehicle by a fan, generating the cushion that greatly reduces friction between the moving vehicle and surface. The air is delivered through ducts and injected at the periphery of the vehicle in a downward and inward direction. This type of vehicle can equally ride over ice, water, marsh, or relatively level land.
2. HOVERCRAFT
2.1. HOVERCRAFT 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.
2.3. HOW DOES A HOVERCRAFT WORK
Hovercrafts work on the two main principles of lift and propulsion. When dealing with a hovercraft, the existence of lift is imperative for the proper function of the vehicle. Lift is an essential factor because it is that which allows the craft to ride on a cushion of air several inches off the ground. This process, the process of attaining lift begins by directing airflow under the craft. In order to quarantine the air under the air cushion, a skirt is required. This is done in order to create pressure under the hovercraft which forces the vehicle off the ground. Attaining the proper amount of airflow is imperative for the maintenance of the craftâ„¢s stability. If too much airflow is directed under the craft, it will then hover too high above the ground, resulting in the hovercraft to tip. Not enough lift will cause the craft to remain on the ground which defeats the very purpose of the hovercraft altogether. The source of the airflow which propels the craft of the ground is a fan. The fan can be used for lift and thrust. It can be dedicated to lift or thrust or even both simultaneously. In either case the passage where the air flows through to reach the air cushion affects the stability of the hovercraft. This passage is a hole located on the base of the craft. Another vital component is the motor. The motor is usually located in the rear of the vehicle and is the heaviest of the components. Due to the weight of the motor, extra pressure is required under the area where the motor is positioned in order to attain hovering capabilities.
That which makes hovercrafts so efficient and different from other vehicles of its category is that very little force is required for it to move. Propulsion is that which makes the craft move. The source of this effect is the fan, which is used to move the air for propulsion. However odd as it may seem, the fan produces more than enough force for the hovercraft to move. This is achieved through the existence of another major factor:
Hovercrafts have no contact with the ground; therefore any resistance the ground may produce under other circumstances is now non-existent for the craft. As explained above, the propulsion of the craft requires a fan but a normal fan is not sufficient. This is because a normal fan does not blow air straight back. Instead it spins the air in a spiral shape. Therefore engineers decided to use turbines or stationary blades, that un-spin the air. When air does not spin more of its kinetic energy can be used for translation and less is required for rotation.
The shape of the body also affects the stability of the hovercraft. The larger the area of the base, the more stable it will be. Wider base implies greater stability. Longer and narrower shapes increase speed but decrease stability. Most hovercrafts have rounded ends, and offer both stability and speed.
The skirt is another vital component. The common skirt is known as a bag skirt. It is comprised of a bag that covers the bottom of the base and has holes in it to allow air to escape and push the craft off the ground. Each part of the skirt inflates independently which makes repairs much easier and improves stability. Unfortunately, the more stable a skirt, the slower it will go.
When the hovercraft is finally able to move it will most definitely require steering capabilities. This is achieved through the use of rudders. These rudders can be controlled by a variety of devices including computers. Rudders cannot be too heavy otherwise they will weigh down the craft because they are located very close to the motor. The shape of the rudder dictates how well it will be able to move air.
When riding a hovercraft the natural state of motion is easily seen to be constant vector velocity with a constant rate of rotation. A sloping floor will definitely change your velocity vector without changing your rate of rotation. In addition to Newtonâ„¢s three laws of motion it will become obvious that to avoid spinning or tilting the hovercraft you must apply the forces in line with the center of mass of the combination of the craft and your body.
2.4. PARTS OF A HOVERCRAFT
2.4.1. 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.
2.4.2. 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.
2.4.3. Momentum Curtain
When early models were built and analysis was done on the airflow using the plenum chamber type of hovercraft it showed that there were problems with stability. In addition the craft would require enormous power to maintain a reasonable hover height.Stability of the hovercraft on its cushion of air remained a real problem despite some design efforts and a new approach was needed. To solve these problems, a plenum chamber with a momentum curtain was developed by Sir Christopher Cockerall.
2.4.4. 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.
For the skirt to meet all of its requirements the design and use of new materials has slowly evolved. The current skirts use Ëœfingers at the lower edge of the skirt envelope which can be unbolted and replaced. By doing this there is a quick and easy way to counter the effects of wear without having to replace the whole skirt structure. A shocking example of the costs is the replacement of the skirt assembly on the SRN 4â„¢s which used to cross the English Channel from the UK to France. The replacement cost for a set of skirts for this craft is over 5 million US Dollars.
2.4.5. The Engine
The SRN 1 and other early hovercrafts used piston type engines. As models like the SRN 4 and SRN 6 were brought into service they tended to favor the use of gas turbines. This type of engine is smaller and lighter for a given horsepower and has been used extensively in turbo prop aircraft.

The engine has a main shaft on which is mounted a compressor and a turbine. A starter motor is connected to one end of the shaft and the other end is connected to the lift fan and propeller gearboxes. Both compressor and turbine look like fans with a large number of blades.
When the engine is started, the compressor compresses air from the engine intakes and pushes it into combustion chambers mounted around the engine. Fuel is squirted into the combustion chambers and ignited. The compressed air then rapidly expands as it is heated and forces its way out through the turbine to the exhaust. As the gas pressure rises, the turbine speeds up, thereby driving the compressor faster. The engine speed increases until it reaches the engine's normal operating speed.
However, the use of these engines results in a very high level of engine noise outside the craft. In the SRN 6 this meant that it was possible to hear the craft traveling across the Solent between Portsmouth and the Isle of Wight in the UK several miles away. With the newer generation of craft close attention was paid to engine noise and fuel efficiency. The current AP188craft that runs on the old SRN6 routes has now moved back towards piston engines and uses marine diesel engines that are much quieter and fuel efficient.
2.4.6. 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.
2.5. HOVERING POWER
Take a hovercraft which, complete with crew, fuel and load, weighs 2,000 pounds (lbs.), and is 15 feet (ft.) long and 7 ft. wide. Its area would be 15 ft x 7 ft. = 105 square (sq.) ft.
If the craft is to hover, the pressure of air forming the cushion must be 2,000 pounds or greater. This represents 19 pounds. Per sq. ft. Yes, only 19 pounds per square feet is required to lift the hovercraft which seems much smaller than you might imagine. From existing designs of Hovercraft that have been developed, it is possible to make some simple estimate of the power needed to lift a Hovercraft. Using 19 pounds per square foot it is estimated 4 horsepower for each sq. ft. of curtain or skirt area can maintain that hover.
Curtain area is its length times its height. A hovercraft 15 ft. long by 7 ft. wide would have a curtain length of 44 ft.-twice the length plus twice the width.
If we want it to hover one foot high we would need sufficient power to provide a curtain of 44 x 1 sq. ft. At 4 horsepower per sq, ft. we would need 176 horsepower Just to lift the craft up to hover one foot above the ground. Don't forget we now need to push the craft along as well so that engine is the minimum size we can use.
2.6. RUDDERS
When the hovercraft is finally able to move it will most definitely require steering capabilities. This is achieved through the use of rudders. These rudders can be controlled by a variety of devices including computers. 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.
Rudders cannot be too heavy otherwise they will weigh down the craft because they are located very close to the motor. The shape of the rudder dictates how well it will be able to move air.
2.7. HOVERCRAFT OPERATION
Piloting a hovercraft is an interesting proposition. Since very little of it actually touches the ground, there isn't much friction, making it very difficult to steer and also very susceptible to strong winds. Imagine trying to drive around on top of an air-hockey puck! We've discovered that the best way to drive it is treat it like a jetski, i.e. leaning back and forth and steering very carefully. It is also possible to do a 360-degree turn without stopping, which is quite a sight.
2.8. AERODYNAMICS
Aerodynamics is defined as the branch of fluid physics that studies the forces exerted by air or other gases in motion. Examples include the airflow around bodies moving at speed through the atmosphere (such as land vehicles, bullets, rockets, and aircraft), the behavior of gas in engines and furnaces, air conditioning of buildings, the deposition of snow, the operation of air-cushion vehicles (hovercraft), wind loads on buildings and bridges, bird and insect flight, musical wind instruments, and meteorology. For maximum efficiency, the aim is usually to design the shape of an object to produce a streamlined flow, with a minimum of turbulence in the moving air. The behavior of aerosols or the pollution of the atmosphere by foreign particles are other aspects of aerodynamics.
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.
4. REFERENCE
1. quicktechhobby.com
2. hovertechnics.com
3. rescuehovercraft.com

[attachment=4805]




HOVERCRAFT
abstract
A Hovercraft can travel over all types of

surfaces including grass, mud, muskeg, sand,

quicksand, water and ice .Hovercraft prefer

gentle terrain although they are capable of

climbing slopes up to 20%, depending upon

surface characteristics.

[attachment=4956]

hovercraft full report

. INRODUCTION

Vehicles designed to travel close to but above ground or water. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect). Hovercrafts are usually supported by fans that force air down under the vehicle to create lift, Air propellers, water propellers, or water jets usually provide forward propulsion. Air-cushion vehicles can attain higher speeds than can either ships or most land vehicles and use much less power than helicopters of the same weight. Air-cushion suspension has also been applied to other forms of transportation, in particular trains, such as the French Aero train and the British hover train.

Hovercraft is a transportation vehicle that rides slightly above the earth’s surface. The air is continuously forced under the vehicle by a fan, generating the cushion that greatly reduces friction between the moving vehicle and surface. The air is delivered through ducts and injected at the periphery of the vehicle in a downward and inward direction. This type of vehicle can equally ride over ice, water, marsh, or relatively level land.

[attachment=7134]
HOVERCRAFT

INRODUCTION

Vehicles designed to travel close to but above ground or water. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect). Hovercrafts are usually supported by fans that force air down under the vehicle to create lift, Air propellers, water propellers, or water jets usually provide forward propulsion. Air-cushion vehicles can attain higher speeds than can either ships or most land vehicles and use much less power than helicopters of the same weight. Air-cushion suspension has also been applied to other forms of transportation, in particular trains, such as the French Aero train and the British hover train.

Hovercraft is a transportation vehicle that rides slightly above the earth’s surface. The air is continuously forced under the vehicle by a fan, generating the cushion that greatly reduces friction between the moving vehicle and surface. The air is delivered through ducts and injected at the periphery of the vehicle in a downward and inward direction. This type of vehicle can equally ride over ice, water, marsh, or relatively level land.

INTRODUCTION

A Hovercraft is a vehicle that flies like a plane but can float like a boat, can drive like a car but will traverse ditches and gullies as it is a flat terrain. A Hovercraft also sometimes called an air cushion vehicle because it can hover over or move across land or water surfaces while being held off from the surfaces by a cushion of air. A Hovercraft can travel over all types of surfaces including grass, mud, muskeg, sand, quicksand, water and ice .Hovercraft prefer gentle terrain although they are capable of climbing slopes up to 20%, depending upon surface characteristics. Modern Hovercrafts are used for many applications where people and equipment need to travel at speed over water but be able load and unload on land. For example they are used as passenger or freight carriers, as recreational machines and even use as warships. Hovercrafts are very exciting to fly and feeling of effortlessly traveling from land to water and back again is unique.

[attachment=8036]

CONTENTS
• INTRODUCTION 2
• HISTORY 3
• PRINCIPLE OF WORKING 5
• MAIN PARTS 6
• DEVELOPMENT OF AIR CUSHION 7
• HULL CONSTRUGTION 9
• HOVERCRAFT SKIRTS 10
• THE LIFTING FAN 12
• THE ENGINE 13
• THE THRUST PROPELLER 15
• RUDDERS AND CONTROL OF HOVERCRAFT 16
• ADVANTAGES 17
• FUTURE DEVELOPMENTS 18
• APPLICATIONS 19
• CONCLUSION 20
• REFERENCES 21



HISTORY

In the beginning……

Hovercraft as we know them today started life as an experimental design to reduce the drag that was placed on boats and ships as they ploughed through water. The first recorded design for an air cushion vehicle was put forwarded by Swedish designer and philosopher Emmanuel Swedenborg in 1716. The craft resembled an upturned dinghy with a cockpit in the centre. Apertures on either side of this allowed the operator to raise or lower a pair of oar-like air scoops, which on downward strokes would force compressed air beneath the hull, thus raising it above the surface. The project was short-lived because it was never built, for soon Swedenborg soon realized that to operate such a machine required a source of energy far greater than that could be supplied by single human equipment. Not until the early20th century was a Hovercraft practically possible, because only the internal combustion engine had the very high power to weight ratio suitable for Hover flight.
In the mid 1950s Christopher Cockrell, a brilliant British radio engineer and French engineer John Bertin, worked along with similar line of research, although they used different approaches to the problem of maintaining the air cushion. Cockrell while running a small boatyard in Norfolk Boards in the early 1950s began by exploring the use of air lubrication to reduce the hydrodynamic drag, first by employing a punt, then a 20 knot ex-naval launch as a test craft.

PRINCIPLE OF WORKING
The principle of working of a Hovercraft is to lift the craft by a cushion of air to propel it using propellers. The idea of supporting the vehicle on a cushion of air developed from the idea to increase the speed of boat by feeding air beneath them. The air beneath the hull would lubricate the surface and reduce the water drag on boat and so increasing its speed through water. The air sucked in through a port by large lifting fans which are fitted to the primary structure of the craft. They are powered by gas turbine or diesel engine. The air is pushed to the under side of the craft. On the way apportion of air from the lift fan is used to inflate the skirt and rest is ducted down under the craft to fill area enclosed by the skirt.
At the point when the pressure equals the weight of the craft, the craft lifts up and air is escaped around the edges of the skirt. So a constant feed of air is needed to lift the craft and compensate for the losses. Thus craft is lifted up. After the propulsion is provided by the propellers mounted on the Hovercraft. The airs from the propellers are passed over rudders, which are used to steer the craft similar to an aircraft. Hovercraft is thus propelled and controlled and its powerful engine makes it to fly.


MAIN PARTS

Lower hull- It is the basic structure on which the Hovercraft floats when the engine is stopped while moving over water. It supports the whole weight of the craft.

Skirts- They are air bags inflated by air are fitted around the perimeter of the craft hold air under the craft and thus upon a cushion of air. It enables to obtain greater Hover height. The material used is rib stop nylon or Terylene.

Lift fan-It is fitted to the primary structure of the Hovercraft. The air is pumped under the craft between the skirt space to produce a cushion of air.

Propeller-It is used to obtain the forward motion of the craft. It is fitted to the top of the craft and is powered by a powerful gas turbine or diesel engine.

Rudders-They are similar to that used in an aircraft. Rudders are moved by hydraulic systems. By moving the rudders we can change the direction of the craft.


DEVELOPMENT OF AIR CUSHION BY MOMENTUM CURTAIN EFFECT
Stability of the Hovercraft on its cushion of air remained a real problem despite some design efforts and new approach was needed. To solve these problems, plenum chamber with a momentum curtain was developed by Sir Christopher Cockrell.
His first experiments were conducted with the aid of two cans and a vacuum cleaner (with blower end). The cans were drilled and bolted so that one can was inside the other with open ends facing down to some weighing scales, the top of the larger can was open and had a tube connected to it so that air could be forced in to the top can and around the smaller can inside.




The air traveled around between the inside of the bigger can and outside of the smaller can and was then let out towards the scales in a narrow ring of air, the cans were mad4e so that it was possible to remove inner can so the air could be directed in two ways.
The experiment was conducted in two steps. First the smaller can was removed and blower switched on. The scales measured the amount of thrust the air from the one can produced down onto the scales. The smaller can was now replaced inside the larger can so that the ring of air was produced. Again the blower was switched on and the scales measured amount of thrust the ring of air produced down onto the scales. Here is the key discovery because Cockrell observed that the two cans nested inside each other produced more thrust onto the scales than the simple open can or plenum chamber did, he had discovered the momentum curtain effect and this was the key ingredient that he patented.
In the full size craft the plenum chamber was also filled in so that a slot round the bottom edge of plenum chamber wall was former where the air fed in at the top. The slot produced a curtain of flowing air that was inclined. The high pressure air from the slot angled inwards towards the centre of the craft helped to contains and sustains the air cushion. Using this method a stable air cushion could be created. The craft was still riding on a plenum chamber of sorts but it was created and maintained by the high pressure ring of air surrounding the lower pressure air in the center.
The momentum curtain arrangement achieved higher hover heights with less power. It also solved some of the stability problems. The box structure in the center of the craft around which air escaped was closed to

form a buoyancy tank to enable the craft to float on water when it came to rest.
The design was exactly what was used in first publicly demonstrated Hovercraft the SRN1, built by Saunders Roe in the United Kingdom it served as a test bed for many years during Hovercraft development.


HULL CONSTRUCTION
The lower hull of the craft includes the craft floor, side panels, forward and aft panels till the top skirt attachment line. Most commercially build craft in polyester resin will use this section to transfer to the top hull. The lower hull
• Needs to have adequate size for the total weight of the craft and payload
• Must be strong enough to support craft off cushion (on landing pads)
• Have enough freeboard to support craft in displacement mode on water
• Must be watertight and as smooth as possible.
The lower hull can be build out of all boat building materials. From simple ply to very complicated composite panels.

HOVERCRAFT SKIRTS

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
A skirt is a flexible shaped strip fitted below the bottom edges of the plenum chamber slot. As the Hovercraft lifts, the skirt extends below it to retain much deeper cushion of air. The development of skirts enables a Hovercraft to maintain its normal operating speed through large waves and also allows it to pass over rocks, ridges and gullies.
Skirt is one of the most design sensitive parts. The design must be just right or an uncomfortable ride for passengers or damage to craft and skirts results. The skirt material has to be light flexible and durable all at the same time. For skirt to meet all of the requirements the design and use of new materials has slowly evolved.
There are three types of skirts
• Bag skirt
• Finger skirt
• Bag and finger skirt

A Hovercraft skirt is required to fulfill the following functions

• Contain the cushion of air beneath the craft at required Hover height
• Have the ability to confirm or contour effectively over obstacles so as to keep minimum, the loss of cushion air
• Return to its original shape after having been deformed
• Give adequate stability
• Offer little resistance to passage of obstacles beneath it
• Have the ability to absorb a large portion of the energy which is produced on impacts or collision with obstacles greater than hover height or cushion depth.

THE LIFTING FAN
In the enclosed space fan operates in a propeller would not be suitable. 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. 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 on most Hovercraft uses what is known as a centrifugal fan. This is a fan in which two discs are fitted together and looks rather like a doughnut with angled slat at their edges.
When the assembly is rotated at high speed air is sucked in to 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 back pressure 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.








THE ENGINE

The engines used in Hovercraft have evolved like the skirt design. The SRN 1 and other early craft used piston type engines. As models like the SRN 4 and SRN 6 were brought into service they tended to favor the use of gas turbines. This type of engine is smaller and lighter for a given horsepower and has been used extensively in turbo prop aircraft.

The engine has a main shaft on which is mounted a compressor and turbine. A starter motor is connected to one end and the other end is connected to the lift fan. Both the compressor and turbine look like fans with large number of blades.
When the engine is started the compressor compresses air from the engine intakes and pushes into the combustion chambers mounted around the engine. Fuel is squirted into the combustion chamber and is ignited. The compressed air then rapidly expands as it is heated and forces its way out through the turbine to the exhaust. As the gas pressure raises the turbine speeds up, there by driving the compressor faster. The engine speed increases until it reaches engines normal operating speed.


However the use of these engines results in very high level of engine noise outside the craft. In the SRN6 this meant that it was possible to hear the craft traveling across the Solent between the Portsmouth and the isle of Wight in the UK several miles away. The current AP188 crafts that runs on the old SRN6 routes has now moved back towards the piston engines and uses marine diesel engines that are much quieter and fuel efficient.

THE THRUST PROPELLER
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 the engine and the lift fan. This is because the amount of lift air requires dictates the engine speed to drive 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. 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.

RUDDERS AND CONTROL OF
HOVERCRAFT
Control of a Hovercraft is accomplished by primarily though the use of rudders like the type used on aircraft. The main difference would be, however, that Hovercraft generally utilizes many rudders rather than just one.
On the SRN4 the pylons on which they are mounted can be rotated to change the direction of thrust. Another method of control is through ‘puff ports’ or dual thrust fans where you would slow one down and speed up the other to turn in the direction desired.
The hovercrafts are designed to float like a boat with the engine turned off. To stop the Hovercraft-Reducing engine RPM will reduce the air cushion height and increased drag between the skirt and the surface will slow and stop the Hovercraft. Alternatively, the Hovercraft can be turned 180 degrees and the engine accelerated till the craft stops. In an emergency situation on most surfaces turning the engine off will stop the Hovercraft immediately

APPLICATIONS
As technology improves, performance improves and reduces noise levels; Hovercrafts are becoming increasingly popular as recreational machines. From cabins year round to cruising, fishing, driving and racing-the possibilities are endless. With the advantage of loading and unloading on land they are used for transporting people and equipment over water.
Hovercrafts are also used as warships to carry out rapid sea-lift and beach landing. They also provide fire support for troop on the shore. They are also capable of lying active minefields.

FUTURE DEVELOPMENTS
By using the hover principle many designs have arise. One is the hover concept by replacing the cushion of low pressure air as inform the modern Hovercraft by high pressure pad it was thought that the pads of high pressure could replace the wheels of the car. but there are two difficulties.
1. It is difficult to lift
2. New method of propulsion is required
Then moved towards Hover train. Here rails provide smooth surface for high pressure air and guidance from the track overcomes the problem of steering.

APPLICATIONS

• It will travel against a current of river with no reduction of speed
• A Hovercraft travel over the surface of water without concern for depth or hidden obstacles
• It can travel with great speed of up to 60 knots
• Hovercraft are unaffected by small waves and offer a comfortable smooth ride
• It is safe around swimmers as there are no propellers in water

• Many Hovercraft have sufficient hover height, ranging from 8 in to 18 in to pass right over a person in water
• The air cushion enables Hovercraft to operate over environmentally sensitive areas such as mudflats without disturbing the surface
• The lack of wake on water minimizes the potential for bank erosion
• It can load and unload peoples and equipments on land
• Unlike many boats, engine exhaust fumes are not directed into water and poisonous antifouling compounds are not required on Hovercraft


CONCLUSION

The unique capabilities of Hovercrafts are recognized and appreciated by a diverse group of its end users. Hovercrafts are in use worldwide with search research groups, fire departments, airport emergency response units and scientific research teams. Hovercrafts are an integral part of numerous commercial operations including driving, tourism, water taxi, ferry service, ice breaking, goods delivery, survey, environmental monitoring and guide outfitting. The Hovercraft as a vehicle is still in common use but not in large volumes. As engine and materials technology progress the Hovercraft may yet make a comeback but for now it is a special vehicle for special applications.

REFERENCES
WEBSITES
• hovercraft.com
• howhovercraftwork.com
• hover.globelinternet.co.uk
• jameshovercraft.com
• hovercraft.org.uk
• hovercraft-world.com
• universalhovercraft.com
• rescuehovercraft.com
• hovercrafttechnics.com

Sir,
Please send me the report
Thank you

hi
please download the attached file from the below given thread.

http://studentbank.in/report-hovercraft-...1#pid10851