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HOVERCRAFT                          ACKNOWLEDGEMENTI express my sincere gratitude to Mr. Sojan P Antony, Lecturer in mecha...
Hovercraft                                              Seminar Report         2012-13Head of Department, Mr. M V Revi and...
Hovercraft                                              Seminar Report             2012-13                                ...
Hovercraft                               Seminar Report         2012-13TOPIC NO.TOPIC NAME                                ...
Hovercraft                                              Seminar Report            2012-13                               1....
Hovercraft                                              Seminar Report            2012-13                                 ...
Hovercraft                                            Seminar Report            2012-133.CONSTRUCTIONAL FEATURES OF HOVERC...
Hovercraft                                               Seminar Report             2012-13    LIFTING FAN :-The volume o...
Hovercraft                                              Seminar Report            2012-13    HOVERCRAFT SKIRT :-Despite t...
Hovercraft                                                Seminar Report             2012-13    AIR BOX :-The box-like st...
Hovercraft                                             Seminar Report             2012-13      4. WORKING PRINCIPLE OF HOV...
Hovercraft                                           Seminar Report            2012-13      When the hovercraft is finally...
Hovercraft                                           Seminar Report          2012-13                       5. ADVANTAGES O...
Hovercraft                                           Seminar Report           2012-13                    7. APPLICATIONS O...
Hovercraft                               Seminar Report         2012-13                            SOME APPLICATIONSSingle...
Hovercraft                                 Seminar Report         2012-13The Hovertravel Service.Fire Department Using A H...
Hovercraft                                            Seminar Report            2012-13                      8. FUTURE OF ...
Hovercraft                                            Seminar Report            2012-13       9. CONCLUSION        Hovercr...
Hovercraft                                            Seminar Report         2012-13                                10. RE...
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Hovercraft seminar report

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Hovercraft seminar report

  1. 1. HOVERCRAFT ACKNOWLEDGEMENTI express my sincere gratitude to Mr. Sojan P Antony, Lecturer in mechanicalEngineering, on this occasion for his suggestion of this topic and presentation of thisseminar. I also take this opportunity to express my sincere thanks to Mr. Jayachandran,
  2. 2. Hovercraft Seminar Report 2012-13Head of Department, Mr. M V Revi and Mr. P.P Devdas for their valuable advice andguidance in completion this seminar in pristine form. At this juncture, I gratefully remember the moral support and co-operationextended by my classmates on this seminar presentation. Their active participationreally brought life to my seminar. My sincere thanks to one and allMIDHUN VIJAYDept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 1
  3. 3. Hovercraft Seminar Report 2012-13 ABSTRACT The air cushion vehicle or “HOVERCRAFT”, as it is popularly known is thenewest vehicle in today„s transport scene. As well as being new, this vehicle is differentfrom other more conventional, terrestrial vehicle in that it requires no surface contact fortraction and it is able to move freely over a variety of surface while supportedcontinuously on a self-generated cushion of air. Though the concept is new, the rate ofdevelopment of hovercraft has been outstandingly faster than that of any other mode oftransport. Modern Hovercrafts are used for many applications where people and equipmentneed to travel at speed over water but be able load and unload on land. For example theyare used as passenger or freight carriers, as recreational machines and even use aswarships. Hovercrafts are very exciting to fly and feeling of effortlessly traveling fromland to water and back again is unique. A hovercraft or air-cushion vehicle (ACV) is a craft designed to travel over anysmooth surface supported by a cushion of slow moving, high-pressure air, ejecteddownwards against the surface below, and contained within a "skirt." Hovercrafts are used throughout the world as a method of specialized transportwherever there is the need to travel over multiple types of surfaces. Because they aresupported by a cushion of air, hovercraft are unique among all forms of groundtransportation in their ability to travel equally well over land, ice, and water. Smallhovercraft are often used in physical activity, combustion, or passenger service, whilegiant hovercraft have been built for civilian and military applications to transport cars,tanks, and large equipment into difficult or hostile environments and terrain.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 2
  4. 4. Hovercraft Seminar Report 2012-13TOPIC NO.TOPIC NAME PAGE-NO. 1. INTRODUCTION 4 2. HISTORY 5 3. CONSTRUCTION FEATURES OF HOVERCRAFT 6 4. WORKING PRINCIPLE OF HOVERCRAFT 10 5. ADVANTAGES OF HOVERCRAFT 12 6. DISADVANTAGES OF HOVERCRAFT 12 7. APPLICATIONS OF HOVERCRAFT 13 8. FUTURE OF HOVERCRAFT 16 9. CONCLUSION 17 10. REFRENCE 18Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 3
  5. 5. Hovercraft Seminar Report 2012-13 1. INTRODUCTION Vehicles designed to travel close to but above ground or water. These vehiclesare supported in various ways. Some of them have a specially designed wing that willlift them just off the surface over which they travel when they have reached a sufficienthorizontal speed (the ground effect).Hovercraft is such a vehicle. Basically hovercraft is a vehicle that,  Drive like a car but  Flies like a plane.  It can hover over or move across land or water surfaces while being held off from the surfaces by a cushion of air.  Float like a boat. A hovercraft, also known as an air-cushion vehicle or ACV, is a craft capable oftravelling over land, water, mud or ice and other surfaces both at speed and whenstationary. Hovercrafts are hybrid vessels operated by a pilot as an aircraft rather than acaptain as a marine vessel. Hovercrafts are usually supported by fans that force air downunder the vehicle to create lift, Air propellers, water propellers, or water jets usuallyprovide forward propulsion. Air-cushion vehicles can attain higher speeds than caneither ships or most land vehicles and use much less power than helicopters of the sameweight. Hovercraft is a transportation vehicle that rides slightly above the earth‟ssurface. The air is continuously forced under the vehicle by a fan, generating thecushion that greatly reduces friction between the moving vehicle and surface. The air isdelivered through ducts and injected at the periphery of the vehicle in a downward andinward direction. This type of vehicle can equally ride over ice, water, marsh, orrelatively level land They operate by creating a cushion of high-pressure air between the hull of thevessel and the surface below. Typically this cushion is contained within a flexible"skirt". They typically hover at heights between 200 mm and 600 mm above any surfaceand operate above 20 knots and can clear gradients up to 20 degrees. The first practical design for hovercraft derived from a British invention in the1950s to 1960s. They are now used throughout the world as specialized transports indisaster relief, coastguard, military and survey applications as well as for sport orpassenger service. Very large versions have been used to transport hundreds of peopleand vehicles across the English Channel whilst others have military applications used totransport tanks, soldiers and large equipment in hostile environments and terrain.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 4
  6. 6. Hovercraft Seminar Report 2012-13 2.HISTORY2.1 EARLIEST EFFORTS :- The first recorded design for a hovercraft was in 1716 put forward by EmmanuelSwedenborg, a Swedish designer and philosopher. The project was short-lived and acraft was never built. Swedenborg realized that to operate such a machine required asource of energy far greater than any available at that time. In the mid-1870s, the Britishengineer Sir John Thorneycroft built a number of model craft to check the air-cushioneffects and even filed patents involving air-lubricated hulls, although the technologyrequired to implement the concept did not yet exist.. In 1915 Austrian Dagobert Müllerbuilt the worlds first "water effect" vehicle. In 1931 Finnish aero engineer Toivo J. Kaario began designing a developedversion of a vessel using an air cushion and built a prototype. During World War II an engineer in the United States of America, CharlesFletcher, invented a walled air cushion vehicle. Because the project was classified bythe U.S. government, Fletcher could not file a patent. From this time both American andEuropean engineers continued work on the problems of designing a practical craft.2.2 INVENTION OF MODERN HOVERCRAFT :- In the early 1950s the British inventorChristopher Cockerell began to experimentwith such vehicles, and in 1955 he obtained apatent for a vehicle that was "neither anairplane, nor a boat, nor a wheeled landcraft." He had a boat builder produce a two-foot prototype, which he demonstrated to themilitary in 1956 without arousing interest.Cockerell persevered, and in 1959 acommercially built one-person Hovercraftcrossed the English Channel. In 1962 aBritish vehicle became the first to go intoactive service. CHRISTOPHER COCKERELLDept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 5
  7. 7. Hovercraft Seminar Report 2012-133.CONSTRUCTIONAL FEATURES OF HOVERCRAFT  Radar: apparatus that detects objects through the use of microwaves.  Pylon: supporting post.  Dynamic propeller: two-bladed apparatus that provides motion.  Fin: steering device.  Rudder: apparatus that prevents drift.  Lift-fan air intake: opening to allow air to enter.  Main level drive gear box: compartment that contains and protects the gear mechanism.  Skirt finger: part of the flexible skirt.  Passenger entrance: opening on the side wall that provides access to the passenger cabin.  Flexible skirt: lower flexible part.  Bow door ramp: opening at the front.  Control deck: cubicle from which a hovercraft is operated.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 6
  8. 8. Hovercraft Seminar Report 2012-13  LIFTING FAN :-The volume of air needed is very large and a propeller is designed to be most efficientin open air like on an aircraft. Also the fan needs to force air into the chamber below thecraft so creating a specific pressure under the craft. Propellers again are not efficient inapplications when an air backpressure will be applied to the propeller blades as theyrotate. Because of this the lifting fan on most Hovercraft uses what is known as acentrifugal fan. This is a fan in which two discs and fitted together and looks rather likea doughnut with angled slats at their edges.When the assembly is rotated at high-speed air is sucked into the center hole in the fanand the slats force it out at the edges. The advantages of the fan are twofold. Theyoperate efficiently in an environment when backpressure is high and they will movelarger volumes of air for a given rotation speed than a propeller with the same speed andpower input. The lifting fan is coupled via a gearbox to the engine. The engine alsodrives the propeller on the craft, which provides thrust for forward motion of theHovercraft.  THRUST PROPELLERS :-The propeller used to drive the hovercraft along is usually an aircraft type with variablepitch 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 liftfan. In turn the amount of propulsion, which the propellers provide, must be obtained byvarying the propeller pitch and not its rate of rotation. This system is termed integratedlift/propulsion. A Hovercraft having more than one lift fan and propeller generally has aseparate engine for each fan-and propeller unit.The propellers used on hovercraft can vary from four-bladed versions and about ninefeet in diameter on the smaller craft to the four propellers on the SRN4 cross-Channelhovercraft. These are four-bladed and nineteen feet in diameter! On the SRN 4 thepylons on which they are mounted can be rotated to change the direction of thrust. Onsmaller craft, rudders like on aircraft, are used for direction control.  MOMENTUM CURTAIN :-When early models were built and analysis was done on the airflow using the plenumchamber type of hovercraft it showed that there were problems with stability. Inaddition 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 somedesign efforts and a new approach was needed. To solve these problems, a plenumchamber with a momentum curtain was developed by Sir Christopher Cockerall.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 7
  9. 9. Hovercraft Seminar Report 2012-13  HOVERCRAFT SKIRT :-Despite the momentum curtain being very effective the hover height was still too lowunless 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 hoverheight 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 plenumchamber slot. As the hovercraft lifts, the skirt extends below it to retain a much deepercushion of air. The development of the skirt enables a hovercraft to maintain its normaloperating speed through large waves and also allows it to pass over rocks, ridges andgullies.The skirt of a hovercraft is one of its most design sensitive parts. The design must bejust right or an uncomfortable ride for passengers or damage to the craft and the skirtsresults. Also, excessive wear of the skirt can occur if its edges are flapping up and downon the surface of the water. The skirt material has to be light flexible and durable all atthe same time.For the skirt to meet all of its requirements the design and use of new materials hasslowly evolved. The current skirts use „fingers at the lower edge of the skirt envelopewhich can be unbolted and replaced. By doing this there is a quick and easy way tocounter the effects of wear without having to replace the whole skirt structure. Ashocking example of the costs is the replacement of the skirt assembly on the SRN 4‟swhich used to cross the English Channel from the UK to France. The replacement costfor a set of skirts for this craft is over 5 million US Dollars.  THE ENGINE :-The SRN 1 and other early hovercrafts used piston type engines and gas turbines. Thistype of engine is smaller and lighter for a given horsepower and has been usedextensively in turbo prop aircraft.The engine has a main shaft on which is mounted a compressor and a turbine. A startermotor is connected to one end of the shaft and the other end is connected to the lift fanand propeller gearboxes. Both compressor and turbine look like fans with a largenumber of blades.When the engine is started, the compressor compresses air from the engine intakes andpushes it into combustion chambers mounted around the engine. Fuel is squirted intothe combustion chambers and ignited. The compressed air then rapidly expands as it isheated and forces its way out through the turbine to the exhaust. As the gas pressurerises, the turbine speeds up, thereby driving the compressor faster. The engine speedincreases until it reaches the engines normal operating speed.However, the use of these engines results in a very high level of engine noise outsidethe craft. Also uses marine diesel engines that are much quieter and fuel efficient.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 8
  10. 10. Hovercraft Seminar Report 2012-13  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 pushedbackward by the propeller and forces it downward, underneath the hovercraft. There arethree small ducts cut into the base of the hovercraft, underneath the air box. Two ofthese ducts lead into the skirt, which is basically a bag that goes all the way around theperimeter of the craft, while the third duct leads directly underneath the hovercraft.  RUDDERS :-When the hovercraft is finally able to move it will most definitely require steeringcapabilities. This is achieved through the use of rudders. These rudders can becontrolled by a variety of devices including computers. The rudders must be wellweighed out in order to avoid weighing down your hovercraft and also well shaped inorder to move air as efficiently as possible.Rudders cannot be too heavy otherwise they will weigh down the craft because they arelocated very close to the motor. The shape of the rudder dictates how well it will be ableto move air.  HOVERCRAFT OPERATION :-Piloting a hovercraft is an interesting proposition. Since very little of it actually touchesthe ground, there isnt much friction, making it very difficult to steer and also verysusceptible to strong winds. Imagine trying to drive around on top of an air-hockeypuck! Weve discovered that the best way to drive it is treat it like a jet ski, i.e. leaningback and forth and steering very carefully. It is also possible to do a 360-degree turnwithout stopping, which is quite a sight.  AERODYNAMICS :-Aerodynamics is defined as the branch of fluid physics that studies the forces exerted byair or other gases in motion. Examples include the airflow around bodies moving atspeed through the atmosphere (such as land vehicles, bullets, rockets, and aircraft), thebehavior of gas in engines and furnaces, air conditioning of buildings, the deposition ofsnow, the operation of air-cushion vehicles (hovercraft), wind loads on buildings andbridges, bird and insect flight, musical wind instruments, and meteorology. Formaximum efficiency, the aim is usually to design the shape of an object to produce astreamlined flow, with a minimum of turbulence in the moving air. The behavior ofaerosols or the pollution of the atmosphere by foreign particles are other aspects ofaerodynamics.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 9
  11. 11. Hovercraft Seminar Report 2012-13 4. WORKING PRINCIPLE OF HOVERCRAFT 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 crafts 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. It is 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. The fan produces more than enough force for the hovercraft to move. 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.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 10
  12. 12. Hovercraft Seminar Report 2012-13 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.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 11
  13. 13. Hovercraft Seminar Report 2012-13 5. ADVANTAGES OF HOVERCRAFT  Can carry a relative size payload.  Can be launched from ship (ex: interception, deploying troops to shore from a carrier, travel where larger/mother vessels cannot).  Travel over any surface.  Shortcutting routes.  Travel Rivers up as fast as down, irrespective of the current.  Travel in dry water-beds.  No collision with debris, logs etc.  Access to 75% of coastal area instead of only 5% with conventional vessels.  Hovercrafts are very fuel efficient (CO² friendly) as Hovercraft do not have to plough through the water but "fly" above the surface. At maximum speed fuel consumption of a Hovercraft is approx. 70% less than of a fast patrol boat with similar payload capacity.  No turbulence or impact in water as no propeller churns up the water so sea life remains untouched.  Travel in dry water-beds independent from harbors, piers and jetties. 6. DISADVANTAGES OF HOVERCRAFT :-  They move a lot of air and can be relatively loud.  Steep grades can be issue.  Potential of skirt damage/puncture.  Not exactly agile (e.g.: cornering).  The Hovercraft is bulky and its high speed makes it difficult to control while on land.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 12
  14. 14. Hovercraft Seminar Report 2012-13 7. APPLICATIONS OF HOVERCRAFT  Downdraft associated with helicopters, & a fraction of the cost to purchase, operate & maintain. Rescuers can reach floods, mud, and sand& ice victims without exposing rescuers to life threatening danger.  Distribution of famine or flood aid support craft. Relief work (United Nations).  Civil emergency & infrastructure support  Oil industry survey, exploration & pipeline patrol.  Electrical Power-line patrol & safety.  Remote mining access support vehicle.  River, lake & port geological surveys.  Mud & riverbed sampling.  Environmental projects & clean-up operations.  Airport bird scaring/support/rescue services.  Coastal civil engineering & bridge construction & repair/maintenance.  Transport, service & safety craft for river & low tide coastal work where 24-hour access is vital for staff safety.  Fish farm & low tide access.  Leisure & family fun. Rental Operations, Corporate entertainment. Education, schools. Summer fetes & shows.  Access to Riverside, lakeside & island properties. Hovercraft travel over mud, sand & ice. Hovercrafts are not restricted by tide, or fast running water. Or shallow water, or submerged rocks, coral, or marine life.  Super Yacht Tenders  Filming & TV work. Store sales & advertising (Harrods).  It can be used on fast flowing water e.g. flooded rivers as current has little Effect on craft when hovering. This means the pilots able to maintain speed and direction or even remain stationary, maintaining position to carry out Rescues etc. without fighting the water current. Can be launched onto rivers and floodwater without use of a slipway Providing a reasonably low bank can be accessed. No need to back a trailer down into the water. The hovercraft can be flown (or reversed) in. Hovercraft can be operated over underwater obstructions such as fences, Walls and debris without hindrance and there is no propeller to foul. No propeller in the water means less risk to casualties in the water or crew when working close to the craft. Extreme maneuverability and controlled reverse capability as well as the ability to stop quickly means this type of hovercraft can be operated in Confined spaces such as narrow streets and flooded caravan sites. Hovercraft can be used on mixed surfaces where boats cannot be effective.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 13
  15. 15. Hovercraft Seminar Report 2012-13 SOME APPLICATIONSSingle Seated Racing Hovercraft.A Formula 1 Racing Hovercraft.Passenger-Carrying HovercraftHovercraft Lifeboats.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 14
  16. 16. Hovercraft Seminar Report 2012-13The Hovertravel Service.Fire Department Using A Hovercraft To Practice A Rescue.Military Hovercraft.Coastal Cruising.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 15
  17. 17. Hovercraft Seminar Report 2012-13 8. FUTURE OF HOVERCRAFT The future of hovercraft seems uncertain, but there is a good chance there will be huge hover ports all over the world, like the one in the picture. Thinner hovercraft might be built so civilians can drive safely on roads. Hovercrafts, once only used by military bureaus and coast guard agencies that delivered disaster relief, and carried out rescue missions, are now used in commercial capacities, both public and private, as well as for personal transport and sporting activities, including fishing, hunting and group outings. The future is more promising than ever before, exciting outdoor enthusiasts across the globe with their possibilities of maintaining the craft for their personal enjoyment, realizing the dream of owning a craft that was once relegated to professionals.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 16
  18. 18. Hovercraft Seminar Report 2012-13 9. CONCLUSION Hovercrafts are generally simple mechanisms in theory. Yet the process fromtheory to manifestation is not as easy as it may seem. A plethora of problems exist andmust be faced in order to attain a well-functioning hovercraft. The plans and designsmust be flawless. One must take under consideration the weight and the shape of eachcomponent in order to avoid problems such as instability and dysfunction. This is amarvelous machine which greatly cuts down the friction which in turn helps it to attaingreater speed and more stability. Varieties of problems and factors have to be taken into account in designing andconstructing a hovercraft. The difficulties involved in maintaining stability andfunctional competency has limited the application to only transportation or for militarypurpose. The cost involved in the developing of a hovercraft is also another impedimentto the widespread use of this machine.Dept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 17
  19. 19. Hovercraft Seminar Report 2012-13 10. REFRENCES 1. http://www.totalairdominance.50megs.com/articles/hovercraft.htm 2. http://en.wikipedia.org/wiki/hovercraft 3. http://en.wikipedia.org/wiki/acv 4. http://en.wikipedia.org/wiki/hovercraft 5. http://www.resonancepub.com/images/hovercraft.gif 6. http://images.google.co.in/images 7. http://science.howstuffworks.com/question69.htm 8. http://www.espionageinfo.com/Sp-Te/hovercraft.html 9. http://www.hovercraft design.info/51.htm 10. http://www.hitechweb.genezis.eu/hovercraft /image013.jpg 11. http://www.maruine.com/hovercraft /scm.html 12. http://www.razorworks.com/enemyengaged/chguide/images/hovercraft.gif 13. htp://www.x20.org/library/thermal/pdm/hovercraft.htm 14. http://en.wikipedia.org/wiki/aircushion 15. http://www.military-heat.com/43/hovercraft / 16. http://homepage.mac.com/ardeshir/hovercraft.pdf 17. http://www.scribd.com/doc/7393272/hovercraft 18. http://www.megaessays.com/essay_search/hovercraft 19. http://www.termpapersmonthly.com 20. http://www.marinetalk.com/articles-marine-companies/art/hovercraftDept. of Mechanical Engg. S.R.G.P.T.C Triprayar Page 18

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