INTRODUCTION

HOVERCRAFT

A hovercraft is a vehicle supported on a cushion of air, able to traverse many different types of sufficiently smooth terrain – including, in some cases, water. These are machines that slide along while balancing on top of an “air cushion” bubble. This bubble is generated by an air pump (fan) while a flexible “skirt” helps retain the bubble beneath the machine by limiting the air loss. A pocket of air is formed and the resulting pressure pushes the hull of the hovercraft up and away from the surface it is sitting on. Since the friction against the bottom of the craft has been significantly reduced because of this pocket of air, less energy is required to move it across a surface. This “air cushion” gives the object a much smoother ride compared to other vehicles across rough surfaces. Hovercraft have one or more separate engines - one engine drives the fan on the bottom of the hovercraft, (the impeller) which is responsible for lifting the vehicle by forcing high pressure air under the craft. The air then exits the apparatus through the "skirt", lifting the craft above the area on which the craft resides. One or more additional engines are used to provide thrust in order to propel the craft in the desired direction (these engines help push the hovercraft). A plethora of different directional utilities exist, but the most popular are thrust vectoring devices (such as rudders or differential thrusts), side thrust devices (such as puff ports or thrusters), and cushion tilt devices (skirt shifts, skirt lifts). Some hovercraft utilize 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. In preparing to design our own working hovercraft, we attempted to incorporate this one as well as many other proven designs in order to build a successful vehicle.

HISTORY

Hovercraft similar to those of today’s world started as an attempt in an experimental design to reduce the drag on boats and ships as they went through water. The first recorded design for an air cushion vehicle was from the work of Emmanual Swedenborg, the Swedish designer and philosopher, in 1716. The craft was similar in shape to that of an upturned dinghy with a cockpit at the center. Devices on either side of the model 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 and therefore raise the vehicle above the surface. The project was short-lived and was never built, for Swedenborg soon realized that to operate such a machine required a source of energy far greater than that which could be supplied by a single human occupant. In later hovercraft history, Sir John Thornycroft built a number of model craft in the mid 1870s to check the ‘air cushion' effects and even filed patents involving air lubricated hulls. From this time, both American and European engineers continued work on the problems of designing a practical craft. Not until the early 20th century was a hovercraft possible, because only the internal combustion engine had the very high power to weight ratio suitable for hover flight. “Hovercraft” was the name coined for this air-cushion vehicle by its inventor, Sir Christopher Cockerell. Cockerell was born in Cambridge, United Kingdom, where his father, Sir Sydney Cockerell, was curator of the Fitzwilliam Museum, Christopher Cockerell was educated at Gresham's School. He then entered Cambridge University, England, as an undergraduate, where he studied engineering and was tutored by William Dobson Womersley.

In 1953, Cockerell tested his theories of an air-cushion device using an empty KiteKat cat food tin inside a coffee tin, an industrial air blower, and a pair of kitchen scales. His idea was to build a vehicle that would move over the water’s surface, floating on a layer of air. This would reduce friction between the water and vehicle. To test his hypothesis, he placed the smaller can inside the larger can and used a hairdryer to blow air into them. By 1955, he had built a working model from balsa wood and had taken out his first patent. Although there have been many variations (leading to the development of a typical hovercraft design, as seen below), Cockerell developed the first practical hovercraft designs – leading to the launch of the first hovercraft to be produced commercially, the SRN1, in 1959.

OBJECTIVE

On our first day in groups, we were introduced to what had been expected of past hovercraft groups and how successful those groups were in accomplishing their objectives. When all hovercraft groups decided which objectives we would attempt to accomplish, the final objectives for all groups were the ability to lift (or hover), to move forward (thrust), to change directions, to stop, and (possibly) to hover over water. Our craft was designed to be able to traverse both solid cement as well as many other surfaces, such as grass, sand, and possibly an obstacle course.

DESIGNING

1. Design Requirements

The primary objective of the project is to be able to carry a payload of 40kg across various types of terrain while demonstrating the advantages of a hovercraft over that of either a typical land or water vehicle. There are many secondary design requirements which are also proposed as follows: Powered by a single engine and propeller fan (lift and thrust) to achieve maximum possible performance Maximum Size 6' x 4' so that transportation can be obtained in any ordinary quarter ton pickup trick. Must move a payload of a minimum 40kg. This weight is selected because 15 kg/ft2 is the upper range of operation pressure for hovercrafts. The 40kg payloadbrings our hovercraft weight to approximately this value as well as is heavy enough to carry many standard items or tools to assist in use. Fans must be covered and shrouded for safety. Controlled by remote control, two thrust controls, and steering. Must include one kill switch for safety shut off of both engines. Maximum speed of at least 10 km/h. Steering controlled by rotating the thrust fan. Craft must be buoyant in case of emergency engine shut down. Minimum hover height of 3”. This value was selected as it will allow the craft to traverse most small obstacles in its way and discrepancy in terrain. Capable of operation on various terrain including: Asphalt, Grass, Sand, Snow, Water, and Rough land – gravel.

3. Design Selection

Engine Selection In order to provide the proper amount of thrust and lift the selection of number of gas engines as well as required horsepower had to be selected. The options which

were weighed were a single 1.0 hp horizontal shaft engine. Due to the carrying capacity of the hovercraft the first option of a single 1.5 hp engine was not selected. Although this would be the most inexpensive option it is not viable due to the lift requirements of the hovercraft. The second alternative of two 1.0 hp horizontal shaft engines provided the necessary amount of horsepower for both thrust and lift but due to the construction difficulties it was not selected. Ducting the air from a horizontal fan for lift posed additional flow consideration problems as well as an increase in cost.

Thrust Powertrain In order for the hovercraft to make turns, it was determined that the thrust fan, which is located at the back of hovercraft, would have to rotate. With a rotating thrust fan, the thrust force could be applied at different angles on the back of the hovercraft in order to rotate the hovercraft and make turns. The thrust fan and motor will be placed on a rotating platform. In order to rotate the platform, three separate designs were examined in terms of performance, vibration resistance, center of gravity, construction and cost.

3. Lift Powertrain

Lift Powertrain is a very important aspect of the hovercraft since it creates the air pressure beneath the hovercraft that lifts it off the ground. This (combined with skirt design) will give the hovercraft its ability to float and traverse the various specified terrain. A front mounted engine for lift has been chosen to balance the rear thrust engine so longitudinal center of gravity remains even without payload. The results of the design

4.FABRICATION OF HOVERCRAFT

Hull: Hull is made by sandwiching polystyrene sheets between plywood sheets; Industrial grade glue is used for this purpose. First plywood is cut according to the specifications given in Fig 7

glue is applied to a plywood sheet and 4 polystyrene sheets, and they are stuck together. Polystyrene sheets are kept horizontal to the plywood. Then another layer of polystyrene is adhered to the existing one to get required thickness, this time sheet are arranged vertical to plywood. Then another plywood sheet is adhered at the top. Prepared hull is kept under pressure overnight.

Air box: We constructed two air boxes. One was of a height to divert 33% of the air driven by the propeller into the duct, and another one to divert 60% of the air. In the Fig 8 .First one was constructed by plywood of thickness 10mm and air tightened by applying fevicol glue at the edges. Second air box was constructed wooden pieces of 10mm thickness. Curved shape is cut in the wood, PVC sheet is fixed along the edges of the two curves with nails. Supports are provided at the base this makes the second air box. Both air boxes are used in the testing phase and appropriate one is selected.

Skirt: Skirt is kept along the craft and extra material is cut off. Both ends of useful part of skirt material is stitched so that it forms a loop. First, hole of 5mm are drilled along the hull, each 10 cm apart. Same is done on the aluminium sheets that are to be placed above the skirt so that it sticks to the hull completely. Skirt is held on the hull and holes are drilled at the same distance as before to match holes on the skirt, hull and aluminium sheets as shown in Fig 9. All three are fixed with the help of nuts and bolts. At the other end of the skirt is stitched so that a passage forms along the skirt, in which a rope is passed. Two ends of rope are tightened.

Skirt

Engine: Engine mount is acquired from the scrap yard, we found that it does not support whole engine, but provides support to the base. Extra material of the mount is cut off to save the weight and to provide space for the new frame. New frame is constructed so as to support the upper part of the engine and hold two bearings that are kept parallel and aligned. Bearings house the shaft of diameter 25mm.diameter at one end of the shaft is reduced to 20 mm so as to fit into the fan hub. Manufacturer provided a 20 mm hole in the fan hub for the shaft to fit in, with a key of 6 mm. Shaft is also provided with the same dimension of key grove. Transmission consists of sprocket and chain arrangement. Sprocket is fixed on the shaft with Allen screws. Chain is fitted on the sprocket teeth and teeth on direct output of the engine. Bearing and frames so constructed that shaft is at the height of 475mm from the hull, or the centre of the duct. Washers are fitted so as not to leave the chain slack. Engine, engine mount, transmission assembly parts are fitted together to form power house assembly. This power house assembly is fixed on the hull 4 M10 bolts. To absorb the vibrations of the engine 4 rubber pads, 2 pads for each leg of the mount, of 8mm thickness are provided between engine mount base and hull.

5.PRINCIPLE OF OPERATION

Principle Of Operation The hovercraft floats above the ground surface on a cushion of air supplied by the lift fan. The air cushion makes the hovercraft essentially frictionless. The hovercraft relies on a stable cushion of air to maintain sufficient lift. The air ejected from the propeller is separated by a horizontal divider into pressurized air utilized for the air cushion and momentum used for thrust. The weight distribution on top of the deck is arranged so that the air is distributed the air from the rear of the deck throughout the cushion volume in an approximately even fashion to provide the necessary support. The skirt extending below the deck provides containment, improves balance, and allows the craft to traverse more varied terrain. We maintain the rigidity of the skirt by filling the air-tight skirt with the same pressurized air diverted towards lift. The skirt inflates and the increasing air pressure acts on the base of the hull thereby pushing up (lifting) the unit. Small air gaps are left underneath the skirt prevent it from bursting and provide the cushion of air needed. A little effort on the hovercraft propels it in the direction of the push [7]. Steering effect is achieved by mounting rudders in the airflow from the blower or propeller. A change in direction of the rudders changes the direction of air flow thereby resulting in a change in direction of the vehicle. This is achieved by connecting wire cables and pulleys to a handle. When the handle is pushed it changes the direction of the rudders.

6.USES OF HOVERCRAFT

Hovercraft have been around in some form for over 50 years, and have been used for many different purposes. Here's a brief look at how hovercraft are used around the world today. The most practical and consistent use of hovercraft since their invention has been transporting people, vehicles, and equipment across terrain that other vehicles simply can't negotiate. Large hovercraft ferries take hundreds of people and cars at a time across the choppy and dangerous waters of the English Channel every day. Hovercraft ferries are common in many countries, sometimes replacing traditional boats for this purpose. Hovercraft used in this capacity are both safer and faster than boats. Similar to ferries are touring operations. Touring hovercraft usually seat about 20 people and operate much like boats. Some serve food and are heated in the winter and air-conditioned in the summer. The captain will frequently narrate the tour, which might be 20 minutes to an hour long. In the past there have been tours like this in Ottawa, Gananoque, Vancouver, and across the desert sands of southern Alberta near Dinosaur Provincial Park. Many oilfield companies use large flat-deck hovercraft to transport equipment across swamps and mud. This saves the oil industry considerable time and money since they don't have to wait for the ground to dry, and they can reach places that are otherwise impossible to reach. Helicopters are sometimes used to haul equipment, but hovercraft built for this purpose are able to carry significant weight and are far safer and cheaper than helicopters. The U.S. Navy uses hovercraft so massive they can each carry several full-sized armoured tanks (do a search for LCAC vehicles). They have an entire fleet of these monster hovercraft. They can carry tanks across terrain where boats and other vehicles can't go, and into locations where tanks have never gone before. I've even heard rumours of the U.S. Navy having their own secret hovercraft research and development program. Other military hovercraft include the British Royal Marines, who apparently operate over 50 military hovercraft of various sizes. Some of their larger vessels were in the first wave of vehicles to go to Iraq when the war began in early 2003 (photo to right). They are ideal for safely patrolling both land and water, and are fully outfitted with armour plating and gigantic machine guns. The Canadian Coast Guard has been using hovercraft for patrol and rescue missions for several decades, again because they are safer and faster than boats, especially in the rough and sometimes frigid waters off the coast of B.C. These craft have performed countless rescues of capsized boats. Golf courses can use small hovercraft instead of other utility vehicles because they don't damage the ground. A hovercraft can fly over waterlogged or freshly laid grass, as well as water and sand traps without causing any damage. They are used to spray fertilizer and grass seed, and can even be used to give rides in the winter when most golf courses are closed and making no money. Many environmental and geological companies use hovercraft to take water and soil samples in remote or sensitive areas because they don't affect the water and ground that is being sampled. They are used to spray insecticides, and can fly over fish breeding ponds, dry riverbeds, mud flats, and shallows without disturbance or damage. A very common use for small hovercraft is thin ice and rapids rescue.

Hovercraft are the only vehicles in the world that can fly safely and freely over thin ice and rapid water without difficulty. They can easily go from open water, to ice, to rapids, to pavement. They aren't affected by currents or tides, and can even fly upstream on rapids. They don't break the ice that a victim may be clinging to, and can deploy onto rapids much faster than it takes to string up ropes and harnesses using old techniques. This is a real breakthrough for the rescue community, and makes the whole process so much safer by keeping the rescuers themselves out of danger.

BIBLIOGRAPHY

“Combination of Hovercraft Skirts.” Google.com. 2007. http://4wings.com.phtemp.com/tip/image/bfdetail01.jpg (19 July 2008).

Elsley, Gordon H. Hovercraft Design and Construction. Great Britain: David & Charles Ltd, 1968.

Fitzgerald, Christopher and Robert Wilson. Light Hovercraft Design, 3rd Edition. Alabama: The Hoverclub of America, Inc., 1995.

“Hovercraft.” Wikipedia. 21 July 2008. http://en.wikipedia.org/wiki/Image:Hovercraft_-_scheme.svg (22 July 2008).

“The History of Hovercraft and Air Cushion Vehicles.” Hovercraft. 13 August 2007. http://links999.net/hovercraft/h overcraft_history.html. (19 July 2008).

V. Fabrication Of The Hover Craft