Final Report on Hovercraft

7/31/2019 Final Report on Hovercraft

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1.INTRODUCTION

Transportation is of prime importance for any human being. Humans' first means

of transport were walking and swimming. The domestication of animals introduced

a new way to lay the burden of transport on more powerful creatures, allowing

heavier loads to be hauled, or humans to ride the animals for higher speed and

duration. Inventions such as the wheel and sled helped make animal transport more

efficient through the introduction of vehicles. Also water transport, including

rowed and sailed vessels, dates back to time immemorial, and was the only

efficient way to transport large quantities or over large distances prior to the

Industrial Revolution.

The first forms of road transport were horses, oxen or even humans carrying goods

over dirt tracks that often followed game trails. The first watercraft were canoes

cut out from tree trunks. Early water transport was accomplished with ships that

were either rowed or used the wind for propulsion, or a combination of the two.

The importance of water has led to most cities that grew up as sites for trading,

being located on rivers or at sea, often at the intersection of two bodies of water.

Until the Industrial Revolution, transport remained slow and costly, and production

and consumption were located as close to each other as feasible.

The Industrial Revolution in the 19th century saw a number of inventions

fundamentally change transport. With telegraphy, communication became instant

and independent of transport. The invention of the steam engine, closely followed

by its application in rail transport, made land transport independent of human or

animal muscles. Both speed and capacity increased rapidly, allowing specialization
http://en.wikipedia.org/wiki/Windhttp://en.wikipedia.org/wiki/Wind


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through manufacturing being located independent of natural resources. The 19th

century also saw the development of the steam ship that sped up global transport.

The development of the combustion engine and the automobile at the turn into the

20th century, road transport became more viable, allowing the introduction of

mechanical private transport.

1.1 OBJECTIVES

1. To design and fabricate a hovercraft.

2. To select an engine and modifying it to suit the needs of a hovercraft.

3. Studying the performance of the engine.

4. Selection of a suitable material for both the hull and skirt.

5. Selection of a propeller to provide both thrust and lift.

6. To effectively conduct the effective run of the hovercraft.

1.2 SCOPE OF THE STUDY

Hovercraft are so versatile that their applications are as diverse as the people who

use them. They are most often used to reach areas that are inaccessible on foot or

by conventional vehicles. These can be used in exploring the vast number of

shallow and narrow waterways that cannot be reached by boat, rescue work on

swift water, ice, snow, mud flats, deserts, wetlands, shallow water, swamps, bogs,

marshes and floodwaters, transport in environmentally sensitive areas where

habitat, erosion and soil compaction are a concern, wildlife conservation and

research, military services, oil spill cleanup, survey work etc.


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1.3 JUSTIFICATION OF THE STUDY

The present work focuses on the feasibility of using hovercraft as a mode oftransportation. Human transportation involves around many factors. One of these is

transportation. The ability to move from place to place for various purposes was

and is of paramount importance for human survival and expansion. Therefore, as

different methods for transportation increase due to need to traverse multiple

terrains including ice, snow, grass, water, sand, dirt, mud and more, ingenuity

plays an important role in fulfilling those demands. One of the physical

manifestations of such ingenuity is the hovercraft. Thus, this project is justified in

the present context as this aims at fulfilling all the above mentioned needs.

1.4 LIMITATIONS

1. Use of hovercraft is not possible at public places.

2. Lack of speed is a concern.

3. The craft does not meet prescribed emission norms.

4. Hovercraft cannot operate over surfaces which do not seal in the air cushion.

5. Noise levels of the hovercraft are an issue.

2. HOVERCRAFT


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A hovercraft, or air-cushion vehicle (ACV), is a craft designed to travel over any

smooth surface supported by a cushion of slow moving, high-pressure air, ejected

downwards against the surface below, and contained within a "skirt." Hovercraft is

used throughout the world as a method of specialized transport wherever there is

the need to travel over multiple types of surfaces. Because they are supported by a

cushion of air, hovercraft are unique among all forms of ground transportation in

their ability to travel equally well over land, ice, and water. Small hovercraft are

often used in physical activity, combustion, or passenger service, while giant

hovercraft have been built for civilian and military applications to transport cars,

tanks, and large equipment into difficult or hostile environments and terrain.

According to Webster, a hovercraft is "a transport vehicle which rides on a

cushion of air ejected from an annular ring beneath it without any contact with the

land or sea over which it travels."

Simply stated, an air cushion vehicle is one whose weight is supported only by air

pressure that is trapped beneath it. It has no wings, wheels, or anything else below

it other than pressurized air. This has been shown in Fig. 2.3

People often confuse a hydrofoil with a hovercraft. A hydrofoil, however, has little

in common with an ACV. The hydrofoil as shown in Fig. 2.1 requires two wings

"flying" under the surface of the water at all times, a requirement that is certainly

not at all an amphibious design like a hovercraft.

Another vessel that is sometimes confused with a hovercraft is the airboat. An

airboat as shown in Fig. 2.2 is little more than a flat-bottomed boat that is propelled

with an air propeller and rudder in the rear of the craft, rather than a propeller and

rudder in the water.


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Fig. 2.1 HYDROFOIL Fig 2.2 AIRBOAT

Fig. 2.3 HOVERCRAFT

2.1 WHY A HOVERCRAFT?

First of all, operating a hovercraft (also called an air cushion vehicle or ACV for

short) is a lot of fun. Thinking is really needed when piloting one of these

machines. They do not respond to controls like any other vehicle. The person

piloting the hovercraft really is not in a positive control of an ACV at all times,

because it is influenced so greatly by winds and any unevenness in the surface

under them, That is why hovercraft are operated over water or over large fields,

you need a lot of room to maneuver them,


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One of the fascinating things about a hovercraft is that it is amphibious. A properly

designed ACV can run at 40 miles per hour. On the way, it does very minimal

damage to the environment. It is possible to clear typically, obstacles of about 6

inches in height at full speed without touching the hard part of the craft. This

includes floating logs too. These are not felt even if they are lying flat along the

water's surface.

A hovercraft at speed doesn't leave a wake, either. Unlike a power boat, no energy

is expended in making the huge waves that trail behind a boat. The hovercraft

leaves a few ripples and some bubbles thats all. Wake damage from an ACV is

nonexistent. Collision damage, however, is a very real possibility.

A hovercraft is environmentally safe, too. An ACVcan operate directly over clam

beds without disturbing them. The hovercraft can travel equally well on hard sur-

faces or on the water. All that is required is a reasonably flat surface. An ACV will

travel very well over any depth of mud, leaving only tiny ripples as tracks behind

it.

A hovercraft will work very well on sand, too. Sand dunes, as long as they are

gently sloped, make a fine roller-coaster ride in an ACV. The humps are smoothed

out by the pressured air beneath the craft, and it is possible to glide along as

though you were riding on a magic carpet.

Hovercraft love to run on ice. There is almost no friction at all, especially if it is a

warm day and the ice is not frozen extremely hard at the surface. In fact, ice

operation is where the ACV is really unique. An ACV is the only really safe

vehicle to use for water/ice rescues, because it makes no difference to the ACV

whether it is on water, ice l/16fh of an inch thick, or 6 inches thick. It still zips

along, comfortably and safely.


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2.2 HISTORY

2.2.1 EARLY THOUGHTS

The idea of using an air-cushion as a means or aid to acceleration and reduction in

hydrodynamic drag was first explored by Sir John Thornycroft, a British engineer,

who, in the 1870's built some experimental models on the basis of an air cushion

system that would reduce the drag of water on boats and ships.

In 1877 he successfully patented the idea and his theory was that if a ship's hull

was given a concave bottom, which could be filled - and replenished - with air, it

would create significant additional lift. And so the air cushion effect was born.

Decades later scientists and inventors were still busy with his ideas but without any

practical applications. With the coming of the airplane however, it was noticed that

additional lift was obtained if the plane flew closer to land or water, creating a

"funnel effect", a cushion of air.

2.2.2 EARLY USES OF AIR CUSHION

The Germans built a flying boat that proved the reality of Thornycroft's theory in

1929, when, during an Atlantic crossing, it flew much closer to the ocean's surface

than was usual in order to take advantage of the air cushion effect. The trip time

was significantly reduced as a result and the aircraft's performance that much

greater.

The flying Boat became quite a legend in its day, especially when it started flying

around the world in 1926. Despite its enormous fuel consumption - around 400

gallons of gas per hour - it was one of the first large passenger planes capable of


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150 seats. But its engines tended to overheat and, after its arrival in New York in

1931 it was refitted with American water-cooled engines.

2.2.3 THE FIRST REAL HOVERCRAFT

The successful use of the air cushion effect was not lost on engineers after World

War 2 was over and in the early 1950's British, American and Swiss engineers

started to rethink Sir John Thornycroft's problem.

The Englishman Christopher Cockerell, commonly seen as the father of the

hovercraft, being retired from the army, settled into boat building where he soongot captivated by Thornycroft's problem of reducing the hydrodynamic drag on the

hull of a boat by using some kind of air cushion.

Cockerell`s theory was that, instead of using the plenum chamber - an empty box

with an open bottom as Thornycroft had devised - air was instead pumped into a

narrow tunnel circumnavigating the entire bottom, it would flow towards the center

and form a more effective air cushion. This peripheral jet would cause the air to

build up enough pressure to equal the weight of the craft and, as it would have

nowhere to go, the pressure would force the craft up, clearing it off the ground

altogether.

Cockerell successfully tested his theory and filed his first patent in 1955. The year

after he formed a company called Hovercraft Ltd. He further envisioned and

partially worked out other problems of the hovercraft principle that still have to be

fully exploited by modern hovercraft builders. One of these was to re-use the air

for greater overall efficiency. Soon after, in 1956, the air cushion vehicle was


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classified as "secret" and a construction contract was placed with a British aircraft

and seaplane manufacturer.

The result was the birth of the first hovercraft in 1959. It weighed four tons and

could carry three men. Its maximum speed was 25 knots (1 knot = 1.15 miles or

1.85 kilometers per hour) on calm water. It had a 6-inch (15 cm) rubberized skirt to

make it easier to contain the air cushion on uneven ground.

The first air cushion vehicles operated by pumping such a huge quantity of air

beneath them that the air did not have sufficient time to get out from under the

craft before more air was pumped in. This required huge amount of power, and thecraft could not attain great clearance off the ground, but the basic principle of

levitating on air actually worked. The weight of an air-cushion vehicle is supported

only by the slightly compressed air pumped beneath the craft by means of a large

fan of some sort. Significant wear and tear of the skirt through friction with the

water at high speeds made it necessary to use more durable material and a rubber

and plastic mixture was developed by 1963. The length of the skirt had also been

extended to about 4 feet (1.2 m

2.3 ENERGY, POWER & FORCE

Moving a vehicle, may it be on the ground, on the water, or in the air, requires a

force to overcome the friction and inertia forces and to lift the vehicle to a different

elevation. A force can be created from any kind of energy, like the energy

contained in liquid fuel for an internal combustion engine, the electric energy

stored in a battery or like solar energy being transformed by solar cells into electric

power. To actually move an object, the force must be must be translated by some

sort of engine into power, pushing the vehicle forward. Burning fuel in an open


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pan does not create a force and having a bottle of compressed air lying around

creates a force on the bottles walls, but does not create any power output. Energy

may be static (fuel) or dynamic (flywheel), force is static and power is always

dynamic. Power equalsforce times distance per time.

Fig. 2.4 Process of converting stored energy into power which can be used to move

a craft.

2.4 MAIN COMPONENTS OF THE HOVERCRAFT


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Fig. 2.5 Components of the hovercraft

MAIN COMPONENTS

Fig. 2.6Engine

2.4.1 ENGINE

Engine is the heart of the hovercraft. It

is the one which produces power,

which is very much essential for the

propeller to provide lift and thrust to

the craft. The lack of powerful,

lightweight engines was one of the

reasons for the failures of early

attempts to hover. Steam engines

proved to be too heavy; compressed

air engines can be used for smaller

demonstrations, but are not practical.

Finally the evolution of the piston

engine, proved to be very useful, thus

providing a successful power plant.


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Fig. 2.7 Propeller

Fig. 2.8Hull

2.4.2 THE THRUST

SYSTEM

Hovercraft thrust is generated by

either a propeller or a ducted fan.Both devices move air from in

front of the craft and accelerate it

out of the back. This accelerated

mass of air then generates thrust

which pushes the craft forwards

2.4.3 THE HULL

A hull is the watertight body of a

hovercraft, ship or boat. Above

the hull comes the superstructure

and deckhouse. The line where

the hull meets the water surface

is called the waterline.


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Fig 2.9 Skirt

Fig 2.10Duct

2.4.4 SKIRT

A skirt is that part of the

hovercraft which helps in

entrapping the air, which isuseful for the movement and

also the lift of the craft.

2.4.5 DUCT

Duct is that part of the hovercraft

which is used as the passage for

the flow of air. This is done into

order to trap the air inside the

skirt, which is necessary for the

lift of the craft.


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Fig 2.11Rudder

.

2.4.6 THE STEERING

SYSTEM

Hovercraft has the steering

rudders or vanes placed directly

in the thrust air flow at the back

of the craft. For rudders to workeffectively they must have a lot

of air flowing over them. An

aircraft has high airflow over its

control surfaces caused by its

relatively high forward speed - a

hovercraft rudder has to work at

zero forward speed. The

hovercraft thrust is effectively re-

directed by the rudders to provide

steering.


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3. CONSTRUCTION OF THE HOVERCRAFT

3.1 ENGINE

An engine is a mechanical device that produces some form of output from a given

input. An engine whose purpose is to produce kinetic energy output from a fuel

source is called a prime mover; alternatively, a motor is a device which produces

kinetic energy from other forms of energy (such as electricity, a flow of hydraulic

fluid or compressed air).

The internal combustion engine is an engine in which the combustion of a fuel

occurs with an oxidizer (usually air) in a combustion chamber. In an internal

combustion engine the expansion of the high temperature and pressure gases that

are produced by the combustion directly apply force to a movable component of

the engine, such as the pistons or turbine blades and by moving it over a distance,

generate useful mechanical energy

The term internal combustion engine usually refers to an engine in which

combustion is intermittent, such as the more familiar four-stroke and two-stroke

piston engines, along with variants, such as the Wankel rotary engine. A second

class of internal combustion engines use continuous combustion: gas turbines, jet

engines and most rocket engines, each of which are internal combustion engines on

the same principle as previously described.

Basically there are two types of I.C. Engines: 2-Stroke and 4-Stroke engines.

3.1.1 2-STROKE ENGINE

Engines based on the two-stroke cycle use two strokes (one up, one down) for

every power stroke. Since there are no dedicated intake or exhaust strokes,


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alternative methods must be used to scavenge the cylinders. The most common

method in spark-ignition two-strokes is to use the downward motion of the piston

to pressurize fresh charge in the crankcase, which is then blown through the

cylinder through ports in the cylinder walls. The working is as shown in fig 3.1

Spark-ignition two-strokes are small and light for their power output and

mechanically very simple; however, they are also generally less efficient and more

polluting than their four-stroke counterparts. In terms of power per cubic

centimeter, a single-cylinder small motor application like a two-stroke engine

produces much more power than an equivalent four-stroke engine due to the

enormous advantage of having one power stroke for every 360 degrees of

crankshaft rotation (compared to 720 degrees in a 4 stroke motor).

Small displacement, crankcase-scavenged two-stroke engines have been less fuel-

efficient than other types of engines when the fuel is mixed with the air prior to

scavenging allowing some of it to escape out of the exhaust port. Modern designs

use air-assisted fuel injection which avoids this loss, and are more efficient than

comparably sized four-stroke engines. Fuel injection is essential for a modern two-

stroke engine in order to meet ever more stringent emission standards. Two-stroke

engines have the advantage of an increased specific power ratio (i.e. power to

volume ratio), typically around 1.5 times that of a typical four-stroke engine.


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Fig 3.1 Working of 2- Stroke Engine

Fig 3.2 Working of 4- Stroke Engine


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3.1.2 4- STROKE ENGINE

As their name implies, operation of a four stroke internal combustion engines have

4 basic steps that repeat with every two revolutions of the engine. A 4- Stroke

engine is depicted in the fig 3.2

1. Intake

Combustible mixtures are emplaced in the combustion chamber

2. Compression

The mixtures are placed under pressure

3. Combustion/Expansion

The mixture is burnt, almost invariably a deflagration, although a few

systems involve detonation. The hot mixture is expanded, pressing on and

moving parts of the engine and performing useful work.

4. Exhaust

The cooled combustion products are exhausted into the atmosphere

3.1.3 BASIC DIFFERENCES BETWEEN 2-STROKE AND 4-STROKE ENGINES


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Table 3.1

2- STROKE ENGINE 4- STROKE ENGINE

1)

here are two strokes one is intake

and compression 2nd is ignition

and exhaust.

2)

here is one working stroke forevery revolution of crankshaft.

3)

Engine consists of inlet and

exhaust ports.

4)

stroke engines have high engine

RPM's.

5)

ower developed is more.

6)

Lubricating oil is mixed with the

fuel.

7) Mechanical Efficiency is less.

1)

n a 4 stroke 1st is Intake 2nd is

compression 3rd is ignition 4th is

exhaust.

2)

no working stroke for every tworevolutions of crankshaft.

3)

Engine consists of inlet and

exhaust valves.

4)

stroke engines have less engine

RPM's.

5)

ower developed is less.

6)

Lubricating oil is filled up into the

crankcase.

7) Mechanical Efficiency is more.


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Hovercraft engines vary depend on the environment condition.With the exception

of human powered craft, a hovercraft needs at least one engine. With a

conventional hovercraft, air needs to be supplied to lift (to make the cushion) and

thrust (to propel the craft). The supply of air to lift and thrust can be accomplished

using only one engine by either powering a single fan and then splitting an amount

of air off to lift (about 33%) and the rest for thrust (called an 'integrated' system) or

the one engine can be used to power separate lift and thrust fans.

Most hovercraft, however, use a dual engine system where one large engine is

used for thrust and another, smaller engine is dedicated to lift. Unlike the

integrated system, this allows the craft to remain hovering while the thrust engine

is turned off.

Larger, commercial craft may use as many as 6 or 8 engines for power of the lift

and thrust systems. Engines types range from diesel to gas turbine.

Here, a single engine is used to run the propeller, which in turn provides both the

thrust and the lift i.e. Integrated System. There are some factors which are to be

taken into account while choosing the engine for a hovercraft. They include:

The engine with the best power to weight ratio

The other point of concern is the engine`s weight.

Also, a gear, belt, or chain reduction system is required to match the fan or

propeller to the engine.

The torque produced by the engine.

Also, it depends on the design of the hovercraft.

The power of the engine required is based on the lift and thrust systems, which are

calculated as follows:


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3.1.4 THEORETICAL LIFT HORSE POWER

The energy required to move the air is the product of the volume of the air and its

pressure.

(1.529 m3/s) *(733.39 Pa) = 1121.362 Nm/s

Converting this into HP, 1121.362 / 746 = 1.5 HP

3.1.5 ACTUAL HP

There is some inefficiency, which are to be overcome. Therefore, 25% increment is

given by:

1.5 / (25%) = 6 HP

The dimensions of the hovercraft are: 8feet in length, 5 feet in width and a hover

height of 2 inches. Choosing the engine was a bit difficult. There were many

engines to choose from, but it was to be a right engine. The engine is used to runthe propeller (Integrated System).It should provide so much power as to propel as

well lift the craft.

The process of selecting the engine started with a 98 cc Kinetic engine, 98.2 cc

Suzuki Samurai engine, Yamaha RXZ etc.

It was the turn of the good old Yezdi Road king engine. It is a 246.3 cc engine,

produces about 16 BHP. It turned out to be a good engine with high power output,

which was suitable for the craft.

3.1.6 POWER FOR THE THRUST SYSTEM

Horse power available = 16


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Lift power required = 6

Thrust power available = 10

The power required for the lift is 6 HP; the engine produces about 16 HP. Theremaining horse power is utilized for providing the thrust to the engine which is

about 10 HP.

3.1.7 ENGINE SPECIFICATIONS

Type : 2 stroke Air cooled CDI engine

No. of Cylinders : one

Bore Diameter : 70mm

Stroke Length : 64mm

Cubic Capacity : 246.3cc

Output (BHP) : 16 at 5500 rpm

Max. Speed : 105 kmph

Compression Ratio : 8.2:1

Torque : 2.43 Kgm at 4250 rpm

Air Filter : Cone type.


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3.1.8 WORKING OF CDI ENGINE

A conventional induction ignition creates a spark by applying electric potential (12

volts) to the primary side of the coil. The coil steps the primary potential up to asmuch as 10,000 volts and delivers this high voltage to the spark plugs. However,

this "step up" process is relatively slow, and as crank speed (rpm) increases, the

secondary voltage declines dramatically.

This limitation is partially solved by the development of capacitive-discharge

ignition (CDI) systems. Instead of applying 12 volts to the coil, a CD ignition

increases the primary current by storing it in a kind of miniature battery called a

capacitor. When this higher primary current is applied to the coil, the secondary

voltage is dramatically increased.

The principal advantage of a CDI system is the ability to present a superior spark

to the air/fuel mixture inside the combustion chamber, thus maximizing burn

efficiency. The easiest way to get a bigger spark is to increase the spark plug gap

size. However, increasing the gap distance also increases the voltage necessary to

ionize the air/fuel mixture. And the resistance of the air/fuel mixture increases as

the mixture is pressurized in the cylinder, requiring even higher voltage to spark

across a plug. A CDI system provides the higher voltage required by the increased

spark plug gap size, thus providing very intense spark.

A CDI ignition system can create spark potential as high as 37,000 volts. Most

engines only need about 20,000 volts for reliable ignition. The stock system begins

to 'droop' as the rpm goes up. At highway speeds, the spark voltage becomes more

and more marginal, averaging about 18,000 volts. With a CDI system, the step up

process is very fast compared to a conventional 12-volt induction. This assures a


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more consistent spark delivery across the plug gap, even at very high crank speeds

(rpm).

3.1.9 CONSTRUCTION OF THE ENGINE

Fig 3.3 Cylinder

Fig 3.4 Piston and Piston rings

3.1.9.1 CYLINDER: is the heart of the

engine, in which the fuel is burnt and the

power is developed. The inside diameter is

called the bore. To prevent the wearing of

cylinder block, a sleeve will be fitted tightly

in the cylinder. The piston reciprocates inside

the cylinder.

3.1.9.2 PISTON: is a close fitting hollow

cylindrical plunger moving to and fro in the

cylinder. The power developed by combustion

of the fuel is transmitted to the crank shaft

through the connecting rod.

3.1.9.3 PISTON RINGS: These are the

metallic rings inserted into the circumferentialgrooves provided at the top end of the piston.

These rings maintain a gas tight joint between

the piston and the cylinder while the piston is

reciprocating in the cylinder. They also help in

conducting heat into the cylinder.


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Fig 3.5 ConnectingRod

Fig 3.6 Crank and

crankshaft

3.1.9.4 CONNECTING ROD: Itis a link that connects the piston

and the crank shaft by means of

pin joints. It converts the

rectilinear motion of the piston

into the rotary motion of the

crank shaft.

3.1.9.5 CRANK & CRANK SHAFT:

The crank is a lever that is connected to the

end of the connecting rod by a pin joint

with its other end connected rigidly to a

shaft called crankshaft. It rotates about theaxis of the crankshaft and causes the

connecting rod to oscillate.


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Fig 3.7 Crankcase

3.2 MODIFICATIONS OF THE ENGINE

The next uphill task was the modification of this engine. This is necessary to

optimize the performance. Since an Integrated System i.e. the same propeller is

used to provide both the lift and the thrust is being used, there were some

modifications to be done.

The modifications include:

1) Re-boring of the engine.

2) Elimination of the Clutch Assembly.

3) Elimination of the Gear Assembly.

4) Introduction of the pulleys and belt drive.

3.2.1 RE-BORING OF THE ENGINE

Re- boring is the operation which is used to bring more power to the engine. This

is done when the clearance between the piston and cylinder is increased which may

be due to wear.

This process includes:

3.1.9.6 CRANKCASE: is the

lower part of the engine serving

as an enclosure for the

crankshaft.


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Removal of burrs in the cylinder.

Increasing the diameter of the cylinder.

Increasing the piston size.

To maintain a constant diameter throughout the stroke length.

To obtain a high quality finish on the worn out surface.

3.2.1.1 CONDITION OF THE ENGINE AFTER RE-BORING

Volumetric capacity of the engine is increased.

Combustion volume of the fuel is increased.

Power of the engine is increased.

Smooth running of the engine is achieved.

Efficiency of the engine decreases.

3.2.2 ELIMINATION OF CLUTCH ASSEMBLY

3.2.2.1 CLUTCH

Mechanical clutches are equipment drive assemblies that contain mechanically

actuated components for connecting two shafts so that they can either be locked

together and spin at the same speed, or decoupled and spin at different speeds.

Engaging the clutch transfers power from an engine to devices such as a

transmission and drive wheels. Disengaging the clutch stops the power transfer, but

allows the engine to continue turning. Mechanical clutches are less expensive than

pneumatic or hydraulic clutches, but do not provide the same range of torque. On

most motorcycles, the clutch is operated by the clutch lever, located on the left

handlebar. No pressure on the lever means that the clutch plates are engaged

(driving), while pulling the lever back towards the rider will disengage the clutch

plates, allowing the rider to shift gears..


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3.2.2.2 MULTIPLE PLATE FRICTION CLUTCH

Motorcycle clutches are usually made up of a stack of alternating plain steel and

friction plates. Friction clutches generate friction between contact surfaces. Onetype of plate has lugs on its inner diameter that key it to the engine crankshaft,

while the other type of plate has lugs on its outer diameter that key it to a basket

that turns the transmission input shaft. The plates are forced together by a set of

coil springs when the clutch is engaged. It is used in motorcycles and in some

diesel locomotives with mechanical transmission. It is also used in some

electronically-controlled all-wheel drive systems. This is the most common type of

clutch on modern types of vehicles.

Clutch assembly is needed only when there is transmission of power from

one shaft to the other.

The original crankshaft assembly is modified.

To the extension of the crankshaft, the customized shaft is directly coupled

with the help of grub screw.

There is a provision for the attachment of the pulley.

Aluminum pulleys are used to transmit the power to the propeller.

3.2.3 ELIMINATION OF GEAR ASSEMBLY

A gear is a component within a transmission device that transmits rotational force

to another gear or device. A gear is different from a pulley in that a gear is a round

wheel that has linkages ("teeth" or "cogs") that mesh with other gear teeth,

allowing force to be fully transferred without slippage. Depending on their

construction and arrangement, geared devices can transmit forces at different

speeds, torques, or in a different direction, from the power source.


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3.2.3.1 MECHANICAL ADVANTAGES

High transmission ratio.

No slip occurs.

Creep does not occur.

Effective transmission of power takes place.

They are positive non slip drives.

Most convenient for small centre- distance.

Velocity ratio will remain constant throughout.

For the hovercraft, an engine is needed which is light in weight and also which

provides more power. In the context of this, the gear assembly is eliminated.

By eliminating the gear assembly,

Power loss is reduced to the maximum extent possible

The power that is being harnessed is the INDICATED POWER.

The friction power is reduced.

The TORQUE is improved.

Noise and vibrations are reduced to some extent.

Weight of the engine is reduced.

Effective utilization of engine power.

The lubrication to the gear assembly is not required.

Constant speed is achieved throughout the working of the craft.

Fuel efficiency is improved to some extent.

3.2.4 INTRODUCTION OF THE PULLEY SYSTEM AND BELT

DRIVES


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The power output from the engine is available. The question is how to transmit it

to the fan, here the pulley and belt drives are made use of.

3.2.4.1 BELT AND PULLEY SYSTEMS

A belt and pulley system is characterized by two or more pulleys in common to a

belt. This allows for mechanical power, torque, and speed to be transmitted across

axes and, if the pulleys are of differing diameters, a mechanical advantage to be

realized.

A belt drive is analogous to that of a chain drive, however a belt sheave may be

smooth (devoid of discrete interlocking members as would be found on a chain

sprocket, spur gear, or timing belt) so that the mechanical advantage is given by

the ratio of the pitch diameter of the sheaves only.

Fig 3.8Belt and Pulley System

A Belt is a looped strip of flexible material, used to mechanically link two or more

rotating shafts. They may be used as a source of motion, to efficiently transmit


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power, or to track relative movement. Belts are looped over pulleys. In a two

pulley system, the belt can either drive the pulleys in the same direction, or the belt

may be crossed, so that the direction of the shafts is opposite. As a source of

motion, a conveyor belt is one application where the belt is adapted to continually

carry a load between two points.

A pulley (also called a block and tackle) is a mechanism composed of a wheel

(called a sheave) with a groove between two flanges around the wheel's

circumference. A rope, cable , belt or chain usually runs inside the groove. Pulleys

are used to change the direction of an applied force, transmit rotational motion, or

realize a mechanical advantage in either a linear or rotational system of motion.

The rotary motion of the crankshaft is directly used to run the fan. Two pulleys and

a V-belt drive are used for the completion of this task.

3.2.4.2 PULLEY & BELT SPECIFICATIONS

NUMBER OF PULLEYS : Two

MATERIALS : Aluminium and Cast Iron

DIAMETER OF CAST IRON PULLEY : 114mm

DIAMETER OF ALUMINIUM PULLEY: 50.8mm

KEYWAY LENGTH : 70mm

KEYWAY WIDTH : 8mm

KEYWAY DEPTH : 8 mm

SHAFT DIAMETER: 34 mm connecting the propeller & 24mm shaft

housing the pulley (STEPPED SHAFT).


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MOUNTING: Cast Iron pulley on the shaft of diameter 24 mm, and

Aluminium pulley on the shaft which is coupled to the

crankshaft.

TYPE OF BELT : V- Belt

LENGTH OF THE BELT = Centre distance between the two pulleys + Radius

of the Aluminium Pulley +

Radius of the Cast Iron Pulley

= 1003.3mm + 25.4mm + 57.15mm = 1085.85mm

3.2.4.3 REASONS FOR USING THE V-BELT DRIVE

Reliable and positive drive.

They can transmit high power.

Compact and high velocity ratio.

Can be used for small center distance.

Permit large speed ratio.

No slipping of the belt from the pulley.

Possible to operate the shaft axis in any position.

Less maintenance.

3.3 PHOTOGRAPHS OF THE ENGINE AFTER

MODIFICATION


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Fig 3.9

Fig 3.10

3.4 HULL

A hull is the watertight body of a hovercraft, ship or boat. Above the hull comes

the superstructure and deckhouse. The line where the hull meets the water surface

is called the waterline.


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The structure of the hull varies depending on the type of application. In a typical

modern steel ship, the structure consists of major transverse and longitudinal

members called watertight bulkheads, intermediate members such as girders,

stringers and webs, and minor members called ordinary transverse frames, frames,

or longitudinal, depending on the structural arrangement.

In a typical wooden sailboat, the hull is constructed of wooden planking, supported

by transverse frames and bulkheads, which are further tied together by longitudinal

stringers or ceiling. Often but not always, there is a centerline longitudinal member

called a keel. In fiberglass or composite hulls, the structure may resemble wooden

or steel vessels to some extent, or be of a monocoque arrangement. In many cases,

composite hulls are built by sandwiching thin fiber-reinforced skins over a

lightweight but reasonably rigid core of foam, balsa wood, impregnated paper

honeycomb or other material.

In a hovercraft, the hull of the craft will include the craft floor, side panels, forward

and aft panels till the top skirt attachment line. The hull:

Needs to have adequate size for the total weight of 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

Before starting, the craft weight as well as payload to get an idea of your crafts

actual size should be known. The size of the craft will be from round to rectangular

till triangular shape. Hover pressure will be about 689.48 Pa on most recreational

craft.


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The following table will give an idea of the relation between Width and Length v/s

Lifting Capability at 70.308 kg/m2.

(Wft x Lft x 144 x 70.308 kg/m2.)

on square craft measured at GcL (Ground contact Line) of skirt

W / L in Feet ( 0.305m) Lift in lb (0.457 kg)

3 x 5 216 lb

3 x 6 260 lb

4 x 6 346 lb

4 x 7 403 lb

4 x 8 461 lb5 x 7 504 lb

5 x 8 576 lb

5 x 9 648 lb

6 x 10 864 lb

6 x 11 950 lb

6 x 12 1036 lb

7 x 12 1210 lb

Table 3.11

The table 3.11 helps in getting a quick idea of the size needed for a given payload.

Selected craft dimension: 1.2192m x 2.4384m Lift: 209.1096 kg

One of the biggest dangers operating a Hovercraft are "plow in" (having the nose

dig in the water or sand during high end speed cruising) and "overturn" accidents.

While overturn accidents can be mostly avoided by proper operation - plow in has

to be held in mind while building the forward section (bow section) of the craft.


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Try to hold a boat like slope in the bow section of the craft which can extend

forward of the skirt attachment line - this will not avoid plow in - but will make it a

lot more comfortable than with a steep bow section.

The sides of the hull should have a 15 - 30 degree angle between lower skirt

attachment line and top skirt attachment line to minimize side "plow in", or in case

of a "plow in" to provide less damage to the skirt and a softer impact.

This will provide more or less with the shape of the hull - at this point the need is

the location for the lift air as well as lift unit ( engine, duct and prop or fan

location) . It should be noted in mind that the lift propeller should not be at, or

below the waterline if craft is floating under maximum payload.

3.4.1 MATERIALS THAT CAN BE USED

The hull can be build out of all boat building materials. From simple ply to very

complicated composite panels. The hull is as well the section of the craft which

might get the highest abuse during operation and especially landing in unknown

areas. As long as the craft is on cushion there is no major harm against the

hull - once the lift unit fails during operation - one can hope for a rigid

floor or a soft landing. Even if the craft has landing pads - if the center

floor is nearly at the same level as the landing pads - you will only be able to set

the craft smooth on a parking lot or water. All other surfaces will not be leveled

enough to provide a smooth surface.

To increase the strength of the floor wood stringer in the center core can beencapsulated. This will increase the weight to a minimal extent, while providing

maximum strength over the whole length of the hull. Once the hull is shaped, you

have to select to provide inner strength to your craft by ribs (as well known in boat

building as bulkheads) or just wooden supports.


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Fig 3.11Hull

3.4.2 HULL SPECIFICATIONS

Material Used : Ply board

Overall Length : 2.4384m

Overall Width : 1.2192m

Thickness : 19mm

Density (Kg/m) : 625 Kg/m

Moisture Content : 8-10 %


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Glue Adhesion : Excellent

Screw Holding Strength : >225 Kgs

Nail Holding Strength : >100 Kgs

The hull is filleted on the front by 1foot on both the sides. This is done to avoid the

sharp edges in the hovercraft, which obstructs the movement of the craft.

Resin coating has been given on to both sides of the hull. This is done in order to

provide more strength to the hull, also it provides waterproof surface.

Resin used: General Purpose Resin

Reinforcement: Glass Fiber of density 450 grams per cu. meter on the front side

and glass fiber of density 200 grams per cu. Meter on the bottom side of the hull.

The process used for resin coating is HAND LAY UP process. In this process, the

resins are stored at low temperatures with stabilizers added to it. This is done in

order to prevent gelling. The resins are in liquid state. The resin used here is

commercially available general purpose resin. Catalyst is added to the resin. When

the catalyst is added, the curing action or gelling will start. The accelerator speeds

up the process.

The layers of reinforcement and resins are laid. It is necessary to calculate the

correct ratio of rein to reinforcement. While laying, care should be taken to ensure

air bubbles are not trapped and also wrinkling should not occur. The brushes used

are of hard type, which permits air bubbles to escape between fibers.


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3.4.3 REASONS FOR USING PLY BOARD

Light weight.

Strong enough to bear load.

Anti-skid surface.

Anti-Free properties.

Unique resistance to weather & water.

Even in adverse weather condition, the dimensional change will be within 2-

3%

Cost Effective.

3.5 PROPELLER

3.5.1 SELECTION OF A PROPELLER

Based on the theory of the optimum, only a small number of design parameters

must be specified. These are:

the number of bladesB,

the axial velocity v of the flow,

the diameterD of the propeller,

the selected distribution of airfoil lift and drag coefficients Cl and Cdalong

the radius,

the desired thrust Tor the available shaft power P,

the density rho of the medium (air-1.22 kg/m, water-1000 kg/m).

The design procedure creates the blade geometry in terms of the chord distribution

along the radius as well as the distribution of the blade angle. The local chord

length c depends mainly on the prescribed lift coefficient Cl - if you would like to


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have wider blades, you have to chose a smaller design lift coefficient (resp. angle

of attack) and vice versa. It should be noted, that the design procedure does not

work accurately for high thrust loadings as they occur under static conditions. If

you receive nonsense values for the blade chord, the power loading of the propeller

is probably too high. Check if the power coefficient Pc is less that 1.5, otherwise

the theory is not fully applicable and may lead to errors.

3.5.1.1 Number of Blades

The number of blades has a small effect on the efficiency only. Usually a propeller

with more blades will perform slightly better, as it distributes its power and thrust

more evenly in its wake. But for a given power or thrust, more blades also mean

more narrow blades with reduced chord length, so practical limits have to be

considered here. The chord length can be increased while decreasing the diameter

to keep the power consumption constant, but a diameter reduction is usually a bad

idea in terms of efficiency, as long as the tip mach number or tip cavitations is not

an issue.

3.5.1.2 The Velocity

The velocity of the incoming fluid together with the velocity of rotation (RPM)

determines the pitch distribution of the propeller. Large pitch propellers may have

a good efficiency in their design point, but may run into trouble when they have to

operate at axial velocity. In this case, the blades tend to stall. Usually the best

overall propellers will have a pitch to diameter ratio in the order of 1.

3.5.1.3 The Diameter


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The propeller diameter has a big impact on performance. Usually a larger propeller

will have a higher efficiency, as it catches more incoming fluid and distributes its

power and thrust on a larger fluid volume. The same effect can be shown for lifting

surfaces, which results in sailplanes having large span but slender wings.

3.5.1.4 Lift and Drag Distributions

Instead of the lift and drag coefficients, it usually convenient to specify an airfoil

with a prescribed polar and the design angle of attack at each radius. The

distribution ofCl and Cdalong the radius can be examined later by performing an

analysis for the design point. For maximum performance, the airfoils must operate

at maximum L/D. But if the propeller should also work reasonable well under off-

design conditions, it is usually necessary to use a lower angle of attack for the

design. Again, you can check the Cl and Cddistributions for off-design cases by

performing several single off-design analysis for different settings of the flow

velocity v. Stall should occur gently when the velocity is reduced. The analysis

code will probably give unreliable results for very small velocities.

3.5.1.5 The Fluid Density

The density of the fluid has no influence on the efficiency of a propeller, but

strongly affects its size and shape. As the forces and the power are directly

proportional to the fluid density, a hydro-propeller will have much smaller

dimensions than a propeller working in air. Also, lifting surface under water tend

to develop cavitation when the local pressure of the flow field falls below the

vapour pressure. Therefore it is not possible to use high lift coefficients in hydro-

props, usually they have to stay below Cl = 0.5. The same is true for high speed

tips of aircraft propellers, where not cavitation, but supersonic regions may occur if


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the pressure gets too low. Therefore the tip sections of propellers operating at

Mach numbers above 0.7 should be designed to operate at small lift coefficients

below 0.5 too. The dimensionless coefficients Ct and Cp are not affected by a

variation of density, but the values for thrust and power are. Thus a propeller

engine combination will find different operating points depending on the fluid

density. This makes a difference for aircraft propellers, where the performance of

propeller and engine depends on the altitude.

3.5.2 WORKING

The working of a propeller is as shown in fig 3.12. As Newton stated, "actio est

reactio". For the propulsion problem, this means that a device accelerating air or

water in one direction, feels a force in the opposite direction. A propeller

accelerates incoming air particles, "throwing" them towards the rear of the craft,

and thus feels a force on itself - this force is called thrust. Looking more closely at

propellers shows, that a propeller adds a velocity to then incoming velocity v.

The first half of this acceleration takes place in front of the propeller, and the

second half behind the propeller. Because the mass of air passing through the

stream tube must be constant (conservation of mass), the increased velocity leads

to a contraction of the stream tube passing through the propeller disk (neglecting

compressibility).


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Side view of the stream tube

passing through a propeller,

showing the acceleration in front

and behind the propeller. The

propeller also induces swirl into

it's wake.

Fig 3.12

Besides the contraction of the stream tube, a propeller also adds a swirl component

to its outflow (wake). The amount of swirl depends on the rotational speed of the

engine and eats up energy, which is not available for thrust anymore. Typical, well

designed propellers loose about 1% to 5% of their power in the swirl of the

propeller wake. The swirl angle (about 1- 10) may cause non symmetrical flow

conditions on parts behind the propeller, e.g. at the tail planes.

The stream tube of a low bypass ratio turbo jet engine looks completely different,

because the acceleration of the flow is mainly performed through the thermal

expansion of the heated air. Here the incoming stream tube will usually have a

smaller diameter than the exhaust stream, depending on operating conditions. The

final extreme is the rocket engine, which has no incoming stream tube, but createsits exhaust jet only by expanding the gases created by a chemical reaction (e.g. by

burning a fuel/oxygen mixture).


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3.5.3 PROPELLER INPUT DETAILS

The propeller used is a multi blade propeller. The blades are made up of PAG

(Glass reinforced Poly Amide). The table 3.12 provides the propeller

specifications.

Table 3.12

PROPELLER DIA DIA 1000mm

PROPELLER RPM 2000 rpm

AIR FLOW 20000CFM

TEMPERATURE 35 degree Celsius

STATIC PRESSURE 15 mm WC

POWER AVAILABLE 16 H.P

TIP CLEARANCE % 0.5 %

NOISE LEVEL 8085 db

APPLICATION HOVERCRAFT

PROPELLER DRIVE ENGINE

DIRECTION OF ROTATION of

PROPELLER From Drive Side CLOCK WISE

DIRECTION OF AIR FLOW

Drive> PROPELLER >Radiator

OR Radiator> PROPELLER >Drive

PUSHER / SUCKER

PUSHER

MOUNTING DETAILS SHAFT MOUNTING


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PROPELLER MOUNTING

DIRECTIONHORIZONTAL

3.5.4 MAJOR FORCES ON THE PROPELLER

The main forces acting on a prop or fan are Centrifugal, Thrust, Gyroscopic, and

Torque Bending. Minor forces are aerodynamic twisting and centrifugal

twisting.

3.5.4.1 CENTRIFUGAL FORCE

Centrifugal forces tend to tear the blades off a prop or fan hub. Briefly, one can

calculate the forces involved for a given blade if the weight of the blade in

pounds is known, the center of gravity of the blade is known (or estimated) as

measured from the hub center, and the RPM at which it is rotating is specified.

Use the following formula:

F= 0.00002842*W*R*RPM

RPM affects the centrifugal force greatly; doubling the RPM will quadruple the

centrifugal force.

3.5.4.2 THRUST FORCE

The total thrust force propelling the craft forward is d ivided equally among the

blades. Each of the blades is being forced forward, toward the front of the vessel,

by this action. Thus a force of 400# thrust on a prop means that each blade is being

forced forward by 200# of dynamic pressure.


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3.5.4.3 GYROSCOPIC FORCE

When a craft is operating at full power and suddenly turned to a new direction by

the rudder, the rotating mass of the propeller or fan will react in such a way as to

either lift or depress the stern of the vehicle. While the amount of lift or

depression is not a lot, it is concentrated at a point on the driveshaft which is be-

tween the last bearing and the prop. Gyroscopic force acts to break the driveshaft

by trying to snap it off, at that location. So a sufficiently strong driveshaft is

needed to withstand this force.

3.5.4.4 TORQUE BENDING

Torque bending is the twisting of the driveshaft produced by each power stroke of

every piston. Rotational power is delivered in pulses and is not a smooth rota-

tional force. This is especially true for a single-cylinder, 4-stroke engine. These

pulses of torque can create a winding-up action of the driveshaft. Between power

pulses, the driveshaft unwinds. A long driveshaft directly coupled to a propeller

and engine can develop extremely high twisting loads, sufficient in some cases to

momentarily reverse the engine between power pulses. These loads can snap a

driveshaft or universal joints. The use of a V-belt transmission can absorb much

of these torque forces, but the belts may suffer from excessive wear if operated at

low RPMs.

In any case, torque bending can produce high stresses on the drive system. These

forces can be minimized by using an engine with multiple cylinders (the more,

the better), us ing a heavy flywheel (smoothes out the pulses considerably), using

a short driveshaft (stores less energy between pulses), using a bell system that

can smooth out excessive pulses by sl ipping a little during operation at cr it ical


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RPMs, and operating at RPMs that simply do not resonate with the driveshaft and

engine/propeller combination.

Fig 3.12 (a) Propeller

3.6 DUCTS

Ducts are the components of the hovercraft which are used to converge or diverge

flow of air. Normally for hovercrafts, there will be two ducts provided namely

thrust duct and lift duct. The thrust duct is used for providing the thrust for the


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hovercraft and the lift duct helps in passage of air into the skirt. The lift duct traps

almost half the air and the air passes into the skirt thus providing the required lift.

3.6.1 DUCT SPECIFICATIONS

3.6.1.1 LIFT DUCT

MATERIAL USED: GALVANIZED IRON

Fig 3.13 (a)Duct


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Fig 3.13 (b)Duct

THICKNESS: 0.5 mm

DIMENSIONS: 600mm * 200 mm

HEIGHT: 500mm


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3.6.1.2 THRUST DUCT

MATERIAL: GALVANIZED IRON

THICKNESS: 0.8 mm

RADIUS: 513 mm

WIDTH: 590 mm

3.7 SKIRT

Skirt is the material which is used at the bottom of the hull to trap the air. It serves

as an air cushion. It serves as a suspension system so that the power required to lift

the craft can be minimized. As far as Hovercrafts go, there are three types of skirts

to be concerned with: Bag Skirts, Wall Skirts, and Finger Skirts. Bag Skirts are

generally used in small projects, recreational hovercrafts use Wall Skirts, and

large-scale professional hovercrafts for racing usually use finger skirts. You can

mix and match the type of skirt with the type of hovercraft you are making.

3.7.1 FUNCTIONS OF THE SKIRT

Contain the cushion of air beneath the craft at the required hover height.

Have the ability to conform or contour efficiently over obstacles so as to

keep to minimum, the loss of cushion air.

Return to its original shape after having been deformed.

Give adequate stability.

Offer little resistance to the passage of obstacles beneath it.

Have the ability to absorb a large proportion of the energy which is produced

on impacts or collisions with obstacles greater than hover height or cushion

depth


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3.7.2 THINGS TO BE TAKEN INTO ACCOUNT WHILE

CHOOSING THE SKIRT MATERIAL

The material is going to be dragged along the ground a lot.

Make sure it can take a lot of wear and tear.

Needs to maintain the air for lifting the craft.

Be easily maintained on site without the need to lift or jack-up the craft.

Have a long operating life.

Be relatively simple to make and fit.

Have a low maintenance cost. The initial cost of making the skirt may not

be very low but it is important that once made and fitted, the skirt be cheaply

maintained.

Be tailored so that it is even in height above the ground all the way around

the craft. One part of the skirt should not drag whilst another is 20 or 30

millimeters above the ground.

3.7.3 SKIRT CHARACTERISTICS

Simple in construction.

Usually it gives fairly high drag over undulating surfaces.

The inflated loop skirt is very stiff in roll and pitch.

The disadvantage is that it gives a harder ride than the segmented type and

has more limited obstacle clearance, depending upon the pressure

differential between the loop and the air cushion.


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3.7.4 SKIRT SPECIFICATIONS

SKIRT MATERIAL: PVC POLYESTER

DIMENSIONS: 1.2192m x 2.4384m

THICKNESS: 2mm

WEIGHT: 1.99 kg

DENSITY: 560 GSM(gram per square meter)

The bag skirt requires a number of holes on the inner fact to transfer air from

the skirt to the cushion. These holes vary in size but are generally 70mm to

150mm in diameter.

3.7.5 CALCULATIONS

3.7.5.1 HOVER GAP

A small air gap of 12.7mm under the skirt through which the air escapes is

assumed.

(1 foot = 12 inches)

In order to get the area of the entire gap, the height of air gap is multiplied with the

length of the hover gap. i.e. (12.7mm)*(7315.2mm) = 92903.04mm2

= 0.093m2

3.7.5.2 CUSHION PRESSURE

Estimated gross weight of the fully loaded craft is 200 Kg

Total area = 8 * 4 = 32 sq. ft = 2.972 m2

Craft cushion Lift Area = 90% * 32 = 28.8 sq. ft = 2.675m2


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The weight per sq. ft of lift area is the total weight divided by the cushion lift in sq.

ft

Lift Pressure required = 200 / 2.675 = 74.76 kg /m2

This is converted into pascals

74.76 * 9.81 = 733.39 Pa

3.7.5.3 ESCAPING LIFT AIR VELOCITY

Table 3.13

Cushion air pressure in

pounds per sq. in(1 psi=

6894.757 Pa)

Air velocity in feet per

second

0.050

0.075

0.100

0.125

0.150

0.175

0.200

78

96

111

123

135

146

156

From the table 3.13, for cushion air pressure 0.106 psi (733.39 Pa), Air Velocity =108.5 fps= 33.07m

3/s(this is obtained by iterations)

To convert this into a more realistic value, it is reduced to 60% of the shown value

100 * 60% = 60 fps(18.28m3/s) which is the expected actual air velocity.


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3.7.5.4 LIFT AIR VOLUME

The total volume of the air that will be required to pass under the craft and out of

the hover gap is calculated by multiplying the air velocity by the area through

which it is escaping.

60 fps * 0.9 sq. ft = 54 cu. ft / sec = 1.529m3/s

Based on these calculations made, the diameter of the hole, distance between the

holes are determined and accordingly holes are cut.

Some of the properties which helped in choosing PVC coated Polyester as the skirt

material are:

Good Hydrostatic resistance, which is of much importance as the craft

hovers on the surface of water.

Good Tensile strength.

Has a temperature range of -22 F to +165 F.

Has good abrasion resistance.

Light in weight.

Can withstand wear to a larger extent.

It is Mildew Resistant.

It is UV Stabilized.

Heat Sealable.

Ideal for Outdoor applications.


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3.7.6 CONSTRUCTION

The material is generally available in any length but limited to a certain width.

Adjustments have to be made to this. As per the design of the craft, it is 8 feet * 5

feet, with a hover height of 2 inches i.e. half a feet. This material is split into two

parts and sewn together to have a single skirt. Joints in the skirt material should be

sewn. Vinyl-coated material is a tough weave of nylon or similar material with the

vinyl heat-bonded to the weave, the vinyl may delaminate if only glue is used for

the seams. Sewing goes through the inner weave, providing a much stronger seam

than simply gluing the pieces together. Also polyester reinforcements are given at

the sewn joints.

The exact steps necessary to fit a skirt to a craft are detailed in order as follows:

The craft is elevated to a level position on a very flat floor.

Determination of the operating ground contact line above the floor.

The height must first be established and this should be about one eighthof the craft width. The height of the ground contact line above the floor

is determined by the working height of the craft above the floor and the

maximum width of material that will be used.

To design the cross section.

The cross section of the bag is comprised of two radii, the outer curve

and the inner curve. For simplicity it can be assumed that the ground

contact point is directly beneath the outer extremity of the hull and

therefore the outer radius is equal to half the distance between the

ground and the upper fixing point.


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The ground contact point can in fact be positioned fractionally in from

the outer hull edge but for the sake of stability, it must never be outside.

To design the cross section, we make a scale drawing of the craft hull at

the appropriate hover height and draw in the outer semi-circle.

The choice of pressure differential is based upon the degree of stability

required. The higher the ratio the greater the stability, but at the

expense of undulating surface performance and higher skirt

After calculating the inner radius, we draw in the inner circle.

This will give the inner skirt fixing point and note that the changeover

from the small radius to the larger radius is at a point 15 degrees in from

the ground point. The skirt cross section calculated in this way has

balanced geometry and will automatically take up this shape, provided

that the pressure differential is accurately predicted.

The skirt is then fitted to the hull with the help of beadings. This will

help to trap the required amount air and also provides the required hover

height.

DESIGN OF SKIRT

IMPORTANT PARAMETERS:

Outer Radius(R2): The visible portion of the skirt

Inner Radius(R1): The inner portion of the skirt

Skirt height: The distance between outer edge of the hull and the lower end

of the skirt.

Air gap: The air cushion which causes the hovercraft to float


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3.7.7 WORKING

A bag skirt is like an inter tube with a piece of plywood on top,

holes feeding into the middle, and an air supply. When it inflates it is the same

principle as simply sitting on an O-shaped balloon, since that's essentially what it

is.

Air goes into the bag, inflating it so it is about two inches high. The air inflating it

goes out of the holes located towards the center, making the air also build up

pressure in a chamber between the ground, the plywood, and the inflated ring of

the bag skirt (plenum chamber). The pressure eventually build up enough so that it

and the bag skirt is lifting the plywood, and the air slides out underneath the bag,

creating a nearly frictionless environment.

3.8 STEERING SYSTEM


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A rudder is a device used to steer a hovercraft, or other conveyance that moves

through a fluid. They provide harmonized means of control, which when used will

ensure the right response from the craft under all conditions, with minimum of

sideways skidding. They are used to control the direction of the hovercraft by

controlling the airflow from the shafts.

The steering system comprises of a handle, two door rudders, four 1 pulleys and

nylon rope. The pulleys are fixed at suitable points on the hull. The rudders are

mounted on the lift duct with the help of bolts and nuts. An MS-angle is used to

support the rudders from the bottom. The rudders and the handle are connected

with the help of a rope. The handle is mounted on the hull in front of the engine

assembly. This along with the rudders and rope will help in turning the craft in

whichever direction needed.

The rudders will turn in the opposite direction to that of the handle. For example, if

left turn should be taken, the handle is moved to the left. This in turn moves the

rudders in the opposite direction i.e. in the right direction.

4.TRIALS

4.1TRIALS ON LAND

The first thing to practice is stopping, by cu tting the lift and skidding to a stop on

the landing pads. Notice that it takes a considerable time for the craft to settle

after the lift throttle is cut. This is the amount of time must be allowed for in an

emergency ditching situation. As the lift engine is operating at higher RPMs,

the time it takes to make contact with the landing skids becomes longer.


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4.2 WATER TRIALS

The only place where one will have a really comfortable amount of room will

probably be when on the water. A large lake or bay is ideal for hovercraft

practice.

Piloting a hovercraft is not an exact science, it will take a considerable amount

of time and practice before you can honestly consider yourself a safe pilot.

After all, it is not possible to learn to drive a car or bike in an hour.

Before operating a hovercraft over water, be sure to have the required safety

equipment aboard, such as life jackets, flares, and fire extinguishers.

The amount of water required to float the craft can be calculated as follows:

Draft= Weight/(0.036* Length* Width)

For example, a craft of 2200lbs having a width of 96" and a length of 240"

will have a flotation of:

2200 / (0.036*96*240) = 2.65"

or only a little more than 2-1/2 inches of water is required to float this vessel

weighing over a ton. Although the above formula will tell you how much water

the craft will require, it is advised to draw slightly more because of the landing

pads and skirt material that will hang down below the craft's hull.

5.MAIDEN FLIGHT AND CHECKOUT

Learning to competently operate a hovercraft is about as difficult as learning to

ride a bicycle; it takes time and practice. Handling the craft can be learnt only by


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trial-and-error method. The progress can be achieved slowly with simple exercises

first, progressing to higher speeds and more complicated maneuvers later. Make

the first outing or two in calm wind conditions.

The main thing to remember is to avoid like the plague three conditions which in

combination spell disaster:I) the loss of skirt pressure 2) sliding sideways and 3)

travelling at high speed.This combination can cause a bad overturn accident on

land, and at least a thorough dunking on water.

A certain amount of preparation must be taken care of before making the first

flight in a hovercraft:

Be sure the belt drives are properly adjusted for the first run. A loose belt

will slip, causing wear and loss of thrust or lift as the case may be.

Be sure that there are no miscellaneous nuts or bolts left loose. These can

cause expensive and dangerous damage to your propellers or fans.

Be sure that nearby objects, particularly paper and other light material, is

weighted down or removed from the area. Fans can kick up and suck in

such things very easily.

A 2-Stroke engine requires a fuel/oil mixture of the proper ratio. Never

run either engine without the proper kind and amount of oil.

Have sufficient fuel for the run. Unexpectedly running out of fuel in a

hovercraft could cause damage if lift is lost during forward movement

The maiden flight should be over land, so that there will be no need of

the required marine safety equipment.


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It should be noted that no one should be there near the propellers or fans

before starting the engine.

A motorcycle helmet is necessary.

5.1 PREFLIGHT CHECKS

There are mainly two types of checks: Mechanical and Operating checks.

5.1.1 MECHANICAL CHECKS

Propellers, fans not cracked, chipped, or loose.

Engine mounts secure without cracks.

Mufflers secure.

Air filters secure.

Bearings and mounts secure.

Gust lock removed.

Nuts and bolts tight.

Control Cables taut and on their pulleys.

Belts clean, tight, and in good condition.

Rudders and other controls normal and free.

Electrical system in good repair without corrosion.

Guards secure with proper tip clearance.

Hull intact, without cracks, holes.

Skirt condition, without tears, esp. across rear.

Fuel lines leak-free.

5.1.2 OPERATING CHECKS


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Check Fuel

Fuel Pumps On

Clear Area

Ignition On

Start Engine

6. OPERATIONAL PROBLEMS AND CURES

6.1 ENGINE

When lift or thrust is not up to expectations, the amount of throttle that must

be used with the engine is a primary means of analyzing the problem.

If the engine throttle is used as a means of troubleshooting, it is always advised to

first disconnect all loads to the engine in question, and then run it. If the engine is

"lively" running and accelerating easily without a load, it can be assumed that the

problem is not in the engine itself. On the other hand, if the engine is "sluggish," andaccelerates slowly, there is a problem in the engine which does not involve the

load at all. Engine problems must be fixed before load (propeller and transmission)

problems can be tackled.

If the craft is kept light, it should hover easily at reasonable lift engine rpm.

6.1.1 BAD FUEL MIX

A bad fuel mix can occur in several ways:

When there is no fuel, only air gets into the engine.

The air intake might be clogged, so there is fuel but not enough air.


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The fuel system might be supplying too much or too little fuel to the mix,

meaning that combustion does not occur properly.

There might be an impurity in the fuel (like water in the fuel tank) that

makes the fuel not burn.

6.1.2 LACK OF COMPRESSION

If the charge of air and fuel cannot be compressed properly, the combustion

process will not work like it should. Lack of compression might occur for these

reasons:

The piston rings are worn, allowing air/fuel to leak past the piston during

compression.

There is a hole in the cylinder.

6.1.3 LACK OF SPARK

The spark might be nonexistent or weak for a number of reasons:

If the spark plug or the wire leading to it is worn out, the spark will be weak.

If the wire is cut or missing, or if the system that sends a spark down the

wire is not working properly, there will be no spark.

If the spark occurs either too early or too late in the cycle (i.e. if the ignition

timing is off), the fuel will not ignite at the right time, and this can cause all

sorts of problems.

6.1.4 UNDERPOWERED THRUST SYSTEM

Engine won't rev up, requires full throttle. Raise engine RPM into power

band.


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Increase drive transmission ratio, this also increases torque to the thrust

fan.

Check and/or decrease thrust fan blade pitch angle. 5 less blade angle

drops required under power by about 25%.

Re-calculate the thrust fan requirements.

6.2 PROPELLERS AND SKIRT

For a moment, consider some of the things that can be a big disappointment

during the first trials. Analyzing most of the problems involves a simple principle:

If the craft is kept light, it should hover easily at reasonable lift engine RPM. If

the craft doesn't lift properly, here are some reasons why and their cures:

6.2.1 CRAFT TOO HEAVY

Lighten the craft as much as possible. Use manometer to measure lift pressure.

Actual pressure should be checked against lift calculations.

6.2.2 LIFT AREA TOO SMALL

Skirt is not properly shaped i.e. can't "bag out" on sides.

6.2.3 LIFT SYSTEM IS UNDERPOWERED i.e. Engine wont rev

up, even at full throttle

Propeller blade pitch angle should be checked.

Propeller requirements are to be re-calculated.

6.2.4 PROPELLER NOT ABSORBING ENOUGH POWER i.e.

Engine revs up very easily at partial throttle.


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Hub is slipping on the shaft.

Blade pitch angle to be increased.

Propeller should run at a faster speed.

Transmission ratio should be decreased.

6.2.5 NON SEALING OF SKIRT

Skirt is not at higher-than-lift pressure.

Bad pressure leak, skirt-to-lift area under craft.

Skirt leaking excessive pressure in only one area.

7. SAFETY

7.1 PERSONAL FLOTATION DEVICE

Both boating law and common sense dictate that personal flotation devices be

worn by anyone in an operating hovercraft. Not only do they reduce the risk of

drowning, they cushion impact in the event of collision. Coast Guard standards call

for a minimum of 15.5 pounds flotation for a Type III PFD. The average vest

exceeds this minimum by some 3 to 4 pounds, but specialized vests may provide

upward of 30 pounds flotation.

When selecting a vest always pay particular attention to fit. Most water skiers can

tell stories of loosely fitting vests coming off over their head during a hard fall.

Secure straps are essential and in the case of infants or large adults a vest with a

crotch strap may be appropriate. Specialized vests are available designed for

whitewater paddling or boat racing which include features such as additional


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flotation (and padding), limits on head/neck travel and retrieval straps. These are

significantly more expensive, but do yield additional protection. Finally, it is

advisable to check the equipment regularly. When straps or fabric become sun

bleached or frayed, the vest should be replaced before it fails.

7.2 HELMET

Cruising operation typically doesn't warrant the use of a helmet designed for

protection from high speed collision. No protection at all, however, opens the

possibility of a serious head injury if an unplanned exit is made from the craft. It is

best to avoid bicycle helmets as they generally do not provide adequate protection.

7.3 HEARING PROTECTION

Noise levels are often in the range 100 to 120 db. Unless there is an

extraordinarily quiet hovercraft or are already deaf, one should always use hearing

protection.

7.4 COLD WATER GEAR

An unexpected swim can be part of a hovercraft cruise for even the most seasoned

operator. If the water temperature happens to be below 60 degrees F, the event is

decisively more intense. While few operators pursue wetsuits for warm weather

cruises, they can be highly beneficial during cool weather or in cool water rivers.

Remember that fast moving water can extract heat from a body dramatically faster

than still water. A diver's wet suit will improve your body's performance dealing

with the situation while in the river and also allow you to warm up quickly after

getting out. Not only is this more comfortable, it yields a greatly reduced risk of


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hypothermia.

Dry suits are also available which provide a waterproof covering over various

layers of thermal insulation.

7.6 FIRST AID KIT

A waterproof container is great for first aid essentials such as bandages,

iodine, antiseptic, gauze, etc. Don't forget that the things that seem the least critical

may be the most likely to be used. This is also a good place to keep waterproof

matches.

7.7 FRESH WATER

While soft drinks may quench the thirst, they are hardly suitable to clean a wound.

Bring a bottle of water along too.

HOVERCRAFT SPECIFICATIONS

LENGTH 2.4384m

WIDTH 1.2192m

HOVER HEIGHT 15.24 cm

CRUISE SPEED 2030 KMPH


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PROPULSION ENGINE 250 CC YEZDI ROAD KING

ENGINE

ENGINE POWER 16 BHP

PROPELLER MULTI BLADE PROPELLER

DRIVE SYSTEM VBELT DRIVE SYSTEM,

CONNECTING PROPELLER TO

ENGINE USING PULLEYS

STEERING SYSTEM 2 DOOR RUDER SYSTEM

CONNECTED TO A HANDLE

8.FUTURE

The future of hovercraft is about increasing their use and operation throughout

more parts of the world, and not necessarily inventing or discovering new

technology. Obviously the industry relies on continual improvement, incorporatingmodern technologies and materials, however the benefits will come as more semi-

skilled workers can produce high quality hovercraft quickly and easily, in even the

remotest regions of the planet.

The new design and manufacturing technologies available today should be used to

create simple and effective hovercraft designs which will help improve the lives of

many people in many ways. If focus is kept on this, then hovercraft will continueto improve and become more affordable for everyone.


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9. CONCLUSION

Due to the fact that the Hovercraft can travel over terrains, mud, ice, water etc they

have gained popularity in all aspects of transportation world. They have relatively

a simple design, which allows for a wide range of uses. Hovercraft travel best over

flat.

The rudders can be modified to provide reverse thrust, allowing the craft to have

reverse options and breaks. Skirt flexibility is a change that can completely alter

the performance of the vehicle for the better. An add-on of segmented skirt would

be ideal for such a change.

Areas where normal vehicles would be rendered useless like mud, ice, or places

where terrains have many changes, such as swamps, give the hovercraft a real

meaning.

REFERENCESa. James Perozzo, Hovercrafting as a Hobby,2001 Edition, Maverick

Publications

b. http://en.wikipedia.org/wiki/Hovercraft

c. http://groups.physics.umn.edu/pforce/hovercraft.html

d.

http://www.xinventions.com/main/hovercrafts/second.htm

e. http://www.jameshovercraft.co.uk/hover/interest/concept.htm

f. http://www.neoterichovercraft.com/general_info/questions_and_answ

ers.htm


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g. http://4wings.com.phtemp.com/tip/lh.html

h. http://www.instructables.com/id/Hovercraft

i. K.R. Gopalakrishna, Elements of Mechanical Engineering, 2003Edition, Sudha Publications.

j. Kirpal Singh, Automotive Engineering, 2004 Edition, Standard

Publishers.

Documents

Final Report on Hovercraft