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INTRODUCTION
A Hovercraft is a vehicle that flies like a plane but can float like a boat, can drive like a car but will traverse ditches and gullies as it is a flat terrain. A Hovercraft also sometimes called an air cushion vehicle because it can hover over or move across land or water surfaces while being held off from the surfaces by a cushion of air. A Hovercraft can travel over all types of surfaces including grass, mud, muskeg, sand, quicksand, water and ice .Hovercraft prefer gentle terrain although they are capable of climbing slopes up to 20%, depending upon surface characteristics. Modern Hovercrafts are used for many applications where people and equipment need to travel at speed over water but be able load and unload on land. For example they are used as passenger or freight carriers, as recreational machines and even use as warships. Hovercrafts are very exciting to fly and feeling of effortlessly traveling from land to water and back again is unique.
Hovercraft as we know them today started life as an experimental design to reduce the drag that was placed on boats and ships as they ploughed through water. The first recorded design for an air cushion vehicle was put forwarded by Swedish designer and philosopher Emmanuel Swedenborg in 1716. The craft resembled an upturned dinghy with a cockpit in the centre. Apertures on either side of this allowed the operator to raise or lower a pair of oar-like air scoops, which on downward strokes would force compressed air beneath the hull, thus raising it above the surface. The project was short-lived because it was never built, for soon Swedenborg soon realized that to operate such a machine required a source of energy far greater than that could be supplied by single human equipment. Not until the early20th century was a Hovercraft practically possible, because only the internal combustion engine had the very high power to weight ratio suitable for Hover flight.
WORKING PRINCIPLE AND CONSTRUCTIONAL
DETAILS OF HOVERCFAFT
PRINCIPLE-
The principle of working of a Hovercraft is to lift the craft by a cushion of air to propel it using propellers. The idea of supporting the vehicle on a cushion of air developed from the idea to increase the speed of boat by feeding air beneath them. The air beneath the hull would lubricate the surface and reduce the water drag on boat and so increasing its speed through water. The air sucked in through a port by large lifting fans which are fitted to the primary structure of the craft. They are powered by gas turbine or diesel engine. The air is pushed to the under side of the craft. On the way apportion of air from the lift fan is used to inflate the skirt and rest is ducted down under the craft to fill area enclosed by the skirt.
At the point when the pressure equals the weight of the craft, the craft lifts up and air is escaped around the edges of the skirt. So a constant feed of air is needed to lift the craft and compensate for the losses. Thus craft is lifted up. After the propulsion is provided by the propellers mounted on the Hovercraft. The airs from the propellers are passed over rudders, which are used to steer the craft similar to an aircraft. Hovercraft is thus propelled and controlled and its powerful engine makes it to fly.
MAIN PARTS OF HOVEFCRAFT-
Lower hull- It is the basic structure on which the Hovercraft floats when the engine is stopped while moving over water. It supports the whole weight of the craft. The lower hull of the craft includes the craft floor, side panels, forward and aft panels till the top skirt attachment line. Most commercially build craft in polyester resin will use this section to transfer to the top hull. The lower hull
1. Needs to have adequate size for the total weight of the craft and payload
2. Must be strong enough to support craft off cushion (on landing pads)
3. Have enough freeboard to support craft in displacement mode on water
4. Must be watertight and as smooth as possible. The lower hull can be build out of all boat building materials, From simple ply to very complicated composite panels.
1. Skirts- They are air bags inflated by air are fitted around the perimeter of the craft hold air under the craft and thus upon a cushion of air. It enables to obtain greater Hover height. The material used is rib stop nylon or Terylene.
2. Lift fan-It is fitted to the primary structure of the Hovercraft. The air is pumped under the craft between the skirt space to produce a cushion of air.
3. Propeller-It is used to obtain the forward motion of the craft. It is fitted to the top of the craft and is powered by a powerful gas turbine or diesel engine.
4. Rudders-They are similar to that used in an aircraft. Rudders are moved by hydraulic systems. By moving the rudders we can change the direction of the craft.
5. The lifting fans-In the enclosed space fan operates in a propeller would not be suitable. Firstly the volume of air needed is very large and a propeller is designed to be most efficient in open air like on an aircraft. Propellers again are not efficient in applications when an air backpressure will be applied to the propeller blades as they rotate.Because of this the lifting on most Hovercraft uses what is known as a centrifugal fan. This is a fan in which two discs are fitted together and looks rather like a doughnut with angled slat at their edges .When the assembly is rotated at high speed air is sucked in to the center hole in the fan and the slats force it out at the edges. The advantages of the fan are two fold. The lifting fan is coupled via a gearbox to the engine. The engine also drives the propeller on the craft, which provides thrust for forward motion of the Hovercraft.
7. The engine-The engines used in Hovercraft have evolved like the skirt design. The SRN 1 and other early craft used piston type engines. As models like the SRN 4 and SRN 6 were brought into service they tended to favor the use of gas turbines. This type of engine is smaller and lighter for a given horsepower and has been used extensively in turbo prop aircraft.
The engine has a main shaft on which is mounted a compressor and turbine. A starter motor is connected to one end and the other end is connected to the lift fan. Both the compressor and turbine look like fans with large number of blades.
When the engine is started the compressor compresses air from the engine intakes and pushes into the combustion chambers mounted around the engine. Fuel is squirted into the combustion chamber and is ignited. The compressed air then rapidly expands as it is heated and forces its way out through the turbine to the exhaust. As the gas pressure raises the turbine speeds up, there by driving the compressor faster. The engine speed increases until it reaches engines normal operating speed.
However the use of these engines results in very high level of engine noise outside the craft. In the SRN6 this meant that it was possible to hear the craft traveling across the Solent between the Portsmouth and the isle of Wight in the UK several miles away.
8. Thrust propeller-The propeller used to drive the Hovercraft along is usually an aircraft type with variable pitch blades. Its speed of rotation must remain fixed to that the engine and the lift fan. This is because the amount of lift air requires dictates the engine speed to drive the lift fan. In turn the amount of propulsion which the propellers provide must be obtained by varying the propeller pitch and not its rate of rotation. This system is termed integrated lift. Hovercraft having more than one lift fan and propeller generally has a separate engine for each fan and propeller unit.
The propellers used on hovercraft can vary from four bladed versions and about nine feet in diameter on the smaller craft to the four propellers on the SRN4 cross-channel Hovercraft. These are four bladed and nineteen feet in diameter.
9. Air Box- As the name suggests, the trapdoor opens and closes. When the trapdoor is closed, all of the air from the rear duct is sent out the back to provide thrust. The GH-2 is normally operated with its trapdoor closed.When the airbox trapdoor is opened, half of the air from the rear duct is sent underneath the craft to provide lift. Since the craft is now getting lift from its airbox, the lift duct in front can be removed
10. Rudders-Control of a Hovercraft is accomplished by primarily though the use of rudders like the type used on aircraft. The main difference would be, however, that Hovercraft generally utilizes many rudders rather than just one.
On the SRN4 the pylons on which they are mounted can be rotated to change the direction of thrust. Another method of control is through ‘puff ports’ or dual thrust fans where you would slow one down and speed up the other to turn in the direction desired.
11.Hovercraft skirt-Despite the momentum curtain being very effective the hover height was still too low unless great, and uneconomical, power was used. Simple obstacles such as small waves, or tide-formed ridges of shingle on a beach, could prove to be too much for the hover height of the craft. These problems led to the development of the skirt.
A skirt is a flexible shaped strip fitted below the bottom edges of the plenum chamber slot. As the Hovercraft lifts, the skirt extends below it to retain much deeper cushion of air. The development of skirts enables a Hovercraft to maintain its normal operating speed through large waves and also allows it to pass over rocks, ridges and gullies.
Skirt is one of the most design sensitive parts. The design must be just right or an uncomfortable ride for passengers or damage to craft and skirts results. The skirt material has to be light flexible and durable all at the same time. For skirt to meet all of the requirements the design and use of new materials has slowly evolved.
There are three types of skirts
1. Bag skirt
2. Finger skirt
3. Bag and finger skirt
. DIFFERENT SYSTEM OF HOVERCRAFT
1. Thrust System
In the case of the thrust system, it is the acceleration of a mass of air that produces the thrust. Newtonian physics state that every action has an equal and opposite reaction and this is exactly what causes the thrust, accelerating air out the back of the hovercraft causes a thrust reaction propelling the hovercraft forward. Now this can be achieved different ways, for example a small area high velocity jet of air might be seen to be equivalent to a large area low velocity jet of air with the same power input. However there are many other factors such as frictional losses in the intake, the propeller, the outlet, the rudders and within the air itself. Without going into detailed analysis, these can be expressed, as efficiencies at each stage but suffice to say for this discussion that most losses are proportional to the air velocity squared. That means in simplistic terms that if you double the air velocity you will double the thrust but you will get four times the losses! In other words, you can get much more thrust per Kilowatt (or horsepower) of engine power input with a system that passes a larger quantity of air at a relatively low velocity compared with a system (same power input) that passes a smaller quantity of air at a higher velocity.
Therefore the factors to consider mostly when designing the thrust system is air mass flow and reduction of losses. This leads to large area thrust devices (propellers and ducts) with very clean through-flow characteristics such as streamlined ducts, nice fairing on belt drives, struts and other obstructions and minimal surface areas. The propeller itself will typically have between two and five blades, a small hub diameter and an aerofoil without camber such as 'Clark Y' or similar. In the case of the ducted propeller, the pitch (blade angle) will typically be coarse to minimise tip speed and reduce operating noise. This can be achieved because the propeller is mainly concerned with volume flow and not too much concerned with static pressure rise.
2. Lift System
In the case of the hovercraft lift system, it is the air pressure that acts upon the underside of the hovercraft that provides the lifting force to make the hovercraft hover. Also, because the skirt does not form a perfect seal around the hovercraft perimeter there is leakage of the air and that must be balanced by constant air input from the lift fan.
Hovercraft lift systems usually have a rather 'dirty' airflow characteristic so most of the dynamic pressure of air moving is lost as it passes around bends and obstructions in the ducting system. The amount of lost dynamic air pressure can be minimised by designing a lift system that operates at low velocity. That leaves static air pressure as the over-riding consideration combined with getting enough air into the lift system to allow for skirt leakage.
Therefore the factors to consider mostly when designing the lift air input system are the pressure characteristics of the fan and the volume flow capabilities of it at the required pressure. This is a completely different design criterion than what is needed for an efficient thrust system and the design of the fan results in a completely different device to that which would be chosen for maximum thrust efficiency. Comparing an Axial flow lift fan with an efficient propeller you would note different blade aerofoil sections, considerably more camber in the aerofoil, more blades, larger hub diameter and different operating tip speeds. Also you would note that the blades run at a finer pitch and the tip speed needs to be relatively high to achieve pressure generation compared with what would be needed to achieve the required volume flow. To use a lift fan for thrust is to accept the considerable aerodynamic losses that will result.
Additional and equally important factors found in a well-designed lift system will be it's flexibility in operation because it will have a favourable pressure volume curve, it will be without nasty stall characteristics and it will be as quiet as possible. Additionally it will be easily controlled so more air can be delivered to cross porous-terrain or accommodate torn skirt fingers and it can be slowed down to save power (fuel and noise) when the hovercraft is operating on water with a well-maintained skirt system. The best result is only possible if the lift system is designed for lift alone without regard for the provision of thrust air.
3. Steering System
The steering for a hovercraft is done through air deflectors placed behind the thrust duct. For these I just used the circle I had already cut out for the duct itself. I cut one of them in half and screwed a 1x2 on the rounded edge of each half. L-brackets were attached to the duct hold them on and allow them to turn. Then rope was attached to the deflectors and run through eye-bolts. The rope was criss-crossed under the duct so that moving the control stick left would turn the hovercraft left and right turns right. The control stick was just a piece of PVC pipe with a hole drilled through the bottom so that it could pivot.
The Thrust engine is controlled by a lawnmower throttle cable and the lift engine was locked into full throttle. I could have mounted the bike brake on the control stick but it would have been just one more thing to worry about so I just left it wide open.