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.
PRINCIPLE OF WORKING
The principle of working of a Hovercraft is to lift the craft by a cushion of air to propel it using propellers. The idea of supporting the vehicle on a cushion of air developed from the idea to increase the speed of boat by feeding air beneath them. The air beneath the hull would lubricate the surface and reduce the water drag on boat and so increasing its speed through water. The air sucked in through a port by large lifting fans which are fitted to the primary structure of the craft. They are powered by gas turbine or diesel engine. The air is pushed to the under side of the craft. On the way apportion of air from the lift fan is used to inflate the skirt and rest is ducted down under the craft to fill area enclosed by the skirt.
At the point when the pressure equals the weight of the craft, the craft lifts up and air is escaped around the edges of the skirt. So a constant feed of air is needed to lift the craft and compensate for the losses. Thus craft is lifted up. After the propulsion is provided by the propellers mounted on the Hovercraft. The airs from the propellers are passed over rudders, which are used to steer the craft similar to an aircraft. Hovercraft is thus propelled and controlled and its powerful engine makes it to fly.
MAIN PARTS
Lower hull- It is the basic structure on which the Hovercraft floats when the engine is stopped while moving over water. It supports the whole weight of the craft.
Skirts- They are air bags inflated by air are fitted around the perimeter of the craft hold air under the craft and thus upon a cushion of air. It enables to obtain greater Hover height. The material used is rib stop nylon or Terylene.
Lift fan-It is fitted to the primary structure of the Hovercraft. The air is pumped under the craft between the skirt space to produce a cushion of air.
Propeller-It is used to obtain the forward motion of the craft. It is fitted to the top of the craft and is powered by a powerful gas turbine or diesel engine.
Rudders-They are similar to that used in an aircraft. Rudders are moved by hydraulic systems. By moving the rudders we can change the direction of the craft.
DEVELOPMENT OF AIR CUSHION BY MOMENTUM CURTAIN EFFECT
Stability of the Hovercraft on its cushion of air remained a real problem despite some design efforts and new approach was needed. To solve these problems, plenum chamber with a momentum curtain was developed by Sir Christopher Cockrell.
His first experiments were conducted with the aid of two cans and a vacuum cleaner (with blower end). The cans were drilled and bolted so that one can was inside the other with open ends facing down to some weighing scales, the top of the larger can was open and had a tube connected to it so that air could be forced in to the top can and around the smaller can inside.
The air traveled around between the inside of the bigger can and outside of the smaller can and was then let out towards the scales in a narrow ring of air, the cans were mad4e so that it was possible to remove inner can so the air could be directed in two ways.
The experiment was conducted in two steps. First the smaller can was removed and blower switched on. The scales measured the amount of thrust the air from the one can produced down onto the scales. The smaller can was now replaced inside the larger can so that the ring of air was produced. Again the blower was switched on and the scales measured amount of thrust the ring of air produced down onto the scales. Here is the key discovery because Cockrell observed that the two cans nested inside each other produced more thrust onto the scales than the simple open can or plenum chamber did, he had discovered the momentum curtain effect and this was the key ingredient that he patented.
In the full size craft the plenum chamber was also filled in so that a slot round the bottom edge of plenum chamber wall was former where the air fed in at the top. The slot produced a curtain of flowing air that was inclined. The high pressure air from the slot angled inwards towards the centre of the craft helped to contains and sustains the air cushion. Using this method a stable air cushion could be created. The craft was still riding on a plenum chamber of sorts but it was created and maintained by the high pressure ring of air surrounding the lower pressure air in the center.
The momentum curtain arrangement achieved higher hover heights with less power. It also solved some of the stability problems. The box structure in the center of the craft around which air escaped was closed to form a buoyancy tank to enable the craft to float on water when it came to rest.
The design was exactly what was used in first publicly demonstrated Hovercraft the SRN1, built by Saunders Roe in the United Kingdom it served as a test bed for many years during Hovercraft development.
HULL CONSTRUCTION
The lower hull of the craft includes the craft floor, side panels, forward and aft panels till the top skirt attachment line. Most commercially build craft in polyester resin will use this section to transfer to the top hull. The lower hull
· Needs to have adequate size for the total weight of the craft and payload
· Must be strong enough to support craft off cushion (on landing pads)
· Have enough freeboard to support craft in displacement mode on water
· Must be watertight and as smooth as possible.
The lower hull can be build out of all boat building materials. From simple ply to very complicated composite panels.
HOVERCRAFT SKIRTS
Despite the momentum curtain being very effective the hover height was still too low unless great, and uneconomical, power was used. Simple obstacles such as small waves, or tide-formed ridges of shingle on a beach, could prove to be too much for the hover height of the craft. These problems led to the development of the skirt
A skirt is a flexible shaped strip fitted below the bottom edges of the plenum chamber slot. As the Hovercraft lifts, the skirt extends below it to retain much deeper cushion of air. The development of skirts enables a Hovercraft to maintain its normal operating speed through large waves and also allows it to pass over rocks, ridges and gullies.
Skirt is one of the most design sensitive parts. The design must be just right or an uncomfortable ride for passengers or damage to craft and skirts results. The skirt material has to be light flexible and durable all at the same time. For skirt to meet all of the requirements the design and use of new materials has slowly evolved.
There are three types of skirts
· Bag skirt
· Finger skirt
· Bag and finger skirt
A Hovercraft skirt is required to fulfill the following functions
