HOVERCRAFT - ADVANCING MULTI TERRAIN VEHICLE

                                    HOVERCRAFT

A hovercraft is a vehicle that glides over a smooth surface by hovering upon an air cushion. Because of this, a hovercraft is also called an Air-Cushion Vehicle, or ACV. How is the air cushion made? The hovercraft creates vents or currents of slow-moving, low-pressure air that are pushed downward against the surface below the hovercraft. Modern ACVs often have propellers on top that create the air currents. These currents are pushed beneath the vehicle with the use of fans. Surrounding the base of the ACV is a flexible skirt, also called the curtain, which traps the air currents, keeping them underneath the hovercraft. These trapped air currents can create an air cushion on any smooth surface, land or water! Since a hovercraft can travel upon the surface of water, it is also called an amphibious vehicle. Figure 1 below shows a picture of a modern hovercraft and a diagram showing how the air vents create the air cushion.

HOVERCRAFT

Commercial hovercraft usually have an oval or rectangular platform, a motorized fan and a large skirt material to trap the air underneath the vehicle. This air cushion underneath the hovercraft is called the plenum chamber. This plenum chamber is formed by the bottom of the craft and the skirt material. The air flowing into the plenum chamber will form a ring of air circulating around the base of the skirt to insulate the air cushion from the lower pressure air outside the skirt. This ring of air keeps the air under the craft from escaping.

Most large hovercraft have a large propeller attached to the back of it to propel it forward. Ruddersattached to the propeller's housing allow drivers to steer the vehicle. On some smaller hovercraft, steering is performed by the driver leaning left or right. Steering a hovercraft is a little tricky: There's no contact with the ground, so steering the craft will feel slippery. In order to stop the vehicle, you just have to slow down the engine and the craft comes to a rest on the ground. One problem with driving a hovercraft is that the faster you go, the harder it is to maintain the cushion of air underneath the craft.

The Airboard

The Airboard is just a small version of a conventional hovercraft that is ridden standing up. It uses the same air cushion principles to glide just above the ground. However, there are some differences between a conventional hovercraft and the Airboard. For instance, the Airboard is unable to hover over water like other hovercraft, and it uses a drive wheel, which touches the ground, to accelerate. Here's a look at all of the components that make up the Airboard:

The fan underneath the shell of the vehicle provides both a cushion of air and a stream of air that exits through the back of the vehicle to provide thrust. To accelerate, the rider shifts his or her weight forward to allow more air to exit the back of the vehicle. By shifting backward, the rider will activate the drive wheel. The drive wheel actually contacts the ground to move the Airboard forward. Conventional hovercraft don't use any type of a drive wheel.

Controlling the Airboard is done by shifting your weight from side-to-side, similar to how you would ride a skateboard or surfboard. By varying the amount of weight transfer, the driver can make the vehicle turn sharply or softly. Sliding and 360-degree turns are also possible. To ensure the best steering performance, riders are recommended to be at least 5 feet (1.3 m) tall and about 14 years old. Airboard's developers believe young adults have the sufficient amount of weight to safely control the vehicle.

The Airboard should be ridden on level ground, but can glide over many surfaces, including grass, concrete, asphalt and packed dense materials such as salt pans. The developers say it shouldn't be ridden over loose or littered surfaces, where debris could be lifted into the air stream. And, while it can ride over wet surfaces, it cannot ride over bodies of water because of its limited air-generating capacity.

In traditional hovercraft, drivers simply stop the engine and the vehicle slowly comes to a rest. This new hoverboard vehicle works the same way. In order to stop, you simply release the levers on the handlebar, at which point it will slide to a stop. Leave yourself a few meters to stop. It's also possible to stop faster by using a sliding turn.

PRINCIPLE OF OPERATION

Air cushion:

 Hovercraft floats on a cushion of air that is chased by a propeller craft. After starting to lift the hovercraft and is ready to ride. The size of stroke ranges from 15 cm in the smallest personal hovercraft to 2.8 meters for large transport machines. The air pocket under the hovercraft is surrounded by plastic to air from leaking out from under the hovercraft. Implementation of the mantle differ may be either in the form of a compact bag or can be divided into individual cells - so-called segments. Most professional hovercraft using Segmented casing, because each piece is in transit through the inequality diverges separately. It is very convenient, because the lifter loses only a very small amount of air. 

Movement hovercraft:

After the hoist lifter can move forward. It must provide a separate air operator, which takes a hovercraft. Many of the vessels used to move a separate engine, but some have only one engine for both functions - that is, for blowing air under the hovercraft and also to move forward. In this case, the airflow split propeller, which in part drives the flyer for floatability, whilemajority of the air is used to move the hovercraft.

EARLY INVENTIONS

Sir Christopher Cockerell (June 4, 1910 – June 1, 1999) 

The name of the famous British Inventor known worldwide as synonymous with the invention of the Hovercraft as we know it. Christopher Sydney Cockerell was born in Cambridge and studied engineering at Cambridge University. He conceived the hovercraft by discovering the Momentum Curtain theory (explained below) in 1953, demonstrating it in 1955 with a hair dryer and two coffee cans in an experiment famous in the hovercraft world (also explained below). He built a larger-scale demonstration of his concept and demonstrated it to as many parties as possible. Boating manufacturers claimed his invention to be an aeroplane, and nothing to do with them; Aeroplane companies claimed the opposite. Eventually a demonstration of the concept in front of officials of the British Government secured the invention's future, albeit through a period of its being sworn to secrecy.

In 1959, funded and developed with the National Research and Development Corporation and built by Saunders Roe (Aviation) Ltd., Cockerell designed the first full-sized hovercraft, the SRN1 (Saunders Roe Nautical One). Following numerous on-land and on-sea trials on the Solent, the SRN1 made its historic cross-channel crossing between Calais (France) and Dover (England) on 25th July later that year. More about the SRN1 later in the tour.

FUNCTION

Movement: Forward, backward and side-to-side 

It's all very well and good having a craft which can eliminate all friction between it and the land surface, but what good is this if it doesn't move! And, contrarily, how do you make a vehicle move if it is not in contact with the surface which it is required to move over? The answer is thrust, in the form of redirected air from the lift fan (such as in the first model of the SRN1), or from a seperate system of either thrust propellers or jet engines. Most craft now use propellers.

Propellers work by generating a lift force from their blades as they move through the air in a circle, much the same was as a swimmer kicks or pushes against the water as they swim. The action of the propeller moving against the air creates a force in line with Newton's 3rd Law, a.k.a., every action has an equal and opposite reaction. The air being pushed out of the way by the propeller exerts a force upon the propeller, in the opposite direction. This force is then carried through the propeller blade, through its centre spinner and into the craft's structure, propelling it forward. The pitch of the propeller blade, or its Angle of Attack, is the angle between the blade's cross-sectional longest axis (leading edge to trailing edge) and its direction of travel (parallel to the length of the blade). This pitch is responsible for the amount of force exerted by/on the air particles - the higher the pitch (to a limiting extent), the higher the force generated. Because force is proportional to acceleration, in Newton's 2nd Law (Force = mass x acceleration), the higher the pitch angle, the more the craft can accelerate or go faster.

Now we have thrust, and therefore movement, we need steering to complete the process. Steering can be achieved in three ways on a hovercraft. By means of rudders behind the propeller or at the back of the craft, by means of moving the propellers themselves to change the direction of the thrust generated, or by means of moveable air ducts such as the bow thrusters on the AP1-88 (later in the tour).

Rudders act much like the steerable wheels on a car. The direction in which a rudder points affects the direction the craft goes, by deflecting the air hitting it in the opposite direction. Rudders normally have about 30 - 45° maximum range of movement either side of their normal direction (aligned with the craft's forward direction). They are usually located directly behind the propellers of a craft (such as in Tiger 12 and AP1-88) or at the rear of the craft, like in the SRN4 or later version of the VA-3.

Rotating the propellers can be a useful form of steering also, most popularly as utilized on the SRN4 hovercraft. This craft had four propellers mounted on swivelling pylons, each capable of moving 30° either side of their normal heading. The swivelling pylons meant that the craft could turn or even counteract drift caused by a crosswind. Directional control was provided by the direction of the thrust emitted by the propellers.

Bow thrusters are used along with rudders on the AP1-88. These use air bled from the lift system to push the craft in any direction. On this craft the bow thrusters can swivel up to 180° from their normal direction, allowing fine-tuned steering and even reverse thrust.

    THE SKIRT

    Back to that ol' topic of obstacle clearance. The SRN1 hovercraft proved a great proof-of-concept        model as it crossed the English Channel. However, to be of any practical use it had to be able to cope with  more than a few small waves. To really take off, hovercraft would have to prove to be versatile and robust  in all manners of weather or terrain conditions. It was thus required to find a way to increase the hoverheight  significantly. Cecil Latimer-Needham was the engineer to do just that. He proposed a flexible skirt of rubber  to be placed around the vents of the plenum chamber, trapping the air inside it as it inflated. This partly  replaced the need for the momentum curtain, instead generating a second curtain at its base by means of  inward-pointing flexible extensions known as fingers.

 With the skirt, the hoverheight was increased more than tenfold, and obstacle clearance also greatly  improved. Upon striking an obstacle or traversing uneven land or choppy seas, the skirt could give, or  change shape, whilst still containing the air cushion within it, allowing the craft to continue a friction-free  voyage over whatever it was moving across.

CONCLUSION

Thus the basic functionalities and working of an hovercraft has been seen above. Hovercraft has its application in many wide areas such as Military, Marine , Shipping, Navy and yet it has it great advantageos feature flood disasters and coping up with all activity need of multi terrain functinality!