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The Hovercraft Principle Of Operation Is Not As Straight Forward As It Appears

A single statement tells us that air pressure created by a huge fan causes a hovercraft to float above the surface of dry land or water, but this is the very tip of the iceberg. A working hovercraft of any size is the result of many years of trail and error before the concepts were full understand. The air flow and pressure underneath an air cushioned vehicle depends on several important factors complex. The power required to life the craft can vary considerably according to the type of terrain it is flying over.

The hovercraft is quite unique in the respect that it, unlike most flying machines, it does not require forward motion to lift from the ground. In fact, it must lift off before moving at all, as the friction of air against air is very small at that time. Contrary to what it might seem, it isn't the speed and the amount of air pushed downwards by the lift engines that determines the lifting power of an ACV, but the shape of the underneath of the hull and the skirt material surrounding and containing the air flow.

Large vehicles have a split skirt in two sections with an inner and outer skin. Air is blown down through this and also down through slots along the underneath of the hull. An effect was found by one of the earliest inventors, known as the annular effect. When air is blown between two walls a ridge of pressure forms in the shape of an annulus or three dimensional ring. This ring of pressure has the tendency to form a barrier between the air being blown down to the ground, and the air outside the skirt. Basically, less lift thrust is needed due to this effect, which makes a huge difference to the power of the engines needed for any given weight of hovercraft. Small hovercraft have a generally less complex design srtucture.

Although capable of flying over many types of surface, the power required and the flight characteristics are not the same. On a perfectly flat surface, the power required to hover is about 25% of the power needed by a helicopter of the same weight! This represents a massive saving both in engine size (and therefore cost) and in fuel requirements. Both factors mean that an ACV can be lighter and carry a bigger payload. However, on rougher terrain and water, this changes dramatically, because of the turbulence introduced by obstructions and undulations on the ground.


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