On the face of it, a hovercraft has a simple principle - simply provide a pressurized cushion of air underneath the hull, contain it with a skirt attached to the periphery of the hull and it will fly. As with most things in life, it's never as easy as all that. There are many things that hovercraft design needs to take into account, both in terms of safety and practicalities. Assuming that we have a good theoretical design and a working prototype machine, what are the practical tests required to ensure correct operation in all conditions?
An air cushioned craft can lift off from two basic types of surface either dry or wet. The thrust power required to lift off from the two are just not the same. In fact, it takes more power to lift a craft from the surface of a river, lake or the sea than it does from dry land, ice or any other solid surface. Lifting off from water is known as 'getting over the hump' and is a particular problem for hovercraft designers. The power to weight ratio is important and unfortunately an engine needs to be designed with extra capacity to give up to 25% more power than needed for normal flight, just to lift off from a wet surface if needed.
A low powered vehicle may have difficulty lifting off from water, but this is generally an inconvenience. Lightening the load a little will ensure lift off and it's not a safety issue unless you happen to be stranded miles out to sea. However, there is one feature that is potentially hazardous and this is called 'Plow-In'. When an ACV is moving at full speed over the sea, or other turbulent water, the surface may not always be flat. On the sea, waves are an important factor for smooth hovercraft flight. Similarly over river rapids, the same principles apply. Actually, the problem on river rapids is probably more acute because rocks may be jutting up or just below the surface of the water.
On the sea, it's common for a wave to rise up and strike the bow of a small boat or hovercraft. A boat will simply bounce over the top of it, but in the case of an ACV this can have disastrous effect. Hovercraft design is crucial - if not designed correctly, the wave will cause the front to dip into the water, which is called 'Plow-In'. The bow plows into the water, raising the stern and coming to a very abrupt stop. This is obviously a major safety concern, as any passengers will be thrown violently into the sea, or come into contact with the craft causing injury.
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