John Hughes, age 12, of Baltimore, Md., for his question:
HOW DOES A HOVERCRAFT WORK?
A hovercraft, which is also called an air cushion vehicle, is a craft that operates above the surface of water or land while supported on a cushion of air four to eight feet thick.
The air cushion of a hovercroaft is provided by a large fan that pushes air downward within a flexible skirt attached to the perimeter of the vehicle. The skirt makes the vehicle appear to be operating only a few inches above the surface.
The vehicle is moved forward either by propellers mounted above the vehicle of by control of the air exhaust through small openings around the skirt. Braking action is controlled by reversing the pitch of the propeller or by changing the direction of airflow through the skirt vents.
The air cushion phenomenon is also known in aerodynamics as ground effect.
The operating controls are essentially like those found in an airplane. A control wheel adjusts the pitch of the vehicle and rudder pedals control yaw (side to side movement).
The speed of a hovercraft over water is limited by wave height and wind speed. Operation of the vehicle is much the same when moving from water to land or vice versa. The air cushion prevents the hull from striking the ground when the terrain changes.
Some air cushion vehicles, such as the Bertin Rovertruck, have conventional wheel systems for travel over highways and solid ground, but for movement over marshland a built in air cushion system supports three fouths of the weight of the truck.
Most air cushion vehicles, however, are designed for amphibious operation over water, marshy ground and beaches.
An experimental air cushion vehicle, designed to run on a track similar to a monorail, reached a speed of 215 miles per hour on a four mile concrete test track in France. This so called aerotrain rode on a cushion of air less than an inch thick.
An air cushion vehicle (CV) is actually a craft that travels on a layer of compressed air just above any kind of surface. The compressed air serves as an invisible cushion that eliminates almost ail friction between the vehicle and the surface.
In the late 1960s, the United States Navy began to use hovercraft in the Vietnam War for patrol duty and rescue missions.
In 1968, the British put several large ACVs into commercial use. Each of these vehicles carried about 175 passengers and 35 automobiles across the English channel between England and France.
In 1969, Canadian and American scientists used hovercraft to travel over the frozen surface of Lake Erie while they conducted a study of water pollution.
Today, manufacturers produce a number of models of ACVs that are used for many purposes. These models differ in size, speed and the power of their engines.