air cushion(redirected from air-cushion)
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a region of increased air pressure between the base of a machine and a supporting surface or between the movable and fixed elements of mechanisms in instruments and machine tools. It is used in transportation equipment (hovercraft, ram-wing craft, and other flying vehicles), various instruments (gyroscopes), and mechanisms as an”air bearing” to reduce the friction between contiguous surfaces .
Static and dynamic methods of forming an air cushion exist. In the static method, pressure is created in the air cushion by a fan or compressor, and in the dynamic method it is developed by the relative velocity of an airflow. Of the many known methods for producing an air cushion, the principal ones are the chamber, nozzle, slot, and wing types.
In the chamber method, the lifting force is produced by the static air pressure forced under the base of the chamber by a fan. Owing to the lifting force, the chamber is raised, and the air then escapes through the gap formed between the chamber rim and the supporting surface. Since the leakage area is fairly large, a substantial air flow rate is needed even with relatively small gaps. In the nozzle method, the air cushion is created by an airflow from an orifice located around the perimeter of the nozzle assembly. The lifting force is made up of the static pressure force on the base of the nozzle assembly and the reaction forces from the outflow of air through the nozzle. This method permits larger gaps between the base of the nozzle assembly and the supporting surface with lower airflow rates. In the slot method, the air cushion is created in a thin, slotlike cavity from which the air flows out in all directions. This cavity is located between specially shaped carrying and supporting surfaces in the case of transportation equipment and between the movable and fixed elements of mechanisms in machine tools. The increased pressure of the air cushion is maintained as a result of relative motion between the surfaces and because of the viscosity of the air passing through the slot. In the wing method an air cushion is formed by the increased air pressure under the wing of a flying vehicle (such as a ram-wing craft) as it moves with a certain angle of attack close to a supporting surface.
REFERENCEStepanov, G. lu. Gidrodinamicheskaia teoriia apparatov na vozdushnoi podushke. Moscow, 1963.
K. P. VASHKEVICH