Skirts

The flow of air leaving the air cushion under early hovercraft was directly proportional to the craft peripheral length and hovercraft height. In order to reduce the flow, and the power required to maintain it, flexible skirts were developed, allowing the hovercraft to maintain a reasonable hover height with a few centimetres air escape gap. Thus craft like the SRN-4 can maintain their structure at over two metres over the water, with only 20 kW/t.

The cushion, circumscribed by its flexible skirts, must also insure the stability of the craft in the heave, roll and pitch modes. There are two methods to insure stability:

• By skirt deformation: Skirt segments are conical so that when collapsed the actual cushion area increases. In heave, the overall cushion area increases as the vehicle comes down so that the extra lift force returns the craft to its equilibrium position. In roll and pitch, differential collapse of skirts around the periphery shift the centre of pressure of the cushion toward the low side creating thus a moment which bring back the craft on even keel.
• By differential pressure: the cushion area is subdivided in cells. When the hovercraft moves down or lean, the cells close to the surface have their exit area reduced so that the pressure increases in these cells. The differential pressure in the cells produces the moment required to right the craft.

For small vehicles, or slow moving air cushion platforms, stability through simple skirt deformation is sufficient. If a greater stiffness is required, for high speed or to counter a high centre of gravity, cells are created through the use of internal flexible barriers. The British Hovercraft Corporation (BHC) subdivides its cushion in this manner, for instance the SRN-4 and the AP.1-88. Bertin and Cie first developed multiple conical cells for its hovercraft, later surrounded by a peripheral skirt (N-300). For larger vehicles such as the SEDAM N-500, the peripheral skirt was replaced by a number of large segments, each enveloping a conical cell. In theory, the outside spaces are at lower pressure than the central spaces so that escape velocity should be reduced.

The different geometries used in the British and French skirt systems lead to the name «skirts» for the first one and «jupes» for the second!

Previous page... Version française...

Page created on August 24, 1997 by MALINA Conseil inc.
Last update on December 4, 1997.