Page 72 - Theory and Design of Air Cushion Craft

56 Air cushion theory

• In order to simplify the formulae, the viscosity of air is not considered in the
theory. This was because it was too difficult analytically to predict the air cushion
characteristics including the viscosity (turbulence) of air flow. For air jet cushions,
some approximations may be made, but for a segmented skirt, this becomes too
complex for analytical treatment.
• The hydrodynamic performance of the bag and finger type flexible skirt will change
dramatically from the above formulae, therefore some experts, such as Professor P.
Kaplan [11], considered that the simple theory for craft with a plenum chamber is
better applied to such skirts even though this approach is somewhat conservative.
The plenum chamber theory does not include the enclosing wall effect of the flow and
for this reason experimentally derived methods for predicting static air cushion per-
formance may be used to reduce the inherent conservatism. MARIC commenced the
experimental investigation on static air cushion performance for two-dimensional
flexible skirts on rigid surfaces in 1974. Since then the theories for three-dimensional
skirts and also performance over a water surface have been investigated regarding the
influence of the seal effect. This will now be outlined.

Experimental method and equipment
MARIC used two skirt test rigs as the main experimental method; a small rig (Fig.
2.6) was used for qualitative analysis and a larger one for quantitative analysis (Figs
2.7-2.9) to obtain a higher Reynolds number at its jet nozzles. The principal dimen-
sions of the two rigs are listed in Table 2.2.
The full-scale skirt of a 50-80 t ACV can be simulated in the big test rig and the air
flow rate can be measured by means of conversion from static pressure measurements

Fig. 2.6 MARIC's small skirt test rig.