Page 183 - Theory and Design of Air Cushion Craft

166 Stability

Fig. 4.31 Transverse movement of centre of cushion area of ACV with bag and finger type skirt on heeling, c,
c', centre of area; d, skirt contact with ground level; d', skirt contact heeled.

Statically this may be estimated in two dimensions by determining the static equilib-
rium of the downgoing skirt as the ground point is raised from d to d'. In general this
will result in just a small increase compared with the cruder assumption that the skirt
does not move at all. The exception to this is if segments are replaced by pericells.
Larger ACVs (above around 5 t displacement) commonly use longitudinal and
transverse stability skirts to improve craft stability; we will present analysis of sta-
bility including cushion compartmentation, with some additional guidance regarding
the choice available to a designer to avoid such complexity.

Calculation of transverse stability for an ACV

It is very complicated to calculate the transverse stability of an ACV hovering over
water because of the deformation of the water surface. The suggested analysis pro-
cedure is therefore to investigate the transverse stability of an ACV hovering on a rigid
surface, followed by the corrections necessary due to the water surface deformation,
obtained with the aid of model experiments.
The coordinate system and the basic assumptions can be written as follows:

1. Since the ACV is supported on a rigid surface, we neglect the effect of hydrody-
namic force acting on the skirt and the deformation of skirts (i.e. we assume that
the skirts are not deformable or at least have small deformations which can be
neglected).
2. We simplify the cushion plane as rectangular and adopt a longitudinal stability
skirt to compartmentalize the air cushion, assuming that the cushion pressure dis-
tributes uniformly both in left/right cushion compartments.
3. We consider the GXYZ system as the body coordinate system and the O^C system
as the fixed coordinate system as shown in Fig. 4.32.
The calculation method for predicting the static transverse stability of an ACV on
cushion is very similar to that for an SES, i.e. in the case where the ACV is heeling,
the lift due to the cushion pressure has to be equal to the weight of craft and the heel-
ing moment equivalent to the restoring moment about the CG caused by the cushion
pressure which satisfies the fan duct characteristic equation. The flow of the fan has