Page 180 - Theory and Design of Air Cushion Craft

Calculation of ACV transverse stability 163

Commentary
It has been shown by Blyth's test programme that the on-cushion stability of an SES
should be principally assessed in relation to rolling behaviour in synchronous beam
seas and in relation to the hydrodynamic forces developed in high-speed turns.
In a seaway, capsizing of an SES is most probable in steepness-limited beam seas
with a period close to resonance. An alteration in course and/or a reduction in lift
power both substantially reduce the probability of capsize occurring. It has been
shown that for each design, there is a VCG below which capsizing becomes
improbable.
In high-speed turns, the hitherto unidentified possibility of large amplitude roll/yaw
oscillations occurring has been detected and examined. It seems probable that this
behaviour is associated with a zone of negative roll stiffness in turns, created by the
manner in which hydrodynamic forces vary with roll attitude.

of ACV transverse stability

Introduction

An ACV has no natural restoring moment from the cushion (plenum chamber) itself
while heeling on cushion. Air jet craft derived stability moments from the increased
force of the jet on the downgoing side and reduced force on the upgoing side, though
these were small and so such craft were very sensitive to movements of craft pay load.
As an ACV heels, due perhaps to the movement to one side of a person on board
creating an overturning moment, a negative restoring moment will act on the ACV if
no other stability moments are created by deformation of the peripheral skirts, as
shown in Fig. 4.26. This is because the cushion pressure will be the same across the
craft width in the case of no cushion compartmentation.
has
The skirt geometry and in the case of skirts with pressurized loops or bags, p\lp c
a strong influence on the righting moment which is generated for an ACV. When
travelling over water, the skirt stiffness will then affect the water displaced on the
downgoing side, in a similar way to the action of SES sidewalls, below hump speed.
Above hump speed, the skirt surface presented to the water acts as a planing surface,
though the force that can be generated is limited to that which can be transmitted
around the fabric membrane back to the craft's hard structure.

Cushion compartmentation
In the case where an ACV compartmented longitudinally hovers statically on a rigid
surface, cushion pressure on the side heeling down increases due to reduced air flow
and the cushion pressure decreases for the other side because of increased escape area
and therefore flow rate. Thus the different cushion pressures give a direct restoring
moment, moving the effective centre of pressure to the downward side of the craft, as
shown in Fig. 4.27.
Meanwhile, the transverse component of P ci)smO of the cushion pressure resultant
will also lead to a drifting motion. For this reason, drifting in general always
P ce