Page 193 - Theory and Design of Air Cushion Craft

176 Stability

significantly affect trim. As a result it is possible to experiment with a craft in suitable
conditions, to test the stability of a bow or side skirt at different speeds. Beyond a
small bow-down trim (1-2°) most skirts begin to wet (i.e. air flow is partly blocked and
some of the segment surface is not lubricated by the air flow), leading to a sharp rise
in drag. The beginning of this process can be seen on skirts without a swept-back bow,
as the segments or bag appear to 'nibble' which can be seen directly or via movement
of the loop. Beyond a further small trim-down, the rate of increase of drag causes
rapid deceleration of the craft and in the case where the skirt system is not stable,
either plough-in or overturning.
Sideways drift can cause a similar effect on the side skirts. Payload (CG) shift away
from the drift to bank the craft can be very helpful. In this respect, provision of ele-
vators or skirt shift mechanisms on utility size craft can be very important in main-
taining dynamic stability. The CG shifts required are too great and required too
quickly for larger craft, which have to rely on cushion compartmentation to keep the
skirt stability envelope outside the normal operating conditions.
The increasing hydrodynamic force (and moment) due to contact of the flexible
skirt with the water surface is the main reason leading to plough-in.
Before development of special skirt geometries, plough-in could be avoided only
with aid of driver operating rules formulated by users, or research and design
bureaux. Thus it can be seen that it is very important to study the rationale of plough-
in and the overturning phenomenon.
Some ACV plough-in and overturning incidents will be examined below and a
rationale developed. We will not present a theoretical analysis of this field due to its
complicated hydromechanics. Readers may find [48] useful as background material,
developed by the UK Department of Transport after the SR.N6 accident in 1973.

ACV overturning at low speed

In a similar way to an SES, the trough is so deformed on the water surface underneath
an ACV during take-off as to reduce the stability skirt effectiveness. Figure 4.42 shows
the inner water surface of a two dimensional ACV model at various Fr; it can be seen
that the trough is deep at Fr = 0.5-0.7, which causes the detrimental influence on the
transverse (or longitudinal) stability of the craft.
Figure 4.43 shows results of an investigation by W.A. Crago. He found that the
transverse stability deteriorated dramatically at Fr } = 0.33-0.56. Figure 4.44 shows
that the heeling moment and heeling angle increased at Fr { = 0.4 and the craft would
capsize at overturning moments exceeding M^WS^ = 0.022.
For this reason, as far as the drivers are concerned, great attention is required
during take-off, particularly in the case of long time duration for take-off due to
shortage of lift and propulsion power, or if for other reasons the craft stability is low,
due for example to a large amount of free surface liquid existing on the craft.
As far as designers are concerned, attention has to be paid to design skirts with a
stable geometry for the hydrodynamic forces expected at hump speed and a realistic
range of overturning moments and resultant craft trim.