Page 86 - Theory and Design of Air Cushion Craft

70 Air cushion theory

The air cushion characteristic curves for both ACV/SES are shown by Fig. 2.21 (the
calculation in detail can be found in Chapter 11, Lift system design), where

• Hj-Q represents the characteristic curve of lift fans, p t-Q represents the character-
istic of the air ducting, i.e. the characteristic curve of a fan at any given revolution
minus the pressure loss of flow in air duct, p v represents the bag pressure of skirts
a.ndp t~Q also represents the characteristic of the bag.
• P~Q represents the characteristic for static air cushion performance, namely the
relation between flow and bag pressure at various hovering heights, which can be
obtained by the foregoing formula. For this reason, the curve p-Q represents the
relation between the bag pressure and flow rate andp t-Q denotes the total pressure
of air duct (or bag) at various hovering heights and fan revolutions.
The intersection point of both curves represents the hovering height of the craft at a
given craft weight (a given cushion pressure) and any given fan speed. Hence, the air
cushion characteristic curve for an ACV can be described as follows (also similar for
an SES):
1. The minimum fan speed for inflating the skirt of an ACV (similar to the hovering
attitude 4 in Fig. 2.20), will be that at which the total pressure of the lift fan equals
the cushion pressure at the zero flow rate. At this point the craft weight is sup-
ported by cushion lift perfectly, but without having risen from the static condition.
In the case of zero flow rate the total pressure of the fan is equal to the total pres-
sure of the duct bag and thus to the cushion pressure.
2. The factors necessary for hovering the craft, i.e. from attitude 1 transient to atti-
tude 3, is that the bottom of the buoyancy tank has to leave the water surface in
order to exert the cushion pressure to the bottom and lift the craft. At MARIC

p,

H-Q

Fig. 2.21 Air duct and air cushion characteristics curves of ACV/SES.