Page 151 - Theory and Design of Air Cushion Craft

134 Steady drag forces

30 G, =0.00205 03=0.00615
£> 2=0.0041

§20

0.5 1.0 Fr

Fig. 3.50 Influence of cushion air flow rate on Lift/drag ratio of SES on calm water.

importance of air flow rate of a modern ACV/SES is rather more significant than
early ACV/SES craft, which were generally designed with high cushion flow rates. In
the latter case, a large gap height was pursued in order to reduce the contact drag of
the bow/stern skirt, but in the former, it seems unnecessary to design a large gap
height due to the beneficial properties of the flexible skirt.
The air flow still affects spray drag and water friction drag at post-hump speeds,
because a large air flow rate will increase the air component of the two-phase flow
blown from the air cushion, producing air lubrication effects on the stern skirt/seal.
This can be clearly seen in Fig. 3.50.
In the case where the Q is so small as to cause significant immersion of skirts, the
take-off performance will deteriorate. For example, a Chinese ACV with too small air
flow could run over the hump speed on calm water, but could not take off even in
small head winds and waves. Take-off performance was improved by increasing the
lift power by 15%.

Effect of skirt configuration

The configurations of skirts are strongly sensitive to the craft drag. Figure 3.51 shows
the effect of both rigid and flexible seals on drag of an SES. It can be clearly seen that
the drag of craft with a flexible skirt is less than that with a rigid seal by a significant
margin. Table 3.6 demonstrates that the drag coefficient for skirts of French ACVs
improved significantly over eight years.

Table 3.6 The improvement of skirt drag coefficient of French ACVs over several years
Time interval Craft model Skirt drag coefficient
June 1968-June 1969 N102 0.36-0.42
Sept. 1967-March 1969 N300-02A 0.16-0.22
March 1969-June 1970 N102C 0.26-0.32
July 1970-June 1971 N102L 0.26-0.32
June 1973-Feb 1975 N300-02B 0.05-0.12
1973-1976 N500 (model) 0.05-0.07